ATMEGA16-16MQ_技术文档

Features

• High-performance, Low-power Atmel^®AVR^®8-bit Microcontroller

• Advanced RISC Architecture

– 131 Powerful Instructions – Most Single-clock Cycle Execution

– 32 x 8 General Purpose Working Registers

– Fully Static Operation

– Up to 16 MIPS Throughput at 16 MHz

– On-chip 2-cycle Multiplier

• High Endurance Non-volatile Memory segments

– 16 Kbytes of In-System Self-programmable Flash program memory

– 512 Bytes EEPROM

8-bit

– 1 Kbyte Internal SRAM

– Write/Erase Cycles: 10,000 Flash/100,000 EEPROM

– Data retention: 20 years at 85°C/100 years at 25°C⁽¹⁾

– Optional Boot Code Section with Independent Lock Bits

In-System Programming by On-chip Boot Program

True Read-While-Write Operation

Microcontroller

with 16K Bytes

In-System

Programmable

Flash

– Programming Lock for Software Security

• JTAG (IEEE std. 1149.1 Compliant) Interface

– Boundary-scan Capabilities According to the JTAG Standard

– Extensive On-chip Debug Support

– Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface

• Peripheral Features

– Two 8-bit Timer/Counters with Separate Prescalers and Compare Modes

– One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture

Mode

– Real Time Counter with Separate Oscillator

– Four PWM Channels

ATmega16

ATmega16L

– 8-channel, 10-bit ADC

8 Single-ended Channels

7 Differential Channels in TQFP Package Only

2 Differential Channels with Programmable Gain at 1x, 10x, or 200x

– Byte-oriented Two-wire Serial Interface

– Programmable Serial USART

– Master/Slave SPI Serial Interface

– Programmable Watchdog Timer with Separate On-chip Oscillator

– On-chip Analog Comparator

• Special Microcontroller Features

– Power-on Reset and Programmable Brown-out Detection

– Internal Calibrated RC Oscillator

– External and Internal Interrupt Sources

– Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby

and Extended Standby

• I/O and Packages

– 32 Programmable I/O Lines

– 40-pin PDIP, 44-lead TQFP, and 44-pad QFN/MLF

• Operating Voltages

– 2.7V - 5.5V for ATmega16L

– 4.5V - 5.5V for ATmega16

• Speed Grades

– 0 - 8 MHz for ATmega16L

– 0 - 16 MHz for ATmega16

• Power Consumption @ 1 MHz, 3V, and 25°C for ATmega16L

– Active: 1.1 mA

– Idle Mode: 0.35 mA

– Power-down Mode: < 1 µA

Rev. 2466T–AVR–07/10

ATmega16(L)

Figure 1. Pinout ATmega16

Configurations

PDIP

(XCK/T0) PB0

(T1) PB1

(INT2/AIN0) PB2

(OC0/AIN1) PB3

(SS) PB4

PA0 (ADC0)

PA1 (ADC1)

PA2 (ADC2)

PA3 (ADC3)

PA4 (ADC4)

PA5 (ADC5)

PA6 (ADC6)

PA7 (ADC7)

AREF

(MOSI) PB5

(MISO) PB6

(SCK) PB7

RESET

VCC

GND

XTAL2

XTAL1

GND

AVCC

PC7 (TOSC2)

PC6 (TOSC1)

PC5 (TDI)

PC4 (TDO)

PC3 (TMS)

PC2 (TCK)

PC1 (SDA)

PC0 (SCL)

PD7 (OC2)

(RXD) PD0

(TXD) PD1

(INT0) PD2

(INT1) PD3

(OC1B) PD4

(OC1A) PD5

(ICP1) PD6

TQFP/QFN/MLF

(MOSI) PB5

PA4 (ADC4)

(MISO) PB6

(SCK) PB7

RESET

PA5 (ADC5)

PA6 (ADC6)

PA7 (ADC7)

AREF

GND

AVCC

PC7 (TOSC2)

PC6 (TOSC1)

PC5 (TDI)

PC4 (TDO)

VCC

GND

XTAL2

XTAL1

(RXD) PD0

(TXD) PD1

(INT0) PD2

NOTE:

Bottom pad should

be soldered to ground.

Disclaimer

Typical values contained in this datasheet are based on simulations and characterization of

other AVR microcontrollers manufactured on the same process technology. Min and Max values

will be available after the device is characterized.

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ATmega16(L)

Overview

The ATmega16 is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC

architecture. By executing powerful instructions in a single clock cycle, the ATmega16 achieves

throughputs approaching 1 MIPS per MHz allowing the system designer to optimize power con-

sumption versus processing speed.

Block Diagram

Figure 2. Block Diagram

PA0 - PA7

PC0 - PC7

VCC

PORTA DRIVERS/BUFFERS

PORTA DIGITAL INTERFACE

PORTC DRIVERS/BUFFERS

PORTC DIGITAL INTERFACE

GND

AVCC

AREF

ADC

INTERFACE

MUX &

ADC

TWI

TIMERS/

COUNTERS

OSCILLATOR

PROGRAM

COUNTER

STACK

POINTER

PROGRAM

FLASH

INTERNAL

OSCILLATOR

SRAM

XTAL1

INSTRUCTION

REGISTER

WATCHDOG

TIMER

GENERAL

PURPOSE

REGISTERS

OSCILLATOR

XTAL2

X

Y

Z

INSTRUCTION

DECODER

MCU CTRL.

& TIMING

RESET

INTERNAL

CALIBRATED

OSCILLATOR

CONTROL

LINES

INTERRUPT

UNIT

ALU

STATUS

REGISTER

AVR CPU

EEPROM

USART

PROGRAMMING

LOGIC

SPI

+

-

COMP.

INTERFACE

PORTB DIGITAL INTERFACE

PORTD DIGITAL INTERFACE

PORTB DRIVERS/BUFFERS

PORTD DRIVERS/BUFFERS

PB0 - PB7

PD0 - PD7

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ATmega16(L)

The AVR core combines a rich instruction set with 32 general purpose working registers. All the

32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent

registers to be accessed in one single instruction executed in one clock cycle. The resulting

architecture is more code efficient while achieving throughputs up to ten times faster than con-

ventional CISC microcontrollers.

The ATmega16 provides the following features: 16 Kbytes of In-System Programmable Flash

Program memory with Read-While-Write capabilities, 512 bytes EEPROM, 1 Kbyte SRAM, 32

general purpose I/O lines, 32 general purpose working registers, a JTAG interface for Boundary-

scan, On-chip Debugging support and programming, three flexible Timer/Counters with com-

pare modes, Internal and External Interrupts, a serial programmable USART, a byte oriented

Two-wire Serial Interface, an 8-channel, 10-bit ADC with optional differential input stage with

programmable gain (TQFP package only), a programmable Watchdog Timer with Internal Oscil-

lator, an SPI serial port, and six software selectable power saving modes. The Idle mode stops

the CPU while allowing the USART, Two-wire interface, A/D Converter, SRAM, Timer/Counters,

SPI port, and interrupt system to continue functioning. The Power-down mode saves the register

contents but freezes the Oscillator, disabling all other chip functions until the next External Inter-

rupt or Hardware Reset. In Power-save mode, the Asynchronous Timer continues to run,

allowing the user to maintain a timer base while the rest of the device is sleeping. The ADC

Noise Reduction mode stops the CPU and all I/O modules except Asynchronous Timer and

ADC, to minimize switching noise during ADC conversions. In Standby mode, the crystal/reso-

nator Oscillator is running while the rest of the device is sleeping. This allows very fast start-up

combined with low-power consumption. In Extended Standby mode, both the main Oscillator

and the Asynchronous Timer continue to run.

The device is manufactured using Atmel’s high density nonvolatile memory technology. The On-

chip ISP Flash allows the program memory to be reprogrammed in-system through an SPI serial

interface, by a conventional nonvolatile memory programmer, or by an On-chip Boot program

running on the AVR core. The boot program can use any interface to download the application

program in the Application Flash memory. Software in the Boot Flash section will continue to run

while the Application Flash section is updated, providing true Read-While-Write operation. By

combining an 8-bit RISC CPU with In-System Self-Programmable Flash on a monolithic chip,

the Atmel ATmega16 is a powerful microcontroller that provides a highly-flexible and cost-effec-

tive solution to many embedded control applications.

The ATmega16 AVR is supported with a full suite of program and system development tools

including: C compilers, macro assemblers, program debugger/simulators, in-circuit emulators,

and evaluation kits.

Pin Descriptions

VCC

Digital supply voltage.

GND

Ground.

Port A (PA7..PA0)

Port A serves as the analog inputs to the A/D Converter.

Port A also serves as an 8-bit bi-directional I/O port, if the A/D Converter is not used. Port pins

can provide internal pull-up resistors (selected for each bit). The Port A output buffers have sym-

metrical drive characteristics with both high sink and source capability. When pins PA0 to PA7

are used as inputs and are externally pulled low, they will source current if the internal pull-up

resistors are activated. The Port A pins are tri-stated when a reset condition becomes active,

even if the clock is not running.

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ATmega16(L)

Port B (PB7..PB0)

Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The

Port B output buffers have symmetrical drive characteristics with both high sink and source

capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up

resistors are activated. The Port B pins are tri-stated when a reset condition becomes active,

even if the clock is not running.

Port B also serves the functions of various special features of the ATmega16 as listed on page

58.

Port C (PC7..PC0)

Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The

Port C output buffers have symmetrical drive characteristics with both high sink and source

capability. As inputs, Port C pins that are externally pulled low will source current if the pull-up

resistors are activated. The Port C pins are tri-stated when a reset condition becomes active,

even if the clock is not running. If the JTAG interface is enabled, the pull-up resistors on pins

PC5(TDI), PC3(TMS) and PC2(TCK) will be activated even if a reset occurs.

Port C also serves the functions of the JTAG interface and other special features of the

ATmega16 as listed on page 61.

Port D (PD7..PD0)

Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The

Port D output buffers have symmetrical drive characteristics with both high sink and source

capability. As inputs, Port D pins that are externally pulled low will source current if the pull-up

resistors are activated. The Port D pins are tri-stated when a reset condition becomes active,

even if the clock is not running.

Port D also serves the functions of various special features of the ATmega16 as listed on page

63.

RESET

Reset Input. A low level on this pin for longer than the minimum pulse length will generate a

reset, even if the clock is not running. The minimum pulse length is given in Table 15 on page

38. Shorter pulses are not guaranteed to generate a reset.

XTAL1

XTAL2

AVCC

Input to the inverting Oscillator amplifier and input to the internal clock operating circuit.

Output from the inverting Oscillator amplifier.

AVCC is the supply voltage pin for Port A and the A/D Converter. It should be externally con-

nected to V_CC, even if the ADC is not used. If the ADC is used, it should be connected to V_CC

through a low-pass filter.

AREF

AREF is the analog reference pin for the A/D Converter.

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ATmega16(L)

Resources

A comprehensive set of development tools, application notes and datasheets are available for

download on http://www.atmel.com/avr.

Note:

1.

Data Retention

Reliability Qualification results show that the projected data retention failure rate is much less

than 1 PPM over 20 years at 85°C or 100 years at 25°C.

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ATmega16(L)

About Code

Examples

This documentation contains simple code examples that briefly show how to use various parts of

the device. These code examples assume that the part specific header file is included before

compilation. Be aware that not all C Compiler vendors include bit definitions in the header files

and interrupt handling in C is compiler dependent. Please confirm with the C Compiler documen-

tation for more details.

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ATmega16(L)

AVR CPU Core

Introduction

This section discusses the AVR core architecture in general. The main function of the CPU core

is to ensure correct program execution. The CPU must therefore be able to access memories,

perform calculations, control peripherals, and handle interrupts.

Architectural

Overview

Figure 3. Block Diagram of the AVR MCU Architecture

Data Bus 8-bit

Program

Counter

Status

and Control

Flash

Program

Memory

Interrupt

Unit

32 x 8

General

Purpose

Registrers

Instruction

Register

SPI

Unit

Instruction

Decoder

Watchdog

Timer

ALU

Analog

Comparator

Control Lines

I/O Module1

I/O Module 2

I/O Module n

Data

SRAM

EEPROM

I/O Lines

In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with

separate memories and buses for program and data. Instructions in the program memory are

executed with a single level pipelining. While one instruction is being executed, the next instruc-

tion is pre-fetched from the program memory. This concept enables instructions to be executed

in every clock cycle. The program memory is In-System Reprogrammable Flash memory.

The fast-access Register File contains 32 × 8-bit general purpose working registers with a single

clock cycle access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typ-

ical ALU operation, two operands are output from the Register File, the operation is executed,

and the result is stored back in the Register File – in one clock cycle.

Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data

Space addressing – enabling efficient address calculations. One of the these address pointers

can also be used as an address pointer for look up tables in Flash Program memory. These

added function registers are the 16-bit X-register, Y-register, and Z-register, described later in

this section.

The ALU supports arithmetic and logic operations between registers or between a constant and

a register. Single register operations can also be executed in the ALU. After an arithmetic opera-

tion, the Status Register is updated to reflect information about the result of the operation.

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ATmega16(L)

Program flow is provided by conditional and unconditional jump and call instructions, able to

directly address the whole address space. Most AVR instructions have a single 16-bit word for-

mat. Every program memory address contains a 16-bit or 32-bit instruction.

Program Flash memory space is divided in two sections, the Boot program section and the

Application Program section. Both sections have dedicated Lock bits for write and read/write

protection. The SPM instruction that writes into the Application Flash memory section must

reside in the Boot Program section.

During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the

Stack. The Stack is effectively allocated in the general data SRAM, and consequently the Stack

size is only limited by the total SRAM size and the usage of the SRAM. All user programs must

initialize the SP in the reset routine (before subroutines or interrupts are executed). The Stack

Pointer SP is read/write accessible in the I/O space. The data SRAM can easily be accessed

through the five different addressing modes supported in the AVR architecture.

The memory spaces in the AVR architecture are all linear and regular memory maps.

A flexible interrupt module has its control registers in the I/O space with an additional global

interrupt enable bit in the Status Register. All interrupts have a separate interrupt vector in the

interrupt vector table. The interrupts have priority in accordance with their interrupt vector posi-

tion. The lower the interrupt vector address, the higher the priority.

The I/O memory space contains 64 addresses for CPU peripheral functions as Control Regis-

ters, SPI, and other I/O functions. The I/O Memory can be accessed directly, or as the Data

Space locations following those of the Register File, $20 - $5F.

ALU – Arithmetic

Logic Unit

The high-performance AVR ALU operates in direct connection with all the 32 general purpose

working registers. Within a single clock cycle, arithmetic operations between general purpose

registers or between a register and an immediate are executed. The ALU operations are divided

into three main categories – arithmetic, logical, and bit-functions. Some implementations of the

architecture also provide a powerful multiplier supporting both signed/unsigned multiplication

and fractional format. See the “Instruction Set” section for a detailed description.

Status Register

The Status Register contains information about the result of the most recently executed arithme-

tic instruction. This information can be used for altering program flow in order to perform

conditional operations. Note that the Status Register is updated after all ALU operations, as

specified in the Instruction Set Reference. This will in many cases remove the need for using the

dedicated compare instructions, resulting in faster and more compact code.

The Status Register is not automatically stored when entering an interrupt routine and restored

when returning from an interrupt. This must be handled by software.

The AVR Status Register – SREG – is defined as:

Bit

7

I

6

T

5

H

4

S

3

V

2

N

1

Z

0

C

SREG

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

• Bit 7 – I: Global Interrupt Enable

The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual inter-

rupt enable control is then performed in separate control registers. If the Global Interrupt Enable

Register is cleared, none of the interrupts are enabled independent of the individual interrupt

enable settings. The I-bit is cleared by hardware after an interrupt has occurred, and is set by

the RETI instruction to enable subsequent interrupts. The I-bit can also be set and cleared by

the application with the SEI and CLI instructions, as described in the instruction set reference.

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ATmega16(L)

• Bit 6 – T: Bit Copy Storage

The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or desti-

nation for the operated bit. A bit from a register in the Register File can be copied into T by the

BST instruction, and a bit in T can be copied into a bit in a register in the Register File by the

BLD instruction.

• Bit 5 – H: Half Carry Flag

The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry is useful

in BCD arithmetic. See the “Instruction Set Description” for detailed information.

• Bit 4 – S: Sign Bit, S = N ⊕ V

The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement

Overflow Flag V. See the “Instruction Set Description” for detailed information.

• Bit 3 – V: Two’s Complement Overflow Flag

The Two’s Complement Overflow Flag V supports two’s complement arithmetics. See the

“Instruction Set Description” for detailed information.

• Bit 2 – N: Negative Flag

The Negative Flag N indicates a negative result in an arithmetic or logic operation. See the

“Instruction Set Description” for detailed information.

• Bit 1 – Z: Zero Flag

The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the “Instruction

Set Description” for detailed information.

• Bit 0 – C: Carry Flag

The Carry Flag C indicates a carry in an arithmetic or logic operation. See the “Instruction Set

Description” for detailed information.

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ATmega16(L)

General Purpose

Register File

The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve

the required performance and flexibility, the following input/output schemes are supported by the

Register File:

•

One 8-bit output operand and one 8-bit result input

Two 8-bit output operands and one 8-bit result input

Two 8-bit output operands and one 16-bit result input

One 16-bit output operand and one 16-bit result input

Figure 4 shows the structure of the 32 general purpose working registers in the CPU.

Figure 4. AVR CPU General Purpose Working Registers

7

0

Addr.

R0

R1

$00

$01

$02

R2

…

R13

R14

R15

R16

R17

…

$0D

$0E

$0F

$10

$11

General

Purpose

Working

Registers

R26

R27

R28

R29

R30

R31

$1A

$1B

$1C

$1D

$1E

$1F

X-register Low Byte

X-register High Byte

Y-register Low Byte

Y-register High Byte

Z-register Low Byte

Z-register High Byte

Most of the instructions operating on the Register File have direct access to all registers, and

most of them are single cycle instructions.

As shown in Figure 4, each register is also assigned a data memory address, mapping them

directly into the first 32 locations of the user Data Space. Although not being physically imple-

mented as SRAM locations, this memory organization provides great flexibility in access of the

registers, as the X-, Y-, and Z-pointer Registers can be set to index any register in the file.

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ATmega16(L)

The X-register, Y-

The registers R26..R31 have some added functions to their general purpose usage. These reg-

register and Z-register isters are 16-bit address pointers for indirect addressing of the Data Space. The three indirect

address registers X, Y, and Z are defined as described in Figure 5.

Figure 5. The X-register, Y-register, and Z-register

15

XH

YH

XL

0

X - register

7

0

7

R27 ($1B)

R26 ($1A)

15

YL

ZL

0

Y - register

Z - register

7

R29 ($1D)

R28 ($1C)

15

ZH

0

7

0

R31 ($1F)

R30 ($1E)

In the different addressing modes these address registers have functions as fixed displacement,

automatic increment, and automatic decrement (see the Instruction Set Reference for details).

Stack Pointer

The Stack is mainly used for storing temporary data, for storing local variables and for storing

return addresses after interrupts and subroutine calls. The Stack Pointer Register always points

to the top of the Stack. Note that the Stack is implemented as growing from higher memory loca-

tions to lower memory locations. This implies that a Stack PUSH command decreases the Stack

Pointer. If software reads the Program Counter from the Stack after a call or an interrupt, unused

bits (15:13) should be masked out.

The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt

Stacks are located. This Stack space in the data SRAM must be defined by the program before

any subroutine calls are executed or interrupts are enabled. The Stack Pointer must be set to

point above $60. The Stack Pointer is decremented by one when data is pushed onto the Stack

with the PUSH instruction, and it is decremented by two when the return address is pushed onto

the Stack with subroutine call or interrupt. The Stack Pointer is incremented by one when data is

popped from the Stack with the POP instruction, and it is incremented by two when data is

popped from the Stack with return from subroutine RET or return from interrupt RETI.

The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of

bits actually used is implementation dependent. Note that the data space in some implementa-

tions of the AVR architecture is so small that only SPL is needed. In this case, the SPH Register

will not be present.

Bit

15

SP15

SP7

7

14

SP14

SP6

6

13

SP13

SP5

5

12

SP12

SP4

4

11

SP11

SP3

3

10

SP10

SP2

2

9

SP9

SP1

1

8

SP8

SP0

0

SPH

SPL

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

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ATmega16(L)

Instruction

Execution Timing

This section describes the general access timing concepts for instruction execution. The AVR

CPU is driven by the CPU clock clk_CPU, directly generated from the selected clock source for the

chip. No internal clock division is used.

Figure 6 shows the parallel instruction fetches and instruction executions enabled by the Har-

vard architecture and the fast-access Register File concept. This is the basic pipelining concept

to obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost,

functions per clocks, and functions per power-unit.

Figure 6. The Parallel Instruction Fetches and Instruction Executions

T1

T2

T3

T4

clk_CPU

1st Instruction Fetch

1st Instruction Execute

2nd Instruction Fetch

2nd Instruction Execute

3rd Instruction Fetch

3rd Instruction Execute

4th Instruction Fetch

Figure 7 shows the internal timing concept for the Register File. In a single clock cycle an ALU

operation using two register operands is executed, and the result is stored back to the destina-

tion register.

Figure 7. Single Cycle ALU Operation

T1

T2

T3

T4

clk_CPU

Total Execution Time

Register Operands Fetch

ALU Operation Execute

Result Write Back

Reset and

Interrupt Handling

The AVR provides several different interrupt sources. These interrupts and the separate reset

vector each have a separate program vector in the program memory space. All interrupts are

assigned individual enable bits which must be written logic one together with the Global Interrupt

Enable bit in the Status Register in order to enable the interrupt. Depending on the Program

Counter value, interrupts may be automatically disabled when Boot Lock bits BLB02 or BLB12

are programmed. This feature improves software security. See the section “Memory Program-

ming” on page 259 for details.

The lowest addresses in the program memory space are by default defined as the Reset and

Interrupt Vectors. The complete list of vectors is shown in “Interrupts” on page 45. The list also

determines the priority levels of the different interrupts. The lower the address the higher is the

priority level. RESET has the highest priority, and next is INT0 – the External Interrupt Request

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ATmega16(L)

0. The Interrupt Vectors can be moved to the start of the Boot Flash section by setting the IVSEL

bit in the General Interrupt Control Register (GICR). Refer to “Interrupts” on page 45 for more

information. The Reset Vector can also be moved to the start of the boot Flash section by pro-

gramming the BOOTRST Fuse, see “Boot Loader Support – Read-While-Write Self-

Programming” on page 246.

When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are dis-

abled. The user software can write logic one to the I-bit to enable nested interrupts. All enabled

interrupts can then interrupt the current interrupt routine. The I-bit is automatically set when a

Return from Interrupt instruction – RETI – is executed.

There are basically two types of interrupts. The first type is triggered by an event that sets the

Interrupt Flag. For these interrupts, the Program Counter is vectored to the actual Interrupt Vec-

tor in order to execute the interrupt handling routine, and hardware clears the corresponding

Interrupt Flag. Interrupt Flags can also be cleared by writing a logic one to the flag bit position(s)

to be cleared. If an interrupt condition occurs while the corresponding interrupt enable bit is

cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled, or the flag is

cleared by software. Similarly, if one or more interrupt conditions occur while the Global Interrupt

Enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the

global interrupt enable bit is set, and will then be executed by order of priority.

The second type of interrupts will trigger as long as the interrupt condition is present. These

interrupts do not necessarily have Interrupt Flags. If the interrupt condition disappears before the

interrupt is enabled, the interrupt will not be triggered.

When the AVR exits from an interrupt, it will always return to the main program and execute one

more instruction before any pending interrupt is served.

Note that the Status Register is not automatically stored when entering an interrupt routine, nor

restored when returning from an interrupt routine. This must be handled by software.

When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled.

No interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the

CLI instruction. The following example shows how this can be used to avoid interrupts during the

timed EEPROM write sequence.

Assembly Code Example

in r16, SREG

; store SREG value

cli ; disable interrupts during timed sequence

sbi EECR, EEMWE

sbi EECR, EEWE

out SREG, r16

; start EEPROM write

; restore SREG value (I-bit)

C Code Example

char cSREG;

cSREG = SREG; /* store SREG value */

/* disable interrupts during timed sequence */

_CLI();

EECR |= (1<<EEMWE); /* start EEPROM write */

EECR |= (1<<EEWE);

SREG = cSREG; /* restore SREG value (I-bit) */

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ATmega16(L)

When using the SEI instruction to enable interrupts, the instruction following SEI will be exe-

cuted before any pending interrupts, as shown in this example.

Assembly Code Example

sei

; set global interrupt enable

sleep ; enter sleep, waiting for interrupt

; note: will enter sleep before any pending

; interrupt(s)

C Code Example

_SEI(); /* set global interrupt enable */

_SLEEP(); /* enter sleep, waiting for interrupt */

/* note: will enter sleep before any pending interrupt(s) */

Interrupt Response

Time

The interrupt execution response for all the enabled AVR interrupts is four clock cycles mini-

mum. After four clock cycles the program vector address for the actual interrupt handling routine

is executed. During this four clock cycle period, the Program Counter is pushed onto the Stack.

The vector is normally a jump to the interrupt routine, and this jump takes three clock cycles. If

an interrupt occurs during execution of a multi-cycle instruction, this instruction is completed

before the interrupt is served. If an interrupt occurs when the MCU is in sleep mode, the interrupt

execution response time is increased by four clock cycles. This increase comes in addition to the

start-up time from the selected sleep mode.

A return from an interrupt handling routine takes four clock cycles. During these four clock

cycles, the Program Counter (two bytes) is popped back from the Stack, the Stack Pointer is

incremented by two, and the I-bit in SREG is set.

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ATmega16(L)

AVR ATmega16 This section describes the different memories in the ATmega16. The AVR architecture has two

main memory spaces, the Data Memory and the Program Memory space. In addition, the

ATmega16 features an EEPROM Memory for data storage. All three memory spaces are linear

and regular.

Memories

In-System

The ATmega16 contains 16 Kbytes On-chip In-System Reprogrammable Flash memory for pro-

gram storage. Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as 8K ×

16. For software security, the Flash Program memory space is divided into two sections, Boot

Program section and Application Program section.

Reprogrammable

Flash Program

Memory

The Flash memory has an endurance of at least 10,000 write/erase cycles. The ATmega16 Pro-

gram Counter (PC) is 13 bits wide, thus addressing the 8K program memory locations. The

operation of Boot Program section and associated Boot Lock bits for software protection are

described in detail in “Boot Loader Support – Read-While-Write Self-Programming” on page

246. “Memory Programming” on page 259 contains a detailed description on Flash data serial

downloading using the SPI pins or the JTAG interface.

Constant tables can be allocated within the entire program memory address space (see the LPM

– Load Program Memory Instruction Description).

Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Tim-

ing” on page 13.

Figure 8. Program Memory Map

$0000

Application Flash Section

Boot Flash Section

$1FFF

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ATmega16(L)

SRAM Data

Memory

Figure 9 shows how the ATmega16 SRAM Memory is organized.

The lower 1120 Data Memory locations address the Register File, the I/O Memory, and the inter-

nal data SRAM. The first 96 locations address the Register File and I/O Memory, and the next

1024 locations address the internal data SRAM.

The five different addressing modes for the data memory cover: Direct, Indirect with Displace-

ment, Indirect, Indirect with Pre-decrement, and Indirect with Post-increment. In the Register

File, registers R26 to R31 feature the indirect addressing pointer registers.

The direct addressing reaches the entire data space.

The Indirect with Displacement mode reaches 63 address locations from the base address given

by the Y-register or Z-register.

When using register indirect addressing modes with automatic pre-decrement and post-incre-

ment, the address registers X, Y, and Z are decremented or incremented.

The 32 general purpose working registers, 64 I/O Registers, and the 1024 bytes of internal data

SRAM in the ATmega16 are all accessible through all these addressing modes. The Register

File is described in “General Purpose Register File” on page 11.

Figure 9. Data Memory Map

Register File

Data Address Space

R0

R1

$0000

$0001

R2

...

$0002

...

R29

R30

R31

$001D

$001E

$001F

I/O Registers

$00

$01

$0020

$0021

$02

...

$0022

...

$3D

$3E

$3F

$005D

$005E

$005F

Internal SRAM

$0060

$0061

...

$045E

$045F

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Data Memory Access

Times

This section describes the general access timing concepts for internal memory access. The

internal data SRAM access is performed in two clk_CPUcycles as described in Figure 10.

Figure 10. On-chip Data SRAM Access Cycles

T1

T2

T3

clk_CPU

Address Valid

Compute Address

Address

Data

WR

Data

RD

Memory Access Instruction

Next Instruction

EEPROM Data

Memory

The ATmega16 contains 512 bytes of data EEPROM memory. It is organized as a separate data

space, in which single bytes can be read and written. The EEPROM has an endurance of at

least 100,000 write/erase cycles. The access between the EEPROM and the CPU is described

in the following, specifying the EEPROM Address Registers, the EEPROM Data Register, and

the EEPROM Control Register.

For a detailed description of SPI, JTAG, and Parallel data downloading to the EEPROM, see

page 273, page 278, and page 262, respectively.

EEPROM Read/Write

Access

The EEPROM Access Registers are accessible in the I/O space.

The write access time for the EEPROM is given in Table 1. A self-timing function, however, lets

the user software detect when the next byte can be written. If the user code contains instructions

that write the EEPROM, some precautions must be taken. In heavily filtered power supplies, V_CC

is likely to rise or fall slowly on Power-up/down. This causes the device for some period of time

to run at a voltage lower than specified as minimum for the clock frequency used. See “Prevent-

ing EEPROM Corruption” on page 22 for details on how to avoid problems in these situations.

In order to prevent unintentional EEPROM writes, a specific write procedure must be followed.

Refer to the description of the EEPROM Control Register for details on this.

When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is

executed. When the EEPROM is written, the CPU is halted for two clock cycles before the next

instruction is executed.

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ATmega16(L)

The EEPROM Address

Register – EEARH and

EEARL

Bit

15

14

13

12

11

10

9

8

–

EEAR8

EEAR0

0

EEARH

EEARL

EEAR7

EEAR6

EEAR5

EEAR4

EEAR3

EEAR2

EEAR1

7

R

6

R

5

R

4

R

3

R

2

R

1

R

Read/Write

Initial Value

R/W

X

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

X

• Bits 15..9 – Res: Reserved Bits

These bits are reserved bits in the ATmega16 and will always read as zero.

• Bits 8..0 – EEAR8..0: EEPROM Address

The EEPROM Address Registers ^–EEARH and EEARL – specify the EEPROM address in the

512 bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and

511. The initial value of EEAR is undefined. A proper value must be written before the EEPROM

may be accessed.

The EEPROM Data

Register – EEDR

Bit

7

6

5

4

3

2

1

0

MSB

R/W

0

LSB

R/W

0

EEDR

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

• Bits 7..0 – EEDR7.0: EEPROM Data

For the EEPROM write operation, the EEDR Register contains the data to be written to the

EEPROM in the address given by the EEAR Register. For the EEPROM read operation, the

EEDR contains the data read out from the EEPROM at the address given by EEAR.

The EEPROM Control

Register – EECR

Bit

7

–

6

–

5

–

4

–

3

EERIE

R/W

0

2

EEMWE

R/W

0

1

EEWE

R/W

X

0

EERE

R/W

0

EECR

Read/Write

Initial Value

R

0

R

0

R

0

R

0

• Bits 7..4 – Res: Reserved Bits

These bits are reserved bits in the ATmega16 and will always read as zero.

• Bit 3 – EERIE: EEPROM Ready Interrupt Enable

Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set. Writing

EERIE to zero disables the interrupt. The EEPROM Ready interrupt generates a constant inter-

rupt when EEWE is cleared.

• Bit 2 – EEMWE: EEPROM Master Write Enable

The EEMWE bit determines whether setting EEWE to one causes the EEPROM to be written.

When EEMWE is set, setting EEWE within four clock cycles will write data to the EEPROM at

the selected address If EEMWE is zero, setting EEWE will have no effect. When EEMWE has

been written to one by software, hardware clears the bit to zero after four clock cycles. See the

description of the EEWE bit for an EEPROM write procedure.

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ATmega16(L)

• Bit 1 – EEWE: EEPROM Write Enable

The EEPROM Write Enable Signal EEWE is the write strobe to the EEPROM. When address

and data are correctly set up, the EEWE bit must be written to one to write the value into the

EEPROM. The EEMWE bit must be written to one before a logical one is written to EEWE, oth-

erwise no EEPROM write takes place. The following procedure should be followed when writing

the EEPROM (the order of steps 3 and 4 is not essential):

1. Wait until EEWE becomes zero.

2. Wait until SPMEN in SPMCR becomes zero.

3. Write new EEPROM address to EEAR (optional).

4. Write new EEPROM data to EEDR (optional).

5. Write a logical one to the EEMWE bit while writing a zero to EEWE in EECR.

6. Within four clock cycles after setting EEMWE, write a logical one to EEWE.

The EEPROM can not be programmed during a CPU write to the Flash memory. The software

must check that the Flash programming is completed before initiating a new EEPROM write.

Step 2 is only relevant if the software contains a Boot Loader allowing the CPU to program the

Flash. If the Flash is never being updated by the CPU, step 2 can be omitted. See “Boot Loader

Support – Read-While-Write Self-Programming” on page 246 for details about boot

programming.

Caution: An interrupt between step 5 and step 6 will make the write cycle fail, since the

EEPROM Master Write Enable will time-out. If an interrupt routine accessing the EEPROM is

interrupting another EEPROM Access, the EEAR or EEDR reGister will be modified, causing the

interrupted EEPROM Access to fail. It is recommended to have the Global Interrupt Flag cleared

during all the steps to avoid these problems.

When the write access time has elapsed, the EEWE bit is cleared by hardware. The user soft-

ware can poll this bit and wait for a zero before writing the next byte. When EEWE has been set,

the CPU is halted for two cycles before the next instruction is executed.

• Bit 0 – EERE: EEPROM Read Enable

The EEPROM Read Enable Signal – EERE – is the read strobe to the EEPROM. When the cor-

rect address is set up in the EEAR Register, the EERE bit must be written to a logic one to

trigger the EEPROM read. The EEPROM read access takes one instruction, and the requested

data is available immediately. When the EEPROM is read, the CPU is halted for four cycles

before the next instruction is executed.

The user should poll the EEWE bit before starting the read operation. If a write operation is in

progress, it is neither possible to read the EEPROM, nor to change the EEAR Register.

The calibrated Oscillator is used to time the EEPROM accesses. Table 1 lists the typical pro-

gramming time for EEPROM access from the CPU.

Table 1. EEPROM Programming Time

Number of Calibrated RC

Symbol

Oscillator Cycles⁽¹⁾

Typ Programming Time

EEPROM write (from CPU)

8448

8.5 ms

Note:

1. Uses 1 MHz clock, independent of CKSEL Fuse setting.

The following code examples show one assembly and one C function for writing to the

EEPROM. The examples assume that interrupts are controlled (for example by disabling inter-

rupts globally) so that no interrupts will occur during execution of these functions. The examples

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ATmega16(L)

also assume that no Flash Boot Loader is present in the software. If such code is present, the

EEPROM write function must also wait for any ongoing SPM command to finish.

Assembly Code Example

EEPROM_write:

; Wait for completion of previous write

sbic EECR,EEWE

rjmp EEPROM_write

; Set up address (r18:r17) in address register

out EEARH, r18

out EEARL, r17

; Write data (r16) to data register

out EEDR,r16

; Write logical one to EEMWE

sbi EECR,EEMWE

; Start eeprom write by setting EEWE

sbi EECR,EEWE

ret

C Code Example

void EEPROM_write(unsigned int uiAddress, unsigned char ucData)

{

/* Wait for completion of previous write */

while(EECR & (1<<EEWE))

;

/* Set up address and data registers */

EEAR = uiAddress;

EEDR = ucData;

/* Write logical one to EEMWE */

EECR |= (1<<EEMWE);

/* Start eeprom write by setting EEWE */

EECR |= (1<<EEWE);

}

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ATmega16(L)

The next code examples show assembly and C functions for reading the EEPROM. The exam-

ples assume that interrupts are controlled so that no interrupts will occur during execution of

these functions.

Assembly Code Example

EEPROM_read:

; Wait for completion of previous write

sbic EECR,EEWE

rjmp EEPROM_read

; Set up address (r18:r17) in address register

out EEARH, r18

out EEARL, r17

; Start eeprom read by writing EERE

sbi EECR,EERE

; Read data from data register

in

r16,EEDR

ret

C Code Example

unsigned char EEPROM_read(unsigned int uiAddress)

{

/* Wait for completion of previous write */

while(EECR & (1<<EEWE))

;

/* Set up address register */

EEAR = uiAddress;

/* Start eeprom read by writing EERE */

EECR |= (1<<EERE);

/* Return data from data register */

return EEDR;

}

EEPROM Write During When entering Power-down Sleep mode while an EEPROM write operation is active, the

Power-down Sleep

Mode

EEPROM write operation will continue, and will complete before the Write Access time has

passed. However, when the write operation is completed, the Oscillator continues running, and

as a consequence, the device does not enter Power-down entirely. It is therefore recommended

to verify that the EEPROM write operation is completed before entering Power-down.

Preventing EEPROM

Corruption

During periods of low V_CC,the EEPROM data can be corrupted because the supply voltage is

too low for the CPU and the EEPROM to operate properly. These issues are the same as for

board level systems using EEPROM, and the same design solutions should be applied.

An EEPROM data corruption can be caused by two situations when the voltage is too low. First,

a regular write sequence to the EEPROM requires a minimum voltage to operate correctly. Sec-

ondly, the CPU itself can execute instructions incorrectly, if the supply voltage is too low.

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ATmega16(L)

EEPROM data corruption can easily be avoided by following this design recommendation:

Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This

can be done by enabling the internal Brown-out Detector (BOD). If the detection level of the

internal BOD does not match the needed detection level, an external low V_CCReset Protec-

tion circuit can be used. If a reset occurs while a write operation is in progress, the write

operation will be completed provided that the power supply voltage is sufficient.

I/O Memory

The I/O space definition of the ATmega16 is shown in “Register Summary” on page 331.

All ATmega16 I/Os and peripherals are placed in the I/O space. The I/O locations are accessed

by the IN and OUT instructions, transferring data between the 32 general purpose working regis-

ters and the I/O space. I/O Registers within the address range $00 - $1F are directly bit-

accessible using the SBI and CBI instructions. In these registers, the value of single bits can be

checked by using the SBIS and SBIC instructions. Refer to the Instruction Set section for more

details. When using the I/O specific commands IN and OUT, the I/O addresses $00 - $3F must

be used. When addressing I/O Registers as data space using LD and ST instructions, $20 must

be added to these addresses.

For compatibility with future devices, reserved bits should be written to zero if accessed.

Reserved I/O memory addresses should never be written.

Some of the Status Flags are cleared by writing a logical one to them. Note that the CBI and SBI

instructions will operate on all bits in the I/O Register, writing a one back into any flag read as

set, thus clearing the flag. The CBI and SBI instructions work with registers $00 to $1F only.

The I/O and Peripherals Control Registers are explained in later sections.

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ATmega16(L)

System Clock

and Clock

Options

Clock Systems

and their

Distribution

Figure 11 presents the principal clock systems in the AVR and their distribution. All of the clocks

need not be active at a given time. In order to reduce power consumption, the clocks to modules

not being used can be halted by using different sleep modes, as described in “Power Manage-

ment and Sleep Modes” on page 32. The clock systems are detailed Figure 11.

Figure 11. Clock Distribution

Asynchronous

Timer/Counter

General I/O

Modules

Flash and

EEPROM

ADC

CPU Core

RAM

clk_ADC

clk_I/O

clk_CPU

AVR Clock

Control Unit

clk_ASY

clk_FLASH

Reset Logic

Watchdog Timer

Source Clock

Watchdog Clock

Clock

Multiplexer

Watchdog

Oscillator

Timer/Counter

Oscillator

External RC

Oscillator

Crystal

Oscillator

Low-frequency

Crystal Oscillator

Calibrated RC

Oscillator

External Clock

CPU Clock – clk_CPU

I/O Clock – clk_I/O

The CPU clock is routed to parts of the system concerned with operation of the AVR core.

Examples of such modules are the General Purpose Register File, the Status Register and the

data memory holding the Stack Pointer. Halting the CPU clock inhibits the core from performing

general operations and calculations.

The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART.

The I/O clock is also used by the External Interrupt module, but note that some external inter-

rupts are detected by asynchronous logic, allowing such interrupts to be detected even if the I/O

clock is halted. Also note that address recognition in the TWI module is carried out asynchro-

nously when clk_I/Ois halted, enabling TWI address reception in all sleep modes.

Flash Clock – clk_FLASHThe Flash clock controls operation of the Flash interface. The Flash clock is usually active simul-

taneously with the CPU clock.

Asynchronous Timer

Clock – clk_ASY

The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly

from an external 32 kHz clock crystal. The dedicated clock domain allows using this

Timer/Counter as a real-time counter even when the device is in sleep mode.

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ATmega16(L)

ADC Clock – clk_ADC

The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks

in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion

results.

Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as shown

below. The clock from the selected source is input to the AVR clock generator, and routed to the

appropriate modules.

Table 2. Device Clocking Options Select⁽¹⁾

Device Clocking Option

External Crystal/Ceramic Resonator

External Low-frequency Crystal

External RC Oscillator

CKSEL3..0

1111 - 1010

1001

1000 - 0101

0100 - 0001

0000

Calibrated Internal RC Oscillator

External Clock

Note:

1. For all fuses “1” means unprogrammed while “0” means programmed.

The various choices for each clocking option is given in the following sections. When the CPU

wakes up from Power-down or Power-save, the selected clock source is used to time the start-

up, ensuring stable Oscillator operation before instruction execution starts. When the CPU starts

from Reset, there is as an additional delay allowing the power to reach a stable level before

commencing normal operation. The Watchdog Oscillator is used for timing this real-time part of

the start-up time. The number of WDT Oscillator cycles used for each time-out is shown in Table

3. The frequency of the Watchdog Oscillator is voltage dependent as shown in “ATmega16 Typ-

ical Characteristics” on page 299.

Table 3. Number of Watchdog Oscillator Cycles

Typ Time-out (V_CC= 5.0V)

Typ Time-out (V_CC= 3.0V)

Number of Cycles

4K (4,096)

4.1 ms

65 ms

4.3 ms

69 ms

64K (65,536)

Default Clock

Source

The device is shipped with CKSEL = “0001” and SUT = “10”. The default clock source setting is

therefore the 1 MHz Internal RC Oscillator with longest startup time. This default setting ensures

that all users can make their desired clock source setting using an In-System or Parallel

Programmer.

Crystal Oscillator

XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be con-

figured for use as an On-chip Oscillator, as shown in Figure 12. Either a quartz crystal or a

ceramic resonator may be used. The CKOPT Fuse selects between two different Oscillator

amplifier modes. When CKOPT is programmed, the Oscillator output will oscillate will a full rail-

to-rail swing on the output. This mode is suitable when operating in a very noisy environment or

when the output from XTAL2 drives a second clock buffer. This mode has a wide frequency

range. When CKOPT is unprogrammed, the Oscillator has a smaller output swing. This reduces

power consumption considerably. This mode has a limited frequency range and it can not be

used to drive other clock buffers.

For resonators, the maximum frequency is 8 MHz with CKOPT unprogrammed and 16 MHz with

CKOPT programmed. C1 and C2 should always be equal for both crystals and resonators. The

optimal value of the capacitors depends on the crystal or resonator in use, the amount of stray

capacitance, and the electromagnetic noise of the environment. Some initial guidelines for

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ATmega16(L)

choosing capacitors for use with crystals are given in Table 4. For ceramic resonators, the

capacitor values given by the manufacturer should be used.

Figure 12. Crystal Oscillator Connections

C2

XTAL2

C1

XTAL1

GND

The Oscillator can operate in three different modes, each optimized for a specific frequency

range. The operating mode is selected by the fuses CKSEL3..1 as shown in Table 4.

Table 4. Crystal Oscillator Operating Modes

Frequency Range

(MHz)

Recommended Range for Capacitors

C1 and C2 for Use with Crystals (pF)

CKOPT

CKSEL3..1

101⁽¹⁾

1

0

0.4 - 0.9

0.9 - 3.0

3.0 - 8.0

1.0 ≤

–

110

12 - 22

111

101, 110, 111

Note:

1. This option should not be used with crystals, only with ceramic resonators.

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ATmega16(L)

The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in Table

5.

Table 5. Start-up Times for the Crystal Oscillator Clock Selection

Start-up Time from Additional Delay

Power-down and

Power-save

from Reset

(V_CC= 5.0V)

CKSEL0 SUT1..0

Recommended Usage

Ceramic resonator, fast

rising power

0

1

00

01

10

11

00

01

10

11

258 CK⁽¹⁾

1K CK⁽²⁾

16K CK

4.1 ms

65 ms

–

Ceramic resonator, slowly

rising power

Ceramic resonator, BOD

enabled

Ceramic resonator, fast

rising power

4.1 ms

65 ms

–

Ceramic resonator, slowly

rising power

Crystal Oscillator, BOD

enabled

Crystal Oscillator, fast

rising power

4.1 ms

65 ms

Crystal Oscillator, slowly

rising power

Notes: 1. These options should only be used when not operating close to the maximum frequency of the

device, and only if frequency stability at start-up is not important for the application. These

options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability

at start-up. They can also be used with crystals when not operating close to the maximum fre-

quency of the device, and if frequency stability at start-up is not important for the application.

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ATmega16(L)

Low-frequency

Crystal Oscillator

To use a 32.768 kHz watch crystal as the clock source for the device, the Low-frequency Crystal

Oscillator must be selected by setting the CKSEL Fuses to “1001”. The crystal should be con-

nected as shown in Figure 12. By programming the CKOPT Fuse, the user can enable internal

capacitors on XTAL1 and XTAL2, thereby removing the need for external capacitors. The inter-

nal capacitors have a nominal value of 36 pF.

When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in

Table 6.

Table 6. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

SUT1..0

00

(V_CC= 5.0V)

Recommended Usage

1K CK⁽¹⁾

32K CK

4.1 ms

65 ms

Fast rising power or BOD enabled

Slowly rising power

01

10

Stable frequency at start-up

11

Reserved

Note:

1. These options should only be used if frequency stability at start-up is not important for the

application.

External RC

Oscillator

For timing insensitive applications, the external RC configuration shown in Figure 13 can be

used. The frequency is roughly estimated by the equation f = 1/(3RC). C should be at least 22

pF. By programming the CKOPT Fuse, the user can enable an internal 36 pF capacitor between

XTAL1 and GND, thereby removing the need for an external capacitor. For more information on

Oscillator operation and details on how to choose R and C, refer to the External RC Oscillator

application note.

Figure 13. External RC Configuration

V

CC

NC

XTAL2

XTAL1

GND

R

C

The Oscillator can operate in four different modes, each optimized for a specific frequency

range. The operating mode is selected by the fuses CKSEL3..0 as shown in Table 7.

Table 7. External RC Oscillator Operating Modes

CKSEL3..0

0101

Frequency Range (MHz)

0.1 ≤ 0.9

0110

0.9 - 3.0

0111

3.0 - 8.0

1000

8.0 - 12.0

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ATmega16(L)

When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in

Table 8.

Table 8. Start-up Times for the External RC Oscillator Clock Selection

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

SUT1..0

00

(V_CC= 5.0V)

Recommended Usage

BOD enabled

18 CK

6 CK⁽¹⁾

–

01

4.1 ms

65 ms

4.1 ms

Fast rising power

10

Slowly rising power

11

Fast rising power or BOD enabled

Note:

1. This option should not be used when operating close to the maximum frequency of the device.

Calibrated Internal The Calibrated Internal RC Oscillator provides a fixed 1.0 MHz, 2.0 MHz, 4.0 MHz, or 8.0 MHz

clock. All frequencies are nominal values at 5V and 25°C. This clock may be selected as the

RC Oscillator

system-clock by programming the CKSEL Fuses as shown in Table 9. If selected, it will operate

with no external components. The CKOPT Fuse should always be unpro-grammed when using

this clock option. During Reset, hardware loads the calibration byte into the OSCCAL Register

and thereby automatically calibrates the RC Oscillator. At 5V, 25°C and 1.0 MHz, 2.0 MHz, 4.0

MHz or 8.0 MHz Oscillator frequency selected, this calibration gives a frequency within ±3% of

the nominal frequency. Using calibration methods as described in application notes available at

www.atmel.com/avr it is possible to achieve ±1% accuracy at any given V_CCand Temperature.

When this Oscillator is used as the Chip Clock, the Watchdog Oscillator will still be used for the

Watchdog Timer and for the reset time-out. For more information on the pre-programmed cali-

bration value, see the section “Calibration Byte” on page 261.

Table 9. Internal Calibrated RC Oscillator Operating Modes

CKSEL3..0

0001⁽¹⁾

0010

Nominal Frequency (MHz)

1.0

2.0

4.0

8.0

0011

0100

Note:

1. The device is shipped with this option selected.

When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in

Table 10. XTAL1 and XTAL2 should be left unconnected (NC).

Table 10. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

SUT1..0

00

(V_CC= 5.0V)

Recommended Usage

BOD enabled

6 CK

–

01

4.1 ms

65 ms

Reserved

Fast rising power

Slowly rising power

10⁽¹⁾

11

Note:

1. The device is shipped with this option selected.

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ATmega16(L)

Oscillator Calibration

Register – OSCCAL

Bit

7

6

5

4

3

2

1

0

CAL7

R/W

CAL6

R/W

CAL5

R/W

CAL4

R/W

CAL3

R/W

CAL2

R/W

CAL1

R/W

CAL0

OSCCAL

Read/Write

Initial Value

R/W

Device Specific Calibration Value

• Bits 7..0 – CAL7..0: Oscillator Calibration Value

Writing the calibration byte to this address will trim the Internal Oscillator to remove process vari-

ations from the Oscillator frequency. This is done automatically during Chip Reset. When

OSCCAL is zero, the lowest available frequency is chosen. Writing non-zero values to this regis-

ter will increase the frequency of the Internal Oscillator. Writing $FF to the register gives the

highest available frequency. The calibrated Oscillator is used to time EEPROM and Flash

access. If EEPROM or Flash is written, do not calibrate to more than 10% above the nominal fre-

quency. Otherwise, the EEPROM or Flash write may fail. Note that the Oscillator is intended for

calibration to 1.0 MHz, 2.0 MHz, 4.0 MHz, or 8.0 MHz. Tuning to other values is not guaranteed,

as indicated in Table 11.

Table 11. Internal RC Oscillator Frequency Range.

Min Frequency in Percentage of

Nominal Frequency (%)

Max Frequency in Percentage of

Nominal Frequency (%)

OSCCAL Value

$00

$7F

$FF

50

75

100

150

200

100

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ATmega16(L)

External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in Figure

14. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.

By programming the CKOPT Fuse, the user can enable an internal 36 pF capacitor between

XTAL1 and GND.

Figure 14. External Clock Drive Configuration

EXTERNAL

CLOCK

SIGNAL

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in

Table 12.

Table 12. Start-up Times for the External Clock Selection

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

SUT1..0

00

(V_CC= 5.0V)

Recommended Usage

BOD enabled

6 CK

–

01

4.1 ms

65 ms

Reserved

Fast rising power

Slowly rising power

10

11

When applying an external clock, it is required to avoid sudden changes in the applied clock fre-

quency to ensure stable operation of the MCU. A variation in frequency of more than 2% from

one clock cycle to the next can lead to unpredictable behavior. It is required to ensure that the

MCU is kept in reset during such changes in the clock frequency.

Timer/Counter

Oscillator

For AVR microcontrollers with Timer/Counter Oscillator pins (TOSC1 and TOSC2), the crystal is

connected directly between the pins. No external capacitors are needed. The Oscillator is opti-

mized for use with a 32.768 kHz watch crystal. Applying an external clock source to TOSC1 is

not recommended.

Note:

The Timer/Counter Oscillator uses the same type of crystal oscillator as Low-Frequency Oscillator

and the internal capacitors have the same nominal value of 36 pF.

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ATmega16(L)

Power

Sleep modes enable the application to shut down unused modules in the MCU, thereby saving

power. The AVR provides various sleep modes allowing the user to tailor the power consump-

tion to the application’s requirements.

Management

and Sleep

Modes

To enter any of the six sleep modes, the SE bit in MCUCR must be written to logic one and a

SLEEP instruction must be executed. The SM2, SM1, and SM0 bits in the MCUCR Register

select which sleep mode (Idle, ADC Noise Reduction, Power-down, Power-save, Standby, or

Extended Standby) will be activated by the SLEEP instruction. See Table 13 for a summary. If

an enabled interrupt occurs while the MCU is in a sleep mode, the MCU wakes up. The MCU is

then halted for four cycles in addition to the start-up time, it executes the interrupt routine, and

resumes execution from the instruction following SLEEP. The contents of the Register File and

SRAM are unaltered when the device wakes up from sleep. If a Reset occurs during sleep

mode, the MCU wakes up and executes from the Reset Vector.

Figure 11 on page 24 presents the different clock systems in the ATmega16, and their distribu-

tion. The figure is helpful in selecting an appropriate sleep mode.

MCU Control Register The MCU Control Register contains control bits for power management.

– MCUCR

Bit

7

6

SE

R/W

0

5

4

3

ISC11

R/W

0

2

ISC10

R/W

0

1

ISC01

R/W

0

ISC00

R/W

0

SM2

R/W

0

SM1

R/W

0

SM0

R/W

0

MCUCR

Read/Write

Initial Value

• Bits 7, 5, 4 – SM2..0: Sleep Mode Select Bits 2, 1, and 0

These bits select between the six available sleep modes as shown in Table 13.

Table 13. Sleep Mode Select

SM2

SM1

SM0

Sleep Mode

Idle

0

1

0

1

0

1

0

1

0

1

0

1

0

1

ADC Noise Reduction

Power-down

Power-save

Reserved

Standby⁽¹⁾

Extended Standby⁽¹⁾

Note:

1. Standby mode and Extended Standby mode are only available with external crystals or

resonators.

• Bit 6 – SE: Sleep Enable

The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP

instruction is executed. To avoid the MCU entering the sleep mode unless it is the programmers

purpose, it is recommended to write the Sleep Enable (SE) bit to one just before the execution of

the SLEEP instruction and to clear it immediately after waking up.

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ATmega16(L)

Idle Mode

When the SM2..0 bits are written to 000, the SLEEP instruction makes the MCU enter Idle

mode, stopping the CPU but allowing SPI, USART, Analog Comparator, ADC, Two-wire Serial

Interface, Timer/Counters, Watchdog, and the interrupt system to continue operating. This sleep

mode basically halts clk_CPUand clk_FLASH, while allowing the other clocks to run.

Idle mode enables the MCU to wake up from external triggered interrupts as well as internal

ones like the Timer Overflow and USART Transmit Complete interrupts. If wake-up from the

Analog Comparator interrupt is not required, the Analog Comparator can be powered down by

setting the ACD bit in the Analog Comparator Control and Status Register – ACSR. This will

reduce power consumption in Idle mode. If the ADC is enabled, a conversion starts automati-

cally when this mode is entered.

ADC Noise

Reduction Mode

When the SM2..0 bits are written to 001, the SLEEP instruction makes the MCU enter ADC

Noise Reduction mode, stopping the CPU but allowing the ADC, the External Interrupts, the

Two-wire Serial Interface address watch, Timer/Counter2 and the Watchdog to continue operat-

ing (if enabled). This sleep mode basically halts clk_I/O, clk_CPU, and clk_FLASH, while allowing the

other clocks to run.

This improves the noise environment for the ADC, enabling higher resolution measurements. If

the ADC is enabled, a conversion starts automatically when this mode is entered. Apart form the

ADC Conversion Complete interrupt, only an External Reset, a Watchdog Reset, a Brown-out

Reset, a Two-wire Serial Interface Address Match Interrupt, a Timer/Counter2 interrupt, an

SPM/EEPROM ready interrupt, an External level interrupt on INT0 or INT1, or an external inter-

rupt on INT2 can wake up the MCU from ADC Noise Reduction mode.

Power-down Mode When the SM2..0 bits are written to 010, the SLEEP instruction makes the MCU enter Power-

down mode. In this mode, the External Oscillator is stopped, while the External interrupts, the

Two-wire Serial Interface address watch, and the Watchdog continue operating (if enabled).

Only an External Reset, a Watchdog Reset, a Brown-out Reset, a Two-wire Serial Interface

address match interrupt, an External level interrupt on INT0 or INT1, or an External interrupt on

INT2 can wake up the MCU. This sleep mode basically halts all generated clocks, allowing oper-

ation of asynchronous modules only.

Note that if a level triggered interrupt is used for wake-up from Power-down mode, the changed

level must be held for some time to wake up the MCU. Refer to “External Interrupts” on page 68

for details.

When waking up from Power-down mode, there is a delay from the wake-up condition occurs

until the wake-up becomes effective. This allows the clock to restart and become stable after

having been stopped. The wake-up period is defined by the same CKSEL Fuses that define the

reset time-out period, as described in “Clock Sources” on page 25.

Power-save Mode

When the SM2..0 bits are written to 011, the SLEEP instruction makes the MCU enter Power-

save mode. This mode is identical to Power-down, with one exception:

If Timer/Counter2 is clocked asynchronously, that is, the AS2 bit in ASSR is set, Timer/Counter2

will run during sleep. The device can wake up from either Timer Overflow or Output Compare

event from Timer/Counter2 if the corresponding Timer/Counter2 interrupt enable bits are set in

TIMSK, and the Global Interrupt Enable bit in SREG is set.

If the Asynchronous Timer is NOT clocked asynchronously, Power-down mode is recommended

instead of Power-save mode because the contents of the registers in the Asynchronous Timer

should be considered undefined after wake-up in Power-save mode if AS2 is 0.

This sleep mode basically halts all clocks except clk_ASY, allowing operation only of asynchronous

modules, including Timer/Counter2 if clocked asynchronously.

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ATmega16(L)

Standby Mode

When the SM2..0 bits are 110 and an external crystal/resonator clock option is selected, the

SLEEP instruction makes the MCU enter Standby mode. This mode is identical to Power-down

with the exception that the Oscillator is kept running. From Standby mode, the device wakes up

in six clock cycles.

Extended Standby When the SM2..0 bits are 111 and an external crystal/resonator clock option is selected, the

SLEEP instruction makes the MCU enter Extended Standby mode. This mode is identical to

Power-save mode with the exception that the Oscillator is kept running. From Extended Standby

mode, the device wakes up in six clock cycles..

Mode

Table 14. Active Clock Domains and Wake Up Sources in the Different Sleep Modes

Active Clock domains

Oscillators

Wake-up Sources

SPM /

INT2

INT1

INT0

TWI

Address

Match

Sleep

Mode

Main Clock

clk_CPUclk_FLASHclk_IOclk_ADCclk_ASYSource Enabled

Timer Osc.

Enabled

Timer

2

EEPROM

Ready

Other

I/O

ADC

Idle

X

X⁽²⁾

X

ADC

Noise

Redu-

ction

X

X⁽²⁾

X⁽³⁾

X

Power

Down

X⁽³⁾

X

Power

Save

X⁽²⁾

X⁽³⁾

X

X⁽²⁾

Standby⁽¹⁾

X

Exten-

ded

X⁽²⁾

X⁽³⁾

X

X⁽²⁾

Standby⁽¹⁾

Notes: 1. External Crystal or resonator selected as clock source.

2. If AS2 bit in ASSR is set.

3. Only INT2 or level interrupt INT1 and INT0.

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ATmega16(L)

Minimizing Power There are several issues to consider when trying to minimize the power consumption in an AVR

controlled system. In general, sleep modes should be used as much as possible, and the sleep

Consumption

mode should be selected so that as few as possible of the device’s functions are operating. All

functions not needed should be disabled. In particular, the following modules may need special

consideration when trying to achieve the lowest possible power consumption.

Analog to Digital

Converter

If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be dis-

abled before entering any sleep mode. When the ADC is turned off and on again, the next

conversion will be an extended conversion. Refer to “Analog to Digital Converter” on page 204

for details on ADC operation.

Analog Comparator

When entering Idle mode, the Analog Comparator should be disabled if not used. When entering

ADC Noise Reduction mode, the Analog Comparator should be disabled. In the other sleep

modes, the Analog Comparator is automatically disabled. However, if the Analog Comparator is

set up to use the Internal Voltage Reference as input, the Analog Comparator should be dis-

abled in all sleep modes. Otherwise, the Internal Voltage Reference will be enabled,

independent of sleep mode. Refer to “Analog Comparator” on page 201 for details on how to

configure the Analog Comparator.

Brown-out Detector

If the Brown-out Detector is not needed in the application, this module should be turned off. If the

Brown-out Detector is enabled by the BODEN Fuse, it will be enabled in all sleep modes, and

hence, always consume power. In the deeper sleep modes, this will contribute significantly to

the total current consumption. Refer to “Brown-out Detection” on page 40 for details on how to

configure the Brown-out Detector.

Internal Voltage

Reference

The Internal Voltage Reference will be enabled when needed by the Brown-out Detector, the

Analog Comparator or the ADC. If these modules are disabled as described in the sections

above, the internal voltage reference will be disabled and it will not be consuming power. When

turned on again, the user must allow the reference to start up before the output is used. If the

reference is kept on in sleep mode, the output can be used immediately. Refer to “Internal Volt-

age Reference” on page 42 for details on the start-up time.

Watchdog Timer

Port Pins

If the Watchdog Timer is not needed in the application, this module should be turned off. If the

Watchdog Timer is enabled, it will be enabled in all sleep modes, and hence, always consume

power. In the deeper sleep modes, this will contribute significantly to the total current consump-

tion. Refer to “Watchdog Timer” on page 42 for details on how to configure the Watchdog Timer.

When entering a sleep mode, all port pins should be configured to use minimum power. The

most important thing is then to ensure that no pins drive resistive loads. In sleep modes where

the both the I/O clock (clk_I/O) and the ADC clock (clk_ADC) are stopped, the input buffers of the

device will be disabled. This ensures that no power is consumed by the input logic when not

needed. In some cases, the input logic is needed for detecting wake-up conditions, and it will

then be enabled. Refer to the section “Digital Input Enable and Sleep Modes” on page 54 for

details on which pins are enabled. If the input buffer is enabled and the input signal is left floating

or have an analog signal level close to V_CC/2, the input buffer will use excessive power.

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ATmega16(L)

JTAG Interface and

On-chip Debug

System

If the On-chip debug system is enabled by the OCDEN Fuse and the chip enter Power down or

Power save sleep mode, the main clock source remains enabled. In these sleep modes, this will

contribute significantly to the total current consumption. There are three alternative ways to

avoid this:

•

Disable OCDEN Fuse.

Disable JTAGEN Fuse.

Write one to the JTD bit in MCUCSR.

The TDO pin is left floating when the JTAG interface is enabled while the JTAG TAP controller is

not shifting data. If the hardware connected to the TDO pin does not pull up the logic level,

power consumption will increase. Note that the TDI pin for the next device in the scan chain con-

tains a pull-up that avoids this problem. Writing the JTD bit in the MCUCSR register to one or

leaving the JTAG fuse unprogrammed disables the JTAG interface.

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ATmega16(L)

System Control

and Reset

Resetting the AVR

During Reset, all I/O Registers are set to their initial values, and the program starts execution

from the Reset Vector. The instruction placed at the Reset Vector must be a JMP – absolute

jump – instruction to the reset handling routine. If the program never enables an interrupt

source, the Interrupt Vectors are not used, and regular program code can be placed at these

locations. This is also the case if the Reset Vector is in the Application section while the Interrupt

Vectors are in the Boot section or vice versa. The circuit diagram in Figure 15 shows the reset

logic. Table 15 defines the electrical parameters of the reset circuitry.

The I/O ports of the AVR are immediately reset to their initial state when a reset source goes

active. This does not require any clock source to be running.

After all reset sources have gone inactive, a delay counter is invoked, stretching the Internal

Reset. This allows the power to reach a stable level before normal operation starts. The time-out

period of the delay counter is defined by the user through the CKSEL Fuses. The different selec-

tions for the delay period are presented in “Clock Sources” on page 25.

Reset Sources

The ATmega16 has five sources of reset:

•

Power-on Reset.

The MCU is reset when the supply voltage is below the Power-on Reset threshold (V_POT).

•

External Reset.

The MCU is reset when a low level is present on the RESET pin for longer than the minimum

pulse length.

•

Watchdog Reset.

The MCU is reset when the Watchdog Timer period expires and the Watchdog is enabled.

Brown-out Reset.

The MCU is reset when the supply voltage V_CCis below the Brown-out Reset threshold

(V_BOT) and the Brown-out Detector is enabled.

•

JTAG AVR Reset.

The MCU is reset as long as there is a logic one in the Reset Register, one of the scan

chains of the JTAG system. Refer to the section “IEEE 1149.1 (JTAG) Boundary-scan” on

page 228 for details.

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ATmega16(L)

Figure 15. Reset Logic

DATA BUS

MCU Control and Status

Register (MCUCSR)

Power-on

Reset Circuit

Brown-out

Reset Circuit

BODEN

BODLEVEL

Pull-up Resistor

SPIKE

FILTER

Reset Circuit

JTAG Reset

Register

Watchdog

Timer

Watchdog

Oscillator

Delay Counters

Clock

CK

TIMEOUT

Generator

CKSEL[3:0]

SUT[1:0]

Table 15. Reset Characteristics

Symbol Parameter

Condition

Min

Typ

Max

Units

Power-on Reset

Threshold Voltage (rising)

1.4

1.3

2.3

V_POT

Power-on Reset

Threshold Voltage

2.3

V

(falling)⁽¹⁾

RESET Pin Threshold

Voltage

V_RST

0.1V_CC

0.9V_CC

1.5

Minimum pulse width on

RESET Pin

t_RST

µs

V

Brown-out Reset

BODLEVEL = 1

2.5

3.6

2.7

4.0

2

3.2

4.5

Threshold Voltage⁽²⁾

V_BOT

BODLEVEL = 0

BODLEVEL = 1

Minimum low voltage

t_BOD

period for Brown-out

Detection

µs

BODLEVEL = 0

2

Brown-out Detector

hysteresis

V_HYST

50

mV

Notes: 1. The Power-on Reset will not work unless the supply voltage has been below V_POT(falling).

2. V_BOTmay be below nominal minimum operating voltage for some devices. For devices where

this is the case, the device is tested down to V_CC= V_BOTduring the production test. This guar-

antees that a Brown-out Reset will occur before V_CCdrops to a voltage where correct

operation of the microcontroller is no longer guaranteed. The test is performed using

BODLEVEL = 1 for ATmega16L and BODLEVEL = 0 for ATmega16. BODLEVEL = 1 is not

applicable for ATmega16.

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ATmega16(L)

Power-on Reset

A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level

is defined in Table 15. The POR is activated whenever V_CCis below the detection level. The

POR circuit can be used to trigger the Start-up Reset, as well as to detect a failure in supply

voltage.

A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the

Power-on Reset threshold voltage invokes the delay counter, which determines how long the

device is kept in RESET after V_CCrise. The RESET signal is activated again, without any delay,

when V_CCdecreases below the detection level.

Figure 16. MCU Start-up, RESET Tied to V_CC

.

V_POT

V_CC

V_RST

RESET

t_TOUT

TIME-OUT

INTERNAL

RESET

Figure 17. MCU Start-up, RESET Extended Externally

V_POT

V_CC

V_RST

RESET

t_TOUT

TIME-OUT

INTERNAL

RESET

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ATmega16(L)

External Reset

An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the

minimum pulse width (see Table 15) will generate a reset, even if the clock is not running.

Shorter pulses are not guaranteed to generate a reset. When the applied signal reaches the

Reset Threshold Voltage – V_RST– on its positive edge, the delay counter starts the MCU after

the Time-out period t_TOUThas expired.

Figure 18. External Reset During Operation

CC

Brown-out Detection

ATmega16 has an On-chip Brown-out Detection (BOD) circuit for monitoring the V_CClevel dur-

ing operation by comparing it to a fixed trigger level. The trigger level for the BOD can be

selected by the fuse BODLEVEL to be 2.7V (BODLEVEL unprogrammed), or 4.0V (BODLEVEL

programmed). The trigger level has a hysteresis to ensure spike free Brown-out Detection. The

hysteresis on the detection level should be interpreted as V_BOT+= V_BOT+ V_HYST/2 and V_BOT-

_BOT- V_HYST/2.

=

V

The BOD circuit can be enabled/disabled by the fuse BODEN. When the BOD is enabled

(BODEN programmed), and V_CCdecreases to a value below the trigger level (V_BOT-in Figure

19), the Brown-out Reset is immediately activated. When V_CCincreases above the trigger level

(V_BOT+in Figure 19), the delay counter starts the MCU after the Time-out period t_TOUThas

expired.

The BOD circuit will only detect a drop in V_CCif the voltage stays below the trigger level for lon-

ger than t_BODgiven in Table 15.

Figure 19. Brown-out Reset During Operation

V

BOT+

CC

V

BOT-

RESET

t

TOUT

TIME-OUT

INTERNAL

RESET

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ATmega16(L)

Watchdog Reset

When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On

the falling edge of this pulse, the delay timer starts counting the Time-out period t_TOUT. Refer to

page 42 for details on operation of the Watchdog Timer.

Figure 20. Watchdog Reset During Operation

CC

CK

MCU Control and

Status Register –

MCUCSR

The MCU Control and Status Register provides information on which reset source caused an

MCU Reset.

Bit

7

6

ISC2

R/W

0

5

–

4

3

2

1

0

JTD

R/W

0

JTRF

R/W

WDRF

R/W

BORF

R/W

EXTRF

R/W

PORF

R/W

MCUCSR

Read/Write

Initial Value

R

0

See Bit Description

• Bit 4 – JTRF: JTAG Reset Flag

This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by

the JTAG instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic

zero to the flag.

• Bit 3 – WDRF: Watchdog Reset Flag

This bit is set if a Watchdog Reset occurs. The bit is reset by a Power-on Reset, or by writing a

logic zero to the flag.

• Bit 2 – BORF: Brown-out Reset Flag

This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a

logic zero to the flag.

• Bit 1 – EXTRF: External Reset Flag

This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a

logic zero to the flag.

• Bit 0 – PORF: Power-on Reset Flag

This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag.

To make use of the Reset Flags to identify a reset condition, the user should read and then reset

the MCUCSR as early as possible in the program. If the register is cleared before another reset

occurs, the source of the reset can be found by examining the Reset Flags.

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ATmega16(L)

Internal Voltage

Reference

ATmega16 features an internal bandgap reference. This reference is used for Brown-out Detec-

tion, and it can be used as an input to the Analog Comparator or the ADC. The 2.56V reference

to the ADC is generated from the internal bandgap reference.

Voltage Reference

Enable Signals and

Start-up Time

The voltage reference has a start-up time that may influence the way it should be used. The

start-up time is given in Table 16. To save power, the reference is not always turned on. The ref-

erence is on during the following situations:

1. When the BOD is enabled (by programming the BODEN Fuse).

2. When the bandgap reference is connected to the Analog Comparator (by setting the

ACBG bit in ACSR).

3. When the ADC is enabled.

Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user

must always allow the reference to start up before the output from the Analog Comparator or

ADC is used. To reduce power consumption in Power-down mode, the user can avoid the three

conditions above to ensure that the reference is turned off before entering Power-down mode.

Table 16. Internal Voltage Reference Characteristics

Symbol

V_BG

Parameter

Min

Typ

1.23

40

Max

1.4

70

Units

V

Bandgap reference voltage

Bandgap reference start-up time

Bandgap reference current consumption

1.15

t_BG

µs

I_BG

10

µA

Watchdog Timer

The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1 MHz. This is

the typical value at V_CC= 5V. See characterization data for typical values at other V_CClevels. By

controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as

shown in Table 17 on page 43. The WDR – Watchdog Reset – instruction resets the Watchdog

Timer. The Watchdog Timer is also reset when it is disabled and when a Chip Reset occurs.

Eight different clock cycle periods can be selected to determine the reset period. If the reset

period expires without another Watchdog Reset, the ATmega16 resets and executes from the

Reset Vector. For timing details on the Watchdog Reset, refer to page 41.

To prevent unintentional disabling of the Watchdog, a special turn-off sequence must be fol-

lowed when the Watchdog is disabled. Refer to the description of the Watchdog Timer Control

Register for details.

Figure 21. Watchdog Timer

WATCHDOG

OSCILLATOR

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ATmega16(L)

Watchdog Timer

Control Register –

WDTCR

Bit

7

–

6

–

5

–

4

WDTOE

R/W

0

3

WDE

R/W

0

2

WDP2

R/W

0

1

WDP1

R/W

0

WDP0

R/W

0

WDTCR

Read/Write

Initial Value

R

0

R

0

R

0

• Bits 7..5 – Res: Reserved Bits

These bits are reserved bits in the ATmega16 and will always read as zero.

• Bit 4 – WDTOE: Watchdog Turn-off Enable

This bit must be set when the WDE bit is written to logic zero. Otherwise, the Watchdog will not

be disabled. Once written to one, hardware will clear this bit after four clock cycles. Refer to the

description of the WDE bit for a Watchdog disable procedure.

• Bit 3 – WDE: Watchdog Enable

When the WDE is written to logic one, the Watchdog Timer is enabled, and if the WDE is written

to logic zero, the Watchdog Timer function is disabled. WDE can only be cleared if the WDTOE

bit has logic level one. To disable an enabled Watchdog Timer, the following procedure must be

followed:

1. In the same operation, write a logic one to WDTOE and WDE. A logic one must be writ-

ten to WDE even though it is set to one before the disable operation starts.

2. Within the next four clock cycles, write a logic 0 to WDE. This disables the Watchdog.

• Bits 2..0 – WDP2, WDP1, WDP0: Watchdog Timer Prescaler 2, 1, and 0

The WDP2, WDP1, and WDP0 bits determine the Watchdog Timer prescaling when the Watch-

dog Timer is enabled. The different prescaling values and their corresponding Timeout Periods

are shown in Table 17.

Table 17. Watchdog Timer Prescale Select

Number of WDT

Oscillator Cycles

TypicalTime-out

at V_CC= 3.0V

Typical Time-out

at V_CC= 5.0V

WDP2 WDP1 WDP0

0

1

0

1

0

1

0

1

0

1

0

1

0

1

16K (16,384)

32K (32,768)

17.1 ms

34.3 ms

68.5 ms

0.14 s

0.27 s

0.55 s

1.1 s

16.3 ms

32.5 ms

65 ms

0.13 s

0.26 s

0.52 s

1.0 s

64K (65,536)

128K (131,072)

256K (262,144)

512K (524,288)

1,024K (1,048,576)

2,048K (2,097,152)

2.2 s

2.1 s

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ATmega16(L)

The following code example shows one assembly and one C function for turning off the WDT.

The example assumes that interrupts are controlled (for example by disabling interrupts globally)

so that no interrupts will occur during execution of these functions.

Assembly Code Example

WDT_off:

; Reset WDT

WDR

; Write logical one to WDTOE and WDE

in

r16, WDTCR

ori r16, (1<<WDTOE)|(1<<WDE)

out WDTCR, r16

; Turn off WDT

ldi r16, (0<<WDE)

out WDTCR, r16

ret

C Code Example

void WDT_off(void)

{

/* Reset WDT*/

_WDR();

/* Write logical one to WDTOE and WDE */

WDTCR |= (1<<WDTOE) | (1<<WDE);

/* Turn off WDT */

WDTCR = 0x00;

}

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ATmega16(L)

Interrupts

This section describes the specifics of the interrupt handling as performed in ATmega16. For a

general explanation of the AVR interrupt handling, refer to “Reset and Interrupt Handling” on

page 13.

Interrupt Vectors

in ATmega16

Table 18. Reset and Interrupt Vectors

Program

Vector No. Address⁽²⁾

Source

Interrupt Definition

1

$000⁽¹⁾

RESET

External Pin, Power-on Reset, Brown-out

Reset, Watchdog Reset, and JTAG AVR

Reset

2

3

$002

$004

$006

$008

$00A

$00C

$00E

$010

$012

$014

$016

$018

$01A

$01C

$01E

$020

$022

$024

$026

$028

INT0

External Interrupt Request 0

External Interrupt Request 1

Timer/Counter2 Compare Match

Timer/Counter2 Overflow

INT1

4

TIMER2 COMP

TIMER2 OVF

TIMER1 CAPT

5

6

Timer/Counter1 Capture Event

7

TIMER1 COMPA Timer/Counter1 Compare Match A

TIMER1 COMPB Timer/Counter1 Compare Match B

8

9

TIMER1 OVF

TIMER0 OVF

SPI, STC

Timer/Counter1 Overflow

Timer/Counter0 Overflow

Serial Transfer Complete

USART, Rx Complete

10

11

12

13

14

15

16

17

18

19

20

21

USART, RXC

USART, UDRE

USART, TXC

ADC

USART Data Register Empty

USART, Tx Complete

ADC Conversion Complete

EEPROM Ready

EE_RDY

ANA_COMP

TWI

Analog Comparator

Two-wire Serial Interface

External Interrupt Request 2

Timer/Counter0 Compare Match

Store Program Memory Ready

INT2

TIMER0 COMP

SPM_RDY

Notes: 1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at

reset, see “Boot Loader Support – Read-While-Write Self-Programming” on page 246.

