ATMEGA8-16PC_技术文档

Features

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

• Advanced RISC Architecture

– 130 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

• Nonvolatile Program and Data Memories

– 8K Bytes of In-System Self-Programmable Flash

Endurance: 10,000 Write/Erase Cycles

– Optional Boot Code Section with Independent Lock Bits

In-System Programming by On-chip Boot Program

True Read-While-Write Operation

8-bit

with 8K Bytes

In-System

Programmable

Flash

– 512 Bytes EEPROM

Endurance: 100,000 Write/Erase Cycles

– 1K Byte Internal SRAM

– Programming Lock for Software Security

• Peripheral Features

– Two 8-bit Timer/Counters with Separate Prescaler, one Compare Mode

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

Mode

– Real Time Counter with Separate Oscillator

– Three PWM Channels

– 8-channel ADC in TQFP and MLF package

Six Channels 10-bit Accuracy

ATmega8

ATmega8L

Two Channels 8-bit Accuracy

– 6-channel ADC in PDIP package

Four Channels 10-bit Accuracy

Two Channels 8-bit Accuracy

– 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

– Five Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, and

Standby

• I/O and Packages

– 23 Programmable I/O Lines

– 28-lead PDIP, 32-lead TQFP, and 32-pad MLF

• Operating Voltages

– 2.7 - 5.5V (ATmega8L)

– 4.5 - 5.5V (ATmega8)

• Speed Grades

– 0 - 8 MHz (ATmega8L)

– 0 - 16 MHz (ATmega8)

• Power Consumption at 4 Mhz, 3V, 25°C

– Active: 3.6 mA

– Idle Mode: 1.0 mA

– Power-down Mode: 0.5 µA

Rev. 2486M–AVR–12/03

Pin Configurations

PDIP

(RESET) PC6

(RXD) PD0

1

2

3

4

5

6

7

8

9

28 PC5 (ADC5/SCL)

27 PC4 (ADC4/SDA)

26 PC3 (ADC3)

25 PC2 (ADC2)

24 PC1 (ADC1)

23 PC0 (ADC0)

22 GND

(TXD) PD1

(INT0) PD2

(INT1) PD3

(XCK/T0) PD4

VCC

GND

21 AREF

(XTAL1/TOSC1) PB6

20 AVCC

(XTAL2/TOSC2) PB7 10

(T1) PD5 11

19 PB5 (SCK)

18 PB4 (MISO)

17 PB3 (MOSI/OC2)

16 PB2 (SS/OC1B)

15 PB1 (OC1A)

(AIN0) PD6 12

(AIN1) PD7 13

(ICP1) PB0 14

TQFP Top View

(INT1) PD3

(XCK/T0) PD4

GND

1

2

3

4

5

6

7

8

24 PC1 (ADC1)

23 PC0 (ADC0)

22 ADC7

VCC

21 GND

GND

20 AREF

VCC

19 ADC6

(XTAL1/TOSC1) PB6

(XTAL2/TOSC2) PB7

18 AVCC

17 PB5 (SCK)

MLF Top View

(INT1) PD3

(XCK/T0) PD4

GND

PC1 (ADC1)

PC0 (ADC0)

ADC7

1

24

23

22

21

20

19

18

17

2

3

4

5

6

7

8

VCC

GND

AREF

VCC

ADC6

(XTAL1/TOSC1) PB6

(XTAL2/TOSC2) PB7

AVCC

PB5 (SCK)

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ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Overview

The ATmega8 is a low-power CMOS 8-bit microcontroller based on the AVR RISC

architecture. By executing powerful instructions in a single clock cycle, the ATmega8

achieves throughputs approaching 1 MIPS per MHz, allowing the system designer to

optimize power consumption versus processing speed.

Block Diagram

Figure 1. Block Diagram

XTAL1

RESET

PC0 - PC6

PB0 - PB7

VCC

XTAL2

PORTC DRIVERS/BUFFERS

PORTC DIGITAL INTERFACE

PORTB DRIVERS/BUFFERS

PORTB DIGITAL INTERFACE

GND

ADC

INTERFACE

MUX &

ADC

TWI

AGND

AREF

TIMERS/

COUNTERS

OSCILLATOR

PROGRAM

COUNTER

STACK

POINTER

PROGRAM

FLASH

INTERNAL

OSCILLATOR

SRAM

INSTRUCTION

REGISTER

WATCHDOG

TIMER

GENERAL

PURPOSE

REGISTERS

OSCILLATOR

X

Y

Z

INSTRUCTION

DECODER

MCU CTRL.

& TIMING

CONTROL

LINES

INTERRUPT

UNIT

ALU

STATUS

REGISTER

AVR CPU

EEPROM

USART

PROGRAMMING

LOGIC

SPI

+

-

COMP.

INTERFACE

PORTD DIGITAL INTERFACE

PORTD DRIVERS/BUFFERS

PD0 - PD7

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2486M–AVR–12/03

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 conventional CISC microcontrollers.

The ATmega8 provides the following features: 8K bytes of In-System Programmable

Flash with Read-While-Write capabilities, 512 bytes of EEPROM, 1K byte of SRAM, 23

general purpose I/O lines, 32 general purpose working registers, three flexible

Timer/Counters with compare modes, internal and external interrupts, a serial program-

mable USART, a byte oriented Two-wire Serial Interface, a 6-channel ADC (eight

channels in TQFP and MLF packages) where four (six) channels have 10-bit accuracy

and two channels have 8-bit accuracy, a programmable Watchdog Timer with Internal

Oscillator, an SPI serial port, and five software selectable power saving modes. The Idle

mode stops the CPU while allowing the 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 Interrupt or Hard-

ware 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 crys-

tal/resonator Oscillator is running while the rest of the device is sleeping. This allows

very fast start-up combined with low-power consumption.

The device is manufactured using Atmel’s high density non-volatile memory technology.

The Flash Program memory can be reprogrammed In-System through an SPI serial

interface, by a conventional non-volatile 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 ATmega8 is a powerful microcon-

troller that provides a highly-flexible and cost-effective solution to many embedded

control applications.

The ATmega8 AVR is supported with a full suite of program and system development

tools, including C compilers, macro assemblers, program debugger/simulators, In-Cir-

cuit Emulators, and evaluation kits.

Disclaimer

Typical values contained in this datasheet are based on simulations and characteriza-

tion of other AVR microcontrollers manufactured on the same process technology. Min

and Max values will be available after the device is characterized.

4

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Pin Descriptions

VCC

GND

Digital supply voltage.

Ground.

Port B (PB7..PB0) XTAL1/

XTAL2/TOSC1/TOSC2

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.

Depending on the clock selection fuse settings, PB6 can be used as input to the invert-

ing Oscillator amplifier and input to the internal clock operating circuit.

Depending on the clock selection fuse settings, PB7 can be used as output from the

inverting Oscillator amplifier.

If the Internal Calibrated RC Oscillator is used as chip clock source, PB7..6 is used as

TOSC2..1 input for the Asynchronous Timer/Counter2 if the AS2 bit in ASSR is set.

The various special features of Port B are elaborated in “Alternate Functions of Port B”

on page 56 and “System Clock and Clock Options” on page 23.

Port C (PC5..PC0)

PC6/RESET

Port C is an 7-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 RSTDISBL Fuse is programmed, PC6 is used as an I/O pin. Note that the electri-

cal characteristics of PC6 differ from those of the other pins of Port C.

If the RSTDISBL Fuse is unprogrammed, PC6 is used as a 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 36. Shorter

pulses are not guaranteed to generate a Reset.

The various special features of Port C are elaborated on page 59.

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 ATmega8 as listed on

page 61.

RESET

Reset input. A low level on this pin for longer than the minimum pulse length will gener-

ate a reset, even if the clock is not running. The minimum pulse length is given in Table

15 on page 36. Shorter pulses are not guaranteed to generate a reset.

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2486M–AVR–12/03

AVCC

AREF

AVCC is the supply voltage pin for the A/D Converter, Port C (3..0), and ADC (7..6). It

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

it should be connected to V_CCthrough a low-pass filter. Note that Port C (5..4) use digital

supply voltage, V_CC

.

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

ADC7..6 (TQFP and MLF

Package Only)

In the TQFP and MLF package, ADC7..6 serve as analog inputs to the A/D converter.

These pins are powered from the analog supply and serve as 10-bit ADC channels.

About Code

Examples

This datasheet 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 defini-

tions in the header files and interrupt handling in C is compiler dependent. Please

confirm with the C compiler documentation for more details.

6

ATmega8(L)

2486M–AVR–12/03

ATmega8(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 2. 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 exe-

cuted, the next instruction 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 x 8-bit general purpose working registers with

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

operation. In a typical 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.

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2486M–AVR–12/03

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 Pro-

gram memory. These added function registers are the 16-bit X-, Y-, and Z-register,

described later in this section.

The ALU supports arithmetic and logic operations between registers or between a con-

stant and a register. Single register operations can also be executed in the ALU. After

an arithmetic operation, the Status Register is updated to reflect information about the

result of the operation.

The Program flow is provided by conditional and unconditional jump and call instruc-

tions, able to directly address the whole address space. Most AVR instructions have a

single 16-bit word format. Every Program memory address contains a 16- 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 position. The lower the Interrupt Vector address, the higher the priority.