2. When the IVSEL bit in GICR is set, interrupt vectors will be moved to the start of the Boot

Flash section. The address of each Interrupt Vector will then be the address in this table added

to the start address of the Boot Flash section.

Table 19 shows Reset and Interrupt Vectors placement for the various combinations of

BOOTRST and IVSEL settings. If the program never enables an interrupt source, the Interrupt

Vectors are not used, and regular program code can be placed at these locations. This is also

the case if the Reset Vector is in the Application section while the Interrupt Vectors are in the

Boot section or vice versa.

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ATmega16(L)

Table 19. Reset and Interrupt Vectors Placement⁽¹⁾

BOOTRST

IVSEL

Reset address

$0000

Interrupt Vectors Start Address

1

0

1

0

1

$0002

$0000

Boot Reset Address + $0002

$0002

Boot Reset Address

Boot Reset Address + $0002

Note:

1. The Boot Reset Address is shown in Table 100 on page 257. For the BOOTRST Fuse “1”

means unprogrammed while “0” means programmed.

The most typical and general program setup for the Reset and Interrupt Vector Addresses in

ATmega16 is:

Address Labels

$000

$002

$004

$006

$008

$00A

$00C

$00E

$010

$012

$014

$016

$018

$01A

$01C

$01E

$020

$022

$024

$026

$028

;

Code

jmp

Comments

RESET

; Reset Handler

EXT_INT0

EXT_INT1

TIM2_COMP

TIM2_OVF

TIM1_CAPT

TIM1_COMPA

TIM1_COMPB

TIM1_OVF

TIM0_OVF

SPI_STC

; IRQ0 Handler

; IRQ1 Handler

; Timer2 Compare Handler

; Timer2 Overflow Handler

; Timer1 Capture Handler

; Timer1 CompareA Handler

; Timer1 CompareB Handler

; Timer1 Overflow Handler

; Timer0 Overflow Handler

; SPI Transfer Complete Handler

; USART RX Complete Handler

; UDR Empty Handler

USART_RXC

USART_UDRE

USART_TXC

ADC

; USART TX Complete Handler

; ADC Conversion Complete Handler

; EEPROM Ready Handler

; Analog Comparator Handler

; Two-wire Serial Interface Handler

; IRQ2 Handler

EE_RDY

ANA_COMP

TWSI

EXT_INT2

TIM0_COMP

SPM_RDY

; Timer0 Compare Handler

; Store Program Memory Ready Handler

$02A

$02B

$02C

$02D

$02E

$02F

...

RESET:

ldi

out

ldi

out

sei

r16,high(RAMEND) ; Main program start

SPH,r16

; Set Stack Pointer to top of RAM

r16,low(RAMEND)

SPL,r16

; Enable interrupts

<instr> xxx

...

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ATmega16(L)

When the BOOTRST Fuse is unprogrammed, the Boot section size set to 2 Kbytes and the

IVSEL bit in the GICR Register is set before any interrupts are enabled, the most typical and

general program setup for the Reset and Interrupt Vector Addresses is:

Address Labels

Code

Comments

$000

$001

$002

$003

$004

$005

;

RESET:

ldi

r16,high(RAMEND) ; Main program start

out

SPH,r16

; Set Stack Pointer to top of RAM

ldi

out

sei

<instr> xxx

r16,low(RAMEND)

SPL,r16

; Enable interrupts

.org $1C02

$1C02

jmp

..

EXT_INT0

EXT_INT1

; IRQ0 Handler

$1C04

; IRQ1 Handler

...

....

;

$1C28

jmp

SPM_RDY

; Store Program Memory Ready Handler

When the BOOTRST Fuse is programmed and the Boot section size set to 2 Kbytes, the most

typical and general program setup for the Reset and Interrupt Vector Addresses is:

Address Labels

.org $002

$002

Code

Comments

jmp

..

EXT_INT0

EXT_INT1

; IRQ0 Handler

$004

; IRQ1 Handler

...

$028

;

....

;

jmp

SPM_RDY

; Store Program Memory Ready Handler

.org $1C00

$1C00

$1C01

$1C02

$1C03

$1C04

$1C05

RESET:

ldi

out

ldi

out

sei

r16,high(RAMEND) ; Main program start

SPH,r16

; Set Stack Pointer to top of RAM

r16,low(RAMEND)

SPL,r16

; Enable interrupts

<instr> xxx

When the BOOTRST Fuse is programmed, the Boot section size set to 2 Kbytes and the IVSEL

bit in the GICR Register is set before any interrupts are enabled, the most typical and general

program setup for the Reset and Interrupt Vector Addresses is:

Address Labels

Code

Comments

.org $1C00

$1C00

$1C02

jmp

RESET

EXT_INT0

; Reset handler

; IRQ0 Handler

$1C04

jmp

..

EXT_INT1

SPM_RDY

; IRQ1 Handler

...

....

;

$1C28

;

jmp

; Store Program Memory Ready Handler

$1C2A

$1C2B

$1C2C

$1C2D

$1C2E

$1C2F

RESET:

ldi

out

ldi

out

sei

r16,high(RAMEND) ; Main program start

SPH,r16

; Set Stack Pointer to top of RAM

r16,low(RAMEND)

SPL,r16

; Enable interrupts

<instr> xxx

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ATmega16(L)

Moving Interrupts

Between Application

and Boot Space

The General Interrupt Control Register controls the placement of the Interrupt Vector table.

General Interrupt

Control Register –

GICR

Bit

7

6

5

4

–

3

–

2

–

1

IVSEL

R/W

0

IVCE

R/W

0

INT1

R/W

0

INT0

R/W

0

INT2

R/W

0

GICR

Read/Write

Initial Value

R

0

R

0

R

0

• Bit 1 – IVSEL: Interrupt Vector Select

When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash

memory. When this bit is set (one), the interrupt vectors are moved to the beginning of the Boot

Loader section of the Flash. The actual address of the start of the Boot Flash section is deter-

mined by the BOOTSZ Fuses. Refer to the section “Boot Loader Support – Read-While-Write

Self-Programming” on page 246 for details. To avoid unintentional changes of Interrupt Vector

tables, a special write procedure must be followed to change the IVSEL bit:

1. Write the Interrupt Vector Change Enable (IVCE) bit to one.

2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.

Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled

in the cycle IVCE is set, and they remain disabled until after the instruction following the write to

IVSEL. If IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status

Register is unaffected by the automatic disabling.

Note:

If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is programmed,

interrupts are disabled while executing from the Application section. If Interrupt Vectors are placed

in the Application section and Boot Lock bit BLB12 is programed, interrupts are disabled while

executing from the Boot Loader section. Refer to the section “Boot Loader Support – Read-While-

Write Self-Programming” on page 246 for details on Boot Lock bits.

• Bit 0 – IVCE: Interrupt Vector Change Enable

The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by

hardware four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable

interrupts, as explained in the IVSEL description above. See Code Example below

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ATmega16(L)

.

Assembly Code Example

Move_interrupts:

; Enable change of interrupt vectors

ldi r16, (1<<IVCE)

out GICR, r16

; Move interrupts to boot Flash section

ldi r16, (1<<IVSEL)

out GICR, r16

ret

C Code Example

void Move_interrupts(void)

{

/* Enable change of interrupt vectors */

GICR = (1<<IVCE);

/* Move interrupts to boot Flash section */

GICR = (1<<IVSEL);

}

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ATmega16(L)

I/O Ports

Introduction

All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports.

This means that the direction of one port pin can be changed without unintentionally changing

the direction of any other pin with the SBI and CBI instructions. The same applies when chang-

ing drive value (if configured as output) or enabling/disabling of pull-up resistors (if configured as

input). Each output buffer has symmetrical drive characteristics with both high sink and source

capability. The pin driver is strong enough to drive LED displays directly. All port pins have indi-

vidually selectable pull-up resistors with a supply-voltage invariant resistance. All I/O pins have

protection diodes to both V_CCand Ground as indicated in Figure 22. Refer to “Electrical Charac-

teristics” on page 291 for a complete list of parameters.

Figure 22. I/O Pin Equivalent Schematic

R_pu

Pxn

Logic

C_pin

See Figure 23

"General Digital I/O" for

Details

All registers and bit references in this section are written in general form. A lower case “x” repre-

sents the numbering letter for the port, and a lower case “n” represents the bit number. However,

when using the register or bit defines in a program, the precise form must be used, that is,

PORTB3 for bit no. 3 in Port B, here documented generally as PORTxn. The physical I/O Regis-

ters and bit locations are listed in “Register Description for I/O Ports” on page 66.

Three I/O memory address locations are allocated for each port, one each for the Data Register

– PORTx, Data Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins

I/O location is read only, while the Data Register and the Data Direction Register are read/write.

In addition, the Pull-up Disable – PUD bit in SFIOR disables the pull-up function for all pins in all

ports when set.

Using the I/O port as General Digital I/O is described in “Ports as General Digital I/O” on page

50. Most port pins are multiplexed with alternate functions for the peripheral features on the

device. How each alternate function interferes with the port pin is described in “Alternate Port

Functions” on page 55. Refer to the individual module sections for a full description of the alter-

nate functions.

Note that enabling the alternate function of some of the port pins does not affect the use of the

other pins in the port as general digital I/O.

Ports as General

Digital I/O

The ports are bi-directional I/O ports with optional internal pull-ups. Figure 23 shows a functional

description of one I/O-port pin, here generically called Pxn.

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ATmega16(L)

Figure 23. General Digital I/O⁽¹⁾

PUD

Q

D

DDxn

Q _CLR

WDx

RESET

RDx

Q

D

Pxn

PORTxn

Q _CLR

WPx

RRx

RESET

SLEEP

SYNCHRONIZER

RPx

D

Q

D

L

Q

PINxn

Q

clk _I/O

WDx:

RDx:

WPx:

RRx:

RPx:

WRITE DDRx

PUD:

PULLUP DISABLE

READ DDRx

SLEEP:

SLEEP CONTROL

I/O CLOCK

WRITE PORTx

clk_I/O

:

READ PORTx REGISTER

READ PORTx PIN

Note:

1. WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clk_I/O, SLEEP,

and PUD are common to all ports.

Configuring the Pin

Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in “Register

Description for I/O Ports” on page 66, the DDxn bits are accessed at the DDRx I/O address, the

PORTxn bits at the PORTx I/O address, and the PINxn bits at the PINx I/O address.

The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one,

Pxn is configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input

pin.

If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is

activated. To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to

be configured as an output pin. The port pins are tri-stated when a reset condition becomes

active, even if no clocks are running.

If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven

high (one). If PORTxn is written logic zero when the pin is configured as an output pin, the port

pin is driven low (zero).

When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn}

= 0b11), an intermediate state with either pull-up enabled ({DDxn, PORTxn} = 0b01) or output

low ({DDxn, PORTxn} = 0b10) must occur. Normally, the pull-up enabled state is fully accept-

able, as a high-impedant environment will not notice the difference between a strong high driver

and a pull-up. If this is not the case, the PUD bit in the SFIOR Register can be set to disable all

pull-ups in all ports.

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2466T–AVR–07/10

ATmega16(L)

Switching between input with pull-up and output low generates the same problem. The user

must use either the tri-state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn}

= 0b11) as an intermediate step.

Table 20 summarizes the control signals for the pin value.

Table 20. Port Pin Configurations

PUD

DDxn PORTxn (in SFIOR)

I/O

Pull-up Comment

0

X

Input

No

Tri-state (Hi-Z)

Pxn will source current if ext. pulled

low.

0

1

0

1

0

1

Input

Yes

No

Tri-state (Hi-Z)

X

Output

Output Low (Sink)

Output High (Source)

Reading the Pin Value Independent of the setting of Data Direction bit DDxn, the port pin can be read through the

PINxn Register bit. As shown in Figure 23, the PINxn Register bit and the preceding latch consti-

tute a synchronizer. This is needed to avoid metastability if the physical pin changes value near

the edge of the internal clock, but it also introduces a delay. Figure 24 shows a timing diagram of

the synchronization when reading an externally applied pin value. The maximum and minimum

propagation delays are denoted t_pd,maxand t_pd,minrespectively.

Figure 24. Synchronization when Reading an Externally Applied Pin Value

SYSTEM CLK

in r17, PINx

XXX

INSTRUCTIONS

SYNC LATCH

PINxn

0xFF

r17

0x00

t_{pd, max}

t_{pd, min}

Consider the clock period starting shortly after the first falling edge of the system clock. The latch

is closed when the clock is low, and goes transparent when the clock is high, as indicated by the

shaded region of the “SYNC LATCH” signal. The signal value is latched when the system clock

goes low. It is clocked into the PINxn Register at the succeeding positive clock edge. As

indicated by the two arrows t_pd,maxand t_pd,min, a single signal transition on the pin will be delayed

between ½ and 1½ system clock period depending upon the time of assertion.

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ATmega16(L)

When reading back a software assigned pin value, a nop instruction must be inserted as indi-

cated in Figure 25. The out instruction sets the “SYNC LATCH” signal at the positive edge of the

clock. In this case, the delay t_pdthrough the synchronizer is one system clock period.

Figure 25. Synchronization when Reading a Software Assigned Pin Value

SYSTEM CLK

r16

0xFF

nop

in r17, PINx

out PORTx, r16

INSTRUCTIONS

SYNC LATCH

PINxn

0xFF

0x00

t_pd

r17

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ATmega16(L)

The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and define

the port pins from 4 to 7 as input with pull-ups assigned to port pins 6 and 7. The resulting pin

values are read back again, but as previously discussed, a nop instruction is included to be able

to read back the value recently assigned to some of the pins.

Assembly Code Example⁽¹⁾

...

; Define pull-ups and set outputs high

; Define directions for port pins

ldi

out

r16,(1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0)

r17,(1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0)

PORTB,r16

DDRB,r17

; Insert nop for synchronization

nop

; Read port pins

in

r16,PINB

...

C Code Example⁽¹⁾

unsigned char i;

...

/* Define pull-ups and set outputs high */

/* Define directions for port pins */

PORTB = (1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0);

DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0);

/* Insert nop for synchronization*/

_NOP();

/* Read port pins */

i = PINB;

...

Note:

1. For the assembly program, two temporary registers are used to minimize the time from pull-

ups are set on pins 0, 1, 6, and 7, until the direction bits are correctly set, defining bit 2 and 3

as low and redefining bits 0 and 1 as strong high drivers.

Digital Input Enable

and Sleep Modes

As shown in Figure 23, the digital input signal can be clamped to ground at the input of the

schmitt-trigger. The signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in

Power-down mode, Power-save mode, Standby mode, and Extended Standby mode to avoid

high power consumption if some input signals are left floating, or have an analog signal level

close to V_CC/2.

SLEEP is overridden for port pins enabled as External Interrupt pins. If the External Interrupt

Request is not enabled, SLEEP is active also for these pins. SLEEP is also overridden by vari-

ous other alternate functions as described in “Alternate Port Functions” on page 55.

If a logic high level (“one”) is present on an Asynchronous External Interrupt pin configured as

“Interrupt on Rising Edge, Falling Edge, or Any Logic Change on Pin” while the External Inter-

rupt is not enabled, the corresponding External Interrupt Flag will be set when resuming from the

above mentioned sleep modes, as the clamping in these sleep modes produces the requested

logic change.

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ATmega16(L)

Unconnected pins

If some pins are unused, it is recommended to ensure that these pins have a defined level. Even

though most of the digital inputs are disabled in the deep sleep modes as described above, float-

ing inputs should be avoided to reduce current consumption in all other modes where the digital

inputs are enabled (Reset, Active mode and Idle mode).

The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up.

In this case, the pull-up will be disabled during reset. If low power consumption during reset is

important, it is recommended to use an external pull-up or pull-down. Connecting unused pins

directly to V_CCor GND is not recommended, since this may cause excessive currents if the pin is

accidentally configured as an output.

Alternate Port

Functions

Most port pins have alternate functions in addition to being General Digital I/Os. Figure 26

shows how the port pin control signals from the simplified Figure 23 can be overridden by alter-

nate functions. The overriding signals may not be present in all port pins, but the figure serves

as a generic description applicable to all port pins in the AVR microcontroller family.

Figure 26. Alternate Port Functions⁽¹⁾

PUOExn

PUOVxn

1

PUD

0

DDOExn

DDOVxn

1

Q

D

0

DDxn

Q _CLR

WDx

RDx

PVOExn

PVOVxn

RESET

1

0

Pxn

Q

D

PORTxn

Q _CLR

DIEOExn

DIEOVxn

SLEEP

WPx

RRx

RESET

1

0

SYNCHRONIZER

RPx

SET

D

Q

D

L

Q

PINxn

_CLRQ

CLR

clk _I/O

DIxn

AIOxn

PUOExn: Pxn PULL-UP OVERRIDE ENABLE

PUOVxn: Pxn PULL-UP OVERRIDE VALUE

DDOExn: Pxn DATA DIRECTION OVERRIDE ENABLE

DDOVxn: Pxn DATA DIRECTION OVERRIDE VALUE

PVOExn: Pxn PORT VALUE OVERRIDE ENABLE

PVOVxn: Pxn PORT VALUE OVERRIDE VALUE

PUD:

WDx:

RDx:

RRx:

WPx:

RPx:

PULLUP DISABLE

WRITE DDRx

READ DDRx

READ PORTx REGISTER

WRITE PORTx

READ PORTx PIN

DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE

DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE

clk_I/O

DIxn:

AIOxn:

:

I/O CLOCK

DIGITAL INPUT PIN n ON PORTx

ANALOG INPUT/OUTPUT PIN n ON PORTx

SLEEP:

SLEEP CONTROL

Note:

1. WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clk_I/O, SLEEP,

and PUD are common to all ports. All other signals are unique for each pin.

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ATmega16(L)

Table 21 summarizes the function of the overriding signals. The pin and port indexes from Fig-

ure 26 are not shown in the succeeding tables. The overriding signals are generated internally in

the modules having the alternate function.

Table 21. Generic Description of Overriding Signals for Alternate Functions

Signal Name Full Name

Description

PUOE

PUOV

DDOE

DDOV

PVOE

Pull-up Override

Enable

If this signal is set, the pull-up enable is controlled by

the PUOV signal. If this signal is cleared, the pull-up is

enabled when {DDxn, PORTxn, PUD} = 0b010.

Pull-up Override

Value

If PUOE is set, the pull-up is enabled/disabled when

PUOV is set/cleared, regardless of the setting of the

DDxn, PORTxn, and PUD Register bits.

Data Direction

Override Enable

If this signal is set, the Output Driver Enable is

controlled by the DDOV signal. If this signal is cleared,

the Output driver is enabled by the DDxn Register bit.

Data Direction

Override Value

If DDOE is set, the Output Driver is enabled/disabled

when DDOV is set/cleared, regardless of the setting of

the DDxn Register bit.

Port Value Override

Enable

If this signal is set and the Output Driver is enabled,

the port value is controlled by the PVOV signal. If

PVOE is cleared, and the Output Driver is enabled, the

port Value is controlled by the PORTxn Register bit.

PVOV

Port Value Override

Value

If PVOE is set, the port value is set to PVOV,

regardless of the setting of the PORTxn Register bit.

DIEOE

Digital Input Enable

Override Enable

If this bit is set, the Digital Input Enable is controlled by

the DIEOV signal. If this signal is cleared, the Digital

Input Enable is determined by MCU-state (Normal

Mode, sleep modes).

DIEOV

DI

Digital Input Enable

Override Value

If DIEOE is set, the Digital Input is enabled/disabled

when DIEOV is set/cleared, regardless of the MCU

state (Normal Mode, sleep modes).

Digital Input

This is the Digital Input to alternate functions. In the

figure, the signal is connected to the output of the

schmitt trigger but before the synchronizer. Unless the

Digital Input is used as a clock source, the module with

the alternate function will use its own synchronizer.

AIO

Analog Input/ output This is the Analog Input/output to/from alternate

functions. The signal is connected directly to the pad,

and can be used bi-directionally.

The following subsections shortly describe the alternate functions for each port, and relate the

overriding signals to the alternate function. Refer to the alternate function description for further

details.

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Special Function I/O

Register – SFIOR

Bit

7

ADTS2

R/W

0

6

ADTS1

R/W

0

5

ADTS0

R/W

0

4

–

3

ACME

R/W

0

2

1

PSR2

R/W

0

PUD

R/W

0

PSR10

R/W

0

SFIOR

Read/Write

Initial Value

R

0

• Bit 2 – PUD: Pull-up disable

When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and

PORTxn Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See “Con-

figuring the Pin” on page 51 for more details about this feature.

Alternate Functions of Port A has an alternate function as analog input for the ADC as shown in Table 22. If some Port

Port A

A pins are configured as outputs, it is essential that these do not switch when a conversion is in

progress. This might corrupt the result of the conversion.

Table 22. Port A Pins Alternate Functions

Port Pin

PA7

Alternate Function

ADC7 (ADC input channel 7)

ADC6 (ADC input channel 6)

ADC5 (ADC input channel 5)

ADC4 (ADC input channel 4)

ADC3 (ADC input channel 3)

ADC2 (ADC input channel 2)

ADC1 (ADC input channel 1)

ADC0 (ADC input channel 0)

PA6

PA5

PA4

PA3

PA2

PA1

PA0

Table 23 and Table 24 relate the alternate functions of Port A to the overriding signals shown in

Figure 26 on page 55.

Table 23. Overriding Signals for Alternate Functions in PA7..PA4

Signal Name

PUOE

PUOV

DDOE

DDOV

PVOE

PVOV

DIEOE

DIEOV

DI

PA7/ADC7

PA6/ADC6

PA5/ADC5

PA4/ADC4

0

–

AIO

ADC7 INPUT

ADC6 INPUT

ADC5 INPUT

ADC4 INPUT

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Table 24. Overriding Signals for Alternate Functions in PA3..PA0

Signal Name

PUOE

PUOV

DDOE

DDOV

PVOE

PVOV

DIEOE

DIEOV

DI

PA3/ADC3

PA2/ADC2

PA1/ADC1

PA0/ADC0

0

–

AIO

ADC3 INPUT

ADC2 INPUT

ADC1 INPUT

ADC0 INPUT

Alternate Functions of The Port B pins with alternate functions are shown in Table 25.

Port B

Table 25. Port B Pins Alternate Functions

Port Pin

PB7

Alternate Functions

SCK (SPI Bus Serial Clock)

PB6

MISO (SPI Bus Master Input/Slave Output)

MOSI (SPI Bus Master Output/Slave Input)

SS (SPI Slave Select Input)

PB5

PB4

AIN1 (Analog Comparator Negative Input)

PB3

OC0 (Timer/Counter0 Output Compare Match Output)

AIN0 (Analog Comparator Positive Input)

INT2 (External Interrupt 2 Input)

PB2

PB1

PB0

T1 (Timer/Counter1 External Counter Input)

T0 (Timer/Counter0 External Counter Input)

XCK (USART External Clock Input/Output)

The alternate pin configuration is as follows:

• SCK – Port B, Bit 7

SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is enabled as a

Slave, this pin is configured as an input regardless of the setting of DDB7. When the SPI is

enabled as a Master, the data direction of this pin is controlled by DDB7. When the pin is forced

by the SPI to be an input, the pull-up can still be controlled by the PORTB7 bit.

• MISO – Port B, Bit 6

MISO: Master Data input, Slave Data output pin for SPI channel. When the SPI is enabled as a

Master, this pin is configured as an input regardless of the setting of DDB6. When the SPI is

enabled as a Slave, the data direction of this pin is controlled by DDB6. When the pin is forced

by the SPI to be an input, the pull-up can still be controlled by the PORTB6 bit.

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• MOSI – Port B, Bit 5

MOSI: SPI Master Data output, Slave Data input for SPI channel. When the SPI is enabled as a

Slave, this pin is configured as an input regardless of the setting of DDB5. When the SPI is

enabled as a Master, the data direction of this pin is controlled by DDB5. When the pin is forced

by the SPI to be an input, the pull-up can still be controlled by the PORTB5 bit.

• SS – Port B, Bit 4

SS: Slave Select input. When the SPI is enabled as a Slave, this pin is configured as an input

regardless of the setting of DDB4. As a Slave, the SPI is activated when this pin is driven low.

When the SPI is enabled as a Master, the data direction of this pin is controlled by DDB4. When

the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB4 bit.

• AIN1/OC0 – Port B, Bit 3

AIN1, Analog Comparator Negative Input. Configure the port pin as input with the internal pull-up

switched off to avoid the digital port function from interfering with the function of the analog

comparator.

OC0, Output Compare Match output: The PB3 pin can serve as an external output for the

Timer/Counter0 Compare Match. The PB3 pin has to be configured as an output (DDB3 set

(one)) to serve this function. The OC0 pin is also the output pin for the PWM mode timer

function.

• AIN0/INT2 – Port B, Bit 2

AIN0, Analog Comparator Positive input. Configure the port pin as input with the internal pull-up

switched off to avoid the digital port function from interfering with the function of the Analog

Comparator.

INT2, External Interrupt Source 2: The PB2 pin can serve as an external interrupt source to the

MCU.

• T1 – Port B, Bit 1

T1, Timer/Counter1 Counter Source.

• T0/XCK – Port B, Bit 0

T0, Timer/Counter0 Counter Source.

XCK, USART External Clock. The Data Direction Register (DDB0) controls whether the clock is

output (DDB0 set) or input (DDB0 cleared). The XCK pin is active only when the USART oper-

ates in Synchronous mode.

Table 26 and Table 27 relate the alternate functions of Port B to the overriding signals shown in

Figure 26 on page 55. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the MISO signal,

while MOSI is divided into SPI MSTR OUTPUT and SPI SLAVE INPUT.

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Table 26. Overriding Signals for Alternate Functions in PB7..PB4

Signal

Name

PUOE

PUOV

DDOE

DDOV

PVOE

PVOV

DIEOE

DIEOV

DI

PB7/SCK

PB6/MISO

PB5/MOSI

PB4/SS

SPE • MSTR

PORTB7 • PUD

PORTB6 • PUD

PORTB5 • PUD

PORTB4 • PUD

SPE • MSTR

0

SPE • MSTR

0

SCK OUTPUT

SPI SLAVE OUTPUT

SPI MSTR OUTPUT

0

SCK INPUT

–

SPI MSTR INPUT

–

SPI SLAVE INPUT

–

SPI SS

–

AIO

Table 27. Overriding Signals for Alternate Functions in PB3..PB0

Signal

Name

PUOE

PUOV

DDOE

DDOV

PVOE

PVOV

DIEOE

DIEOV

DI

PB3/OC0/AIN1

PB2/INT2/AIN0

PB1/T1

PB0/T0/XCK

0

OC0 ENABLE

0

UMSEL

OC0

0

XCK OUTPUT

0

INT2 ENABLE

1

0

–

INT2 INPUT

AIN0 INPUT

T1 INPUT

–

XCK INPUT/T0 INPUT

–

AIO

AIN1 INPUT

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Alternate Functions of The Port C pins with alternate functions are shown in Table 28. If the JTAG interface is enabled,

Port C

the pull-up resistors on pins PC5(TDI), PC3(TMS) and PC2(TCK) will be activated even if a reset

occurs.

Table 28. Port C Pins Alternate Functions

Port Pin

PC7

Alternate Function

TOSC2 (Timer Oscillator Pin 2)

TOSC1 (Timer Oscillator Pin 1)

TDI (JTAG Test Data In)

PC6

PC5

PC4

TDO (JTAG Test Data Out)

PC3

TMS (JTAG Test Mode Select)

TCK (JTAG Test Clock)

PC2

PC1

SDA (Two-wire Serial Bus Data Input/Output Line)

SCL (Two-wire Serial Bus Clock Line)

PC0

The alternate pin configuration is as follows:

• TOSC2 – Port C, Bit 7

TOSC2, Timer Oscillator pin 2: When the AS2 bit in ASSR is set (one) to enable asynchronous

clocking of Timer/Counter2, pin PC7 is disconnected from the port, and becomes the inverting

output of the Oscillator amplifier. In this mode, a Crystal Oscillator is connected to this pin, and

the pin can not be used as an I/O pin.

• TOSC1 – Port C, Bit 6

TOSC1, Timer Oscillator pin 1: When the AS2 bit in ASSR is set (one) to enable asynchronous

clocking of Timer/Counter2, pin PC6 is disconnected from the port, and becomes the input of the

inverting Oscillator amplifier. In this mode, a Crystal Oscillator is connected to this pin, and the

pin can not be used as an I/O pin.

• TDI – Port C, Bit 5

TDI, JTAG Test Data In: Serial input data to be shifted in to the Instruction Register or Data Reg-

ister (scan chains). When the JTAG interface is enabled, this pin can not be used as an I/O pin.

• TDO – Port C, Bit 4

TDO, JTAG Test Data Out: Serial output data from Instruction Register or Data Register. When

the JTAG interface is enabled, this pin can not be used as an I/O pin.

The TD0 pin is tri-stated unless TAP states that shifts out data are entered.

• TMS – Port C, Bit 3

TMS, JTAG Test Mode Select: This pin is used for navigating through the TAP-controller state

machine. When the JTAG interface is enabled, this pin can not be used as an I/O pin.

• TCK – Port C, Bit 2

TCK, JTAG Test Clock: JTAG operation is synchronous to TCK. When the JTAG interface is

enabled, this pin can not be used as an I/O pin.

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• SDA – Port C, Bit 1

SDA, Two-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to enable the

Two-wire Serial Interface, pin PC1 is disconnected from the port and becomes the Serial Data

I/O pin for the Two-wire Serial Interface. In this mode, there is a spike filter on the pin to sup-

press spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver

with slew-rate limitation. When this pin is used by the Two-wire Serial Interface, the pull-up can

still be controlled by the PORTC1 bit.

• SCL – Port C, Bit 0

SCL, Two-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to enable the

Two-wire Serial Interface, pin PC0 is disconnected from the port and becomes the Serial Clock

I/O pin for the Two-wire Serial Interface. In this mode, there is a spike filter on the pin to sup-

press spikes shorter than 50 ns on the input signal, and the pin is driven by an open drain driver

with slew-rate limitation. When this pin is used by the Two-wire Serial Interface, the pull-up can

still be controlled by the PORTC0 bit.

Table 29 and Table 30 relate the alternate functions of Port C to the overriding signals shown in

Figure 26 on page 55.

Table 29. Overriding Signals for Alternate Functions in PC7..PC4

Signal

Name

PUOE

PUOV

DDOE

DDOV

PVOE

PVOV

DIEOE

DIEOV

DI

PC7/TOSC2

PC6/TOSC1

PC5/TDI

PC4/TDO

AS2

JTAGEN

0

1

0

AS2

JTAGEN

0

SHIFT_IR + SHIFT_DR

0

JTAGEN

0

TDO

AS2

JTAGEN

0

–

AIO

T/C2 OSC OUTPUT

T/C2 OSC INPUT

TDI

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Table 30. Overriding Signals for Alternate Functions in PC3..PC0⁽¹⁾

Signal

Name

PUOE

PUOV

DDOE

DDOV

PVOE

PVOV

DIEOE

DIEOV

DI

PC3/TMS

PC2/TCK

PC1/SDA

PC0/SCL

JTAGEN

TWEN

1

PORTC1 • PUD

PORTC0 • PUD

JTAGEN

TWEN

0

SDA_OUT

SCL_OUT

0

TWEN

0

JTAGEN

0

–

AIO

TMS

TCK

SDA INPUT

SCL INPUT

Note:

1. When enabled, the Two-wire Serial Interface enables slew-rate controls on the output pins

PC0 and PC1. This is not shown in the figure. In addition, spike filters are connected between

the AIO outputs shown in the port figure and the digital logic of the TWI module.

Alternate Functions of The Port D pins with alternate functions are shown in Table 31.

Port D

Table 31. Port D Pins Alternate Functions

Port Pin

PD7

Alternate Function

OC2 (Timer/Counter2 Output Compare Match Output)

ICP1 (Timer/Counter1 Input Capture Pin)

OC1A (Timer/Counter1 Output Compare A Match Output)

OC1B (Timer/Counter1 Output Compare B Match Output)

INT1 (External Interrupt 1 Input)

PD6

PD5

PD4

PD3

PD2

INT0 (External Interrupt 0 Input)

PD1

TXD (USART Output Pin)

PD0

RXD (USART Input Pin)

The alternate pin configuration is as follows:

• OC2 – Port D, Bit 7

OC2, Timer/Counter2 Output Compare Match output: The PD7 pin can serve as an external out-

put for the Timer/Counter2 Output Compare. The pin has to be configured as an output (DDD7

set (one)) to serve this function. The OC2 pin is also the output pin for the PWM mode timer

function.

• ICP1 – Port D, Bit 6

ICP1 – Input Capture Pin: The PD6 pin can act as an Input Capture pin for Timer/Counter1.

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• OC1A – Port D, Bit 5

OC1A, Output Compare Match A output: The PD5 pin can serve as an external output for the

Timer/Counter1 Output Compare A. The pin has to be configured as an output (DDD5 set (one))

to serve this function. The OC1A pin is also the output pin for the PWM mode timer function.

• OC1B – Port D, Bit 4

OC1B, Output Compare Match B output: The PD4 pin can serve as an external output for the

Timer/Counter1 Output Compare B. The pin has to be configured as an output (DDD4 set (one))

to serve this function. The OC1B pin is also the output pin for the PWM mode timer function.

• INT1 – Port D, Bit 3

INT1, External Interrupt Source 1: The PD3 pin can serve as an external interrupt source.

• INT0 – Port D, Bit 2

INT0, External Interrupt Source 0: The PD2 pin can serve as an external interrupt source.

• TXD – Port D, Bit 1

TXD, Transmit Data (Data output pin for the USART). When the USART Transmitter is enabled,

this pin is configured as an output regardless of the value of DDD1.

• RXD – Port D, Bit 0

RXD, Receive Data (Data input pin for the USART). When the USART Receiver is enabled this

pin is configured as an input regardless of the value of DDD0. When the USART forces this pin

to be an input, the pull-up can still be controlled by the PORTD0 bit.

Table 32 and Table 33 relate the alternate functions of Port D to the overriding signals shown in

Figure 26 on page 55.

Table 32. Overriding Signals for Alternate Functions PD7..PD4

Signal Name

PUOE

PUOV

DDOE

DDOV

PVOE

PVOV

DIEOE

DIEOV

DI

PD7/OC2

PD6/ICP1

PD5/OC1A

PD4/OC1B

0

OC2 ENABLE

0

OC1A ENABLE

OC1B ENABLE

OC2

0

OC1A

OC1B

0

–

0

–

0

–

0

ICP1 INPUT

–

AIO

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Table 33. Overriding Signals for Alternate Functions in PD3..PD0

Signal Name

PUOE

PUOV

DDOE

DDOV

PVOE

PVOV

DIEOE

DIEOV

DI

PD3/INT1

PD2/INT0

PD1/TXD

PD0/RXD

0

TXEN

RXEN

0

PORTD0 • PUD

0

TXEN

RXEN

0

1

0

TXEN

0

TXD

0

INT1 ENABLE

INT0 ENABLE

0

–

0

1

0

INT1 INPUT

–

INT0 INPUT

–

RXD

–

AIO

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Register

Description for I/O

Ports

Port A Data Register –

PORTA

Bit

7

PORTA7

R/W

0

6

PORTA6

R/W

0

5

PORTA5

R/W

0

4

PORTA4

R/W

0

3

PORTA3

R/W

0

2

PORTA2

R/W

0

1

PORTA1

R/W

0

PORTA0

R/W

0

PORTA

DDRA

PINA

Read/Write

Initial Value

Port A Data Direction

Register – DDRA

Bit

7

DDA7

R/W

0

6

DDA6

R/W

0

5

DDA5

R/W

0

4

DDA4

R/W

0

3

DDA3

R/W

0

2

DDA2

R/W

0

1

DDA1

R/W

0

DDA0

R/W

0

Read/Write

Initial Value

Port A Input Pins

Address – PINA

Bit

7

PINA7

R

6

PINA6

R

5

PINA5

R

4

PINA4

R

3

PINA3

R

2

PINA2

R

1

PINA1

R

0

PINA0

R

Read/Write

Initial Value

N/A

Port B Data Register –

PORTB

Bit

7

PORTB7

R/W

0

6

PORTB6

R/W

0

5

PORTB5

R/W

0

4

PORTB4

R/W

0

3

PORTB3

R/W

0

2

PORTB2

R/W

0

1

PORTB1

R/W

0

PORTB0

R/W

0

PORTB

DDRB

PINB

Read/Write

Initial Value

Port B Data Direction

Register – DDRB

Bit

7

DDB7

R/W

0

6

DDB6

R/W

0

5

DDB5

R/W

0

4

DDB4

R/W

0

3

DDB3

R/W

0

2

DDB2

R/W

0

1

DDB1

R/W

0

DDB0

R/W

0

Read/Write

Initial Value

Port B Input Pins

Address – PINB

Bit

7

PINB7

R

6

PINB6

R

5

PINB5

R

4

PINB4

R

3

PINB3

R

2

PINB2

R

1

PINB1

R

0

PINB0

R

Read/Write

Initial Value

N/A

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Port C Data Register –

PORTC

Bit

7

PORTC7

R/W

0

6

PORTC6

R/W

0

5

PORTC5

R/W

0

4

PORTC4

R/W

0

3

PORTC3

R/W

0

2

PORTC2

R/W

0

1

PORTC1

R/W

0

PORTC0

R/W

0

PORTC

DDRC

PINC

Read/Write

Initial Value

Port C Data Direction

Register – DDRC

Bit

7

DDC7

R/W

0

6

DDC6

R/W

0

5

DDC5

R/W

0

4

DDC4

R/W

0

3

DDC3

R/W

0

2

DDC2

R/W

0

1

DDC1

R/W

0

DDC0

R/W

0

Read/Write

Initial Value

Port C Input Pins

Address – PINC

Bit

7

PINC7

R

6

PINC6

R

5

PINC5

R

4

PINC4

R

3

PINC3

R

2

PINC2

R

1

PINC1

R

0

PINC0

R

Read/Write

Initial Value

N/A

Port D Data Register –

PORTD

Bit

7

PORTD7

R/W

0

6

PORTD6

R/W

0

5

PORTD5

R/W

0

4

PORTD4

R/W

0

3

PORTD3

R/W

0

2

PORTD2

R/W

0

1

PORTD1

R/W

0

PORTD0

R/W

0

PORTD

DDRD

PIND

Read/Write

Initial Value

Port D Data Direction

Register – DDRD

Bit

7

DDD7

R/W

0

6

DDD6

R/W

0

5

DDD5

R/W

0

4

DDD4

R/W

0

3

DDD3

R/W

0

2

DDD2

R/W

0

1

DDD1

R/W

0

DDD0

R/W

0

Read/Write

Initial Value

Port D Input Pins

Address – PIND

Bit

7

PIND7

R

6

PIND6

R

5

PIND5

R

4

PIND4

R

3

PIND3

R

2

PIND2

R

1

PIND1

R

0

PIND0

R

Read/Write

Initial Value

N/A

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External

Interrupts

The External Interrupts are triggered by the INT0, INT1, and INT2 pins. Observe that, if enabled,

the interrupts will trigger even if the INT0..2 pins are configured as outputs. This feature provides

a way of generating a software interrupt. The external interrupts can be triggered by a falling or

rising edge or a low level (INT2 is only an edge triggered interrupt). This is set up as indicated in

the specification for the MCU Control Register – MCUCR – and MCU Control and Status Regis-

ter – MCUCSR. When the external interrupt is enabled and is configured as level triggered (only

INT0/INT1), the interrupt will trigger as long as the pin is held low. Note that recognition of falling

or rising edge interrupts on INT0 and INT1 requires the presence of an I/O clock, described in

“Clock Systems and their Distribution” on page 24. Low level interrupts on INT0/INT1 and the

edge interrupt on INT2 are detected asynchronously. This implies that these interrupts can be

used for waking the part also from sleep modes other than Idle mode. The I/O clock is halted in

all sleep modes except Idle mode.

Note that if a level triggered interrupt is used for wake-up from Power-down mode, the changed

level must be held for some time to wake up the MCU. This makes the MCU less sensitive to

noise. The changed level is sampled twice by the Watchdog Oscillator clock. The period of the

Watchdog Oscillator is 1 µs (nominal) at 5.0V and 25°C. The frequency of the Watchdog Oscilla-

tor is voltage dependent as shown in “Electrical Characteristics” on page 291. The MCU will

wake up if the input has the required level during this sampling or if it is held until the end of the

start-up time. The start-up time is defined by the SUT Fuses as described in “System Clock and

Clock Options” on page 24. If the level is sampled twice by the Watchdog Oscillator clock but

disappears before the end of the start-up time, the MCU will still wake up, but no interrupt will be

generated. The required level must be held long enough for the MCU to complete the wake up to

trigger the level interrupt.

MCU Control Register The MCU Control Register contains control bits for interrupt sense control and general MCU

– MCUCR

functions.

Bit

7

6

SE

R/W

0

5

4

3

ISC11

R/W

0

2

ISC10

R/W

0

1

ISC01

R/W

0

ISC00

R/W

0

SM2

R/W

0

SM1

R/W

0

SM0

R/W

0

MCUCR

Read/Write

Initial Value

• Bit 3, 2 – ISC11, ISC10: Interrupt Sense Control 1 Bit 1 and Bit 0

The External Interrupt 1 is activated by the external pin INT1 if the SREG I-bit and the corre-

sponding interrupt mask in the GICR are set. The level and edges on the external INT1 pin that

activate the interrupt are defined in Table 34. The value on the INT1 pin is sampled before

detecting edges. If edge or toggle interrupt is selected, pulses that last longer than one clock

period will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt. If

low level interrupt is selected, the low level must be held until the completion of the currently

executing instruction to generate an interrupt.

Table 34. Interrupt 1 Sense Control

ISC11

ISC10

Description

0

1

0

1

0

1

The low level of INT1 generates an interrupt request.

Any logical change on INT1 generates an interrupt request.

The falling edge of INT1 generates an interrupt request.

The rising edge of INT1 generates an interrupt request.

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• Bit 1, 0 – ISC01, ISC00: Interrupt Sense Control 0 Bit 1 and Bit 0

The External Interrupt 0 is activated by the external pin INT0 if the SREG I-flag and the corre-

sponding interrupt mask are set. The level and edges on the external INT0 pin that activate the

interrupt are defined in Table 35. The value on the INT0 pin is sampled before detecting edges.

If edge or toggle interrupt is selected, pulses that last longer than one clock period will generate

an interrupt. Shorter pulses are not guaranteed to generate an interrupt. If low level interrupt is

selected, the low level must be held until the completion of the currently executing instruction to

generate an interrupt.

Table 35. Interrupt 0 Sense Control

ISC01

ISC00

Description

0

1

0

1

0

1

The low level of INT0 generates an interrupt request.

Any logical change on INT0 generates an interrupt request.

The falling edge of INT0 generates an interrupt request.

The rising edge of INT0 generates an interrupt request.

MCU Control and

Status Register –

MCUCSR

Bit

7

6

5

–

4

3

2

1

0

JTD

R/W

0

ISC2

R/W

0

JTRF

R/W

WDRF

R/W

BORF

R/W

EXTRF

R/W

PORF

R/W

MCUCSR

Read/Write

Initial Value

R

0

See Bit Description

• Bit 6 – ISC2: Interrupt Sense Control 2

The Asynchronous External Interrupt 2 is activated by the external pin INT2 if the SREG I-bit and

the corresponding interrupt mask in GICR are set. If ISC2 is written to zero, a falling edge on

INT2 activates the interrupt. If ISC2 is written to one, a rising edge on INT2 activates the inter-

rupt. Edges on INT2 are registered asynchronously. Pulses on INT2 wider than the minimum

pulse width given in Table 36 will generate an interrupt. Shorter pulses are not guaranteed to

generate an interrupt. When changing the ISC2 bit, an interrupt can occur. Therefore, it is rec-

ommended to first disable INT2 by clearing its Interrupt Enable bit in the GICR Register. Then,

the ISC2 bit can be changed. Finally, the INT2 Interrupt Flag should be cleared by writing a logi-

cal one to its Interrupt Flag bit (INTF2) in the GIFR Register before the interrupt is re-enabled.

Table 36. Asynchronous External Interrupt Characteristics

Symbol Parameter

Condition Min

Typ

Max

Units

Minimum pulse width for

asynchronous external interrupt

t_INT

50

ns

General Interrupt

Control Register –

GICR

Bit

7

6

5

4

–

3

–

2

–

1

0

INT1

R/W

0

INT0

R/W

0

INT2

R/W

0

IVSEL

R/W

0

IVCE

GICR

Read/Write

Initial Value

R

0

R

0

R

0

R/W

0

• Bit 7 – INT1: External Interrupt Request 1 Enable

When the INT1 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the exter-

nal pin interrupt is enabled. The Interrupt Sense Control1 bits 1/0 (ISC11 and ISC10) in the MCU

General Control Register (MCUCR) define whether the External Interrupt is activated on rising

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and/or falling edge of the INT1 pin or level sensed. Activity on the pin will cause an interrupt

request even if INT1 is configured as an output. The corresponding interrupt of External Interrupt

Request 1 is executed from the INT1 interrupt Vector.

• Bit 6 – INT0: External Interrupt Request 0 Enable

When the INT0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the exter-

nal pin interrupt is enabled. The Interrupt Sense Control0 bits 1/0 (ISC01 and ISC00) in the MCU

General Control Register (MCUCR) define whether the External Interrupt is activated on rising

and/or falling edge of the INT0 pin or level sensed. Activity on the pin will cause an interrupt

request even if INT0 is configured as an output. The corresponding interrupt of External Interrupt

Request 0 is executed from the INT0 interrupt vector.

• Bit 5 – INT2: External Interrupt Request 2 Enable

When the INT2 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the exter-

nal pin interrupt is enabled. The Interrupt Sense Control2 bit (ISC2) in the MCU Control and

Status Register (MCUCSR) defines whether the External Interrupt is activated on rising or falling

edge of the INT2 pin. Activity on the pin will cause an interrupt request even if INT2 is configured

as an output. The corresponding interrupt of External Interrupt Request 2 is executed from the

INT2 Interrupt Vector.

General Interrupt Flag

Register – GIFR

Bit

7

INTF1

R/W

0

6

INTF0

R/W

0

5

INTF2

R/W

0

4

–

3

–

2

–

1

–

0

–

GIFR

Read/Write

Initial Value

R

0

R

0

R

0

R

0

R

0

• Bit 7 – INTF1: External Interrupt Flag 1

When an edge or logic change on the INT1 pin triggers an interrupt request, INTF1 becomes set

(one). If the I-bit in SREG and the INT1 bit in GICR are set (one), the MCU will jump to the corre-

sponding Interrupt Vector. The flag is cleared when the interrupt routine is executed.

Alternatively, the flag can be cleared by writing a logical one to it. This flag is always cleared

when INT1 is configured as a level interrupt.

• Bit 6 – INTF0: External Interrupt Flag 0

When an edge or logic change on the INT0 pin triggers an interrupt request, INTF0 becomes set

(one). If the I-bit in SREG and the INT0 bit in GICR are set (one), the MCU will jump to the corre-

sponding interrupt vector. The flag is cleared when the interrupt routine is executed.

Alternatively, the flag can be cleared by writing a logical one to it. This flag is always cleared

when INT0 is configured as a level interrupt.

• Bit 5 – INTF2: External Interrupt Flag 2

When an event on the INT2 pin triggers an interrupt request, INTF2 becomes set (one). If the I-

bit in SREG and the INT2 bit in GICR are set (one), the MCU will jump to the corresponding

Interrupt Vector. The flag is cleared when the interrupt routine is executed. Alternatively, the flag

can be cleared by writing a logical one to it. Note that when entering some sleep modes with the

INT2 interrupt disabled, the input buffer on this pin will be disabled. This may cause a logic

change in internal signals which will set the INTF2 Flag. See “Digital Input Enable and Sleep

Modes” on page 54 for more information.

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8-bit

Timer/Counter0

with PWM

Timer/Counter0 is a general purpose, single compare unit, 8-bit Timer/Counter module. The

main features are:

• Single Compare Unit Counter

• Clear Timer on Compare Match (Auto Reload)

• Glitch-free, Phase Correct Pulse Width Modulator (PWM)

• Frequency Generator

• External Event Counter

• 10-bit Clock Prescaler

• Overflow and Compare Match Interrupt Sources (TOV0 and OCF0)

Overview

A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 27. For the actual place-

ment of I/O pins, refer to “Pinout ATmega16” on page 2. CPU accessible I/O Registers, including

I/O bits and I/O pins, are shown in bold. The device-specific I/O Register and bit locations are

listed in the “8-bit Timer/Counter Register Description” on page 83.

Figure 27. 8-bit Timer/Counter Block Diagram

TCCRn

count

TOVn

(Int.Req.)

clear

Control Logic

TOP

Clock Select

direction

clk

Tn

Edge

Detector

Tn

BOTTOM

( From Prescaler )

Timer/Counter

TCNTn

= 0

= 0xFF

OCn

(Int.Req.)

Waveform

Generation

OCn

=

OCRn

Registers

The Timer/Counter (TCNT0) and Output Compare Register (OCR0) are 8-bit registers. Interrupt

request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag

Register (TIFR). All interrupts are individually masked with the Timer Interrupt Mask Register

(TIMSK). TIFR and TIMSK are not shown in the figure since these registers are shared by other

timer units.

The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on

the T0 pin. The Clock Select logic block controls which clock source and edge the Timer/Counter

uses to increment (or decrement) its value. The Timer/Counter is inactive when no clock source

is selected. The output from the Clock Select logic is referred to as the timer clock (clk_T0).

The double buffered Output Compare Register (OCR0) is compared with the Timer/Counter

value at all times. The result of the compare can be used by the waveform generator to generate

a PWM or variable frequency output on the Output Compare Pin (OC0). See “Output Compare

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Unit” on page 73. for details. The compare match event will also set the Compare Flag (OCF0)

which can be used to generate an output compare interrupt request.

Definitions

Many register and bit references in this document are written in general form. A lower case “n”

replaces the Timer/Counter number, in this case 0. However, when using the register or bit

defines in a program, the precise form must be used, that is, TCNT0 for accessing

Timer/Counter0 counter value and so on.

The definitions in Table 37 are also used extensively throughout the document.

Table 37. Definitions

BOTTOM The counter reaches the BOTTOM when it becomes 0x00.

MAX

TOP

The counter reaches its MAXimum when it becomes 0xFF (decimal 255).

The counter reaches the TOP when it becomes equal to the highest

value in the count sequence. The TOP value can be assigned to be the

fixed value 0xFF (MAX) or the value stored in the OCR0 Register. The

assignment is dependent on the mode of operation.

Timer/Counter

Clock Sources

The Timer/Counter can be clocked by an internal or an external clock source. The clock source

is selected by the clock select logic which is controlled by the clock select (CS02:0) bits located

in the Timer/Counter Control Register (TCCR0). For details on clock sources and prescaler, see

“Timer/Counter0 and Timer/Counter1 Prescalers” on page 87.

Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure

28 shows a block diagram of the counter and its surroundings.

Figure 28. Counter Unit Block Diagram

TOVn

(Int. Req.)

DATA BUS

Clock Select

count

clear

Edge

Detector

Tn

clk_Tn

TCNTn

Control Logic

direction

( From Prescaler )

BOTTOM

TOP

Signal description (internal signals):

count Increment or decrement TCNT0 by 1.

direction Select between increment and decrement.

clear

clk_Tn

TOP

Clear TCNT0 (set all bits to zero).

Timer/Counter clock, referred to as clk_T0in the following.

Signalize that TCNT0 has reached maximum value.

BOTTOM Signalize that TCNT0 has reached minimum value (zero).

Depending of the mode of operation used, the counter is cleared, incremented, or decremented

at each timer clock (clk_T0). clk_T0can be generated from an external or internal clock source,

selected by the Clock Select bits (CS02:0). When no clock source is selected (CS02:0 = 0) the

timer is stopped. However, the TCNT0 value can be accessed by the CPU, regardless of

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whether clk_T0is present or not. A CPU write overrides (has priority over) all counter clear or

count operations.

The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in

the Timer/Counter Control Register (TCCR0). There are close connections between how the

counter behaves (counts) and how waveforms are generated on the Output Compare output

OC0. For more details about advanced counting sequences and waveform generation, see

“Modes of Operation” on page 76.

The Timer/Counter Overflow (TOV0) Flag is set according to the mode of operation selected by

the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.

Output Compare

Unit

The 8-bit comparator continuously compares TCNT0 with the Output Compare Register

(OCR0). Whenever TCNT0 equals OCR0, the comparator signals a match. A match will set the

Output Compare Flag (OCF0) at the next timer clock cycle. If enabled (OCIE0 = 1 and Global

Interrupt Flag in SREG is set), the Output Compare Flag generates an output compare interrupt.

The OCF0 Flag is automatically cleared when the interrupt is executed. Alternatively, the OCF0

Flag can be cleared by software by writing a logical one to its I/O bit location. The waveform gen-

erator uses the match signal to generate an output according to operating mode set by the

WGM01:0 bits and Compare Output mode (COM01:0) bits. The max and bottom signals are

used by the waveform generator for handling the special cases of the extreme values in some

modes of operation (See “Modes of Operation” on page 76.).

Figure 29 shows a block diagram of the output compare unit.

Figure 29. Output Compare Unit, Block Diagram

DATA BUS

OCRn

TCNTn

=

(8-bit Comparator )

OCFn (Int.Req.)

top

bottom

FOCn

Waveform Generator

OCn

WGMn1:0

COMn1:0

The OCR0 Register is double buffered when using any of the Pulse Width Modulation (PWM)

modes. For the normal and Clear Timer on Compare (CTC) modes of operation, the double buff-

ering is disabled. The double buffering synchronizes the update of the OCR0 Compare Register

to either top or bottom of the counting sequence. The synchronization prevents the occurrence

of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free.

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The OCR0 Register access may seem complex, but this is not case. When the double buffering

is enabled, the CPU has access to the OCR0 Buffer Register, and if double buffering is disabled

the CPU will access the OCR0 directly.

Force Output

Compare

In non-PWM waveform generation modes, the match output of the comparator can be forced by

writing a one to the Force Output Compare (FOC0) bit. Forcing compare match will not set the

OCF0 Flag or reload/clear the timer, but the OC0 pin will be updated as if a real compare match

had occurred (the COM01:0 bits settings define whether the OC0 pin is set, cleared or toggled).

Compare Match

Blocking by TCNT0

Write

All CPU write operations to the TCNT0 Register will block any compare match that occur in the

next timer clock cycle, even when the timer is stopped. This feature allows OCR0 to be initialized

to the same value as TCNT0 without triggering an interrupt when the Timer/Counter clock is

enabled.

Using the Output

Compare Unit

Since writing TCNT0 in any mode of operation will block all compare matches for one timer clock

cycle, there are risks involved when changing TCNT0 when using the output compare unit, inde-

pendently of whether the Timer/Counter is running or not. If the value written to TCNT0 equals

the OCR0 value, the compare match will be missed, resulting in incorrect waveform generation.

Similarly, do not write the TCNT0 value equal to BOTTOM when the counter is downcounting.

The setup of the OC0 should be performed before setting the Data Direction Register for the port

pin to output. The easiest way of setting the OC0 value is to use the Force Output Compare

(FOC0) strobe bits in Normal mode. The OC0 Register keeps its value even when changing

between waveform generation modes.

Be aware that the COM01:0 bits are not double buffered together with the compare value.

Changing the COM01:0 bits will take effect immediately.

Compare Match

Output Unit

The Compare Output mode (COM01:0) bits have two functions. The Waveform Generator uses

the COM01:0 bits for defining the Output Compare (OC0) state at the next compare match. Also,

the COM01:0 bits control the OC0 pin output source. Figure 30 shows a simplified schematic of

the logic affected by the COM01:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the fig-

ure are shown in bold. Only the parts of the general I/O port Control Registers (DDR and PORT)

that are affected by the COM01:0 bits are shown. When referring to the OC0 state, the reference

is for the internal OC0 Register, not the OC0 pin. If a System Reset occur, the OC0 Register is

reset to “0”.

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Figure 30. Compare Match Output Unit, Schematic

COMn1

Waveform

Generator

COMn0

FOCn

D

Q

1

0

OCn

D

PORT

D

Q

DDR

clk_I/O

The general I/O port function is overridden by the Output Compare (OC0) from the Waveform

Generator if either of the COM01:0 bits are set. However, the OC0 pin direction (input or output)

is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Regis-

ter bit for the OC0 pin (DDR_OC0) must be set as output before the OC0 value is visible on the

pin. The port override function is independent of the Waveform Generation mode.

The design of the output compare pin logic allows initialization of the OC0 state before the out-

put is enabled. Note that some COM01:0 bit settings are reserved for certain modes of

operation. See “8-bit Timer/Counter Register Description” on page 83.

Compare Output Mode The Waveform Generator uses the COM01:0 bits differently in normal, CTC, and PWM modes.

and Waveform

Generation

For all modes, setting the COM01:0 = 0 tells the waveform generator that no action on the OC0

Register is to be performed on the next compare match. For compare output actions in the non-

PWM modes refer to Table 39 on page 84. For fast PWM mode, refer to Table 40 on page 84,

and for phase correct PWM refer to Table 41 on page 84.

A change of the COM01:0 bits state will have effect at the first compare match after the bits are

written. For non-PWM modes, the action can be forced to have immediate effect by using the

FOC0 strobe bits.

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Modes of

Operation

The mode of operation, that is, the behavior of the Timer/Counter and the Output Compare pins,

is defined by the combination of the Waveform Generation mode (WGM01:0) and Compare Out-

put mode (COM01:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COM01:0 bits control whether the PWM out-

put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes

the COM01:0 bits control whether the output should be set, cleared, or toggled at a compare

match (See “Compare Match Output Unit” on page 74.).

For detailed timing information refer to Figure 34, Figure 35, Figure 36 and Figure 37 in

“Timer/Counter Timing Diagrams” on page 81.

Normal Mode

The simplest mode of operation is the normal mode (WGM01:0 = 0). In this mode the counting

direction is always up (incrementing), and no counter clear is performed. The counter simply

overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bot-

tom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV0) will be set in the same

timer clock cycle as the TCNT0 becomes zero. The TOV0 Flag in this case behaves like a ninth

bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt

that automatically clears the TOV0 Flag, the timer resolution can be increased by software.

There are no special cases to consider in the normal mode, a new counter value can be written

anytime.

The output compare unit can be used to generate interrupts at some given time. Using the out-

put compare to generate waveforms in Normal mode is not recommended, since this will occupy

too much of the CPU time.

Clear Timer on

In Clear Timer on Compare or CTC mode (WGM01:0 = 2), the OCR0 Register is used to manip-

Compare Match (CTC) ulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value

Mode

(TCNT0) matches the OCR0. The OCR0 defines the top value for the counter, hence also its

resolution. This mode allows greater control of the compare match output frequency. It also sim-

plifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 31. The counter value (TCNT0)

increases until a compare match occurs between TCNT0 and OCR0, and then counter (TCNT0)

is cleared.

Figure 31. CTC Mode, Timing Diagram

OCn Interrupt Flag Set

TCNTn

OCn

(Toggle)

(COMn1:0 = 1)

1

2

3

4

Period

An interrupt can be generated each time the counter value reaches the TOP value by using the

OCF0 Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating the

TOP value. However, changing TOP to a value close to BOTTOM when the counter is running

with none or a low prescaler value must be done with care since the CTC mode does not have

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the double buffering feature. If the new value written to OCR0 is lower than the current value of

TCNT0, the counter will miss the compare match. The counter will then have to count to its max-

imum value (0xFF) and wrap around starting at 0x00 before the compare match can occur.

For generating a waveform output in CTC mode, the OC0 output can be set to toggle its logical

level on each compare match by setting the Compare Output mode bits to toggle mode

(COM01:0 = 1). The OC0 value will not be visible on the port pin unless the data direction for the

pin is set to output. The waveform generated will have a maximum frequency of f_OC0= f_{clk_I/O}/2

when OCR0 is set to zero (0x00). The waveform frequency is defined by the following equation:

f

clk_I/O

f

= ----------------------------------------------

OCn

2 ⋅ N ⋅ (1 + OCRn)

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle that the

counter counts from MAX to 0x00.

Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGM01:0 = 3) provides a high frequency

PWM waveform generation option. The fast PWM differs from the other PWM option by its sin-

gle-slope operation. The counter counts from BOTTOM to MAX then restarts from BOTTOM. In

non-inverting Compare Output mode, the Output Compare (OC0) is cleared on the compare

match between TCNT0 and OCR0, and set at BOTTOM. In inverting Compare Output mode, the

output is set on compare match and cleared at BOTTOM. Due to the single-slope operation, the

operating frequency of the fast PWM mode can be twice as high as the phase correct PWM

mode that use dual-slope operation. This high frequency makes the fast PWM mode well suited

for power regulation, rectification, and DAC applications. High frequency allows physically small

sized external components (coils, capacitors), and therefore reduces total system cost.

In fast PWM mode, the counter is incremented until the counter value matches the MAX value.

The counter is then cleared at the following timer clock cycle. The timing diagram for the fast

PWM mode is shown in Figure 32. The TCNT0 value is in the timing diagram shown as a histo-

gram for illustrating the single-slope operation. The diagram includes non-inverted and inverted

PWM outputs. The small horizontal line marks on the TCNT0 slopes represent compare

matches between OCR0 and TCNT0.

Figure 32. Fast PWM Mode, Timing Diagram

OCRn Interrupt Flag Set

OCRn Update and

TOVn Interrupt Flag Set

TCNTn

(COMn1:0 = 2)

(COMn1:0 = 3)

OCn

1

2

3

4

5

6

7

Period

The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches MAX. If the inter-

rupt is enabled, the interrupt handler routine can be used for updating the compare value.

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In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0 pin. Set-

ting the COM01:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can

be generated by setting the COM01:0 to 3 (See Table 40 on page 84). The actual OC0 value will

only be visible on the port pin if the data direction for the port pin is set as output. The PWM

waveform is generated by setting (or clearing) the OC0 Register at the compare match between

OCR0 and TCNT0, and clearing (or setting) the OC0 Register at the timer clock cycle the coun-

ter is cleared (changes from MAX to BOTTOM).

The PWM frequency for the output can be calculated by the following equation:

f

clk_I/O

f

= -----------------

OCnPWM

N ⋅ 256

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

The extreme values for the OCR0 Register represents special cases when generating a PWM

waveform output in the fast PWM mode. If the OCR0 is set equal to BOTTOM, the output will be

a narrow spike for each MAX+1 timer clock cycle. Setting the OCR0 equal to MAX will result in a

constantly high or low output (depending on the polarity of the output set by the COM01:0 bits.)

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Phase Correct PWM

Mode

The phase correct PWM mode (WGM01:0 = 1) provides a high resolution phase correct PWM

waveform generation option. The phase correct PWM mode is based on a dual-slope operation.

The counter counts repeatedly from BOTTOM to MAX and then from MAX to BOTTOM. In non-

inverting Compare Output mode, the Output Compare (OC0) is cleared on the compare match

between TCNT0 and OCR0 while upcounting, and set on the compare match while downcount-

ing. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has

lower maximum operation frequency than single slope operation. However, due to the symmet-

ric feature of the dual-slope PWM modes, these modes are preferred for motor control

applications.

The PWM resolution for the phase correct PWM mode is fixed to eight bits. In phase correct

PWM mode the counter is incremented until the counter value matches MAX. When the counter

reaches MAX, it changes the count direction. The TCNT0 value will be equal to MAX for one

timer clock cycle. The timing diagram for the phase correct PWM mode is shown on Figure 33.

The TCNT0 value is in the timing diagram shown as a histogram for illustrating the dual-slope

operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal

line marks on the TCNT0 slopes represent compare matches between OCR0 and TCNT0.

Figure 33. Phase Correct PWM Mode, Timing Diagram

OCn Interrupt Flag Set

OCRn Update

TOVn Interrupt Flag Set

TCNTn

(COMn1:0 = 2)

OCn

(COMn1:0 = 3)

OCn

1

2

3

Period

The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches BOTTOM. The

Interrupt Flag can be used to generate an interrupt each time the counter reaches the BOTTOM

value.

In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the

OC0 pin. Setting the COM01:0 bits to 2 will produce a non-inverted PWM. An inverted PWM out-

put can be generated by setting the COM01:0 to 3 (see Table 41 on page 84). The actual OC0

value will only be visible on the port pin if the data direction for the port pin is set as output. The

PWM waveform is generated by clearing (or setting) the OC0 Register at the compare match

between OCR0 and TCNT0 when the counter increments, and setting (or clearing) the OC0

Register at compare match between OCR0 and TCNT0 when the counter decrements. The

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PWM frequency for the output when using phase correct PWM can be calculated by the follow-

ing equation:

f

clk_I/O

f

= -----------------

OCnPCPWM

N ⋅ 510

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

The extreme values for the OCR0 Register represent special cases when generating a PWM

waveform output in the phase correct PWM mode. If the OCR0 is set equal to BOTTOM, the out-

put will be continuously low and if set equal to MAX the output will be continuously high for non-

inverted PWM mode. For inverted PWM the output will have the opposite logic values.

At the very start of Period 2 in Figure 33 OCn has a transition from high to low even though there

is no Compare Match. The point of this transition is to guarantee symmetry around BOTTOM.

There are two cases that give a transition without Compare Match:

•

OCR0A changes its value from MAX, like in Figure 33. When the OCR0A value is MAX the

OCn pin value is the same as the result of a down-counting Compare Match. To ensure

symmetry around BOTTOM the OCn value at MAX must be correspond to the result of an

up-counting Compare Match.

•

The Timer starts counting from a value higher than the one in OCR0A, and for that reason

misses the Compare Match and hence the OCn change that would have happened on the

way up.

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Timer/Counter

Timing Diagrams

The Timer/Counter is a synchronous design and the timer clock (clk_T0) is therefore shown as a

clock enable signal in the following figures. The figures include information on when Interrupt

Flags are set. Figure 34 contains timing data for basic Timer/Counter operation. The figure

shows the count sequence close to the MAX value in all modes other than phase correct PWM

mode.

Figure 34. Timer/Counter Timing Diagram, no Prescaling

clk_I/O

clk_Tn

(clk_I/O/1)

TCNTn

TOVn

MAX - 1

MAX

BOTTOM

BOTTOM + 1

Figure 35 shows the same timing data, but with the prescaler enabled.

Figure 35. Timer/Counter Timing Diagram, with Prescaler (f_{clk_I/O}/8)

clk_I/O

clk_Tn

(clk_I/O/8)

TCNTn

TOVn

MAX - 1

MAX

BOTTOM

BOTTOM + 1

Figure 36 shows the setting of OCF0 in all modes except CTC mode.

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Figure 36. Timer/Counter Timing Diagram, Setting of OCF0, with Prescaler (f_{clk_I/O}/8)

clk_I/O

clk_Tn

(clk_I/O/8)

TCNTn

OCRn

OCFn

OCRn - 1

OCRn

OCRn + 1

OCRn + 2

OCRn Value

Figure 37 shows the setting of OCF0 and the clearing of TCNT0 in CTC mode.

Figure 37. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with Pres-

caler (f_{clk_I/O}/8)

clk_I/O

clk_Tn

(clk_I/O/8)

TCNTn

(CTC)

TOP - 1

TOP

BOTTOM

BOTTOM + 1

OCRn

TOP

OCFn

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8-bit

Timer/Counter

Register

Description

Timer/Counter Control

Register – TCCR0

Bit

7

FOC0

W

6

WGM00

R/W

0

5

COM01

R/W

0

4

COM00

R/W

0

3

WGM01

R/W

0

2

CS02

R/W

0

1

CS01

R/W

0

CS00

R/W

0

TCCR0

Read/Write

Initial Value

0

• Bit 7 – FOC0: Force Output Compare

The FOC0 bit is only active when the WGM00 bit specifies a non-PWM mode. However, for

ensuring compatibility with future devices, this bit must be set to zero when TCCR0 is written

when operating in PWM mode. When writing a logical one to the FOC0 bit, an immediate com-

pare match is forced on the Waveform Generation unit. The OC0 output is changed according to

its COM01:0 bits setting. Note that the FOC0 bit is implemented as a strobe. Therefore it is the

value present in the COM01:0 bits that determines the effect of the forced compare.

A FOC0 strobe will not generate any interrupt, nor will it clear the timer in CTC mode using

OCR0 as TOP.

The FOC0 bit is always read as zero.

• Bit 3, 6 – WGM01:0: Waveform Generation Mode

These bits control the counting sequence of the counter, the source for the maximum (TOP)

counter value, and what type of Waveform Generation to be used. Modes of operation sup-

ported by the Timer/Counter unit are: Normal mode, Clear Timer on Compare Match (CTC)

mode, and two types of Pulse Width Modulation (PWM) modes. See Table 38 and “Modes of

Operation” on page 76.

Table 38. Waveform Generation Mode Bit Description⁽¹⁾

WGM01

(CTC0)

WGM00 Timer/Counter Mode

(PWM0) of Operation

Update of

OCR0

TOV0 Flag

Set-on

Mode

TOP

0xFF

0

1

2

3

0

1

0

1

0

1

Normal

Immediate

TOP

MAX

PWM, Phase Correct

CTC

BOTTOM

MAX

OCR0 Immediate

0xFF BOTTOM

Fast PWM

MAX

Note:

1. The CTC0 and PWM0 bit definition names are now obsolete. Use the WGM01:0 definitions.

However, the functionality and location of these bits are compatible with previous versions of

the timer.

• Bit 5:4 – COM01:0: Compare Match Output Mode

These bits control the Output Compare pin (OC0) behavior. If one or both of the COM01:0 bits

are set, the OC0 output overrides the normal port functionality of the I/O pin it is connected to.

However, note that the Data Direction Register (DDR) bit corresponding to the OC0 pin must be

set in order to enable the output driver.

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When OC0 is connected to the pin, the function of the COM01:0 bits depends on the WGM01:0

bit setting. Table 39 shows the COM01:0 bit functionality when the WGM01:0 bits are set to a

normal or CTC mode (non-PWM).

Table 39. Compare Output Mode, non-PWM Mode

COM01

COM00

Description

0

1

0

1

0

1

Normal port operation, OC0 disconnected.

Toggle OC0 on compare match

Clear OC0 on compare match

Set OC0 on compare match

Table 40 shows the COM01:0 bit functionality when the WGM01:0 bits are set to fast PWM

mode.

Table 40. Compare Output Mode, Fast PWM Mode⁽¹⁾

COM01

COM00

Description

0

1

0

1

0

Normal port operation, OC0 disconnected.

Reserved

Clear OC0 on compare match, set OC0 at BOTTOM,

(non-inverting mode)

1

Set OC0 on compare match, clear OC0 at BOTTOM,

(inverting mode)

Note:

1. A special case occurs when OCR0 equals TOP and COM01 is set. In this case, the compare

match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page 77 for

more details.

Table 41 shows the COM01:0 bit functionality when the WGM01:0 bits are set to phase correct

PWM mode.

Table 41. Compare Output Mode, Phase Correct PWM Mode⁽¹⁾

COM01 COM00 Description

0

1

0

1

0

Normal port operation, OC0 disconnected.

Reserved

Clear OC0 on compare match when up-counting. Set OC0 on compare

match when downcounting.

1

Set OC0 on compare match when up-counting. Clear OC0 on compare

match when downcounting.

Note:

1. A special case occurs when OCR0 equals TOP and COM01 is set. In this case, the compare

match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page

79 for more details.

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• Bit 2:0 – CS02:0: Clock Select

The three Clock Select bits select the clock source to be used by the Timer/Counter.

Table 42. Clock Select Bit Description

CS02

CS01

CS00

Description

0

1

0

1

0

1

0

1

0

1

0

1

0

1

No clock source (Timer/Counter stopped).

clk_I/O/(No prescaling)

clk_I/O/8 (From prescaler)

clk_I/O/64 (From prescaler)

clk_I/O/256 (From prescaler)

clk_I/O/1024 (From prescaler)

External clock source on T0 pin. Clock on falling edge.

External clock source on T0 pin. Clock on rising edge.

If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the

counter even if the pin is configured as an output. This feature allows software control of the

counting.

Timer/Counter

Register – TCNT0

Bit

7

6

5

4

3

2

1

0

TCNT0[7:0]

TCNT0

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

The Timer/Counter Register gives direct access, both for read and write operations, to the

Timer/Counter unit 8-bit counter. Writing to the TCNT0 Register blocks (removes) the compare

match on the following timer clock. Modifying the counter (TCNT0) while the counter is running,

introduces a risk of missing a compare match between TCNT0 and the OCR0 Register.

Output Compare

Register – OCR0

Bit

7

6

5

4

3

2

1

0

OCR0[7:0]

OCR0

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

The Output Compare Register contains an 8-bit value that is continuously compared with the

counter value (TCNT0). A match can be used to generate an output compare interrupt, or to

generate a waveform output on the OC0 pin.

Timer/Counter

Interrupt Mask

Register – TIMSK

Bit

7

OCIE2

R/W

0

6

TOIE2

R/W

0

5

TICIE1

R/W

0

4

OCIE1A

R/W

0

3

OCIE1B

R/W

0

2

TOIE1

R/W

0

1

OCIE0

R/W

0

TOIE0

R/W

0

TIMSK

Read/Write

Initial Value

• Bit 1 – OCIE0: Timer/Counter0 Output Compare Match Interrupt Enable

When the OCIE0 bit is written to one, and the I-bit in the Status Register is set (one), the

Timer/Counter0 Compare Match interrupt is enabled. The corresponding interrupt is executed if

a compare match in Timer/Counter0 occurs, that is, when the OCF0 bit is set in the Timer/Coun-

ter Interrupt Flag Register – TIFR.

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• Bit 0 – TOIE0: Timer/Counter0 Overflow Interrupt Enable

When the TOIE0 bit is written to one, and the I-bit in the Status Register is set (one), the

Timer/Counter0 Overflow interrupt is enabled. The corresponding interrupt is executed if an

overflow in Timer/Counter0 occurs, that is, when the TOV0 bit is set in the Timer/Counter Inter-

rupt Flag Register – TIFR.

Timer/Counter

Interrupt Flag Register

– TIFR

Bit

7

OCF2

R/W

0

6

TOV2

R/W

0

5

4

OCF1A

R/W

0

3

OCF1B

R/W

0

2

TOV1

R/W

0

1

OCF0

R/W

0

TOV0

R/W

0

ICF1

R/W

0

TIFR

Read/Write

Initial Value

• Bit 1 – OCF0: Output Compare Flag 0

The OCF0 bit is set (one) when a compare match occurs between the Timer/Counter0 and the

data in OCR0 – Output Compare Register0. OCF0 is cleared by hardware when executing the

corresponding interrupt handling vector. Alternatively, OCF0 is cleared by writing a logic one to

the flag. When the I-bit in SREG, OCIE0 (Timer/Counter0 Compare Match Interrupt Enable), and

OCF0 are set (one), the Timer/Counter0 Compare Match Interrupt is executed.

• Bit 0 – TOV0: Timer/Counter0 Overflow Flag

The bit TOV0 is set (one) when an overflow occurs in Timer/Counter0. TOV0 is cleared by hard-

ware when executing the corresponding interrupt handling vector. Alternatively, TOV0 is cleared

by writing a logic one to the flag. When the SREG I-bit, TOIE0 (Timer/Counter0 Overflow Inter-

rupt Enable), and TOV0 are set (one), the Timer/Counter0 Overflow interrupt is executed. In

phase correct PWM mode, this bit is set when Timer/Counter0 changes counting direction at

$00.

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Timer/Counter0 Timer/Counter1 and Timer/Counter0 share the same prescaler module, but the Timer/Counters

can have different prescaler settings. The description below applies to both Timer/Counter1 and

Timer/Counter0.

and

Timer/Counter1

Prescalers

Internal Clock Source The Timer/Counter can be clocked directly by the system clock (by setting the CSn2:0 = 1). This

provides the fastest operation, with a maximum Timer/Counter clock frequency equal to system

clock frequency (f_{CLK_I/O}). Alternatively, one of four taps from the prescaler can be used as a

clock source. The prescaled clock has a frequency of either f_{CLK_I/O}/8, f_{CLK_I/O}/64, f_{CLK_I/O}/256, or

f_{CLK_I/O}/1024.