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

Registers, 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, 0x20 - 0x5F.

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ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Arithmetic Logic Unit –

ALU

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-func-

tions. Some implementations of the architecture also provide a powerful multiplier

supporting both signed/unsigned multiplication and fractional format. See the “Instruc-

tion Set” section for a detailed description.

Status Register

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

arithmetic 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 individ-

ual interrupt 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.

• Bit 6 – T: Bit Copy Storage

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

destination 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 Comple-

ment 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

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2486M–AVR–12/03

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 “Instruc-

tion Set Description” for detailed information.

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

Figure 3. AVR CPU General Purpose Working Registers

7

0

Addr.

R0

R1

0x00

0x01

0x02

R2

…

R13

R14

R15

R16

R17

…

0x0D

0x0E

0x0F

0x10

0x11

General

Purpose

Working

Registers

R26

R27

R28

R29

R30

R31

0x1A

0x1B

0x1C

0x1D

0x1E

0x1F

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 3, 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 phys-

ically implemented 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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ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

The X-register, Y-register and The registers R26..R31 have some added functions to their general purpose usage.

Z-register

These registers 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 4.

Figure 4. The X-, Y- and Z-Registers

15

XH

YH

XL

0

X-register

7

0

7

R27 (0x1B)

R26 (0x1A)

15

YL

ZL

0

Y-register

Z-register

7

R29 (0x1D)

R28 (0x1C)

15

ZH

0

7

0

R31 (0x1F)

R30 (0x1E)

In the different addressing modes these address registers have functions as fixed dis-

placement, 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 Regis-

ter always points to the top of the Stack. Note that the Stack is implemented as growing

from higher memory locations to lower memory locations. This implies that a Stack

PUSH command decreases the Stack Pointer.

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

rupt 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 0x60. 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 address 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 num-

ber of bits actually used is implementation dependent. Note that the data space in some

implementations 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

11

2486M–AVR–12/03

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 5 shows the parallel instruction fetches and instruction executions enabled by the

Harvard architecture and the fast-access Register File concept. This is the basic pipelin-

ing 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 5. 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 6 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 destination register.

Figure 6. 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 Programming” on page 219 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 44. 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 0. The Interrupt Vectors can be moved to the start

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ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

of the boot Flash section by setting the Interrupt Vector Select (IVSEL) bit in the General

Interrupt Control Register (GICR). Refer to “Interrupts” on page 44 for more information.

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

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

Programming” on page 206.

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

are disabled. The user software can write logic one to the I-bit to enable nested inter-

rupts. 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 Vector 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 remem-

bered 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 corre-

sponding 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 disap-

pears 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 exe-

cute one more instruction before any pending interrupt is served.

Note that the Status Register is not automatically stored when entering an interrupt rou-

tine, 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 simulta-

neously 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

sbiEECR, EEMWE ; start EEPROM write

sbiEECR, EEWE

outSREG, r16

; 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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2486M–AVR–12/03

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

executed before any pending interrupts, as shown in the following 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

minimum. After four clock cycles, the Program Vector address for the actual interrupt

handling routine is executed. During this 4-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 (2 bytes) is popped back from the Stack, the Stack

Pointer is incremented by 2, and the I-bit in SREG is set.

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ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

AVR ATmega8

Memories

This section describes the different memories in the ATmega8. The AVR architecture

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

addition, the ATmega8 features an EEPROM Memory for data storage. All three mem-

ory spaces are linear and regular.

In-System

Reprogrammable Flash

Program Memory

The ATmega8 contains 8K bytes On-chip In-System Reprogrammable Flash memory

for program storage. Since all AVR instructions are 16- or 32-bits wide, the Flash is

organized as 4K x 16 bits. For software security, the Flash Program memory space is

divided into two sections, Boot Program section and Application Program section.

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

ATmega8 Program Counter (PC) is 12 bits wide, thus addressing the 4K Program mem-

ory 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 206. “Memory Programming” on page 219 contains a

detailed description on Flash Programming in SPI- or Parallel Programming mode.

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 Execu-

tion Timing” on page 12.

Figure 7. Program Memory Map

$000

Application Flash Section

Boot Flash Section

$FFF

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2486M–AVR–12/03

SRAM Data Memory

Figure 8 shows how the ATmega8 SRAM Memory is organized.

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

the internal data SRAM. The first 96 locations address the Register File and I/O Mem-

ory, and the next 1024 locations address the internal data SRAM.

The five different addressing modes for the Data memory cover: Direct, Indirect with

Displacement, 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- or Z-register.

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

increment, 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 inter-

nal data SRAM in the ATmega8 are all accessible through all these addressing modes.

The Register File is described in “General Purpose Register File” on page 10.

Figure 8. 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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ATmega8(L)

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ATmega8(L)

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

9.

Figure 9. On-chip Data SRAM Access Cycles

T1

T2

T3

clk_CPU

Address Valid

Compute Address

Address

Data

WR

Data

RD

Memory Vccess Instruction

Next Instruction

EEPROM Data Memory

The ATmega8 contains 512 bytes of data EEPROM memory. It is organized as a sepa-

rate 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 bellow, specifying the EEPROM Address Registers, the

EEPROM Data Register, and the EEPROM Control Register.

“Memory Programming” on page 219 contains a detailed description on EEPROM Pro-

gramming in SPI- or Parallel Programming mode.

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 on page 19. 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_CCis 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 “Preventing EEPROM Corrup-

tion” on page 21. for details on how to avoid problems in these situations.

In order to prevent unintentional EEPROM writes, a specific write procedure must be fol-

lowed. 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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2486M–AVR–12/03

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 ATmega8 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 lin-

early 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 oper-

ation, 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 ATmega8 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 interrupt 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.

• 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

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ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

value into the EEPROM. The EEMWE bit must be written to one before a logical one is

written to EEWE, otherwise 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 206 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 software 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

correct 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

programming 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 settings.

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2486M–AVR–12/03

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 dis-

abling interrupts globally) so that no interrupts will occur during execution of these

functions. The examples also assume that no Flash boot loader is present in the soft-

ware. 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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ATmega8(L)

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ATmega8(L)

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

examples 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 Power- When entering Power-down sleep mode while an EEPROM write operation is active, the

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 volt-

age 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. Second, the CPU itself can execute instructions incorrectly, if the sup-

ply voltage is too low.

EEPROM data corruption can easily be avoided by following this design

recommendation:

21

2486M–AVR–12/03

Keep the AVR RESET active (low) during periods of insufficient power supply volt-

age. 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 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.

I/O Memory

The I/O space definition of the ATmega8 is shown in “” on page 282.

All ATmega8 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 pur-

pose working registers and the I/O space. I/O Registers within the address range 0x00 -

0x1F 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 0x00 - 0x3F must be used. When addressing I/O Registers

as data space using LD and ST instructions, 0x20 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 reg-

isters 0x00 to 0x1F only.

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

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ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

System Clock and

Clock Options

Clock Systems and their Figure 10 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

Distribution

clocks to modules not being used can be halted by using different sleep modes, as

described in “Power Management and Sleep Modes” on page 31. The clock systems

are detailed Figure 10.

Figure 10. 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 Reg-

ister 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 interrupts 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 asynchronously when clk_I/Ois halted, enabling TWI address recep-

tion in all sleep modes.

Flash Clock – clk_FLASH

The Flash clock controls operation of the Flash interface. The Flash clock is usually

active simultaneously with the CPU clock.

23

2486M–AVR–12/03

Asynchronous Timer Clock – The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked

clk_ASY

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. The

Asynchronous Timer/Counter uses the same XTAL pins as the CPU main clock but

requires a CPU main clock frequency of more than four times the Oscillator frequency.

Thus, asynchronous operation is only available while the chip is clocked on the Internal

Oscillator.

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 accu-

rate 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 “ATmega8 Typical Characteristics”. The

device is shipped with CKSEL = “0001” and SUT = “10” (1 MHz Internal RC Oscillator,

slowly rising power).

Table 3. Number of Watchdog Oscillator Cycles

Typical Time-out (V_CC= 5.0V) Typical 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)

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ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Crystal Oscillator

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

be configured for use as an On-chip Oscillator, as shown in Figure 11. Either a quartz

crystal or a ceramic resonator may be used. The CKOPT Fuse selects between two dif-

ferent Oscillator amplifier modes. When CKOPT is programmed, the Oscillator output

will oscillate 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 cannot 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 environ-

ment. Some initial guidelines for 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 11. Crystal Oscillator Connections

C2

XTAL2

C1

XTAL1

GND

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

quency 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.

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

in Table 5.

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2486M–AVR–12/03

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

Start-up Time

from Power-down

and Power-save

Additional Delay

from Reset

CKSEL0 SUT1..0

(V_CC= 5.0V)

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 fre-

quency 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 fre-

quency stability at start-up. They can also be used with crystals when not operating

close to the maximum frequency of the device, and if frequency stability at start-up is

not important for the application.

Low-frequency Crystal

Oscillator

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

quency Crystal Oscillator must be selected by setting the CKSEL Fuses to “1001”. The

crystal should be connected as shown in Figure 11. By programming the CKOPT Fuse,

the user can enable internal capacitors on XTAL1 and XTAL2, thereby removing the

need for external capacitors. The internal 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.