Prescaler Reset

The prescaler is free running, that is, operates independently of the clock select logic of the

Timer/Counter, and it is shared by Timer/Counter1 and Timer/Counter0. Since the prescaler is

not affected by the Timer/Counter’s clock select, the state of the prescaler will have implications

for situations where a prescaled clock is used. One example of prescaling artifacts occurs when

the timer is enabled and clocked by the prescaler (6 > CSn2:0 > 1). The number of system clock

cycles from when the timer is enabled to the first count occurs can be from 1 to N+1 system

clock cycles, where N equals the prescaler divisor (8, 64, 256, or 1024).

It is possible to use the Prescaler Reset for synchronizing the Timer/Counter to program execu-

tion. However, care must be taken if the other Timer/Counter that shares the same prescaler

also uses prescaling. A prescaler reset will affect the prescaler period for all Timer/Counters it is

connected to.

External Clock Source An external clock source applied to the T1/T0 pin can be used as Timer/Counter clock

(clk_T1/clk_T0). The T1/T0 pin is sampled once every system clock cycle by the pin synchronization

logic. The synchronized (sampled) signal is then passed through the edge detector. Figure 38

shows a functional equivalent block diagram of the T1/T0 synchronization and edge detector

logic. The registers are clocked at the positive edge of the internal system clock (clk_I/O). The latch

is transparent in the high period of the internal system clock.

The edge detector generates one clk_T1/clk_Tpulse for each positive (CSn2:0 = 7) or negative

0

(CSn2:0 = 6) edge it detects.

Figure 38. T1/T0 Pin Sampling

Tn_sync

(To Clock

Tn

D

Q

D

Q

D

Q

Select Logic)

LE

clk_I/O

Synchronization

Edge Detector

The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles

from an edge has been applied to the T1/T0 pin to the counter is updated.

Enabling and disabling of the clock input must be done when T1/T0 has been stable for at least

one system clock cycle, otherwise it is a risk that a false Timer/Counter clock pulse is generated.

Each half period of the external clock applied must be longer than one system clock cycle to

ensure correct sampling. The external clock must be guaranteed to have less than half the sys-

tem clock frequency (f_ExtClk< f_{clk_I/O}/2) given a 50/50% duty cycle. Since the edge detector uses

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sampling, the maximum frequency of an external clock it can detect is half the sampling fre-

quency (Nyquist sampling theorem). However, due to variation of the system clock frequency

and duty cycle caused by Oscillator source (crystal, resonator, and capacitors) tolerances, it is

recommended that maximum frequency of an external clock source is less than f_{clk_I/O}/2.5.

An external clock source can not be prescaled.

Figure 39. Prescaler for Timer/Counter0 and Timer/Counter1⁽¹⁾

clk_I/O

Clear

PSR10

T0

Synchronization

T1

Synchronization

clk_T1

clk_T0

Note:

1. The synchronization logic on the input pins (T1/T0) is shown in Figure 38.

Special Function IO

Register – SFIOR

Bit

7

ADTS2

R/W

0

6

ADTS1

R/W

0

5

ADTS0

R/W

0

4

–

3

ACME

R/W

0

2

1

PSR2

R/W

0

PSR10

R/W

0

PUD

R/W

0

SFIOR

Read/Write

Initial Value

R

0

• Bit 0 – PSR10: Prescaler Reset Timer/Counter1 and Timer/Counter0

When this bit is written to one, the Timer/Counter1 and Timer/Counter0 prescaler will be reset.

The bit will be cleared by hardware after the operation is performed. Writing a zero to this bit will

have no effect. Note that Timer/Counter1 and Timer/Counter0 share the same prescaler and a

reset of this prescaler will affect both timers. This bit will always be read as zero.

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16-bit

Timer/Counter1

The 16-bit Timer/Counter unit allows accurate program execution timing (event management),

wave generation, and signal timing measurement. The main features are:

• True 16-bit Design (that is, allows 16-bit PWM)

• Two Independent Output Compare Units

• Double Buffered Output Compare Registers

• One Input Capture Unit

• Input Capture Noise Canceler

• Clear Timer on Compare Match (Auto Reload)

• Glitch-free, Phase Correct Pulse Width Modulator (PWM)

• Variable PWM Period

• Frequency Generator

• External Event Counter

• Four Independent Interrupt Sources (TOV1, OCF1A, OCF1B, and ICF1)

Overview

Most register and bit references in this section are written in general form. A lower case “n”

replaces the Timer/Counter number, and a lower case “x” replaces the output compare unit.

However, when using the register or bit defines in a program, the precise form must be used

(that is, TCNT1 for accessing Timer/Counter1 counter value and so on).

A simplified block diagram of the 16-bit Timer/Counter is shown in Figure 40. For the actual

placement of I/O pins, refer to Figure 1 on page 2. CPU accessible I/O Registers, including I/O

bits and I/O pins, are shown in bold. The device specific I/O Register and bit locations are listed

in the “16-bit Timer/Counter Register Description” on page 110.

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Figure 40. 16-bit Timer/Counter Block Diagram⁽¹⁾

Count

TOVn

(Int.Req.)

Clear

Control Logic

Direction

Clock Select

clk_Tn

Edge

Detector

Tn

TOP

BOTTOM

( From Prescaler )

Timer/Counter

TCNTn

=

0

OCnA

(Int.Req.)

Waveform

Generation

OCnA

OCnB

=

OCRnA

OCnB

(Int.Req.)

Fixed

TOP

Values

Waveform

Generation

=

OCRnB

( From Analog

Comparator Ouput )

ICFn (Int.Req.)

Edge

Detector

Noise

Canceler

ICRn

ICPn

TCCRnA

TCCRnB

Note:

1. Refer to Figure 1 on page 2, Table 25 on page 58, and Table 31 on page 63 for

Timer/Counter1 pin placement and description.

Registers

The Timer/Counter (TCNT1), Output Compare Registers (OCR1A/B), and Input Capture Regis-

ter (ICR1) are all 16-bit registers. Special procedures must be followed when accessing the 16-

bit registers. These procedures are described in the section “Accessing 16-bit Registers” on

page 92. The Timer/Counter Control Registers (TCCR1A/B) are 8-bit registers and have no CPU

access restrictions. Interrupt requests (abbreviated to Int.Req. in the figure) signals are all visible

in the Timer Interrupt Flag Register (TIFR). All interrupts are individually masked with the Timer

Interrupt Mask Register (TIMSK). TIFR and TIMSK are not shown in the figure since these regis-

ters are shared by other timer units.

The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on

the T1 pin. The Clock Select logic block controls which clock source and edge the Timer/Counter

uses to increment (or decrement) its value. The Timer/Counter is inactive when no clock source

is selected. The output from the clock select logic is referred to as the timer clock (clk ).

1

T

The double buffered Output Compare Registers (OCR1A/B) are compared with the Timer/Coun-

ter value at all time. The result of the compare can be used by the Waveform Generator to

generate a PWM or variable frequency output on the Output Compare pin (OC1A/B). See “Out-

put Compare Units” on page 98. The compare match event will also set the Compare Match

Flag (OCF1A/B) which can be used to generate an output compare interrupt request.

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The Input Capture Register can capture the Timer/Counter value at a given external (edge trig-

gered) event on either the Input Capture Pin (ICP1) or on the Analog Comparator pins (See

“Analog Comparator” on page 201.) The Input Capture unit includes a digital filtering unit (Noise

Canceler) for reducing the chance of capturing noise spikes.

The TOP value, or maximum Timer/Counter value, can in some modes of operation be defined

by either the OCR1A Register, the ICR1 Register, or by a set of fixed values. When using

OCR1A as TOP value in a PWM mode, the OCR1A Register can not be used for generating a

PWM output. However, the TOP value will in this case be double buffered allowing the TOP

value to be changed in run time. If a fixed TOP value is required, the ICR1 Register can be used

as an alternative, freeing the OCR1A to be used as PWM output.

Definitions

The following definitions are used extensively throughout the document:

Table 43. Definitions

BOTTOM

MAX

The counter reaches the BOTTOM when it becomes 0x0000.

The counter reaches its MAXimum when it becomes 0xFFFF (decimal 65535).

The counter reaches the TOP when it becomes equal to the highest value in the

count sequence. The TOP value can be assigned to be one of the fixed values:

0x00FF, 0x01FF, or 0x03FF, or to the value stored in the OCR1A or ICR1 Regis-

ter. The assignment is dependent of the mode of operation.

TOP

Compatibility

The 16-bit Timer/Counter has been updated and improved from previous versions of the 16-bit

AVR Timer/Counter. This 16-bit Timer/Counter is fully compatible with the earlier version

regarding:

•

All 16-bit Timer/Counter related I/O Register address locations, including Timer Interrupt

Registers.

•

Bit locations inside all 16-bit Timer/Counter Registers, including Timer Interrupt Registers.

Interrupt Vectors.

The following control bits have changed name, but have same functionality and register location:

•

PWM10 is changed to WGM10.

PWM11 is changed to WGM11.

CTC1 is changed to WGM12.

The following bits are added to the 16-bit Timer/Counter Control Registers:

•

FOC1A and FOC1B are added to TCCR1A.

WGM13 is added to TCCR1B.

The 16-bit Timer/Counter has improvements that will affect the compatibility in some special

cases.

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Accessing 16-bit

Registers

The TCNT1, OCR1A/B, and ICR1 are 16-bit registers that can be accessed by the AVR CPU via

the 8-bit data bus. The 16-bit register must be byte accessed using two read or write operations.

Each 16-bit timer has a single 8-bit register for temporary storing of the High byte of the 16-bit

access. The same temporary register is shared between all 16-bit registers within each 16-bit

timer. Accessing the Low byte triggers the 16-bit read or write operation. When the Low byte of a

16-bit register is written by the CPU, the High byte stored in the temporary register, and the Low

byte written are both copied into the 16-bit register in the same clock cycle. When the Low byte

of a 16-bit register is read by the CPU, the High byte of the 16-bit register is copied into the tem-

porary register in the same clock cycle as the Low byte is read.

Not all 16-bit accesses uses the temporary register for the High byte. Reading the OCR1A/B 16-

bit registers does not involve using the temporary register.

To do a 16-bit write, the High byte must be written before the Low byte. For a 16-bit read, the

Low byte must be read before the High byte.

The following code examples show how to access the 16-bit Timer Registers assuming that no

interrupts updates the temporary register. The same principle can be used directly for accessing

the OCR1A/B and ICR1 Registers. Note that when using “C”, the compiler handles the 16-bit

access.

Assembly Code Example⁽¹⁾

...

; Set TCNT1 to 0x01FF

ldi r17,0x01

ldi r16,0xFF

out TCNT1H,r17

out TCNT1L,r16

; Read TCNT1 into r17:r16

in r16,TCNT1L

in r17,TCNT1H

...

C Code Example⁽¹⁾

unsigned int i;

...

/* Set TCNT1 to 0x01FF */

TCNT1 = 0x1FF;

/* Read TCNT1 into i */

i = TCNT1;

...

Note:

1. See “About Code Examples” on page 7.

The assembly code example returns the TCNT1 value in the r17:r16 register pair.

It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt

occurs between the two instructions accessing the 16-bit register, and the interrupt code

updates the temporary register by accessing the same or any other of the 16-bit Timer Regis-

ters, then the result of the access outside the interrupt will be corrupted. Therefore, when both

the main code and the interrupt code update the temporary register, the main code must disable

the interrupts during the 16-bit access.

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The following code examples show how to do an atomic read of the TCNT1 Register contents.

Reading any of the OCR1A/B or ICR1 Registers can be done by using the same principle.

Assembly Code Example⁽¹⁾

TIM16_ReadTCNT1:

; Save global interrupt flag

in r18,SREG

; Disable interrupts

cli

; Read TCNT1 into r17:r16

in r16,TCNT1L

in r17,TCNT1H

; Restore global interrupt flag

out SREG,r18

ret

C Code Example⁽¹⁾

unsigned int TIM16_ReadTCNT1( void )

{

unsigned char sreg;

unsigned int i;

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

_CLI();

/* Read TCNT1 into i */

i = TCNT1;

/* Restore global interrupt flag */

SREG = sreg;

return i;

}

Note:

1. See “About Code Examples” on page 7.

The assembly code example returns the TCNT1 value in the r17:r16 register pair.

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The following code examples show how to do an atomic write of the TCNT1 Register contents.

Writing any of the OCR1A/B or ICR1 Registers can be done by using the same principle.

Assembly Code Example⁽¹⁾

TIM16_WriteTCNT1:

; Save global interrupt flag

in r18,SREG

; Disable interrupts

cli

; Set TCNT1 to r17:r16

out TCNT1H,r17

out TCNT1L,r16

; Restore global interrupt flag

out SREG,r18

ret

C Code Example⁽¹⁾

void TIM16_WriteTCNT1 ( unsigned int i )

{

unsigned char sreg;

unsigned int i;

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

_CLI();

/* Set TCNT1 to i */

TCNT1 = i;

/* Restore global interrupt flag */

SREG = sreg;

}

Note:

1. See “About Code Examples” on page 7.

The assembly code example requires that the r17:r16 register pair contains the value to be writ-

ten to TCNT1.

Reusing the

Temporary High Byte

Register

If writing to more than one 16-bit register where the High byte is the same for all registers writ-

ten, then the High byte only needs to be written once. However, note that the same rule of

atomic operation described previously also applies in this case.

Timer/Counter

Clock Sources

The Timer/Counter can be clocked by an internal or an external clock source. The clock source

is selected by the Clock Select logic which is controlled by the Clock Select (CS12:0) bits

located in the Timer/Counter Control Register B (TCCR1B). For details on clock sources and

prescaler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on page 87.

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Counter Unit

The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit.

Figure 41 shows a block diagram of the counter and its surroundings.

Figure 41. Counter Unit Block Diagram

DATA BUS (8-bit)

TOVn

(Int.Req.)

TEMP (8-bit)

Clock Select

Count

Clear

Edge

Detector

Tn

TCNTnH (8-bit)

TCNTnL (8-bit)

clk_Tn

Control Logic

Direction

TCNTn (16-bit Counter)

( From Prescaler )

TOP

BOTTOM

Signal description (internal signals):

Count Increment or decrement TCNT1 by 1.

Direction Select between increment and decrement.

Clear

Clear TCNT1 (set all bits to zero).

Timer/Counter clock.

clk_T

1

TOP

Signalize that TCNT1 has reached maximum value.

BOTTOM Signalize that TCNT1 has reached minimum value (zero).

The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High (TCNT1H) con-

taining the upper eight bits of the counter, and Counter Low (TCNT1L) containing the lower 8

bits. The TCNT1H Register can only be indirectly accessed by the CPU. When the CPU does an

access to the TCNT1H I/O location, the CPU accesses the High byte temporary register

(TEMP). The temporary register is updated with the TCNT1H value when the TCNT1L is read,

and TCNT1H is updated with the temporary register value when TCNT1L is written. This allows

the CPU to read or write the entire 16-bit counter value within one clock cycle via the 8-bit data

bus. It is important to notice that there are special cases of writing to the TCNT1 Register when

the counter is counting that will give unpredictable results. The special cases are described in

the sections where they are of importance.

Depending on the mode of operation used, the counter is cleared, incremented, or decremented

at each timer clock (clk ). The clk ₁can be generated from an external or internal clock source,

1

T

selected by the Clock Select bits (CS12:0). When no clock source is selected (CS12:0 = 0) the

timer is stopped. However, the TCNT1 value can be accessed by the CPU, independent of

whether clk_Tis present or not. A CPU write overrides (has priority over) all counter clear or

1

count operations.

The counting sequence is determined by the setting of the Waveform Generation Mode bits

(WGM13:0) located in the Timer/Counter Control Registers A and B (TCCR1A and TCCR1B).

There are close connections between how the counter behaves (counts) and how waveforms

are generated on the Output Compare outputs OC1x. For more details about advanced counting

sequences and waveform generation, see “Modes of Operation” on page 101.

The Timer/Counter Overflow (TOV1) Flag is set according to the mode of operation selected by

the WGM13:0 bits. TOV1 can be used for generating a CPU interrupt.

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Input Capture Unit The Timer/Counter incorporates an Input Capture unit that can capture external events and give

them a time-stamp indicating time of occurrence. The external signal indicating an event, or mul-

tiple events, can be applied via the ICP1 pin or alternatively, via the Analog Comparator unit.

The time-stamps can then be used to calculate frequency, duty-cycle, and other features of the

signal applied. Alternatively the time-stamps can be used for creating a log of the events.

The Input Capture unit is illustrated by the block diagram shown in Figure 42. The elements of

the block diagram that are not directly a part of the Input Capture unit are gray shaded. The

small “n” in register and bit names indicates the Timer/Counter number.

Figure 42. Input Capture Unit Block Diagram

DATA BUS (8-bit)

TEMP (8-bit)

ICRnH (8-bit)

ICRnL (8-bit)

TCNTnH (8-bit)

TCNTnL (8-bit)

ICRn (16-bit Register)

TCNTn (16-bit Counter)

WRITE

ACO*

ACIC*

ICNC

ICES

Analog

Comparator

Noise

Canceler

Edge

Detector

ICFn (Int.Req.)

ICPn

When a change of the logic level (an event) occurs on the Input Capture pin (ICP1), alternatively

on the Analog Comparator output (ACO), and this change confirms to the setting of the edge

detector, a capture will be triggered. When a capture is triggered, the 16-bit value of the counter

(TCNT1) is written to the Input Capture Register (ICR1). The Input Capture Flag (ICF1) is set at

the same system clock as the TCNT1 value is copied into ICR1 Register. If enabled (TICIE1 =

1), the Input Capture Flag generates an Input Capture Interrupt. The ICF1 Flag is automatically

cleared when the interrupt is executed. Alternatively the ICF1 Flag can be cleared by software

by writing a logical one to its I/O bit location.

Reading the 16-bit value in the Input Capture Register (ICR1) is done by first reading the Low

byte (ICR1L) and then the High byte (ICR1H). When the Low byte is read the High byte is copied

into the High byte temporary register (TEMP). When the CPU reads the ICR1H I/O location it will

access the TEMP Register.

The ICR1 Register can only be written when using a Waveform Generation mode that utilizes

the ICR1 Register for defining the counter’s TOP value. In these cases the Waveform Genera-

tion mode (WGM13:0) bits must be set before the TOP value can be written to the ICR1

Register. When writing the ICR1 Register the High byte must be written to the ICR1H I/O loca-

tion before the Low byte is written to ICR1L.

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For more information on how to access the 16-bit registers refer to “Accessing 16-bit Registers”

on page 92.

Input Capture Pin

Source

The main trigger source for the Input Capture unit is the Input Capture pin (ICP1).

Timer/Counter1 can alternatively use the Analog Comparator output as trigger source for the

Input Capture unit. The Analog Comparator is selected as trigger source by setting the Analog

Comparator Input Capture (ACIC) bit in the Analog Comparator Control and Status Register

(ACSR). Be aware that changing trigger source can trigger a capture. The Input Capture Flag

must therefore be cleared after the change.

Both the Input Capture pin (ICP1) and the Analog Comparator output (ACO) inputs are sampled

using the same technique as for the T1 pin (Figure 38 on page 87). The edge detector is also

identical. However, when the noise canceler is enabled, additional logic is inserted before the

edge detector, which increases the delay by four system clock cycles. Note that the input of the

noise canceler and edge detector is always enabled unless the Timer/Counter is set in a wave-

form generation mode that uses ICR1 to define TOP.

An Input Capture can be triggered by software by controlling the port of the ICP1 pin.

Noise Canceler

The noise canceler improves noise immunity by using a simple digital filtering scheme. The

noise canceler input is monitored over four samples, and all four must be equal for changing the

output that in turn is used by the edge detector.

The noise canceler is enabled by setting the Input Capture Noise Canceler (ICNC1) bit in

Timer/Counter Control Register B (TCCR1B). When enabled the noise canceler introduces addi-

tional four system clock cycles of delay from a change applied to the input, to the update of the

ICR1 Register. The noise canceler uses the system clock and is therefore not affected by the

prescaler.

Using the Input

Capture Unit

The main challenge when using the Input Capture unit is to assign enough processor capacity

for handling the incoming events. The time between two events is critical. If the processor has

not read the captured value in the ICR1 Register before the next event occurs, the ICR1 will be

overwritten with a new value. In this case the result of the capture will be incorrect.

When using the Input Capture Interrupt, the ICR1 Register should be read as early in the inter-

rupt handler routine as possible. Even though the Input Capture Interrupt has relatively high

priority, the maximum interrupt response time is dependent on the maximum number of clock

cycles it takes to handle any of the other interrupt requests.

Using the Input Capture unit in any mode of operation when the TOP value (resolution) is

actively changed during operation, is not recommended.

Measurement of an external signal’s duty cycle requires that the trigger edge is changed after

each capture. Changing the edge sensing must be done as early as possible after the ICR1

Register has been read. After a change of the edge, the Input Capture Flag (ICF1) must be

cleared by software (writing a logical one to the I/O bit location). For measuring frequency only,

the clearing of the ICF1 Flag is not required (if an interrupt handler is used).

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Output Compare

Units

The 16-bit comparator continuously compares TCNT1 with the Output Compare Register

(OCR1x). If TCNT equals OCR1x the comparator signals a match. A match will set the Output

Compare Flag (OCF1x) at the next timer clock cycle. If enabled (OCIE1x = 1), the Output Com-

pare Flag generates an output compare interrupt. The OCF1x Flag is automatically cleared

when the interrupt is executed. Alternatively the OCF1x Flag can be cleared by software by writ-

ing a logical one to its I/O bit location. The Waveform Generator uses the match signal to

generate an output according to operating mode set by the Waveform Generation mode

(WGM13:0) bits and Compare Output mode (COM1x1:0) bits. The TOP and BOTTOM signals

are used by the Waveform Generator for handling the special cases of the extreme values in

some modes of operation (See “Modes of Operation” on page 101.)

A special feature of output compare unit A allows it to define the Timer/Counter TOP value (that

is, counter resolution). In addition to the counter resolution, the TOP value defines the period

time for waveforms generated by the Waveform Generator.

Figure 43 shows a block diagram of the output compare unit. The small “n” in the register and bit

names indicates the device number (n = 1 for Timer/Counter1), and the “x” indicates output com-

pare unit (A/B). The elements of the block diagram that are not directly a part of the output

compare unit are gray shaded.

Figure 43. Output Compare Unit, Block Diagram

DATA BUS (8-bit)

TEMP (8-bit)

OCRnxH Buf. (8-bit)

OCRnxL Buf. (8-bit)

TCNTnH (8-bit)

TCNTnL (8-bit)

OCRnx Buffer (16-bit Register)

TCNTn (16-bit Counter)

OCRnxH (8-bit)

OCRnxL (8-bit)

OCRnx (16-bit Register)

=

(16-bit Comparator )

OCFnx (Int.Req.)

TOP

OCnx

Waveform Generator

BOTTOM

WGMn3:0

COMnx1:0

The OCR1x Register is double buffered when using any of the twelve Pulse Width Modulation

(PWM) modes. For the normal and Clear Timer on Compare (CTC) modes of operation, the dou-

ble buffering is disabled. The double buffering synchronizes the update of the OCR1x Compare

Register to either TOP or BOTTOM of the counting sequence. The synchronization prevents the

occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free.

The OCR1x Register access may seem complex, but this is not case. When the double buffering

is enabled, the CPU has access to the OCR1x Buffer Register, and if double buffering is dis-

abled the CPU will access the OCR1x directly. The content of the OCR1x (Buffer or Compare)

Register is only changed by a write operation (the Timer/Counter does not update this register

automatically as the TCNT1 and ICR1 Register). Therefore OCR1x is not read via the High byte

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temporary register (TEMP). However, it is a good practice to read the Low byte first as when

accessing other 16-bit registers. Writing the OCR1x Registers must be done via the TEMP Reg-

ister since the compare of all 16 bits is done continuously. The High byte (OCR1xH) has to be

written first. When the High byte I/O location is written by the CPU, the TEMP Register will be

updated by the value written. Then when the Low byte (OCR1xL) is written to the lower eight

bits, the High byte will be copied into the upper 8-bits of either the OCR1x buffer or OCR1x Com-

pare Register in the same system clock cycle.

For more information of how to access the 16-bit registers refer to “Accessing 16-bit Registers”

on page 92.

Force Output

Compare

In non-PWM Waveform Generation modes, the match output of the comparator can be forced by

writing a one to the Force Output Compare (FOC1x) bit. Forcing compare match will not set the

OCF1x Flag or reload/clear the timer, but the OC1x pin will be updated as if a real compare

match had occurred (the COM1x1:0 bits settings define whether the OC1x pin is set, cleared or

toggled).

Compare Match

Blocking by TCNT1

Write

All CPU writes to the TCNT1 Register will block any compare match that occurs in the next timer

clock cycle, even when the timer is stopped. This feature allows OCR1x to be initialized to the

same value as TCNT1 without triggering an interrupt when the Timer/Counter clock is enabled.

Using the Output

Compare Unit

Since writing TCNT1 in any mode of operation will block all compare matches for one timer clock

cycle, there are risks involved when changing TCNT1 when using any of the output compare

units, independent of whether the Timer/Counter is running or not. If the value written to TCNT1

equals the OCR1x value, the compare match will be missed, resulting in incorrect waveform

generation. Do not write the TCNT1 equal to TOP in PWM modes with variable TOP values. The

compare match for the TOP will be ignored and the counter will continue to 0xFFFF. Similarly,

do not write the TCNT1 value equal to BOTTOM when the counter is downcounting.

The setup of the OC1x should be performed before setting the Data Direction Register for the

port pin to output. The easiest way of setting the OC1x value is to use the force output compare

(FOC1x) strobe bits in Normal mode. The OC1x Register keeps its value even when changing

between waveform generation modes.

Be aware that the COM1x1:0 bits are not double buffered together with the compare value.

Changing the COM1x1:0 bits will take effect immediately.

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Compare Match

Output Unit

The Compare Output mode (COM1x1:0) bits have two functions. The Waveform Generator uses

the COM1x1:0 bits for defining the Output Compare (OC1x) state at the next compare match.

Secondly the COM1x1:0 bits control the OC1x pin output source. Figure 44 shows a simplified

schematic of the logic affected by the COM1x1:0 bit setting. The I/O Registers, I/O bits, and I/O

pins in the figure are shown in bold. Only the parts of the general I/O Port Control Registers

(DDR and PORT) that are affected by the COM1x1:0 bits are shown. When referring to the

OC1x state, the reference is for the internal OC1x Register, not the OC1x pin. If a System Reset

occur, the OC1x Register is reset to “0”.

Figure 44. Compare Match Output Unit, Schematic

COMnx1

Waveform

Generator

COMnx0

FOCnx

D

Q

1

0

OCnx

D

Q

PORT

D

Q

DDR

clk_I/O

The general I/O port function is overridden by the Output Compare (OC1x) from the Waveform

Generator if either of the COM1x1:0 bits are set. However, the OC1x pin direction (input or out-

put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction

Register bit for the OC1x pin (DDR_OC1x) must be set as output before the OC1x value is visi-

ble on the pin. The port override function is generally independent of the Waveform Generation

mode, but there are some exceptions. Refer to Table 44, Table 45 and Table 46 for details.

The design of the output compare pin logic allows initialization of the OC1x state before the out-

put is enabled. Note that some COM1x1:0 bit settings are reserved for certain modes of

operation. See “16-bit Timer/Counter Register Description” on page 110.

The COM1x1:0 bits have no effect on the Input Capture unit.

Compare Output Mode The Waveform Generator uses the COM1x1:0 bits differently in normal, CTC, and PWM modes.

and Waveform

Generation

For all modes, setting the COM1x1:0 = 0 tells the Waveform Generator that no action on the

OC1x Register is to be performed on the next compare match. For compare output actions in the

non-PWM modes refer to Table 44 on page 110. For fast PWM mode refer to Table 45 on page

111, and for phase correct and phase and frequency correct PWM refer to Table 46 on page

111.

A change of the COM1x1:0 bits state will have effect at the first compare match after the bits are

written. For non-PWM modes, the action can be forced to have immediate effect by using the

FOC1x strobe bits.

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Modes of

Operation

The mode of operation, that is, the behavior of the Timer/Counter and the output compare pins,

is defined by the combination of the Waveform Generation mode (WGM13:0) and Compare Out-

put mode (COM1x1:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COM1x1:0 bits control whether the PWM out-

put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes

the COM1x1:0 bits control whether the output should be set, cleared or toggle at a compare

match (See “Compare Match Output Unit” on page 100.)

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 108.

Normal Mode

The simplest mode of operation is the Normal mode (WGM13:0 = 0). In this mode the counting

direction is always up (incrementing), and no counter clear is performed. The counter simply

overruns when it passes its maximum 16-bit value (MAX = 0xFFFF) and then restarts from the

BOTTOM (0x0000). In normal operation the Timer/Counter Overflow Flag (TOV1) will be set in

the same timer clock cycle as the TCNT1 becomes zero. The TOV1 Flag in this case behaves

like a 17th bit, except that it is only set, not cleared. However, combined with the timer overflow

interrupt that automatically clears the TOV1 Flag, the timer resolution can be increased by soft-

ware. There are no special cases to consider in the Normal mode, a new counter value can be

written anytime.

The Input Capture unit is easy to use in Normal mode. However, observe that the maximum

interval between the external events must not exceed the resolution of the counter. If the interval

between events are too long, the timer overflow interrupt or the prescaler must be used to

extend the resolution for the capture unit.

The output compare units can be used to generate interrupts at some given time. Using the out-

put compare to generate waveforms in Normal mode is not recommended, since this will occupy

too much of the CPU time.

Clear Timer on

In Clear Timer on Compare or CTC mode (WGM13:0 = 4 or 12), the OCR1A or ICR1 Register

Compare Match (CTC) are used to manipulate the counter resolution. In CTC mode the counter is cleared to zero when

Mode

the counter value (TCNT1) matches either the OCR1A (WGM13:0 = 4) or the ICR1 (WGM13:0 =

12). The OCR1A or ICR1 define the top value for the counter, hence also its resolution. This

mode allows greater control of the compare match output frequency. It also simplifies the opera-

tion of counting external events.

The timing diagram for the CTC mode is shown in Figure 45. The counter value (TCNT1)

increases until a compare match occurs with either OCR1A or ICR1, and then counter (TCNT1)

is cleared.

Figure 45. CTC Mode, Timing Diagram

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

TCNTn

OCnA

(Toggle)

(COMnA1:0 = 1)

1

2

3

4

Period

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An interrupt can be generated at each time the counter value reaches the TOP value by either

using the OCF1A or ICF1 Flag according to the register used to define the TOP value. If the

interrupt is enabled, the interrupt handler routine can be used for updating the TOP value. How-

ever, changing the TOP to a value close to BOTTOM when the counter is running with none or a

low prescaler value must be done with care since the CTC mode does not have the double buff-

ering feature. If the new value written to OCR1A or ICR1 is lower than the current value of

TCNT1, the counter will miss the compare match. The counter will then have to count to its max-

imum value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur.

In many cases this feature is not desirable. An alternative will then be to use the fast PWM mode

using OCR1A for defining TOP (WGM13:0 = 15) since the OCR1A then will be double buffered.

For generating a waveform output in CTC mode, the OC1A output can be set to toggle its logical

level on each compare match by setting the compare output mode bits to toggle mode

(COM1A1:0 = 1). The OC1A value will not be visible on the port pin unless the data direction for

the pin is set to output (DDR_OC1A = 1). The waveform generated will have a maximum fre-

quency of f_{OC A}= f_{clk_I/O}/2 when OCR1A is set to zero (0x0000). The waveform frequency is

1

defined by the following equation:

f

clk_I/O

f

= --------------------------------------------------

OCnA

2 ⋅ N ⋅ (1 + OCRnA)

The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).

As for the Normal mode of operation, the TOV1 Flag is set in the same timer clock cycle that the

counter counts from MAX to 0x0000.

Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGM13:0 = 5,6,7,14, or 15) provides a

high frequency PWM waveform generation option. The fast PWM differs from the other PWM

options by its single-slope operation. The counter counts from BOTTOM to TOP then restarts

from BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is cleared

on the compare match between TCNT1 and OCR1x, and set at BOTTOM. In inverting Compare

Output mode output is set on compare match and cleared at BOTTOM. Due to the single-slope

operation, the operating frequency of the fast PWM mode can be twice as high as the phase cor-

rect and phase and frequency correct PWM modes that use dual-slope operation. This high

frequency makes the fast PWM mode well suited for power regulation, rectification, and DAC

applications. High frequency allows physically small sized external components (coils, capaci-

tors), hence reduces total system cost.

The PWM resolution for fast PWM can be fixed to 8-bit, 9-bit, or 10-bit, or defined by either ICR1

or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and the

maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolution in bits can be

calculated by using the following equation:

log(TOP + 1)

R

= ----------------------------------

FPWM

log(2)

In fast PWM mode the counter is incremented until the counter value matches either one of the

fixed values 0x00FF, 0x01FF, or 0x03FF (WGM13:0 = 5, 6, or 7), the value in ICR1 (WGM13:0 =

14), or the value in OCR1A (WGM13:0 = 15). The counter is then cleared at the following timer

clock cycle. The timing diagram for the fast PWM mode is shown in Figure 46. The figure shows

fast PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1 value is in the timing

diagram shown as a histogram for illustrating the single-slope operation. The diagram includes

non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT1 slopes

represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set

when a compare match occurs.

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Figure 46. Fast PWM Mode, Timing Diagram

OCRnx / TOP Update and

TOVn Interrupt Flag Set and

OCnA Interrupt Flag Set

(Interrupt on TOP)

TCNTn

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCnx

1

2

3

4

5

6

7

8

Period

The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches TOP. In addition

the OC1A or ICF1 Flag is set at the same timer clock cycle as TOV1 is set when either OCR1A

or ICR1 is used for defining the TOP value. If one of the interrupts are enabled, the interrupt han-

dler routine can be used for updating the TOP and compare values.

When changing the TOP value the program must ensure that the new TOP value is higher or

equal to the value of all of the Compare Registers. If the TOP value is lower than any of the

Compare Registers, a compare match will never occur between the TCNT1 and the OCR1x.

Note that when using fixed TOP values the unused bits are masked to zero when any of the

OCR1x Registers are written.

The procedure for updating ICR1 differs from updating OCR1A when used for defining the TOP

value. The ICR1 Register is not double buffered. This means that if ICR1 is changed to a low

value when the counter is running with none or a low prescaler value, there is a risk that the new

ICR1 value written is lower than the current value of TCNT1. The result will then be that the

counter will miss the compare match at the TOP value. The counter will then have to count to the

MAX value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur.

The OCR1A Register however, is double buffered. This feature allows the OCR1A I/O location

to be written anytime. When the OCR1A I/O location is written the value written will be put into

the OCR1A Buffer Register. The OCR1A Compare Register will then be updated with the value

in the Buffer Register at the next timer clock cycle the TCNT1 matches TOP. The update is done

at the same timer clock cycle as the TCNT1 is cleared and the TOV1 Flag is set.

Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using

ICR1, the OCR1A Register is free to be used for generating a PWM output on OC1A. However,

if the base PWM frequency is actively changed (by changing the TOP value), using the OCR1A

as TOP is clearly a better choice due to its double buffer feature.

In fast PWM mode, the compare units allow generation of PWM waveforms on the OC1x pins.

Setting the COM1x1:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output

can be generated by setting the COM1x1:0 to 3 (See Table 44 on page 110). The actual OC1x

value will only be visible on the port pin if the data direction for the port pin is set as output

(DDR_OC1x). The PWM waveform is generated by setting (or clearing) the OC1x Register at

the compare match between OCR1x and TCNT1, and clearing (or setting) the OC1x Register at

the timer clock cycle the counter is cleared (changes from TOP to BOTTOM).

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The PWM frequency for the output can be calculated by the following equation:

f

clk_I/O

f

= ----------------------------------

OCnxPWM

N ⋅ (1 + TOP)

The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).

The extreme values for the OCR1x Register represents special cases when generating a PWM

waveform output in the fast PWM mode. If the OCR1x is set equal to BOTTOM (0x0000) the out-

put will be a narrow spike for each TOP+1 timer clock cycle. Setting the OCR1x equal to TOP

will result in a constant high or low output (depending on the polarity of the output set by the

COM1x1:0 bits.)

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-

ting OC1A to toggle its logical level on each compare match (COM1A1:0 = 1). This applies only

if OCR1A is used to define the TOP value (WGM13:0 = 15). The waveform generated will have

a maximum frequency of f_{OC A}= f_{clk_I/O}/2 when OCR1A is set to zero (0x0000). This feature is

1

similar to the OC1A toggle in CTC mode, except the double buffer feature of the output compare

unit is enabled in the fast PWM mode.

Phase Correct PWM

Mode

The phase correct Pulse Width Modulation or phase correct PWM mode (WGM13:0 = 1,2,3,10,

or 11) provides a high resolution phase correct PWM waveform generation option. The phase

correct PWM mode is, like the phase and frequency correct PWM mode, based on a dual-slope

operation. The counter counts repeatedly from BOTTOM (0x0000) to TOP and then from TOP to

BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is cleared on

the compare match between TCNT1 and OCR1x while upcounting, and set on the compare

match while downcounting. In inverting Output Compare mode, the operation is inverted. The

dual-slope operation has lower maximum operation frequency than single slope operation. How-

ever, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for

motor control applications.

The PWM resolution for the phase correct PWM mode can be fixed to 8-bit, 9-bit, or 10-bit, or

defined by either ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set

to 0x0003), and the maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM reso-

lution in bits can be calculated by using the following equation:

log(TOP + 1)

R

= ----------------------------------

PCPWM

log(2)

In phase correct PWM mode the counter is incremented until the counter value matches either

one of the fixed values 0x00FF, 0x01FF, or 0x03FF (WGM13:0 = 1, 2, or 3), the value in ICR1

(WGM13:0 = 10), or the value in OCR1A (WGM13:0 = 11). The counter has then reached the

TOP and changes the count direction. The TCNT1 value will be equal to TOP for one timer clock

cycle. The timing diagram for the phase correct PWM mode is shown on Figure 47. The figure

shows phase correct PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1

value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The

diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on

the TCNT1 slopes represent compare matches between OCR1x and TCNT1. The OC1x Inter-

rupt Flag will be set when a compare match occurs.

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Figure 47. Phase Correct PWM Mode, Timing Diagram

OCRnx/TOP Update and

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

TOVn Interrupt Flag Set

(Interrupt on Bottom)

TCNTn

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCnx

1

2

3

4

Period

The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches BOTTOM. When

either OCR1A or ICR1 is used for defining the TOP value, the OC1A or ICF1 Flag is set accord-

ingly at the same timer clock cycle as the OCR1x Registers are updated with the double buffer

value (at TOP). The Interrupt Flags can be used to generate an interrupt each time the counter

reaches the TOP or BOTTOM value.

When changing the TOP value the program must ensure that the new TOP value is higher or

equal to the value of all of the Compare Registers. If the TOP value is lower than any of the

Compare Registers, a compare match will never occur between the TCNT1 and the OCR1x.

Note that when using fixed TOP values, the unused bits are masked to zero when any of the

OCR1x Registers are written. As the third period shown in Figure 47 illustrates, changing the

TOP actively while the Timer/Counter is running in the phase correct mode can result in an

unsymmetrical output. The reason for this can be found in the time of update of the OCR1x Reg-

ister. Since the OCR1x update occurs at TOP, the PWM period starts and ends at TOP. This

implies that the length of the falling slope is determined by the previous TOP value, while the

length of the rising slope is determined by the new TOP value. When these two values differ the

two slopes of the period will differ in length. The difference in length gives the unsymmetrical

result on the output.

It is recommended to use the phase and frequency correct mode instead of the phase correct

mode when changing the TOP value while the Timer/Counter is running. When using a static

TOP value there are practically no differences between the two modes of operation.

In phase correct PWM mode, the compare units allow generation of PWM waveforms on the

OC1x pins. Setting the COM1x1:0 bits to 2 will produce a non-inverted PWM and an inverted

PWM output can be generated by setting the COM1x1:0 to 3 (See Table 44 on page 110). The

actual OC1x value will only be visible on the port pin if the data direction for the port pin is set as

output (DDR_OC1x). The PWM waveform is generated by setting (or clearing) the OC1x Regis-

ter at the compare match between OCR1x and TCNT1 when the counter increments, and

clearing (or setting) the OC1x Register at compare match between OCR1x and TCNT1 when

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the counter decrements. The PWM frequency for the output when using phase correct PWM can

be calculated by the following equation:

f

clk_I/O

f

= ---------------------------

OCnxPCPWM

2 ⋅ N ⋅ TOP

The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).

The extreme values for the OCR1x Register represent special cases when generating a PWM

waveform output in the phase correct PWM mode. If the OCR1x is set equal to BOTTOM the

output will be continuously low and if set equal to TOP the output will be continuously high for

non-inverted PWM mode. For inverted PWM the output will have the opposite logic values. If

OCR1A is used to define the TOP value (WGM13:0 = 11) and COM1A1:0 = 1, the OC1A output

will toggle with a 50% duty cycle.

Phase and Frequency The phase and frequency correct Pulse Width Modulation, or phase and frequency correct PWM

Correct PWM Mode

mode (WGM13:0 = 8 or 9) provides a high resolution phase and frequency correct PWM wave-

form generation option. The phase and frequency correct PWM mode is, like the phase correct

PWM mode, based on a dual-slope operation. The counter counts repeatedly from BOTTOM

(0x0000) to TOP and then from TOP to BOTTOM. In non-inverting Compare Output mode, the

Output Compare (OC1x) is cleared on the compare match between TCNT1 and OCR1x while

upcounting, and set on the compare match while downcounting. In inverting Compare Output

mode, the operation is inverted. The dual-slope operation gives a lower maximum operation fre-

quency compared to the single-slope operation. However, due to the symmetric feature of the

dual-slope PWM modes, these modes are preferred for motor control applications.

The main difference between the phase correct, and the phase and frequency correct PWM

mode is the time the OCR1x Register is updated by the OCR1x Buffer Register, (see Figure 47

and Figure 48).

The PWM resolution for the phase and frequency correct PWM mode can be defined by either

ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and

the maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolution in bits can

be calculated using the following equation:

log(TOP + 1)

R

= ----------------------------------

PFCPWM

log(2)

In phase and frequency correct PWM mode the counter is incremented until the counter value

matches either the value in ICR1 (WGM13:0 = 8), or the value in OCR1A (WGM13:0 = 9). The

counter has then reached the TOP and changes the count direction. The TCNT1 value will be

equal to TOP for one timer clock cycle. The timing diagram for the phase correct and frequency

correct PWM mode is shown on Figure 48. The figure shows phase and frequency correct PWM

mode when OCR1A or ICR1 is used to define TOP. The TCNT1 value is in the timing diagram

shown as a histogram for illustrating the dual-slope operation. The diagram includes non-

inverted and inverted PWM outputs. The small horizontal line marks on the TCNT1 slopes repre-

sent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a

compare match occurs.

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Figure 48. Phase and Frequency Correct PWM Mode, Timing Diagram

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

OCRnx / TOP Update

and

TOVn Interrupt Flag Set

(Interrupt on Bottom)

TCNTn

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCnx

1

2

3

4

Period

The Timer/Counter Overflow Flag (TOV1) is set at the same timer clock cycle as the OCR1x

Registers are updated with the double buffer value (at BOTTOM). When either OCR1A or ICR1

is used for defining the TOP value, the OC1A or ICF1 Flag set when TCNT1 has reached TOP.

The Interrupt Flags can then be used to generate an interrupt each time the counter reaches the

TOP or BOTTOM value.

When changing the TOP value the program must ensure that the new TOP value is higher or

equal to the value of all of the Compare Registers. If the TOP value is lower than any of the

Compare Registers, a compare match will never occur between the TCNT1 and the OCR1x.

As Figure 48 shows the output generated is, in contrast to the phase correct mode, symmetrical

in all periods. Since the OCR1x Registers are updated at BOTTOM, the length of the rising and

the falling slopes will always be equal. This gives symmetrical output pulses and is therefore fre-

quency correct.

Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using

ICR1, the OCR1A Register is free to be used for generating a PWM output on OC1A. However,

if the base PWM frequency is actively changed by changing the TOP value, using the OCR1A as

TOP is clearly a better choice due to its double buffer feature.

In phase and frequency correct PWM mode, the compare units allow generation of PWM wave-

forms on the OC1x pins. Setting the COM1x1:0 bits to 2 will produce a non-inverted PWM and

an inverted PWM output can be generated by setting the COM1x1:0 to 3 (See Table on page

111). The actual OC1x value will only be visible on the port pin if the data direction for the port

pin is set as output (DDR_OC1x). The PWM waveform is generated by setting (or clearing) the

OC1x Register at the compare match between OCR1x and TCNT1 when the counter incre-

ments, and clearing (or setting) the OC1x Register at compare match between OCR1x and

TCNT1 when the counter decrements. The PWM frequency for the output when using phase

and frequency correct PWM can be calculated by the following equation:

f

clk_I/O

f

= ---------------------------

OCnxPFCPWM

2 ⋅ N ⋅ TOP

The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).

The extreme values for the OCR1x Register represents special cases when generating a PWM

waveform output in the phase correct PWM mode. If the OCR1x is set equal to BOTTOM the

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ATmega16(L)

output will be continuously low and if set equal to TOP the output will be set to high for non-

inverted PWM mode. For inverted PWM the output will have the opposite logic values. If OCR1A

is used to define the TOP value (WGM13:0 = 9) and COM1A1:0 = 1, the OC1A output will toggle

with a 50% duty cycle.

Timer/Counter

Timing Diagrams

The Timer/Counter is a synchronous design and the timer clock (clk_T1) is therefore shown as a

clock enable signal in the following figures. The figures include information on when Interrupt

Flags are set, and when the OCR1x Register is updated with the OCR1x buffer value (only for

modes utilizing double buffering). Figure 49 shows a timing diagram for the setting of OCF1x.

Figure 49. Timer/Counter Timing Diagram, Setting of OCF1x, No Prescaling

clk_I/O

clk_Tn

(clk_I/O/1)

TCNTn

OCRnx

OCFnx

OCRnx - 1

OCRnx

OCRnx + 1

OCRnx + 2

OCRnx Value

Figure 50 shows the same timing data, but with the prescaler enabled.

Figure 50. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (f_{clk_I/O}/8)

clk_I/O

clk_Tn

(clk_I/O/8)

TCNTn

OCRnx

OCFnx

OCRnx - 1

OCRnx

OCRnx + 1

OCRnx + 2

OCRnx Value

Figure 51 shows the count sequence close to TOP in various modes. When using phase and

frequency correct PWM mode the OCR1x Register is updated at BOTTOM. The timing diagrams

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ATmega16(L)

will be the same, but TOP should be replaced by BOTTOM, TOP-1 by BOTTOM+1 and so on.

The same renaming applies for modes that set the TOV1 Flag at BOTTOM.

Figure 51. Timer/Counter Timing Diagram, no Prescaling

clk_I/O

clk_Tn

(clk_I/O/1)

TCNTn

TOP - 1

TOP

BOTTOM

TOP - 1

BOTTOM + 1

TOP - 2

(CTC and FPWM)

TCNTn

(PC and PFC PWM)

TOVn (FPWM)

and ICFn (if used

as TOP)

OCRnx

(Update at TOP)

Old OCRnx Value

New OCRnx Value

Figure 52 shows the same timing data, but with the prescaler enabled.

Figure 52. Timer/Counter Timing Diagram, with Prescaler (f_{clk_I/O}/8)

clk_I/O

clk_Tn

(clk_I/O/8)

TCNTn

TOP - 1

TOP

BOTTOM

TOP - 1

BOTTOM + 1

TOP - 2

(CTC and FPWM)

TCNTn

(PC and PFC PWM)

TOVn (FPWM)

and ICFn (if used

as TOP)

OCRnx

(Update at TOP)

Old OCRnx Value

New OCRnx Value

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16-bit

Timer/Counter

Register

Description

Timer/Counter1

Control Register A –

TCCR1A

Bit

7

COM1A1

R/W

6

COM1A0

R/W

5

COM1B1

R/W

4

COM1B0

R/W

3

FOC1A

W

2

FOC1B

W

1

WGM11

R/W

0

WGM10

R/W

0

TCCR1A

Read/Write

Initial Value

0

• Bit 7:6 – COM1A1:0: Compare Output Mode for Channel A

• Bit 5:4 – COM1B1:0: Compare Output Mode for Channel B

The COM1A1:0 and COM1B1:0 control the Output Compare pins (OC1A and OC1B respec-

tively) behavior. If one or both of the COM1A1:0 bits are written to one, the OC1A output

overrides the normal port functionality of the I/O pin it is connected to. If one or both of the

COM1B1:0 bit are written to one, the OC1B output overrides the normal port functionality of the

I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit correspond-

ing to the OC1A or OC1B pin must be set in order to enable the output driver.

When the OC1A or OC1B is connected to the pin, the function of the COM1x1:0 bits is depen-

dent of the WGM13:0 bits setting. Table 44 shows the COM1x1:0 bit functionality when the

WGM13:0 bits are set to a normal or a CTC mode (non-PWM).

Table 44. Compare Output Mode, non-PWM

COM1A1/COM1B1

COM1A0/COM1B0

Description

0

Normal port operation, OC1A/OC1B

disconnected.

0

1

0

Toggle OC1A/OC1B on compare match

Clear OC1A/OC1B on compare match (Set

output to low level)

1

Set OC1A/OC1B on compare match (Set

output to high level)

Table 45 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the fast PWM

mode.

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Table 45. Compare Output Mode, Fast PWM⁽¹⁾

COM1A1/COM1B1

COM1A0/COM1B0

Description

0

Normal port operation, OC1A/OC1B

disconnected.

0

1

WGM13:0 = 15: Toggle OC1A on Compare

Match, OC1B disconnected (normal port

operation).

For all other WGM13:0 settings, normal port

operation, OCnA/OCnB disconnected.

1

0

1

Clear OC1A/OC1B on compare match, set

OC1A/OC1B at BOTTOM,

(non-inverting mode)

Set OC1A/OC1B on compare match, clear

OC1A/OC1B at BOTTOM,

(inverting mode)

Note:

1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. In

this case the compare match is ignored, but the set or clear is done at BOTTOM. See “Fast

PWM Mode” on page 102. for more details.

Table 46 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the phase cor-

rect or the phase and frequency correct, PWM mode.

Table 46. Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM ⁽¹⁾

COM1A1/COM1B1

COM1A0/COM1B0

Description

0

Normal port operation, OC1A/OC1B

disconnected.

0

1

WGM13:0 = 9 or 14: Toggle OCnA on

Compare Match, OCnB disconnected (normal

port operation).

For all other WGM13:0 settings, normal port

operation, OC1A/OC1B disconnected.

1

0

1

Clear OC1A/OC1B on compare match when

up-counting. Set OC1A/OC1B on compare

match when downcounting.

Set OC1A/OC1B on compare match when up-

counting. Clear OC1A/OC1B on compare

match when downcounting.

Note:

1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. See

“Phase Correct PWM Mode” on page 104. for more details.

• Bit 3 – FOC1A: Force Output Compare for Channel A

• Bit 2 – FOC1B: Force Output Compare for Channel B

The FOC1A/FOC1B bits are only active when the WGM13:0 bits specifies a non-PWM mode.

However, for ensuring compatibility with future devices, these bits must be set to zero when

TCCR1A is written when operating in a PWM mode. When writing a logical one to the

FOC1A/FOC1B bit, an immediate compare match is forced on the Waveform Generation unit.

The OC1A/OC1B output is changed according to its COM1x1:0 bits setting. Note that the

FOC1A/FOC1B bits are implemented as strobes. Therefore it is the value present in the

COM1x1:0 bits that determine the effect of the forced compare.

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A FOC1A/FOC1B strobe will not generate any interrupt nor will it clear the timer in Clear Timer

on Compare match (CTC) mode using OCR1A as TOP.

The FOC1A/FOC1B bits are always read as zero.

• Bit 1:0 – WGM11:0: Waveform Generation Mode

Combined with the WGM13:2 bits found in the TCCR1B Register, these bits control the counting

sequence of the counter, the source for maximum (TOP) counter value, and what type of wave-

form generation to be used, see Table 47. Modes of operation supported by the Timer/Counter

unit are: Normal mode (counter), Clear Timer on Compare match (CTC) mode, and three types

of Pulse Width Modulation (PWM) modes. (See “Modes of Operation” on page 101.)

Table 47. Waveform Generation Mode Bit Description⁽¹⁾

WGM12

(CTC1)

WGM11

(PWM11) (PWM10)

WGM10

Update of

OCR1x

TOV1 Flag Set

on

Mode WGM13

Timer/Counter Mode of Operation

Normal

TOP

0xFFFF

0x00FF

0x01FF

0x03FF

OCR1A

0x00FF

0x01FF

0x03FF

ICR1

Immediate

TOP

MAX

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

PWM, Phase Correct, 8-bit

PWM, Phase Correct, 9-bit

PWM, Phase Correct, 10-bit

CTC

BOTTOM

MAX

TOP

2

TOP

3

Immediate

BOTTOM

TOP

4

Fast PWM, 8-bit

TOP

5

6

Fast PWM, 9-bit

TOP

Fast PWM, 10-bit

TOP

7

PWM, Phase and Frequency Correct

PWM, Phase Correct

CTC

BOTTOM

MAX

8

9

OCR1A

ICR1

10

11

12

13

14

OCR1A

ICR1

TOP

Immediate

–

Reserved

–

Fast PWM

ICR1

BOTTOM

TOP

15

Fast PWM

OCR1A

TOP

Note:

1. The CTC1 and PWM11:0 bit definition names are obsolete. Use the WGM12:0 definitions. However, the functionality and

location of these bits are compatible with previous versions of the timer.

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Timer/Counter1

Control Register B –

TCCR1B

Bit

7

ICNC1

R/W

0

6

ICES1

R/W

0

5

–

4

WGM13

R/W

0

3

WGM12

R/W

0

2

CS12

R/W

0

1

CS11

R/W

0

CS10

R/W

0

TCCR1B

Read/Write

Initial Value

R

0

• Bit 7 – ICNC1: Input Capture Noise Canceler

Setting this bit (to one) activates the Input Capture Noise Canceler. When the Noise Canceler is

activated, the input from the Input Capture Pin (ICP1) is filtered. The filter function requires four

successive equal valued samples of the ICP1 pin for changing its output. The Input Capture is

therefore delayed by four Oscillator cycles when the Noise Canceler is enabled.

• Bit 6 – ICES1: Input Capture Edge Select

This bit selects which edge on the Input Capture Pin (ICP1) that is used to trigger a capture

event. When the ICES1 bit is written to zero, a falling (negative) edge is used as trigger, and

when the ICES1 bit is written to one, a rising (positive) edge will trigger the capture.

When a capture is triggered according to the ICES1 setting, the counter value is copied into the

Input Capture Register (ICR1). The event will also set the Input Capture Flag (ICF1), and this

can be used to cause an Input Capture Interrupt, if this interrupt is enabled.

When the ICR1 is used as TOP value (see description of the WGM13:0 bits located in the

TCCR1A and the TCCR1B Register), the ICP1 is disconnected and consequently the Input Cap-

ture function is disabled.

• Bit 5 – Reserved Bit

This bit is reserved for future use. For ensuring compatibility with future devices, this bit must be

written to zero when TCCR1B is written.

• Bit 4:3 – WGM13:2: Waveform Generation Mode

See TCCR1A Register description.

• Bit 2:0 – CS12:0: Clock Select

The three Clock Select bits select the clock source to be used by the Timer/Counter, see Figure

49 and Figure 50.

Table 48. Clock Select Bit Description

CS12

CS11

CS10

Description

0

1

0

1

0

1

0

1

0

1

0

1

0

1

No clock source (Timer/Counter stopped).

clk_I/O/1 (No prescaling)

clk_I/O/8 (From prescaler)

clk_I/O/64 (From prescaler)

clk_I/O/256 (From prescaler)

clk_I/O/1024 (From prescaler)

External clock source on T1 pin. Clock on falling edge.

External clock source on T1 pin. Clock on rising edge.

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If external pin modes are used for the Timer/Counter1, transitions on the T1 pin will clock the

counter even if the pin is configured as an output. This feature allows software control of the

counting.

Timer/Counter1 –

TCNT1H and TCNT1L

Bit

7

6

5

4

3

2

1

0

TCNT1[15:8]

TCNT1[7:0]

TCNT1H

TCNT1L

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

The two Timer/Counter I/O locations (TCNT1H and TCNT1L, combined TCNT1) give direct

access, both for read and for write operations, to the Timer/Counter unit 16-bit counter. To

ensure that both the high and Low bytes are read and written simultaneously when the CPU

accesses these registers, the access is performed using an 8-bit temporary High Byte Register

(TEMP). This temporary register is shared by all the other 16-bit registers. See “Accessing 16-bit

Registers” on page 92.

Modifying the counter (TCNT1) while the counter is running introduces a risk of missing a com-

pare match between TCNT1 and one of the OCR1x Registers.

Writing to the TCNT1 Register blocks (removes) the compare match on the following timer clock

for all compare units.

Output Compare

Register 1 A –

Bit

7

6

5

4

3

2

1

0

OCR1AH and OCR1AL

OCR1A[15:8]

OCR1A[7:0]

OCR1AH

OCR1AL

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

Output Compare

Register 1 B –

Bit

7

6

5

4

3

2

1

0

OCR1BH and OCR1BL

OCR1B[15:8]

OCR1B[7:0]

OCR1BH

OCR1BL

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

The Output Compare Registers contain a 16-bit value that is continuously compared with the

counter value (TCNT1). A match can be used to generate an output compare interrupt, or to

generate a waveform output on the OC1x pin.

The Output Compare Registers are 16-bit in size. To ensure that both the high and Low bytes

are written simultaneously when the CPU writes to these registers, the access is performed

using an 8-bit temporary High Byte Register (TEMP). This temporary register is shared by all the

other 16-bit registers. See “Accessing 16-bit Registers” on page 92.

Input Capture Register

1 – ICR1H and ICR1L

Bit

7

6

5

4

3

2

1

0

ICR1[15:8]

ICR1[7:0]

ICR1H

ICR1L

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

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The Input Capture is updated with the counter (TCNT1) value each time an event occurs on the

ICP1 pin (or optionally on the analog comparator output for Timer/Counter1). The Input Capture

can be used for defining the counter TOP value.

The Input Capture Register is 16-bit in size. To ensure that both the high and Low bytes are read

simultaneously when the CPU accesses these registers, the access is performed using an 8-bit

temporary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit

registers. See “Accessing 16-bit Registers” on page 92.

Timer/Counter

Interrupt Mask

Bit

7

OCIE2

R/W

0

6

TOIE2

R/W

0

5

TICIE1

R/W

0

4

OCIE1A

R/W

0

3

OCIE1B

R/W

0

2

TOIE1

R/W

0

1

OCIE0

R/W

0

TOIE0

R/W

0

Register – TIMSK⁽¹⁾

TIMSK

Read/Write

Initial Value

Note:

1. This register contains interrupt control bits for several Timer/Counters, but only Timer1 bits are

described in this section. The remaining bits are described in their respective timer sections.

• Bit 5 – TICIE1: Timer/Counter1, Input Capture Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally

enabled), the Timer/Counter1 Input Capture Interrupt is enabled. The corresponding Interrupt

Vector (See “Interrupts” on page 45.) is executed when the ICF1 Flag, located in TIFR, is set.

• Bit 4 – OCIE1A: Timer/Counter1, Output Compare A Match Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally

enabled), the Timer/Counter1 Output Compare A match interrupt is enabled. The corresponding

Interrupt Vector (See “Interrupts” on page 45.) is executed when the OCF1A Flag, located in

TIFR, is set.

• Bit 3 – OCIE1B: Timer/Counter1, Output Compare B Match Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally

enabled), the Timer/Counter1 Output Compare B match interrupt is enabled. The corresponding

Interrupt Vector (See “Interrupts” on page 45.) is executed when the OCF1B Flag, located in

TIFR, is set.

• Bit 2 – TOIE1: Timer/Counter1, Overflow Interrupt Enable

When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally

enabled), the Timer/Counter1 Overflow Interrupt is enabled. The corresponding Interrupt Vector

(See “Interrupts” on page 45.) is executed when the TOV1 Flag, located in TIFR, is set.

Timer/Counter

Interrupt Flag Register

– TIFR

Bit

7

OCF2

R/W

0

6

TOV2

R/W

0

5

4

OCF1A

R/W

0

3

OCF1B

R/W

0

2

TOV1

R/W

0

1

OCF0

R/W

0

TOV0

R/W

0

ICF1

R/W

0

TIFR

Read/Write

Initial Value

Note:

This register contains flag bits for several Timer/Counters, but only Timer1 bits are described in

this section. The remaining bits are described in their respective timer sections.

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• Bit 5 – ICF1: Timer/Counter1, Input Capture Flag

This flag is set when a capture event occurs on the ICP1 pin. When the Input Capture Register

(ICR1) is set by the WGM13:0 to be used as the TOP value, the ICF1 Flag is set when the coun-

ter reaches the TOP value.

ICF1 is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively,

ICF1 can be cleared by writing a logic one to its bit location.

• Bit 4 – OCF1A: Timer/Counter1, Output Compare A Match Flag

This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output

Compare Register A (OCR1A).

Note that a Forced Output Compare (FOC1A) strobe will not set the OCF1A Flag.

OCF1A is automatically cleared when the Output Compare Match A Interrupt Vector is exe-

cuted. Alternatively, OCF1A can be cleared by writing a logic one to its bit location.

• Bit 3 – OCF1B: Timer/Counter1, Output Compare B Match Flag

This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output

Compare Register B (OCR1B).

Note that a forced output compare (FOC1B) strobe will not set the OCF1B Flag.

OCF1B is automatically cleared when the Output Compare Match B Interrupt Vector is exe-

cuted. Alternatively, OCF1B can be cleared by writing a logic one to its bit location.

• Bit 2 – TOV1: Timer/Counter1, Overflow Flag

The setting of this flag is dependent of the WGM13:0 bits setting. In normal and CTC modes, the

TOV1 Flag is set when the timer overflows. Refer to Table 47 on page 112 for the TOV1 Flag

behavior when using another WGM13:0 bit setting.

TOV1 is automatically cleared when the Timer/Counter1 Overflow interrupt vector is executed.

Alternatively, TOV1 can be cleared by writing a logic one to its bit location.

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8-bit

Timer/Counter2 is a general purpose, single compare unit, 8-bit Timer/Counter module. The

main features are:

• Single Compare unit Counter

• Clear Timer on Compare Match (Auto Reload)

• Glitch-free, Phase Correct Pulse Width Modulator (PWM)

• Frequency Generator

• 10-bit Clock Prescaler

• Overflow and Compare Match Interrupt Sources (TOV2 and OCF2)

• Allows clocking from External 32 kHz Watch Crystal Independent of the I/O Clock

Timer/Counter2

with PWM and

Asynchronous

Operation

Overview

A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 53. For the actual place-

ment of I/O pins, refer to “Pinout ATmega16” on page 2. CPU accessible I/O Registers, including

I/O bits and I/O pins, are shown in bold. The device-specific I/O Register and bit locations are

listed in the “8-bit Timer/Counter Register Description” on page 128.

Figure 53. 8-bit Timer/Counter Block Diagram

TCCRn

count

TOVn

(Int.Req.)

clear

Control Logic

TOP

direction

clk_Tn

TOSC1

TOSC2

BOTTOM

T/C

Oscillator

Prescaler

Timer/Counter

TCNTn

= 0

= 0xFF

clk_I/O

OCn

(Int.Req.)

Waveform

Generation

=

OCRn

clk_I/O

Synchronized Status flags

Synchronization Unit

clk_ASY

Status flags

ASSRn

asynchronous mode

select (ASn)

Registers

The Timer/Counter (TCNT2) and Output Compare Register (OCR2) are 8-bit registers. Interrupt

request (shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag Register (TIFR).

All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK). TIFR and

TIMSK are not shown in the figure since these registers are shared by other timer units.

The Timer/Counter can be clocked internally, via the prescaler, or asynchronously clocked from

the TOSC1/2 pins, as detailed later in this section. The asynchronous operation is controlled by

the Asynchronous Status Register (ASSR). The Clock Select logic block controls which clock

source the Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inac-

tive when no clock source is selected. The output from the Clock Select logic is referred to as the

timer clock (clk_T2).

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The double buffered Output Compare Register (OCR2) is compared with the Timer/Counter

value at all times. The result of the compare can be used by the waveform generator to generate

a PWM or variable frequency output on the Output Compare Pin (OC2). See “Output Compare

Unit” on page 119. for details. The compare match event will also set the Compare Flag (OCF2)

which can be used to generate an output compare interrupt request.

Definitions

Many register and bit references in this document are written in general form. A lower case “n”

replaces the Timer/Counter number, in this case 2. However, when using the register or bit

defines in a program, the precise form must be used (that is, TCNT2 for accessing

Timer/Counter2 counter value and so on). The definitions in Table 49 are also used extensively

throughout the document.

Table 49. Definitions

BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00).

MAX

The counter reaches its MAXimum when it becomes 0xFF (decimal

255).

TOP

The counter reaches the TOP when it becomes equal to the highest

value in the count sequence. The TOP value can be assigned to be the

fixed value 0xFF (MAX) or the value stored in the OCR2 Register. The

assignment is dependent on the mode of operation.

Timer/Counter

Clock Sources

The Timer/Counter can be clocked by an internal synchronous or an external asynchronous

clock source. The clock source clk_T2is by default equal to the MCU clock, clk_I/O. When the AS2

bit in the ASSR Register is written to logic one, the clock source is taken from the Timer/Counter

Oscillator connected to TOSC1 and TOSC2. For details on asynchronous operation, see “Asyn-

chronous Status Register – ASSR” on page 131. For details on clock sources and prescaler, see

“Timer/Counter Prescaler” on page 134.

Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure

54 shows a block diagram of the counter and its surrounding environment.

Figure 54. Counter Unit Block Diagram

TOVn

(Int.Req.)

DATA BUS

TOSC1

count

T/C

Oscillator

clk _Tn

clear

TCNTn

Control Logic

Prescaler

direction

TOSC2

clk

bottom

top

I/O

Signal description (internal signals):

count Increment or decrement TCNT2 by 1.

direction Selects between increment and decrement.

clear

clk_T2

top

Clear TCNT2 (set all bits to zero).

Timer/Counter clock.

Signalizes that TCNT2 has reached maximum value.

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bottom

Signalizes that TCNT2 has reached minimum value (zero).

Depending on the mode of operation used, the counter is cleared, incremented, or decremented

at each timer clock (clk_T2). clk_T2can be generated from an external or internal clock source,

selected by the Clock Select bits (CS22:0). When no clock source is selected (CS22:0 = 0) the

timer is stopped. However, the TCNT2 value can be accessed by the CPU, regardless of

whether clk_T2is present or not. A CPU write overrides (has priority over) all counter clear or

count operations.

The counting sequence is determined by the setting of the WGM21 and WGM20 bits located in

the Timer/Counter Control Register (TCCR2). There are close connections between how the

counter behaves (counts) and how waveforms are generated on the Output Compare output

OC2. For more details about advanced counting sequences and waveform generation, see

“Modes of Operation” on page 122.

The Timer/Counter Overflow (TOV2) Flag is set according to the mode of operation selected by

the WGM21:0 bits. TOV2 can be used for generating a CPU interrupt.

Output Compare

Unit

The 8-bit comparator continuously compares TCNT2 with the Output Compare Register

(OCR2). Whenever TCNT2 equals OCR2, the comparator signals a match. A match will set the

Output Compare Flag (OCF2) at the next timer clock cycle. If enabled (OCIE2 = 1), the Output

Compare Flag generates an output compare interrupt. The OCF2 Flag is automatically cleared

when the interrupt is executed. Alternatively, the OCF2 Flag can be cleared by software by writ-

ing a logical one to its I/O bit location. The waveform generator uses the match signal to

generate an output according to operating mode set by the WGM21:0 bits and Compare Output

mode (COM21:0) bits. The max and bottom signals are used by the waveform generator for han-

dling the special cases of the extreme values in some modes of operation (“Modes of Operation”

on page 122). Figure 55 shows a block diagram of the output compare unit.

Figure 55. Output Compare Unit, Block Diagram

DATA BUS

OCRn

TCNTn

=

(8-bit Comparator )

OCFn (Int.Req.)

top

bottom

FOCn

Waveform Generator

OCxy

WGMn1:0

COMn1:0

The OCR2 Register is double buffered when using any of the Pulse Width Modulation (PWM)

modes. For the normal and Clear Timer on Compare (CTC) modes of operation, the double buff-

ering is disabled. The double buffering synchronizes the update of the OCR2 Compare Register

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to either top or bottom of the counting sequence. The synchronization prevents the occurrence

of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free.

The OCR2 Register access may seem complex, but this is not case. When the double buffering

is enabled, the CPU has access to the OCR2 Buffer Register, and if double buffering is disabled

the CPU will access the OCR2 directly.

Force Output

Compare

In non-PWM waveform generation modes, the match output of the comparator can be forced by

writing a one to the Force Output Compare (FOC2) bit. Forcing compare match will not set the

OCF2 Flag or reload/clear the timer, but the OC2 pin will be updated as if a real compare match

had occurred (the COM21:0 bits settings define whether the OC2 pin is set, cleared or toggled).

Compare Match

Blocking by TCNT2

Write

All CPU write operations to the TCNT2 Register will block any compare match that occurs in the

next timer clock cycle, even when the timer is stopped. This feature allows OCR2 to be initialized

to the same value as TCNT2 without triggering an interrupt when the Timer/Counter clock is

enabled.

Using the Output

Compare Unit

Since writing TCNT2 in any mode of operation will block all compare matches for one timer clock

cycle, there are risks involved when changing TCNT2 when using the output compare unit, inde-

pendently of whether the Timer/Counter is running or not. If the value written to TCNT2 equals

the OCR2 value, the compare match will be missed, resulting in incorrect waveform generation.

Similarly, do not write the TCNT2 value equal to BOTTOM when the counter is downcounting.

The setup of the OC2 should be performed before setting the Data Direction Register for the port

pin to output. The easiest way of setting the OC2 value is to use the Force Output Compare

(FOC2) strobe bit in Normal mode. The OC2 Register keeps its value even when changing

between Waveform Generation modes.

Be aware that the COM21:0 bits are not double buffered together with the compare value.

Changing the COM21:0 bits will take effect immediately.

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Compare Match

Output Unit

The Compare Output mode (COM21:0) bits have two functions. The Waveform Generator uses

the COM21:0 bits for defining the Output Compare (OC2) state at the next compare match. Also,

the COM21:0 bits control the OC2 pin output source. Figure 56 shows a simplified schematic of

the logic affected by the COM21:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the fig-

ure are shown in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT)

that are affected by the COM21:0 bits are shown. When referring to the OC2 state, the reference

is for the internal OC2 Register, not the OC2 pin.

Figure 56. Compare Match Output Unit, Schematic

COMn1

Waveform

Generator

COMn0

FOCn

D

Q

1

0

OCn

D

PORT

D

Q

DDR

clk_I/O

The general I/O port function is overridden by the Output Compare (OC2) from the waveform

generator if either of the COM21:0 bits are set. However, the OC2 pin direction (input or output)

is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Regis-

ter bit for the OC2 pin (DDR_OC2) must be set as output before the OC2 value is visible on the

pin. The port override function is independent of the Waveform Generation mode.

The design of the output compare pin logic allows initialization of the OC2 state before the out-

put is enabled. Note that some COM21:0 bit settings are reserved for certain modes of

operation. See “8-bit Timer/Counter Register Description” on page 128.

Compare Output Mode The waveform generator uses the COM21:0 bits differently in Normal, CTC, and PWM modes.

and Waveform

Generation

For all modes, setting the COM21:0 = 0 tells the Waveform Generator that no action on the OC2

Register is to be performed on the next compare match. For compare output actions in the non-

PWM modes refer to Table 51 on page 129. For fast PWM mode, refer to Table 52 on page 129,

and for phase correct PWM refer to Table 53 on page 129.

A change of the COM21:0 bits state will have effect at the first compare match after the bits are

written. For non-PWM modes, the action can be forced to have immediate effect by using the

FOC2 strobe bits.

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Modes of

Operation

The mode of operation, that is, the behavior of the Timer/Counter and the output compare pins,

is defined by the combination of the Waveform Generation mode (WGM21:0) and Compare Out-

put mode (COM21:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COM21:0 bits control whether the PWM out-

put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes

the COM21:0 bits control whether the output should be set, cleared, or toggled at a compare

match (See “Compare Match Output Unit” on page 121.).

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 126.

Normal Mode

The simplest mode of operation is the Normal mode (WGM21:0 = 0). In this mode the counting

direction is always up (incrementing), and no counter clear is performed. The counter simply

overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bot-

tom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV2) will be set in the same

timer clock cycle as the TCNT2 becomes zero. The TOV2 Flag in this case behaves like a ninth

bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt

that automatically clears the TOV2 Flag, the timer resolution can be increased by software. There

are no special cases to consider in the normal mode, a new counter value can be written

anytime.

The Output Compare unit can be used to generate interrupts at some given time. Using the out-

put compare to generate waveforms in normal mode is not recommended, since this will occupy

too much of the CPU time.

Clear Timer on

In Clear Timer on Compare or CTC mode (WGM21:0 = 2), the OCR2 Register is used to manip-

Compare Match (CTC) ulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value

Mode

(TCNT2) matches the OCR2. The OCR2 defines the top value for the counter, hence also its

resolution. This mode allows greater control of the compare match output frequency. It also sim-

plifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 57. The counter value (TCNT2)

increases until a compare match occurs between TCNT2 and OCR2, and then counter (TCNT2)

is cleared.

Figure 57. CTC Mode, Timing Diagram

OCn Interrupt Flag Set

TCNTn

OCn

(Toggle)

(COMn1:0 = 1)

1

2

3

4

Period

An interrupt can be generated each time the counter value reaches the TOP value by using the

OCF2 Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating the

TOP value. However, changing the TOP to a value close to BOTTOM when the counter is run-

ning with none or a low prescaler value must be done with care since the CTC mode does not

have the double buffering feature. If the new value written to OCR2 is lower than the current

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value of TCNT2, the counter will miss the compare match. The counter will then have to count to

its maximum value (0xFF) and wrap around starting at 0x00 before the compare match can

occur.

For generating a waveform output in CTC mode, the OC2 output can be set to toggle its logical

level on each compare match by setting the Compare Output mode bits to toggle mode

(COM21:0 = 1). The OC2 value will not be visible on the port pin unless the data direction for the

pin is set to output. The waveform generated will have a maximum frequency of f_OC2= f_{clk_I/O}/2

when OCR2 is set to zero (0x00). The waveform frequency is defined by the following equation:

f

clk_I/O

f

= ----------------------------------------------

OCn

2 ⋅ N ⋅ (1 + OCRn)

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

As for the Normal mode of operation, the TOV2 Flag is set in the same timer clock cycle that the

counter counts from MAX to 0x00.

Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGM21:0 = 3) provides a high frequency

PWM waveform generation option. The fast PWM differs from the other PWM option by its sin-

gle-slope operation. The counter counts from BOTTOM to MAX then restarts from BOTTOM. In

non-inverting Compare Output mode, the Output Compare (OC2) is cleared on the compare

match between TCNT2 and OCR2, and set at BOTTOM. In inverting Compare Output mode, the

output is set on compare match and cleared at BOTTOM. Due to the single-slope operation, the

operating frequency of the fast PWM mode can be twice as high as the phase correct PWM

mode that uses dual-slope operation. This high frequency makes the fast PWM mode well suited

for power regulation, rectification, and DAC applications. High frequency allows physically small

sized external components (coils, capacitors), and therefore reduces total system cost.

In fast PWM mode, the counter is incremented until the counter value matches the MAX value.

The counter is then cleared at the following timer clock cycle. The timing diagram for the fast

PWM mode is shown in Figure 58. The TCNT2 value is in the timing diagram shown as a histo-

gram for illustrating the single-slope operation. The diagram includes non-inverted and inverted

PWM outputs. The small horizontal line marks on the TCNT2 slopes represent compare

matches between OCR2 and TCNT2.

Figure 58. Fast PWM Mode, Timing Diagram

OCRn Interrupt Flag Set

OCRn Update and

TOVn Interrupt Flag Set

TCNTn

(COMn1:0 = 2)

(COMn1:0 = 3)

OCn

1

2

3

4

5

6

7

Period

The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches MAX. If the inter-

rupt is enabled, the interrupt handler routine can be used for updating the compare value.