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ATmega8(L)

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ATmega8(L)

External RC Oscillator

For timing insensitive applications, the external RC configuration shown in Figure 12

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 12. External RC Configuration

V

CC

NC

XTAL2

XTAL1

GND

R

C

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

quency 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)

≤ 0.9

0110

0.9 - 3.0

0111

3.0 - 8.0

1000

8.0 - 12.0

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.

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2486M–AVR–12/03

Calibrated Internal RC

Oscillator

The calibrated internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All

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

tem 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 Oscillator frequency selected, this calibration gives a frequency

within ± 3% of the nominal frequency. Using run-time calibration methods as described

in application notes available at www.atmel.com/avr it is possible to achieve ± 1% accu-

racy 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 calibration value, see the sec-

tion “Calibration Byte” on page 221.

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. PB6 (XTAL1/TOSC1) and PB7(XTAL2/TOSC2) can be used as

either general I/O pins or Timer Oscillator pins..

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.

28

ATmega8(L)

2486M–AVR–12/03

ATmega8(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

R/W

OSCCAL

Read/Write

Initial Value

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 pro-

cess variations from the Oscillator frequency. During Reset, the 1 MHz calibration value

which is located in the signature row High byte (address 0x00) is automatically loaded

into the OSCCAL Register. If the internal RC is used at other frequencies, the calibration

values must be loaded manually. This can be done by first reading the signature row by

a programmer, and then store the calibration values in the Flash or EEPROM. Then the

value can be read by software and loaded into the OSCCAL Register. When OSCCAL is

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

will increase the frequency of the Internal Oscillator. Writing 0xFF 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 frequency. Otherwise, the EEPROM or Flash write may fail. Note that the

Oscillator is intended for calibration to 1.0, 2.0, 4.0, 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

0x00

50

75

100

150

200

0x7F

0xFF

100

29

2486M–AVR–12/03

External Clock

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

Figure 13. To run the device on an external clock, the CKSEL Fuses must be pro-

grammed to “0000”. By programming the CKOPT Fuse, the user can enable an internal

36 pF capacitor between XTAL1 and GND.

Figure 13. 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 frequency 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. By programming the CKOPT Fuse, the

user can enable internal capacitors on XTAL1 and XTAL2, thereby removing the need

for external capacitors. The Oscillator is optimized for use with a 32.768 kHz watch crys-

tal. Applying an external clock source to TOSC1 is not recommended.

30

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Power Management

and Sleep Modes

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 consumption to the application’s requirements.

To enter any of the five 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, or 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 exe-

cutes 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.

Note that the Extended Standby mode present in many other AVR MCUs has been

removed in the ATmega8, as the TOSC and XTAL inputs share the same physical pins.

Figure 10 on page 23 presents the different clock systems in the ATmega8, and their

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

MCU Control Register –

MCUCR

The MCU Control Register contains control bits for power management.

Bit

7

SE

R/W

0

6

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 7 – 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 programmer’s purpose, it is recommended to set the Sleep Enable (SE) bit just

before the execution of the SLEEP instruction.

• Bits 6..4 – SM2..0: Sleep Mode Select Bits 2, 1, and 0

These bits select between the five 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

ADC Noise Reduction

Power-down

Power-save

Reserved

Standby⁽¹⁾

Note:

1. Standby mode is only available with external crystals or resonators.

31

2486M–AVR–12/03

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 Sta-

tus Register – ACSR. This will reduce power consumption in Idle mode. If the ADC is

enabled, a conversion starts automatically 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 operating (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 measure-

ments. 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 inter-

rupt, a Timer/Counter2 interrupt, an SPM/EEPROM ready interrupt, or an external level

interrupt on INT0 or INT1, 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, or an external level interrupt on INT0

or INT1, can wake up the MCU. This sleep mode basically halts all generated clocks,

allowing operation 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 Inter-

rupts” on page 64 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

24.

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, i.e. 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 recom-

mended instead of Power-save mode because the contents of the registers in the

32

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

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 asyn-

chronous modules, including Timer/Counter 2 if clocked asynchronously.

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 6 clock cycles.

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

Active Clock Domains Oscillators

Wake-up Sources

SPM/

TWI

Main Clock

Timer Osc. INT1 Address Timer EEPROM

Other

Sleep

Mode

clk_CPUclk_FLASHclk_IOclk_ADCclk_ASYSource Enabled Enabled INT0 Match

2

Ready ADC I/O

Idle

X

X⁽²⁾

X

ADC Noise

Reduction

X⁽³⁾

X

Power

Down

X⁽³⁾

X

Power

Save

X⁽²⁾

X⁽³⁾

X

X⁽²⁾

Standby⁽¹⁾

X

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

2. If AS2 bit in ASSR is set.

3. Only level interrupt INT1 and INT0.

Minimizing Power

Consumption

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 possi-

ble, and the sleep 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

(ADC)

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

be disabled 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 Con-

verter” on page 193 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 disabled in all sleep modes. Otherwise, the Internal Voltage Ref-

erence will be enabled, independent of sleep mode. Refer to “Analog Comparator” on

page 190 for details on how to configure the Analog Comparator.

33

2486M–AVR–12/03

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 38 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 Detec-

tor, 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 con-

suming 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 Voltage Reference” on page 40 for details on the

start-up time.

Watchdog Timer

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 consumption. Refer to “Watchdog Timer” on page 41 for details on how

to configure the Watchdog Timer.

Port Pins

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 53 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.

34

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

System Control and

Reset

Resetting the AVR

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

cution from the Reset Vector. If the program never enables an interrupt source, the

Interrupt Vectors are not used, and regular program code can be placed at these loca-

tions. 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 14

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 selections for the delay period are presented in “Clock

Sources” on page 24.

Reset Sources

The ATmega8 has four 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.

35

2486M–AVR–12/03

Figure 14. Reset Logic

DATA BUS

MCU Control and Status

Register (MCUCSR)

Brown-Out

Reset Circuit

BODEN

BODLEVEL

Pull-up Resistor

SPIKE

FILTER

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

1.4

2.3

V

Voltage (rising)⁽¹⁾

V_POT

Power-on Reset Threshold

Voltage (falling)

1.3

2.3

0.9

1.5

V

V_RST

t_RST

RESET Pin Threshold Voltage

0.1

V_CC

µs

Minimum pulse width on

RESET Pin

Brown-out Reset Threshold

Voltage⁽²⁾

BODLEVEL = 1

BODLEVEL = 0

BODLEVEL = 1

BODLEVEL = 0

2.4

3.7

2.6

4.0

2

2.9

4.5

V_BOT

V

Minimum low voltage period for

Brown-out Detection

µs

t_BOD

2

V_HYST

Brown-out Detector hysteresis

130

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 guarantees 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 ATmega8L and BODLEVEL = 0 for

ATmega8. BODLEVEL = 1 is not applicable for ATmega8.

36

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Power-on Reset

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

tion 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. Reach-

ing 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 15. MCU Start-up, RESET Tied to V_CC

V_POT

V_CC

V_RST

RESET

t_TOUT

TIME-OUT

INTERNAL

RESET

Figure 16. MCU Start-up, RESET Extended Externally

V_POT

V_CC

V_RST

RESET

t_TOUT

TIME-OUT

INTERNAL

RESET

37

2486M–AVR–12/03

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_RSTon its positive edge, the delay

counter starts the MCU after the time-out period t_TOUThas expired.

Figure 17. External Reset During Operation

CC

Brown-out Detection

ATmega8 has an On-chip Brown-out Detection (BOD) circuit for monitoring the V_CC

level during 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-= V_BOT- V_HYST/2.

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 18), the Brown-out Reset is immediately activated. When V_CCincreases

above the trigger level (V_BOT+in Figure 18), 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 longer than t_BODgiven in Table 15.

Figure 18. Brown-out Reset During Operation

V_BOT+

V_CC

V_BOT-

RESET

t

TOUT

TIME-OUT

INTERNAL

RESET

38

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Watchdog Reset

When the Watchdog times out, it will generate a short reset pulse of 1 CK cycle dura-

tion. On the falling edge of this pulse, the delay timer starts counting the time-out period

t_TOUT. Refer to page 41 for details on operation of the Watchdog Timer.

Figure 19. 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

–

5

–

4

–

3

2

1

0

WDRF

R/W

BORF

R/W

EXTRF

R/W

PORF

R/W

MCUCSR

Read/Write

Initial Value

R

0

R

0

R

0

R

0

See Bit Description

• Bit 7..4 – Res: Reserved Bits

These bits are reserved bits in the ATmega8 and always read as zero.

• 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.

39

2486M–AVR–12/03

Internal Voltage

Reference

ATmega8 features an internal bandgap reference. This reference is used for Brown-out

Detection, 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 reference 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 Com-

parator 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

1.35

70

Units

V

Bandgap reference voltage

Bandgap reference start-up time

Bandgap reference current consumption

1.15

t_BG

µs

I_BG

10

µA

40

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

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 42. 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 ATmega8 resets and executes from the Reset Vector. For timing

details on the Watchdog Reset, refer to page 39.

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

followed when the Watchdog is disabled. Refer to the description of the Watchdog Timer

Control Register for details.

Figure 20. Watchdog Timer

WATCHDOG

OSCILLATOR

Watchdog Timer Control

Register – WDTCR

Bit

7

–

6

–

5

–

4

WDCE

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 ATmega8 and will always read as zero.