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In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC2 pin. Set-

ting the COM21:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can

be generated by setting the COM21:0 to 3 (see Table 52 on page 129). The actual OC2 value

will only be visible on the port pin if the data direction for the port pin is set as output. The PWM

waveform is generated by setting (or clearing) the OC2 Register at the compare match between

OCR2 and TCNT2, and clearing (or setting) the OC2 Register at the timer clock cycle the coun-

ter is cleared (changes from MAX to BOTTOM).

The PWM frequency for the output can be calculated by the following equation:

f

clk_I/O

f

= -----------------

OCnPWM

N ⋅ 256

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

The extreme values for the OCR2 Register represent special cases when generating a PWM

waveform output in the fast PWM mode. If the OCR2 is set equal to BOTTOM, the output will be

a narrow spike for each MAX+1 timer clock cycle. Setting the OCR2 equal to MAX will result in a

constantly high or low output (depending on the polarity of the output set by the COM21:0 bits.)

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-

ting OC2 to toggle its logical level on each compare match (COM21:0 = 1). The waveform

generated will have a maximum frequency of f_oc2= f_{clk_I/O}/2 when OCR2 is set to zero. This fea-

ture is similar to the OC2 toggle in CTC mode, except the double buffer feature of the output

compare unit is enabled in the fast PWM mode.

Phase Correct PWM

Mode

The phase correct PWM mode (WGM21:0 = 1) provides a high resolution phase correct PWM

waveform generation option. The phase correct PWM mode is based on a dual-slope operation.

The counter counts repeatedly from BOTTOM to MAX and then from MAX to BOTTOM. In non-

inverting Compare Output mode, the Output Compare (OC2) is cleared on the compare match

between TCNT2 and OCR2 while upcounting, and set on the compare match while downcount-

ing. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has

lower maximum operation frequency than single slope operation. However, due to the symmet-

ric feature of the dual-slope PWM modes, these modes are preferred for motor control

applications.

The PWM resolution for the phase correct PWM mode is fixed to 8 bits. In phase correct PWM

mode the counter is incremented until the counter value matches MAX. When the counter

reaches MAX, it changes the count direction. The TCNT2 value will be equal to MAX for one

timer clock cycle. The timing diagram for the phase correct PWM mode is shown on Figure 59.

The TCNT2 value is in the timing diagram shown as a histogram for illustrating the dual-slope

operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal

line marks on the TCNT2 slopes represent compare matches between OCR2 and TCNT2.

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Figure 59. Phase Correct PWM Mode, Timing Diagram

OCn Interrupt Flag Set

OCRn Update

TOVn Interrupt Flag Set

TCNTn

(COMn1:0 = 2)

(COMn1:0 = 3)

OCn

1

2

3

Period

The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches BOTTOM. The

Interrupt Flag can be used to generate an interrupt each time the counter reaches the BOTTOM

value.

In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the

OC2 pin. Setting the COM21:0 bits to 2 will produce a non-inverted PWM. An inverted PWM out-

put can be generated by setting the COM21:0 to 3 (see Table 53 on page 129). The actual OC2

value will only be visible on the port pin if the data direction for the port pin is set as output. The

PWM waveform is generated by clearing (or setting) the OC2 Register at the compare match

between OCR2 and TCNT2 when the counter increments, and setting (or clearing) the OC2

Register at compare match between OCR2 and TCNT2 when the counter decrements. The

PWM frequency for the output when using phase correct PWM can be calculated by the follow-

ing equation:

f

clk_I/O

f

= -----------------

OCnPCPWM

N ⋅ 510

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

The extreme values for the OCR2 Register represent special cases when generating a PWM

waveform output in the phase correct PWM mode. If the OCR2 is set equal to BOTTOM, the out-

put will be continuously low and if set equal to MAX the output will be continuously high for non-

inverted PWM mode. For inverted PWM the output will have the opposite logic values.

At the very start of Period 2 in Figure 59 OCn has a transition from high to l ow even though

there is no Compare Match. The point of this transition is to guarantee symmetry around BOT-

TOM. There are two cases that will give transition without Compare Match:

•

OCR2A changes its value from Max, like in Figure 59. When the OCR2A value is MAX the

OCn pin value is the same as the result of a down-counting Compare Match. To ensure

symmetry around BOTTOM the OCn value at MAX must be correspond the the result of an

up-counting Compare Match.

•

The Timer starts counting from a value higher than the one in OCR2A, and for that reason

misses the Compare Match and hence the OCn that would have happened on the way up.

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Timer/Counter

Timing Diagrams

The following figures show the Timer/Counter in Synchronous mode, and the timer clock (clk_T2)

is therefore shown as a clock enable signal. In Asynchronous mode, clk_I/Oshould be replaced by

the Timer/Counter Oscillator clock. The figures include information on when Interrupt Flags are

set. Figure 60 contains timing data for basic Timer/Counter operation. The figure shows the

count sequence close to the MAX value in all modes other than phase correct PWM mode.

Figure 60. Timer/Counter Timing Diagram, no Prescaling

clk_I/O

clk_Tn

(clk_I/O/1)

TCNTn

TOVn

MAX - 1

MAX

BOTTOM

BOTTOM + 1

Figure 61 shows the same timing data, but with the prescaler enabled.

Figure 61. Timer/Counter Timing Diagram, with Prescaler (f_{clk_I/O}/8)

clk_I/O

clk_Tn

(clk_I/O/8)

TCNTn

TOVn

MAX - 1

MAX

BOTTOM

BOTTOM + 1

Figure 62 shows the setting of OCF2 in all modes except CTC mode.

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Figure 62. Timer/Counter Timing Diagram, Setting of OCF2, with Prescaler (f_{clk_I/O}/8)

clk_I/O

clk_Tn

(clk_I/O/8)

TCNTn

OCRn

OCFn

OCRn - 1

OCRn

OCRn + 1

OCRn + 2

OCRn Value

Figure 63 shows the setting of OCF2 and the clearing of TCNT2 in CTC mode.

Figure 63. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with Pres-

caler (f_{clk_I/O}/8)

clk_I/O

clk_Tn

(clk_I/O/8)

TCNTn

(CTC)

TOP - 1

TOP

BOTTOM

BOTTOM + 1

OCRn

TOP

OCFn

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8-bit

Timer/Counter

Register

Description

Timer/Counter Control

Register – TCCR2

Bit

7

FOC2

W

6

WGM20

R/W

0

5

COM21

R/W

0

4

COM20

R/W

0

3

WGM21

R/W

0

2

CS22

R/W

0

1

CS21

R/W

0

CS20

R/W

0

TCCR2

Read/Write

Initial Value

0

• Bit 7 – FOC2: Force Output Compare

The FOC2 bit is only active when the WGM bits specify a non-PWM mode. However, for ensur-

ing compatibility with future devices, this bit must be set to zero when TCCR2 is written when

operating in PWM mode. When writing a logical one to the FOC2 bit, an immediate compare

match is forced on the waveform generation unit. The OC2 output is changed according to its

COM21:0 bits setting. Note that the FOC2 bit is implemented as a strobe. Therefore it is the

value present in the COM21:0 bits that determines the effect of the forced compare.

A FOC2 strobe will not generate any interrupt, nor will it clear the timer in CTC mode using

OCR2 as TOP.

The FOC2 bit is always read as zero.

• Bit 3, 6 – WGM21:0: Waveform Generation Mode

These bits control the counting sequence of the counter, the source for the maximum (TOP)

counter value, and what type of waveform generation to be used. Modes of operation supported

by the Timer/Counter unit are: Normal mode, Clear Timer on Compare match (CTC) mode, and

two types of Pulse Width Modulation (PWM) modes. See Table 50 and “Modes of Operation” on

page 122.

Table 50. Waveform Generation Mode Bit Description⁽¹⁾

WGM21 WGM20 Timer/Counter Mode of

Update of TOV2 Flag

OCR2 Set on

Mode

(CTC2)

(PWM2) Operation

TOP

0xFF

0

1

2

3

0

1

0

1

0

1

Normal

Immediate MAX

TOP BOTTOM

PWM, Phase Correct

CTC

OCR2 Immediate MAX

0xFF BOTTOM MAX

Fast PWM

Note:

1. The CTC2 and PWM2 bit definition names are now obsolete. Use the WGM21:0 definitions.

However, the functionality and location of these bits are compatible with previous versions of

the timer.

• Bit 5:4 – COM21:0: Compare Match Output Mode

These bits control the Output Compare pin (OC2) behavior. If one or both of the COM21:0 bits

are set, the OC2 output overrides the normal port functionality of the I/O pin it is connected to.

However, note that the Data Direction Register (DDR) bit corresponding to OC2 pin must be set

in order to enable the output driver.

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When OC2 is connected to the pin, the function of the COM21:0 bits depends on the WGM21:0

bit setting. Table 51 shows the COM21:0 bit functionality when the WGM21:0 bits are set to a

normal or CTC mode (non-PWM).

Table 51. Compare Output Mode, non-PWM Mode

COM21

COM20

Description

0

1

0

1

0

1

Normal port operation, OC2 disconnected.

Toggle OC2 on compare match

Clear OC2 on compare match

Set OC2 on compare match

Table 52 shows the COM21:0 bit functionality when the WGM21:0 bits are set to fast PWM

mode.

Table 52. Compare Output Mode, Fast PWM Mode⁽¹⁾

COM21

COM20

Description

0

1

0

1

0

Normal port operation, OC2 disconnected.

Reserved

Clear OC2 on compare match, set OC2 at BOTTOM,

(non-inverting mode)

1

Set OC2 on compare match, clear OC2 at BOTTOM,

(inverting mode)

Note:

1. A special case occurs when OCR2 equals TOP and COM21 is set. In this case, the compare

match is ignored, but the set or clear is done at BOTTOM. See “Fast PWM Mode” on page 123

for more details.

Table 53 shows the COM21:0 bit functionality when the WGM21:0 bits are set to phase correct

PWM mode

.

Table 53. Compare Output Mode, Phase Correct PWM Mode⁽¹⁾

COM21 COM20 Description

0

1

0

1

0

Normal port operation, OC2 disconnected.

Reserved

Clear OC2 on compare match when up-counting. Set OC2 on compare

match when downcounting.

1

Set OC2 on compare match when up-counting. Clear OC2 on compare

match when downcounting.

Note:

1. A special case occurs when OCR2 equals TOP and COM21 is set. In this case, the compare

match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page

124 for more details.

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• Bit 2:0 – CS22:0: Clock Select

The three Clock Select bits select the clock source to be used by the Timer/Counter, see Table

54.

Table 54. Clock Select Bit Description

CS22

CS21

CS20

Description

0

1

0

1

0

1

0

1

0

1

0

1

0

1

No clock source (Timer/Counter stopped).

clk_T2S/(No prescaling)

clk_T2S/8 (From prescaler)

clk_T2S/32 (From prescaler)

clk_T2S/64 (From prescaler)

clk_T2S/128 (From prescaler)

clk_{T S}/256 (From prescaler)

2

clk_{T S}/1024 (From prescaler)

2

Timer/Counter

Register – TCNT2

Bit

7

6

5

4

3

2

1

0

TCNT2[7:0]

TCNT2

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

The Timer/Counter Register gives direct access, both for read and write operations, to the

Timer/Counter unit 8-bit counter. Writing to the TCNT2 Register blocks (removes) the compare

match on the following timer clock. Modifying the counter (TCNT2) while the counter is running,

introduces a risk of missing a compare match between TCNT2 and the OCR2 Register.

Output Compare

Register – OCR2

Bit

7

6

5

4

3

2

1

0

OCR2[7:0]

OCR2

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

The Output Compare Register contains an 8-bit value that is continuously compared with the

counter value (TCNT2). A match can be used to generate an output compare interrupt, or to

generate a waveform output on the OC2 pin.

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Asynchronous

Operation of the

Timer/Counter

Asynchronous Status

Register – ASSR

Bit

7

–

6

–

5

–

4

–

3

2

1

0

AS2

R/W

0

TCN2UB

OCR2UB

TCR2UB

ASSR

Read/Write

Initial Value

R

0

R

0

R

0

R

0

R

0

R

0

R

0

• Bit 3 – AS2: Asynchronous Timer/Counter2

When AS2 is written to zero, Timer/Counter 2 is clocked from the I/O clock, clk_I/O. When AS2 is

written to one, Timer/Counter2 is clocked from a Crystal Oscillator connected to the Timer Oscil-

lator 1 (TOSC1) pin. When the value of AS2 is changed, the contents of TCNT2, OCR2, and

TCCR2 might be corrupted.

• Bit 2 – TCN2UB: Timer/Counter2 Update Busy

When Timer/Counter2 operates asynchronously and TCNT2 is written, this bit becomes set.

When TCNT2 has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that TCNT2 is ready to be updated with a new value.

• Bit 1 – OCR2UB: Output Compare Register2 Update Busy

When Timer/Counter2 operates asynchronously and OCR2 is written, this bit becomes set.

When OCR2 has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that OCR2 is ready to be updated with a new value.

• Bit 0 – TCR2UB: Timer/Counter Control Register2 Update Busy

When Timer/Counter2 operates asynchronously and TCCR2 is written, this bit becomes set.

When TCCR2 has been updated from the temporary storage register, this bit is cleared by hard-

ware. A logical zero in this bit indicates that TCCR2 is ready to be updated with a new value.

If a write is performed to any of the three Timer/Counter2 Registers while its update busy flag is

set, the updated value might get corrupted and cause an unintentional interrupt to occur.

The mechanisms for reading TCNT2, OCR2, and TCCR2 are different. When reading TCNT2,

the actual timer value is read. When reading OCR2 or TCCR2, the value in the temporary stor-

age register is read.

Asynchronous

Operation of

Timer/Counter2

When Timer/Counter2 operates asynchronously, some considerations must be taken.

•

Warning: When switching between asynchronous and synchronous clocking of

Timer/Counter2, the Timer Registers TCNT2, OCR2, and TCCR2 might be corrupted. A

safe procedure for switching clock source is:

1. Disable the Timer/Counter2 interrupts by clearing OCIE2 and TOIE2.

2. Select clock source by setting AS2 as appropriate.

3. Write new values to TCNT2, OCR2, and TCCR2.

4. To switch to asynchronous operation: Wait for TCN2UB, OCR2UB, and TCR2UB.

5. Clear the Timer/Counter2 Interrupt Flags.

6. Enable interrupts, if needed.

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•

The Oscillator is optimized for use with a 32.768 kHz watch crystal. Applying an external

clock to the TOSC1 pin may result in incorrect Timer/Counter2 operation. The CPU main

clock frequency must be more than four times the Oscillator frequency.

When writing to one of the registers TCNT2, OCR2, or TCCR2, the value is transferred to a

temporary register, and latched after two positive edges on TOSC1. The user should not

write a new value before the contents of the temporary register have been transferred to its

destination. Each of the three mentioned registers have their individual temporary register,

which means for example that writing to TCNT2 does not disturb an OCR2 write in progress.

To detect that a transfer to the destination register has taken place, the Asynchronous Status

Register – ASSR has been implemented.

•

When entering Power-save or Extended Standby mode after having written to TCNT2,

OCR2, or TCCR2, the user must wait until the written register has been updated if

Timer/Counter2 is used to wake up the device. Otherwise, the MCU will enter sleep mode

before the changes are effective. This is particularly important if the Output Compare2

interrupt is used to wake up the device, since the output compare function is disabled during

writing to OCR2 or TCNT2. If the write cycle is not finished, and the MCU enters sleep mode

before the OCR2UB bit returns to zero, the device will never receive a compare match

interrupt, and the MCU will not wake up.

If Timer/Counter2 is used to wake the device up from Power-save or Extended Standby

mode, precautions must be taken if the user wants to re-enter one of these modes: The

interrupt logic needs one TOSC1 cycle to be reset. If the time between wake-up and re-

entering sleep mode is less than one TOSC1 cycle, the interrupt will not occur, and the

device will fail to wake up. If the user is in doubt whether the time before re-entering Power-

save or Extended Standby mode is sufficient, the following algorithm can be used to ensure

that one TOSC1 cycle has elapsed:

1. Write a value to TCCR2, TCNT2, or OCR2.

2. Wait until the corresponding Update Busy Flag in ASSR returns to zero.

3. Enter Power-save or Extended Standby mode.

•

When the asynchronous operation is selected, the 32.768 kHz Oscillator for Timer/Counter2

is always running, except in Power-down and Standby modes. After a Power-up Reset or

wake-up from Power-down or Standby mode, the user should be aware of the fact that this

Oscillator might take as long as one second to stabilize. The user is advised to wait for at

least one second before using Timer/Counter2 after power-up or wake-up from Power-down

or Standby mode. The contents of all Timer/Counter2 Registers must be considered lost

after a wake-up from Power-down or Standby mode due to unstable clock signal upon start-

up, no matter whether the Oscillator is in use or a clock signal is applied to the TOSC1 pin.

•

Description of wake up from Power-save or Extended Standby mode when the timer is

clocked asynchronously: When the interrupt condition is met, the wake up process is started

on the following cycle of the timer clock, that is, the timer is always advanced by at least one

before the processor can read the counter value. After wake-up, the MCU is halted for four

cycles, it executes the interrupt routine, and resumes execution from the instruction

following SLEEP.

Reading of the TCNT2 Register shortly after wake-up from Power-save may give an

incorrect result. Since TCNT2 is clocked on the asynchronous TOSC clock, reading TCNT2

must be done through a register synchronized to the internal I/O clock domain.

Synchronization takes place for every rising TOSC1 edge. When waking up from Power-

save mode, and the I/O clock (clk_I/O) again becomes active, TCNT2 will read as the previous

value (before entering sleep) until the next rising TOSC1 edge. The phase of the TOSC

clock after waking up from Power-save mode is essentially unpredictable, as it depends on

the wake-up time. The recommended procedure for reading TCNT2 is thus as follows:

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1. Write any value to either of the registers OCR2 or TCCR2.

2. Wait for the corresponding Update Busy Flag to be cleared.

3. Read TCNT2.

•

During asynchronous operation, the synchronization of the Interrupt Flags for the

asynchronous timer takes three processor cycles plus one timer cycle. The timer is therefore

advanced by at least one before the processor can read the timer value causing the setting

of the Interrupt Flag. The output compare pin is changed on the timer clock and is not

synchronized to the processor clock.

Timer/Counter

Interrupt Mask

Register – TIMSK

Bit

7

OCIE2

R/W

0

6

TOIE2

R/W

0

5

TICIE1

R/W

0

4

OCIE1A

R/W

0

3

OCIE1B

R/W

0

2

TOIE1

R/W

0

1

OCIE0

R/W

0

TOIE0

R/W

0

TIMSK

Read/Write

Initial Value

• Bit 7 – OCIE2: Timer/Counter2 Output Compare Match Interrupt Enable

When the OCIE2 bit is written to one and the I-bit in the Status Register is set (one), the

Timer/Counter2 Compare Match interrupt is enabled. The corresponding interrupt is executed if

a compare match in Timer/Counter2 occurs, that is, when the OCF2 bit is set in the Timer/Coun-

ter Interrupt Flag Register – TIFR.

• Bit 6 – TOIE2: Timer/Counter2 Overflow Interrupt Enable

When the TOIE2 bit is written to one and the I-bit in the Status Register is set (one), the

Timer/Counter2 Overflow interrupt is enabled. The corresponding interrupt is executed if an

overflow in Timer/Counter2 occurs, that is, when the TOV2 bit is set in the Timer/Counter Inter-

rupt Flag Register – TIFR.

Timer/Counter

Interrupt Flag Register

– TIFR

Bit

7

OCF2

R/W

0

6

TOV2

R/W

0

5

4

OCF1A

R/W

0

3

OCF1B

R/W

0

2

TOV1

R/W

0

1

OCF0

R/W

0

TOV0

R/W

0

ICF1

R/W

0

TIFR

Read/Write

Initial Value

• Bit 7 – OCF2: Output Compare Flag 2

The OCF2 bit is set (one) when a compare match occurs between the Timer/Counter2 and the

data in OCR2 – Output Compare Register2. OCF2 is cleared by hardware when executing the

corresponding interrupt handling vector. Alternatively, OCF2 is cleared by writing a logic one to

the flag. When the I-bit in SREG, OCIE2 (Timer/Counter2 Compare match Interrupt Enable), and

OCF2 are set (one), the Timer/Counter2 Compare match Interrupt is executed.

• Bit 6 – TOV2: Timer/Counter2 Overflow Flag

The TOV2 bit is set (one) when an overflow occurs in Timer/Counter2. TOV2 is cleared by hard-

ware when executing the corresponding interrupt handling vector. Alternatively, TOV2 is cleared

by writing a logic one to the flag. When the SREG I-bit, TOIE2 (Timer/Counter2 Overflow Inter-

rupt Enable), and TOV2 are set (one), the Timer/Counter2 Overflow interrupt is executed. In

PWM mode, this bit is set when Timer/Counter2 changes counting direction at $00.

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Timer/Counter

Prescaler

Figure 64. Prescaler for Timer/Counter2

clk_I/O

clk_T2S

10-BIT T/C PRESCALER

Clear

TOSC1

AS2

PSR2

0

CS20

CS21

CS22

TIMER/COUNTER2 CLOCK SOURCE

clk_T2

The clock source for Timer/Counter2 is named clk_T2S. clk_T2Sis by default connected to the main

system I/O clock clk_IO. By setting the AS2 bit in ASSR, Timer/Counter2 is asynchronously

clocked from the TOSC1 pin. This enables use of Timer/Counter2 as a Real Time Counter

(RTC). When AS2 is set, pins TOSC1 and TOSC2 are disconnected from Port C. A crystal can

then be connected between the TOSC1 and TOSC2 pins to serve as an independent clock

source for Timer/Counter2. The Oscillator is optimized for use with a 32.768 kHz crystal. Apply-

ing an external clock source to TOSC1 is not recommended.

For Timer/Counter2, the possible prescaled selections are: clk_T2S/8, clk_T2S/32, clk_T2S/64,

clk_T2S/128, clk_T2S/256, and clk_T2S/1024. Additionally, clk_T2Sas well as 0 (stop) may be selected.

Setting the PSR2 bit in SFIOR resets the prescaler. This allows the user to operate with a pre-

dictable prescaler.

Special Function IO

Register – SFIOR

Bit

7

ADTS2

R/W

0

6

ADTS1

R/W

0

5

ADTS0

R/W

0

4

–

3

ACME

R/W

0

2

1

PSR2

R/W

0

PSR10

R/W

0

PUD

R/W

0

SFIOR

Read/Write

Initial Value

R

0

• Bit 1 – PSR2: Prescaler Reset Timer/Counter2

When this bit is written to one, the Timer/Counter2 prescaler will be reset. The bit will be cleared

by hardware after the operation is performed. Writing a zero to this bit will have no effect. This bit

will always be read as zero if Timer/Counter2 is clocked by the internal CPU clock. If this bit is

written when Timer/Counter2 is operating in asynchronous mode, the bit will remain one until the

prescaler has been reset.

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Serial

Peripheral

Interface – SPI

The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the

ATmega16 and peripheral devices or between several AVR devices. The ATmega16 SPI

includes the following features:

• Full-duplex, Three-wire Synchronous Data Transfer

• Master or Slave Operation

• LSB First or MSB First Data Transfer

• Seven Programmable Bit Rates

• End of Transmission Interrupt Flag

• Write Collision Flag Protection

• Wake-up from Idle Mode

• Double Speed (CK/2) Master SPI Mode

Figure 65. SPI Block Diagram⁽¹⁾

DIVIDER

/2/4/8/16/32/64/128

Note:

1. Refer to Figure 1 on page 2, and Table 25 on page 58 for SPI pin placement.

The interconnection between Master and Slave CPUs with SPI is shown in Figure 66. The sys-

tem consists of two Shift Registers, and a Master clock generator. The SPI Master initiates the

communication cycle when pulling low the Slave Select SS pin of the desired Slave. Master and

Slave prepare the data to be sent in their respective Shift Registers, and the Master generates

the required clock pulses on the SCK line to interchange data. Data is always shifted from Mas-

ter to Slave on the Master Out – Slave In, MOSI, line, and from Slave to Master on the Master In

– Slave Out, MISO, line. After each data packet, the Master will synchronize the Slave by pulling

high the Slave Select, SS, line.

When configured as a Master, the SPI interface has no automatic control of the SS line. This

must be handled by user software before communication can start. When this is done, writing a

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byte to the SPI Data Register starts the SPI clock generator, and the hardware shifts the eight

bits into the Slave. After shifting one byte, the SPI clock generator stops, setting the end of

Transmission Flag (SPIF). If the SPI Interrupt Enable bit (SPIE) in the SPCR Register is set, an

interrupt is requested. The Master may continue to shift the next byte by writing it into SPDR, or

signal the end of packet by pulling high the Slave Select, SS line. The last incoming byte will be

kept in the Buffer Register for later use.

When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated as long

as the SS pin is driven high. In this state, software may update the contents of the SPI Data

Register, SPDR, but the data will not be shifted out by incoming clock pulses on the SCK pin

until the SS pin is driven low. As one byte has been completely shifted, the end of Transmission

Flag, SPIF is set. If the SPI Interrupt Enable bit, SPIE, in the SPCR Register is set, an interrupt

is requested. The Slave may continue to place new data to be sent into SPDR before reading

the incoming data. The last incoming byte will be kept in the Buffer Register for later use.

Figure 66. SPI Master-Slave Interconnection

MSB

MASTER

LSB

MSB

SLAVE

LSB

MISO

MOSI

MISO

MOSI

8 BIT SHIFT REGISTER

SHIFT

ENABLE

SPI

SCK

SS

SCK

SS

CLOCK GENERATOR

The system is single buffered in the transmit direction and double buffered in the receive direc-

tion. This means that bytes to be transmitted cannot be written to the SPI Data Register before

the entire shift cycle is completed. When receiving data, however, a received character must be

read from the SPI Data Register before the next character has been completely shifted in. Oth-

erwise, the first byte is lost.

In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure

correct sampling of the clock signal, the minimum low and high periods should be:

Low periods: Longer than 2 CPU clock cycles.

High periods: Longer than 2 CPU clock cycles.

When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden

according to Table 55 on page 136. For more details on automatic port overrides, refer to “Alter-

nate Port Functions” on page 55.

Table 55. SPI Pin Overrides

Direction, Master SPI

User Defined

Input

Direction, Slave SPI

MOSI

MISO

SCK

SS

Input

User Defined

Input

User Defined

Input

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Note:

See “Alternate Functions of Port B” on page 58 for a detailed description of how to define the

direction of the user defined SPI pins.

The following code examples show how to initialize the SPI as a Master and how to perform a

simple transmission. DDR_SPI in the examples must be replaced by the actual Data Direction

Register controlling the SPI pins. DD_MOSI, DD_MISO and DD_SCK must be replaced by the

actual data direction bits for these pins. For example if MOSI is placed on pin PB5, replace

DD_MOSI with DDB5 and DDR_SPI with DDRB.

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Assembly Code Example⁽¹⁾

SPI_MasterInit:

; Set MOSI and SCK output, all others input

ldi

out

r17,(1<<DD_MOSI)|(1<<DD_SCK)

DDR_SPI,r17

; Enable SPI, Master, set clock rate fck/16

ldi

out

ret

r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0)

SPCR,r17

SPI_MasterTransmit:

; Start transmission of data (r16)

out

SPDR,r16

Wait_Transmit:

; Wait for transmission complete

sbis SPSR,SPIF

rjmp Wait_Transmit

ret

C Code Example⁽¹⁾

void SPI_MasterInit(void)

{

/* Set MOSI and SCK output, all others input */

DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK);

/* Enable SPI, Master, set clock rate fck/16 */

SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);

}

void SPI_MasterTransmit(char cData)

{

/* Start transmission */

SPDR = cData;

/* Wait for transmission complete */

while(!(SPSR & (1<<SPIF)))

;

}

Note:

1. See “About Code Examples” on page 7.

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The following code examples show how to initialize the SPI as a Slave and how to perform a

simple reception.

Assembly Code Example⁽¹⁾

SPI_SlaveInit:

; Set MISO output, all others input

ldi

out

r17,(1<<DD_MISO)

DDR_SPI,r17

; Enable SPI

ldi

out

ret

r17,(1<<SPE)

SPCR,r17

SPI_SlaveReceive:

; Wait for reception complete

sbis SPSR,SPIF

rjmp SPI_SlaveReceive

; Read received data and return

in

r16,SPDR

ret

C Code Example⁽¹⁾

void SPI_SlaveInit(void)

{

/* Set MISO output, all others input */

DDR_SPI = (1<<DD_MISO);

/* Enable SPI */

SPCR = (1<<SPE);

}

char SPI_SlaveReceive(void)

{

/* Wait for reception complete */

while(!(SPSR & (1<<SPIF)))

;

/* Return data register */

return SPDR;

}

Note:

1. See “About Code Examples” on page 7.

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SS Pin

Functionality

Slave Mode

When the SPI is configured as a Slave, the Slave Select (SS) pin is always input. When SS is

held low, the SPI is activated, and MISO becomes an output if configured so by the user. All

other pins are inputs. When SS is driven high, all pins are inputs except MISO which can be user

configured as an output, and the SPI is passive, which means that it will not receive incoming

data. Note that the SPI logic will be reset once the SS pin is driven high.

The SS pin is useful for packet/byte synchronization to keep the Slave Bit Counter synchronous

with the Master Clock generator. When the SS pin is driven high, the SPI Slave will immediately

reset the send and receive logic, and drop any partially received data in the Shift Register.

Master Mode

When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the

direction of the SS pin.

If SS is configured as an output, the pin is a general output pin which does not affect the SPI

system. Typically, the pin will be driving the SS pin of the SPI Slave.

If SS is configured as an input, it must be held high to ensure Master SPI operation. If the SS pin

is driven low by peripheral circuitry when the SPI is configured as a Master with the SS pin

defined as an input, the SPI system interprets this as another Master selecting the SPI as a

Slave and starting to send data to it. To avoid bus contention, the SPI system takes the following

actions:

1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a result of

the SPI becoming a Slave, the MOSI and SCK pins become inputs.

2. The SPIF Flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in SREG is

set, the interrupt routine will be executed.

Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possi-

bility that SS is driven low, the interrupt should always check that the MSTR bit is still set. If the

MSTR bit has been cleared by a Slave Select, it must be set by the user to re-enable SPI Master

mode.

SPI Control Register –

SPCR

Bit

7

SPIE

R/W

0

6

5

DORD

R/W

0

4

MSTR

R/W

0

3

CPOL

R/W

0

2

CPHA

R/W

0

1

SPR1

R/W

0

SPR0

R/W

0

SPE

R/W

0

SPCR

Read/Write

Initial Value

• Bit 7 – SPIE: SPI Interrupt Enable

This bit causes the SPI interrupt to be executed if SPIF bit in the SPSR Register is set and the if

the global interrupt enable bit in SREG is set.

• Bit 6 – SPE: SPI Enable

When the SPE bit is written to one, the SPI is enabled. This bit must be set to enable any SPI

operations.

• Bit 5 – DORD: Data Order

When the DORD bit is written to one, the LSB of the data word is transmitted first.

When the DORD bit is written to zero, the MSB of the data word is transmitted first.

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• Bit 4 – MSTR: Master/Slave Select

This bit selects Master SPI mode when written to one, and Slave SPI mode when written logic

zero. If SS is configured as an input and is driven low while MSTR is set, MSTR will be cleared,

and SPIF in SPSR will become set. The user will then have to set MSTR to re-enable SPI Mas-

ter mode.

• Bit 3 – CPOL: Clock Polarity

When this bit is written to one, SCK is high when idle. When CPOL is written to zero, SCK is low

when idle. Refer to Figure 67 and Figure 68 for an example. The CPOL functionality is summa-

rized below:

Table 56. CPOL Functionality

CPOL

Leading Edge

Rising

Trailing Edge

Falling

0

1

Falling

Rising

• Bit 2 – CPHA: Clock Phase

The settings of the Clock Phase bit (CPHA) determine if data is sampled on the leading (first) or

trailing (last) edge of SCK. Refer to Figure 67 and Figure 68 for an example. The CPHA func-

tionality is summarized below:

Table 57. CPHA Functionality

CPHA

Leading Edge

Sample

Trailing Edge

Setup

0

1

Setup

Sample

• Bits 1, 0 – SPR1, SPR0: SPI Clock Rate Select 1 and 0

These two bits control the SCK rate of the device configured as a Master. SPR1 and SPR0 have

no effect on the Slave. The relationship between SCK and the Oscillator Clock frequency f_oscis

shown in the following table:

Table 58. Relationship Between SCK and the Oscillator Frequency

SPI2X

SPR1

SPR0

SCK Frequency

f_osc/4

0

1

0

1

0

1

0

1

0

1

0

1

0

1

f_osc/16

f_osc/64

f_osc/128

f_osc/2

f_osc/8

f_osc/32

f_osc/64

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SPI Status Register –

SPSR

Bit

7

SPIF

R

6

5

–

4

–

3

–

2

–

1

–

0

WCOL

SPI2X

R/W

0

SPSR

Read/Write

Initial Value

R

0

R

0

R

0

R

0

R

0

R

0

• Bit 7 – SPIF: SPI Interrupt Flag

When a serial transfer is complete, the SPIF Flag is set. An interrupt is generated if SPIE in

SPCR is set and global interrupts are enabled. If SS is an input and is driven low when the SPI is

in Master mode, this will also set the SPIF Flag. SPIF is cleared by hardware when executing the

corresponding interrupt handling vector. Alternatively, the SPIF bit is cleared by first reading the

SPI Status Register with SPIF set, then accessing the SPI Data Register (SPDR).

• Bit 6 – WCOL: Write COLlision Flag

The WCOL bit is set if the SPI Data Register (SPDR) is written during a data transfer. The

WCOL bit (and the SPIF bit) are cleared by first reading the SPI Status Register with WCOL set,

and then accessing the SPI Data Register.

• Bit 5..1 – Res: Reserved Bits

These bits are reserved bits in the ATmega16 and will always read as zero.

• Bit 0 – SPI2X: Double SPI Speed Bit

When this bit is written logic one the SPI speed (SCK Frequency) will be doubled when the SPI

is in Master mode (see Table 58). This means that the minimum SCK period will be two CPU

clock periods. When the SPI is configured as Slave, the SPI is only guaranteed to work at f_osc/4

or lower.

The SPI interface on the ATmega16 is also used for program memory and EEPROM download-

ing or uploading. See page 273 for SPI Serial Programming and Verification.

SPI Data Register –

SPDR

Bit

7

6

5

4

3

2

1

0

MSB

R/W

X

LSB

R/W

X

SPDR

Read/Write

Initial Value

R/W

X

R/W

X

R/W

X

R/W

X

R/W

X

R/W

X

Undefined

The SPI Data Register is a read/write register used for data transfer between the Register File

and the SPI Shift Register. Writing to the register initiates data transmission. Reading the regis-

ter causes the Shift Register Receive buffer to be read.

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Data Modes

There are four combinations of SCK phase and polarity with respect to serial data, which are

determined by control bits CPHA and CPOL. The SPI data transfer formats are shown in Figure

67 and Figure 68. Data bits are shifted out and latched in on opposite edges of the SCK signal,

ensuring sufficient time for data signals to stabilize. This is clearly seen by summarizing Table

56 and Table 57, as done below:

Table 59. CPOL and CPHA Functionality

Leading Edge

Sample (Rising)

Setup (Rising)

Sample (Falling)

Setup (Falling)

Trailing Edge

Setup (Falling)

Sample (Falling)

Setup (Rising)

Sample (Rising)

SPI Mode

CPOL = 0, CPHA = 0

CPOL = 0, CPHA = 1

CPOL = 1, CPHA = 0

CPOL = 1, CPHA = 1

0

1

2

3

Figure 67. SPI Transfer Format with CPHA = 0

SCK (CPOL = 0)

mode 0

SCK (CPOL = 1)

mode 2

SAMPLE I

MOSI/MISO

CHANGE 0

MOSI PIN

CHANGE 0

MISO PIN

SS

MSB first (DORD = 0) MSB

LSB first (DORD = 1) LSB

Bit 6

Bit 1

Bit 5

Bit 2

Bit 4

Bit 3

Bit 4

Bit 2

Bit 5

Bit 1

Bit 6

LSB

MSB

Figure 68. SPI Transfer Format with CPHA = 1

SCK (CPOL = 0)

mode 1

SCK (CPOL = 1)

mode 3

SAMPLE I

MOSI/MISO

CHANGE 0

MOSI PIN

CHANGE 0

MISO PIN

SS

MSB first (DORD = 0)

LSB first (DORD = 1)

MSB

LSB

Bit 6

Bit 1

Bit 5

Bit 2

Bit 4

Bit 3

Bit 4

Bit 2

Bit 5

Bit 1

Bit 6

LSB

MSB

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USART

The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a

highly flexible serial communication device. The main features are:

• Full Duplex Operation (Independent Serial Receive and Transmit Registers)

• Asynchronous or Synchronous Operation

• Master or Slave Clocked Synchronous Operation

• High Resolution Baud Rate Generator

• Supports Serial Frames with 5, 6, 7, 8, or 9 Data Bits and 1 or 2 Stop Bits

• Odd or Even Parity Generation and Parity Check Supported by Hardware

• Data OverRun Detection

• Framing Error Detection

• Noise Filtering Includes False Start Bit Detection and Digital Low Pass Filter

• Three Separate Interrupts on TX Complete, TX Data Register Empty, and RX Complete

• Multi-processor Communication Mode

• Double Speed Asynchronous Communication Mode

Overview

A simplified block diagram of the USART transmitter is shown in Figure 69. CPU accessible I/O

Registers and I/O pins are shown in bold.

Figure 69. USART Block Diagram⁽¹⁾

Clock Generator

UBRR[H:L]

OSC

BAUD RATE GENERATOR

SYNC LOGIC

XCK

CONTROL

Transmitter

TX

CONTROL

UDR (Transmit)

PARITY

GENERATOR

CONTROL

TRANSMIT SHIFT REGISTER

TxD

Receiver

CLOCK

RX

RECOVERY

CONTROL

DATA

RECOVERY

CONTROL

RECEIVE SHIFT REGISTER

RxD

PARITY

CHECKER

UDR (Receive)

UCSRA

UCSRB

UCSRC

Note:

1. Refer to Figure 1 on page 2, Table 33 on page 65, and Table 27 on page 60 for USART pin

placement.

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The dashed boxes in the block diagram separate the three main parts of the USART (listed from

the top): Clock Generator, Transmitter and Receiver. Control Registers are shared by all units.

The clock generation logic consists of synchronization logic for external clock input used by syn-

chronous Slave operation, and the baud rate generator. The XCK (Transfer Clock) pin is only

used by Synchronous Transfer mode. The Transmitter consists of a single write buffer, a serial

Shift Register, parity generator and control logic for handling different serial frame formats. The

write buffer allows a continuous transfer of data without any delay between frames. The

Receiver is the most complex part of the USART module due to its clock and data recovery

units. The recovery units are used for asynchronous data reception. In addition to the recovery

units, the receiver includes a parity checker, control logic, a Shift Register and a two level

receive buffer (UDR). The receiver supports the same frame formats as the transmitter, and can

detect frame error, data overrun and parity errors.

AVR USART vs. AVR

UART – Compatibility

The USART is fully compatible with the AVR UART regarding:

•

Bit locations inside all USART Registers

Baud Rate Generation

Transmitter Operation

Transmit Buffer Functionality

Receiver Operation

However, the receive buffering has two improvements that will affect the compatibility in some

special cases:

•

A second Buffer Register has been added. The two Buffer Registers operate as a circular

FIFO buffer. Therefore the UDR must only be read once for each incoming data! More

important is the fact that the Error Flags (FE and DOR) and the 9th data bit (RXB8) are

buffered with the data in the receive buffer. Therefore the status bits must always be read

before the UDR Register is read. Otherwise the error status will be lost since the buffer state

is lost.

•

The receiver Shift Register can now act as a third buffer level. This is done by allowing the

received data to remain in the serial Shift Register (see Figure 69) if the Buffer Registers are

full, until a new start bit is detected. The USART is therefore more resistant to Data OverRun

(DOR) error conditions.

The following control bits have changed name, but have same functionality and register location:

•

CHR9 is changed to UCSZ2

OR is changed to DOR

Clock Generation

The clock generation logic generates the base clock for the Transmitter and Receiver. The

USART supports four modes of clock operation: Normal Asynchronous, Double Speed Asyn-

chronous, Master Synchronous and Slave Synchronous mode. The UMSEL bit in USART

Control and Status Register C (UCSRC) selects between asynchronous and synchronous oper-

ation. Double Speed (Asynchronous mode only) is controlled by the U2X found in the UCSRA

Register. When using Synchronous mode (UMSEL = 1), the Data Direction Register for the XCK

pin (DDR_XCK) controls whether the clock source is internal (Master mode) or external (Slave

mode). The XCK pin is only active when using Synchronous mode.

Figure 70 shows a block diagram of the clock generation logic.

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Figure 70. Clock Generation Logic, Block Diagram

UBRR

U2X

fosc

UBRR+1

Prescaling

Down-Counter

/ 2

/ 4

/ 2

0

1

0

OSC

txclk

1

DDR_XCK

Sync

Edge

Register

Detector

xcki

0

UMSEL

1

XCK

xcko

DDR_XCK

UCPOL

1

rxclk

0

Signal description:

txclk Transmitter clock (Internal Signal).

rxclk Receiver base clock (Internal Signal).

xcki

Input from XCK pin (Internal Signal). Used for synchronous Slave operation.

xcko Clock output to XCK pin (Internal Signal). Used for synchronous Master

operation.

fosc

XTAL pin frequency (System Clock).

Internal Clock

Generation – The

Baud Rate Generator

Internal clock generation is used for the asynchronous and the synchronous Master modes of

operation. The description in this section refers to Figure 70.

The USART Baud Rate Register (UBRR) and the down-counter connected to it function as a

programmable prescaler or baud rate generator. The down-counter, running at system clock

(fosc), is loaded with the UBRR value each time the counter has counted down to zero or when

the UBRRL Register is written. A clock is generated each time the counter reaches zero. This

clock is the baud rate generator clock output (= fosc/(UBRR+1)). The Transmitter divides the

baud rate generator clock output by 2, 8 or 16 depending on mode. The baud rate generator out-

put is used directly by the receiver’s clock and data recovery units. However, the recovery units

use a state machine that uses 2, 8 or 16 states depending on mode set by the state of the

UMSEL, U2X and DDR_XCK bits.

Table 60 contains equations for calculating the baud rate (in bits per second) and for calculating

the UBRR value for each mode of operation using an internally generated clock source.

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Table 60. Equations for Calculating Baud Rate Register Setting

Equation for

Calculating UBRR

Value

Equation for Calculating

Operating Mode

Baud Rate⁽¹⁾

Asynchronous Normal Mode

(U2X = 0)

f

OSC

BAUD = -------------------------------------- UBRR = ----------------------- – 1

16(UBRR + 1)

16BAUD

Asynchronous Double Speed Mode

(U2X = 1)

f

OSC

BAUD = ----------------------------------- UBRR = -------------------- – 1

8(UBRR + 1)

8BAUD

Synchronous Master Mode

f

OSC

BAUD = ----------------------------------- UBRR = -------------------- – 1

2(UBRR + 1)

1. The baud rate is defined to be the transfer rate in bit per second (bps).

BAUD Baud rate (in bits per second, bps)

f_OSCSystem Oscillator clock frequency

UBRR Contents of the UBRRH and UBRRL Registers, (0 - 4095)

2BAUD

Note:

Some examples of UBRR values for some system clock frequencies are found in Table 68 (see

page 168).

Double Speed

Operation (U2X)

The transfer rate can be doubled by setting the U2X bit in UCSRA. Setting this bit only has effect

for the asynchronous operation. Set this bit to zero when using synchronous operation.

Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling

the transfer rate for asynchronous communication. Note however that the receiver will in this

case only use half the number of samples (reduced from 16 to 8) for data sampling and clock

recovery, and therefore a more accurate baud rate setting and system clock are required when

this mode is used. For the Transmitter, there are no downsides.

External Clock

External clocking is used by the synchronous Slave modes of operation. The description in this

section refers to Figure 70 for details.

External clock input from the XCK pin is sampled by a synchronization register to minimize the

chance of meta-stability. The output from the synchronization register must then pass through

an edge detector before it can be used by the Transmitter and receiver. This process introduces

a two CPU clock period delay and therefore the maximum external XCK clock frequency is lim-

ited by the following equation:

f

OSC

-----------

f

<

XCK

4

Note that f_oscdepends on the stability of the system clock source. It is therefore recommended to

add some margin to avoid possible loss of data due to frequency variations.

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Synchronous Clock

Operation

When Synchronous mode is used (UMSEL = 1), the XCK pin will be used as either clock input

(Slave) or clock output (Master). The dependency between the clock edges and data sampling

or data change is the same. The basic principle is that data input (on RxD) is sampled at the

opposite XCK clock edge of the edge the data output (TxD) is changed.

Figure 71. Synchronous Mode XCK Timing.

UCPOL = 1

XCK

RxD / TxD

Sample

UCPOL = 0

XCK

RxD / TxD

The UCPOL bit UCRSC selects which XCK clock edge is used for data sampling and which is

used for data change. As Figure 71 shows, when UCPOL is zero the data will be changed at ris-

ing XCK edge and sampled at falling XCK edge. If UCPOL is set, the data will be changed at

falling XCK edge and sampled at rising XCK edge.

Frame Formats

A serial frame is defined to be one character of data bits with synchronization bits (start and stop

bits), and optionally a parity bit for error checking. The USART accepts all 30 combinations of

the following as valid frame formats:

•

1 start bit

5, 6, 7, 8, or 9 data bits

no, even or odd parity bit

1 or 2 stop bits

A frame starts with the start bit followed by the least significant data bit. Then the next data bits,

up to a total of nine, are succeeding, ending with the most significant bit. If enabled, the parity bit

is inserted after the data bits, before the stop bits. When a complete frame is transmitted, it can

be directly followed by a new frame, or the communication line can be set to an idle (high) state.

Figure 72 illustrates the possible combinations of the frame formats. Bits inside brackets are

optional.

Figure 72. Frame Formats

FRAME

(IDLE)

St

0

1

2

3

4

[5] [6] [7] [8] [P] Sp1 [Sp2] (St / IDLE)

St

(n)

P

Start bit, always low.

Data bits (0 to 8).

Parity bit. Can be odd or even.

Stop bit, always high.

Sp

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IDLE No transfers on the communication line (RxD or TxD). An IDLE line must be

high.

The frame format used by the USART is set by the UCSZ2:0, UPM1:0, and USBS bits in

UCSRB and UCSRC. The Receiver and Transmitter use the same setting. Note that changing

the setting of any of these bits will corrupt all ongoing communication for both the Receiver and

Transmitter.

The USART Character SiZe (UCSZ2:0) bits select the number of data bits in the frame. The

USART Parity mode (UPM1:0) bits enable and set the type of parity bit. The selection between

one or two stop bits is done by the USART Stop Bit Select (USBS) bit. The receiver ignores the

second stop bit. An FE (Frame Error) will therefore only be detected in the cases where the first

stop bit is zero.

Parity Bit Calculation

The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is used, the

result of the exclusive or is inverted. The relation between the parity bit and data bits is as

follows::

P

= d

⊕ … ⊕ d ⊕ d ⊕ d ⊕ d ⊕ 0

⊕ … ⊕ d ⊕ d ⊕ d ⊕ d ⊕ 1

3 2 1 0

even

n – 1

3 2 1 0

odd

P_evenParity bit using even parity

P_odd

d_n

Parity bit using odd parity

Data bit n of the character

If used, the parity bit is located between the last data bit and first stop bit of a serial frame.

USART

Initialization

The USART has to be initialized before any communication can take place. The initialization pro-

cess normally consists of setting the baud rate, setting frame format and enabling the

Transmitter or the Receiver depending on the usage. For interrupt driven USART operation, the

Global Interrupt Flag should be cleared (and interrupts globally disabled) when doing the

initialization.

Before doing a re-initialization with changed baud rate or frame format, be sure that there are no

ongoing transmissions during the period the registers are changed. The TXC Flag can be used

to check that the Transmitter has completed all transfers, and the RXC Flag can be used to

check that there are no unread data in the receive buffer. Note that the TXC Flag must be

cleared before each transmission (before UDR is written) if it is used for this purpose.

The following simple USART initialization code examples show one assembly and one C func-

tion that are equal in functionality. The examples assume asynchronous operation using polling

(no interrupts enabled) and a fixed frame format. The baud rate is given as a function parameter.

For the assembly code, the baud rate parameter is assumed to be stored in the r17:r16 regis-

ters. When the function writes to the UCSRC Register, the URSEL bit (MSB) must be set due to

the sharing of I/O location by UBRRH and UCSRC.

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Assembly Code Example⁽¹⁾

USART_Init:

; Set baud rate

out

UBRRH, r17

UBRRL, r16

; Enable receiver and transmitter

ldi

out

r16, (1<<RXEN)|(1<<TXEN)

UCSRB,r16

; Set frame format: 8data, 2stop bit

ldi

out

ret

r16, (1<<URSEL)|(1<<USBS)|(3<<UCSZ0)

UCSRC,r16

C Code Example⁽¹⁾

#define FOSC 1843200// Clock Speed

#define BAUD 9600

#define MYUBRR FOSC/16/BAUD-1

void main( void )

{

...

USART_Init ( MYUBRR );

...

}

void USART_Init( unsigned int ubrr)

{

/* Set baud rate */

UBRRH = (unsigned char)(ubrr>>8);

UBRRL = (unsigned char)ubrr;

/* Enable receiver and transmitter */

UCSRB = (1<<RXEN)|(1<<TXEN);

/* Set frame format: 8data, 2stop bit */

UCSRC = (1<<URSEL)|(1<<USBS)|(3<<UCSZ0);

}

Note:

1. See “About Code Examples” on page 7.

More advanced initialization routines can be made that include frame format as parameters, dis-

able interrupts and so on. However, many applications use a fixed setting of the Baud and

Control Registers, and for these types of applications the initialization code can be placed

directly in the main routine, or be combined with initialization code for other I/O modules.

Data Transmission – The USART Transmitter

The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRB

Register. When the Transmitter is enabled, the normal port operation of the TxD pin is overrid-

den by the USART and given the function as the transmitter’s serial output. The baud rate, mode

of operation and frame format must be set up once before doing any transmissions. If synchro-

nous operation is used, the clock on the XCK pin will be overridden and used as transmission

clock.

Sending Frames with

5 to 8 Data Bit

A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The

CPU can load the transmit buffer by writing to the UDR I/O location. The buffered data in the

transmit buffer will be moved to the Shift Register when the Shift Register is ready to send a new

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frame. The Shift Register is loaded with new data if it is in idle state (no ongoing transmission) or

immediately after the last stop bit of the previous frame is transmitted. When the Shift Register is

loaded with new data, it will transfer one complete frame at the rate given by the Baud Register,

U2X bit or by XCK depending on mode of operation.

The following code examples show a simple USART transmit function based on polling of the

Data Register Empty (UDRE) Flag. When using frames with less than eight bits, the most signif-

icant bits written to the UDR are ignored. The USART has to be initialized before the function

can be used. For the assembly code, the data to be sent is assumed to be stored in Register

R16

Assembly Code Example⁽¹⁾

USART_Transmit:

; Wait for empty transmit buffer

sbis UCSRA,UDRE

rjmp USART_Transmit

; Put data (r16) into buffer, sends the data

out

ret

UDR,r16

C Code Example⁽¹⁾

void USART_Transmit( unsigned char data )

{

/* Wait for empty transmit buffer */

while ( !( UCSRA & (1<<UDRE)) )

;

/* Put data into buffer, sends the data */

UDR = data;

}

Note:

1. See “About Code Examples” on page 7.

The function simply waits for the transmit buffer to be empty by checking the UDRE Flag, before

loading it with new data to be transmitted. If the Data Register Empty Interrupt is utilized, the

interrupt routine writes the data into the buffer.

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Sending Frames with

9 Data Bit

If 9-bit characters are used (UCSZ = 7), the ninth bit must be written to the TXB8 bit in UCSRB

before the Low byte of the character is written to UDR. The following code examples show a

transmit function that handles 9-bit characters. For the assembly code, the data to be sent is

assumed to be stored in Registers R17:R16.

Assembly Code Example⁽¹⁾

USART_Transmit:

; Wait for empty transmit buffer

sbis UCSRA,UDRE

rjmp USART_Transmit

; Copy 9th bit from r17 to TXB8

cbi

UCSRB,TXB8

sbrc r17,0

sbi

UCSRB,TXB8

; Put LSB data (r16) into buffer, sends the data

out

ret

UDR,r16

C Code Example⁽¹⁾

void USART_Transmit( unsigned int data )

{

/* Wait for empty transmit buffer */

while ( !( UCSRA & (1<<UDRE))) )

;

/* Copy 9th bit to TXB8 */

UCSRB &= ~(1<<TXB8);

if ( data & 0x0100 )

UCSRB |= (1<<TXB8);

/* Put data into buffer, sends the data */

UDR = data;

}

Note:

1. These transmit functions are written to be general functions. They can be optimized if the con-

tents of the UCSRB is static. (that is, only the TXB8 bit of the UCSRB Register is used after

initialization).

The ninth bit can be used for indicating an address frame when using multi processor communi-

cation mode or for other protocol handling as for example synchronization.

Transmitter Flags and The USART transmitter has two flags that indicate its state: USART Data Register Empty

Interrupts

(UDRE) and Transmit Complete (TXC). Both flags can be used for generating interrupts.

The Data Register Empty (UDRE) Flag indicates whether the transmit buffer is ready to receive

new data. This bit is set when the transmit buffer is empty, and cleared when the transmit buffer

contains data to be transmitted that has not yet been moved into the Shift Register. For compat-

ibility with future devices, always write this bit to zero when writing the UCSRA Register.

When the Data Register empty Interrupt Enable (UDRIE) bit in UCSRB is written to one, the

USART Data Register Empty Interrupt will be executed as long as UDRE is set (provided that

global interrupts are enabled). UDRE is cleared by writing UDR. When interrupt-driven data

transmission is used, the Data Register Empty Interrupt routine must either write new data to

UDR in order to clear UDRE or disable the Data Register empty Interrupt, otherwise a new inter-

rupt will occur once the interrupt routine terminates.

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The Transmit Complete (TXC) Flag bit is set one when the entire frame in the transmit Shift

Register has been shifted out and there are no new data currently present in the transmit buffer.

The TXC Flag bit is automatically cleared when a transmit complete interrupt is executed, or it

can be cleared by writing a one to its bit location. The TXC Flag is useful in half-duplex

communication interfaces (like the RS485 standard), where a transmitting application must enter

receive mode and free the communication bus immediately after completing the transmission.

When the Transmit Compete Interrupt Enable (TXCIE) bit in UCSRB is set, the USART Transmit

Complete Interrupt will be executed when the TXC Flag becomes set (provided that global inter-

rupts are enabled). When the transmit complete interrupt is used, the interrupt handling routine

does not have to clear the TXC Flag, this is done automatically when the interrupt is executed.

Parity Generator

The parity generator calculates the parity bit for the serial frame data. When parity bit is enabled

(UPM1 = 1), the transmitter control logic inserts the parity bit between the last data bit and the

first stop bit of the frame that is sent.

Disabling the

Transmitter

The disabling of the transmitter (setting the TXEN to zero) will not become effective until ongoing

and pending transmissions are completed, that is, when the transmit Shift Register and transmit

Buffer Register do not contain data to be transmitted. When disabled, the transmitter will no lon-

ger override the TxD pin.

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Data Reception –

The USART

Receiver

The USART Receiver is enabled by writing the Receive Enable (RXEN) bit in the UCSRB Regis-

ter to one. When the receiver is enabled, the normal pin operation of the RxD pin is overridden

by the USART and given the function as the receiver’s serial input. The baud rate, mode of oper-

ation and frame format must be set up once before any serial reception can be done. If

synchronous operation is used, the clock on the XCK pin will be used as transfer clock.

Receiving Frames with The receiver starts data reception when it detects a valid start bit. Each bit that follows the start

5 to 8 Data Bits

bit will be sampled at the baud rate or XCK clock, and shifted into the receive Shift Register until

the first stop bit of a frame is received. A second stop bit will be ignored by the receiver. When

the first stop bit is received, that is, a complete serial frame is present in the receive Shift Regis-

ter, the contents of the Shift Register will be moved into the receive buffer. The receive buffer

can then be read by reading the UDR I/O location.

The following code example shows a simple USART receive function based on polling of the

Receive Complete (RXC) Flag. When using frames with less than eight bits the most significant

bits of the data read from the UDR will be masked to zero. The USART has to be initialized

before the function can be used.

Assembly Code Example⁽¹⁾

USART_Receive:

; Wait for data to be received

sbis UCSRA, RXC

rjmp USART_Receive

; Get and return received data from buffer

in

r16, UDR

ret

C Code Example⁽¹⁾

unsigned char USART_Receive( void )

{

/* Wait for data to be received */

while ( !(UCSRA & (1<<RXC)) )

;

/* Get and return received data from buffer */

return UDR;

}

Note:

1. See “About Code Examples” on page 7.

The function simply waits for data to be present in the receive buffer by checking the RXC Flag,

before reading the buffer and returning the value.

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Receiving Frames with If 9 bit characters are used (UCSZ=7) the ninth bit must be read from the RXB8 bit in UCSRB

9 Databits

before reading the low bits from the UDR. This rule applies to the FE, DOR and PE status Flags

as well. Read status from UCSRA, then data from UDR. Reading the UDR I/O location will

change the state of the receive buffer FIFO and consequently the TXB8, FE, DOR and PE bits,

which all are stored in the FIFO, will change.

The following code example shows a simple USART receive function that handles both 9-bit

characters and the status bits.

Assembly Code Example⁽¹⁾

USART_Receive:

; Wait for data to be received

sbis UCSRA, RXC

rjmp USART_Receive

; Get status and 9th bit, then data from buffer

in

r18, UCSRA

r17, UCSRB

r16, UDR

; If error, return -1

andi r18,(1<<FE)|(1<<DOR)|(1<<PE)

breq USART_ReceiveNoError

ldi

r17, HIGH(-1)

r16, LOW(-1)

USART_ReceiveNoError:

; Filter the 9th bit, then return

lsr

r17

andi r17, 0x01

ret

C Code Example⁽¹⁾

unsigned int USART_Receive( void )

{

unsigned char status, resh, resl;

/* Wait for data to be received */

while ( !(UCSRA & (1<<RXC)) )

;

/* Get status and 9th bit, then data */

/* from buffer */

status = UCSRA;

resh = UCSRB;

resl = UDR;

/* If error, return -1 */

if ( status & (1<<FE)|(1<<DOR)|(1<<PE) )

return -1;

/* Filter the 9th bit, then return */

resh = (resh >> 1) & 0x01;

return ((resh << 8) | resl);

}

Note:

1. See “About Code Examples” on page 7.

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The receive function example reads all the I/O Registers into the Register File before any com-

putation is done. This gives an optimal receive buffer utilization since the buffer location read will

be free to accept new data as early as possible.

Receive Compete Flag The USART Receiver has one flag that indicates the receiver state.

and Interrupt

The Receive Complete (RXC) Flag indicates if there are unread data present in the receive buf-

fer. This flag is one when unread data exist in the receive buffer, and zero when the receive

buffer is empty (that is, does not contain any unread data). If the receiver is disabled (RXEN =

0), the receive buffer will be flushed and consequently the RXC bit will become zero.

When the Receive Complete Interrupt Enable (RXCIE) in UCSRB is set, the USART Receive

Complete Interrupt will be executed as long as the RXC Flag is set (provided that global inter-

rupts are enabled). When interrupt-driven data reception is used, the receive complete routine

must read the received data from UDR in order to clear the RXC Flag, otherwise a new interrupt

will occur once the interrupt routine terminates.

Receiver Error Flags

The USART Receiver has three Error Flags: Frame Error (FE), Data OverRun (DOR) and Parity

Error (PE). All can be accessed by reading UCSRA. Common for the Error Flags is that they are

located in the receive buffer together with the frame for which they indicate the error status. Due

to the buffering of the Error Flags, the UCSRA must be read before the receive buffer (UDR),

since reading the UDR I/O location changes the buffer read location. Another equality for the

Error Flags is that they can not be altered by software doing a write to the flag location. How-

ever, all flags must be set to zero when the UCSRA is written for upward compatibility of future

USART implementations. None of the Error Flags can generate interrupts.

The Frame Error (FE) Flag indicates the state of the first stop bit of the next readable frame

stored in the receive buffer. The FE Flag is zero when the stop bit was correctly read (as one),

and the FE Flag will be one when the stop bit was incorrect (zero). This flag can be used for

detecting out-of-sync conditions, detecting break conditions and protocol handling. The FE Flag

is not affected by the setting of the USBS bit in UCSRC since the receiver ignores all, except for

the first, stop bits. For compatibility with future devices, always set this bit to zero when writing to

UCSRA.

The Data OverRun (DOR) Flag indicates data loss due to a receiver buffer full condition. A Data

OverRun occurs when the receive buffer is full (two characters), it is a new character waiting in

the receive Shift Register, and a new start bit is detected. If the DOR Flag is set there was one or

more serial frame lost between the frame last read from UDR, and the next frame read from

UDR. For compatibility with future devices, always write this bit to zero when writing to UCSRA.

The DOR Flag is cleared when the frame received was successfully moved from the Shift Regis-

ter to the receive buffer.

The Parity Error (PE) Flag indicates that the next frame in the receive buffer had a parity error

when received. If parity check is not enabled the PE bit will always be read zero. For compatibil-

ity with future devices, always set this bit to zero when writing to UCSRA. For more details see

“Parity Bit Calculation” on page 149 and “Parity Checker” on page 156.

Parity Checker

The Parity Checker is active when the high USART Parity mode (UPM1) bit is set. Type of parity

check to be performed (odd or even) is selected by the UPM0 bit. When enabled, the parity

checker calculates the parity of the data bits in incoming frames and compares the result with

the parity bit from the serial frame. The result of the check is stored in the receive buffer together

with the received data and stop bits. The Parity Error (PE) Flag can then be read by software to

check if the frame had a parity error.

The PE bit is set if the next character that can be read from the receive buffer had a parity error

when received and the parity checking was enabled at that point (UPM1 = 1). This bit is valid

until the receive buffer (UDR) is read.

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Disabling the Receiver In contrast to the Transmitter, disabling of the Receiver will be immediate. Data from ongoing

receptions will therefore be lost. When disabled (that is, the RXEN is set to zero) the Receiver

will no longer override the normal function of the RxD port pin. The receiver buffer FIFO will be

flushed when the receiver is disabled. Remaining data in the buffer will be lost

Flushing the Receive

Buffer

The receiver buffer FIFO will be flushed when the Receiver is disabled, that is, the buffer will be

emptied of its contents. Unread data will be lost. If the buffer has to be flushed during normal

operation, due to for instance an error condition, read the UDR I/O location until the RXC Flag is

cleared. The following code example shows how to flush the receive buffer.

Assembly Code Example⁽¹⁾

USART_Flush:

sbis UCSRA, RXC

ret

in

r16, UDR

rjmp USART_Flush

C Code Example⁽¹⁾

void USART_Flush( void )

{

unsigned char dummy;

while ( UCSRA & (1<<RXC) ) dummy = UDR;

}

Note:

1. See “About Code Examples” on page 7.

Asynchronous

Data Reception

The USART includes a clock recovery and a data recovery unit for handling asynchronous data

reception. The clock recovery logic is used for synchronizing the internally generated baud rate

clock to the incoming asynchronous serial frames at the RxD pin. The data recovery logic sam-

ples and low pass filters each incoming bit, thereby improving the noise immunity of the receiver.

The asynchronous reception operational range depends on the accuracy of the internal baud

rate clock, the rate of the incoming frames, and the frame size in number of bits.

Asynchronous Clock

Recovery

The clock recovery logic synchronizes internal clock to the incoming serial frames. Figure 73

illustrates the sampling process of the start bit of an incoming frame. The sample rate is 16 times

the baud rate for Normal mode, and 8 times the baud rate for Double Speed mode. The horizon-

tal arrows illustrate the synchronization variation due to the sampling process. Note the larger

time variation when using the double speed mode (U2X = 1) of operation. Samples denoted zero

are samples done when the RxD line is idle (that is, no communication activity).

Figure 73. Start Bit Sampling

RxD

IDLE

START

BIT 0

Sample

(U2X = 0)

0

1

2

3

2

4

5

3

6

7

4

8

9

5

10

11

6

12

13

7

14

15

8

16

1

2

3

Sample

(U2X = 1)

0

2

When the clock recovery logic detects a high (idle) to low (start) transition on the RxD line, the

start bit detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in

the figure. The clock recovery logic then uses samples 8, 9, and 10 for Normal mode, and sam-

ples 4, 5, and 6 for Double Speed mode (indicated with sample numbers inside boxes on the

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figure), to decide if a valid start bit is received. If two or more of these three samples have logical

high levels (the majority wins), the start bit is rejected as a noise spike and the receiver starts

looking for the next high to low-transition. If however, a valid start bit is detected, the clock recov-

ery logic is synchronized and the data recovery can begin. The synchronization process is

repeated for each start bit.

Asynchronous Data

Recovery

When the receiver clock is synchronized to the start bit, the data recovery can begin. The data

recovery unit uses a state machine that has 16 states for each bit in normal mode and 8 states

for each bit in Double Speed mode. Figure 74 shows the sampling of the data bits and the parity

bit. Each of the samples is given a number that is equal to the state of the recovery unit.

Figure 74. Sampling of Data and Parity Bit

RxD

BIT n

Sample

(U2X = 0)

1

2

3

2

4

5

3

6

7

4

8

9

5

10

11

6

12

13

7

14

15

8

16

1

Sample

(U2X = 1)

The decision of the logic level of the received bit is taken by doing a majority voting of the logic

value to the three samples in the center of the received bit. The center samples are emphasized

on the figure by having the sample number inside boxes. The majority voting process is done as

follows: If two or all three samples have high levels, the received bit is registered to be a logic 1.

If two or all three samples have low levels, the received bit is registered to be a logic 0. This

majority voting process acts as a low pass filter for the incoming signal on the RxD pin. The

recovery process is then repeated until a complete frame is received. Including the first stop bit.

Note that the receiver only uses the first stop bit of a frame.

Figure 75 shows the sampling of the stop bit and the earliest possible beginning of the start bit of

the next frame.

Figure 75. Stop Bit Sampling and Next Start Bit Sampling

(A)

(B)

(C)

RxD

STOP 1

Sample

(U2X = 0)

1

2

3

2

4

5

3

6

7

4

8

9

5

10

0/1 0/1 0/1

Sample

(U2X = 1)

6

0/1

The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop

bit is registered to have a logic 0 value, the Frame Error (FE) Flag will be set.

A new high to low transition indicating the start bit of a new frame can come right after the last of

the bits used for majority voting. For Normal Speed mode, the first low level sample can be at

point marked (A) in Figure 75. For Double Speed mode the first low level must be delayed to (B).

(C) marks a stop bit of full length. The early start bit detection influences the operational range of

the receiver.

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Asynchronous

Operational Range

The operational range of the receiver is dependent on the mismatch between the received bit

rate and the internally generated baud rate. If the Transmitter is sending frames at too fast or too

slow bit rates, or the internally generated baud rate of the receiver does not have a similar (see

Table 61) base frequency, the receiver will not be able to synchronize the frames to the start bit.

The following equations can be used to calculate the ratio of the incoming data rate and internal

receiver baud rate.

(D + 1)S

S – 1 + D ⋅ S + S

R

= ------------------------------------------

slow

F

(D + 2)S

(D + 1)S + S

R

= -----------------------------------

fast

M

D

S

Sum of character size and parity size (D = 5 to 10 bit)

Samples per bit. S = 16 for Normal Speed mode and S = 8 for

Double Speed mode.

S_F

First sample number used for majority voting. S_F= 8 for Normal Speed and

S_F= 4 for Double Speed mode.

S_MMiddle sample number used for majority voting. S_M= 9 for Normal Speed and

S_M= 5 for Double Speed mode.

R_slowis the ratio of the slowest incoming data rate that can be accepted in relation to the

receiver baud rate. R_fastis the ratio of the fastest incoming data rate that can be

accepted in relation to the receiver baud rate.

Table 61 and Table 62 list the maximum receiver baud rate error that can be tolerated. Note that

Normal Speed mode has higher toleration of baud rate variations.

Table 61. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2X =

0)

D

Max Total

Error (%)

Recommended Max

Receiver Error (%)

# (Data+Parity Bit)

R_slow(%)

93.20

94.12

94.81

95.36

95.81

96.17

R_fast(%)

106.67

105.79

105.11

104.58

104.14

103.78

5

6

+6.67/-6.8

+5.79/-5.88

+5.11/-5.19

+4.58/-4.54

+4.14/-4.19

+3.78/-3.83

±3.0

±2.5

±2.0

±1.5

7

8

9

10

Table 62. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2X =

1)

D

Max Total

Error (%)

Recommended Max

Receiver Error (%)

# (Data+Parity Bit)

R_slow(%) R_fast(%)

5

6

7

94.12

94.92

95.52

105.66

104.92

104.35

+5.66/-5.88

+4.92/-5.08

+4.35/-4.48

±2.5

±2.0

±1.5

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Table 62. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2X =

1)

D

Max Total

Error (%)

Recommended Max

Receiver Error (%)

# (Data+Parity Bit)

R_slow(%) R_fast(%)

8

9

96.00

96.39

96.70

103.90

103.53

103.23

+3.90/-4.00

+3.53/-3.61

+3.23/-3.30

±1.5

±1.0

10

The recommendations of the maximum receiver baud rate error was made under the assump-

tion that the receiver and transmitter equally divides the maximum total error.

There are two possible sources for the receivers baud rate error. The receiver’s system clock

(XTAL) will always have some minor instability over the supply voltage range and the tempera-

ture range. When using a crystal to generate the system clock, this is rarely a problem, but for a

resonator the system clock may differ more than 2% depending of the resonators tolerance. The

second source for the error is more controllable. The baud rate generator can not always do an

exact division of the system frequency to get the baud rate wanted. In this case an UBRR value

that gives an acceptable low error can be used if possible.

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Multi-processor

Communication

Mode

Setting the Multi-processor Communication mode (MPCM) bit in UCSRA enables a filtering

function of incoming frames received by the USART Receiver. Frames that do not contain

address information will be ignored and not put into the receive buffer. This effectively reduces

the number of incoming frames that has to be handled by the CPU, in a system with multiple

MCUs that communicate via the same serial bus. The Transmitter is unaffected by the MPCM

setting, but has to be used differently when it is a part of a system utilizing the Multi-processor

Communication mode.

If the receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop bit indi-

cates if the frame contains data or address information. If the receiver is set up for frames with

nine data bits, then the ninth bit (RXB8) is used for identifying address and data frames. When

the frame type bit (the first stop or the ninth bit) is one, the frame contains an address. When the

frame type bit is zero the frame is a data frame.

The Multi-processor Communication mode enables several Slave MCUs to receive data from a

Master MCU. This is done by first decoding an address frame to find out which MCU has been

addressed. If a particular Slave MCU has been addressed, it will receive the following data

frames as normal, while the other Slave MCUs will ignore the received frames until another

address frame is received.

Using MPCM

For an MCU to act as a Master MCU, it can use a 9-bit character frame format (UCSZ = 7). The

ninth bit (TXB8) must be set when an address frame (TXB8 = 1) or cleared when a data frame

(TXB = 0) is being transmitted. The Slave MCUs must in this case be set to use a 9-bit character

frame format.

The following procedure should be used to exchange data in Multi-processor Communication

mode:

1. All Slave MCUs are in Multi-processor Communication mode (MPCM in UCSRA is set).

2. The Master MCU sends an address frame, and all Slaves receive and read this frame. In

the Slave MCUs, the RXC Flag in UCSRA will be set as normal.

3. Each Slave MCU reads the UDR Register and determines if it has been selected. If so, it

clears the MPCM bit in UCSRA, otherwise it waits for the next address byte and keeps

the MPCM setting.

4. The addressed MCU will receive all data frames until a new address frame is received.

The other Slave MCUs, which still have the MPCM bit set, will ignore the data frames.

5. When the last data frame is received by the addressed MCU, the addressed MCU sets

the MPCM bit and waits for a new address frame from Master. The process then repeats

from 2.

Using any of the 5-bit to 8-bit character frame formats is possible, but impractical since the

receiver must change between using n and n+1 character frame formats. This makes full-duplex

operation difficult since the transmitter and receiver uses the same character size setting. If 5-bit

to 8-bit character frames are used, the transmitter must be set to use two stop bit (USBS = 1)

since the first stop bit is used for indicating the frame type.

Do not use Read-Modify-Write instructions (SBI and CBI) to set or clear the MPCM bit. The

MPCM bit shares the same I/O location as the TXC Flag and this might accidentally be cleared

when using SBI or CBI instructions.

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Accessing

UBRRH/ UCSRC

Registers

The UBRRH Register shares the same I/O location as the UCSRC Register. Therefore some

special consideration must be taken when accessing this I/O location.

Write Access

When doing a write access of this I/O location, the high bit of the value written, the USART Reg-

ister Select (URSEL) bit, controls which one of the two registers that will be written. If URSEL is

zero during a write operation, the UBRRH value will be updated. If URSEL is one, the UCSRC

setting will be updated.

The following code examples show how to access the two registers.

Assembly Code Example⁽¹⁾

...

; Set UBRRH to 2

ldi r16,0x02

out UBRRH,r16

...

; Set the USBS and the UCSZ1 bit to one, and

; the remaining bits to zero.

ldi r16,(1<<URSEL)|(1<<USBS)|(1<<UCSZ1)

out UCSRC,r16

...

C Code Example⁽¹⁾

...

/* Set UBRRH to 2 */

UBRRH = 0x02;

...

/* Set the USBS and the UCSZ1 bit to one, and */

/* the remaining bits to zero. */

UCSRC = (1<<URSEL)|(1<<USBS)|(1<<UCSZ1);

...

Note:

1. See “About Code Examples” on page 7.

As the code examples illustrate, write accesses of the two registers are relatively unaffected of

the sharing of I/O location.

Read Access

Doing a read access to the UBRRH or the UCSRC Register is a more complex operation. How-

ever, in most applications, it is rarely necessary to read any of these registers.

The read access is controlled by a timed sequence. Reading the I/O location once returns the

UBRRH Register contents. If the register location was read in previous system clock cycle, read-

ing the register in the current clock cycle will return the UCSRC contents. Note that the timed

sequence for reading the UCSRC is an atomic operation. Interrupts must therefore be controlled

(for example by disabling interrupts globally) during the read operation.

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The following code example shows how to read the UCSRC Register contents.

Assembly Code Example⁽¹⁾

USART_ReadUCSRC:

; Read UCSRC

in r16,UBRRH

in r16,UCSRC

ret

C Code Example⁽¹⁾

unsigned char USART_ReadUCSRC( void )

{

unsigned char ucsrc;

/* Read UCSRC */

ucsrc = UBRRH;

ucsrc = UCSRC;

return ucsrc;

}

Note:

1. See “About Code Examples” on page 7.

The assembly code example returns the UCSRC value in r16.

Reading the UBRRH contents is not an atomic operation and therefore it can be read as an ordi-

nary register, as long as the previous instruction did not access the register location.

USART Register

Description

USART I/O Data

Register – UDR

Bit

7

6

5

4

3

2

1

0

RXB[7:0]

TXB[7:0]

UDR (Read)

UDR (Write)

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

The USART Transmit Data Buffer Register and USART Receive Data Buffer Registers share the

same I/O address referred to as USART Data Register or UDR. The Transmit Data Buffer Reg-

ister (TXB) will be the destination for data written to the UDR Register location. Reading the

UDR Register location will return the contents of the Receive Data Buffer Register (RXB).

For 5-bit, 6-bit, or 7-bit characters the upper unused bits will be ignored by the Transmitter and

set to zero by the Receiver.

The transmit buffer can only be written when the UDRE Flag in the UCSRA Register is set. Data

written to UDR when the UDRE Flag is not set, will be ignored by the USART Transmitter. When

data is written to the transmit buffer, and the Transmitter is enabled, the Transmitter will load the

data into the transmit Shift Register when the Shift Register is empty. Then the data will be seri-

ally transmitted on the TxD pin.

The receive buffer consists of a two level FIFO. The FIFO will change its state whenever the

receive buffer is accessed. Due to this behavior of the receive buffer, do not use read modify

write instructions (SBI and CBI) on this location. Be careful when using bit test instructions (SBIC

and SBIS), since these also will change the state of the FIFO.