• Bit 4 – WDCE: Watchdog Change 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. In

Safety Level 1 and 2, this bit must also be set when changing the prescaler bits. See the

Code Examples on page 43.

41

2486M–AVR–12/03

• 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 WDCE bit has logic level one. To disable an enabled Watchdog Timer, the follow-

ing procedure must be followed:

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

written 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

Watchdog 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

Typical Time-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

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.

42

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Timed Sequences for

Changing the

Configuration of the

Watchdog Timer

The sequence for changing the Watchdog Timer configuration differs slightly between

the safety levels. Separate procedures are described for each level.

Assembly Code Example

WDT_off:

; reset WDT

WDR

; Write logical one to WDCE and WDE

in r16, WDTCR

ori r16, (1<<WDCE)|(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 WDCE and WDE */

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

/* Turn off WDT */

WDTCR = 0x00;

}

Safety Level 1 (WDTON Fuse

Unprogrammed)

In this mode, the Watchdog Timer is initially disabled, but can be enabled by writing the

WDE bit to 1 without any restriction. A timed sequence is needed when changing the

Watchdog Time-out period or disabling an enabled Watchdog Timer. To disable an

enabled Watchdog Timer and/or changing the Watchdog Time-out, the following proce-

dure must be followed:

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

written to WDE regardless of the previous value of the WDE bit.

2. Within the next four clock cycles, in the same operation, write the WDE and

WDP bits as desired, but with the WDCE bit cleared.

Safety Level 2 (WDTON Fuse

Programmed)

In this mode, the Watchdog Timer is always enabled, and the WDE bit will always read

as one. A timed sequence is needed when changing the Watchdog Time-out period. To

change the Watchdog Time-out, the following procedure must be followed:

1. In the same operation, write a logical one to WDCE and WDE. Even though the

WDE always is set, the WDE must be written to one to start the timed sequence.

Within the next four clock cycles, in the same operation, write the WDP bits as desired,

but with the WDCE bit cleared. The value written to the WDE bit is irrelevant.

43

2486M–AVR–12/03

Interrupts

This section describes the specifics of the interrupt handling performed by the

ATmega8. For a general explanation of the AVR interrupt handling, refer to “Reset and

Interrupt Handling” on page 12.

Interrupt Vectors in

ATmega8

Table 18. Reset and Interrupt Vectors

Program

Vector No. Address⁽²⁾Source

Interrupt Definition

1

0x000⁽¹⁾

RESET

External Pin, Power-on Reset, Brown-out

Reset, and Watchdog Reset

2

3

0x001

0x002

0x003

0x004

0x005

0x006

0x007

0x008

0x009

0x00A

0x00B

0x00C

0x00D

0x00E

0x00F

0x010

0x011

0x012

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

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

Store Program Memory Ready

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 206.

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 loca-

tions. 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.

44

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Table 19. Reset and Interrupt Vectors Placement

BOOTRST⁽¹⁾

IVSEL

Reset Address

0x000

Interrupt Vectors Start Address

1

0

1

0

1

0x001

0x000

Boot Reset Address + 0x001

0x001

Boot Reset Address

Boot Reset Address + 0x001

Note:

1. The Boot Reset Address is shown in Table 82 on page 217. 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 ATmega8 is:

addressLabelsCode

Comments

$000

$001

$002

$003

$004

$005

$006

$007

$008

$009

$00a

$00b

$00c

$00d

$00e

$00f

$010

rjmp

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

EE_RDY

ANA_COMP

TWSI

$011

Handler

$012

rjmp

SPM_RDY

; Store Program Memory Ready

Handler

;

$013

$014

$015

$016

$017

$018

...

RESET: ldi

out

r16,high(RAMEND); Main program start

SPH,r16

; Set Stack Pointer to top of RAM

ldi

r16,low(RAMEND)

SPL,r16

out

sei

; Enable interrupts

<instr> xxx

...

45

2486M–AVR–12/03

When the BOOTRST Fuse is unprogrammed, the boot section size set to 2K bytes 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:

AddressLabelsCode

Comments

$000

;

rjmp

RESET

; Reset handler

$001

$002

$003

$004

$005

$006

;

RESET:ldi

r16,high(RAMEND); Main program start

out

ldi

out

sei

SPH,r16

; Set Stack Pointer to top of RAM

r16,low(RAMEND)

SPL,r16

; Enable interrupts

<instr> xxx

.org $c01

$c01

$c02

...

rjmp

...