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USART Control and

Status Register A –

UCSRA

Bit

7

RXC

R

6

5

UDRE

R

4

FE

R

3

DOR

R

2

PE

R

1

0

TXC

R/W

0

U2X

R/W

0

MPCM

R/W

0

UCSRA

Read/Write

Initial Value

0

1

0

• Bit 7 – RXC: USART Receive Complete

This flag bit is set when there are unread data in the receive buffer and cleared when the receive

buffer is empty (that is, does not contain any unread data). If the receiver is disabled, the receive

buffer will be flushed and consequently the RXC bit will become zero. The RXC Flag can be

used to generate a Receive Complete interrupt (see description of the RXCIE bit).

• Bit 6 – TXC: USART Transmit Complete

This flag bit is set when the entire frame in the transmit Shift Register has been shifted out and

there are no new data currently present in the transmit buffer (UDR). The TXC Flag bit is auto-

matically cleared when a transmit complete interrupt is executed, or it can be cleared by writing

a one to its bit location. The TXC Flag can generate a Transmit Complete interrupt (see descrip-

tion of the TXCIE bit).

• Bit 5 – UDRE: USART Data Register Empty

The UDRE Flag indicates if the transmit buffer (UDR) is ready to receive new data. If UDRE is

one, the buffer is empty, and therefore ready to be written. The UDRE Flag can generate a Data

Register empty Interrupt (see description of the UDRIE bit).

UDRE is set after a reset to indicate that the transmitter is ready.

• Bit 4 – FE: Frame Error

This bit is set if the next character in the receive buffer had a Frame Error when received. that is,

when the first stop bit of the next character in the receive buffer is zero. This bit is valid until the

receive buffer (UDR) is read. The FE bit is zero when the stop bit of received data is one. Always

set this bit to zero when writing to UCSRA.

• Bit 3 – DOR: Data OverRun

This bit is set if a Data OverRun condition is detected. A Data OverRun occurs when the receive

buffer is full (two characters), it is a new character waiting in the receive Shift Register, and a

new start bit is detected. This bit is valid until the receive buffer (UDR) is read. Always set this bit

to zero when writing to UCSRA.

• Bit 2 – PE: Parity Error

This bit is set if the next character in the receive buffer had a Parity Error when received and the

parity checking was enabled at that point (UPM1 = 1). This bit is valid until the receive buffer

(UDR) is read. Always set this bit to zero when writing to UCSRA.

• Bit 1 – U2X: Double the USART Transmission Speed

This bit only has effect for the asynchronous operation. Write this bit to zero when using syn-

chronous operation.

Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively dou-

bling the transfer rate for asynchronous communication.

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• Bit 0 – MPCM: Multi-processor Communication Mode

This bit enables the Multi-processor Communication mode. When the MPCM bit is written to

one, all the incoming frames received by the USART receiver that do not contain address infor-

mation will be ignored. The transmitter is unaffected by the MPCM setting. For more detailed

information see “Multi-processor Communication Mode” on page 161.

USART Control and

Status Register B –

UCSRB

Bit

7

RXCIE

R/W

0

6

TXCIE

R/W

0

5

UDRIE

R/W

0

4

RXEN

R/W

0

3

TXEN

R/W

0

2

UCSZ2

R/W

0

1

RXB8

R

0

TXB8

R/W

0

UCSRB

Read/Write

Initial Value

0

• Bit 7 – RXCIE: RX Complete Interrupt Enable

Writing this bit to one enables interrupt on the RXC Flag. A USART Receive Complete Interrupt

will be generated only if the RXCIE bit is written to one, the Global Interrupt Flag in SREG is writ-

ten to one and the RXC bit in UCSRA is set.

• Bit 6 – TXCIE: TX Complete Interrupt Enable

Writing this bit to one enables interrupt on the TXC Flag. A USART Transmit Complete Interrupt

will be generated only if the TXCIE bit is written to one, the Global Interrupt Flag in SREG is writ-

ten to one and the TXC bit in UCSRA is set.

• Bit 5 – UDRIE: USART Data Register Empty Interrupt Enable

Writing this bit to one enables interrupt on the UDRE Flag. A Data Register Empty Interrupt will

be generated only if the UDRIE bit is written to one, the Global Interrupt Flag in SREG is written

to one and the UDRE bit in UCSRA is set.

• Bit 4 – RXEN: Receiver Enable

Writing this bit to one enables the USART Receiver. The Receiver will override normal port oper-

ation for the RxD pin when enabled. Disabling the Receiver will flush the receive buffer

invalidating the FE, DOR, and PE Flags.

• Bit 3 – TXEN: Transmitter Enable

Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port

operation for the TxD pin when enabled. The disabling of the Transmitter (writing TXEN to zero)

will not become effective until ongoing and pending transmissions are completed, that is, when

the transmit Shift Register and transmit Buffer Register do not contain data to be transmitted.

When disabled, the transmitter will no longer override the TxD port.

• Bit 2 – UCSZ2: Character Size

The UCSZ2 bits combined with the UCSZ1:0 bit in UCSRC sets the number of data bits (Char-

acter Size) in a frame the receiver and transmitter use.

• Bit 1 – RXB8: Receive Data Bit 8

RXB8 is the ninth data bit of the received character when operating with serial frames with nine

data bits. Must be read before reading the low bits from UDR.

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ATmega16(L)

• Bit 0 – TXB8: Transmit Data Bit 8

TXB8 is the ninth data bit in the character to be transmitted when operating with serial frames

with nine data bits. Must be written before writing the low bits to UDR.

USART Control and

Status Register C –

UCSRC

Bit

7

URSEL

R/W

1

6

UMSEL

R/W

0

5

UPM1

R/W

0

4

UPM0

R/W

0

3

USBS

R/W

0

2

UCSZ1

R/W

1

UCSZ0

R/W

1

0

UCPOL

R/W

0

UCSRC

Read/Write

Initial Value

The UCSRC Register shares the same I/O location as the UBRRH Register. See the “Accessing

UBRRH/ UCSRC Registers” on page 162 section which describes how to access this register.

• Bit 7 – URSEL: Register Select

This bit selects between accessing the UCSRC or the UBRRH Register. It is read as one when

reading UCSRC. The URSEL must be one when writing the UCSRC.

• Bit 6 – UMSEL: USART Mode Select

This bit selects between Asynchronous and Synchronous mode of operation.

Table 63. UMSEL Bit Settings

UMSEL

Mode

0

1

Asynchronous Operation

Synchronous Operation

• Bit 5:4 – UPM1:0: Parity Mode

These bits enable and set type of parity generation and check. If enabled, the transmitter will

automatically generate and send the parity of the transmitted data bits within each frame. The

Receiver will generate a parity value for the incoming data and compare it to the UPM0 setting.

If a mismatch is detected, the PE Flag in UCSRA will be set.

Table 64. UPM Bits Settings

UPM1

UPM0

Parity Mode

0

1

0

1

0

1

Disabled

Reserved

Enabled, Even Parity

Enabled, Odd Parity

• Bit 3 – USBS: Stop Bit Select

This bit selects the number of Stop Bits to be inserted by the Transmitter. The Receiver ignores

this setting.

Table 65. USBS Bit Settings

USBS

Stop Bit(s)

1-bit

0

1

2-bit

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ATmega16(L)

• Bit 2:1 – UCSZ1:0: Character Size

The UCSZ1:0 bits combined with the UCSZ2 bit in UCSRB sets the number of data bits (Char-

acter Size) in a frame the Receiver and Transmitter use.

Table 66. UCSZ Bits Settings

UCSZ2

UCSZ1

UCSZ0

Character Size

5-bit

0

1

0

1

0

1

0

1

0

1

0

1

0

1

6-bit

7-bit

8-bit

Reserved

9-bit

• Bit 0 – UCPOL: Clock Polarity

This bit is used for Synchronous mode only. Write this bit to zero when Asynchronous mode is

used. The UCPOL bit sets the relationship between data output change and data input sample,

and the synchronous clock (XCK).

Table 67. UCPOL Bit Settings

Transmitted Data Changed (Output of

UCPOL TxD Pin)

Received Data Sampled (Input on

RxD Pin)

0

1

Rising XCK Edge

Falling XCK Edge

Rising XCK Edge

USART Baud Rate

Registers – UBRRL

and UBRRH

Bit

15

14

13

12

11

10

9

8

URSEL

–

UBRR[11:8]

UBRRH

UBRRL

UBRR[7:0]

7

R/W

0

6

R

5

R

4

R

3

R/W

0

2

R/W

0

1

R/W

0

R/W

0

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

The UBRRH Register shares the same I/O location as the UCSRC Register. See the “Accessing

UBRRH/ UCSRC Registers” on page 162 section which describes how to access this register.

• Bit 15 – URSEL: Register Select

This bit selects between accessing the UBRRH or the UCSRC Register. It is read as zero when

reading UBRRH. The URSEL must be zero when writing the UBRRH.

• Bit 14:12 – Reserved Bits

These bits are reserved for future use. For compatibility with future devices, these bit must be

written to zero when UBRRH is written.

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ATmega16(L)

• Bit 11:0 – UBRR11:0: USART Baud Rate Register

This is a 12-bit register which contains the USART baud rate. The UBRRH contains the four

most significant bits, and the UBRRL contains the 8 least significant bits of the USART baud

rate. Ongoing transmissions by the transmitter and receiver will be corrupted if the baud rate is

changed. Writing UBRRL will trigger an immediate update of the baud rate prescaler.

Examples of Baud For standard crystal and resonator frequencies, the most commonly used baud rates for asyn-

chronous operation can be generated by using the UBRR settings in Table 68. UBRR values

Rate Setting

which yield an actual baud rate differing less than 0.5% from the target baud rate, are bold in the

table. Higher error ratings are acceptable, but the receiver will have less noise resistance when

the error ratings are high, especially for large serial frames (see “Asynchronous Operational

Range” on page 159). The error values are calculated using the following equation:

BaudRate_{Closest Match}

⎛

⎞

Error[%] = ------------------------------------------------------- – 1 • 100%

⎝

⎠

BaudRate

Table 68. Examples of UBRR Settings for Commonly Used Oscillator Frequencies

f_osc= 1.0000 MHz f_osc= 1.8432 MHz

U2X = 0 U2X = 1 U2X = 0 U2X = 1

UBRR UBRR UBRR UBRR

f_osc= 2.0000 MHz

U2X = 0 U2X = 1

UBRR

Baud

Rate

(bps)

Error

0.2%

-7.0%

8.5%

-18.6%

8.5%

–

Error

0.2%

-3.5%

-7.0%

8.5%

-18.6%

8.5%

–

Error

0.0%

-25.0%

0.0%

–

Error

0.0%

–

UBRR

Error

0.2%

-3.5%

-7.0%

8.5%

-18.6%

8.5%

–

Error

0.2%

2.1%

0.2%

-3.5%

-7.0%

8.5%

–

2400

25

12

6

51

25

12

8

47

23

11

7

95

47

23

15

11

7

51

25

12

8

103

51

25

16

12

8

4800

9600

14.4k

19.2k

28.8k

38.4k

57.6k

76.8k

115.2k

230.4k

250k

3

2

6

5

6

1

3

1

2

5

2

6

0

1

3

1

3

–

1

2

1

2

–

0

1

0

1

–

0

–

0

0.0%

Max ⁽¹⁾

62.5 Kbps

UBRR = 0, Error = 0.0%

125 Kbps

115.2 Kbps

230.4 Kbps

125 Kbps

250 Kbps

1.

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ATmega16(L)

Table 69. Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)

f_osc= 3.6864 MHz f_osc= 4.0000 MHz

U2X = 0 U2X = 1 U2X = 0 U2X = 1

UBRR UBRR UBRR UBRR

f_osc= 7.3728 MHz

Baud

Rate

(bps)

U2X = 0

U2X = 1

UBRR

Error

0.0%

-7.8%

–

Error

0.0%

-7.8%

–

Error

0.2%

2.1%

0.2%

-3.5%

-7.0%

8.5%

0.0%

–

Error

0.2%

-0.8%

0.2%

2.1%

0.2%

-3.5%

-7.0%

8.5%

0.0%

–

UBRR

191

95

47

31

23

15

11

7

Error

0.0%

-7.8%

–

Error

0.0%

-7.8%

2400

4800

9600

14.4k

19.2k

28.8k

38.4k

57.6k

76.8k

115.2k

230.4k

250k

95

47

23

15

11

7

191

95

47

31

23

15

11

7

103

51

25

16

12

8

207

103

51

34

25

16

12

8

383

191

95

63

47

31

23

15

11

7

5

6

3

2

5

2

6

5

1

3

1

3

0

1

0

1

3

0

1

0

1

3

0.5M

–

0

–

0

1

1M

–

0

Max ⁽¹⁾

230.4 Kbps

UBRR = 0, Error = 0.0%

460.8 Kbps

250 Kbps

0.5 Mbps

460.8 Kbps

921.6 Kbps

1.

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ATmega16(L)

Table 70. Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)

f_osc= 8.0000 MHz f_osc= 11.0592 MHz

U2X = 0 U2X = 1 U2X = 0 U2X = 1

UBRR UBRR UBRR UBRR

f_osc= 14.7456 MHz

Baud

Rate

(bps)

U2X = 0

U2X = 1

UBRR

Error

0.2%

-0.8%

0.2%

2.1%

0.2%

-3.5%

-7.0%

8.5%

0.0%

–

Error

-0.1%

0.2%

0.6%

0.2%

-0.8%

0.2%

2.1%

0.2%

-3.5%

8.5%

0.0%

Error

0.0%

-7.8%

–

Error

0.0%

-7.8%

–

UBRR

383

191

95

63

47

31

23

15

11

7

Error

0.0%

-7.8%

Error

0.0%

5.3%

-7.8%

2400

4800

9600

14.4k

19.2k

28.8k

38.4k

57.6k

76.8k

115.2k

230.4k

250k

207

103

51

34

25

16

12

8

416

207

103

68

51

34

25

16

12

8

287

143

71

47

35

23

17

11

8

575

287

143

95

71

47

35

23

17

11

5

767

383

191

127

95

63

47

31

23

15

7

6

3

5

1

3

2

3

1

3

2

5

3

6

0.5M

0

1

–

2

1

3

1M

–

0

–

0

1

Max ⁽¹⁾

0.5 Mbps

UBRR = 0, Error = 0.0%

1 Mbps

691.2 Kbps

1.3824 Mbps

921.6 Kbps

1.8432 Mbps

1.

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Table 71. Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)

f_osc= 16.0000 MHz f_osc= 18.4320 MHz

U2X = 0 U2X = 1 U2X = 0 U2X = 1

UBRR UBRR UBRR UBRR

f_osc= 20.0000 MHz

Baud

Rate

(bps)

U2X = 0

U2X = 1

UBRR

Error

-0.1%

0.2%

0.6%

0.2%

-0.8%

0.2%

2.1%

0.2%

-3.5%

8.5%

0.0%

Error

0.0%

-0.1%

0.2%

-0.1%

0.2%

0.6%

0.2%

-0.8%

0.2%

2.1%

-3.5%

0.0%

Error

0.0%

-7.8%

–

Error

0.0%

2.4%

-7.8%

–

UBRR

520

259

129

86

64

42

32

21

15

10

4

Error

0.0%

0.2%

-0.2%

0.2%

0.9%

-1.4%

1.7%

-1.4%

8.5%

0.0%

–

Error

0.0%

0.2%

-0.2%

0.2%

-0.2%

0.2%

0.9%

-1.4%

0.0%

–

2400

4800

9600

14.4k

19.2k

28.8k

38.4k

57.6k

76.8k

115.2k

230.4k

250k

416

207

103

68

51

34

25

16

12

8

832

416

207

138

103

68

51

34

25

16

8

479

239

119

79

59

39

29

19

14

9

959

479

239

159

119

79

59

39

29

19

9

1041

520

259

173

129

86

64

42

32

21

3

4

10

3

7

4

8

4

9

0.5M

1

3

–

4

–

4

1M

0

1

–

Max ⁽¹⁾

1 Mbps

UBRR = 0, Error = 0.0%

2 Mbps

1.152 Mbps

2.304 Mbps

1.25 Mbps

2.5 Mbps

1.

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ATmega16(L)

Two-wire Serial

Interface

Features

• Simple Yet Powerful and Flexible Communication Interface, Only Two Bus Lines Needed

• Both Master and Slave Operation Supported

• Device Can Operate as Transmitter or Receiver

• 7-bit Address Space allows up to 128 Different Slave Addresses

• Multi-master Arbitration Support

• Up to 400 kHz Data Transfer Speed

• Slew-rate Limited Output Drivers

• Noise Suppression Circuitry Rejects Spikes on Bus Lines

• Fully Programmable Slave Address with General Call Support

• Address Recognition causes Wake-up when AVR is in Sleep Mode

Two-wire Serial

Interface Bus

Definition

The Two-wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The

TWI protocol allows the systems designer to interconnect up to 128 different devices using only

two bi-directional bus lines, one for clock (SCL) and one for data (SDA). The only external hard-

ware needed to implement the bus is a single pull-up resistor for each of the TWI bus lines. All

devices connected to the bus have individual addresses, and mechanisms for resolving bus

contention are inherent in the TWI protocol.

Figure 76. TWI Bus Interconnection

V_CC

Device 1

Device 3

R1

R2

Device 2

Device n

........

SDA

SCL

TWI Terminology

The following definitions are frequently encountered in this section.

Table 72. TWI Terminology

Term

Description

Master

The device that initiates and terminates a transmission. The Master also

generates the SCL clock.

Slave

The device addressed by a Master.

The device placing data on the bus.

The device reading data from the bus.

Transmitter

Receiver

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ATmega16(L)

Electrical

Interconnection

As depicted in Figure 76, both bus lines are connected to the positive supply voltage through

pull-up resistors. The bus drivers of all TWI-compliant devices are open-drain or open-collector.

This implements a wired-AND function which is essential to the operation of the interface. A low

level on a TWI bus line is generated when one or more TWI devices output a zero. A high level

is output when all TWI devices tri-state their outputs, allowing the pull-up resistors to pull the line

high. Note that all AVR devices connected to the TWI bus must be powered in order to allow any

bus operation.

The number of devices that can be connected to the bus is only limited by the bus capacitance

limit of 400 pF and the 7-bit Slave address space. A detailed specification of the electrical char-

acteristics of the TWI is given in “Two-wire Serial Interface Characteristics” on page 294. Two

different sets of specifications are presented there, one relevant for bus speeds below 100 kHz,

and one valid for bus speeds up to 400 kHz.

Data Transfer and

Frame Format

Transferring Bits

Each data bit transferred on the TWI bus is accompanied by a pulse on the clock line. The level

of the data line must be stable when the clock line is high. The only exception to this rule is for

generating start and stop conditions.

Figure 77. Data Validity

SDA

SCL

Data Stable

Data Change

START and STOP

Conditions

The Master initiates and terminates a data transmission. The transmission is initiated when the

Master issues a START condition on the bus, and it is terminated when the Master issues a

STOP condition. Between a START and a STOP condition, the bus is considered busy, and no

other Master should try to seize control of the bus. A special case occurs when a new START

condition is issued between a START and STOP condition. This is referred to as a REPEATED

START condition, and is used when the Master wishes to initiate a new transfer without releas-

ing control of the bus. After a REPEATED START, the bus is considered busy until the next

STOP. This is identical to the START behavior, and therefore START is used to describe both

START and REPEATED START for the remainder of this datasheet, unless otherwise noted. As

depicted below, START and STOP conditions are signalled by changing the level of the SDA

line when the SCL line is high.

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ATmega16(L)

Figure 78. START, REPEATED START, and STOP Conditions

SDA

SCL

START

STOP START

REPEATED START

STOP

Address Packet

Format

All address packets transmitted on the TWI bus are nine bits long, consisting of seven address

bits, one READ/WRITE control bit and an acknowledge bit. If the READ/WRITE bit is set, a read

operation is to be performed, otherwise a write operation should be performed. When a Slave

recognizes that it is being addressed, it should acknowledge by pulling SDA low in the ninth SCL

(ACK) cycle. If the addressed Slave is busy, or for some other reason can not service the Mas-

ter’s request, the SDA line should be left high in the ACK clock cycle. The Master can then

transmit a STOP condition, or a REPEATED START condition to initiate a new transmission. An

address packet consisting of a Slave address and a READ or a WRITE bit is called SLA+R or

SLA+W, respectively.

The MSB of the address byte is transmitted first. Slave addresses can freely be allocated by the

designer, but the address 0000 000 is reserved for a general call.

When a general call is issued, all Slaves should respond by pulling the SDA line low in the ACK

cycle. A general call is used when a Master wishes to transmit the same message to several

Slaves in the system. When the general call address followed by a Write bit is transmitted on the

bus, all Slaves set up to acknowledge the general call will pull the SDA line low in the ack cycle.

The following data packets will then be received by all the Slaves that acknowledged the general

call. Note that transmitting the general call address followed by a Read bit is meaningless, as

this would cause contention if several Slaves started transmitting different data.

All addresses of the format 1111 xxx should be reserved for future purposes.

Figure 79. Address Packet Format

Addr MSB

Addr LSB

R/W

ACK

SDA

SCL

1

2

7

8

9

START

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ATmega16(L)

Data Packet Format

All data packets transmitted on the TWI bus are nine bits long, consisting of one data byte and

an acknowledge bit. During a data transfer, the Master generates the clock and the START and

STOP conditions, while the receiver is responsible for acknowledging the reception. An

Acknowledge (ACK) is signalled by the receiver pulling the SDA line low during the ninth SCL

cycle. If the receiver leaves the SDA line high, a NACK is signalled. When the receiver has

received the last byte, or for some reason cannot receive any more bytes, it should inform the

transmitter by sending a NACK after the final byte. The MSB of the data byte is transmitted first.

Figure 80. Data Packet Format

Data MSB

Data LSB

ACK

Aggregate

SDA

SDA from

Transmitter

SDA from

receiverR

SCL from

Master

1

2

7

8

9

STOP, REPEATED

START or Next

Data Byte

SLA+R/W

Data Byte

Combining Address

A transmission basically consists of a START condition, a SLA+R/W, one or more data packets

and Data Packets into and a STOP condition. An empty message, consisting of a START followed by a STOP condi-

a Transmission

tion, is illegal. Note that the wired-ANDing of the SCL line can be used to implement

handshaking between the Master and the Slave. The Slave can extend the SCL low period by

pulling the SCL line low. This is useful if the clock speed set up by the Master is too fast for the

Slave, or the Slave needs extra time for processing between the data transmissions. The Slave

extending the SCL low period will not affect the SCL high period, which is determined by the

Master. As a consequence, the Slave can reduce the TWI data transfer speed by prolonging the

SCL duty cycle.

Figure 81 shows a typical data transmission. Note that several data bytes can be transmitted

between the SLA+R/W and the STOP condition, depending on the software protocol imple-

mented by the application software.

Figure 81. Typical Data Transmission

Addr MSB

Addr LSB R/W

ACK

Data MSB

Data LSB ACK

SDA

SCL

1

2

7

8

9

1

2

7

8

9

START

SLA+R/W

Data Byte

STOP

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ATmega16(L)

Multi-master Bus

Systems,

Arbitration and

Synchronization

The TWI protocol allows bus systems with several Masters. Special concerns have been taken

in order to ensure that transmissions will proceed as normal, even if two or more Masters initiate

a transmission at the same time. Two problems arise in multi-master systems:

•

An algorithm must be implemented allowing only one of the Masters to complete the

transmission. All other Masters should cease transmission when they discover that they

have lost the selection process. This selection process is called arbitration. When a

contending Master discovers that it has lost the arbitration process, it should immediately

switch to Slave mode to check whether it is being addressed by the winning Master. The fact

that multiple Masters have started transmission at the same time should not be detectable to

the Slaves, that is, the data being transferred on the bus must not be corrupted.

•

Different Masters may use different SCL frequencies. A scheme must be devised to

synchronize the serial clocks from all Masters, in order to let the transmission proceed in a

lockstep fashion. This will facilitate the arbitration process.

The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from

all Masters will be wired-ANDed, yielding a combined clock with a high period equal to the one

from the Master with the shortest high period. The low period of the combined clock is equal to

the low period of the Master with the longest low period. Note that all Masters listen to the SCL

line, effectively starting to count their SCL high and low time-out periods when the combined

SCL line goes high or low, respectively.

Figure 82. SCL Synchronization between Multiple Masters

TA _low

TA _high

SCL from

Master A

SCL from

Master B

SCL bus

Line

TB_low

TB_high

Masters Start

Counting Low Period

Counting High Period

Arbitration is carried out by all Masters continuously monitoring the SDA line after outputting

data. If the value read from the SDA line does not match the value the Master had output, it has

lost the arbitration. Note that a Master can only lose arbitration when it outputs a high SDA value

while another Master outputs a low value. The losing Master should immediately go to Slave

mode, checking if it is being addressed by the winning Master. The SDA line should be left high,

but losing Masters are allowed to generate a clock signal until the end of the current data or

address packet. Arbitration will continue until only one Master remains, and this may take many

bits. If several Masters are trying to address the same Slave, arbitration will continue into the

data packet.

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Figure 83. Arbitration between Two Masters

START

Master A Loses

Arbitration, SDA_ASDA

SDA from

Master A

SDA from

Master B

SDA Line

Synchronized

SCL Line

Note that arbitration is not allowed between:

•

A REPEATED START condition and a data bit

A STOP condition and a data bit

A REPEATED START and a STOP condition

It is the user software’s responsibility to ensure that these illegal arbitration conditions never

occur. This implies that in multi-master systems, all data transfers must use the same composi-

tion of SLA+R/W and data packets. In other words: All transmissions must contain the same

number of data packets, otherwise the result of the arbitration is undefined.

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Overview of the

TWI Module

The TWI module is comprised of several submodules, as shown in Figure 84. All registers drawn

in a thick line are accessible through the AVR data bus.

Figure 84. Overview of the TWI Module

SCL

SDA

Spike

Filter

Spike

Filter

Slew-rate

Control

Slew-rate

Control

Bus Interface Unit

Bit Rate Generator

START / STOP

Spike Suppression

Prescaler

Control

Address/Data Shift

Register (TWDR)

Bit Rate Register

Arbitration detection

Ack

(TWBR)

Address Match Unit

Control Unit

Address Register

Status Register

Control Register

(TWAR)

(TWSR)

(TWCR)

TWI Unit

State Machine and

Status control

Address Comparator

SCL and SDA Pins

These pins interface the AVR TWI with the rest of the MCU system. The output drivers contain a

slew-rate limiter in order to conform to the TWI specification. The input stages contain a spike

suppression unit removing spikes shorter than 50 ns. Note that the internal pull-ups in the AVR

pads can be enabled by setting the PORT bits corresponding to the SCL and SDA pins, as

explained in the I/O Port section. The internal pull-ups can in some systems eliminate the need

for external ones.

Bit Rate Generator

Unit

This unit controls the period of SCL when operating in a Master mode. The SCL period is con-

trolled by settings in the TWI Bit Rate Register (TWBR) and the Prescaler bits in the TWI Status

Register (TWSR). Slave operation does not depend on Bit Rate or Prescaler settings, but the

CPU clock frequency in the Slave must be at least 16 times higher than the SCL frequency. Note

that Slaves may prolong the SCL low period, thereby reducing the average TWI bus clock

period. The SCL frequency is generated according to the following equation:

CPU Clock frequency

SCL frequency = -----------------------------------------------------------

TWPS

16 + 2(TWBR) ⋅ 4

•

TWBR = Value of the TWI Bit Rate Register

TWPS = Value of the prescaler bits in the TWI Status Register

Note:

Note: Pull-up resistor values should be selected according to the SCL frequency and the capaci-

tive bus line load. See Table 120 on page 294 for value of pull-up resistor.

Bus Interface Unit

This unit contains the Data and Address Shift Register (TWDR), a START/STOP Controller and

Arbitration detection hardware. The TWDR contains the address or data bytes to be transmitted,

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or the address or data bytes received. In addition to the 8-bit TWDR, the Bus Interface Unit also

contains a register containing the (N)ACK bit to be transmitted or received. This (N)ACK Regis-

ter is not directly accessible by the application software. However, when receiving, it can be set

or cleared by manipulating the TWI Control Register (TWCR). When in Transmitter mode, the

value of the received (N)ACK bit can be determined by the value in the TWSR.

The START/STOP Controller is responsible for generation and detection of START, REPEATED

START, and STOP conditions. The START/STOP controller is able to detect START and STOP

conditions even when the AVR MCU is in one of the sleep modes, enabling the MCU to wake up

if addressed by a Master.

If the TWI has initiated a transmission as Master, the Arbitration Detection hardware continu-

ously monitors the transmission trying to determine if arbitration is in process. If the TWI has lost

an arbitration, the Control Unit is informed. Correct action can then be taken and appropriate

status codes generated.

Address Match Unit

The Address Match unit checks if received address bytes match the 7-bit address in the TWI

Address Register (TWAR). If the TWI General Call Recognition Enable (TWGCE) bit in the

TWAR is written to one, all incoming address bits will also be compared against the General Call

address. Upon an address match, the Control Unit is informed, allowing correct action to be

taken. The TWI may or may not acknowledge its address, depending on settings in the TWCR.

The Address Match unit is able to compare addresses even when the AVR MCU is in sleep

mode, enabling the MCU to wake up if addressed by a Master.

Control Unit

The Control unit monitors the TWI bus and generates responses corresponding to settings in the

TWI Control Register (TWCR). When an event requiring the attention of the application occurs

on the TWI bus, the TWI Interrupt Flag (TWINT) is asserted. In the next clock cycle, the TWI Sta-

tus Register (TWSR) is updated with a status code identifying the event. The TWSR only

contains relevant status information when the TWI Interrupt Flag is asserted. At all other times,

the TWSR contains a special status code indicating that no relevant status information is avail-

able. As long as the TWINT Flag is set, the SCL line is held low. This allows the application

software to complete its tasks before allowing the TWI transmission to continue.

The TWINT Flag is set in the following situations:

•

After the TWI has transmitted a START/REPEATED START condition

After the TWI has transmitted SLA+R/W

After the TWI has transmitted an address byte

After the TWI has lost arbitration

After the TWI has been addressed by own Slave address or general call

After the TWI has received a data byte

After a STOP or REPEATED START has been received while still addressed as a Slave.

When a bus error has occurred due to an illegal START or STOP condition

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TWI Register

Description

TWI Bit Rate Register

– TWBR

Bit

7

TWBR7

R/W

0

6

TWBR6

R/W

0

5

TWBR5

R/W

0

4

TWBR4

R/W

0

3

TWBR3

R/W

0

2

TWBR2

R/W

0

1

TWBR1

R/W

0

TWBR0

R/W

0

TWBR

Read/Write

Initial Value

• Bits 7..0 – TWI Bit Rate Register

TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency

divider which generates the SCL clock frequency in the Master modes. See “Bit Rate Generator

Unit” on page 178 for calculating bit rates.

TWI Control Register –

TWCR

Bit

7

TWINT

R/W

0

6

TWEA

R/W

0

5

TWSTA

R/W

0

4

TWSTO

R/W

0

3

2

TWEN

R/W

0

1

–

0

TWIE

R/W

0

TWWC

TWCR

Read/Write

Initial Value

R

0

R

0

The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to initiate a

Master access by applying a START condition to the bus, to generate a receiver acknowledge,

to generate a stop condition, and to control halting of the bus while the data to be written to the

bus are written to the TWDR. It also indicates a write collision if data is attempted written to

TWDR while the register is inaccessible.

• Bit 7 – TWINT: TWI Interrupt Flag

This bit is set by hardware when the TWI has finished its current job and expects application

software response. If the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the

TWI Interrupt Vector. While the TWINT Flag is set, the SCL low period is stretched. The TWINT

Flag must be cleared by software by writing a logic one to it. Note that this flag is not automati-

cally cleared by hardware when executing the interrupt routine. Also note that clearing this flag

starts the operation of the TWI, so all accesses to the TWI Address Register (TWAR), TWI Sta-

tus Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing this

flag.

• Bit 6 – TWEA: TWI Enable Acknowledge Bit

The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to

one, the ACK pulse is generated on the TWI bus if the following conditions are met:

1. The device’s own Slave address has been received.

2. A general call has been received, while the TWGCE bit in the TWAR is set.

3. A data byte has been received in Master Receiver or Slave Receiver mode.

By writing the TWEA bit to zero, the device can be virtually disconnected from the Two-wire

Serial Bus temporarily. Address recognition can then be resumed by writing the TWEA bit to one

again.

• Bit 5 – TWSTA: TWI START Condition Bit

The application writes the TWSTA bit to one when it desires to become a Master on the Two-

wire Serial Bus. The TWI hardware checks if the bus is available, and generates a START con-

dition on the bus if it is free. However, if the bus is not free, the TWI waits until a STOP condition

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is detected, and then generates a new START condition to claim the bus Master status. TWSTA

must be cleared by software when the START condition has been transmitted.

• Bit 4 – TWSTO: TWI STOP Condition Bit

Writing the TWSTO bit to one in Master mode will generate a STOP condition on the Two-wire

Serial Bus. When the STOP condition is executed on the bus, the TWSTO bit is cleared auto-

matically. In Slave mode, setting the TWSTO bit can be used to recover from an error condition.

This will not generate a STOP condition, but the TWI returns to a well-defined unaddressed

Slave mode and releases the SCL and SDA lines to a high impedance state.

• Bit 3 – TWWC: TWI Write Collision Flag

The TWWC bit is set when attempting to write to the TWI Data Register – TWDR when TWINT is

low. This flag is cleared by writing the TWDR Register when TWINT is high.

• Bit 2 – TWEN: TWI Enable Bit

The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to

one, the TWI takes control over the I/O pins connected to the SCL and SDA pins, enabling the

slew-rate limiters and spike filters. If this bit is written to zero, the TWI is switched off and all TWI

transmissions are terminated, regardless of any ongoing operation.

• Bit 1 – Res: Reserved Bit

This bit is a reserved bit and will always read as zero.

• Bit 0 – TWIE: TWI Interrupt Enable

When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be acti-

vated for as long as the TWINT Flag is high.

TWI Status Register –

TWSR

Bit

7

TWS7

R

6

TWS6

R

5

TWS5

R

4

TWS4

R

3

TWS3

R

2

–

1

TWPS1

R/W

0

TWPS0

R/W

0

TWSR

Read/Write

Initial Value

R

0

1

• Bits 7..3 – TWS: TWI Status

These five bits reflect the status of the TWI logic and the Two-wire Serial Bus. The different sta-

tus codes are described later in this section. Note that the value read from TWSR contains both

the 5-bit status value and the 2-bit prescaler value. The application designer should mask the

prescaler bits to zero when checking the Status bits. This makes status checking independent of

prescaler setting. This approach is used in this datasheet, unless otherwise noted.

• Bit 2 – Res: Reserved Bit

This bit is reserved and will always read as zero.

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• Bits 1..0 – TWPS: TWI Prescaler Bits

These bits can be read and written, and control the bit rate prescaler.

Table 73. TWI Bit Rate Prescaler

TWPS1

TWPS0

Prescaler Value

0

1

0

1

0

1

4

16

64

To calculate bit rates, see “Bit Rate Generator Unit” on page 178. The value of TWPS1..0 is

used in the equation.

TWI Data Register –

TWDR

Bit

7

TWD7

R/W

1

6

TWD6

R/W

1

5

TWD5

R/W

1

4

TWD4

R/W

1

3

TWD3

R/W

1

2

TWD2

R/W

1

TWD1

R/W

1

0

TWD0

R/W

1

TWDR

Read/Write

Initial Value

In Transmit mode, TWDR contains the next byte to be transmitted. In Receive mode, the TWDR

contains the last byte received. It is writable while the TWI is not in the process of shifting a byte.

This occurs when the TWI Interrupt Flag (TWINT) is set by hardware. Note that the Data Regis-

ter cannot be initialized by the user before the first interrupt occurs. The data in TWDR remains

stable as long as TWINT is set. While data is shifted out, data on the bus is simultaneously

shifted in. TWDR always contains the last byte present on the bus, except after a wake up from

a sleep mode by the TWI interrupt. In this case, the contents of TWDR is undefined. In the case

of a lost bus arbitration, no data is lost in the transition from Master to Slave. Handling of the

ACK bit is controlled automatically by the TWI logic, the CPU cannot access the ACK bit directly.

• Bits 7..0 – TWD: TWI Data Register

These eight bits contain the next data byte to be transmitted, or the latest data byte received on

the Two-wire Serial Bus.

TWI (Slave) Address

Register – TWAR

Bit

7

TWA6

R/W

1

6

TWA5

R/W

1

5

TWA4

R/W

1

4

TWA3

R/W

1

3

TWA2

R/W

1

2

TWA1

R/W

1

TWA0

R/W

1

0

TWGCE

R/W

0

TWAR

Read/Write

Initial Value

The TWAR should be loaded with the 7-bit Slave address (in the seven most significant bits of

TWAR) to which the TWI will respond when programmed as a Slave Transmitter or receiver. In

multi-master systems, TWAR must be set in Masters which can be addressed as Slaves by

other Masters.

The LSB of TWAR is used to enable recognition of the general call address ($00). There is an

associated address comparator that looks for the Slave address (or general call address if

enabled) in the received serial address. If a match is found, an interrupt request is generated.

• Bits 7..1 – TWA: TWI (Slave) Address Register

These seven bits constitute the Slave address of the TWI unit.

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• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit

If set, this bit enables the recognition of a General Call given over the Two-wire Serial Bus.

Using the TWI

The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like

reception of a byte or transmission of a START condition. Because the TWI is interrupt-based,

the application software is free to carry on other operations during a TWI byte transfer. Note that

the TWI Interrupt Enable (TWIE) bit in TWCR together with the Global Interrupt Enable bit in

SREG allow the application to decide whether or not assertion of the TWINT Flag should gener-

ate an interrupt request. If the TWIE bit is cleared, the application must poll the TWINT Flag in

order to detect actions on the TWI bus.

When the TWINT Flag is asserted, the TWI has finished an operation and awaits application

response. In this case, the TWI Status Register (TWSR) contains a value indicating the current

state of the TWI bus. The application software can then decide how the TWI should behave in

the next TWI bus cycle by manipulating the TWCR and TWDR Registers.

Figure 85 is a simple example of how the application can interface to the TWI hardware. In this

example, a Master wishes to transmit a single data byte to a Slave. This description is quite

abstract, a more detailed explanation follows later in this section. A simple code example imple-

menting the desired behavior is also presented.

Figure 85. Interfacing the Application to the TWI in a Typical Transmission

3. Check TWSR to see if START was

sent.

Application loads SLA+W into TWDR, and

loads appropriate control signalsinto

TWCR, making sure that TWINT is written

to one, and TWSTA is written to zero

5. Check TWSR to see if SLA+W was

1. Application

writes to TWCR to

initiate

transmission of

START

7. Check TWSR to see if data was sent

and ACK received.

Application loads appropriate control

signals to send STOP into TWCR,

making sure that TWINT is written to one

sent and ACK received.

Application loads data into TWDR, and

loads appropriate control signals into

TWCR, making sure that TWINT is

written to one

Application

Action

TWI bus

START

SLA+W

A

Data

A

STOP

Indicates

TWINT set

4. TWINT set.

Status code indicates

SLA+W sent, ACK

received

2. TWINT set.

6. TWINT set.

Status code indicates

data sent, ACK received

TWI

Hardware

Action

Status code indicates

START condition sent

1. The first step in a TWI transmission is to transmit a START condition. This is done by

writing a specific value into TWCR, instructing the TWI hardware to transmit a START

condition. Which value to write is described later on. However, it is important that the

TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will

not start any operation as long as the TWINT bit in TWCR is set. Immediately after the

application has cleared TWINT, the TWI will initiate transmission of the START condition.

2. When the START condition has been transmitted, the TWINT Flag in TWCR is set, and

TWSR is updated with a status code indicating that the START condition has success-

fully been sent.

3. The application software should now examine the value of TWSR, to make sure that the

START condition was successfully transmitted. If TWSR indicates otherwise, the applica-

tion software might take some special action, like calling an error routine. Assuming that

the status code is as expected, the application must load SLA+W into TWDR. Remember

that TWDR is used both for address and data. After TWDR has been loaded with the

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desired SLA+W, a specific value must be written to TWCR, instructing the TWI hardware

to transmit the SLA+W present in TWDR. Which value to write is described later on.

However, it is important that the TWINT bit is set in the value written. Writing a one to

TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in

TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate

transmission of the address packet.

4. When the address packet has been transmitted, the TWINT Flag in TWCR is set, and

TWSR is updated with a status code indicating that the address packet has successfully

been sent. The status code will also reflect whether a Slave acknowledged the packet or

not.

5. The application software should now examine the value of TWSR, to make sure that the

address packet was successfully transmitted, and that the value of the ACK bit was as

expected. If TWSR indicates otherwise, the application software might take some special

action, like calling an error routine. Assuming that the status code is as expected, the

application must load a data packet into TWDR. Subsequently, a specific value must be

written to TWCR, instructing the TWI hardware to transmit the data packet present in

TWDR. Which value to write is described later on. However, it is important that the

TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will

not start any operation as long as the TWINT bit in TWCR is set. Immediately after the

application has cleared TWINT, the TWI will initiate transmission of the data packet.

6. When the data packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR

is updated with a status code indicating that the data packet has successfully been sent.

The status code will also reflect whether a Slave acknowledged the packet or not.

7. The application software should now examine the value of TWSR, to make sure that the

data packet was successfully transmitted, and that the value of the ACK bit was as

expected. If TWSR indicates otherwise, the application software might take some special

action, like calling an error routine. Assuming that the status code is as expected, the

application must write a specific value to TWCR, instructing the TWI hardware to transmit

a STOP condition. Which value to write is described later on. However, it is important that

the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI

will not start any operation as long as the TWINT bit in TWCR is set. Immediately after

the application has cleared TWINT, the TWI will initiate transmission of the STOP condi-

tion. Note that TWINT is NOT set after a STOP condition has been sent.

Even though this example is simple, it shows the principles involved in all TWI transmissions.

These can be summarized as follows:

•

When the TWI has finished an operation and expects application response, the TWINT Flag

is set. The SCL line is pulled low until TWINT is cleared.

•

When the TWINT Flag is set, the user must update all TWI Registers with the value relevant

for the next TWI bus cycle. As an example, TWDR must be loaded with the value to be

transmitted in the next bus cycle.

•

After all TWI Register updates and other pending application software tasks have been

completed, TWCR is written. When writing TWCR, the TWINT bit should be set. Writing a

one to TWINT clears the flag. The TWI will then commence executing whatever operation

was specified by the TWCR setting.

In the following an assembly and C implementation of the example is given. Note that the code

below assumes that several definitions have been made, for example by using include-files.

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Assembly code example

C example

Comments

ldi

r16, (1<<TWINT)|(1<<TWSTA)|

TWCR = (1<<TWINT)|(1<<TWSTA)|

1

2

Send START condition

(1<<TWEN)

TWCR, r16

(1<<TWEN)

out

wait1:

while (!(TWCR & (1<<TWINT)))

Wait for TWINT Flag set. This indicates

that the START condition has been

transmitted

in

r16,TWCR

;

sbrs r16,TWINT

rjmp wait1

in

andi r16, 0xF8

cpi r16, START

brne ERROR

r16,TWSR

if ((TWSR & 0xF8) != START)

3

Check value of TWI Status Register. Mask

prescaler bits. If status different from

START go to ERROR

ERROR();

ldi

out

ldi

out

r16, SLA_W

TWDR = SLA_W;

Load SLA_W into TWDR Register. Clear

TWINT bit in TWCR to start transmission

of address

TWDR, r16

TWCR = (1<<TWINT) | (1<<TWEN);

r16, (1<<TWINT) | (1<<TWEN)

TWCR, r16

wait2:

while (!(TWCR & (1<<TWINT)))

4

5

Wait for TWINT Flag set. This indicates

that the SLA+W has been transmitted,

and ACK/NACK has been received.

in

r16,TWCR

;

sbrs r16,TWINT

rjmp wait2

in

r16,TWSR

if ((TWSR & 0xF8) != MT_SLA_ACK)

Check value of TWI Status Register. Mask

prescaler bits. If status different from

MT_SLA_ACK go to ERROR

andi r16, 0xF8

ERROR();

cpi

r16, MT_SLA_ACK

brne ERROR

ldi

out

ldi

out

r16, DATA

TWDR = DATA;

Load DATA into TWDR Register. Clear

TWINT bit in TWCR to start transmission

of data

TWDR, r16

TWCR = (1<<TWINT) | (1<<TWEN);

r16, (1<<TWINT) | (1<<TWEN)

TWCR, r16

wait3:

while (!(TWCR & (1<<TWINT)))

6

7

Wait for TWINT Flag set. This indicates

that the DATA has been transmitted, and

ACK/NACK has been received.

in

r16,TWCR

;

sbrs r16,TWINT

rjmp wait3

in

r16,TWSR

if ((TWSR & 0xF8) != MT_DATA_ACK)

Check value of TWI Status Register. Mask

prescaler bits. If status different from

MT_DATA_ACK go to ERROR

andi r16, 0xF8

ERROR();

cpi

r16, MT_DATA_ACK

brne ERROR

ldi

out

r16, (1<<TWINT)|(1<<TWEN)|

TWCR = (1<<TWINT)|(1<<TWEN)|

(1<<TWSTO);

Transmit STOP condition

(1<<TWSTO)

TWCR, r16

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Transmission

Modes

The TWI can operate in one of four major modes. These are named Master Transmitter (MT),

Master Receiver (MR), Slave Transmitter (ST) and Slave Receiver (SR). Several of these

modes can be used in the same application. As an example, the TWI can use MT mode to write

data into a TWI EEPROM, MR mode to read the data back from the EEPROM. If other Masters

are present in the system, some of these might transmit data to the TWI, and then SR mode

would be used. It is the application software that decides which modes are legal.

The following sections describe each of these modes. Possible status codes are described

along with figures detailing data transmission in each of the modes. These figures contain the

following abbreviations:

S: START condition

Rs: REPEATED START condition

R: Read bit (high level at SDA)

W: Write bit (low level at SDA)

A: Acknowledge bit (low level at SDA)

A: Not acknowledge bit (high level at SDA)

Data: 8-bit data byte

P: STOP condition

SLA: Slave Address

In Figure 87 to Figure 93, circles are used to indicate that the TWINT Flag is set. The numbers in

the circles show the status code held in TWSR, with the prescaler bits masked to zero. At these

points, actions must be taken by the application to continue or complete the TWI transfer. The

TWI transfer is suspended until the TWINT Flag is cleared by software.

When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate soft-

ware action. For each status code, the required software action and details of the following serial

transfer are given in Table 74 to Table 77. Note that the prescaler bits are masked to zero in

these tables.

Master Transmitter

Mode

In the Master Transmitter mode, a number of data bytes are transmitted to a Slave Receiver

(see Figure 86). In order to enter a Master mode, a START condition must be transmitted. The

format of the following address packet determines whether Master Transmitter or Master

Receiver mode is to be entered. If SLA+W is transmitted, MT mode is entered, if SLA+R is trans-

mitted, MR mode is entered. All the status codes mentioned in this section assume that the

prescaler bits are zero or are masked to zero.

Figure 86. Data Transfer in Master Transmitter Mode

V_CC

Device 1

MASTER

TRANSMITTER

Device 2

SLAVE

RECEIVER

Device 3

Device n

R1

R2

........

SDA

SCL

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A START condition is sent by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

1

X

1

0

X

1

0

X

TWEN must be set to enable the Two-wire Serial Interface, TWSTA must be written to one to

transmit a START condition and TWINT must be written to one to clear the TWINT Flag. The

TWI will then test the Two-wire Serial Bus and generate a START condition as soon as the bus

becomes free. After a START condition has been transmitted, the TWINT Flag is set by hard-

ware, and the status code in TWSR will be $08 (See Table 74). In order to enter MT mode,

SLA+W must be transmitted. This is done by writing SLA+W to TWDR. Thereafter the TWINT bit

should be cleared (by writing it to one) to continue the transfer. This is accomplished by writing

the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

1

X

0

X

1

0

X

When SLA+W have been transmitted and an acknowledgement bit has been received, TWINT is

set again and a number of status codes in TWSR are possible. Possible status codes in Master

mode are $18, $20, or $38. The appropriate action to be taken for each of these status codes is

detailed in Table 74.

When SLA+W has been successfully transmitted, a data packet should be transmitted. This is

done by writing the data byte to TWDR. TWDR must only be written when TWINT is high. If not,

the access will be discarded, and the Write Collision bit (TWWC) will be set in the TWCR Regis-

ter. After updating TWDR, the TWINT bit should be cleared (by writing it to one) to continue the

transfer. This is accomplished by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

1

X

0

X

1

0

X

This scheme is repeated until the last byte has been sent and the transfer is ended by generat-

ing a STOP condition or a repeated START condition. A STOP condition is generated by writing

the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

1

X

0

1

X

1

0

X

A REPEATED START condition is generated by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

1

X

1

0

X

1

0

X

After a repeated START condition (state $10) the Two-wire Serial Interface can access the

same Slave again, or a new Slave without transmitting a STOP condition. Repeated START

enables the Master to switch between Slaves, Master Transmitter mode and Master Receiver

mode without losing control of the bus.

Table 74. Status Codes for Master Transmitter Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Application Software Response

To TWCR

Status of the Two-wire Serial

Bus and Two-wire Serial Inter-

face Hardware

To/from TWDR

STA

0

STO

0

TWINT

1

TWEA

X

Next Action Taken by TWI Hardware

$08

A START condition has been Load SLA+W

transmitted

SLA+W will be transmitted;

ACK or NOT ACK will be received

$10

A

repeated START condition Load SLA+W or

0

1

X

SLA+W will be transmitted;

ACK or NOT ACK will be received

SLA+R will be transmitted;

has been transmitted

Load SLA+R

Logic will switch to Master Receiver mode

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Table 74. Status Codes for Master Transmitter Mode

$18

$20

$28

$30

$38

SLA+W has been transmitted;

ACK has been received

Load data byte or

0

1

X

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be Reset

No TWDR action or

1

0

1

X

No TWDR action

Load data byte or

1

0

1

0

1

X

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be Reset

SLA+W has been transmitted;

NOT ACK has been received

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

No TWDR action or

1

0

1

X

No TWDR action

1

0

1

0

1

X

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

Data byte has been transmitted; Load data byte or

ACK has been received

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

No TWDR action or

1

0

1

X

No TWDR action

1

0

1

0

1

X

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

Data byte has been transmitted; Load data byte or

NOT ACK has been received

No TWDR action or

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

1

0

1

X

No TWDR action or

No TWDR action

1

X

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

Arbitration lost in SLA+W or data No TWDR action or

bytes

0

1

0

1

X

Two-wire Serial Bus will be released and not addressed

Slave mode entered

A START condition will be transmitted when the bus be-

comes free

No TWDR action

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Figure 87. Formats and States in the Master Transmitter Mode

MT

Successfull

S

SLA

W

A

DATA

A

P

transmission

to a slave

receiver

$08

$18

$28

Next transfer

started with a

repeated start

condition

R_S

SLA

W

R

$10

Not acknowledge

received after the

slave address

A

P

$20

MR

Not acknowledge

received after a data

byte

A

P

$30

Arbitration lost in slave

address or data byte

Other master

continues

Other master

continues

A or A

$38

A

$38

Arbitration lost and

addressed as slave

Other master

continues

To corresponding

states in slave mode

$68 $78 $B0

Any number of data bytes

and their associated acknowledge bits

From master to slave

From slave to master

DATA

A

This number (contained in TWSR) corresponds

to a defined state of the Two-wire Serial Bus. The

prescaler bits are zero or masked to zero

n

Master Receiver Mode In the Master Receiver mode, a number of data bytes are received from a Slave Transmitter

(see Figure 88). In order to enter a Master mode, a START condition must be transmitted. The

format of the following address packet determines whether Master Transmitter or Master

Receiver mode is to be entered. If SLA+W is transmitted, MT mode is entered, if SLA+R is trans-

mitted, MR mode is entered. All the status codes mentioned in this section assume that the

prescaler bits are zero or are masked to zero.

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Figure 88. Data Transfer in Master Receiver Mode

V_CC

Device 1

MASTER

RECEIVER

Device 2

SLAVE

TRANSMITTER

Device 3

Device n

R1

R2

........

SDA

SCL

A START condition is sent by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

1

X

1

0

X

1

0

X

TWEN must be written to one to enable the Two-wire Serial Interface, TWSTA must be written to

one to transmit a START condition and TWINT must be set to clear the TWINT Flag. The TWI

will then test the Two-wire Serial Bus and generate a START condition as soon as the bus

becomes free. After a START condition has been transmitted, the TWINT Flag is set by hard-

ware, and the status code in TWSR will be $08 (See Table 74). In order to enter MR mode,

SLA+R must be transmitted. This is done by writing SLA+R to TWDR. Thereafter the TWINT bit

should be cleared (by writing it to one) to continue the transfer. This is accomplished by writing

the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

1

X

0

X

1

0

X

When SLA+R have been transmitted and an acknowledgement bit has been received, TWINT is

set again and a number of status codes in TWSR are possible. Possible status codes in Master

mode are $38, $40, or $48. The appropriate action to be taken for each of these status codes is

detailed in Table 75. Received data can be read from the TWDR Register when the TWINT Flag

is set high by hardware. This scheme is repeated until the last byte has been received. After the

last byte has been received, the MR should inform the ST by sending a NACK after the last

received data byte. The transfer is ended by generating a STOP condition or a repeated START

condition. A STOP condition is generated by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

1

X

0

1

X

1

0

X

A REPEATED START condition is generated by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

1

X

1

0

X

1

0

X

After a repeated START condition (state $10) the Two-wire Serial Interface can access the

same Slave again, or a new Slave without transmitting a STOP condition. Repeated START

enables the Master to switch between Slaves, Master Transmitter mode and Master Receiver

mode without losing control over the bus.

Table 75. Status Codes for Master Receiver Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Application Software Response

Status of the Two-wire Serial

Bus and Two-wire Serial Inter-

face Hardware

To TWCR

To/from TWDR

STA

STO

TWINT

TWEA

Next Action Taken by TWI Hardware

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Table 75. Status Codes for Master Receiver Mode (Continued)

$08

A START condition has been Load SLA+R

transmitted

0

1

X

SLA+R will be transmitted

ACK or NOT ACK will be received

$10

A

repeated START condition Load SLA+R or

0

1

X

SLA+R will be transmitted

ACK or NOT ACK will be received

SLA+W will be transmitted

has been transmitted

Load SLA+W

Logic will switch to masTer Transmitter mode

$38

$40

$48

Arbitration lost in SLA+R or NOT No TWDR action or

ACK bit

0

1

0

1

X

Two-wire Serial Bus will be released and not addressed

Slave mode will be entered

A START condition will be transmitted when the bus

becomes free

No TWDR action

SLA+R has been transmitted;

ACK has been received

No TWDR action or

No TWDR action

0

1

0

1

Data byte will be received and NOT ACK will be

returned

Data byte will be received and ACK will be returned

SLA+R has been transmitted;

NOT ACK has been received

No TWDR action or

1

0

1

X

Repeated START will be transmitted

STOP condition will be transmitted and TWSTO Flag will

be reset

No TWDR action

1

X

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

$50

$58

Data byte has been received;

ACK has been returned

Read data byte or

Read data byte

0

1

0

Data byte will be received and NOT ACK will be

returned

Data byte will be received and ACK will be returned

0

1

Data byte has been received;

NOT ACK has been returned

Read data byte or

1

0

1

X

Repeated START will be transmitted

STOP condition will be transmitted and TWSTO Flag will

be reset

Read data byte

1

X

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

Figure 89. Formats and States in the Master Receiver Mode

MR

Successfull

S

SLA

R

A

DATA

A

DATA

A

P

reception

from a slave

receiver

$08

$40

$50

$58

Next transfer

started with a

repeated start

condition

R_S

SLA

R

$10

Not acknowledge

received after the

slave address

W

A

P

$48

MT

Arbitration lost in slave

address or data byte

Other master

continues

Other master

continues

A or A

A

$38

A

$38

Arbitration lost and

addressed as slave

Other master

continues

To corresponding

states in slave mode

$68 $78 $B0

Any number of data bytes

and their associated acknowledge bits

From master to slave

From slave to master

DATA

A

This number (contained in TWSR) corresponds

to a defined state of the Two-wire Serial Bus. The

prescaler bits are zero or masked to zero

n

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Slave Receiver Mode

In the Slave Receiver mode, a number of data bytes are received from a Master Transmitter

(see Figure 90). All the status codes mentioned in this section assume that the prescaler bits are

zero or are masked to zero.

Figure 90. Data Transfer in Slave Receiver Mode

V_CC

Device 1

SLAVE

RECEIVER

Device 2

MASTER

TRANSMITTER

Device 3

Device n

R1

R2

........

SDA

SCL

To initiate the Slave Receiver mode, TWAR and TWCR must be initialized as follows:

TWAR

TWA6

TWA5

TWA4

TWA3

TWA2

TWA1

TWA0

TWGCE

Value

Device’s Own Slave Address

The upper seven bits are the address to which the Two-wire Serial Interface will respond when

addressed by a Master. If the LSB is set, the TWI will respond to the general call address ($00),

otherwise it will ignore the general call address.

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

0

1

0

1

0

X

TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable

the acknowledgement of the device’s own Slave address or the general call address. TWSTA

and TWSTO must be written to zero.

When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own

Slave address (or the general call address if enabled) followed by the data direction bit. If the

direction bit is “0” (write), the TWI will operate in SR mode, otherwise ST mode is entered. After

its own Slave address and the write bit have been received, the TWINT Flag is set and a valid

status code can be read from TWSR. The status code is used to determine the appropriate soft-

ware action. The appropriate action to be taken for each status code is detailed in Table 76. The

Slave Receiver mode may also be entered if arbitration is lost while the TWI is in the Master

mode (see states $68 and $78).

If the TWEA bit is reset during a transfer, the TWI will return a “Not Acknowledge” (“1”) to SDA

after the next received data byte. This can be used to indicate that the Slave is not able to

receive any more bytes. While TWEA is zero, the TWI does not acknowledge its own Slave

address. However, the Two-wire Serial Bus is still monitored and address recognition may

resume at any time by setting TWEA. This implies that the TWEA bit may be used to temporarily

isolate the TWI from the Two-wire Serial Bus.

In all sleep modes other than Idle Mode, the clock system to the TWI is turned off. If the TWEA

bit is set, the interface can still acknowledge its own Slave address or the general call address

by using the Two-wire Serial Bus clock as a clock source. The part will then wake up from sleep

and the TWI will hold the SCL clock low during the wake up and until the TWINT Flag is cleared

(by writing it to one). Further data reception will be carried out as normal, with the AVR clocks

running as normal. Observe that if the AVR is set up with a long start-up time, the SCL line may

be held low for a long time, blocking other data transmissions.

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Note that the Two-wire Serial Interface Data Register – TWDR does not reflect the last byte

present on the bus when waking up from these sleep modes.

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Table 76. Status Codes for Slave Receiver Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Application Software Response

To TWCR

Status of the Two-wire Serial Bus

and Two-wire Serial Interface

Hardware

To/from TWDR

STA

X

STO

0

TWINT

1

TWEA

0

Next Action Taken by TWI Hardware

$60

$68

$70

$78

Own SLA+W has been received;

ACK has been returned

No TWDR action or

No TWDR action

Data byte will be received and NOT ACK will be

returned

X

0

1

0

Data byte will be received and ACK will be returned

Arbitration lost in SLA+R/W as No TWDR action or

Master; own SLA+W has been

Data byte will be received and NOT ACK will be

returned

received; ACK has been returned

No TWDR action

X

0

1

0

Data byte will be received and ACK will be returned

General call address has been

received; ACK has been returned

No TWDR action or

Data byte will be received and NOT ACK will be

returned

No TWDR action

X

0

1

0

Data byte will be received and ACK will be returned

Arbitration lost in SLA+R/W as No TWDR action or

Master; General call address has

Data byte will be received and NOT ACK will be

returned

been received; ACK has been

returned

No TWDR action

X

0

1

0

Data byte will be received and ACK will be returned

$80

$88

Previously addressed with own Read data byte or

SLA+W; data has been received;

Data byte will be received and NOT ACK will be

returned

ACK has been returned

Read data byte

X

0

1

0

Data byte will be received and ACK will be returned

Previously addressed with own Read data byte or

SLA+W; data has been received;

NOT ACK has been returned

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA;

a START condition will be transmitted when the bus

becomes free

Read data byte or

0

1

0

1

0

Read data byte or

Read data byte

1

0

1

0

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted when the bus

becomes free

$90

$98

Previously addressed with

general call; data has been re-

ceived; ACK has been returned

Read data byte or

X

Data byte will be received and NOT ACK will be

returned

Read data byte

X

0

1

0

Data byte will be received and ACK will be returned

Previously addressed with

general call; data has been

received; NOT ACK has been

returned

Read data byte or

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA;

a START condition will be transmitted when the bus

becomes free

Read data byte or

0

1

0

1

0

Read data byte

1

0

1

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted when the bus

becomes free

$A0

A

STOP condition or repeated No action

0

1

0

1

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave mode;

own SLA will be recognized;

START condition has been

received while still addressed as

Slave

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA;

a START condition will be transmitted when the bus

becomes free

1

0

1

0

1

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted when the bus

becomes free

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Figure 91. Formats and States in the Slave Receiver Mode

Reception of the own

S

SLA

W

A

DATA

A

DATA

A

P or S

slave address and one or

more data bytes. All are

acknowledged

$60

$80

A

$A0

Last data byte received

is not acknowledged

P or S

$88

Arbitration lost as master

and addressed as slave

A

$68

A

Reception of the general call

address and one or more data

bytes

General Call

DATA

A

DATA

A

P or S

$70

$90

A

$A0

Last data byte received is

not acknowledged

P or S

$98

Arbitration lost as master and

addressed as slave by general call

A

$78

Any number of data bytes

and their associated acknowledge bits

From master to slave

From slave to master

DATA

A

This number (contained in TWSR) corresponds

to a defined state of the Two-wire Serial Bus. The

prescaler bits are zero or masked to zero

n

Slave Transmitter

Mode

In the Slave Transmitter mode, a number of data bytes are transmitted to a Master Receiver

(see Figure 92). All the status codes mentioned in this section assume that the prescaler bits are

zero or are masked to zero.

Figure 92. Data Transfer in Slave Transmitter Mode

V_CC

Device 1

SLAVE

TRANSMITTER

Device 2

MASTER

RECEIVER

Device 3

Device n

R1

R2

........

SDA

SCL

To initiate the Slave Transmitter mode, TWAR and TWCR must be initialized as follows:

TWAR

TWA6

TWA5

TWA4

TWA3

TWA2

TWA1

TWA0

TWGCE

Value

Device’s Own Slave Address

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The upper seven bits are the address to which the Two-wire Serial Interface will respond when

addressed by a Master. If the LSB is set, the TWI will respond to the general call address ($00),

otherwise it will ignore the general call address.

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

0

1

0

1

0

X

TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable

the acknowledgement of the device’s own Slave address or the general call address. TWSTA

and TWSTO must be written to zero.

When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own

Slave address (or the general call address if enabled) followed by the data direction bit. If the

direction bit is “1” (read), the TWI will operate in ST mode, otherwise SR mode is entered. After

its own Slave address and the write bit have been received, the TWINT Flag is set and a valid

status code can be read from TWSR. The status code is used to determine the appropriate soft-

ware action. The appropriate action to be taken for each status code is detailed in Table 77. The

Slave Transmitter mode may also be entered if arbitration is lost while the TWI is in the Master

mode (see state $B0).

If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the trans-

fer. State $C0 or state $C8 will be entered, depending on whether the Master Receiver transmits

a NACK or ACK after the final byte. The TWI is switched to the not addressed Slave mode, and

will ignore the Master if it continues the transfer. Thus the Master Receiver receives all “1” as

serial data. State $C8 is entered if the Master demands additional data bytes (by transmitting

ACK), even though the Slave has transmitted the last byte (TWEA zero and expecting NACK

from the Master).

While TWEA is zero, the TWI does not respond to its own Slave address. However, the Two-

wire Serial Bus is still monitored and address recognition may resume at any time by setting

TWEA. This implies that the TWEA bit may be used to temporarily isolate the TWI from the Two-

wire Serial Bus.

In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA

bit is set, the interface can still acknowledge its own Slave address or the general call address

by using the Two-wire Serial Bus clock as a clock source. The part will then wake up from sleep

and the TWI will hold the SCL clock will low during the wake up and until the TWINT Flag is

cleared (by writing it to one). Further data transmission will be carried out as normal, with the

AVR clocks running as normal. Observe that if the AVR is set up with a long start-up time, the

SCL line may be held low for a long time, blocking other data transmissions.

Note that the Two-wire Serial Interface Data Register – TWDR does not reflect the last byte

present on the bus when waking up from these sleep modes.

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Table 77. Status Codes for Slave Transmitter Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Application Software Response

To TWCR

STO TWINT

Status of the Two-wire Serial Bus

and Two-wire Serial Interface

Hardware

To/from TWDR

Load data byte or

Load data byte

STA

X

TWEA

0

Next Action Taken by TWI Hardware

$A8

$B0

$B8

$C0

Own SLA+R has been received;

ACK has been returned

0

1

Last data byte will be transmitted and NOT ACK should

be received

X

1

Data byte will be transmitted and ACK should be re-

ceived

Arbitration lost in SLA+R/W as Load data byte or

Master; own SLA+R has been

X

0

1

0

1

Last data byte will be transmitted and NOT ACK should

be received

Data byte will be transmitted and ACK should be re-

ceived

received; ACK has been returned

Load data byte

Data byte in TWDR has been

transmitted; ACK has been

received

Load data byte or

Load data byte

X

0

1

0

1

Last data byte will be transmitted and NOT ACK should

be received

Data byte will be transmitted and ACK should be re-

ceived

Data byte in TWDR has been

transmitted; NOT ACK has been

received

No TWDR action or

0

1

0

1

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA;

a START condition will be transmitted when the bus

becomes free

No TWDR action or

No TWDR action

1

0

1

0

1

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted when the bus

becomes free

$C8

Last data byte in TWDR has been No TWDR action or

transmitted (TWEA = “0”); ACK

0

1

0

1

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA

Switched to the not addressed Slave mode;

own SLA will be recognized;

has been received

No TWDR action or

GCA will be recognized if TWGCE = “1”

Switched to the not addressed Slave mode;

no recognition of own SLA or GCA;

a START condition will be transmitted when the bus

becomes free

No TWDR action or

1

0

1

0

1

No TWDR action

Switched to the not addressed Slave mode;

own SLA will be recognized;

GCA will be recognized if TWGCE = “1”;

a START condition will be transmitted when the bus

becomes free

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ATmega16(L)

Figure 93. Formats and States in the Slave Transmitter Mode

Reception of the own

slave address and one or

more data bytes

S

SLA

R

A

DATA

A

DATA

A

P or S

$A8

A

$B8

$C0

Arbitration lost as master

and addressed as slave

$B0

Last data byte transmitted.

Switched to not addressed

slave (TWEA = '0')

A

All 1's

P or S

$C8

Any number of data bytes

and their associated acknowledge bits

From master to slave

From slave to master

DATA

A

This number (contained in TWSR) corresponds

to a defined state of the Two-wire Serial Bus. The

prescaler bits are zero or masked to zero

n

Miscellaneous States

There are two status codes that do not correspond to a defined TWI state, see Table 78.

Status $F8 indicates that no relevant information is available because the TWINT Flag is not set.

This occurs between other states, and when the TWI is not involved in a serial transfer.

Status $00 indicates that a bus error has occurred during a Two-wire Serial Bus transfer. A bus

error occurs when a START or STOP condition occurs at an illegal position in the format frame.

Examples of such illegal positions are during the serial transfer of an address byte, a data byte,

or an acknowledge bit. When a bus error occurs, TWINT is set. To recover from a bus error, the

TWSTO Flag must set and TWINT must be cleared by writing a logic one to it. This causes the

TWI to enter the not addressed Slave mode and to clear the TWSTO Flag (no other bits in

TWCR are affected). The SDA and SCL lines are released, and no STOP condition is

transmitted.

Table 78. Miscellaneous States

Status Code

(TWSR)

Prescaler Bits

are 0

Application Software Response

To TWCR

Status of the Two-wire Serial

Bus and Two-wire Serial Inter-

face Hardware

To/from TWDR

STA

STO

TWINT

TWEA

X

Next Action Taken by TWI Hardware

Wait or proceed current transfer

$F8

No relevant state information No TWDR action

available; TWINT = “0”

No TWCR action

$00

Bus error due to an illegal No TWDR action

START or STOP condition

0

1

Only the internal hardware is affected, no STOP condi-

tion is sent on the bus. In all cases, the bus is released

and TWSTO is cleared.

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Combining Several

TWI Modes

In some cases, several TWI modes must be combined in order to complete the desired action.

Consider for example reading data from a serial EEPROM. Typically, such a transfer involves

the following steps:

1. The transfer must be initiated

2. The EEPROM must be instructed what location should be read

3. The reading must be performed

4. The transfer must be finished

Note that data is transmitted both from Master to Slave and vice versa. The Master must instruct

the Slave what location it wants to read, requiring the use of the MT mode. Subsequently, data

must be read from the Slave, implying the use of the MR mode. Thus, the transfer direction must

be changed. The Master must keep control of the bus during all these steps, and the steps

should be carried out as an atomical operation. If this principle is violated in a multi-master sys-

tem, another Master can alter the data pointer in the EEPROM between steps 2 and 3, and the

Master will read the wrong data location. Such a change in transfer direction is accomplished by

transmitting a REPEATED START between the transmission of the address byte and reception

of the data. After a REPEATED START, the Master keeps ownership of the bus. The following

figure shows the flow in this transfer.

Figure 94. Combining Several TWI Modes to Access a Serial EEPROM

Master Transmitter

Master Receiver

S

SLA+W

A

ADDRESS

A

Rs

SLA+R

A

DATA

A

P

S = START

Transmitted from Master to Slave

Rs = REPEATED START

Transmitted from Slave to Master

P = STOP

Multi-master

Systems and

Arbitration

If multiple Masters are connected to the same bus, transmissions may be initiated simultane-

ously by one or more of them. The TWI standard ensures that such situations are handled in

such a way that one of the Masters will be allowed to proceed with the transfer, and that no data

will be lost in the process. An example of an arbitration situation is depicted below, where two

Masters are trying to transmit data to a Slave Receiver.

Figure 95. An Arbitration Example

V_CC

Device 1

MASTER

TRANSMITTER

Device 3

SLAVE

RECEIVER

Device 2

MASTER

TRANSMITTER

Device n

R1

R2

........

SDA

SCL

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Several different scenarios may arise during arbitration, as described below:

•

Two or more Masters are performing identical communication with the same Slave. In this

case, neither the Slave nor any of the Masters will know about the bus contention.

•

Two or more Masters are accessing the same Slave with different data or direction bit. In this

case, arbitration will occur, either in the READ/WRITE bit or in the data bits. The Masters

trying to output a one on SDA while another Master outputs a zero will lose the arbitration.

Losing Masters will switch to not addressed Slave mode or wait until the bus is free and

transmit a new START condition, depending on application software action.

•

Two or more Masters are accessing different Slaves. In this case, arbitration will occur in the

SLA bits. Masters trying to output a one on SDA while another Master outputs a zero will

lose the arbitration. Masters losing arbitration in SLA will switch to Slave mode to check if

they are being addressed by the winning Master. If addressed, they will switch to SR or ST

mode, depending on the value of the READ/WRITE bit. If they are not being addressed, they

will switch to not addressed Slave mode or wait until the bus is free and transmit a new

START condition, depending on application software action.

This is summarized in Figure 96. Possible status values are given in circles.

Figure 96. Possible Status Codes Caused by Arbitration

START

SLA

Data

STOP

Arbitration lost in SLA

Arbitration lost in Data

Own

No

38

TWI bus will be released and not addressed slave mode will be entered

A START condition will be transmitted when the bus becomes free

Address / General Call

received

Yes

Write

68/78

B0

Data byte will be received and NOT ACK will be returned

Data byte will be received and ACK will be returned

Direction

Read

Last data byte will be transmitted and NOT ACK should be received

Data byte will be transmitted and ACK should be received

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Analog

Comparator

The Analog Comparator compares the input values on the positive pin AIN0 and negative pin

AIN1. When the voltage on the positive pin AIN0 is higher than the voltage on the negative pin

AIN1, the Analog Comparator Output, ACO, is set. The comparator’s output can be set to trigger

the Timer/Counter1 Input Capture function. In addition, the comparator can trigger a separate

interrupt, exclusive to the Analog Comparator. The user can select Interrupt triggering on com-

parator output rise, fall or toggle. A block diagram of the comparator and its surrounding logic is

shown in Figure 97.

Figure 97. Analog Comparator Block Diagram⁽²⁾

BANDGAP

REFERENCE

ACBG

ACME

ADEN

ADC MULTIPLEXER

OUTPUT⁽¹⁾

Notes: 1. See Table 80 on page 203.

2. Refer to Figure 1 on page 2 and Table 25 on page 58 for Analog Comparator pin placement.

Special Function IO

Register – SFIOR

Bit

7

ADTS2

R/W

0

6

ADTS1

R/W

0

5

ADTS0

R/W

0

4

–

3

ACME

R/W

0

2

1

PSR2

R/W

0

PSR10

R/W

0

PUD

R/W

0

SFIOR

Read/Write

Initial Value

R

0

• Bit 3 – ACME: Analog Comparator Multiplexer Enable

When this bit is written logic one and the ADC is switched off (ADEN in ADCSRA is zero), the

ADC multiplexer selects the negative input to the Analog Comparator. When this bit is written

logic zero, AIN1 is applied to the negative input of the Analog Comparator. For a detailed

description of this bit, see “Analog Comparator Multiplexed Input” on page 203.

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Analog Comparator

Control and Status

Register – ACSR

Bit

7

6

ACBG

R/W

0

5

ACO

R

4

ACI

R/W

0

3

ACIE

R/W

0

2

ACIC

R/W

0

1

ACIS1

R/W

0

ACD

R/W

0

ACIS0

R/W

0

ACSR

Read/Write

Initial Value

N/A

• Bit 7 – ACD: Analog Comparator Disable

When this bit is written logic one, the power to the Analog Comparator is switched off. This bit

can be set at any time to turn off the Analog Comparator. This will reduce power consumption in

active and Idle mode. When changing the ACD bit, the Analog Comparator Interrupt must be

disabled by clearing the ACIE bit in ACSR. Otherwise an interrupt can occur when the bit is

changed.

• Bit 6 – ACBG: Analog Comparator Bandgap Select

When this bit is set, a fixed bandgap reference voltage replaces the positive input to the Analog

Comparator. When this bit is cleared, AIN0 is applied to the positive input of the Analog Compar-

ator. See “Internal Voltage Reference” on page 42.

• Bit 5 – ACO: Analog Comparator Output

The output of the Analog Comparator is synchronized and then directly connected to ACO. The

synchronization introduces a delay of 1 - 2 clock cycles.

• Bit 4 – ACI: Analog Comparator Interrupt Flag

This bit is set by hardware when a comparator output event triggers the interrupt mode defined

by ACIS1 and ACIS0. The Analog Comparator Interrupt routine is executed if the ACIE bit is set

and the I-bit in SREG is set. ACI is cleared by hardware when executing the corresponding inter-

rupt handling vector. Alternatively, ACI is cleared by writing a logic one to the flag.

• Bit 3 – ACIE: Analog Comparator Interrupt Enable

When the ACIE bit is written logic one and the I-bit in the Status Register is set, the Analog Com-

parator Interrupt is activated. When written logic zero, the interrupt is disabled.

• Bit 2 – ACIC: Analog Comparator Input Capture Enable

When written logic one, this bit enables the Input Capture function in Timer/Counter1 to be trig-

gered by the Analog Comparator. The comparator output is in this case directly connected to the

Input Capture front-end logic, making the comparator utilize the noise canceler and edge select

features of the Timer/Counter1 Input Capture interrupt. When written logic zero, no connection

between the Analog Comparator and the Input Capture function exists. To make the comparator

trigger the Timer/Counter1 Input Capture interrupt, the TICIE1 bit in the Timer Interrupt Mask

Register (TIMSK) must be set.

• Bits 1, 0 – ACIS1, ACIS0: Analog Comparator Interrupt Mode Select

These bits determine which comparator events that trigger the Analog Comparator interrupt. The

different settings are shown in Table 79.

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Table 79. ACIS1/ACIS0 Settings

ACIS1

ACIS0

Interrupt Mode

0

1

0

1

0

1

Comparator Interrupt on Output Toggle

Reserved

Comparator Interrupt on Falling Output Edge

Comparator Interrupt on Rising Output Edge

When changing the ACIS1/ACIS0 bits, the Analog Comparator Interrupt must be disabled by

clearing its Interrupt Enable bit in the ACSR Register. Otherwise an interrupt can occur when the

bits are changed.

Analog

Comparator

Multiplexed Input

It is possible to select any of the ADC7..0 pins to replace the negative input to the Analog Com-

parator. The ADC multiplexer is used to select this input, and consequently, the ADC must be

switched off to utilize this feature. If the Analog Comparator Multiplexer Enable bit (ACME in

SFIOR) is set and the ADC is switched off (ADEN in ADCSRA is zero), MUX2..0 in ADMUX

select the input pin to replace the negative input to the Analog Comparator, as shown in Table

80. If ACME is cleared or ADEN is set, AIN1 is applied to the negative input to the Analog

Comparator.

Table 80. Analog Comparator Multiplexed Input

ACME

ADEN

MUX2..0

xxx

Analog Comparator Negative Input

0

1

x

1

0

AIN1

xxx

AIN1

000

ADC0

ADC1

ADC2

ADC3

ADC4

ADC5

ADC6

ADC7

001

010

011

100

101

110

111

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Analog to

Digital

Converter

Features

• 10-bit Resolution

• 0.5 LSB Integral Non-linearity

• ±2 LSB Absolute Accuracy

• 13 µs- 260 µs Conversion Time

• Up to 15 kSPS at Maximum Resolution

• 8 Multiplexed Single Ended Input Channels

• 7 Differential Input Channels

• 2 Differential Input Channels with Optional Gain of 10x and 200x

• Optional Left adjustment for ADC Result Readout

• 0 - V_CCADC Input Voltage Range

• Selectable 2.56V ADC Reference Voltage

• Free Running or Single Conversion Mode

• ADC Start Conversion by Auto Triggering on Interrupt Sources

• Interrupt on ADC Conversion Complete

• Sleep Mode Noise Canceler

The ATmega16 features a 10-bit successive approximation ADC. The ADC is connected to an

8-channel Analog Multiplexer which allows 8 single-ended voltage inputs constructed from the

pins of Port A. The single-ended voltage inputs refer to 0V (GND).

The device also supports 16 differential voltage input combinations. Two of the differential inputs

(ADC1, ADC0 and ADC3, ADC2) are equipped with a programmable gain stage, providing

amplification steps of 0 dB (1x), 20 dB (10x), or 46 dB (200x) on the differential input voltage

before the A/D conversion. Seven differential analog input channels share a common negative

terminal (ADC1), while any other ADC input can be selected as the positive input terminal. If 1x

or 10x gain is used, 8-bit resolution can be expected. If 200x gain is used, 7-bit resolution can be

expected.

The ADC contains a Sample and Hold circuit which ensures that the input voltage to the ADC is

held at a constant level during conversion. A block diagram of the ADC is shown in Figure 98.

The ADC has a separate analog supply voltage pin, AVCC. AVCC must not differ more than

±0.3V from V_CC. See the paragraph “ADC Noise Canceler” on page 211 on how to connect this

pin.

Internal reference voltages of nominally 2.56V or AVCC are provided On-chip. The voltage refer-

ence may be externally decoupled at the AREF pin by a capacitor for better noise performance.

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Figure 98. Analog to Digital Converter Block Schematic

ADC CONVERSION

COMPLETE IRQ

INTERRUPT

FLAGS

ADTS[2:0]

8-BIT DATA BUS

15

0

ADC MULTIPLEXER

SELECT (ADMUX)

ADC CTRL. & STATUS

REGISTER (ADCSRA)

ADC DATA REGISTER

(ADCH/ADCL)

TRIGGER

SELECT

MUX DECODER

PRESCALER

START

CONVERSION LOGIC

AVCC

INTERNAL 2.56V

REFERENCE

SAMPLE & HOLD

COMPARATOR

AREF

GND

10-BIT DAC

-

+

BANDGAP

REFERENCE

ADC7

ADC6

ADC5

ADC4

ADC3

ADC2

ADC1

ADC0

SINGLE ENDED / DIFFERENTIAL SELECTION

POS.

INPUT

MUX

ADC MULTIPLEXER

OUTPUT

GAIN

AMPLIFIER

+

-

NEG.

INPUT

MUX

Operation

The ADC converts an analog input voltage to a 10-bit digital value through successive approxi-

mation. The minimum value represents GND and the maximum value represents the voltage on

the AREF pin minus 1 LSB. Optionally, AVCC or an internal 2.56V reference voltage may be

connected to the AREF pin by writing to the REFSn bits in the ADMUX Register. The internal

voltage reference may thus be decoupled by an external capacitor at the AREF pin to improve

noise immunity.

The analog input channel and differential gain are selected by writing to the MUX bits in

ADMUX. Any of the ADC input pins, as well as GND and a fixed bandgap voltage reference, can

be selected as single ended inputs to the ADC. A selection of ADC input pins can be selected as

positive and negative inputs to the differential gain amplifier.

If differential channels are selected, the differential gain stage amplifies the voltage difference

between the selected input channel pair by the selected gain factor. This amplified value then

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becomes the analog input to the ADC. If single ended channels are used, the gain amplifier is

bypassed altogether.

The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Voltage reference and

input channel selections will not go into effect until ADEN is set. The ADC does not consume

power when ADEN is cleared, so it is recommended to switch off the ADC before entering power

saving sleep modes.

The ADC generates a 10-bit result which is presented in the ADC Data Registers, ADCH and

ADCL. By default, the result is presented right adjusted, but can optionally be presented left

adjusted by setting the ADLAR bit in ADMUX.

If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read

ADCH. Otherwise, ADCL must be read first, then ADCH, to ensure that the content of the Data

Registers belongs to the same conversion. Once ADCL is read, ADC access to Data Registers

is blocked. This means that if ADCL has been read, and a conversion completes before ADCH is

read, neither register is updated and the result from the conversion is lost. When ADCH is read,

ADC access to the ADCH and ADCL Registers is re-enabled.

The ADC has its own interrupt which can be triggered when a conversion completes. When ADC

access to the Data Registers is prohibited between reading of ADCH and ADCL, the interrupt

will trigger even if the result is lost.

Starting a

Conversion

A single conversion is started by writing a logical one to the ADC Start Conversion bit, ADSC.

This bit stays high as long as the conversion is in progress and will be cleared by hardware

when the conversion is completed. If a different data channel is selected while a conversion is in

progress, the ADC will finish the current conversion before performing the channel change.

Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is

enabled by setting the ADC Auto Trigger Enable bit, ADATE in ADCSRA. The trigger source is

selected by setting the ADC Trigger Select bits, ADTS in SFIOR (see description of the ADTS

bits for a list of the trigger sources). When a positive edge occurs on the selected trigger signal,

the ADC prescaler is reset and a conversion is started. This provides a method of starting con-

versions at fixed intervals. If the trigger signal still is set when the conversion completes, a new

conversion will not be started. If another positive edge occurs on the trigger signal during con-

version, the edge will be ignored. Note that an Interrupt Flag will be set even if the specific

interrupt is disabled or the global interrupt enable bit in SREG is cleared. A conversion can thus

be triggered without causing an interrupt. However, the Interrupt Flag must be cleared in order to

trigger a new conversion at the next interrupt event.

Figure 99. ADC Auto Trigger Logic

ADTS[2:0]

PRESCALER

CLK_ADC

START

ADIF

ADATE

SOURCE 1

.

CONVERSION

LOGIC

EDGE

DETECTOR

SOURCE n

ADSC

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Using the ADC Interrupt Flag as a trigger source makes the ADC start a new conversion as soon

as the ongoing conversion has finished. The ADC then operates in Free Running mode, con-

stantly sampling and updating the ADC Data Register. The first conversion must be started by

writing a logical one to the ADSC bit in ADCSRA. In this mode the ADC will perform successive

conversions independently of whether the ADC Interrupt Flag, ADIF is cleared or not.

If Auto Triggering is enabled, single conversions can be started by writing ADSC in ADCSRA to

one. ADSC can also be used to determine if a conversion is in progress. The ADSC bit will be

read as one during a conversion, independently of how the conversion was started.

Prescaling and

Figure 100. ADC Prescaler

Conversion Timing

ADEN

START

Reset

7-BIT ADC PRESCALER

CK

ADPS0

ADPS1

ADPS2

ADC CLOCK SOURCE

By default, the successive approximation circuitry requires an input clock frequency between 50

kHz and 200 kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the

input clock frequency to the ADC can be higher than 200 kHz to get a higher sample rate.

The ADC module contains a prescaler, which generates an acceptable ADC clock frequency

from any CPU frequency above 100 kHz. The prescaling is set by the ADPS bits in ADCSRA.

The prescaler starts counting from the moment the ADC is switched on by setting the ADEN bit

in ADCSRA. The prescaler keeps running for as long as the ADEN bit is set, and is continuously

reset when ADEN is low.

When initiating a single ended conversion by setting the ADSC bit in ADCSRA, the conversion

starts at the following rising edge of the ADC clock cycle. See “Differential Gain Channels” on

page 209 for details on differential conversion timing.

A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched

on (ADEN in ADCSRA is set) takes 25 ADC clock cycles in order to initialize the analog circuitry.

The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conver-

sion and 13.5 ADC clock cycles after the start of a first conversion. When a conversion is

complete, the result is written to the ADC Data Registers, and ADIF is set. In single conversion

mode, ADSC is cleared simultaneously. The software may then set ADSC again, and a new

conversion will be initiated on the first rising ADC clock edge.

When Auto Triggering is used, the prescaler is reset when the trigger event occurs. This assures

a fixed delay from the trigger event to the start of conversion. In this mode, the sample-and-hold

takes place 2 ADC clock cycles after the rising edge on the trigger source signal. Three addi-

tional CPU clock cycles are used for synchronization logic. When using Differential mode, along

with Auto triggering from a source other than the ADC Conversion Complete, each conversion

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will require 25 ADC clocks. This is because the ADC must be disabled and re-enabled after

every conversion.

In Free Running mode, a new conversion will be started immediately after the conversion com-

pletes, while ADSC remains high. For a summary of conversion times, see Table 81.

Figure 101. ADC Timing Diagram, First Conversion (Single Conversion Mode)

Conversion

Cycle Number

1

2

12

13

14

15

16

17

18

19

20

21

22

23

24

25

1

2

3

ADC Clock

ADEN

ADSC

ADIF

MSB of Result

LSB of Result

ADCH

ADCL

MUX and REFS

Update

Conversion

Complete

Sample & Hold

MUX and REFS

Update

Figure 102. ADC Timing Diagram, Single Conversion

One Conversion

Next Conversion

1

2

3

4

5

6

7

8

9

10

11

12

13

1

2

3

Cycle Number

ADC Clock

ADSC

ADIF

ADCH

MSB of Result

LSB of Result

ADCL

Sample & Hold

Conversion

MUX and REFS

Update

Complete

MUX and REFS

Update

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Figure 103. ADC Timing Diagram, Auto Triggered Conversion

One Conversion

Next Conversion

1

2

3

4

5

6

7

8

9

10

11

12

13

1

2

Cycle Number

ADC Clock

Trigger

Source

ADATE

ADIF

ADCH

ADCL

MSB of Result

LSB of Result

Sample & Hold

Prescaler

Reset

Conversion

Complete

Prescaler

Reset

MUX and REFS

Update

Figure 104. ADC Timing Diagram, Free Running Conversion

One Conversion

Next Conversion

11

12

13

1

2

3

4

Cycle Number

ADC Clock

ADSC

ADIF

ADCH

ADCL

MSB of Result

LSB of Result

Sample & Hold

MUX and REFS

Update

Conversion

Complete

Table 81. ADC Conversion Time

Sample & Hold (Cycles

from Start of

Condition

Conversion)

Conversion Time (Cycles)

First conversion

13.5

1.5

25

13

Normal conversions, single ended

Auto Triggered conversions

Normal conversions, differential

2

13.5

13/14

1.5/2.5

Differential Gain

Channels

When using differential gain channels, certain aspects of the conversion need to be taken into

consideration.

Differential conversions are synchronized to the internal clock CK_ADC2equal to half the ADC

clock. This synchronization is done automatically by the ADC interface in such a way that the

sample-and-hold occurs at a specific phase of CK_ADC2. A conversion initiated by the user (that is,

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all single conversions, and the first free running conversion) when CK_ADC2is low will take the

same amount of time as a single ended conversion (13 ADC clock cycles from the next pres-

caled clock cycle). A conversion initiated by the user when CK_ADC2is high will take 14 ADC clock

cycles due to the synchronization mechanism. In Free Running mode, a new conversion is initi-

ated immediately after the previous conversion completes, and since CK_ADC2is high at this time,

all automatically started (that is, all but the first) free running conversions will take 14 ADC clock

cycles.

The gain stage is optimized for a bandwidth of 4 kHz at all gain settings. Higher frequencies may

be subjected to non-linear amplification. An external low-pass filter should be used if the input

signal contains higher frequency components than the gain stage bandwidth. Note that the ADC

clock frequency is independent of the gain stage bandwidth limitation. For example, the ADC

clock period may be 6 µs, allowing a channel to be sampled at 12 kSPS, regardless of the band-

width of this channel.

If differential gain channels are used and conversions are started by Auto Triggering, the ADC

must be switched off between conversions. When Auto Triggering is used, the ADC prescaler is

reset before the conversion is started. Since the gain stage is dependent of a stable ADC clock

prior to the conversion, this conversion will not be valid. By disabling and then re-enabling the

ADC between each conversion (writing ADEN in ADCSRA to “0” then to “1”), only extended con-

versions are performed. The result from the extended conversions will be valid. See “Prescaling

and Conversion Timing” on page 207 for timing details.

Changing Channel The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a temporary

register to which the CPU has random access. This ensures that the channels and reference

selection only takes place at a safe point during the conversion. The channel and reference

or Reference

Selection

selection is continuously updated until a conversion is started. Once the conversion starts, the

channel and reference selection is locked to ensure a sufficient sampling time for the ADC. Con-

tinuous updating resumes in the last ADC clock cycle before the conversion completes (ADIF in

ADCSRA is set). Note that the conversion starts on the following rising ADC clock edge after

ADSC is written. The user is thus advised not to write new channel or reference selection values

to ADMUX until one ADC clock cycle after ADSC is written.

If Auto Triggering is used, the exact time of the triggering event can be indeterministic. Special

care must be taken when updating the ADMUX Register, in order to control which conversion

will be affected by the new settings.

If both ADATE and ADEN is written to one, an interrupt event can occur at any time. If the

ADMUX Register is changed in this period, the user cannot tell if the next conversion is based

on the old or the new settings. ADMUX can be safely updated in the following ways:

1. When ADATE or ADEN is cleared.

2. During conversion, minimum one ADC clock cycle after the trigger event.

3. After a conversion, before the Interrupt Flag used as trigger source is cleared.

When updating ADMUX in one of these conditions, the new settings will affect the next ADC

conversion.

Special care should be taken when changing differential channels. Once a differential channel

has been selected, the gain stage may take as much as 125 µs to stabilize to the new value.

Thus conversions should not be started within the first 125 µs after selecting a new differential

channel. Alternatively, conversion results obtained within this period should be discarded.

The same settling time should be observed for the first differential conversion after changing

ADC reference (by changing the REFS1:0 bits in ADMUX).

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ADC Input Channels

When changing channel selections, the user should observe the following guidelines to ensure

that the correct channel is selected:

In Single Conversion mode, always select the channel before starting the conversion. The chan-

nel selection may be changed one ADC clock cycle after writing one to ADSC. However, the

simplest method is to wait for the conversion to complete before changing the channel selection.

In Free Running mode, always select the channel before starting the first conversion. The chan-

nel selection may be changed one ADC clock cycle after writing one to ADSC. However, the

simplest method is to wait for the first conversion to complete, and then change the channel

selection. Since the next conversion has already started automatically, the next result will reflect

the previous channel selection. Subsequent conversions will reflect the new channel selection.

When switching to a differential gain channel, the first conversion result may have a poor accu-

racy due to the required settling time for the automatic offset cancellation circuitry. The user

should preferably disregard the first conversion result.

ADC Voltage

Reference

The reference voltage for the ADC (V_REF) indicates the conversion range for the ADC. Single

ended channels that exceed V_REFwill result in codes close to 0x3FF. V_REFcan be selected as

either AVCC, internal 2.56V reference, or external AREF pin.

AVCC is connected to the ADC through a passive switch. The internal 2.56V reference is gener-

ated from the internal bandgap reference (V_BG) through an internal amplifier. In either case, the

external AREF pin is directly connected to the ADC, and the reference voltage can be made

more immune to noise by connecting a capacitor between the AREF pin and ground. V_REFcan

also be measured at the AREF pin with a high impedant voltmeter. Note that V_REFis a high

impedant source, and only a capacitive load should be connected in a system.

If the user has a fixed voltage source connected to the AREF pin, the user may not use the other

reference voltage options in the application, as they will be shorted to the external voltage. If no

external voltage is applied to the AREF pin, the user may switch between AVCC and 2.56V as

reference selection. The first ADC conversion result after switching reference voltage source

may be inaccurate, and the user is advised to discard this result.

If differential channels are used, the selected reference should not be closer to AVCC than

indicated in Table 122 on page 297.

ADC Noise

Canceler

The ADC features a noise canceler that enables conversion during sleep mode to reduce noise

induced from the CPU core and other I/O peripherals. The noise canceler can be used with ADC

Noise Reduction and Idle mode. To make use of this feature, the following procedure should be

used:

1. Make sure that the ADC is enabled and is not busy converting. Single Conversion

Mode must be selected and the ADC conversion complete interrupt must be

enabled.

2. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion

once the CPU has been halted.

3. If no other interrupts occur before the ADC conversion completes, the ADC interrupt

will wake up the CPU and execute the ADC Conversion Complete interrupt routine. If

another interrupt wakes up the CPU before the ADC conversion is complete, that

interrupt will be executed, and an ADC Conversion Complete interrupt request will be

generated when the ADC conversion completes. The CPU will remain in active mode

until a new sleep command is executed.

Note that the ADC will not be automatically turned off when entering other sleep modes than Idle

mode and ADC Noise Reduction mode. The user is advised to write zero to ADEN before enter-

ing such sleep modes to avoid excessive power consumption. If the ADC is enabled in such

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sleep modes and the user wants to perform differential conversions, the user is advised to

switch the ADC off and on after waking up from sleep to prompt an extended conversion to get a

valid result.

Analog Input Circuitry The Analog Input Circuitry for single ended channels is illustrated in Figure 105. An analog

source applied to ADCn is subjected to the pin capacitance and input leakage of that pin, regard-

less of whether that channel is selected as input for the ADC. When the channel is selected, the

source must drive the S/H capacitor through the series resistance (combined resistance in the

input path).

The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or

less. If such a source is used, the sampling time will be negligible. If a source with higher imped-

ance is used, the sampling time will depend on how long time the source needs to charge the

S/H capacitor, with can vary widely. The user is recommended to only use low impedant sources

with slowly varying signals, since this minimizes the required charge transfer to the S/H

capacitor.

If differential gain channels are used, the input circuitry looks somewhat different, although

source impedances of a few hundred kΩ or less is recommended.

Signal components higher than the Nyquist frequency (f_ADC/2) should not be present for either

kind of channels, to avoid distortion from unpredictable signal convolution. The user is advised

to remove high frequency components with a low-pass filter before applying the signals as

inputs to the ADC.

Figure 105. Analog Input Circuitry

I_IH

ADCn

1..100 kΩ

C_S/H= 14 pF

I_IL

V_CC/2

Analog Noise

Digital circuitry inside and outside the device generates EMI which might affect the accuracy of

Canceling Techniques analog measurements. If conversion accuracy is critical, the noise level can be reduced by

applying the following techniques:

1. Keep analog signal paths as short as possible. Keep them well away from high-

speed switching digital tracks.

2. The AVCC pin on the device should be connected to the digital V_CCsupply voltage

via an LC network as shown in Figure 106.

3. Use the ADC noise canceler function to reduce induced noise from the CPU.

4. If any ADC port pins are used as digital outputs, it is essential that these do not

switch while a conversion is in progress.

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Figure 106. ADC Power Connections

PA4 (ADC4)

PA5 (ADC5)

PA6 (ADC6)

PA7 (ADC7)

AREF

GND

AVCC

PC7

Offset Compensation

Schemes

The gain stage has a built-in offset cancellation circuitry that nulls the offset of differential mea-

surements as much as possible. The remaining offset in the analog path can be measured

directly by selecting the same channel for both differential inputs. This offset residue can be then

subtracted in software from the measurement results. Using this kind of software based offset

correction, offset on any channel can be reduced below one LSB.

ADC Accuracy

Definitions

An n-bit single-ended ADC converts a voltage linearly between GND and V_REFin 2ⁿsteps

(LSBs). The lowest code is read as 0, and the highest code is read as 2ⁿ-1.

Several parameters describe the deviation from the ideal behavior:

•

Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition

(at 0.5 LSB). Ideal value: 0 LSB.

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Figure 107. Offset Error

Output Code

Ideal ADC

Actual ADC

Offset

Error

V_REF

Input Voltage

•

Gain Error: After adjusting for offset, the Gain Error is found as the deviation of the last

transition (0x3FE to 0x3FF) compared to the ideal transition (at 1.5 LSB below maximum).

Ideal value: 0 LSB

Figure 108. Gain Error

Gain

Error

Output Code

Ideal ADC

Actual ADC

V_REF

Input Voltage

•

Integral Non-linearity (INL): After adjusting for offset and gain error, the INL is the maximum

deviation of an actual transition compared to an ideal transition for any code. Ideal value: 0

LSB.

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Figure 109. Integral Non-linearity (INL)

Output Code

Ideal ADC

Actual ADC

V

Input Voltage

REF

•

Differential Non-linearity (DNL): The maximum deviation of the actual code width (the

interval between two adjacent transitions) from the ideal code width (1 LSB). Ideal value: 0

LSB.

Figure 110. Differential Non-linearity (DNL)

Output Code

0x3FF

1 LSB

DNL

0x000

0

V_REFInput Voltage

•

Quantization Error: Due to the quantization of the input voltage into a finite number of codes,

a range of input voltages (1 LSB wide) will code to the same value. Always ±0.5 LSB.

Absolute Accuracy: The maximum deviation of an actual (unadjusted) transition compared

to an ideal transition for any code. This is the compound effect of Offset, Gain Error,

Differential Error, Non-linearity, and Quantization Error. Ideal value: ±0.5 LSB.

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ADC Conversion

Result

After the conversion is complete (ADIF is high), the conversion result can be found in the ADC

Result Registers (ADCL, ADCH).

For single ended conversion, the result is

V

⋅ 1024

IN

ADC = --------------------------

V

REF

where V_INis the voltage on the selected input pin and V_REFthe selected voltage reference (see

Table 83 on page 217 and Table 84 on page 218). 0x000 represents ground, and 0x3FF repre-

sents the selected reference voltage minus one LSB.

If differential channels are used, the result is

(V

– V

) ⋅ GAIN ⋅ 512

POS

NEG

ADC = ------------------------------------------------------------------------

V

REF

where V_POSis the voltage on the positive input pin, V_NEGthe voltage on the negative input pin,

GAIN the selected gain factor, and V_REFthe selected voltage reference. The result is presented

in two’s complement form, from 0x200 (-512d) through 0x1FF (+511d). Note that if the user

wants to perform a quick polarity check of the results, it is sufficient to read the MSB of the result

(ADC9 in ADCH). If this bit is one, the result is negative, and if this bit is zero, the result is posi-

tive. Figure 111 shows the decoding of the differential input range.

Table 82 shows the resulting output codes if the differential input channel pair (ADCn - ADCm) is

selected with a gain of GAIN and a reference voltage of V_REF

.

Figure 111. Differential Measurement Range

Output Code

0x1FF

0x000

0

Differential Input

Voltage (Volts)

- V_REF/GAIN

0x3FF

V

_REF/GAIN

0x200

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Table 82. Correlation between Input Voltage and Output Codes

V_ADCn

Read code

0x1FF

0x1FE

...

Corresponding Decimal Value

V_ADCm+ V_REF/GAIN

511

510

...

V_ADCm+ 511/512 V_REF/GAIN

V_ADCm+ 510/512 V_REF/GAIN

...

V

_ADCm+ 1/512 V_REF/GAIN

V_ADCm

_ADCm- 1/512 V_REF/GAIN

0x001

0x000

0x3FF

...

1

0

V

-1

...

V_ADCm- 511/512 V_REF/GAIN

0x201

0x200

-511

-512

V_ADCm- V_REF/GAIN

Example:

ADMUX = 0xED (ADC3 - ADC2, 10x gain, 2.56V reference, left adjusted result)

Voltage on ADC3 is 300 mV, voltage on ADC2 is 500 mV.

ADCR = 512 × 10 × (300 - 500) / 2560 = -400 = 0x270

ADCL will thus read 0x00, and ADCH will read 0x9C. Writing zero to ADLAR right adjusts

the result: ADCL = 0x70, ADCH = 0x02.

ADC Multiplexer

Selection Register –

ADMUX

Bit

7

REFS1

R/W

0

6

REFS0

R/W

0

5

ADLAR

R/W

0

4

MUX4

R/W

0

3

MUX3

R/W

0

2

MUX2

R/W

0

1

MUX1

R/W

0

MUX0

R/W

0

ADMUX

Read/Write

Initial Value

• Bit 7:6 – REFS1:0: Reference Selection Bits

These bits select the voltage reference for the ADC, as shown in Table 83. If these bits are

changed during a conversion, the change will not go in effect until this conversion is complete

(ADIF in ADCSRA is set). The internal voltage reference options may not be used if an external

reference voltage is being applied to the AREF pin.

Table 83. Voltage Reference Selections for ADC

REFS1

REFS0 Voltage Reference Selection

0

1

0

1

0

1

AREF, Internal Vref turned off

AVCC with external capacitor at AREF pin

Reserved

Internal 2.56V Voltage Reference with external capacitor at AREF pin

•

Bit 5 – ADLAR: ADC Left Adjust Result

The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register.

Write one to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the

ADLAR bit will affect the ADC Data Register immediately, regardless of any ongoing conver-

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sions. For a complete description of this bit, see “The ADC Data Register – ADCL and ADCH” on

page 220.

• Bits 4:0 – MUX4:0: Analog Channel and Gain Selection Bits

The value of these bits selects which combination of analog inputs are connected to the ADC.

These bits also select the gain for the differential channels. See Table 84 for details. If these bits

are changed during a conversion, the change will not go in effect until this conversion is

complete (ADIF in ADCSRA is set).

Table 84. Input Channel and Gain Selections

Single Ended

Input

Positive Differential

Input

Negative Differential

Input

MUX4..0

00000

00001

00010

00011

00100

00101

00110

00111

01000

01001

01010

01011

01100

01101

01110

01111

10000

10001

10010

10011

10100

10101

10110

10111

11000

11001

11010

11011

11100

Gain

ADC0

ADC1

ADC2

ADC3

ADC4

ADC5

ADC6

ADC7

N/A

ADC0

ADC1

ADC0

ADC1

ADC2

ADC3

ADC2

ADC3

ADC0

ADC1

ADC2

ADC3

ADC4

ADC5

ADC6

ADC7

ADC0

ADC1

ADC2

ADC3

ADC4

ADC0

ADC2

ADC1

ADC2

10x

200x

10x

200x

1x

N/A

1x

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Table 84. Input Channel and Gain Selections (Continued)

Single Ended

Input

Positive Differential

Input

Negative Differential

Input

MUX4..0

11101

Gain

ADC5

N/A

ADC2

1x

11110

11111

1.22V (V_BG

0 V (GND)

)

ADC Control and

Status Register A –

ADCSRA

Bit

7

6

5

ADATE

R/W

0

4

ADIF

R/W

0

3

ADIE

R/W

0

2

ADPS2

R/W

0

1

ADPS1

R/W

0

ADPS0

R/W

0

ADEN

R/W

0

ADSC

R/W

0

ADCSRA

Read/Write

Initial Value

• Bit 7 – ADEN: ADC Enable

Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the

ADC off while a conversion is in progress, will terminate this conversion.

• Bit 6 – ADSC: ADC Start Conversion

In Single Conversion mode, write this bit to one to start each conversion. In Free Running Mode,

write this bit to one to start the first conversion. The first conversion after ADSC has been written

after the ADC has been enabled, or if ADSC is written at the same time as the ADC is enabled,

will take 25 ADC clock cycles instead of the normal 13. This first conversion performs initializa-

tion of the ADC.

ADSC will read as one as long as a conversion is in progress. When the conversion is complete,

it returns to zero. Writing zero to this bit has no effect.

• Bit 5 – ADATE: ADC Auto Trigger Enable

When this bit is written to one, Auto Triggering of the ADC is enabled. The ADC will start a con-

version on a positive edge of the selected trigger signal. The trigger source is selected by setting

the ADC Trigger Select bits, ADTS in SFIOR.

• Bit 4 – ADIF: ADC Interrupt Flag

This bit is set when an ADC conversion completes and the Data Registers are updated. The

ADC Conversion Complete Interrupt is executed if the ADIE bit and the I-bit in SREG are set.

ADIF is cleared by hardware when executing the corresponding interrupt handling vector. Alter-

natively, ADIF is cleared by writing a logical one to the flag. Beware that if doing a Read-Modify-

Write on ADCSRA, a pending interrupt can be disabled. This also applies if the SBI and CBI

instructions are used.

• Bit 3 – ADIE: ADC Interrupt Enable

When this bit is written to one and the I-bit in SREG is set, the ADC Conversion Complete Inter-

rupt is activated.

• Bits 2:0 – ADPS2:0: ADC Prescaler Select Bits

These bits determine the division factor between the XTAL frequency and the input clock to the

ADC.

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Table 85. ADC Prescaler Selections

ADPS2

ADPS1

ADPS0

Division Factor

0

1

0

1

0

1

0

1

0

1

0

1

0

1

2

4

8

16

32

64

128

The ADC Data

Register – ADCL and

ADCH

ADLAR = 0

Bit

15

14

13

12

11

10

9

8

–

ADC9

ADC8

ADCH

ADC7

ADC6

ADC5

ADC4

ADC3

ADC2

ADC1

ADC0

ADCL

7

R

0

6

R

0

5

R

0

4

R

0

3

R

0

2

R

0

1

R

0

R

0

Read/Write

Initial Value

0

ADLAR = 1

Bit

15

14

13

12

11

10

9

8

ADC9

ADC8

ADC7

ADC6

ADC5

ADC4

ADC3

ADC2

ADCH

ADC1

ADC0

–

5

–

4

–

3

–

2

–

1

–

0

ADCL

7

R

0

6

R

0

Read/Write

Initial Value

R

0

R

0

R

0

R

0

R

0

R

0

When an ADC conversion is complete, the result is found in these two registers. If differential

channels are used, the result is presented in two’s complement form.

When ADCL is read, the ADC Data Register is not updated until ADCH is read. Consequently, if

the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read

ADCH. Otherwise, ADCL must be read first, then ADCH.

The ADLAR bit in ADMUX, and the MUXn bits in ADMUX affect the way the result is read from

the registers. If ADLAR is set, the result is left adjusted. If ADLAR is cleared (default), the result

is right adjusted.

• ADC9:0: ADC Conversion Result

These bits represent the result from the conversion, as detailed in “ADC Conversion Result” on

page 216.

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Special FunctionIO

Register – SFIOR

Bit

7

ADTS2

R/W

0

6

ADTS1

R/W

0

5

ADTS0

R/W

0

4

–

3

ACME

R/W

0

2

1

PSR2

R/W

0

PUD

R/W

0

PSR10

R/W

0

SFIOR

Read/Write

Initial Value

R

0

• Bit 7:5 – ADTS2:0: ADC Auto Trigger Source

If ADATE in ADCSRA is written to one, the value of these bits selects which source will trigger

an ADC conversion. If ADATE is cleared, the ADTS2:0 settings will have no effect. A conversion

will be triggered by the rising edge of the selected Interrupt Flag. Note that switching from a trig-

ger source that is cleared to a trigger source that is set, will generate a positive edge on the

trigger signal. If ADEN in ADCSRA is set, this will start a conversion. Switching to Free Running

mode (ADTS[2:0]=0) will not cause a trigger event, even if the ADC Interrupt Flag is set.

Table 86. ADC Auto Trigger Source Selections

ADTS2

ADTS1

ADTS0

Trigger Source

0

1

0

1

0

1

0

1

0

1

0

1

0

1

Free Running mode

Analog Comparator

External Interrupt Request 0

Timer/Counter0 Compare Match

Timer/Counter0 Overflow

Timer/Counter1 Compare Match B

Timer/Counter1 Overflow

Timer/Counter1 Capture Event

• Bit 4 – Res: Reserved Bit

This bit is reserved for future use. To ensure compatibility with future devices, this bit must be

written to zero when SFIOR is written.

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JTAG Interface

and On-chip

Debug System

Features

• JTAG (IEEE std. 1149.1 Compliant) Interface

• Boundary-scan Capabilities According to the IEEE std. 1149.1 (JTAG) Standard

• Debugger Access to:

– All Internal Peripheral Units

– Internal and External RAM

– The Internal Register File

– Program Counter

– EEPROM and Flash Memories

– Extensive On-chip Debug Support for Break Conditions, Including

– AVR Break Instruction

– Break on Change of Program Memory Flow

– Single Step Break

– Program Memory Breakpoints on Single Address or Address Range

– Data Memory Breakpoints on Single Address or Address Range

• Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface

• On-chip Debugging Supported by AVR Studio^®

Overview

The AVR IEEE std. 1149.1 compliant JTAG interface can be used for

•

Testing PCBs by using the JTAG Boundary-scan capability

Programming the non-volatile memories, Fuses and Lock bits

On-chip Debugging

A brief description is given in the following sections. Detailed descriptions for Programming via

the JTAG interface, and using the Boundary-scan Chain can be found in the sections “Program-

ming via the JTAG Interface” on page 278 and “IEEE 1149.1 (JTAG) Boundary-scan” on page

228, respectively. The On-chip Debug support is considered being private JTAG instructions,

and distributed within ATMEL and to selected third party vendors only.

Figure 112 shows a block diagram of the JTAG interface and the On-chip Debug system. The

TAP Controller is a state machine controlled by the TCK and TMS signals. The TAP Controller

selects either the JTAG Instruction Register or one of several Data Registers as the scan chain

(Shift Register) between the TDI input and TDO output. The Instruction Register holds JTAG

instructions controlling the behavior of a Data Register.

The ID-Register, Bypass Register, and the Boundary-scan Chain are the Data Registers used

for board-level testing. The JTAG Programming Interface (actually consisting of several physical

and virtual Data Registers) is used for JTAG Serial Programming via the JTAG interface. The

Internal Scan Chain and Break Point Scan Chain are used for On-chip Debugging only.

Test Access Port – The JTAG interface is accessed through four of the AVR’s pins. In JTAG terminology, these pins

constitute the Test Access Port – TAP. These pins are:

TAP

•

TMS: Test Mode Select. This pin is used for navigating through the TAP-controller state

machine.

•

TCK: Test Clock. JTAG operation is synchronous to TCK.

TDI: Test Data In. Serial input data to be shifted in to the Instruction Register or Data

Register (Scan Chains).

•

TDO: Test Data Out. Serial output data from Instruction register or Data Register.

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The IEEE std. 1149.1 also specifies an optional TAP signal; TRST – Test ReSeT – which is not

provided.

When the JTAGEN fuse is unprogrammed, these four TAP pins are normal port pins and the

TAP controller is in reset. When programmed and the JTD bit in MCUCSR is cleared, the TAP

input signals are internally pulled high and the JTAG is enabled for Boundary-scan and program-

ming. In this case, the TAP output pin (TDO) is left floating in states where the JTAG TAP

controller is not shifting data, and must therefore be connected to a pull-up resistor or other

hardware having pull-ups (for instance the TDI-input of the next device in the scan chain). The

device is shipped with this fuse programmed.

For the On-chip Debug system, in addition to the JTAG interface pins, the RESET pin is moni-

tored by the debugger to be able to detect external reset sources. The debugger can also pull

the RESET pin low to reset the whole system, assuming only open collectors on the reset line

are used in the application.

Figure 112. Block Diagram

I/O PORT 0

DEVICE BOUNDARY

BOUNDARY SCAN CHAIN

TDI

JTAG PROGRAMMING

TDO

TAP

INTERFACE

TCK

TMS

CONTROLLER

AVR CPU

INTERNAL

SCAN

CHAIN

FLASH

MEMORY

Address

Data

PC

Instruction

INSTRUCTION

REGISTER

ID

REGISTER

BREAKPOINT

UNIT

M

U

X

FLOW CONTROL

UNIT

BYPASS

REGISTER

DIGITAL

PERIPHERAL

UNITS

BREAKPOINT

SCAN CHAIN

JTAG / AVR CORE

COMMUNICATION

INTERFACE

ADDRESS

DECODER

OCD STATUS

AND CONTROL

I/O PORT n

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Figure 113. TAP Controller State Diagram

1

Test-Logic-Reset

0

1

0

Run-Test/Idle

Select-DR Scan

Select-IR Scan

0

Capture-IR

0

1

Capture-DR

0

Shift-DR

0

Shift-IR

0

1

Exit1-DR

0

1

Exit1-IR

0

Pause-IR

1

Pause-DR

1

0

Exit2-DR

1

Exit2-IR

1

Update-DR

Update-IR

1

0

TAP Controller

The TAP controller is a 16-state finite state machine that controls the operation of the Boundary-

scan circuitry, JTAG programming circuitry, or On-chip Debug system. The state transitions

depicted in Figure 113 depend on the signal present on TMS (shown adjacent to each state tran-

sition) at the time of the rising edge at TCK. The initial state after a Power-On Reset is Test-

Logic-Reset.

As a definition in this document, the LSB is shifted in and out first for all Shift Registers.

Assuming Run-Test/Idle is the present state, a typical scenario for using the JTAG interface is:

•

At the TMS input, apply the sequence 1, 1, 0, 0 at the rising edges of TCK to enter the Shift

Instruction Register – Shift-IR state. While in this state, shift the four bits of the JTAG

instructions into the JTAG Instruction Register from the TDI input at the rising edge of TCK.

The TMS input must be held low during input of the 3 LSBs in order to remain in the Shift-IR

state. The MSB of the instruction is shifted in when this state is left by setting TMS high.

While the instruction is shifted in from the TDI pin, the captured IR-state 0x01 is shifted out

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on the TDO pin. The JTAG Instruction selects a particular Data Register as path between

TDI and TDO and controls the circuitry surrounding the selected Data Register.

•

Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. The instruction is

latched onto the parallel output from the Shift Register path in the Update-IR state. The Exit-

IR, Pause-IR, and Exit2-IR states are only used for navigating the state machine.

At the TMS input, apply the sequence 1, 0, 0 at the rising edges of TCK to enter the Shift

Data Register – Shift-DR state. While in this state, upload the selected Data Register

(selected by the present JTAG instruction in the JTAG Instruction Register) from the TDI

input at the rising edge of TCK. In order to remain in the Shift-DR state, the TMS input must

be held low during input of all bits except the MSB. The MSB of the data is shifted in when

this state is left by setting TMS high. While the Data Register is shifted in from the TDI pin,

the parallel inputs to the Data Register captured in the Capture-DR state is shifted out on the

TDO pin.

•

Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. If the selected Data

Register has a latched parallel-output, the latching takes place in the Update-DR state. The

Exit-DR, Pause-DR, and Exit2-DR states are only used for navigating the state machine.

As shown in the state diagram, the Run-Test/Idle state need not be entered between selecting

JTAG instruction and using Data Registers, and some JTAG instructions may select certain

functions to be performed in the Run-Test/Idle, making it unsuitable as an Idle state.

Note:

Independent of the initial state of the TAP Controller, the Test-Logic-Reset state can always be

entered by holding TMS high for five TCK clock periods.

For detailed information on the JTAG specification, refer to the literature listed in “Bibliography”

on page 227.

Using the

Boundary-scan

Chain

A complete description of the Boundary-scan capabilities are given in the section “IEEE 1149.1

(JTAG) Boundary-scan” on page 228.

Using the On-chip As shown in Figure 112, the hardware support for On-chip Debugging consists mainly of:

Debug System

•

A scan chain on the interface between the internal AVR CPU and the internal peripheral

units

•

Break Point unit

Communication interface between the CPU and JTAG system

All read or modify/write operations needed for implementing the Debugger are done by applying

AVR instructions via the internal AVR CPU Scan Chain. The CPU sends the result to an I/O

memory mapped location which is part of the communication interface between the CPU and the

JTAG system.

The Break Point Unit implements Break on Change of Program Flow, Single Step Break, 2 Pro-

gram Memory Break Points, and 2 combined Break Points. Together, the 4 Break Points can be

configured as either:

•

4 single Program Memory Break Points

3 Single Program Memory Break Point + 1 single Data Memory Break Point

2 single Program Memory Break Points + 2 single Data Memory Break Points

2 single Program Memory Break Points + 1 Program Memory Break Point with mask (“range

Break Point”)

•

2 single Program Memory Break Points + 1 Data Memory Break Point with mask (“range

Break Point”)

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A debugger, like the AVR Studio, may however use one or more of these resources for its inter-

nal purpose, leaving less flexibility to the end-user.

A list of the On-chip Debug specific JTAG instructions is given in “On-chip Debug Specific JTAG

Instructions” on page 226.

The JTAGEN Fuse must be programmed to enable the JTAG Test Access Port. In addition, the

OCDEN Fuse must be programmed and no Lock bits must be set for the On-chip Debug system

to work. As a security feature, the On-chip Debug system is disabled when any Lock bits are set.

Otherwise, the On-chip Debug system would have provided a back-door into a secured device.

The AVR JTAG ICE from Atmel is a powerful development tool for On-chip Debugging of all

AVR 8-bit RISC Microcontrollers with IEEE 1149.1 compliant JTAG interface. The JTAG ICE

and the AVR Studio user interface give the user complete control of the internal resources of the

microcontroller, helping to reduce development time by making debugging easier. The JTAG

ICE performs real-time emulation of the microcontroller while it is running in a target system.

Please refer to the Support Tools section on the AVR pages on www.atmel.com for a full

description of the AVR JTEG ICE. AVR Studio can be downloaded free from Software section

on the same web site.

All necessary execution commands are available in AVR Studio, both on source level and on

disassembly level. The user can execute the program, single step through the code either by

tracing into or stepping over functions, step out of functions, place the cursor on a statement and

execute until the statement is reached, stop the execution, and reset the execution target. In

addition, the user can have an unlimited number of code breakpoints (using the BREAK instruc-

tion) and up to two data memory breakpoints, alternatively combined as a mask (range) Break

Point.

On-chip Debug

Specific JTAG

Instructions

The On-chip Debug support is considered being private JTAG instructions, and distributed within

ATMEL and to selected third party vendors only. Instruction opcodes are listed for reference.

PRIVATE0; $8

PRIVATE1; $9

PRIVATE2; $A

PRIVATE3; $B

Private JTAG instruction for accessing On-chip Debug system.

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On-chip Debug

Related Register in

I/O Memory

On-chip Debug

Register – OCDR

Bit

7

MSB/IDRD

R/W

6

5

4

3

2

1

0

LSB

R/W

0

OCDR

Read/Write

Initial Value

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

R/W

0

The OCDR Register provides a communication channel from the running program in the micro-

controller to the debugger. The CPU can transfer a byte to the debugger by writing to this

location. At the same time, an Internal Flag; I/O Debug Register Dirty – IDRD – is set to indicate

to the debugger that the register has been written. When the CPU reads the OCDR Register the

7 LSB will be from the OCDR Register, while the MSB is the IDRD bit. The debugger clears the

IDRD bit when it has read the information.

In some AVR devices, this register is shared with a standard I/O location. In this case, the OCDR

Register can only be accessed if the OCDEN Fuse is programmed, and the debugger enables

access to the OCDR Register. In all other cases, the standard I/O location is accessed.

Refer to the debugger documentation for further information on how to use this register.

Using the JTAG

Programming

Capabilities

Programming of AVR parts via JTAG is performed via the 4-pin JTAG port, TCK, TMS, TDI and

TDO. These are the only pins that need to be controlled/observed to perform JTAG program-

ming (in addition to power pins). It is not required to apply 12V externally. The JTAGEN Fuse

must be programmed and the JTD bit in the MCUSR Register must be cleared to enable the

JTAG Test Access Port.

The JTAG programming capability supports:

•

Flash programming and verifying

EEPROM programming and verifying

Fuse programming and verifying

Lock bit programming and verifying

The Lock bit security is exactly as in Parallel Programming mode. If the Lock bits LB1 or LB2 are

programmed, the OCDEN Fuse cannot be programmed unless first doing a chip erase. This is a

security feature that ensures no back-door exists for reading out the content of a secured

device.

The details on programming through the JTAG interface and programming specific JTAG

instructions are given in the section “Programming via the JTAG Interface” on page 278.

Bibliography

For more information about general Boundary-scan, the following literature can be consulted:

•

IEEE: IEEE Std. 1149.1-1990. IEEE Standard Test Access Port and Boundary-scan

Architecture, IEEE, 1993

•

Colin Maunder: The Board Designers Guide to Testable Logic Circuits, Addison-Wesley,

1992

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IEEE 1149.1

(JTAG)

Boundary-scan

Features

• JTAG (IEEE std. 1149.1 Compliant) Interface

• Boundary-scan Capabilities According to the JTAG Standard

• Full Scan of all Port Functions as well as Analog Circuitry having Off-chip Connections

• Supports the Optional IDCODE Instruction

• Additional Public AVR_RESET Instruction to Reset the AVR

System Overview

The Boundary-scan chain has the capability of driving and observing the logic levels on the digi-

tal I/O pins, as well as the boundary between digital and analog logic for analog circuitry having

Off-chip connections. At system level, all ICs having JTAG capabilities are connected serially by

the TDI/TDO signals to form a long Shift Register. An external controller sets up the devices to

drive values at their output pins, and observe the input values received from other devices. The

controller compares the received data with the expected result. In this way, Boundary-scan pro-

vides a mechanism for testing interconnections and integrity of components on Printed Circuits

Boards by using the four TAP signals only.

The four IEEE 1149.1 defined mandatory JTAG instructions IDCODE, BYPASS, SAMPLE/PRE-

LOAD, and EXTEST, as well as the AVR specific public JTAG instruction AVR_RESET can be

used for testing the Printed Circuit Board. Initial scanning of the Data Register path will show the

ID-code of the device, since IDCODE is the default JTAG instruction. It may be desirable to have

the AVR device in Reset during Test mode. If not reset, inputs to the device may be determined

by the scan operations, and the internal software may be in an undetermined state when exiting

the Test mode. Entering reset, the outputs of any Port Pin will instantly enter the high impedance

state, making the HIGHZ instruction redundant. If needed, the BYPASS instruction can be

issued to make the shortest possible scan chain through the device. The device can be set in

the reset state either by pulling the external RESET pin low, or issuing the AVR_RESET instruc-

tion with appropriate setting of the Reset Data Register.

The EXTEST instruction is used for sampling external pins and loading output pins with data.

The data from the output latch will be driven out on the pins as soon as the EXTEST instruction

is loaded into the JTAG IR-Register. Therefore, the SAMPLE/PRELOAD should also be used for

setting initial values to the scan ring, to avoid damaging the board when issuing the EXTEST

instruction for the first time. SAMPLE/PRELOAD can also be used for taking a snapshot of the

external pins during normal operation of the part.

The JTAGEN Fuse must be programmed and the JTD bit in the I/O Register MCUCSR must be

cleared to enable the JTAG Test Access Port.

When using the JTAG interface for Boundary-scan, using a JTAG TCK clock frequency higher

than the internal chip frequency is possible. The chip clock is not required to run.

Data Registers

The Data Registers relevant for Boundary-scan operations are:

•

Bypass Register

Device Identification Register

Reset Register

Boundary-scan Chain

Bypass Register

The Bypass Register consists of a single Shift Register stage. When the Bypass Register is

selected as path between TDI and TDO, the register is reset to 0 when leaving the Capture-DR

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controller state. The Bypass Register can be used to shorten the scan chain on a system when

the other devices are to be tested.

Device Identification

Register

Figure 114 shows the structure of the Device Identification Register.

Figure 114. The Format of the Device Identification Register

MSB

31

LSB

0

Bit

28

27

Part Number

16 bits

12

11

1

Device ID

Version

4 bits

Manufacturer ID

1

11 bits

1 bit

Version

Version is a 4-bit number identifying the revision of the component. The JTAG version number

follows the revision of the device. Revision A is 0x0, revision B is 0x1 and so on. However, some

revisions deviate from this rule, and the relevant version number is shown in Table 87.

Table 87. JTAG Version Numbers

Version

JTAG Version Number (Hex)

ATmega16 revision G

ATmega16 revision H

ATmega16 revision I

ATmega16 revision J

ATmega16 revision K

ATmega16 revision L

0x6

0xE

0x8

0x9

0xA

0xB

Part Number

The part number is a 16-bit code identifying the component. The JTAG Part Number for

ATmega16 is listed in Table 88.

Table 88. AVR JTAG Part Number

Part Number

JTAG Part Number (Hex)

ATmega16

0x9403

Manufacturer ID

Reset Register

The Manufacturer ID is a 11 bit code identifying the manufacturer. The JTAG manufacturer ID

for ATMEL is listed in Table 89.

Table 89. Manufacturer ID

Manufacturer

JTAG Manufacturer ID (Hex)

ATMEL

0x01F

The Reset Register is a Test Data Register used to reset the part. Since the AVR tri-states Port

Pins when reset, the Reset Register can also replace the function of the unimplemented optional

JTAG instruction HIGHZ.

A high value in the Reset Register corresponds to pulling the External Reset low. The part is

reset as long as there is a high value present in the Reset Register. Depending on the Fuse set-

tings for the clock options, the part will remain reset for a Reset Time-Out Period (refer to “Clock

Sources” on page 25) after releasing the Reset Register. The output from this Data Register is

not latched, so the reset will take place immediately, as shown in Figure 115.

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Figure 115. Reset Register

To

TDO

From other Internal and

External Reset Sources

From

TDI

Internal Reset

D

Q

ClockDR · AVR_RESET

Boundary-scan Chain The Boundary-scan Chain has the capability of driving and observing the logic levels on the dig-

ital I/O pins, as well as the boundary between digital and analog logic for analog circuitry having

Off-chip connections.

See “Boundary-scan Chain” on page 232 for a complete description.

Boundary-scan

Specific JTAG

Instructions

The instruction register is 4-bit wide, supporting up to 16 instructions. Listed below are the JTAG

instructions useful for Boundary-scan operation. Note that the optional HIGHZ instruction is not

implemented, but all outputs with tri-state capability can be set in high-impedant state by using

the AVR_RESET instruction, since the initial state for all port pins is tri-state.

As a definition in this datasheet, the LSB is shifted in and out first for all Shift Registers.

The OPCODE for each instruction is shown behind the instruction name in hex format. The text

describes which Data Register is selected as path between TDI and TDO for each instruction.

EXTEST; $0

Mandatory JTAG instruction for selecting the Boundary-scan Chain as Data Register for testing

circuitry external to the AVR package. For port-pins, Pull-up Disable, Output Control, Output

Data, and Input Data are all accessible in the scan chain. For Analog circuits having Off-chip

connections, the interface between the analog and the digital logic is in the scan chain. The con-

tents of the latched outputs of the Boundary-scan chain is driven out as soon as the JTAG IR-

register is loaded with the EXTEST instruction.

The active states are:

•

Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain.

Shift-DR: The Internal Scan Chain is shifted by the TCK input.

Update-DR: Data from the scan chain is applied to output pins.

IDCODE; $1

Optional JTAG instruction selecting the 32-bit ID-register as Data Register. The ID-register con-

sists of a version number, a device number and the manufacturer code chosen by JEDEC. This

is the default instruction after power-up.

The active states are:

•

Capture-DR: Data in the IDCODE-register is sampled into the Boundary-scan Chain.

Shift-DR: The IDCODE scan chain is shifted by the TCK input.

SAMPLE_PRELOAD;

$2

Mandatory JTAG instruction for pre-loading the output latches and talking a snap-shot of the

input/output pins without affecting the system operation. However, the output latches are not

connected to the pins. The Boundary-scan Chain is selected as Data Register.

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The active states are:

•

Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain.

Shift-DR: The Boundary-scan Chain is shifted by the TCK input.

Update-DR: Data from the Boundary-scan Chain is applied to the output latches. However,

the output latches are not connected to the pins.

AVR_RESET; $C

The AVR specific public JTAG instruction for forcing the AVR device into the Reset mode or

releasing the JTAG Reset source. The TAP controller is not reset by this instruction. The one bit

Reset Register is selected as Data Register. Note that the reset will be active as long as there is

a logic 'one' in the Reset Chain. The output from this chain is not latched.

The active states are:

•

Shift-DR: The Reset Register is shifted by the TCK input.

BYPASS; $F

Mandatory JTAG instruction selecting the Bypass Register for Data Register.

The active states are:

•

Capture-DR: Loads a logic “0” into the Bypass Register.

Shift-DR: The Bypass Register cell between TDI and TDO is shifted.

Boundary-scan

Related Register in I/O

Memory

MCU Control and

Status Register –

MCUCSR

The MCU Control and Status Register contains control bits for general MCU functions, and pro-

vides information on which reset source caused an MCU Reset.

Bit

7

6

ISC2

R/W

0

5

–

4

3

2

1

0

JTD

R/W

0

JTRF

R/W

WDRF

R/W

BORF

R/W

EXTRF

R/W

PORF

R/W

MCUCSR

Read/Write

Initial Value

R

0

See Bit Description

• Bit 7 – JTD: JTAG Interface Disable

When this bit is zero, the JTAG interface is enabled if the JTAGEN Fuse is programmed. If this

bit is one, the JTAG interface is disabled. In order to avoid unintentional disabling or enabling of

the JTAG interface, a timed sequence must be followed when changing this bit: The application

software must write this bit to the desired value twice within four cycles to change its value.

If the JTAG interface is left unconnected to other JTAG circuitry, the JTD bit should be set to

one. The reason for this is to avoid static current at the TDO pin in the JTAG interface.

• Bit 4 – JTRF: JTAG Reset Flag

This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by

the JTAG instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic

zero to the flag.

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Boundary-scan

Chain

The Boundary-scan chain has the capability of driving and observing the logic levels on the digi-

tal I/O pins, as well as the boundary between digital and analog logic for analog circuitry having

Off-chip connection.

Scanning the Digital

Port Pins

Figure 116 shows the Boundary-scan Cell for a bi-directional port pin with pull-up function. The

cell consists of a standard Boundary-scan cell for the Pull-up Enable – PUExn – function, and a

bi-directional pin cell that combines the three signals Output Control – OCxn, Output Data –

ODxn, and Input Data – IDxn, into only a two-stage Shift Register. The port and pin indexes are

not used in the following description.

The Boundary-scan logic is not included in the figures in the datasheet. Figure 117 shows a sim-

ple digital Port Pin as described in the section “I/O Ports” on page 50. The Boundary-scan

details from Figure 116 replaces the dashed box in Figure 117.

When no alternate port function is present, the Input Data – ID – corresponds to the PINxn Reg-

ister value (but ID has no synchronizer), Output Data corresponds to the PORT Register, Output

Control corresponds to the Data Direction – DD Register, and the Pull-up Enable – PUExn – cor-

responds to logic expression PUD · DDxn · PORTxn.

Digital alternate port functions are connected outside the dotted box in Figure 117 to make the

scan chain read the actual pin value. For Analog function, there is a direct connection from the

external pin to the analog circuit, and a scan chain is inserted on the interface between the digi-

tal logic and the analog circuitry.

Figure 116. Boundary-scan Cell for Bidirectional Port Pin with Pull-up Function.

ShiftDR

To Next Cell

EXTEST

Vcc

Pullup Enable (PUE)

0

1

FF2

Q

LD2

0

1

D

Q

G

Output Control (OC)

FF1

D Q

LD1

0

1

0

1

D

G

Q

Output Data (OD)

0

1

FF0

D

LD0

0

1

Port Pin (PXn)

0

1

Q

D

G

Q

Input Data (ID)

From Last Cell

ClockDR

UpdateDR

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Figure 117. General Port Pin Schematic Diagram⁽¹⁾

PUExn

PUD

Q

D

DDxn

Q _CLR

WDx

RDx

RESET

OCxn

Q

D

Pxn

PORTxn

ODxn

Q _CLR

WPx

RRx

IDxn

RESET

SLEEP

SYNCHRONIZER

RPx

D

Q

D

L

Q

PINxn

Q

CLK _I/O

PUD:

PULLUP DISABLE

WDx:

RDx:

WPx:

RRx:

RPx:

WRITE DDRx

PUExn:

OCxn:

ODxn:

IDxn:

PULLUP ENABLE for pin Pxn

OUTPUT CONTROL for pin Pxn

OUTPUT DATA to pin Pxn

INPUT DATA from pin Pxn

SLEEP CONTROL

READ DDRx

WRITE PORTx

READ PORTx REGISTER

READ PORTx PIN

I/O CLOCK

SLEEP:

CLK _I/O

:

Note:

1. See Boundary-scan description for details.

Boundary-scan and

The 2 Two-wire Interface pins SCL and SDA have one additional control signal in the scan-

the Two-wire Interface chain; Two-wire Interface Enable – TWIEN. As shown in Figure 118, the TWIEN signal enables

a tri-state buffer with slew-rate control in parallel with the ordinary digital port pins. A general

scan cell as shown in Figure 122 is attached to the TWIEN signal.

Notes: 1. A separate scan chain for the 50 ns spike filter on the input is not provided. The ordinary scan

support for digital port pins suffice for connectivity tests. The only reason for having TWIEN in

the scan path, is to be able to disconnect the slew-rate control buffer when doing boundary-

scan.

2. Make sure the OC and TWIEN signals are not asserted simultaneously, as this will lead to

drive contention.

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Figure 118. Additional Scan Signal for the Two-wire Interface

PUExn

OCxn

ODxn

TWIEN

Pxn

SRC

Slew-rate Limited

IDxn

Scanning the RESET

The RESET pin accepts 5V active low logic for standard reset operation, and 12V active high

logic for High Voltage Parallel Programming. An observe-only cell as shown in Figure 119 is

inserted both for the 5V reset signal; RSTT, and the 12V reset signal; RSTHV.

Figure 119. Observe-only Cell

To

Cell

From System Pin

To System Logic

FF1

0

1

D

Q

From

ClockDR

Scanning the Clock

Pins

The AVR devices have many clock options selectable by fuses. These are: Internal RC Oscilla-

tor, External RC, External Clock, (High Frequency) Crystal Oscillator, Low Frequency Crystal

Oscillator, and Ceramic Resonator.

Figure 120 shows how each Oscillator with external connection is supported in the scan chain.

The Enable signal is supported with a general boundary-scan cell, while the Oscillator/Clock out-

put is attached to an observe-only cell. In addition to the main clock, the Timer Oscillator is

scanned in the same way. The output from the internal RC Oscillator is not scanned, as this

Oscillator does not have external connections.

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Figure 120. Boundary-scan Cells for Oscillators and Clock Options

XTAL1/TOSC1

XTAL2/TOSC2

To

ShiftDR

EXTEST

ShiftDR

Cell

0

1

ENABLE

OUTPUT

0

1

FF1

D

Q

D

G

Q

0

1

D

Q

From

ClockDR

UpdateDR

ClockDR

Cell

Table 90 summaries the scan registers for the external clock pin XTAL1, Oscillators with

XTAL1/XTAL2 connections as well as 32 kHz Timer Oscillator.

Table 90. Scan Signals for the Oscillators^(1)(2)(3)

Scanned Clock Line

Enable Signal Scanned Clock Line Clock Option

when not Used

EXTCLKEN

OSCON

EXTCLK (XTAL1)

OSCCK

External Clock

0

External Crystal

External Ceramic

Resonator

RCOSCEN

OSC32EN

TOSKON

RCCK

External RC

1

0

OSC32CK

TOSCK

Low Freq. External Crystal

32 kHz Timer Oscillator

Notes: 1. Do not enable more than one clock source as main clock at a time.

2. Scanning an Oscillator output gives unpredictable results as there is a frequency drift between

the Internal Oscillator and the JTAG TCK clock. If possible, scanning an external clock is

preferred.

3. The clock configuration is programmed by fuses. As a fuse is not changed run-time, the clock

configuration is considered fixed for a given application. The user is advised to scan the same

clock option as to be used in the final system. The enable signals are supported in the scan

chain because the system logic can disable clock options in sleep modes, thereby disconnect-

ing the Oscillator pins from the scan path if not provided. The INTCAP Fuses are not

supported in the scan-chain, so the boundary scan chain can not make a XTAL Oscillator

requiring internal capacitors to run unless the fuse is correctly programmed.

Scanning the Analog

Comparator

The relevant Comparator signals regarding Boundary-scan are shown in Figure 121. The

Boundary-scan cell from Figure 122 is attached to each of these signals. The signals are

described in Table 91.

The Comparator need not be used for pure connectivity testing, since all analog inputs are

shared with a digital port pin as well.

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Figure 121. Analog Comparator

BANDGAP

REFERENCE

ACBG

ACO

AC_IDLE

ACME

ADCEN

ADC MULTIPLEXER

OUTPUT

Figure 122. General Boundary-scan Cell used for Signals for Comparator and ADC

To

Cell

EXTEST

From Digital Logic/

From Analog Ciruitry

0

1

To Analog Circuitry/

To Digital Logic

0

1

D

Q

D

G

Q

From

ClockDR

UpdateDR

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Table 91. Boundary-scan Signals for the Analog Comparator

Signal

Name

Direction as Seen from

the Comparator

Recommended Input

when Not in Use

Output Values when

Recommended Inputs are Used

Description

AC_IDLE

Input

Output

Input

Turns off Analog

comparator when true

1

Depends upon µC code being

executed

ACO

Analog Comparator

Output

Will become input to µC

code being executed

0

ACME

ACBG

Uses output signal from

ADC mux when true

0

Depends upon µC code being

executed

Bandgap Reference

enable

0

Depends upon µC code being

executed

Scanning the ADC

Figure 123 shows a block diagram of the ADC with all relevant control and observe signals. The Boundary-scan cell from

Figure 122 is attached to each of these signals. The ADC need not be used for pure connectivity testing, since all analog

inputs are shared with a digital port pin as well.

Figure 123. Analog to Digital Converter

VCCREN

AREF

IREFEN

2.56V

ref

To Comparator

PASSEN

MUXEN_7

ADC_7

MUXEN_6

ADC_6

MUXEN_5

ADC_5

MUXEN_4

ADC_4

ADCBGEN

SCTEST

1.22V

ref

EXTCH

MUXEN_3

ADC_3

PRECH

AREF

DACOUT

COMP

MUXEN_2

ADC_2

MUXEN_1

ADC_1

MUXEN_0

ADC_0

DAC_9..0

ADCEN

10-bit DAC

+

-

G10

G20

ACTEN

GNDEN

+

NEGSEL_2

NEGSEL_1

NEGSEL_0

10x

-

20x

-

HOLD

ADC_2

ADC_1

ST

ACLK

AMPEN

ADC_0

The signals are described briefly in Table 92.

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Table 92. Boundary-scan Signals for the ADC

Recommended

Input when Not

in Use

Output Values when Recommended

Inputs are used, and CPU is not

Using the ADC

Signal

Name

Direction as Seen

from the ADC

Description

COMP

ACLK

Output

Input

Comparator Output

0

Clock signal to gain stages

implemented as Switch-cap filters

ACTEN

Input

Enable path from gain stages to

the comparator

0

ADCBGEN

Enable Band-gap reference as

negative input to comparator

ADCEN

AMPEN

DAC_9

DAC_8

DAC_7

DAC_6

DAC_5

DAC_4

DAC_3

DAC_2

DAC_1

DAC_0

EXTCH

Input

Power-on signal to the ADC

Power-on signal to the gain stages

Bit 9 of digital value to DAC

Bit 8 of digital value to DAC

Bit 7 of digital value to DAC

Bit 6 of digital value to DAC

Bit 5 of digital value to DAC

Bit 4 of digital value to DAC

Bit 3 of digital value to DAC

Bit 2 of digital value to DAC

Bit 1 of digital value to DAC

Bit 0 of digital value to DAC

0

1

0

1

0

1

0

1

Connect ADC channels 0 - 3 to by-

pass path around gain stages

G10

Input

Enable 10x gain

Enable 20x gain

0

G20

GNDEN

Ground the negative input to

comparator when true

HOLD

Input

Sample&Hold signal. Sample

analog signal when low. Hold

signal when high. If gain stages

are used, this signal must go

active when ACLK is high.

1

IREFEN

Input

Enables Band-gap reference as

AREF signal to DAC

0

MUXEN_7

MUXEN_6

MUXEN_5

MUXEN_4

MUXEN_3

Input

Input Mux bit 7

Input Mux bit 6

Input Mux bit 5

Input Mux bit 4

Input Mux bit 3

0

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Table 92. Boundary-scan Signals for the ADC (Continued)

Recommended

Input when Not

in Use

Output Values when Recommended

Inputs are used, and CPU is not

Using the ADC

Signal

Name

Direction as Seen

from the ADC

Description

MUXEN_2

MUXEN_1

MUXEN_0

Input

Input Mux bit 2

Input Mux bit 1

Input Mux bit 0

0

1

0

1

0

NEGSEL_2 Input

NEGSEL_1 Input

NEGSEL_0 Input

Input Mux for negative input for

differential signal, bit 2

Input Mux for negative input for

differential signal, bit 1

0

Input Mux for negative input for

differential signal, bit 0

PASSEN

PRECH

Input

Enable pass-gate of gain stages.

1

Precharge output latch of

comparator. (Active low)

SCTEST

ST

Input

Switch-cap TEST enable. Output

from x10 gain stage send out to

Port Pin having ADC_4

0

Output of gain stages will settle

faster if this signal is high first two

ACLK periods after AMPEN goes

high.

VCCREN

Input

Selects Vcc as the ACC reference

voltage.

0

Note:

Incorrect setting of the switches in Figure 123 will make signal contention and may damage the part. There are several input

choices to the S&H circuitry on the negative input of the output comparator in Figure 123. Make sure only one path is selected

from either one ADC pin, Bandgap reference source, or Ground.

If the ADC is not to be used during scan, the recommended input values from Table 92 should

be used. The user is recommended not to use the Differential Gain stages during scan. Switch-

cap based gain stages require fast operation and accurate timing which is difficult to obtain

when used in a scan chain. Details concerning operations of the differential gain stage is there-

fore not provided.

The AVR ADC is based on the analog circuitry shown in Figure 123 with a successive approxi-

mation algorithm implemented in the digital logic. When used in Boundary-scan, the problem is

usually to ensure that an applied analog voltage is measured within some limits. This can easily

be done without running a successive approximation algorithm: apply the lower limit on the digi-

tal DAC[9:0] lines, make sure the output from the comparator is low, then apply the upper limit

on the digital DAC[9:0] lines, and verify the output from the comparator to be high.

The ADC need not be used for pure connectivity testing, since all analog inputs are shared with

a digital port pin as well.

When using the ADC, remember the following:

•

The Port Pin for the ADC channel in use must be configured to be an input with pull-up

disabled to avoid signal contention.

•

In Normal mode, a dummy conversion (consisting of 10 comparisons) is performed when

enabling the ADC. The user is advised to wait at least 200 ns after enabling the ADC before

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ATmega16(L)

controlling/observing any ADC signal, or perform a dummy conversion before using the first

result.

•

The DAC values must be stable at the midpoint value 0x200 when having the HOLD signal

low (Sample mode).

As an example, consider the task of verifying a 1.5V ±5% input signal at ADC channel 3 when

the power supply is 5.0V and AREF is externally connected to V_CC

.

The lower limit is:

The upper limit is:

1024 ⋅ 1,5V ⋅ 0,95 ⁄ 5V = 291 = 0x123

1024 ⋅ 1,5V ⋅ 1,05 ⁄ 5V = 323 = 0x143

The recommended values from Table 92 are used unless other values are given in the algorithm

in Table 93. Only the DAC and Port Pin values of the Scan-chain are shown. The column

“Actions” describes what JTAG instruction to be used before filling the Boundary-scan Register

with the succeeding columns. The verification should be done on the data scanned out when

scanning in the data on the same row in the table.

Table 93. Algorithm for Using the ADC

PA3.

Data

PA3.

Control

Pullup_

Enable

Step

Actions

ADCEN

DAC

MUXEN

HOLD

PRECH

1

SAMPLE

_PRELO

AD

1

0x200

0x08

1

0

2

3

4

5

6

EXTEST

1

0x200

0x123

0x200

0x08

0

1

0

1

0

Verify the

COMPbit

scanned

out to be

0

7

8

1

0x200

0x143

0x200

0x08

0

1

0

1

0

9

10

11

Verify the

COMPbit

scanned

out to be

1

Using this algorithm, the timing constraint on the HOLD signal constrains the TCK clock fre-

quency. As the algorithm keeps HOLD high for five steps, the TCK clock frequency has to be at

least five times the number of scan bits divided by the maximum hold time, t_hold,max

.

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ATmega16

Boundary-scan

Order

Table 94 shows the scan order between TDI and TDO when the Boundary-scan chain is

selected as data path. Bit 0 is the LSB; the first bit scanned in, and the first bit scanned out. The

scan order follows the pin-out order as far as possible. Therefore, the bits of Port A is scanned in

the opposite bit order of the other ports. Exceptions from the rules are the Scan chains for the

analog circuits, which constitute the most significant bits of the scan chain regardless of which

physical pin they are connected to. In Figure 116, PXn. Data corresponds to FF0, PXn. Control

corresponds to FF1, and PXn. Pullup_enable corresponds to FF2. Bit 2, 3, 4, and 5 of Port C is

not in the scan chain, since these pins constitute the TAP pins when the JTAG is enabled.

Table 94. ATmega16 Boundary-scan Order

Bit Number

140

139

138

137

136

135

134

133

132

131

130

129

128

127

126

125

124

123

122

121

120

119

118

117

116

115

114

113

112

Signal Name

AC_IDLE

ACO

Module

Comparator

ACME

ACBG

COMP

ADC

PRIVATE_SIGNAL1⁽¹⁾

ACLK

ACTEN

PRIVATE_SIGNAL2⁽²⁾

ADCBGEN

ADCEN

AMPEN

DAC_9

DAC_8

DAC_7

DAC_6

DAC_5

DAC_4

DAC_3

DAC_2

DAC_1

DAC_0

EXTCH

G10

G20

GNDEN

HOLD

IREFEN

MUXEN_7

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Table 94. ATmega16 Boundary-scan Order (Continued)

Bit Number

111

110

109

108

107

106

105

104

103

102

101

100

99

Signal Name

MUXEN_6

Module

MUXEN_5

MUXEN_4

MUXEN_3

MUXEN_2

MUXEN_1

MUXEN_0

NEGSEL_2

NEGSEL_1

NEGSEL_0

PASSEN

PRECH

SCTEST

98

ST

97

VCCREN

96

PB0.Data

Port B

95

PB0.Control

PB0.Pullup_Enable

PB1.Data

94

93

92

PB1.Control

PB1.Pullup_Enable

PB2.Data

91

90

89

PB2.Control

PB2.Pullup_Enable

PB3.Data

88

87

86

PB3.Control

PB3.Pullup_Enable

PB4.Data

85

84

83

PB4.Control

PB4.Pullup_Enable

PB5.Data

82

81

80

PB5.Control

PB5.Pullup_Enable

PB6.Data

79

78

77

PB6.Control

PB6.Pullup_Enable

76

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Table 94. ATmega16 Boundary-scan Order (Continued)

Bit Number

75

Signal Name

PB7.Data

Module

74

PB7.Control

PB7.Pullup_Enable

RSTT

73

72

Reset Logic

(Observe-Only)

71

RSTHV

70

EXTCLKEN

OSCON

Enable signals for main clock/Oscillators

69

68

RCOSCEN

67

OSC32EN

66

EXTCLK (XTAL1)

OSCCK

Clock input and Oscillators for the main clock

(Observe-Only)

65

64

RCCK

63

OSC32CK

62

TWIEN

TWI

61

PD0.Data

Port D

60

PD0.Control

PD0.Pullup_Enable

PD1.Data

59

58

57

PD1.Control

PD1.Pullup_Enable

PD2.Data

56

55

54

PD2.Control

PD2.Pullup_Enable

PD3.Data

53

52

51

PD3.Control

PD3.Pullup_Enable

PD4.Data

50

49

48

PD4.Control

PD4.Pullup_Enable

PD5.Data

47

46

45

PD5.Control

PD5.Pullup_Enable

PD6.Data

44

43

42

PD6.Control

PD6.Pullup_Enable

PD7.Data

41

40

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Table 94. ATmega16 Boundary-scan Order (Continued)

Bit Number

39

38

37

36

35

34

33

32

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

Signal Name

PD7.Control

PD7.Pullup_Enable

PC0.Data

Module

Port C

PC0.Control

PC0.Pullup_Enable

PC1.Data

PC1.Control

PC1.Pullup_Enable

PC6.Data

PC6.Control

PC6.Pullup_Enable

PC7.Data

PC7.Control

PC7.Pullup_Enable

TOSC

32 kHz Timer Oscillator

Port A

TOSCON

PA7.Data

PA7.Control

PA7.Pullup_Enable

PA6.Data

PA6.Control

PA6.Pullup_Enable

PA5.Data

PA5.Control

PA5.Pullup_Enable

PA4.Data

PA4.Control

PA4.Pullup_Enable

PA3.Data

PA3.Control

PA3.Pullup_Enable

PA2.Data

8

7

PA2.Control

6

PA2.Pullup_Enable

PA1.Data

5

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Table 94. ATmega16 Boundary-scan Order (Continued)

Bit Number

Signal Name

PA1.Control

Module

4

3

2

1

0

PA1.Pullup_Enable

PA0.Data

PA0.Control

PA0.Pullup_Enable

Notes: 1. PRIVATE_SIGNAL1 should always be scanned in as zero.

2. PRIVATE:SIGNAL2 should always be scanned in as zero.

Boundary-scan

Description

Language Files

Boundary-scan Description Language (BSDL) files describe Boundary-scan capable devices in

a standard format used by automated test-generation software. The order and function of bits in

the Boundary-scan Data Register are included in this description. A BSDL file for ATmega16 is

available.

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Boot Loader

Support – Read-

While-Write

Self-

The Boot Loader Support provides a real Read-While-Write Self-Programming mechanism for

downloading and uploading program code by the MCU itself. This feature allows flexible applica-

tion software updates controlled by the MCU using a Flash-resident Boot Loader program. The

Boot Loader program can use any available data interface and associated protocol to read code

and write (program) that code into the Flash memory, or read the code from the Program mem-

ory. The program code within the Boot Loader section has the capability to write into the entire

Flash, including the Boot Loader memory. The Boot Loader can thus even modify itself, and it

can also erase itself from the code if the feature is not needed anymore. The size of the Boot

Loader memory is configurable with Fuses and the Boot Loader has two separate sets of Boot

Lock bits which can be set independently. This gives the user a unique flexibility to select differ-

ent levels of protection.

Programming

Features

• Read-While-Write Self-Programming

• Flexible Boot Memory size

• High Security (Separate Boot Lock Bits for a Flexible Protection)

• Separate Fuse to Select Reset Vector

• Optimized Page⁽¹⁾Size

• Code Efficient Algorithm

• Efficient Read-Modify-Write Support

Note:

1. A page is a section in the flash consisting of several bytes (see Table 107 on page 262)

used during programming. The page organization does not affect normal operation.

Application and

Boot Loader Flash

Sections

The Flash memory is organized in two main sections, the Application section and the Boot

Loader section (see Figure 125). The size of the different sections is configured by the BOOTSZ

Fuses as shown in Table 100 on page 257 and Figure 125. These two sections can have differ-

ent level of protection since they have different sets of Lock bits.

Application Section

The Application section is the section of the Flash that is used for storing the application code.

The protection level for the application section can be selected by the Application Boot Lock bits

(Boot Lock bits 0), see Table 96 on page 249. The Application section can never store any Boot

Loader code since the SPM instruction is disabled when executed from the Application section.

BLS – Boot Loader

Section

While the Application section is used for storing the application code, the The Boot Loader soft-

ware must be located in the BLS since the SPM instruction can initiate a programming when

executing from the BLS only. The SPM instruction can access the entire Flash, including the

BLS itself. The protection level for the Boot Loader section can be selected by the Boot Loader

Lock bits (Boot Lock bits 1), see Table 97 on page 249.

Read-While-Write

and no Read-

While-Write Flash

Sections

Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader soft-

ware update is dependent on which address that is being programmed. In addition to the two

sections that are configurable by the BOOTSZ Fuses as described above, the Flash is also

divided into two fixed sections, the Read-While-Write (RWW) section and the No Read-While-

Write (NRWW) section. The limit between the RWW- and NRWW sections is given in Table 101

on page 257 and Figure 125 on page 248. The main difference between the two sections is:

•

When erasing or writing a page located inside the RWW section, the NRWW section can be

read during the operation.

•

When erasing or writing a page located inside the NRWW section, the CPU is halted during

the entire operation.

Note that the user software can never read any code that is located inside the RWW section dur-

ing a Boot Loader software operation. The syntax “Read-While-Write section” refers to which

section that is being programmed (erased or written), not which section that actually is being

read during a Boot Loader software update.

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RWW – Read-While-

Write Section

If a Boot Loader software update is programming a page inside the RWW section, it is possible

to read code from the Flash, but only code that is located in the NRWW section. During an on-

going programming, the software must ensure that the RWW section never is being read. If the

user software is trying to read code that is located inside the RWW section (that is, by a

call/jmp/lpm or an interrupt) during programming, the software might end up in an unknown

state. To avoid this, the interrupts should either be disabled or moved to the Boot Loader sec-

tion. The Boot Loader section is always located in the NRWW section. The RWW Section Busy

bit (RWWSB) in the Store Program Memory Control Register (SPMCR) will be read as logical

one as long as the RWW section is blocked for reading. After a programming is completed, the

RWWSB must be cleared by software before reading code located in the RWW section. See

“Store Program Memory Control Register – SPMCR” on page 250. for details on how to clear

RWWSB.

NRWW – No Read-

While-Write Section

The code located in the NRWW section can be read when the Boot Loader software is updating

a page in the RWW section. When the Boot Loader code updates the NRWW section, the CPU

is halted during the entire page erase or page write operation.

Table 95. Read-While-Write Features

Which Section does the Z-

pointer Address during the

Programming?

Which Section can be

Read during

Read-While-

Write

Supported?

Is the CPU

Halted?

Programming?

RWW section

NRWW section

None

No

Yes

No

NRWW section

Yes

Figure 124. Read-While-Write vs. No Read-While-Write

Read-While-Write

(RWW) Section

Z-pointer

Addresses NRWW

Section

Z-pointer

No Read-While-Write

Addresses RWW

(NRWW) Section

Section

CPU is Halted

during the Operation

Code Located in

NRWW Section

Can be Read during

the Operation

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Figure 125. Memory Sections⁽¹⁾

Program Memory

BOOTSZ = '10'

Program Memory

BOOTSZ = '11'

$0000

Application Flash Section

End RWW

Start NRWW

Application Flash Section

Boot Loader Flash Section

Application Flash Section

Boot Loader Flash Section

End Application

Start Boot Loader

Flashend

Start Boot Loader

Flashend

Program Memory

BOOTSZ = '01'

Program Memory

BOOTSZ = '00'

$0000

Application Flash Section

Application flash Section

End RWW, End Application

End RWW

Start NRWW, Start Boot Loader

Start NRWW

Application Flash Section

Boot Loader Flash Section

End Application

Boot Loader Flash Section

Start Boot Loader

Flashend

Note:

1. The parameters in the figure above are given in Table 100 on page 257.

Boot Loader Lock If no Boot Loader capability is needed, the entire Flash is available for application code. The

Boot Loader has two separate sets of Boot Lock bits which can be set independently. This gives

the user a unique flexibility to select different levels of protection.

Bits

The user can select:

•

To protect the entire Flash from a software update by the MCU

To protect only the Boot Loader Flash section from a software update by the MCU

To protect only the Application Flash section from a software update by the MCU

Allow software update in the entire Flash

See Table 96 and Table 97 for further details. The Boot Lock bits can be set in software and in

Serial or Parallel Programming mode, but they can be cleared by a Chip Erase command only.

The general Write Lock (Lock Bit mode 2) does not control the programming of the Flash mem-

ory by SPM instruction. Similarly, the general Read/Write Lock (Lock Bit mode 3) does not

control reading nor writing by LPM/SPM, if it is attempted.

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Table 96. Boot Lock Bit0 Protection Modes (Application Section)⁽¹⁾

BLB0 Mode BLB02 BLB01 Protection

No restrictions for SPM or LPM accessing the Application

section.

1

2

1

0

SPM is not allowed to write to the Application section.

SPM is not allowed to write to the Application section, and

LPM executing from the Boot Loader section is not

allowed to read from the Application section. If interrupt

vectors are placed in the Boot Loader section, interrupts

are disabled while executing from the Application section.

3

4

0

1

LPM executing from the Boot Loader section is not

allowed to read from the Application section. If interrupt

vectors are placed in the Boot Loader section, interrupts

are disabled while executing from the Application section.

Note:

1. “1” means unprogrammed, “0” means programmed

Table 97. Boot Lock Bit1 Protection Modes (Boot Loader Section)⁽¹⁾

BLB1 mode BLB12 BLB11 Protection

No restrictions for SPM or LPM accessing the Boot Loader

section.

1

2

1

0

SPM is not allowed to write to the Boot Loader section.

SPM is not allowed to write to the Boot Loader section,

and LPM executing from the Application section is not

allowed to read from the Boot Loader section. If interrupt

vectors are placed in the Application section, interrupts

are disabled while executing from the Boot Loader section.

3

4

0

LPM executing from the Application section is not allowed

to read from the Boot Loader section. If interrupt vectors

are placed in the Application section, interrupts are

disabled while executing from the Boot Loader section.

0

1

Note:

1. “1” means unprogrammed, “0” means programmed

Entering the Boot Entering the Boot Loader takes place by a jump or call from the application program. This may

be initiated by a trigger such as a command received via USART, or SPI interface. Alternatively,

Loader Program

the Boot Reset Fuse can be programmed so that the Reset Vector is pointing to the Boot Flash

start address after a reset. In this case, the Boot Loader is started after a reset. After the applica-

tion code is loaded, the program can start executing the application code. Note that the fuses

cannot be changed by the MCU itself. This means that once the Boot Reset Fuse is pro-

grammed, the Reset Vector will always point to the Boot Loader Reset and the fuse can only be

changed through the serial or parallel programming interface.

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Table 98. Boot Reset Fuse⁽¹⁾

BOOTRST

Reset Address

1

0

Reset Vector = Application reset (address $0000)

Reset Vector = Boot Loader reset (see Table 100 on page 257)

Note:

1. “1” means unprogrammed, “0” means programmed

Store Program

Memory Control

Register – SPMCR

The Store Program Memory Control Register contains the control bits needed to control the Boot

Loader operations.

Bit

7

SPMIE

R/W

0

6

5

–

4

RWWSRE

R/W

3

BLBSET

R/W

0

2

PGWRT

R/W

0

1

PGERS

R/W

0

SPMEN

R/W

0

RWWSB

SPMCR

Read/Write

Initial value

R

0

R

0

• Bit 7 – SPMIE: SPM Interrupt Enable

When the SPMIE bit is written to one, and the I-bit in the Status Register is set (one), the SPM

ready interrupt will be enabled. The SPM ready Interrupt will be executed as long as the SPMEN

bit in the SPMCR Register is cleared.

• Bit 6 – RWWSB: Read-While-Write Section Busy

When a self-programming (Page Erase or Page Write) operation to the RWW section is initiated,

the RWWSB will be set (one) by hardware. When the RWWSB bit is set, the RWW section can-

not be accessed. The RWWSB bit will be cleared if the RWWSRE bit is written to one after a

Self-Programming operation is completed. Alternatively the RWWSB bit will automatically be

cleared if a page load operation is initiated.

• Bit 5 – Res: Reserved Bit

This bit is a reserved bit in the ATmega16 and always read as zero.

• Bit 4 – RWWSRE: Read-While-Write Section Read Enable

When programming (Page Erase or Page Write) to the RWW section, the RWW section is

blocked for reading (the RWWSB will be set by hardware). To re-enable the RWW section, the

user software must wait until the programming is completed (SPMEN will be cleared). Then, if

the RWWSRE bit is written to one at the same time as SPMEN, the next SPM instruction within

four clock cycles re-enables the RWW section. The RWW section cannot be re-enabled while

the Flash is busy with a page erase or a page write (SPMEN is set). If the RWWSRE bit is writ-

ten while the Flash is being loaded, the Flash load operation will abort and the data loaded will

be lost.

• Bit 3 – BLBSET: Boot Lock Bit Set

If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock

cycles sets Boot Lock bits, according to the data in R0. The data in R1 and the address in the Z-

pointer are ignored. The BLBSET bit will automatically be cleared upon completion of the Lock

bit set, or if no SPM instruction is executed within four clock cycles.

An LPM instruction within three cycles after BLBSET and SPMEN are set in the SPMCR Regis-

ter, will read either the Lock bits or the Fuse bits (depending on Z0 in the Z-pointer) into the

destination register. See “Reading the Fuse and Lock Bits from Software” on page 254 for

details.

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ATmega16(L)

• Bit 2 – PGWRT: Page Write

If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock

cycles executes Page Write, with the data stored in the temporary buffer. The page address is

taken from the high part of the Z-pointer. The data in R1 and R0 are ignored. The PGWRT bit

will auto-clear upon completion of a page write, or if no SPM instruction is executed within four

clock cycles. The CPU is halted during the entire page write operation if the NRWW section is

addressed.

• Bit 1 – PGERS: Page Erase

If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock

cycles executes Page Erase. The page address is taken from the high part of the Z-pointer. The

data in R1 and R0 are ignored. The PGERS bit will auto-clear upon completion of a page erase,

or if no SPM instruction is executed within four clock cycles. The CPU is halted during the entire

page write operation if the NRWW section is addressed.

• Bit 0 – SPMEN: Store Program Memory Enable

This bit enables the SPM instruction for the next four clock cycles. If written to one together with

either RWWSRE, BLBSET, PGWRT’ or PGERS, the following SPM instruction will have a spe-

cial meaning, see description above. If only SPMEN is written, the following SPM instruction will

store the value in R1:R0 in the temporary page buffer addressed by the Z-pointer. The LSB of

the Z-pointer is ignored. The SPMEN bit will auto-clear upon completion of an SPM instruction,

or if no SPM instruction is executed within four clock cycles. During page erase and page write,

the SPMEN bit remains high until the operation is completed.

Writing any other combination than “10001”, “01001”, “00101”, “00011” or “00001” in the lower

five bits will have no effect.

Addressing the

Flash during Self-

Programming

The Z-pointer is used to address the SPM commands.

Bit

15

Z15

Z7

7

14

Z14

Z6

6

13

Z13

Z5

5

12

Z12

Z4

4

11

Z11

Z3

3

10

Z10

Z2

2

9

Z9

Z1

1

8

Z8

Z0

0

ZH (R31)

ZL (R30)

Since the Flash is organized in pages (see Table 107 on page 262), the Program Counter can

be treated as having two different sections. One section, consisting of the least significant bits, is

addressing the words within a page, while the most significant bits are addressing the pages.

This is shown in Figure 126. Note that the Page Erase and Page Write operations are addressed

independently. Therefore it is of major importance that the Boot Loader software addresses the

same page in both the Page Erase and Page Write operation. Once a programming operation is

initiated, the address is latched and the Z-pointer can be used for other operations.

The only SPM operation that does not use the Z-pointer is Setting the Boot Loader Lock bits.

The content of the Z-pointer is ignored and will have no effect on the operation. The LPM

instruction does also use the Z pointer to store the address. Since this instruction addresses the

Flash byte by byte, also the LSB (bit Z0) of the Z-pointer is used.

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Figure 126. Addressing the Flash during SPM⁽¹⁾

BIT 15

ZPCMSB

ZPAGEMSB

1

0

Z - REGISTER

PCMSB

PAGEMSB

PCWORD

PROGRAM

COUNTER

PCPAGE

PAGE ADDRESS

WITHIN THE FLASH

WORD ADDRESS

WITHIN A PAGE

PROGRAM MEMORY

PAGE

INSTRUCTION WORD

PCWORD[PAGEMSB:0]:

00

01

02

PAGEEND

Notes: 1. The different variables used in Figure 126 are listed in Table 102 on page 258.

2. PCPAGE and PCWORD are listed in Table 107 on page 262.

Self-Programming The program memory is updated in a page by page fashion. Before programming a page with

the data stored in the temporary page buffer, the page must be erased. The temporary page buf-

fer is filled one word at a time using SPM and the buffer can be filled either before the page

erase command or between a page erase and a page write operation:

the Flash

Alternative 1, fill the buffer before a Page Erase

•

Fill temporary page buffer

Perform a Page Erase

Perform a Page Write

Alternative 2, fill the buffer after Page Erase

•

Perform a Page Erase

Fill temporary page buffer

Perform a Page Write

If only a part of the page needs to be changed, the rest of the page must be stored (for example

in the temporary page buffer) before the erase, and then be rewritten. When using alternative 1,

the Boot Loader provides an effective Read-Modify-Write feature which allows the user software

to first read the page, do the necessary changes, and then write back the modified data. If alter-

native 2 is used, it is not possible to read the old data while loading since the page is already

erased. The temporary page buffer can be accessed in a random sequence. It is essential that

the page address used in both the page erase and page write operation is addressing the same

page. See “Simple Assembly Code Example for a Boot Loader” on page 256 for an assembly

code example.

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Performing Page

Erase by SPM

To execute Page Erase, set up the address in the Z-pointer, write “X0000011” to SPMCR and

execute SPM within four clock cycles after writing SPMCR. The data in R1 and R0 is ignored.

The page address must be written to PCPAGE in the Z-register. Other bits in the Z-pointer must

be written zero during this operation.

•

Page Erase to the RWW section: The NRWW section can be read during the page erase.

Page Erase to the NRWW section: The CPU is halted during the operation.

Filling the Temporary

To write an instruction word, set up the address in the Z-pointer and data in R1:R0, write

Buffer (Page Loading) “00000001” to SPMCR and execute SPM within four clock cycles after writing SPMCR. The con-

tent of PCWORD in the Z-register is used to address the data in the temporary buffer. The

temporary buffer will auto-erase after a page write operation or by writing the RWWSRE bit in

SPMCR. It is also erased after a system reset. Note that it is not possible to write more than one

time to each address without erasing the temporary buffer.

Note:

If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be

lost.

Performing a Page

Write

To execute Page Write, set up the address in the Z-pointer, write “X0000101” to SPMCR and

execute SPM within four clock cycles after writing SPMCR. The data in R1 and R0 is ignored.

The page address must be written to PCPAGE. Other bits in the Z-pointer must be written zero

during this operation.

•

Page Write to the RWW section: The NRWW section can be read during the Page Write.

Page Write to the NRWW section: The CPU is halted during the operation.

Using the SPM

Interrupt

If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the

SPMEN bit in SPMCR is cleared. This means that the interrupt can be used instead of polling

the SPMCR Register in software. When using the SPM interrupt, the Interrupt Vectors should be

moved to the BLS section to avoid that an interrupt is accessing the RWW section when it is

blocked for reading. How to move the interrupts is described in “Interrupts” on page 45.

Consideration while

Updating BLS

Special care must be taken if the user allows the Boot Loader section to be updated by leaving

Boot Lock bit11 unprogrammed. An accidental write to the Boot Loader itself can corrupt the

entire Boot Loader, and further software updates might be impossible. If it is not necessary to

change the Boot Loader software itself, it is recommended to program the Boot Lock bit11 to

protect the Boot Loader software from any internal software changes.

Prevent Reading the

RWW Section during

Self-Programming

During Self-Programming (either Page Erase or Page Write), the RWW section is always

blocked for reading. The user software itself must prevent that this section is addressed during

the Self-Programming operation. The RWWSB in the SPMCR will be set as long as the RWW

section is busy. During self-programming the Interrupt Vector table should be moved to the BLS

as described in “Interrupts” on page 45, or the interrupts must be disabled. Before addressing

the RWW section after the programming is completed, the user software must clear the

RWWSB by writing the RWWSRE. See “Simple Assembly Code Example for a Boot Loader” on

page 256 for an example.

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Setting the Boot

Loader Lock Bits by

SPM

To set the Boot Loader Lock bits, write the desired data to R0, write “X0001001” to SPMCR and

execute SPM within four clock cycles after writing SPMCR. The only accessible Lock bits are the

Boot Lock bits that may prevent the Application and Boot Loader section from any software

update by the MCU.

Bit

7

6

5

4

3

2

1

0

R0

1

BLB12

BLB11

BLB02

BLB01

1

See Table 96 and Table 97 for how the different settings of the Boot Loader bits affect the Flash

access.

If bits 5..2 in R0 are cleared (zero), the corresponding Boot Lock bit will be programmed if an

SPM instruction is executed within four cycles after BLBSET and SPMEN are set in SPMCR.

The Z-pointer is don’t care during this operation, but for future compatibility it is recommended to

load the Z-pointer with $0001 (same as used for reading the Lock bits). For future compatibility It

is also recommended to set bits 7, 6, 1, and 0 in R0 to “1” when writing the Lock bits. When pro-

gramming the Lock bits the entire Flash can be read during the operation.

EEPROM Write

Prevents Writing to

SPMCR

Note that an EEPROM write operation will block all software programming to Flash. Reading the

Fuses and Lock bits from software will also be prevented during the EEPROM write operation. It

is recommended that the user checks the status bit (EEWE) in the EECR Register and verifies

that the bit is cleared before writing to the SPMCR Register.

Reading the Fuse and It is possible to read both the Fuse and Lock bits from software. To read the Lock bits, load the

Lock Bits from

Software

Z-pointer with $0001 and set the BLBSET and SPMEN bits in SPMCR. When an LPM instruction

is executed within three CPU cycles after the BLBSET and SPMEN bits are set in SPMCR, the

value of the Lock bits will be loaded in the destination register. The BLBSET and SPMEN bits

will auto-clear upon completion of reading the Lock bits or if no LPM instruction is executed

within three CPU cycles or no SPM instruction is executed within four CPU cycles. When BLB-

SET and SPMEN are cleared, LPM will work as described in the Instruction set Manual.

Bit

Rd

7

6

5

4

3

2

1

0

–

BLB12

BLB11

BLB02

BLB01

LB2

LB1

The algorithm for reading the Fuse Low bits is similar to the one described above for reading the

Lock bits. To read the Fuse Low bits, load the Z-pointer with $0000 and set the BLBSET and

SPMEN bits in SPMCR. When an LPM instruction is executed within three cycles after the BLB-

SET and SPMEN bits are set in the SPMCR, the value of the Fuse Low bits (FLB) will be loaded

in the destination register as shown below. Refer to Table 106 on page 261 for a detailed

description and mapping of the Fuse Low bits.

Bit

Rd

7

6

5

4

3

2

1

0

FLB7

FLB6

FLB5

FLB4

FLB3

FLB2

FLB1

FLB0

Similarly, when reading the Fuse High bits, load $0003 in the Z-pointer. When an LPM instruc-

tion is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCR,

the value of the Fuse High bits (FHB) will be loaded in the destination register as shown below.

Refer to Table 105 on page 260 for detailed description and mapping of the Fuse High bits.

Bit

Rd

7

6

5

4

3

2

1

0

FHB7

FHB6

FHB5

FHB4

FHB3

FHB2

FHB1

FHB0

Fuse and Lock bits that are programmed, will be read as zero. Fuse and Lock bits that are

unprogrammed, will be read as one.

Preventing Flash

Corruption

During periods of low V_CC,the Flash program can be corrupted because the supply voltage is too

low for the CPU and the Flash to operate properly. These issues are the same as for board level

systems using the Flash, and the same design solutions should be applied.

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A Flash program corruption can be caused by two situations when the voltage is too low. First, a

regular write sequence to the Flash requires a minimum voltage to operate correctly. Secondly,

the CPU itself can execute instructions incorrectly, if the supply voltage for executing instructions

is too low.

Flash corruption can easily be avoided by following these design recommendations (one is

sufficient):

1. If there is no need for a Boot Loader update in the system, program the Boot Loader Lock

bits to prevent any Boot Loader software updates.

2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage.

This can be done by enabling the internal Brown-out Detector (BOD) if the operating volt-

age matches the detection level. If not, an external low V_CCReset Protection circuit can

be used. If a reset occurs while a write operation is in progress, the write operation will be

completed provided that the power supply voltage is sufficient.

3. Keep the AVR core in Power-down Sleep mode during periods of low V_CC. This will pre-

vent the CPU from attempting to decode and execute instructions, effectively protecting

the SPMCR Register and thus the Flash from unintentional writes.

Programming Time for The Calibrated RC Oscillator is used to time Flash accesses. Table 99 shows the typical pro-

Flash when using SPM gramming time for Flash accesses from the CPU.

Table 99. SPM Programming Time.

Symbol

Min Programming Time

Max Programming Time

Flash write (Page Erase, Page

Write, and write Lock bits by SPM)

3.7 ms

4.5 ms

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Simple Assembly

Code Example for a

Boot Loader

;-the routine writes one page of data from RAM to Flash

; the first data location in RAM is pointed to by the Y pointer

; the first data location in Flash is pointed to by the Z pointer

;-error handling is not included

;-the routine must be placed inside the boot space

; (at least the Do_spm sub routine). Only code inside NRWW section can

; be read during self-programming (page erase and page write).

;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24),

; loophi (r25), spmcrval (r20)

; storing and restoring of registers is not included in the routine

; register usage can be optimized at the expense of code size

;-It is assumed that either the interrupt table is moved to the Boot

; loader section or that the interrupts are disabled.

.equ PAGESIZEB = PAGESIZE*2

; PAGESIZEB is page size in BYTES, not

; words

.org SMALLBOOTSTART

Write_page:

; page erase

ldi

spmcrval, (1<<PGERS) | (1<<SPMEN)

call Do_spm

; re-enable the RWW section

ldi

spmcrval, (1<<RWWSRE) | (1<<SPMEN)

call Do_spm

; transfer data from RAM to Flash page buffer

ldi

looplo, low(PAGESIZEB)

loophi, high(PAGESIZEB)

;init loop variable

;not required for PAGESIZEB<=256

Wrloop:

ld

r0, Y+

ld

r1, Y+

ldi

spmcrval, (1<<SPMEN)

call Do_spm

adiw ZH:ZL, 2

sbiw loophi:looplo, 2

brne Wrloop

;use subi for PAGESIZEB<=256

; execute page write

subi ZL, low(PAGESIZEB)

sbci ZH, high(PAGESIZEB)

;restore pointer

;not required for PAGESIZEB<=256

ldi

spmcrval, (1<<PGWRT) | (1<<SPMEN)

call Do_spm

; re-enable the RWW section

ldi

spmcrval, (1<<RWWSRE) | (1<<SPMEN)

call Do_spm

; read back and check, optional

ldi

looplo, low(PAGESIZEB)

loophi, high(PAGESIZEB)

;init loop variable

;not required for PAGESIZEB<=256

;restore pointer

subi YL, low(PAGESIZEB)

sbci YH, high(PAGESIZEB)

Rdloop:

lpm

ld

r0, Z+

r1, Y+

cpse r0, r1

jmp Error

sbiw loophi:looplo, 1

brne Rdloop

;use subi for PAGESIZEB<=256

; return to RWW section

; verify that RWW section is safe to read

Return:

in

temp1, SPMCR

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sbrs temp1, RWWSB

; If RWWSB is set, the RWW section is not

; ready yet

ret

; re-enable the RWW section

ldi

spmcrval, (1<<RWWSRE) | (1<<SPMEN)

call Do_spm

rjmp Return

Do_spm:

; check for previous SPM complete

Wait_spm:

in

temp1, SPMCR

sbrc temp1, SPMEN

rjmp Wait_spm

; input: spmcrval determines SPM action

; disable interrupts if enabled, store status

in

temp2, SREG

cli

; check that no EEPROM write access is present

Wait_ee:

sbic EECR, EEWE

rjmp Wait_ee

; SPM timed sequence

out

spm

SPMCR, spmcrval

; restore SREG (to enable interrupts if originally enabled)

out

ret

SREG, temp2

ATmega16 Boot

Loader Parameters

In Table 100 through Table 102, the parameters used in the description of the self programming

are given.

Table 100. Boot Size Configuration⁽¹⁾

Boot Reset

Boot

Loader

Flash

Address

(start Boot

Loader

Application

Flash

Section

End

Application

section

Boot

Size

BOOTSZ1

BOOTSZ0

Pages

Section

Section)

128

words

$0000 -

$1F7F

$1F80 -

$1FFF

1

2

$1F7F

$1EFF

$1DFF

$1BFF

$1F80

$1F00

$1E00

$1C00

256

words

$0000 -

$1EFF

$1F00 -

$1FFF

1

0

1

0

4

512

words

$0000 -

$1DFF

$1E00 -

$1FFF

8

1024

words

$0000 -

$1BFF

$1C00 -

$1FFF

16

Note:

1. The different BOOTSZ Fuse configurations are shown in Figure 125

Table 101. Read-While-Write Limit⁽¹⁾

Section

Pages

112

Address

Read-While-Write section (RWW)

No Read-While-Write section (NRWW)

$0000 - $1BFF

$1C00 - $1FFF

16

Note:

1. For details about these two section, see “NRWW – No Read-While-Write Section” on page

247 and “RWW – Read-While-Write Section” on page 247

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Table 102. Explanation of Different Variables used in Figure 126 and the Mapping to the Z-

pointer

Corresponding

Variable

Z-value⁽¹⁾

Description

12

5

Most significant bit in the Program Counter.

(The Program Counter is 13 bits PC[12:0])

PCMSB

Most significant bit which is used to address the

words within one page (64 words in a page

requires 6 bits PC [5:0]).

PAGEMSB

ZPCMSB

Z13

Z6

Bit in Z-register that is mapped to PCMSB.

Because Z0 is not used, the ZPCMSB equals

PCMSB + 1.

Bit in Z-register that is mapped to PAGEMSB.

Because Z0 is not used, the ZPAGEMSB

equals PAGEMSB + 1.

ZPAGEMSB

PCPAGE

PC[12:6]

PC[5:0]

Z13:Z7

Z6:Z1

Program Counter page address: Page select,

for Page Erase and Page Write

Program Counter word address: Word select,

for filling temporary buffer (must be zero during

page write operation)

PCWORD

Note:

1. Z15:Z14: always ignored

Z0: should be zero for all SPM commands, byte select for the LPM instruction.

See “Addressing the Flash during Self-Programming” on page 251 for details about the use of

Z-pointer during Self-Programming.

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Memory

Programming

Program And Data The ATmega16 provides six Lock bits which can be left unprogrammed (“1”) or can be pro-

grammed (“0”) to obtain the additional features listed in Table 104. The Lock bits can only be

erased to “1” with the Chip Erase command.

Memory Lock Bits

Table 103. Lock Bit Byte⁽¹⁾

Lock Bit Byte

Bit No.

Description

–

Default Value

7

6

5

4

3

2

1

0

1 (unprogrammed)

–

BLB12

Boot Lock bit

Lock bit

BLB11

BLB02

BLB01

LB2

LB1

Lock bit

Note:

1. “1” means unprogrammed, “0” means programmed

Table 104. Lock Bit Protection Modes

Memory Lock Bits⁽²⁾

Protection Type

LB Mode

LB2

LB1

1

No memory lock features enabled.

Further programming of the Flash and EEPROM is

disabled in Parallel and SPI/JTAG Serial Programming

mode. The Fuse bits are locked in both Serial and Parallel

Programming mode.⁽¹⁾

2

1

0

Further programming and verification of the Flash and

EEPROM is disabled in Parallel and SPI/JTAG Serial

Programming mode. The Fuse bits are locked in both

Serial and Parallel Programming mode.⁽¹⁾

3

BLB0 Mode BLB02 BLB01

No restrictions for SPM or LPM accessing the Application

section.

1

2

1

0

SPM is not allowed to write to the Application section.

SPM is not allowed to write to the Application section, and

LPM executing from the Boot Loader section is not

allowed to read from the Application section. If interrupt

vectors are placed in the Boot Loader section, interrupts

are disabled while executing from the Application section.

3

4

0

1

LPM executing from the Boot Loader section is not

allowed to read from the Application section. If interrupt

vectors are placed in the Boot Loader section, interrupts

are disabled while executing from the Application section.

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Table 104. Lock Bit Protection Modes (Continued)

Memory Lock Bits⁽²⁾

Protection Type

BLB1 Mode BLB12 BLB11

No restrictions for SPM or LPM accessing the Boot Loader

section.

1

2

1

0

SPM is not allowed to write to the Boot Loader section.

SPM is not allowed to write to the Boot Loader section,

and LPM executing from the Application section is not

allowed to read from the Boot Loader section. If interrupt

vectors are placed in the Application section, interrupts

are disabled while executing from the Boot Loader section.

3

0

LPM executing from the Application section is not allowed

to read from the Boot Loader section. If interrupt vectors

are placed in the Application section, interrupts are

disabled while executing from the Boot Loader section.

4

0

1

Notes: 1. Program the Fuse bits before programming the Lock bits.

2. “1” means unprogrammed, “0” means programmed

Fuse Bits

The ATmega16 has two fuse bytes. Table 105 and Table 106 describe briefly the functionality of

all the fuses and how they are mapped into the fuse bytes. Note that the fuses are read as logi-

cal zero, “0”, if they are programmed.

Table 105. Fuse High Byte

FuseHigh

Byte

Bit

No.

Description

Enable OCD

Enable JTAG

Default Value

OCDEN⁽⁴⁾

JTAGEN⁽⁵⁾

7

6

1 (unprogrammed, OCD disabled)

0 (programmed, JTAG enabled)

Enable SPI Serial Program and

Data Downloading

SPIEN⁽¹⁾

CKOPT⁽²⁾

EESAVE

5

4

3

0 (programmed, SPI prog. enabled)

1 (unprogrammed)

Oscillator options

EEPROM memory is preserved

through the Chip Erase

1 (unprogrammed, EEPROM not

preserved)

Select Boot Size (see Table 100

for details)

BOOTSZ1

2

0 (programmed)⁽³⁾

Select Boot Size (see Table 100

for details)

BOOTSZ0

BOOTRST

1

0

0 (programmed)⁽³⁾

1 (unprogrammed)

Select reset vector

Notes: 1. The SPIEN Fuse is not accessible in SPI Serial Programming mode.

2. The CKOPT Fuse functionality depends on the setting of the CKSEL bits. See See “Clock

Sources” on page 25. for details.

3. The default value of BOOTSZ1..0 results in maximum Boot Size. See Table 100 on page 257.

4. Never ship a product with the OCDEN Fuse programmed regardless of the setting of Lock bits

and the JTAGEN Fuse. A programmed OCDEN Fuse enables some parts of the clock system

to be running in all sleep modes. This may increase the power consumption.

5. If the JTAG interface is left unconnected, the JTAGEN fuse should if possible be disabled. This

to avoid static current at the TDO pin in the JTAG interface.

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Table 106. Fuse Low Byte

Fuse Low

Byte

Bit

No.

Description

Default Value

BODLEVEL

BODEN

SUT1

7

6

5

4

3

2

1

0

Brown-out Detector trigger level 1 (unprogrammed)

Brown-out Detector enable

Select start-up time

Select Clock source

1 (unprogrammed, BOD disabled)

1 (unprogrammed)⁽¹⁾

0 (programmed)⁽¹⁾

0 (programmed)⁽²⁾

1 (unprogrammed)⁽²⁾

SUT0

CKSEL3

CKSEL2

CKSEL1

CKSEL0

Notes: 1. The default value of SUT1..0 results in maximum start-up time. SeeTable 10 on page 29 for

details.

2. The default setting of CKSEL3..0 results in internal RC Oscillator @ 1 MHz. See Table 2 on

page 25 for details.

The status of the Fuse bits is not affected by Chip Erase. Note that the Fuse bits are locked if

Lock bit1 (LB1) is programmed. Program the Fuse bits before programming the Lock bits.

Latching of Fuses

The Fuse values are latched when the device enters programming mode and changes of the

Fuse values will have no effect until the part leaves Programming mode. This does not apply to

the EESAVE Fuse which will take effect once it is programmed. The fuses are also latched on

Power-up in Normal mode.

Signature Bytes

All Atmel microcontrollers have a three-byte signature code which identifies the device. This

code can be read in both serial and parallel mode, also when the device is locked. The three

bytes reside in a separate address space.

For the ATmega16 the signature bytes are:

1. $000: $1E (indicates manufactured by Atmel)

2. $001: $94 (indicates 16 Kbytes Flash memory)

3. $002: $03 (indicates ATmega16 device when $001 is $94)

Calibration Byte

The ATmega16 stores four different calibration values for the internal RC Oscillator. These bytes

resides in the signature row High Byte of the addresses 0x0000, 0x0001, 0x0002, and 0x0003

for 1 MHz, 2 MHz, 4 MHz, and 8 Mhz respectively. During Reset, the 1 MHz value is automati-

cally loaded into the OSCCAL Register. If other frequencies are used, the calibration value has

to be loaded manually, see “Oscillator Calibration Register – OSCCAL” on page 30 for details.

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Page Size

Table 107. No. of Words in a Page and no. of Pages in the Flash

Flash Size

Page Size

PCWORD

No. of Pages

PCPAGE

PC[12:6]

PCMSB

8K words (16 Kbytes)

64 words

PC[5:0]

128

12

Table 108. No. of Words in a Page and no. of Pages in the EEPROM

EEPROM Size

Page Size

PCWORD

No. of Pages

PCPAGE

EEAMSB

512 bytes

4 bytes

EEA[1:0]

128

EEA[8:2]

8

Parallel

This section describes how to parallel program and verify Flash Program memory, EEPROM

Data memory, Memory Lock bits, and Fuse bits in the ATmega16. Pulses are assumed to be at

least 250 ns unless otherwise noted.

Programming

Parameters, Pin

Mapping, and

Commands

Signal Names

In this section, some pins of the ATmega16 are referenced by signal names describing their

functionality during parallel programming, see Figure 127 and Table 109. Pins not described in

the following table are referenced by pin names.

The XA1/XA0 pins determine the action executed when the XTAL1 pin is given a positive pulse.

The bit coding is shown in Table 111.

When pulsing WR or OE, the command loaded determines the action executed. The different

Commands are shown in Table 112.

Figure 127. Parallel Programming

+5V

RDY/BSY

OE

PD1

PD2

PD3

PD4

PD5

PD6

PD7

VCC

+5V

WR

AVCC

BS1

PB7 - PB0

DATA

XA0

XA1

PAGEL

+12 V

BS2

RESET

PA0

XTAL1

GND

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Table 109. Pin Name Mapping

Signal Name in

Programming Mode Pin Name I/O Function

0: Device is busy programming, 1: Device is ready

for new command

RDY/BSY

PD1

O

OE

PD2

PD3

I

Output Enable (Active low)

Write Pulse (Active low)

WR

Byte Select 1 (“0” selects Low byte, “1” selects

High byte)

BS1

PD4

I

XA0

PD5

PD6

PD7

I

XTAL Action Bit 0

XA1

XTAL Action Bit 1

PAGEL

Program Memory and EEPROM data Page Load

Byte Select 2 (“0” selects Low byte, “1” selects

2’nd High byte)

BS2

PA0

I

DATA

PB7-0

I/O Bidirectional Data bus (Output when OE is low)

Table 110. Pin Values used to Enter Programming Mode

PAGEL

XA1

Symbol

Value

Prog_enable[3]

Prog_enable[2]

Prog_enable[1]

Prog_enable[0]

0

XA0

BS1

Table 111. XA1 and XA0 Coding

XA1 XA0 Action when XTAL1 is Pulsed

0

1

0

1

0

1

Load Flash or EEPROM Address (High or low address byte determined by BS1)

Load Data (High or Low data byte for Flash determined by BS1)

Load Command

No Action, Idle

Table 112. Command Byte Bit Coding

Command Byte

1000 0000

0100 0000

0010 0000

0001 0000

0001 0001

Command Executed

Chip Erase

Write Fuse Bits

Write Lock Bits

Write Flash

Write EEPROM

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Table 112. Command Byte Bit Coding

Command Byte

0000 1000

Command Executed

Read Signature Bytes and Calibration byte

Read Fuse and Lock bits

Read Flash

0000 0100

0000 0010

0000 0011

Read EEPROM

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ATmega16(L)

Parallel

Programming

Enter Programming

Mode

The following algorithm puts the device in Parallel Programming mode:

1. Apply 4.5V - 5.5V between V_CCand GND, and wait at least 100 µs.

2. Set RESET to “0” and toggle XTAL1 at least 6 times

3. Set the Prog_enable pins listed in Table 110 on page 263 to “0000” and wait at least 100

ns.

4. Apply 11.5V - 12.5V to RESET. Any activity on Prog_enable pins within 100 ns after

+12V has been applied to RESET, will cause the device to fail entering Programming

mode.

Note, if External Crystal or External RC configuration is selected, it may not be possible to apply

qualified XTAL1 pulses. In such cases, the following algorithm should be followed:

1. Set Prog_enable pins listed in Table 110 on page 263 to “0000”.

2. Apply 4.5V - 5.5V between V_CCand GND simultaneously as 11.5V - 12.5V is applied to

RESET.

3. Wait 100 µs.

4. Re-program the fuses to ensure that External Clock is selected as clock source

(CKSEL3:0 = 0b0000) If Lock bits are programmed, a Chip Erase command must be

executed before changing the fuses.

5. Exit Programming mode by power the device down or by bringing RESET pin to 0b0.

6. Entering Programming mode with the original algorithm, as described above.

Considerations for

The loaded command and address are retained in the device during programming. For efficient

Efficient Programming programming, the following should be considered.

•

The command needs only be loaded once when writing or reading multiple memory

locations.

Skip writing the data value $FF, that is the contents of the entire EEPROM (unless the

EESAVE Fuse is programmed) and Flash after a Chip Erase.

Address High byte needs only be loaded before programming or reading a new 256 word

window in Flash or 256 byte EEPROM. This consideration also applies to Signature bytes

reading.

Chip Erase

The Chip Erase will erase the Flash and EEPROM⁽¹⁾memories plus Lock bits. The Lock bits are

not reset until the program memory has been completely erased. The Fuse bits are not

changed. A Chip Erase must be performed before the Flash and/or the EEPROM are

reprogrammed.

Note:

1. The EEPRPOM memory is preserved during chip erase if the EESAVE Fuse is programmed.

Load Command “Chip Erase”

1. Set XA1, XA0 to “10”. This enables command loading.

2. Set BS1 to “0”.

3. Set DATA to “1000 0000”. This is the command for Chip Erase.

4. Give XTAL1 a positive pulse. This loads the command.

5. Give WR a negative pulse. This starts the Chip Erase. RDY/BSY goes low.

6. Wait until RDY/BSY goes high before loading a new command.

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Programming the

Flash

The Flash is organized in pages, see Table 107 on page 262. When programming the Flash, the

program data is latched into a page buffer. This allows one page of program data to be pro-

grammed simultaneously. The following procedure describes how to program the entire Flash

memory:

A. Load Command “Write Flash”

1. Set XA1, XA0 to “10”. This enables command loading.

2. Set BS1 to “0”.

3. Set DATA to “0001 0000”. This is the command for Write Flash.

4. Give XTAL1 a positive pulse. This loads the command.

B. Load Address Low byte

1. Set XA1, XA0 to “00”. This enables address loading.

2. Set BS1 to “0”. This selects low address.

3. Set DATA = Address Low byte ($00 - $FF).

4. Give XTAL1 a positive pulse. This loads the address Low byte.

C. Load Data Low Byte

1. Set XA1, XA0 to “01”. This enables data loading.

2. Set DATA = Data Low byte ($00 - $FF).

3. Give XTAL1 a positive pulse. This loads the data byte.

D. Load Data High Byte

1. Set BS1 to “1”. This selects high data byte.

2. Set XA1, XA0 to “01”. This enables data loading.

3. Set DATA = Data High byte ($00 - $FF).

4. Give XTAL1 a positive pulse. This loads the data byte.

E. Latch Data

1. Set BS1 to “1”. This selects high data byte.

2. Give PAGEL a positive pulse. This latches the data bytes. (See Figure 129 for signal

waveforms)

F. Repeat B through E until the entire buffer is filled or until all data within the page is loaded.

While the lower bits in the address are mapped to words within the page, the higher bits address

the pages within the FLASH. This is illustrated in Figure 128 on page 267. Note that if less than

8 bits are required to address words in the page (pagesize < 256), the most significant bit(s) in

the address Low byte are used to address the page when performing a page write.

G. Load Address High byte

1. Set XA1, XA0 to “00”. This enables address loading.

2. Set BS1 to “1”. This selects high address.

3. Set DATA = Address High byte ($00 - $FF).

4. Give XTAL1 a positive pulse. This loads the address High byte.

H. Program Page

1. Set BS1 = “0”

2. Give WR a negative pulse. This starts programming of the entire page of data. RDY/BSY

goes low.

3. Wait until RDY/BSY goes high. (See Figure 129 for signal waveforms)

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I. Repeat B through H until the entire Flash is programmed or until all data has been

programmed.

J. End Page Programming

1. 1. Set XA1, XA0 to “10”. This enables command loading.

2. Set DATA to “0000 0000”. This is the command for No Operation.

3. Give XTAL1 a positive pulse. This loads the command, and the internal write signals are

reset.

Figure 128. Addressing the Flash which is Organized in Pages

PCMSB

PAGEMSB

PCWORD

PROGRAM

COUNTER

PCPAGE

PAGE ADDRESS

WITHIN THE FLASH

WORD ADDRESS

WITHIN A PAGE

PROGRAM MEMORY

PAGE

INSTRUCTION WORD

PCWORD[PAGEMSB:0]:

00

01

02

PAGEEND

Note:

1. PCPAGE and PCWORD are listed in Table 107 on page 262.

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Figure 129. Programming the Flash Waveforms⁽¹⁾

F

A

B

C

D

E

B

C

D

E

G

H

$10

ADDR. LOW DATA LOW DATA HIGH

XX

ADDR. HIGH

XX

DATA

XA1

XA0

BS1

XTAL1

WR

RDY/BSY

RESET +12V

OE

PAGEL

BS2

Note:

1. “XX” is don’t care. The letters refer to the programming description above.

Programming the

EEPROM

The EEPROM is organized in pages, see Table 108 on page 262. When programming the

EEPROM, the program data is latched into a page buffer. This allows one page of data to be

programmed simultaneously. The programming algorithm for the EEPROM data memory is as

follows (refer to “Programming the Flash” on page 266 for details on Command, Address and

Data loading):

1. A: Load Command “0001 0001”.

2. G: Load Address High Byte ($00 - $FF)

3. B: Load Address Low Byte ($00 - $FF)

4. C: Load Data ($00 - $FF)

5. E: Latch data (give PAGEL a positive pulse)

K: Repeat 3 through 5 until the entire buffer is filled

L: Program EEPROM page

1. Set BS1 to “0”.

2. Give WR a negative pulse. This starts programming of the EEPROM page. RDY/BSY

goes low.

3. Wait until to RDY/BSY goes high before programming the next page. (See Figure 130 for

signal waveforms)

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Figure 130. Programming the EEPROM Waveforms

K

A

G

B

C

E

B

C

E

L

0x11

ADDR. HIGH ADDR. LOW

DATA

ADDR. LOW

DATA

XX

DATA

XA1

XA0

BS1

XTAL1

WR

RDY/BSY

RESET +12V

OE

PAGEL

BS2

Reading the Flash

The algorithm for reading the Flash memory is as follows (refer to “Programming the Flash” on

page 266 for details on Command and Address loading):

1. A: Load Command “0000 0010”.

2. G: Load Address High Byte ($00 - $FF)

3. B: Load Address Low Byte ($00 - $FF)

4. Set OE to “0”, and BS1 to “0”. The Flash word Low byte can now be read at DATA.

5. Set BS1 to “1”. The Flash word High byte can now be read at DATA.

6. Set OE to “1”.

Reading the EEPROM The algorithm for reading the EEPROM memory is as follows (refer to “Programming the Flash”

on page 266 for details on Command and Address loading):

1. A: Load Command “0000 0011”.

2. G: Load Address High Byte ($00 - $FF)

3. B: Load Address Low Byte ($00 - $FF)

4. Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at DATA.

5. Set OE to “1”.

Programming the

Fuse Low Bits

The algorithm for programming the Fuse Low bits is as follows (refer to “Programming the Flash”

on page 266 for details on Command and Data loading):

1. A: Load Command “0100 0000”.

2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.

3. Set BS1 to “0” and BS2 to “0”.

4. Give WR a negative pulse and wait for RDY/BSY to go high.

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Programming the

Fuse High Bits

The algorithm for programming the Fuse high bits is as follows (refer to “Programming the Flash”

on page 266 for details on Command and Data loading):

1. A: Load Command “0100 0000”.

2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.

3. Set BS1 to “1” and BS2 to “0”. This selects high data byte.

4. Give WR a negative pulse and wait for RDY/BSY to go high.

5. Set BS1 to “0”. This selects low data byte.

Figure 131. Programming the Fuses

Write Fuse Low byte

Write Fuse high byte

A

C

A

C

$40

DATA

XX

$40

DATA

XX

DATA

XA1

XA0

BS1

BS2

XTAL1

WR

RDY/BSY

RESET +12V

OE

PAGEL

Programming the Lock The algorithm for programming the Lock bits is as follows (refer to “Programming the Flash” on

Bits

page 266 for details on Command and Data loading):

1. A: Load Command “0010 0000”.

2. C: Load Data Low Byte. Bit n = “0” programs the Lock bit.

3. Give WR a negative pulse and wait for RDY/BSY to go high.

The Lock bits can only be cleared by executing Chip Erase.

Reading the Fuse and The algorithm for reading the Fuse and Lock bits is as follows (refer to “Programming the Flash”

Lock Bits

on page 266 for details on Command loading):

1. A: Load Command “0000 0100”.

2. Set OE to “0”, BS2 to “0” and BS1 to “0”. The status of the Fuse Low bits can now be

read at DATA (“0” means programmed).

3. Set OE to “0”, BS2 to “1” and BS1 to “1”. The status of the Fuse High bits can now be

read at DATA (“0” means programmed).

4. Set OE to “0”, BS2 to “0” and BS1 to “1”. The status of the Lock bits can now be read at

DATA (“0” means programmed).

5. Set OE to “1”.

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Figure 132. Mapping between BS1, BS2 and the Fuse- and Lock Bits during Read

Fuse Low Byte

0

DATA

0

1

Lock Bits

1

BS1

Fuse High Byte

BS2

Reading the Signature The algorithm for reading the Signature bytes is as follows (refer to “Programming the Flash” on

Bytes

page 266 for details on Command and Address loading):

1. A: Load Command “0000 1000”.

2. B: Load Address Low Byte ($00 - $02).

3. Set OE to “0”, and BS1 to “0”. The selected Signature byte can now be read at DATA.

4. Set OE to “1”.

Reading the

Calibration Byte

The algorithm for reading the Calibration byte is as follows (refer to “Programming the Flash” on

page 266 for details on Command and Address loading):

1. A: Load Command “0000 1000”.

2. B: Load Address Low Byte, $00.

3. Set OE to “0”, and BS1 to “1”. The Calibration byte can now be read at DATA.

4. Set OE to “1”.

Parallel Programming Figure 133. Parallel Programming Timing, Including some General Timing Requirements

Characteristics

t_XLWL

t_XHXL

XTAL1

t_DVXH

t_XLDX

Data & Contol

(DATA, XA0/1, BS1, BS2)

t_BVPH

t_PLBX

t _BVWL

t_WLBX

PAGEL

t_PHPL

t_{WL WH}

WR

t_PLWL

WLRL

RDY/BSY

t_WLRH

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Figure 134. Parallel Programming Timing, Loading Sequence with Timing Requirements⁽¹⁾

LOAD DATA

LOAD ADDRESS

(LOW BYTE)

LOAD DATA

(LOW BYTE)

LOAD DATA

(HIGH BYTE)

LOAD ADDRESS

(LOW BYTE)

t_XLPH

t_XLXH

t_PLXH

XTAL1

BS1

PAGEL

DATA

ADDR0 (Low Byte)

DATA (Low Byte)

DATA (High Byte)

ADDR1 (Low Byte)

XA0

XA1

Note:

1. The timing requirements shown in Figure 133 (that is, t_DVXH, t_XHXL, and t_XLDX) also apply to

loading operation.

Figure 135. Parallel Programming Timing, Reading Sequence (within the Same Page) with

Timing Requirements⁽¹⁾

LOAD ADDRESS

(LOW BYTE)

READ DATA

(LOW BYTE)

READ DATA

(HIGH BYTE)

LOAD ADDRESS

(LOW BYTE)

t_XLOL

XTAL1

BS1

t_BVDV

t_OLDV

OE

t_OHDZ

ADDR1 (Low Byte)

DATA (High Byte)

DATA

ADDR0 (Low Byte)

DATA (Low Byte)

XA0

XA1

Note:

1. The timing requirements shown in Figure 133 (that is, t_DVXH, t_XHXL, and t_XLDX) also apply to

reading operation.

Table 113. Parallel Programming Characteristics, V_CC= 5V ±10%

Symbol

V_PP

Parameter

Min

Typ

Max

12.5

250

Units

V

Programming Enable Voltage

Programming Enable Current

11.5

I_PP

μA

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Table 113. Parallel Programming Characteristics, V_CC= 5V ±10% (Continued)

Symbol

t_DVXH

t_XLXH

t_XHXL

t_XLDX

t_XLWL

t_XLPH

t_PLXH

t_BVPH

t_PHPL

t_PLBX

Parameter

Min

67

200

150

67

0

Typ

Max

Units

Data and Control Valid before XTAL1 High

XTAL1 Low to XTAL1 High

XTAL1 Pulse Width High

Data and Control Hold after XTAL1 Low

XTAL1 Low to WR Low

XTAL1 Low to PAGEL high

PAGEL low to XTAL1 high

BS1 Valid before PAGEL High

PAGEL Pulse Width High

BS1 Hold after PAGEL Low

BS2/1 Hold after WR Low

PAGEL Low to WR Low

0

150

67

150

67

150

0

ns

t_WLBX

t_PLWL

t_BVWL

t_WLWH

t_WLRL

t_WLRH

t_{WLRH_CE}

t_XLOL

BS1 Valid to WR Low

WR Pulse Width Low

WR Low to RDY/BSY Low

WR Low to RDY/BSY High⁽¹⁾

WR Low to RDY/BSY High for Chip Erase⁽²⁾

XTAL1 Low to OE Low

1

4.5

9

μs

3.7

7.5

0

ms

t_BVDV

t_OLDV

t_OHDZ

BS1 Valid to DATA valid

0

250

ns

OE Low to DATA Valid

OE High to DATA Tri-stated

Notes: 1. t_WLRHis valid for the Write Flash, Write EEPROM, Write Fuse bits and Write Lock bits

commands.

2. t_{WLRH_CE}is valid for the Chip Erase command.

Serial

Downloading

Both the Flash and EEPROM memory arrays can be programmed using the serial SPI bus while

RESET is pulled to GND. The serial interface consists of pins SCK, MOSI (input), and MISO

(output). After RESET is set low, the Programming Enable instruction needs to be executed first

before program/erase operations can be executed. NOTE, in Table 114 on page 273, the pin

mapping for SPI programming is listed. Not all parts use the SPI pins dedicated for the internal

SPI interface.

SPI Serial

Programming Pin

Mapping

Table 114. Pin Mapping SPI Serial Programming

Symbol

MOSI

MISO

SCK

Pins

PB5

PB6

PB7

I/O

Description

Serial Data in

Serial Data out

Serial Clock

I

O

I

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Figure 136. SPI Serial Programming and Verify⁽¹⁾

+2.7 - 5.5V

VCC

+2.7 - 5.5V⁽²⁾

PB5

PB6

PB7

MOSI

MISO

AVCC

SCK

XTAL1

RESET

GND

Notes: 1. If the device is clocked by the Internal Oscillator, it is no need to connect a clock source to the

XTAL1 pin.

2. V_CC-0.3V < AVCC < V_CC+0.3V, however, AVCC should always be within 2.7V - 5.5V

When programming the EEPROM, an auto-erase cycle is built into the self-timed programming

operation (in the serial mode ONLY) and there is no need to first execute the Chip Erase instruc-

tion. The Chip Erase operation turns the content of every memory location in both the Program

and EEPROM arrays into $FF.

Depending on CKSEL Fuses, a valid clock must be present. The minimum low and high periods

for the serial clock (SCK) input are defined as follows:

Low: > 2 CPU clock cycles for f_ck< 12 MHz, 3 CPU clock cycles for f_ck≥ 12 MHz

High: > 2 CPU clock cycles for f_ck< 12 MHz, 3 CPU clock cycles for f_ck≥ 12 MHz

SPI Serial

When writing serial data to the ATmega16, data is clocked on the rising edge of SCK.

Programming

Algorithm

When reading data from the ATmega16, data is clocked on the falling edge of SCK. See Figure

138 for timing details.

To program and verify the ATmega16 in the SPI Serial Programming mode, the following

sequence is recommended (See four byte instruction formats in Figure 116 on page 276):

1. Power-up sequence:

Apply power between V_CCand GND while RESET and SCK are set to “0”. In some sys-

tems, the programmer can not guarantee that SCK is held low during power-up. In this

case, RESET must be given a positive pulse of at least two CPU clock cycles duration

after SCK has been set to “0”.

2. Wait for at least 20 ms and enable SPI Serial Programming by sending the Programming

Enable serial instruction to pin MOSI.

3. The SPI Serial Programming instructions will not work if the communication is out of syn-

chronization. When in sync. the second byte ($53), will echo back when issuing the third

byte of the Programming Enable instruction. Whether the echo is correct or not, all four

bytes of the instruction must be transmitted. If the $53 did not echo back, give RESET a

positive pulse and issue a new Programming Enable command.

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4. The Flash is programmed one page at a time. The page size is found in Table 107 on page

262. The memory page is loaded one byte at a time by supplying the 6 LSB of the

address and data together with the Load Program Memory Page instruction. To ensure

correct loading of the page, the data Low byte must be loaded before data High byte is

applied for a given address. The Program Memory Page is stored by loading the Write

Program Memory Page instruction with the 7 MSB of the address. If polling is not used,

the user must wait at least t_{WD_FLASH}before issuing the next page. (See Table 115).

Accessing the SPI Serial Programming interface before the Flash write operation com-

pletes can result in incorrect programming.

5. The EEPROM array is programmed one byte at a time by supplying the address and data

together with the appropriate Write instruction. An EEPROM memory location is first

automatically erased before new data is written. If polling is not used, the user must wait

at least t_{WD_EEPROM}before issuing the next byte. (See Table 115). In a chip erased device,

no $FFs in the data file(s) need to be programmed.

6. Any memory location can be verified by using the Read instruction which returns the con-

tent at the selected address at serial output MISO.

7. At the end of the programming session, RESET can be set high to commence normal

operation.

8. Power-off sequence (if needed):

Set RESET to “1”.

Turn V_CCpower off.

Data Polling Flash

When a page is being programmed into the Flash, reading an address location within the page

being programmed will give the value $FF. At the time the device is ready for a new page, the

programmed value will read correctly. This is used to determine when the next page can be writ-

ten. Note that the entire page is written simultaneously and any address within the page can be

used for polling. Data polling of the Flash will not work for the value $FF, so when programming

this value, the user will have to wait for at least t_{WD_FLASH}before programming the next page. As

a chip erased device contains $FF in all locations, programming of addresses that are meant to

contain $FF, can be skipped. See Table 115 for t_{WD_FLASH}value

Data Polling EEPROM When a new byte has been written and is being programmed into EEPROM, reading the

address location being programmed will give the value $FF. At the time the device is ready for a

new byte, the programmed value will read correctly. This is used to determine when the next

byte can be written. This will not work for the value $FF, but the user should have the following in

mind: As a chip erased device contains $FF in all locations, programming of addresses that are

meant to contain $FF, can be skipped. This does not apply if the EEPROM is re-programmed

without chip erasing the device. In this case, data polling cannot be used for the value $FF, and

the user will have to wait at least t_{WD_EEPROM}before programming the next byte. See Table 115

for t_{WD_EEPROM}value.

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Table 115. Minimum Wait Delay before Writing the Next Flash or EEPROM Location

Symbol

Minimum Wait Delay

4.5 ms

t_{WD_FUSE}

t_{WD_FLASH}

t_{WD_EEPROM}

t_{WD_ERASE}

4.5 ms

9.0 ms

Serial Programming

Instruction set

Table 116 on page 276 and Figure 137 on page 277 describes the Instruction set.

Table 116. Serial Programming Instruction Set (Hexadecimal values)

Instruction Format

Instruction⁽¹⁾/Operation

Programming Enable

Byte 1

$AC

Byte 2

Byte 3

$00

Byte 4

$00

$53

$80

$00

Chip Erase (Program Memory/EEPROM)

Poll RDY/BSY

$AC

$00

$F0

$00

data byte out

Load Instructions

Load Extended Address byte⁽¹⁾

Load Program Memory Page, High byte

Load Program Memory Page, Low byte

Load EEPROM Memory Page (page access)⁽¹⁾

Read Instructions

$4D

$48

$40

$C1

$00

adr MSB

$00

Extended adr

adr LSB

$00

high data byte in

low data byte in

data byte in

adr LSB

Read Program Memory, High byte

Read Program Memory, Low byte

Read EEPROM Memory

Read Lock bits

$28

$20

$A0

$58

$30

$50

$58

$50

$38

adr MSB

$00

adr LSB

$00

high data byte out

low data byte out

data byte out

Read Signature Byte

$00

0000 000aa

$00

Read Fuse bits

$00

Read Fuse High bits

$08

$00

Read Extended Fuse Bits

Read Calibration Byte

$08

$00

$0b00 000bb

Write Instructions

Write Program Memory Page

Write EEPROM Memory

Write EEPROM Memory Page (page access)⁽¹⁾

Write Lock bits

$4C

$C0

$C2

$AC

000a aaaa

adr MSB

$E0

aa00 0000

adr LSB

$00

data byte in

$00

data byte in

Write Fuse bits

$A0

$00

Write Fuse High bits

$A8

$00

Write Extended Fuse Bits

$A4

$00

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Notes: 1. Not all instructions are applicable for all parts.

2. a = address

3. Bits are programmed ‘0’, unprogrammed ‘1’.

4. To ensure future compatibility, unused Fuses and Lock bits should be unprogrammed (‘1’) .

5. Refer to the correspondig section for Fuse and Lock bits, Calibration and Signature bytes and

Page size.

6. See htt://www.atmel.com/avr for Application Notes regarding programming and programmers.

If the LSB in RDY/BSY data byte out is ‘1’, a programming operation is still pending. Wait until

this bit returns ‘0’ before the next instruction is carried out.

Within the same page, the low data byte must be loaded prior to the high data byte.

After data is loaded to the page buffer, program the EEPROM page, see Figure 137 on page

277.

Figure 137. Serial Programming Instruction example

Serial Programming Instruction

Load Program Memory Page (High/Low Byte)/

Load EEPROM Memory Page (page access)

Write Program Memory Page/

Write EEPROM Memory Page

Byte 1

Byte 2

Byte 3

Byte 4

Byte 1

Byte 2

Byte 3

Byte 4

Adr MSB

Adr LSB

Adr MSB

Adr LSB

Bit 15 B

0

Bit 15 B

0

Page Buffer

Page Offset

Page 0

Page 1

Page 2

Page Number

Page N-1

Program Memory/

EEPROM Memory

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SPI Serial

For characteristics of the SPI module, see “SPI Timing Characteristics” on page 295.

Programming

Characteristics

Figure 138. SPI Serial Programming Waveforms

SERIAL DATA INPUT

MSB

LSB

(MOSI)

SERIAL DATA OUTPUT

MSB

(MISO)

SERIAL CLOCK INPUT

(SCK)

SAMPLE

Programming via

the JTAG Interface

Programming through the JTAG interface requires control of the four JTAG specific pins: TCK,

TMS, TDI and TDO. Control of the reset and clock pins is not required.

To be able to use the JTAG interface, the JTAGEN Fuse must be programmed. The device is

default shipped with the fuse programmed. In addition, the JTD bit in MCUCSR must be cleared.

Alternatively, if the JTD bit is set, the External Reset can be forced low. Then, the JTD bit will be

cleared after two chip clocks, and the JTAG pins are available for programming. This provides a

means of using the JTAG pins as normal port pins in running mode while still allowing In-System

Programming via the JTAG interface. Note that this technique can not be used when using the

JTAG pins for Boundary-scan or On-chip Debug. In these cases the JTAG pins must be dedi-

cated for this purpose.

As a definition in this datasheet, the LSB is shifted in and out first of all Shift Registers.

Programming Specific The instruction register is 4-bit wide, supporting up to 16 instructions. The JTAG instructions

JTAG Instructions

useful for Programming are listed below.

The OPCODE for each instruction is shown behind the instruction name in hex format. The text

describes which Data Register is selected as path between TDI and TDO for each instruction.

The Run-Test/Idle state of the TAP controller is used to generate internal clocks. It can also be

used as an idle state between JTAG sequences. The state machine sequence for changing the

instruction word is shown in Figure 139.

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Figure 139. State Machine Sequence for Changing the Instruction Word

1

Test-Logic-Reset

0

1

0

Run-Test/Idle

Select-DR Scan

Select-IR Scan

0

Capture-IR

0

1

Capture-DR

0

Shift-DR

0

Shift-IR

0

1

Exit1-DR

0

1

Exit1-IR

0

Pause-IR

1

Pause-DR

1

0

Exit2-DR

1

Exit2-IR

1

Update-DR

Update-IR

1

0

AVR_RESET ($C)

The AVR specific public JTAG instruction for setting the AVR device in the Reset mode or taking

the device out from the Reset Mode. The TAP controller is not reset by this instruction. The one

bit Reset Register is selected as Data Register. Note that the Reset will be active as long as

there is a logic “one” in the Reset Chain. The output from this chain is not latched.

The active states are:

•

Shift-DR: The Reset Register is shifted by the TCK input.

PROG_ENABLE ($4)

The AVR specific public JTAG instruction for enabling programming via the JTAG port. The 16-

bit Programming Enable Register is selected as Data Register. The active states are the

following:

•

Shift-DR: The programming enable signature is shifted into the Data Register.

Update-DR: The programming enable signature is compared to the correct value, and

Programming mode is entered if the signature is valid.

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PROG_COMMANDS

($5)

The AVR specific public JTAG instruction for entering programming commands via the JTAG

port. The 15-bit Programming Command Register is selected as Data Register. The active

states are the following:

•

Capture-DR: The result of the previous command is loaded into the Data Register.

Shift-DR: The Data Register is shifted by the TCK input, shifting out the result of the

previous command and shifting in the new command.

•

Update-DR: The programming command is applied to the Flash inputs

Run-Test/Idle: One clock cycle is generated, executing the applied command (not always

required, see Table 117 below).

PROG_PAGELOAD

($6)

The AVR specific public JTAG instruction to directly load the Flash data page via the JTAG port.

The 1024 bit Virtual Flash Page Load Register is selected as Data Register. This is a virtual

scan chain with length equal to the number of bits in one Flash page. Internally the Shift Register

is 8-bit. Unlike most JTAG instructions, the Update-DR state is not used to transfer data from the

Shift Register. The data are automatically transferred to the Flash page buffer byte by byte in the

Shift-DR state by an internal state machine. This is the only active state:

•

Shift-DR: Flash page data are shifted in from TDI by the TCK input, and automatically

loaded into the Flash page one byte at a time.

Note:

The JTAG instruction PROG_PAGELOAD can only be used if the AVR device is the first device in

JTAG scan chain. If the AVR cannot be the first device in the scan chain, the byte-wise program-

ming algorithm must be used.

PROG_PAGEREAD

($7)

The AVR specific public JTAG instruction to read one full Flash data page via the JTAG port.

The 1032 bit Virtual Flash Page Read Register is selected as Data Register. This is a virtual

scan chain with length equal to the number of bits in one Flash page plus 8. Internally the Shift

Register is 8-bit. Unlike most JTAG instructions, the Capture-DR state is not used to transfer

data to the Shift Register. The data are automatically transferred from the Flash page buffer byte

by byte in the Shift-DR state by an internal state machine. This is the only active state:

•

Shift-DR: Flash data are automatically read one byte at a time and shifted out on TDO by the

TCK input. The TDI input is ignored.

Note:

The JTAG instruction PROG_PAGEREAD can only be used if the AVR device is the first device in

JTAG scan chain. If the AVR cannot be the first device in the scan chain, the byte-wise program-

ming algorithm must be used.

Data Registers

The Data Registers are selected by the JTAG Instruction Registers described in section “Pro-

gramming Specific JTAG Instructions” on page 278. The Data Registers relevant for

programming operations are:

•

Reset Register

Programming Enable Register

Programming Command Register

Virtual Flash Page Load Register

Virtual Flash Page Read Register

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Reset Register

The Reset Register is a Test Data Register used to reset the part during programming. It is

required to reset the part before entering programming mode.

A high value in the Reset Register corresponds to pulling the external Reset low. The part is

reset as long as there is a high value present in the Reset Register. Depending on the Fuse set-

tings for the clock options, the part will remain reset for a Reset Time-out Period (refer to “Clock

Sources” on page 25) after releasing the Reset Register. The output from this Data Register is

not latched, so the reset will take place immediately, as shown in Figure 115 on page 230.

Programming Enable

Register

The Programming Enable Register is a 16-bit register. The contents of this register is compared

to the programming enable signature, binary code 1010_0011_0111_0000. When the contents

of the register is equal to the programming enable signature, programming via the JTAG port is

enabled. The register is reset to 0 on Power-on Reset, and should always be reset when leaving

Programming mode.

Figure 140. Programming Enable Register

TDI

$A370

D

Q

A

T

A

Programming Enable

=

ClockDR & PROG_ENABLE

TDO

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Programming

Command Register

The Programming Command Register is a 15-bit register. This register is used to serially shift in

programming commands, and to serially shift out the result of the previous command, if any. The

JTAG Programming Instruction Set is shown in Table 117. The state sequence when shifting in

the programming commands is illustrated in Figure 142.

Figure 141. Programming Command Register

TDI

S

T

R

O

B

E

S

Flash

EEPROM

Fuses

Lock Bits

A

D

R

E

S

/

D

A

T

A

TDO

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Table 117. JTAG Programming Instruction Set

a = address high bits, b = address low bits, H = 0 – Low byte, 1 – High Byte, o = data out, i = data in, x = don’t care

Instruction

TDI sequence

TDO sequence

Notes

1a. Chip erase

0100011_10000000

0110001_10000000

0110011_10000000

xxxxxxx_xxxxxxxx

1b. Poll for chip erase complete

2a. Enter Flash Write

0110011_10000000

0100011_00010000

0000111_aaaaaaaa

0000011_bbbbbbbb

0010011_iiiiiiii

xxxxxox_xxxxxxxx

xxxxxxx_xxxxxxxx

(2)

(9)

2b. Load Address High Byte

2c. Load Address Low Byte

2d. Load Data Low Byte

2e. Load Data High Byte

2f. Latch Data

0010111_iiiiiiii

0110111_00000000

1110111_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

(1)

2g. Write Flash Page

0110111_00000000

0110101_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

2h. Poll for Page Write complete

3a. Enter Flash Read

0110111_00000000

0100011_00000010

0000111_aaaaaaaa

0000011_bbbbbbbb

xxxxxox_xxxxxxxx

xxxxxxx_xxxxxxxx

(2)

(9)

3b. Load Address High Byte

3c. Load Address Low Byte

3d. Read Data Low and High Byte

0110010_00000000

0110110_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

Low byte

High byte

4a. Enter EEPROM Write

4b. Load Address High Byte

4c. Load Address Low Byte

4d. Load Data Byte

0100011_00010001

0000111_aaaaaaaa

0000011_bbbbbbbb

0010011_iiiiiiii

xxxxxxx_xxxxxxxx

(9)

4e. Latch Data

0110111_00000000

1110111_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

(1)

4f. Write EEPROM Page

0110011_00000000

0110001_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

4g. Poll for Page Write complete

5a. Enter EEPROM Read

0110011_00000000

0100011_00000011

0000111_aaaaaaaa

0000011_bbbbbbbb

xxxxxox_xxxxxxxx

xxxxxxx_xxxxxxxx

(2)

(9)

5b. Load Address High Byte

5c. Load Address Low Byte

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Table 117. JTAG Programming Instruction Set (Continued)

a = address high bits, b = address low bits, H = 0 – Low byte, 1 – High Byte, o = data out, i = data in, x = don’t care

Instruction

TDI sequence

TDO sequence

Notes

5d. Read Data Byte

0110011_bbbbbbbb

0110010_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

6a. Enter Fuse Write

0100011_01000000

xxxxxxx_xxxxxxxx

6b. Load Data Low Byte⁽⁶⁾

0010011_iiiiiiii

(3)

(1)

6c. Write Fuse High byte

0110111_00000000

0110101_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

6d. Poll for Fuse Write complete

6e. Load Data Low Byte⁽⁷⁾

6f. Write Fuse Low byte

0110111_00000000

xxxxxox_xxxxxxxx

(2)

(3)

(1)

0010011_iiiiiiii

xxxxxxx_xxxxxxxx

0110011_00000000

0110001_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

6g. Poll for Fuse Write complete

7a. Enter Lock Bit Write

7b. Load Data Byte⁽⁸⁾

0110011_00000000

0100011_00100000

0010011_11iiiiii

xxxxxox_xxxxxxxx

xxxxxxx_xxxxxxxx

(2)

(4)

(1)

7c. Write Lock Bits

0110011_00000000

0110001_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

7d. Poll for Lock Bit Write complete

8a. Enter Fuse/Lock Bit Read

8b. Read Fuse High Byte⁽⁶⁾

0110011_00000000

0100011_00000100

xxxxxox_xxxxxxxx

(2)

xxxxxxx_xxxxxxxx

0111110_00000000

0111111_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

8c. Read Fuse Low Byte⁽⁷⁾

8d. Read Lock Bits⁽⁸⁾

0110010_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

0110110_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

(5)

xxxxxxx_xxoooooo

8e. Read Fuses and Lock Bits

0111110_00000000

0110010_00000000

0110110_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

(5)

Fuse High Byte

Fuse Low Byte

Lock bits

9a. Enter Signature Byte Read

9b. Load Address Byte

0100011_00001000

xxxxxxx_xxxxxxxx

0000011_bbbbbbbb

9c. Read Signature Byte

0110010_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

10a. Enter Calibration Byte Read

0100011_00001000

xxxxxxx_xxxxxxxx

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Table 117. JTAG Programming Instruction Set (Continued)

a = address high bits, b = address low bits, H = 0 – Low byte, 1 – High Byte, o = data out, i = data in, x = don’t care

Instruction

TDI sequence

TDO sequence

Notes

10b. Load Address Byte

10c. Read Calibration Byte

0000011_bbbbbbbb

xxxxxxx_xxxxxxxx

0110110_00000000

0110111_00000000

xxxxxxx_xxxxxxxx

xxxxxxx_oooooooo

11a. Load No Operation Command

0100011_00000000

0110011_00000000

xxxxxxx_xxxxxxxx

Notes: 1. This command sequence is not required if the seven MSB are correctly set by the previous command sequence (which is

normally the case).

2. Repeat until o = “1”.

3. Set bits to “0” to program the corresponding fuse, “1” to unprogram the fuse.

4. Set bits to “0” to program the corresponding lock bit, “1” to leave the lock bit unchanged.

5. “0” = programmed, “1” = unprogrammed.

6. The bit mapping for Fuses High byte is listed in Table 105 on page 260

7. The bit mapping for Fuses Low byte is listed in Table 106 on page 261

8. The bit mapping for Lock bits byte is listed in Table 103 on page 259

9. Address bits exceeding PCMSB and EEAMSB (Table 107 and Table 108) are don’t care

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Figure 142. State Machine Sequence for Changing/Reading the Data Word

1

Test-Logic-Reset

0

1

0

Run-Test/Idle

Select-DR Scan

Select-IR Scan

0

Capture-IR

0

1

Capture-DR

0

Shift-DR

0

Shift-IR

0

1

Exit1-DR

0

1

Exit1-IR

0

Pause-IR

1

Pause-DR

1

0

Exit2-DR

1

Exit2-IR

1

Update-DR

Update-IR

1

0

Virtual Flash Page

Load Register

The Virtual Flash Page Load Register is a virtual scan chain with length equal to the number of

bits in one Flash page. Internally the Shift Register is 8-bit, and the data are automatically trans-

ferred to the Flash page buffer byte by byte. Shift in all instruction words in the page, starting

with the LSB of the first instruction in the page and ending with the MSB of the last instruction in

the page. This provides an efficient way to load the entire Flash page buffer before executing

Page Write.

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Figure 143. Virtual Flash Page Load Register

STROBES

ADDRESS

State

Machine

TDI

Flash

EEPROM

Fuses

Lock Bits

D

A

T

A

TDO

Virtual Flash Page

Read Register

The Virtual Flash Page Read Register is a virtual scan chain with length equal to the number of

bits in one Flash page plus 8. Internally the Shift Register is 8-bit, and the data are automatically

transferred from the Flash data page byte by byte. The first 8 cycles are used to transfer the first

byte to the internal Shift Register, and the bits that are shifted out during these 8 cycles should

be ignored. Following this initialization, data are shifted out starting with the LSB of the first

instruction in the page and ending with the MSB of the last instruction in the page. This provides

an efficient way to read one full Flash page to verify programming.

Figure 144. Virtual Flash Page Read Register

STROBES

State

Machine

TDI

ADDRESS

Flash

EEPROM

Fuses

Lock Bits

D

A

T

A

TDO

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Programming

Algorithm

All references below of type “1a”, “1b”, and so on, refer to Table 117.

EnteringProgramming 1. Enter JTAG instruction AVR_RESET and shift 1 in the Reset Register.

Mode

2. Enter instruction PROG_ENABLE and shift 1010_0011_0111_0000 in the Programming

Enable Register.

Leaving Programming 1. Enter JTAG instruction PROG_COMMANDS.

Mode

2. Disable all programming instructions by using no operation instruction 11a.

3. Enter instruction PROG_ENABLE and shift 0000_0000_0000_0000 in the programming

Enable Register.

4. Enter JTAG instruction AVR_RESET and shift 0 in the Reset Register.

Performing Chip Erase 1. Enter JTAG instruction PROG_COMMANDS.

2. Start chip erase using programming instruction 1a.

3. Poll for Chip Erase complete using programming instruction 1b, or wait for t_{WLRH_CE}(refer

to Table 113 on page 272).

Programming the

Flash

Before programming the Flash a Chip Erase must be performed. See “Performing Chip Erase”

on page 288.

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Flash write using programming instruction 2a.

3. Load address High byte using programming instruction 2b.

4. Load address Low byte using programming instruction 2c.

5. Load data using programming instructions 2d, 2e and 2f.

6. Repeat steps 4 and 5 for all instruction words in the page.

7. Write the page using programming instruction 2g.

8. Poll for Flash write complete using programming instruction 2h, or wait for t_WLRH(refer to

Table 113 on page 272).

9. Repeat steps 3 to 7 until all data have been programmed.

A more efficient data transfer can be achieved using the PROG_PAGELOAD instruction:

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Flash write using programming instruction 2a.

3. Load the page address using programming instructions 2b and 2c. PCWORD (refer to

Table 107 on page 262) is used to address within one page and must be written as 0.

4. Enter JTAG instruction PROG_PAGELOAD.

5. Load the entire page by shifting in all instruction words in the page, starting with the LSB

of the first instruction in the page and ending with the MSB of the last instruction in the

page.

6. Enter JTAG instruction PROG_COMMANDS.

7. Write the page using programming instruction 2g.

8. Poll for Flash write complete using programming instruction 2h, or wait for t_WLRH(refer to

Table 113 on page 272).

9. Repeat steps 3 to 8 until all data have been programmed.

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Reading the Flash

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Flash read using programming instruction 3a.

3. Load address using programming instructions 3b and 3c.

4. Read data using programming instruction 3d.

5. Repeat steps 3 and 4 until all data have been read.

A more efficient data transfer can be achieved using the PROG_PAGEREAD instruction:

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Flash read using programming instruction 3a.

3. Load the page address using programming instructions 3b and 3c. PCWORD (refer to

Table 107 on page 262) is used to address within one page and must be written as 0.

4. Enter JTAG instruction PROG_PAGEREAD.

5. Read the entire page by shifting out all instruction words in the page, starting with the

LSB of the first instruction in the page and ending with the MSB of the last instruction in

the page. Remember that the first 8 bits shifted out should be ignored.

6. Enter JTAG instruction PROG_COMMANDS.

7. Repeat steps 3 to 6 until all data have been read.

Programming the

EEPROM

Before programming the EEPROM a Chip Erase must be performed. See “Performing Chip

Erase” on page 288.

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable EEPROM write using programming instruction 4a.

3. Load address High byte using programming instruction 4b.

4. Load address Low byte using programming instruction 4c.

5. Load data using programming instructions 4d and 4e.

6. Repeat steps 4 and 5 for all data bytes in the page.

7. Write the data using programming instruction 4f.

8. Poll for EEPROM write complete using programming instruction 4g, or wait for t_WLRH

(refer to Table 113 on page 272).

9. Repeat steps 3 to 8 until all data have been programmed.

Note that the PROG_PAGELOAD instruction can not be used when programming the EEPROM

Reading the EEPROM 1. Enter JTAG instruction PROG_COMMANDS.

2. Enable EEPROM read using programming instruction 5a.

3. Load address using programming instructions 5b and 5c.

4. Read data using programming instruction 5d.

5. Repeat steps 3 and 4 until all data have been read.

Note that the PROG_PAGEREAD instruction can not be used when reading the EEPROM

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Programming the

Fuses

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Fuse write using programming instruction 6a.

3. Load data High byte using programming instructions 6b. A bit value of “0” will program

the corresponding fuse, a “1” will unprogram the fuse.

4. Write Fuse High byte using programming instruction 6c.

5. Poll for Fuse write complete using programming instruction 6d, or wait for t_WLRH(refer to

Table 113 on page 272).

6. Load data Low byte using programming instructions 6e. A “0” will program the fuse, a “1”

will unprogram the fuse.

7. Write Fuse Low byte using programming instruction 6f.

8. Poll for Fuse write complete using programming instruction 6g, or wait for t_WLRH(refer to

Table 113 on page 272).

Programming the Lock 1. Enter JTAG instruction PROG_COMMANDS.

Bits

2. Enable Lock bit write using programming instruction 7a.

3. Load data using programming instructions 7b. A bit value of “0” will program the corre-

sponding Lock bit, a “1” will leave the Lock bit unchanged.

4. Write Lock bits using programming instruction 7c.

5. Poll for Lock bit write complete using programming instruction 7d, or wait for t_WLRH(refer

to Table 113 on page 272).

Reading the Fuses

and Lock Bits

1. Enter JTAG instruction PROG_COMMANDS.

2. Enable Fuse/Lock bit read using programming instruction 8a.

3. To read all Fuses and Lock bits, use programming instruction 8e.

To only read Fuse High byte, use programming instruction 8b.

To only read Fuse Low byte, use programming instruction 8c.

To only read Lock bits, use programming instruction 8d.

Reading the Signature 1. Enter JTAG instruction PROG_COMMANDS.

Bytes

2. Enable Signature byte read using programming instruction 9a.

3. Load address $00 using programming instruction 9b.

4. Read first signature byte using programming instruction 9c.

5. Repeat steps 3 and 4 with address $01 and address $02 to read the second and third

signature bytes, respectively.

Reading the

1. Enter JTAG instruction PROG_COMMANDS.

Calibration Byte

2. Enable Calibration byte read using programming instruction 10a.

3. Load address $00 using programming instruction 10b.

4. Read the calibration byte using programming instruction 10c.

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Electrical Characteristics

Absolute Maximum Ratings*

*NOTICE:

Stresses beyond those listed under “Absolute

Maximum Ratings” may cause permanent dam-

age to the device. This is a stress rating only and

functional operation of the device at these or

other conditions beyond those indicated in the

operational sections of this specification is not

implied. Exposure to absolute maximum rating

conditions for extended periods may affect

device reliability.

Operating Temperature.................................. -55°C to +125°C

Storage Temperature..................................... -65°C to +150°C

Voltage on any Pin except RESET

with respect to Ground ................................-0.5V to V_CC+0.5V

Voltage on RESET with respect to Ground......-0.5V to +13.0V

Maximum Operating Voltage ............................................ 6.0V

DC Current per I/O Pin ............................................... 40.0 mA

DC Current V_CCand GND Pins................200.0 mA PDIP and

400.0 mA TQFP/MLF

DC Characteristics

T_A= -40°C to 85°C, V_CC= 2.7V to 5.5V (Unless Otherwise Noted)

Symbol

Parameter

Condition

Min

Typ

Max

Units

Input Low Voltage except

XTAL1 and RESET pins

(1)

V_IL

V_CC= 2.7 - 5.5

-0.5

0.2 V_CC

Input High Voltage except

XTAL1 and RESET pins

(2)

V_IH

V_CC= 2.7 - 5.5

0.6 V_CC

0.7 V_CC

-0.5

V_CC+0.5

Input High Voltage

XTAL1 pin

V_IH1

V_IL1

V_IH2

V_CC= 2.7 - 5.5

V

Input Low Voltage

XTAL1 pin

(1)

0.1 V_CC

Input High Voltage

RESET pin

(2)

0.9 V_CC

-0.5

V_CC+0.5

0.2 V_CC

Input Low Voltage

RESET pin

V_IL2

V_OL

V_OH

I_IL

Output Low Voltage⁽³⁾

(Ports A,B,C,D)

I

_OL= 20 mA, V_CC= 5V

0.7

0.5

V

I_OL= 10 mA, V_CC= 3V

Output High Voltage⁽⁴⁾

(Ports A,B,C,D)

I_OH= -20 mA, V_CC= 5V

4.2

2.2

V

I

_OH= -10 mA, V_CC= 3V

Input Leakage

Current I/O Pin

Vcc = 5.5V, pin low

(absolute value)

1

µA

Input Leakage

Current I/O Pin

Vcc = 5.5V, pin high

(absolute value)

I_IH

R_RST

R_pu

Reset Pull-up Resistor

I/O Pin Pull-up Resistor

30

20

60

50

kΩ

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T_A= -40°C to 85°C, V_CC= 2.7V to 5.5V (Unless Otherwise Noted) (Continued)

Symbol

Parameter

Condition

Min

Typ

Max

Units

Active 1 MHz, V_CC= 3V

(ATmega16L)

1.1

Active 4 MHz, V_CC= 3V

(ATmega16L)

3.8

12

5

Active 8 MHz, V_CC= 5V

(ATmega16)

15

Power Supply Current

mA

Idle 1 MHz, V_CC= 3V

(ATmega16L)

0.35

1.2

5.5

I_CC

Idle 4 MHz, V_CC= 3V

(ATmega16L)

2

7

Idle 8 MHz, V_CC= 5V

(ATmega16)

WDT enabled, V_CC= 3V

WDT disabled, V_CC= 3V

<8

15

4

Power-down Mode⁽⁵⁾

µA

< 1

V_CC= 5V

Analog Comparator

Input Offset Voltage

V_ACIO

I_ACLK

t_ACPD

40

50

mV

nA

ns

V_in= V_CC/2

Analog Comparator

Input Leakage Current

V_CC= 5V

V_in= V_CC/2

-50

Analog Comparator

Propagation Delay

V_CC= 2.7V

V_CC= 4.0V

750

500

Notes: 1. “Max” means the highest value where the pin is guaranteed to be read as low

2. “Min” means the lowest value where the pin is guaranteed to be read as high

3. Although each I/O port can sink more than the test conditions (20 mA at Vcc = 5V, 10 mA at Vcc = 3V) under steady state

conditions (non-transient), the following must be observed:

PDIP Package:

1] The sum of all IOL, for all ports, should not exceed 200 mA.

2] The sum of all IOL, for port A0 - A7, should not exceed 100 mA.

3] The sum of all IOL, for ports B0 - B7,C0 - C7, D0 - D7 and XTAL2, should not exceed 100 mA.

TQFP and QFN/MLF Package:

1] The sum of all IOL, for all ports, should not exceed 400 mA.

2] The sum of all IOL, for ports A0 - A7, should not exceed 100 mA.

3] The sum of all IOL, for ports B0 - B4, should not exceed 100 mA.

4] The sum of all IOL, for ports B3 - B7, XTAL2, D0 - D2, should not exceed 100 mA.

5] The sum of all IOL, for ports D3 - D7, should not exceed 100 mA.

6] The sum of all IOL, for ports C0 - C7, should not exceed 100 mA.

If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater

than the listed test condition.

4. Although each I/O port can source more than the test conditions (20 mA at Vcc = 5V, 10 mA at Vcc = 3V) under steady state

conditions (non-transient), the following must be observed:

PDIP Package:

1] The sum of all IOH, for all ports, should not exceed 200 mA.

2] The sum of all IOH, for port A0 - A7, should not exceed 100 mA.

3] The sum of all IOH, for ports B0 - B7,C0 - C7, D0 - D7 and XTAL2, should not exceed 100 mA.

TQFP and QFN/MLF Package:

1] The sum of all IOH, for all ports, should not exceed 400 mA.

2] The sum of all IOH, for ports A0 - A7, should not exceed 100 mA.

3] The sum of all IOH, for ports B0 - B4, should not exceed 100 mA.

4] The sum of all IOH, for ports B3 - B7, XTAL2, D0 - D2, should not exceed 100 mA.

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5] The sum of all IOH, for ports D3 - D7, should not exceed 100 mA.

6] The sum of all IOH, for ports C0 - C7, should not exceed 100 mA.If IOH exceeds the test condition, VOH may exceed the

related specification. Pins are not guaranteed to source current greater than the listed test condition.

5. Minimum V_CCfor Power-down is 2.5V.

External Clock

Figure 145. External Clock Drive Waveforms

Drive Waveforms

V

IH1

V

IL1

External Clock

Drive

Table 118. External Clock Drive⁽¹⁾

V_CC= 2.7V to 5.5V

V_CC= 4.5V to 5.5V

Symbol

1/t_CLCL

t_CLCL

Parameter

Oscillator Frequency

Clock Period

High Time

Min

0

Max

Min

0

Max

Units

8

16

MHz

125

50

62.5

25

t_CHCX

t_CLCX

ns

Low Time

50

25

t_CLCH

Rise Time

1.6

0.5

μs

t_CHCL

Fall Time

Change in period from

one clock cycle to the

2

%

Δt_CLCL

Note:

1. Refer to “External Clock” on page 31 for details.

Table 119. External RC Oscillator, Typical Frequencies (V_CC= 5)

R [kΩ]⁽¹⁾

33

C [pF]

22

f⁽²⁾

650 kHz

2.0 MHz

10

22

Notes: 1. R should be in the range 3 kΩ - 100 kΩ, and C should be at least 20 pF.

2. The frequency will vary with package type and board layout.

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ATmega16(L)

Two-wire Serial Interface Characteristics

Table 120 describes the requirements for devices connected to the Two-wire Serial Bus. The ATmega16 Two-wire Serial

Interface meets or exceeds these requirements under the noted conditions.

Timing symbols refer to Figure 146.

Table 120. Two-wire Serial Bus Requirements

Symbol Parameter

Condition

Min

-0.5

Max

0.3 V_CC

V_CC+ 0.5

–

Units

V_IL

Input Low-voltage

V_IH

V_hys

Input High-voltage

0.7 V_CC

V

(1)

(2)

Hysteresis of Schmitt Trigger Inputs

Output Low-voltage

0.05 V_CC

V_OL

t_r⁽¹⁾

3 mA sink current

10 pF < C_b< 400 pF⁽³⁾

0.1V_CC< V_i< 0.9V_CC

0

0.4

(3)(2)

Rise Time for both SDA and SCL

Output Fall Time from V_IHminto V_ILmax

Spikes Suppressed by Input Filter

Input Current each I/O Pin

Capacitance for each I/O Pin

SCL Clock Frequency

20 + 0.1C_b

300

(1)

t_of

20 + 0.1C_b

250

ns

t_SP

I_i

C_i⁽¹⁾

f_SCL

0

-10

–

50⁽²⁾

(1)

10

µA

pF

10

f_CK⁽⁴⁾> max(16f_SCL, 250kHz)⁽⁵⁾

0

400

kHz

V_CC– 0.4V

----------------------------

3 mA

f_SCL≤ 100 kHz

1000ns

C_b

-------------------

Rp

Value of Pull-up resistor

Ω

V_CC– 0.4V

----------------------------

3 mA

f

_SCL> 100 kHz

300ns

---------------

C_b

f_SCL≤ 100 kHz

f_SCL> 100 kHz

f_SCL≤ 100 kHz

4.0

0.6

4.7

1.3

4.0

0.6

4.7

0.6

0

–

t_HD;STA

Hold Time (repeated) START Condition

Low Period of the SCL Clock

–

t_LOW

f

_SCL> 100 kHz

–

f_SCL≤ 100 kHz

f_SCL> 100 kHz

f_SCL≤ 100 kHz

f_SCL> 100 kHz

f_SCL≤ 100 kHz

–

t_HIGH

High period of the SCL clock

µs

–

t_SU;STA

t_HD;DAT

t_SU;DAT

t_SU;STO

t_BUF

Set-up time for a repeated START condition

Data hold time

–

3.45

0.9

–

f

_SCL> 100 kHz

0

f_SCL≤ 100 kHz

f_SCL> 100 kHz

f_SCL≤ 100 kHz

250

100

4.0

0.6

4.7

1.3

Data setup time

ns

µs

–

Setup time for STOP condition

f

_SCL> 100 kHz

–

f_SCL≤ 100 kHz

–

Bus free time between a STOP and START

condition

f_SCL> 100 kHz

–

Notes: 1. In ATmega16, this parameter is characterized and not 100% tested.

2. Required only for f_SCL> 100 kHz.

3. C_b= capacitance of one bus line in pF.

4. f_CK= CPU clock frequency

294

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ATmega16(L)

5. This requirement applies to all ATmega16 Two-wire Serial Interface operation. Other devices

connected to the Two-wire Serial Bus need only obey the general f_SCLrequirement.

Figure 146. Two-wire Serial Bus Timing

t

HIGH

t

r

of

t

LOW

SCL

SDA

t

HD;DAT

SU;STA

HD;STA

t

SU;DAT

t

SU;STO

t

BUF

SPI Timing

See Figure 147 and Figure 148 for details.

Characteristics

Table 121. SPI Timing Parameters

Description

SCK period

SCK high/low

Rise/Fall time

Setup

Mode

Master

Slave

Min

Typ

See Table 58

Max

1

2

50% duty cycle

3

3.6

10

4

5

Hold

10

ns

6

Out to SCK

SCK to out

SCK to out high

SS low to out

SCK period

SCK high/low

Rise/Fall time

Setup

0.5 • t_SCK

10

7

8

10

9

15

10

11

12

13

14

15

16

17

18

Slave

4 • t_SCK

2 • t_SCK

Slave

1.6

µs

ns

Slave

10

Hold

Slave

SCK to out

SCK to SS high

SS high to tri-state

SS low to SCK

Slave

15

10

Slave

20

Slave

2 • t_SCK

295

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ATmega16(L)

Figure 147. SPI Interface Timing Requirements (Master Mode)

SS

6

1

SCK

(CPOL = 0)

2

SCK

(CPOL = 1)

4

5

3

MISO

(Data Input)

MSB

...

LSB

7

8

MOSI

(Data Output)

MSB

...

LSB

Figure 148. SPI Interface Timing Requirements (Slave Mode)

18

SS

10

16

9

SCK

(CPOL = 0)

11

SCK

(CPOL = 1)

13

14

12

MOSI

(Data Input)

MSB

...

LSB

15

17

MISO

(Data Output)

MSB

...

LSB

X

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ATmega16(L)

ADC Characteristics

Table 122. ADC Characteristics

Symbol Parameter

Condition

Min⁽¹⁾

Typ⁽¹⁾

Max⁽¹⁾

Units

Single Ended Conversion

10

Differential Conversion

Gain = 1x or 10x

8

7

Resolution

Bits

Differential Conversion

Gain = 200x

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 200 kHz

1.5

3

2.5

4

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 1 MHz

Absolute Accuracy (Including INL, DNL,

Quantization Error, Gain, and Offset Error).

Single Ended Conversion

V_REF= 4V, V_CC= 4V

1.5

3

ADC clock = 200 kHz

Noise Reduction mode

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 1 MHz

LSB

Noise Reduction mode

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 200 kHz

Integral Non-linearity (INL)

Differential Non-linearity (DNL)

Gain Error

1

0.5

1

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 200 kHz

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 200 kHz

Single Ended Conversion

Offset Error

V_REF= 4V, V_CC= 4V

ADC clock = 200 kHz

Conversion Time

Free Running Conversion

13

50

260

1000

µs

Clock Frequency

kHz

AVCC

V_REF

Analog Supply Voltage

V_CC- 0.3⁽²⁾

V_CC+ 0.3⁽³⁾

Single Ended Conversion

Differential Conversion

Single ended channels

Differential channels

Single ended channels

Differential channels

2.0

AVCC

Reference Voltage

Input voltage

2.0

AVCC - 0.2

V_REF

V

GND

0

V_REF

V_IN

38.5

4

Input bandwidth

kHz

297

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ATmega16(L)

Table 122. ADC Characteristics (Continued)

Symbol Parameter

Condition

Min⁽¹⁾

Typ⁽¹⁾

2.6

Max⁽¹⁾

Units

V

V_INT

R_REF

R_AIN

Internal Voltage Reference

Reference Input Resistance

Analog Input Resistance

2.3

2.9

32

kΩ

100

MΩ

Notes: 1. Values are guidelines only.

2. Minimum for AVCC is 2.7V.

3. Maximum for AVCC is 5.5V.

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ATmega16(L)

ATmega16

Typical

Characteristics

The following charts show typical behavior. These figures are not tested during manufacturing.

All current consumption measurements are performed with all I/O pins configured as inputs and

with internal pull-ups enabled. A sine wave generator with rail-to-rail output is used as clock

source.

The power consumption in Power-down mode is independent of clock selection.

The current consumption is a function of several factors such as: operating voltage, operating

frequency, loading of I/O pins, switching rate of I/O pins, code executed and ambient tempera-

ture. The dominating factors are operating voltage and frequency.

The current drawn from capacitive loaded pins may be estimated (for one pin) as C_L*V_CC*f where

C_L= load capacitance, V_CC= operating voltage and f = average switching frequency of I/O pin.

The parts are characterized at frequencies higher than test limits. Parts are not guaranteed to

function properly at frequencies higher than the ordering code indicates.

The difference between current consumption in Power-down mode with Watchdog Timer

enabled and Power-down mode with Watchdog Timer disabled represents the differential cur-

rent drawn by the Watchdog Timer.

Active Supply Current Figure 149. Active Supply Current vs. Frequency (0.1 MHz - 1.0 MHz

2

1.8

1.6

1.4

1.2

1

5.5V

5.0V

4.5V

4.0V

3.6V

3.3V

3.0V

2.7V

0.8

0.6

0.4

0.2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Frequency (MHz)

299

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ATmega16(L)

Figure 150. Active Supply Current vs. Frequency (1 MHz - 20 MHz)

35

30

25

20

15

5.5V

5.0V

4.5V

4.0V

3.6V

10

5

3.3V

3.0V

2.7V

0

2

4

6

8

10

Frequency (MHz)

12

14

16

18

20

Figure 151. Active Supply Current vs. V_CC(Internal RC Oscillator, 8 MHz)

16

-40°C

25°C

14

85°C

12

10

8

6

4

2

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

300

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ATmega16(L)

Figure 152. Active Supply Current vs. V_CC(Internal RC Oscillator, 4 MHz)

9

8

7

6

5

4

3

2

1

0

25°C

-40°C

85°C

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

Figure 153. Active Supply Current vs. V_CC(Internal RC Oscillator, 2 MHz)

4.5

4

25°C

85°C

3.5

-40°C

3

2.5

2

1.5

1

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

301

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ATmega16(L)

Figure 154. Active Supply Current vs. V_CC(Internal RC Oscillator, 1 MHz)

2.5

85°C

-40°C

25°C

2

1.5

1

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 155. Active Supply Current vs. V_CC(32 kHz External Oscillator)

180

160

140

120

100

80

85°C

25°C

60

40

20

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

302

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ATmega16(L)

Idle Supply Current

Figure 156. Idle Supply Current vs. Frequency (0.1 MHz - 1.0 MHz)

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

5.5V

5.0V

4.5V

4.0V

3.6V

3.3V

3.0V

2.7V

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Frequency (MHz)

Figure 157. Idle Supply Current vs. Frequency (1 MHz - 20 MHz)

16

14

12

10

8

5.5V

5.0V

4.5V

4.0V

6

3.6V

4

3.3V

3.0V

2

2.7V

0

2

4

6

8

10

12

14

16

18

20

Frequency (MHz)

303

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ATmega16(L)

Figure 158. Idle Supply Current vs. V_CC(Internal RC Oscillator, 8 MHz)

8

7

6

5

4

3

2

1

0

25°C

-40°C

85°C

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 159. Idle Supply Current vs. V_CC(Internal RC Oscillator, 4 MHz)

4

-40°C

25°C

85°C

3.5

3

2.5

2

1.5

1

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

304

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ATmega16(L)

Figure 160. Idle Supply Current vs. V_CC(Internal RC Oscillator, 2 MHz)

2

1.8

1.6

1.4

1.2

1

25°C

85°C

-40°C

0.8

0.6

0.4

0.2

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 161. Idle Supply Current vs. V_CC(Internal RC Oscillator, 1 MHz)

1

25°C

0.9

85°C

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

305

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ATmega16(L)

Figure 162. Idle Supply Current vs. V_CC(32 kHz External Oscillator)

40

35

30

25

20

15

10

5

85°C

25°C

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Power-Down Supply

Current

Figure 163. Power-Down Supply Current vs. V_CC(Watchdog Timer Disabled)

2.5

85°C

2

1.5

1

-40°C

25°C

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

306

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ATmega16(L)

Figure 164. Power-Down Supply Current vs. V_CC(Watchdog Timer Enabled)

20

18

16

14

12

10

8

-40°C

85°C

25°C

6

4

2

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Power-Save Supply

Current

Figure 165. Power-Save Supply Current vs. V_CC(Watchdog Timer Disabled)

18

16

14

12

10

8

85°C

25°C

6

4

2

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

307

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ATmega16(L)

Standby Supply

Current

Figure 166. Standby Supply Current vs. V_CC(455 kHz Resonator, Watchdog Timer Disabled)

60

50

40

30

20

10

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 167. Standby Supply Current vs. V_CC(1 MHz Resonator, Watchdog Timer Disabled)

50

45

40

35

30

25

20

15

10

5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

308

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ATmega16(L)

Figure 168. Standby Supply Current vs. V_CC(2 MHz Resonator, Watchdog Timer Disabled)

80

70

60

50

40

30

20

10

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 169. Standby Supply Current vs. V_CC(2 MHz Xtal, Watchdog Timer Disabled)

80

70

60

50

40

30

20

10

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

309

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ATmega16(L)

Figure 170. Standby Supply Current vs. V_CC(4 MHz Resonator, Watchdog Timer Disabled)

120

100

80

60

40

20

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 171. Standby Supply Current vs. V_CC(4 MHz Xtal, Watchdog Timer Disabled)

120

100

80

60

40

20

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

310

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ATmega16(L)

Figure 172. Standby Supply Current vs. V_CC(6 MHz Resonator, Watchdog Timer Disabled)

140

120

100

80

60

40

20

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

Figure 173. Standby Supply Current vs. V_CC(6 MHz Xtal, Watchdog Timer Disabled)

180

160

140

120

100

80

60

40

20

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

311

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ATmega16(L)

Pin Pullup

Figure 174. I/O Pin Pull-Up Resistor Current vs. Input Voltage (V_CC= 5V)

160

140

85°C

120

25°C

-40°C

100

80

60

40

20

0

1

2

3

V_IO(V)

Figure 175. I/O Pin Pull-Up Resistor Current vs. Input Voltage (V_CC= 2.7V)

100

90

80

85°C

25°C

70

60

50

40

30

20

10

0

-40°C

0

0.5

1

1.5

2

2.5

3

V_IO(V)

312

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ATmega16(L)

Figure 176. Reset Pull-Up Resistor Current vs. Reset Pin Voltage (V_CC= 5V)

120

25°C

-40°C

100

85°C

80

60

40

20

0

1

2

3

V_RESET(V)

Figure 177. Reset Pull-Up Resistor Current vs. Reset Pin Voltage (V_CC= 2.7V)

60

25°C

-40°C

50

85°C

40

30

20

10

0

0.5

1

1.5

2

2.5

3

V_RESET(V)

313

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ATmega16(L)

Pin Driver Strength

Figure 178. I/O Pin Source Current vs. Output Voltage (V_CC= 5V)

90

-40°C

80

25°C

70

85°C

60

50

40

30

20

10

0

1

2

3

4

V_OH(V)

Figure 179. I/O Pin Source Current vs. Output Voltage (V_CC= 2.7V)

30

-40°C

25

25°C

85°C

20

15

10

5

0

0.5

1

1.5

2

2.5

3

V_OH(V)

314

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ATmega16(L)

Figure 180. I/O Pin Sink Current vs. Output Voltage (V_CC= 5V)

90

80

70

60

50

40

30

20

10

0

-40°C

25°C

85°C

0

0.5

1

1.5

2

2.5

V_OL(V)

Figure 181. I/O Pin Sink Current vs. Output Voltage (V_CC= 2.7V)

35

30

25

20

15

10

5

-40°C

25°C

85°C

0

0.5

1

1.5

2

2.5

V_OL(V)

315

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ATmega16(L)

Pin Thresholds And

Hysteresis

Figure 182. I/O Pin Input Threshold Voltage vs. V_CC(V_IH, I/O Pin Read As '1')

2.5

2

-40°C

85°C

25°C

1.5

1

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 183. I/O Pin Input Threshold Voltage vs. V_CC(V_IL, I/O Pin Read As '0')

2.5

2

-40°C

1.5

25°C

85°C

1

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

316

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ATmega16(L)

Figure 184. I/O Pin Input Hysteresis vs. V_CC

0.8

85°C

25°C

0.6

0.4

0.2

0

-40°C

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

Figure 185. Reset Input Threshold Voltage vs. V_CC(V_IH, Reset Pin Read As '1')

2.5

2

-40°C

1.5

25°C

85°C

1

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

317

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ATmega16(L)

Figure 186. Reset Input Threshold Voltage vs. V_CC(V_IL, Reset Pin Read As '0')

2.5

2

1.5

1

85°C

25°C

-40°C

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 187. Reset Input Pin Hysteresis vs. V_CC

0.5

-40°C

0.45

0.4

0.35

0.3

25°C

0.25

0.2

0.15

85°C

0.1

0.05

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

318

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ATmega16(L)

Bod Thresholds And

Analog Comparator

Offset

Figure 188. Bod Thresholds vs. Temperature (Bodlevel is 4.0V)

4.1

4

Rising V_CC

3.9

3.8

Falling V_CC

3.7

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

80

90

100

Temperature (°C)

Figure 189. Bod Thresholds vs. Temperature (Bodlevel is 2.7V)

3

2.9

Rising V_CC

2.8

2.7

2.6

Falling V_CC

2.5

2.4

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

80

90

100

Temperature (°C)

319

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ATmega16(L)

Figure 190. Bandgap Voltage vs. V_CC

1.25

1.245

1.24

85°

25°

-40°

1.235

1.23

1.225

2.5

3

3.5

4

4.5

5

5.5

Vcc (V)

Figure 191. Analog Comparator Offset Voltage vs. Common Mode Voltage (V_CC= 5V)

0

-0.001

-0.002

-0.003

-0.004

-0.005

-0.006

85°C

-0.007

25°C

-0.008

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Common Mode Voltage (V)

320

2466T–AVR–07/10

ATmega16(L)

Figure 192. Analog Comparator Offset Voltage vs. Common Mode Voltage (V_CC= 3V)

0

-0.001

-0.002

-0.003

-0.004

-0.005

-0.006

-0.007

85°C

25°C

0

0.5

1

1.5

2

2.5

3

Common Mode Voltage (V)

Internal Oscillator

Speed

Figure 193. Watchdog Oscillator Frequency vs. V_CC

1060

1050

1040

1030

1020

1010

1000

990

-40°C

25°C

85

°C

980

970

960

950

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

321

2466T–AVR–07/10

ATmega16(L)

Figure 194. Calibrated 8 MHz RC Oscillator Frequency vs. Temperature

9

8.6

8.2

7.8

7.4

7

5.5V

4.0V

2.7V

6.6

-60

-40

-20

0

20

40

60

80

100

Temperature (°C)

Figure 195. Calibrated 8 MHz RC Oscillator Frequency vs. V_CC

8.5

8.3

8.1

7.9

7.7

7.5

7.3

7.1

6.9

6.7

6.5

-40°C

25°C

85°C

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

322

2466T–AVR–07/10

ATmega16(L)

Figure 196. Calibrated 8 MHz RC Oscillator Frequency vs. Osccal Value

13

12

11

10

9

8

7

6

5

4

3

0

16

32

48

64

80

96

112 128 144 160 176 192 208 224 240

OSCCAL VALUE

Figure 197. Calibrated 4 MHz RC Oscillator Frequency vs. Temperature

4.1

4

5.5V

4.0V

3.9

3.8

3.7

3.6

2.7V

-60

-40

-20

0

20

40

60

80

100

Temperature (°C)

323

2466T–AVR–07/10

ATmega16(L)

Figure 198. Calibrated 4 MHz RC Oscillator Frequency vs. V_CC

4.2

4.1

4

-40°C

25°C

85°C

3.9

3.8

3.7

3.6

3.5

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

Figure 199. Calibrated 4 MHz RC Oscillator Frequency vs. Osccal Value

6.5

6

5.5

5

4.5

4

3.5

3

2.5

2

1.5

0

16

32

48

64

80

96

112 128 144 160 176 192 208 224 240

OSCCAL VALUE

324

2466T–AVR–07/10

ATmega16(L)

Figure 200. Calibrated 2 MHz RC Oscillator Frequency vs. Temperature

2.1

2.05

2

5.5V

4.0V

1.95

1.9

1.85

1.8

2.7V

-60

-40

-20

0

20

40

60

80

100

Temperature (°C)

Figure 201. Calibrated 2 MHz RC Oscillator Frequency vs. V_CC

2.2

2.1

2

-40°C

25°C

85°C

1.9

1.8

1.7

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

325

2466T–AVR–07/10

ATmega16(L)

Figure 202. Calibrated 2 MHz RC Oscillator Frequency vs. Osccal Value

3.2

3

2.8

2.6

2.4

2.2

2

1.8

1.6

1.4

1.2

1

0.8

0

16

32

48

64

80

96

112 128 144 160 176 192 208 224 240

OSCCAL VALUE

Figure 203. Calibrated 1 MHz RC Oscillator Frequency vs. Temperature

1.02

1.01

1

5.5V

4.0V

0.99

0.98

0.97

0.96

0.95

0.94

0.93

2.7V

-60

-40

-20

0

20

40

60

80

100

Temperature (°C)

326

2466T–AVR–07/10

ATmega16(L)

Figure 204. Calibrated 1 MHz RC Oscillator Frequency vs. V_CC

1.1

1.05

1

25°C

-40°C

85°C

0.95

0.9

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

Figure 205. Calibrated 1 MHz RC Oscillator Frequency vs. Osccal Value

1.6

1.4

1.2

1

0.8

0.6

0.4

0

16

32

48

64

80

96

112 128 144 160 176 192 208 224 240

OSCCAL VALUE

327

2466T–AVR–07/10

ATmega16(L)

Current Consumption Figure 206. Brownout Detector Current vs. V_CC

Of Peripheral Units

20

18

16

14

12

10

8

-40°C

25°C

85°C

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 207. ADC Current vs. V_CC(Aref = AVCC)

350

300

250

200

150

100

50

25°C

85°C

-40°C

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

328

2466T–AVR–07/10

ATmega16(L)

Figure 208. Aref External Reference Current vs. V_CC

180

160

140

120

100

80

85°C

25°C

-40°C

60

40

20

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

Figure 209. 32khz Tosc Current vs. V_CC(Watchdog Timer Disabled)

16

14

12

10

8

85°C

25°C

6

4

2

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

329

2466T–AVR–07/10

ATmega16(L)

Figure 210. Watchdog Timer Current vs. V_CC

18

16

14

12

10

8

-40°C

25°C

85°C

6

4

2

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

Figure 211. Programming Current vs. V_CC

9

8

7

6

5

4

3

2

1

0

-40°C

25°C

85°C

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

330

2466T–AVR–07/10

ATmega16(L)

Register Summary

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Page

$3F ($5F)

$3E ($5E)

$3D ($5D)

$3C ($5C)

$3B ($5B)

$3A ($5A)

$39 ($59)

$38 ($58)

$37 ($57)

$36 ($56)

$35 ($55)

$34 ($54)

$33 ($53)

$32 ($52)

SREG

SPH

I

–

T

–

H

–

S

–

V

–

N

Z

C

9

SP10

SP2

SP9

SP1

SP8

SP0

12

SPL

SP7

SP6

SP5

SP4

SP3

12

Timer/Counter0 Output Compare Register

OCR0

85

GICR

INT1

INTF1

OCIE2

OCF2

SPMIE

TWINT

SM2

INT0

INTF0

TOIE2

TOV2

RWWSB

TWEA

SE

INT2

INTF2

TICIE1

ICF1

–

IVSEL

–

IVCE

–

48, 69

GIFR

–

70

TIMSK

TIFR

OCIE1A

OCF1A

RWWSRE

TWSTO

SM0

OCIE1B

OCF1B

BLBSET

TWWC

ISC11

WDRF

WGM01

TOIE1

TOV1

PGWRT

TWEN

ISC10

BORF

CS02

OCIE0

OCF0

PGERS

–

TOIE0

TOV0

SPMEN

TWIE

ISC00

PORF

CS00

85, 115, 133

86, 115, 133

SPMCR

TWCR

MCUCR

MCUCSR

TCCR0

TCNT0

OSCCAL

OCDR

SFIOR

TCCR1A

TCCR1B

TCNT1H

TCNT1L

OCR1AH

OCR1AL

OCR1BH

OCR1BL

ICR1H

ICR1L

250

TWSTA

SM1

180

ISC01

EXTRF

CS01

32, 68

JTD

ISC2

–

JTRF

41, 69, 231

FOC0

WGM00

COM01

COM00

83

85

Timer/Counter0 (8 Bits)

Oscillator Calibration Register

On-Chip Debug Register

30

$31⁽¹⁾($51)⁽¹⁾

227

57,88,134,201,221

110

113

114

128

130

131

43

$30 ($50)

$2F ($4F)

$2E ($4E)

$2D ($4D)

$2C ($4C)

$2B ($4B)

$2A ($4A)

$29 ($49)

$28 ($48)

$27 ($47)

$26 ($46)

$25 ($45)

$24 ($44)

$23 ($43)

$22 ($42)

$21 ($41)

ADTS2

COM1A1

ICNC1

ADTS1

COM1A0

ICES1

ADTS0

COM1B1

–

ACME

FOC1A

WGM12

PUD

FOC1B

CS12

PSR2

WGM11

CS11

PSR10

WGM10

CS10

COM1B0

WGM13

Timer/Counter1 – Counter Register High Byte

Timer/Counter1 – Counter Register Low Byte

Timer/Counter1 – Output Compare Register A High Byte

Timer/Counter1 – Output Compare Register A Low Byte

Timer/Counter1 – Output Compare Register B High Byte

Timer/Counter1 – Output Compare Register B Low Byte

Timer/Counter1 – Input Capture Register High Byte

Timer/Counter1 – Input Capture Register Low Byte

TCCR2

TCNT2

OCR2

FOC2

Timer/Counter2 (8 Bits)

Timer/Counter2 Output Compare Register

WGM20

COM21

COM20

WGM21

CS22

CS21

CS20

ASSR

–

WDTOE

–

AS2

TCN2UB

WDP2

OCR2UB

WDP1

TCR2UB

WDP0

WDTCR

UBRRH

UCSRC

EEARH

EEARL

EEDR

–

WDE

URSEL

–

UMSEL

–

UPM1

–

UBRR[11:8]

167

166

19

$20⁽²⁾($40)⁽²⁾

UPM0

–

USBS

–

UCSZ1

–

UCSZ0

–

UCPOL

EEAR8

$1F ($3F)

$1E ($3E)

$1D ($3D)

$1C ($3C)

$1B ($3B)

$1A ($3A)

$19 ($39)

$18 ($38)

$17 ($37)

$16 ($36)

$15 ($35)

$14 ($34)

$13 ($33)

$12 ($32)

$11 ($31)

$10 ($30)

$0F ($2F)

$0E ($2E)

$0D ($2D)

$0C ($2C)

$0B ($2B)

$0A ($2A)

$09 ($29)

$08 ($28)

$07 ($27)

$06 ($26)

$05 ($25)

$04 ($24)

$03 ($23)

$02 ($22)

EEPROM Address Register Low Byte

EEPROM Data Register

19

EECR

–

EERIE

PORTA3

DDA3

EEMWE

PORTA2

DDA2

EEWE

PORTA1

DDA1

EERE

PORTA0

DDA0

19

PORTA

DDRA

PORTA7

DDA7

PORTA6

DDA6

PORTA5

DDA5

PORTA4

DDA4

66

PINA

PINA7

PINA6

PORTB6

DDB6

PINA5

PORTB5

DDB5

PINA4

PORTB4

DDB4

PINA3

PINA2

PINA1

PINA0

66

PORTB

DDRB

PORTB7

DDB7

PORTB3

DDB3

PORTB2

DDB2

PORTB1

DDB1

PORTB0

DDB0

66

PINB

PINB7

PINB6

PORTC6

DDC6

PINB5

PORTC5

DDC5

PINB4

PORTC4

DDC4

PINB3

PINB2

PINB1

PINB0

66

PORTC

DDRC

PINC

PORTC7

DDC7

PORTC3

DDC3

PORTC2

DDC2

PORTC1

DDC1

PORTC0

DDC0

67

PINC7

PINC6

PORTD6

DDD6

PINC5

PORTD5

DDD5

PINC4

PORTD4

DDD4

PINC3

PORTD3

DDD3

PINC2

PINC1

PORTD1

DDD1

PINC0

PORTD0

DDD0

67

PORTD

DDRD

PIND

PORTD7

DDD7

PORTD2

DDD2

67

PIND7

PIND6

PIND5

PIND4

PIND3

PIND2

PIND1

PIND0

67

SPDR

SPI Data Register

SPIF

142

140

163

164

165

167

202

217

219

220

182

SPSR

WCOL

SPE

–

SPI2X

SPR0

SPCR

SPIE

DORD

MSTR

CPOL

CPHA

SPR1

UDR

USART I/O Data Register

UCSRA

UCSRB

UBRRL

ACSR

RXC

TXC

UDRE

UDRIE

FE

DOR

PE

U2X

MPCM

TXB8

RXCIE

TXCIE

RXEN

TXEN

UCSZ2

RXB8

USART Baud Rate Register Low Byte

ACD

REFS1

ADEN

ACBG

REFS0

ADSC

ACO

ACI

MUX4

ADIF

ACIE

MUX3

ADIE

ACIC

MUX2

ADPS2

ACIS1

MUX1

ADPS1

ACIS0

MUX0

ADPS0

ADMUX

ADCSRA

ADCH

ADLAR

ADATE

ADC Data Register High Byte

ADC Data Register Low Byte

ADCL

TWDR

TWAR

Two-wire Serial Interface Data Register

TWA6 TWA5 TWA4

TWA3

TWA2

TWA1

TWA0

TWGCE

331

2466T–AVR–07/10

ATmega16(L)

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Page

TWS6

TWS5

TWS4

TWS3

TWPS1

TWPS0

$01 ($21)

$00 ($20)

TWSR

TWBR

TWS7

–

181

180

Two-wire Serial Interface Bit Rate Register

Notes: 1. When the OCDEN Fuse is unprogrammed, the OSCCAL Register is always accessed on this address. Refer to the debug-

ger specific documentation for details on how to use the OCDR Register.

2. Refer to the USART description for details on how to access UBRRH and UCSRC.

3. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory addresses

should never be written.

4. Some of the Status Flags are cleared by writing a logical one to them. Note that the CBI and SBI instructions will operate on

all bits in the I/O Register, writing a one back into any flag read as set, thus clearing the flag. The CBI and SBI instructions

work with registers $00 to $1F only.

332

2466T–AVR–07/10

ATmega16(L)

Instruction Set Summary

Mnemonics

Flags

Operands

Description

Operation

#Clocks

ARITHMETIC AND LOGIC INSTRUCTIONS

ADD

Rd, Rr

Rdl,K

Rd, Rr

Rd, K

Rd, Rr

Rd, K

Rdl,K

Rd, Rr

Rd, K

Rd, Rr

Rd, K

Rd, Rr

Rd

Add two Registers

Rd ← Rd + Rr

Z,C,N,V,H

Z,C,N,V,S

Z,C,N,V,H

Z,C,N,V,S

Z,N,V

1

2

1

2

1

2

ADC

ADIW

SUB

Add with Carry two Registers

Add Immediate to Word

Subtract two Registers

Subtract Constant from Register

Subtract with Carry two Registers

Subtract with Carry Constant from Reg.

Subtract Immediate from Word

Logical AND Registers

Logical AND Register and Constant

Logical OR Registers

Rd ← Rd + Rr + C

Rdh:Rdl ← Rdh:Rdl + K

Rd ← Rd - Rr

SUBI

SBC

Rd ← Rd - K

Rd ← Rd - Rr - C

Rd ← Rd - K - C

Rdh:Rdl ← Rdh:Rdl - K

Rd ← Rd • Rr

SBCI

SBIW

AND

ANDI

OR

Rd ← Rd • K

Z,N,V

Rd ← Rd v Rr

Z,N,V

ORI

Logical OR Register and Constant

Exclusive OR Registers

One’s Complement

Rd ← Rd v K

Z,N,V

EOR

COM

NEG

SBR

Rd ← Rd ⊕ Rr

Z,N,V

Rd ← $FF − Rd

Rd ← $00 − Rd

Rd ← Rd v K

Z,C,N,V

Z,C,N,V,H

Z,N,V

Rd

Two’s Complement

Rd,K

Rd

Set Bit(s) in Register

CBR

Clear Bit(s) in Register

Increment

Rd ← Rd • ($FF - K)

Rd ← Rd + 1

Z,N,V

INC

Z,N,V

DEC

Rd

Decrement

Rd ← Rd − 1

Z,N,V

TST

Rd

Test for Zero or Minus

Clear Register

Rd ← Rd • Rd

Z,N,V

CLR

Rd

Rd ← Rd ⊕ Rd

Rd ← $FF

Z,N,V

SER

Rd

Set Register

None

MUL

Rd, Rr

Multiply Unsigned

R1:R0 ← Rd x Rr

^{R1:R0 ← (Rd x Rr)}<< 1

R1:R0 ← (Rd x Rr) << 1

Z,C

MULS

MULSU

FMUL

FMULS

FMULSU

Multiply Signed

Z,C

Multiply Signed with Unsigned

Fractional Multiply Unsigned

Fractional Multiply Signed

Fractional Multiply Signed with Unsigned

Z,C

BRANCH INSTRUCTIONS

RJMP

IJMP

k

Relative Jump

PC ← PC + k + 1

None

I

2

Indirect Jump to (Z)

PC ← Z

JMP

k

Direct Jump

PC ← k

3

RCALL

ICALL

CALL

RET

Relative Subroutine Call

Indirect Call to (Z)

PC ← PC + k + 1

3

PC ← Z

3

k

Direct Subroutine Call

Subroutine Return

PC ← k

4

PC ← STACK

4

RETI

Interrupt Return

PC ← STACK

4

CPSE

CP

Rd,Rr

Compare, Skip if Equal

Compare

if (Rd = Rr) PC ← PC + 2 or 3

Rd − Rr

None

Z, N,V,C,H

None

1 / 2 / 3

1

Rd,Rr

CPC

Rd,Rr

Compare with Carry

Rd − Rr − C

1

CPI

Rd,K

Compare Register with Immediate

Skip if Bit in Register Cleared

Skip if Bit in Register is Set

Skip if Bit in I/O Register Cleared

Skip if Bit in I/O Register is Set

Branch if Status Flag Set

Branch if Status Flag Cleared

Branch if Equal

Rd − K

1

SBRC

SBRS

SBIC

SBIS

Rr, b

if (Rr(b)=0) PC ← PC + 2 or 3

if (Rr(b)=1) PC ← PC + 2 or 3

if (P(b)=0) PC ← PC + 2 or 3

if (P(b)=1) PC ← PC + 2 or 3

if (SREG(s) = 1) then PC←PC+k + 1

if (SREG(s) = 0) then PC←PC+k + 1

if (Z = 1) then PC ← PC + k + 1

if (Z = 0) then PC ← PC + k + 1

if (C = 1) then PC ← PC + k + 1

if (C = 0) then PC ← PC + k + 1

if (C = 1) then PC ← PC + k + 1

if (N = 1) then PC ← PC + k + 1

if (N = 0) then PC ← PC + k + 1

if (N ⊕ V= 0) then PC ← PC + k + 1

if (N ⊕ V= 1) then PC ← PC + k + 1

if (H = 1) then PC ← PC + k + 1

if (H = 0) then PC ← PC + k + 1

if (T = 1) then PC ← PC + k + 1

if (T = 0) then PC ← PC + k + 1

if (V = 1) then PC ← PC + k + 1

if (V = 0) then PC ← PC + k + 1

1 / 2 / 3

1 / 2

Rr, b

P, b

s, k

k

BRBS

BRBC

BREQ

BRNE

BRCS

BRCC

BRSH

BRLO

BRMI

BRPL

BRGE

BRLT

BRHS

BRHC

BRTS

BRTC

BRVS

BRVC

k

Branch if Not Equal

k

Branch if Carry Set

k

Branch if Carry Cleared

Branch if Same or Higher

Branch if Lower

k

Branch if Minus

k

Branch if Plus

k

Branch if Greater or Equal, Signed

Branch if Less Than Zero, Signed

Branch if Half Carry Flag Set

Branch if Half Carry Flag Cleared

Branch if T Flag Set

k

Branch if T Flag Cleared

Branch if Overflow Flag is Set

Branch if Overflow Flag is Cleared

k

333

2466T–AVR–07/10

ATmega16(L)

Mnemonics

Flags

Operands

Description

Operation

#Clocks

BRIE

BRID

k

Branch if Interrupt Enabled

Branch if Interrupt Disabled

if ( I = 1) then PC ← PC + k + 1

if ( I = 0) then PC ← PC + k + 1

None

1 / 2

DATA TRANSFER INSTRUCTIONS

MOV

MOVW

LDI

LD

Rd, Rr

Rd, K

Move Between Registers

Copy Register Word

Rd ← Rr

None

1

2

3

-

Rd+1:Rd ← Rr+1:Rr

Load Immediate

Rd ← K

Rd, X

Load Indirect

Rd ← (X)

LD

Rd, X+

Rd, - X

Rd, Y

Load Indirect and Post-Inc.

Load Indirect and Pre-Dec.

Load Indirect

Rd ← (X), X ← X + 1

X ← X - 1, Rd ← (X)

Rd ← (Y)

LD

Rd, Y+

Rd, - Y

Rd,Y+q

Rd, Z

Load Indirect and Post-Inc.

Load Indirect and Pre-Dec.

Load Indirect with Displacement

Load Indirect

Rd ← (Y), Y ← Y + 1

Y ← Y - 1, Rd ← (Y)

Rd ← (Y + q)

Rd ← (Z)

LD

LDD

LD

Rd, Z+

Rd, -Z

Rd, Z+q

Rd, k

Load Indirect and Post-Inc.

Load Indirect and Pre-Dec.

Load Indirect with Displacement

Load Direct from SRAM

Store Indirect

Rd ← (Z), Z ← Z+1

Z ← Z - 1, Rd ← (Z)

Rd ← (Z + q)

Rd ← (k)

LD

LDD

LDS

ST

X, Rr

(X) ← Rr

ST

X+, Rr

- X, Rr

Y, Rr

Store Indirect and Post-Inc.

Store Indirect and Pre-Dec.

Store Indirect

(X) ← Rr, X ← X + 1

X ← X - 1, (X) ← Rr

(Y) ← Rr

ST

Y+, Rr

- Y, Rr

Y+q,Rr

Z, Rr

Store Indirect and Post-Inc.

Store Indirect and Pre-Dec.

Store Indirect with Displacement

Store Indirect

(Y) ← Rr, Y ← Y + 1

Y ← Y - 1, (Y) ← Rr

(Y + q) ← Rr

ST

STD

ST

(Z) ← Rr

ST

Z+, Rr

-Z, Rr

Z+q,Rr

k, Rr

Store Indirect and Post-Inc.

Store Indirect and Pre-Dec.

Store Indirect with Displacement

Store Direct to SRAM

Load Program Memory

Load Program Memory and Post-Inc

Store Program Memory

In Port

(Z) ← Rr, Z ← Z + 1

Z ← Z - 1, (Z) ← Rr

(Z + q) ← Rr

ST

STD

STS

LPM

SPM

IN

(k) ← Rr

R0 ← (Z)

Rd, Z

Rd ← (Z)

Rd, Z+

Rd ← (Z), Z ← Z+1

(Z) ← R1:R0

Rd, P

P, Rr

Rr

Rd ← P

1

2

OUT

PUSH

POP

Out Port

P ← Rr

Push Register on Stack

Pop Register from Stack

STACK ← Rr

Rd ← STACK

Rd

BIT AND BIT-TEST INSTRUCTIONS

SBI

P,b

Rd

s

Set Bit in I/O Register

Clear Bit in I/O Register

Logical Shift Left

I/O(P,b) ← 1

None

2

1

CBI

I/O(P,b) ← 0

None

LSL

Rd(n+1) ← Rd(n), Rd(0) ← 0

Z,C,N,V

LSR

ROL

ROR

ASR

SWAP

BSET

BCLR

BST

BLD

SEC

CLC

SEN

CLN

SEZ

CLZ

SEI

Logical Shift Right

Rd(n) ← Rd(n+1), Rd(7) ← 0

Z,C,N,V

Rotate Left Through Carry

Rotate Right Through Carry

Arithmetic Shift Right

Swap Nibbles

Rd(0)←C,Rd(n+1)← Rd(n),C←Rd(7)

Z,C,N,V

Rd(7)←C,Rd(n)← Rd(n+1),C←Rd(0)

Z,C,N,V

Rd(n) ← Rd(n+1), n=0..6

Z,C,N,V

Rd(3..0)←Rd(7..4),Rd(7..4)←Rd(3..0)

None

Flag Set

SREG(s) ← 1

SREG(s) ← 0

T ← Rr(b)

Rd(b) ← T

C ← 1

SREG(s)

s

Flag Clear

SREG(s)

Rr, b

Rd, b

Bit Store from Register to T

Bit load from T to Register

Set Carry

T

None

C

N

Z

Clear Carry

C ← 0

Set Negative Flag

N ← 1

Clear Negative Flag

Set Zero Flag

N ← 0

Z ← 1

Clear Zero Flag

Z ← 0

Z

Global Interrupt Enable

Global Interrupt Disable

Set Signed Test Flag

Clear Signed Test Flag

Set Twos Complement Overflow.

Clear Twos Complement Overflow

Set T in SREG

I ← 1

I

CLI

I ← 0

I

SES

CLS

SEV

CLV

SET

CLT

SEH

S ← 1

S

S ← 0

S

V ← 1

V

V ← 0

T ← 1

T ← 0

H ← 1

V

T

H

Clear T in SREG

Set Half Carry Flag in SREG

334

2466T–AVR–07/10

ATmega16(L)

Mnemonics

Flags

Operands

Description

Operation

#Clocks

CLH

Clear Half Carry Flag in SREG

H ← 0

H

1

MCU CONTROL INSTRUCTIONS

NOP

SLEEP

WDR

No Operation

Sleep

Watchdog Reset

Break

None

1

(see specific descr. for Sleep function)

(see specific descr. for WDR/timer)

For On-Chip Debug Only

BREAK

N/A

335

2466T–AVR–07/10

ATmega16(L)

Ordering Information

Speed (MHz)

Power Supply

Ordering Code

Package

Operation Range

ATmega16L-8AU⁽¹⁾

ATmega16L-8PU⁽¹⁾

ATmega16L-8MU⁽¹⁾

ATmega16-16AU⁽¹⁾

ATmega16-16PU⁽¹⁾

ATmega16-16MU⁽¹⁾

44A

40P6

44M1

Industrial

8

2.7V - 5.5V

4.5V - 5.5V

(-40^oC to 85^oC)

44A

40P6

44M1

Industrial

16

(-40^oC to 85^oC)

Note:

1. Pb-free packaging complies to the European Directive for Restriction of Hazardous Substances (RoHS directive). Also

Halide free and fully Green.

Package Type

44A

44-lead, Thin (1.0 mm) Plastic Gull Wing Quad Flat Package (TQFP)

40-pin, 0.600” Wide, Plastic Dual Inline Package (PDIP)

40P6

44M1

44-pad, 7 × 7 × 1.0 mm body, lead pitch 0.50 mm, Quad Flat No-Lead/Micro Lead Frame Package (QFN/MLF)

336

2466T–AVR–07/10

ATmega16(L)

Packaging Information

44A

PIN 1

B

PIN 1 IDENTIFIER

E1

E

e

D1

D

C

0˚~7˚

A2

A

A1

L

COMMON DIMENSIONS

(Unit of Measure = mm)

MIN

–

MAX

1.20

NOM

NOTE

SYMBOL

A

–

A1

A2

D

0.05

0.95

11.75

9.90

11.75

9.90

0.30

0.09

0.45

0.15

1.00

12.00

10.00

12.00

10.00

–

1.05

12.25

D1

E

10.10 Note 2

12.25

Notes:

1. This package conforms to JEDEC reference MS-026, Variation ACB.

2. Dimensions D1 and E1 do not include mold protrusion. Allowable

protrusion is 0.25 mm per side. Dimensions D1 and E1 are maximum

plastic body size dimensions including mold mismatch.

E1

B

10.10 Note 2

0.45

C

–

0.20

3. Lead coplanarity is 0.10 mm maximum.

L

–

0.75

e

0.80 TYP

10/5/2001

TITLE

DRAWING NO. REV.

2325 Orchard Parkway

San Jose, CA 95131

44A, 44-lead, 10 x 10 mm Body Size, 1.0 mm Body Thickness,

0.8 mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP)

44A

B

R

337

2466T–AVR–07/10

ATmega16(L)

40P6

D

1

E1

A

SEATING PLANE

A1

L

B

B1

e

E

COMMON DIMENSIONS

(Unit of Measure = mm)

0º ~ 15º REF

C

MIN

–

MAX

4.826

–

NOM

NOTE

SYMBOL

A

–

eB

A1

D

0.381

52.070

15.240

13.462

0.356

1.041

3.048

0.203

15.494

–

52.578 Note 2

15.875

E

–

E1

B

–

13.970 Note 2

0.559

–

B1

L

–

1.651

Notes:

1. This package conforms to JEDEC reference MS-011, Variation AC.

2. Dimensions D and E1 do not include mold Flash or Protrusion.

Mold Flash or Protrusion shall not exceed 0.25 mm (0.010").

–

3.556

C

–

0.381

eB

e

17.526

2.540 TYP

09/28/01

DRAWING NO. REV.

40P6

TITLE

2325 Orchard Parkway

San Jose, CA 95131

40P6, 40-lead (0.600"/15.24 mm Wide) Plastic Dual

Inline Package (PDIP)

B

R

338

2466T–AVR–07/10

ATmega16(L)

44M1

D

Marked Pin# 1 ID

E

SEATING PLANE

A1

TOP VIEW

A3

A

K

L

Pin #1 Corner

SIDE VIEW

D2

Pin #1

Triangle

Option A

COMMON DIMENSIONS

(Unit of Measure = mm)

1

2

3

MIN

0.80

–

MAX

1.00

0.05

NOM

0.90

NOTE

SYMBOL

A

E2

Option B

Option C

A1

A3

b

0.02

Pin #1

Chamfer

(C 0.30)

0.20 REF

0.23

0.18

6.90

5.00

6.90

0.30

7.10

5.40

7.10

D

7.00

D2

E

5.20

K

Pin #1

Notch

(0.20 R)

e

b

7.00

E2

e

5.00

5.20

0.50 BSC

0.64

5.40

BOTTOM VIEW

L

0.59

0.20

0.69

0.41

Note: JEDEC Standard MO-220, Fig. 1 (SAW Singulation) VKKD-3.

K

0.26

9/26/08

GPC

ZWS

DRAWING NO.

TITLE

REV.

44M1, 44-pad, 7 x 7 x 1.0 mm Body, Lead

Pitch 0.50 mm, 5.20 mm Exposed Pad,Thermally

Enhanced Plastic Very Thin Quad Flat No

Lead Package (VQFN)

Package Drawing Contact:

packagedrawings@atmel.com

44M1

H

339

2466T–AVR–07/10

ATmega16(L)

Errata

The revision letter in this section refers to the revision of the ATmega16 device.

ATmega16(L) Rev. • First Analog Comparator conversion may be delayed

• Interrupts may be lost when writing the timer registers in the asynchronous timer

• IDCODE masks data from TDI input

M

• Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising V_CC, the first Analog Comparator conversion will

take longer than expected on some devices.

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the first conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when the asynchronous Timer/Counter register(TCNTx) is 0x00.

Problem Fix / Workaround

Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones during Update-DR.

Problem Fix / Workaround

–

If ATmega16 is the only device in the scan chain, the problem is not visible.

Select the Device ID Register of the ATmega16 by issuing the IDCODE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

–

If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

ATmega16(L) Rev. • First Analog Comparator conversion may be delayed

• Interrupts may be lost when writing the timer registers in the asynchronous timer

• IDCODE masks data from TDI input

L

• Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising V_CC, the first Analog Comparator conversion will

take longer than expected on some devices.

340

2466T–AVR–07/10

ATmega16(L)

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the first conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when the asynchronous Timer/Counter register(TCNTx) is 0x00.

Problem Fix / Workaround

Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones during Update-DR.

Problem Fix / Workaround

–

If ATmega16 is the only device in the scan chain, the problem is not visible.

Select the Device ID Register of the ATmega16 by issuing the IDCODE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

–

If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

ATmega16(L) Rev. • First Analog Comparator conversion may be delayed

• Interrupts may be lost when writing the timer registers in the asynchronous timer

• IDCODE masks data from TDI input

K

• Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising V_CC, the first Analog Comparator conversion will

take longer than expected on some devices.

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the first conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when the asynchronous Timer/Counter register(TCNTx) is 0x00.

341

2466T–AVR–07/10

ATmega16(L)

Problem Fix / Workaround

Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones during Update-DR.

Problem Fix / Workaround

–

If ATmega16 is the only device in the scan chain, the problem is not visible.

Select the Device ID Register of the ATmega16 by issuing the IDCODE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

–

If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

ATmega16(L) Rev. • First Analog Comparator conversion may be delayed

• Interrupts may be lost when writing the timer registers in the asynchronous timer

• IDCODE masks data from TDI input

J

• Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising V_CC, the first Analog Comparator conversion will

take longer than expected on some devices.

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the first conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when the asynchronous Timer/Counter register(TCNTx) is 0x00.

Problem Fix / Workaround

Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones during Update-DR.

Problem Fix / Workaround

342

2466T–AVR–07/10

ATmega16(L)

–

If ATmega16 is the only device in the scan chain, the problem is not visible.

Select the Device ID Register of the ATmega16 by issuing the IDCODE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

–

If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

ATmega16(L) Rev. • First Analog Comparator conversion may be delayed

• Interrupts may be lost when writing the timer registers in the asynchronous timer

• IDCODE masks data from TDI input

I

• Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising V_CC, the first Analog Comparator conversion will

take longer than expected on some devices.

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the first conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when the asynchronous Timer/Counter register(TCNTx) is 0x00.

Problem Fix / Workaround

Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones during Update-DR.

Problem Fix / Workaround

–

If ATmega16 is the only device in the scan chain, the problem is not visible.

Select the Device ID Register of the ATmega16 by issuing the IDCODE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

–

If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

343

2466T–AVR–07/10

ATmega16(L)

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

ATmega16(L) Rev. • First Analog Comparator conversion may be delayed

• Interrupts may be lost when writing the timer registers in the asynchronous timer

• IDCODE masks data from TDI input

H

• Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt request

1. First Analog Comparator conversion may be delayed

If the device is powered by a slow rising V_CC, the first Analog Comparator conversion will

take longer than expected on some devices.

Problem Fix/Workaround

When the device has been powered or reset, disable then enable theAnalog Comparator

before the first conversion.

2. Interrupts may be lost when writing the timer registers in the asynchronous timer

The interrupt will be lost if a timer register that is synchronized to the asynchronous timer

clock is written when the asynchronous Timer/Counter register(TCNTx) is 0x00.

Problem Fix / Workaround

Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor

0x00 before writing to the asynchronous Timer Control Register(TCCRx), asynchronous

Timer Counter Register(TCNTx), or asynchronous Output Compare Register(OCRx).

3. IDCODE masks data from TDI input

The JTAG instruction IDCODE is not working correctly. Data to succeeding devices are

replaced by all-ones during Update-DR.

Problem Fix / Workaround

–

If ATmega16 is the only device in the scan chain, the problem is not visible.

Select the Device ID Register of the ATmega16 by issuing the IDCODE instruction or

by entering the Test-Logic-Reset state of the TAP controller to read out the contents

of its Device ID Register and possibly data from succeeding devices of the scan

chain. Issue the BYPASS instruction to the ATmega16 while reading the Device ID

Registers of preceding devices of the boundary scan chain.

–

If the Device IDs of all devices in the boundary scan chain must be captured

simultaneously, the ATmega16 must be the fist device in the chain.

4. Reading EEPROM by using ST or STS to set EERE bit triggers unexpected interrupt

request.

Reading EEPROM by using the ST or STS command to set the EERE bit in the EECR reg-

ister triggers an unexpected EEPROM interrupt request.

Problem Fix / Workaround

Always use OUT or SBI to set EERE in EECR.

344

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ATmega16(L)

Datasheet

Revision

History

Please note that the referring page numbers in this section are referred to this document. The

referring revision in this section are referring to the document revision.

Rev. 2466T-07/10

1. Corrected use of comma in formula Rp in Table 120, “Two-wire Serial Bus Require-

ments,” on page 294.

2. Updated document according to Atmel’s Technical Terminology

3. Note 6 and Note 7 under Table 120, “Two-wire Serial Bus Requirements,” on

page 294 have been removed.

Rev. 2466S-05/09

1. Updated “Errata” on page 340.

2. Updated the last page with Atmel’s new adresses.

Rev. 2466R-06/08

Rev. 2466Q-05/08

1. Added “Not recommended for new designs” note in Figure on page 1.

1. Updated “Fast PWM Mode” on page 77 in “8-bit Timer/Counter0 with PWM” on page

71:

– Removed the last section describing how to achieve a frequency with 50% duty

cycle waveform output in fast PWM mode.

2. Removed note from Feature list in “Analog to Digital Converter” on page 204.

3. Removed note from Table 84 on page 218.

4. Updated “Ordering Information” on page 336:

- Commercial ordering codes removed.

- Non Pb-free package option removed.

Rev. 2466P-08/07

Rev. 2466O-03/07

1. Updated “Features” on page 1.

2. Added “Data Retention” on page 6.

3. Updated “Errata” on page 340.

4. Updated “Slave Mode” on page 140.

1. Updated “Calibrated Internal RC Oscillator” on page 29.

2. Updated C code example in “USART Initialization” on page 149.

3. Updated “ATmega16 Boundary-scan Order” on page 241.

4. Removed “premilinary” from “ADC Characteristics” on page 297.

5. Updated from V to mV in “I/O Pin Input Hysteresis vs. VCC” on page 317.

6. Updated from V to mV in “Reset Input Pin Hysteresis vs. VCC” on page 318.

345

2466T–AVR–07/10

ATmega16(L)

Rev. 2466N-10/06

1. Updated “Timer/Counter Oscillator” on page 31.

2. Updated “Fast PWM Mode” on page 102.

3. Updated Table 38 on page 83, Table 40 on page 84, Table 45 on page 111, Table 47 on

page 112, Table 50 on page 128 and Table 52 on page 129.

4. Updated C code example in “USART Initialization” on page 149.

5. Updated “Errata” on page 340.

Rev. 2466M-04/06

Rev. 2466L-06/05

1. Updated typos.

2. Updated “Serial Peripheral Interface – SPI” on page 135.

3. Updated Table 86 on page 221, Table 116 on page 276 ,Table 121 on page 295 and

Table 122 on page 297.

1. Updated note in “Bit Rate Generator Unit” on page 178.

2. Updated values for V_INTin “ADC Characteristics” on page 297.

3. Updated “Serial Programming Instruction set” on page 276.

4. Updated USART init C-code example in “USART” on page 144.

Rev. 2466K-04/05

1. Updated “Ordering Information” on page 336.

2. MLF-package alternative changed to “Quad Flat No-Lead/Micro Lead Frame Package

QFN/MLF”.

3. Updated “Electrical Characteristics” on page 291.

1. Updated “Ordering Information” on page 336.

1. Removed references to analog ground.

Rev. 2466J-10/04

Rev. 2466I-10/04

2. Updated Table 7 on page 28, Table 15 on page 38, Table 16 on page 42, Table 81 on

page 209, Table 116 on page 276, and Table 119 on page 293.

3. Updated “Pinout ATmega16” on page 2.

4. Updated features in “Analog to Digital Converter” on page 204.

5. Updated “Version” on page 229.

6. Updated “Calibration Byte” on page 261.

7. Added “Page Size” on page 262.

Rev. 2466H-12/03

1. Updated “Calibrated Internal RC Oscillator” on page 29.

346

2466T–AVR–07/10

ATmega16(L)

Rev. 2466G-10/03

1. Removed “Preliminary” from the datasheet.

2. Changed ICP to ICP1 in the datasheet.

3. Updated “JTAG Interface and On-chip Debug System” on page 36.

4. Updated assembly and C code examples in “Watchdog Timer Control Register –

WDTCR” on page 43.

5. Updated Figure 46 on page 103.

6. Updated Table 15 on page 38, Table 82 on page 217 and Table 115 on page 276.

7. Updated “Test Access Port – TAP” on page 222 regarding JTAGEN.

8. Updated description for the JTD bit on page 231.

9. Added note 2 to Figure 126 on page 252.

10. Added a note regarding JTAGEN fuse to Table 105 on page 260.

11. Updated Absolute Maximum Ratings* and DC Characteristics in “Electrical Character-

istics” on page 291.

12. Updated “ATmega16 Typical Characteristics” on page 299.

13. Fixed typo for 16 MHz QFN/MLF package in “Ordering Information” on page 336.

14. Added a proposal for solving problems regarding the JTAG instruction IDCODE in

“Errata” on page 340.

Rev. 2466F-02/03

1. Added note about masking out unused bits when reading the Program Counter in

“Stack Pointer” on page 12.

2. Added Chip Erase as a first step in “Programming the Flash” on page 288 and “Pro-

gramming the EEPROM” on page 289.

3. Added the section “Unconnected pins” on page 55.

4. Added tips on how to disable the OCD system in “On-chip Debug System” on page

34.

5. Removed reference to the “Multi-purpose Oscillator” application note and “32 kHz

Crystal Oscillator” application note, which do not exist.

6. Added information about PWM symmetry for Timer0 and Timer2.

7. Added note in “Filling the Temporary Buffer (Page Loading)” on page 253 about writ-

ing to the EEPROM during an SPM Page Load.

8. Removed ADHSM completely.

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ATmega16(L)

9. Added Table 73, “TWI Bit Rate Prescaler,” on page 182 to describe the TWPS bits in

the “TWI Status Register – TWSR” on page 181.

10. Added section “Default Clock Source” on page 25.

11. Added note about frequency variation when using an external clock. Note added in

“External Clock” on page 31. An extra row and a note added in Table 118 on page 293.

12. Various minor TWI corrections.

13. Added “Power Consumption” data in “Features” on page 1.

14. Added section “EEPROM Write During Power-down Sleep Mode” on page 22.

15. Added note about Differential Mode with Auto Triggering in “Prescaling and Conver-

sion Timing” on page 207.

16. Added updated “Packaging Information” on page 337.

1. Updated “DC Characteristics” on page 291.

Rev. 2466E-10/02

Rev. 2466D-09/02

1. Changed all Flash write/erase cycles from 1,000 to 10,000.

2. Updated the following tables: Table 4 on page 26, Table 15 on page 38, Table 42 on

page 85, Table 45 on page 111, Table 46 on page 111, Table 59 on page 143, Table 67

on page 167, Table 90 on page 235, Table 102 on page 258, “DC Characteristics” on

page 291, Table 119 on page 293, Table 121 on page 295, and Table 122 on page 297.

3. Updated “Errata” on page 340.

Rev. 2466C-03/02

1. Updated typical EEPROM programming time, Table 1 on page 20.

2. Updated typical start-up time in the following tables:

Table 3 on page 25, Table 5 on page 27, Table 6 on page 28, Table 8 on page 29, Table 9

on page 29, and Table 10 on page 29.

3. Updated Table 17 on page 43 with typical WDT Time-out.

4. Added Some Preliminary Test Limits and Characterization Data.

Removed some of the TBD's in the following tables and pages:

Table 15 on page 38, Table 16 on page 42, Table 116 on page 272 (table removed in docu-

ment review #D), “Electrical Characteristics” on page 291, Table 119 on page 293, Table

121 on page 295, and Table 122 on page 297.

5. Updated TWI Chapter.

Added the note at the end of the “Bit Rate Generator Unit” on page 178.

6. Corrected description of ADSC bit in “ADC Control and Status Register A – ADCSRA”

on page 219.

7. Improved description on how to do a polarity check of the ADC doff results in “ADC

Conversion Result” on page 216.

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ATmega16(L)

8. Added JTAG version number for rev. H in Table 87 on page 229.

9. Added not regarding OCDEN Fuse below Table 105 on page 260.

10. Updated Programming Figures:

Figure 127 on page 262 and Figure 136 on page 274 are updated to also reflect that AVCC

must be connected during Programming mode. Figure 131 on page 270 added to illustrate

how to program the fuses.

11. Added a note regarding usage of the “PROG_PAGELOAD ($6)” on page 280 and

“PROG_PAGEREAD ($7)” on page 280.

12. Removed alternative algortihm for leaving JTAG Programming mode.

See “Leaving Programming Mode” on page 288.

13. Added Calibrated RC Oscillator characterization curves in section “ATmega16 Typi-

cal Characteristics” on page 299.

14. Corrected ordering code for QFN/MLF package (16 MHz) in “Ordering Information” on

page 336.

15. Corrected Table 90, “Scan Signals for the Oscillators(1)(2)(3),” on page 235.

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ATmega16(L)

Table of Contents

Features 1

Pin Configurations 2

Disclaimer 2

Overview 3

Block Diagram 3

Pin Descriptions 4

Resources 6

Data Retention 6

About Code Examples 7

AVR CPU Core 8

Introduction 8

Architectural Overview 8

ALU – Arithmetic Logic Unit 9

Status Register 9

General Purpose Register File 11

Stack Pointer 12

Instruction Execution Timing 13

Reset and Interrupt Handling 13

AVR ATmega16 Memories 16

In-System Reprogrammable Flash Program Memory 16

SRAM Data Memory 17

EEPROM Data Memory 18

I/O Memory 23

System Clock and Clock Options 24

Clock Systems and their Distribution 24

Clock Sources 25

Default Clock Source 25

Crystal Oscillator 25

Low-frequency Crystal Oscillator 28

External RC Oscillator 28

Calibrated Internal RC Oscillator 29

External Clock 31

Timer/Counter Oscillator 31

Power Management and Sleep Modes 32

i

2466T–AVR–07/10

Idle Mode 33

ADC Noise Reduction Mode 33

Power-down Mode 33

Power-save Mode 33

Standby Mode 34

Extended Standby Mode 34

Minimizing Power Consumption 35

System Control and Reset 37

Internal Voltage Reference 42

Watchdog Timer 42

Interrupts 45

Interrupt Vectors in ATmega16 45

I/O Ports 50

Introduction 50

Ports as General Digital I/O 50

Alternate Port Functions 55

Register Description for I/O Ports 66

External Interrupts 68

8-bit Timer/Counter0 with PWM 71

Overview 71

Timer/Counter Clock Sources 72

Counter Unit 72

Output Compare Unit 73

Compare Match Output Unit 74

Modes of Operation 76

Timer/Counter Timing Diagrams 81

8-bit Timer/Counter Register Description 83

Timer/Counter0 and Timer/Counter1 Prescalers 87

16-bit Timer/Counter1 89

Overview 89

Accessing 16-bit Registers 92

Timer/Counter Clock Sources 94

Counter Unit 95

Input Capture Unit 96

Output Compare Units 98

Compare Match Output Unit 100

Modes of Operation 101

Timer/Counter Timing Diagrams 108

16-bit Timer/Counter Register Description 110

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ATmega16(L)

8-bit Timer/Counter2 with PWM and Asynchronous Operation 117

Overview 117

Timer/Counter Clock Sources 118

Counter Unit 118

Output Compare Unit 119

Compare Match Output Unit 121

Modes of Operation 122

Timer/Counter Timing Diagrams 126

8-bit Timer/Counter Register Description 128

Asynchronous Operation of the Timer/Counter 131

Timer/Counter Prescaler 134

Serial Peripheral Interface – SPI 135

SS Pin Functionality 140

Data Modes 143

USART 144

Overview 144

Clock Generation 145

Frame Formats 148

USART Initialization 149

Data Reception – The USART Receiver 154

Asynchronous Data Reception 157

Multi-processor Communication Mode 161

Accessing UBRRH/ UCSRC Registers 162

USART Register Description 163

Examples of Baud Rate Setting 168

Two-wire Serial Interface 172

Features 172

Two-wire Serial Interface Bus Definition 172

Data Transfer and Frame Format 173

Multi-master Bus Systems, Arbitration and Synchronization 176

Overview of the TWI Module 178

TWI Register Description 180

Using the TWI 183

Transmission Modes 186

Multi-master Systems and Arbitration 199

Analog Comparator 201

Analog Comparator Multiplexed Input 203

Analog to Digital Converter 204

Features 204

Operation 205

Starting a Conversion 206

iii

2466T–AVR–07/10

Prescaling and Conversion Timing 207

Changing Channel or Reference Selection 210

ADC Noise Canceler 211

ADC Conversion Result 216

JTAG Interface and On-chip Debug System 222

Features 222

Overview 222

Test Access Port – TAP 222

TAP Controller 224

Using the Boundary-scan Chain 225

Using the On-chip Debug System 225

On-chip Debug Specific JTAG Instructions 226

On-chip Debug Related Register in I/O Memory 227

Using the JTAG Programming Capabilities 227

Bibliography 227

IEEE 1149.1 (JTAG) Boundary-scan 228

Features 228

System Overview 228

Data Registers 228

Boundary-scan Specific JTAG Instructions 230

Boundary-scan Chain 232

ATmega16 Boundary-scan Order 241

Boundary-scan Description Language Files 245

Boot Loader Support – Read-While-Write Self-Programming 246

Features 246

Application and Boot Loader Flash Sections 246

Read-While-Write and no Read-While-Write Flash Sections 246

Boot Loader Lock Bits 248

Entering the Boot Loader Program 249

Addressing the Flash during Self-Programming 251

Self-Programming the Flash 252

Memory Programming 259

Program And Data Memory Lock Bits 259

Fuse Bits 260

Signature Bytes 261

Calibration Byte 261

Page Size 262

Parallel Programming Parameters, Pin Mapping, and Commands 262

Parallel Programming 265

Serial Downloading 273

Programming via the JTAG Interface 278

iv

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ATmega16(L)

Electrical Characteristics 291

Absolute Maximum Ratings* 291

DC Characteristics 291

External Clock Drive Waveforms 293

External Clock Drive 293

Two-wire Serial Interface Characteristics 294

SPI Timing Characteristics 295

ADC Characteristics 297

ATmega16 Typical Characteristics 299

Register Summary 331

Instruction Set Summary 333

Ordering Information 336

Packaging Information 337

44A 337

40P6 338

44M1 339

Errata 340

ATmega16(L) Rev. M 340

ATmega16(L) Rev. L 340

ATmega16(L) Rev. K 341

ATmega16(L) Rev. J 342

ATmega16(L) Rev. I 343

ATmega16(L) Rev. H 344

Datasheet Revision History 345

Rev. 2466T-07/10 345

Rev. 2466S-05/09 345

Rev. 2466R-06/08 345

Rev. 2466Q-05/08 345

Rev. 2466P-08/07 345

Rev. 2466O-03/07 345

Rev. 2466N-10/06 346

Rev. 2466M-04/06 346

Rev. 2466L-06/05 346

Rev. 2466K-04/05 346

Rev. 2466J-10/04 346

Rev. 2466I-10/04 346

Rev. 2466H-12/03 346

Rev. 2466G-10/03 347

Rev. 2466F-02/03 347

v

2466T–AVR–07/10

Rev. 2466E-10/02 348

Rev. 2466D-09/02 348

Rev. 2466C-03/02 348

Table of Contents i

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ATmega16(L)

vii

2466T–AVR–07/10

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2466T–AVR–07/10


型号：	ATMEGA16-16MQ
厂家：	MICROCHIP
描述：	IC MCU 8BIT 16KB FLASH 44VQFN
文件：	总357页 (文件大小：7302K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

ATMEGA16-16MQ [MICROCHIP]

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