EXT_INT0

; IRQ0 Handler

; IRQ1 Handler

EXT_INT1

...;

$c12

rjmp

SPM_RDY

; Store Program Memory Ready

Handler

When the BOOTRST Fuse is programmed and the boot section size set to 2K bytes, the

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

AddressLabelsCode

Comments

.org $001

$001

$002

...

rjmp

...

EXT_INT0

EXT_INT1

...

; IRQ0 Handler

; IRQ1 Handler

;

$012

rjmp

SPM_RDY

; Store Program Memory Ready

Handler

;

.org $c00

$c00

;

rjmp

RESET

; Reset handler

$c01

$c02

$c03

$c04

$c05

$c06

RESET:ldi

r16,high(RAMEND); Main program start

out

ldi

out

sei

SPH,r16

; Set Stack Pointer to top of RAM

r16,low(RAMEND)

SPL,r16

; Enable interrupts

<instr> xxx

46

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

When the BOOTRST Fuse is programmed, the boot section size set to 2K bytes, 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:

AddressLabels

Code

Comments

;

.org $c00

$c00

$c01

rjmp

RESET

EXT_INT0

; Reset handler

; IRQ0 Handler

rjmp

...

$c02

...

EXT_INT1

...;

; IRQ1 Handler

$c12

rjmp

SPM_RDY

; Store Program Memory Ready

Handler

$c13

$c14

$c15

$c16

$c17

$c18

RESET: ldi

out

r16,high(RAMEND); Main program start

SPH,r16 ; Set Stack Pointer to top of RAM

r16,low(RAMEND)

SPL,r16

ldi

out

sei

; Enable interrupts

<instr> xxx

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

INT1

R/W

0

6

5

–

4

–

3

–

2

–

1

IVSEL

R/W

0

IVCE

R/W

0

INT0

R/W

0

GICR

Read/Write

Initial Value

R

0

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 begin-

ning of the Boot Loader section of the Flash. The actual address of the start of the boot

Flash section is determined by the BOOTSZ Fuses. Refer to the section “Boot Loader

Support – Read-While-Write Self-Programming” on page 206 for details. To avoid unin-

tentional 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 fol-

lowing 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 pro-

gramed, interrupts are disabled while executing from the Boot Loader section. Refer to

the section “Boot Loader Support – Read-While-Write Self-Programming” on page 206

for details on Boot Lock Bits.

47

2486M–AVR–12/03

• 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.

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);

}

48

ATmega8(L)

2486M–AVR–12/03

ATmega8(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 uninten-

tionally changing the direction of any other pin with the SBI and CBI instructions. The

same applies when changing 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 individually 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 21. Refer to “Electrical Characteristics” on page

237 for a complete list of parameters.

Figure 21. I/O Pin Equivalent Schematic

R_pu

Pxn

Logic

C_pin

See Figure

"General Digital I/O" for

Details

All registers and bit references in this section are written in general form. A lower case

“x” represents 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 (i.e., PORTB3 for bit 3 in Port B, here documented generally as PORTxn).

The physical I/O Registers and bit locations are listed in “Register Description for I/O

Ports” on page 63.

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 fea-

tures on the device. How each alternate function interferes with the port pin is described

in “Alternate Port Functions” on page 54. Refer to the individual module sections for a

full description of the alternate 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.

49

2486M–AVR–12/03

Ports as General Digital

I/O

The ports are bi-directional I/O ports with optional internal pull-ups. Figure 22 shows a

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

Figure 22. General Digital I/O⁽¹⁾

PUD

Q

D

DDxn

Q _CLR

WDx

RDx

RESET

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

SLEEP CONTROL

I/O CLOCK

READ DDRx

SLEEP:

WRITE PORTx

READ PORTx REGISTER

READ PORTx PIN

clk_I/O

:

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 3 Register bits: DDxn, PORTxn, and PINxn. As shown in “Reg-

ister Description for I/O Ports” on page 63, 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 config-

ured 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 out-

put pin, the port pin is driven low (zero).

50

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

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 acceptable, as a high-impedant environment will not notice the dif-

ference 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.

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

1

X

0

Input

No

Tri-state (Hi-Z)

Pxn will source current if external

pulled low.

Input

Yes

0

1

0

1

X

Input

Output

No

Tri-state (Hi-Z)

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 22, the PINxn Register bit and the preceding

latch constitute 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

23 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,min, respectively.

Figure 23. Synchronization when Reading an Externally Applied Pin Value

SYSTEM CLK

XXX

in r17, PINx

INSTRUCTIONS

SYNC LATCH

PINxn

0x00

t_{pd, max}

0xFF

r17

t_{pd, min}

51

2486M–AVR–12/03

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 suc-

ceeding 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 1-½ system clock period

depending upon the time of assertion.

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

indicated in Figure 24. 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 1 system clock

period.

Figure 24. Synchronization when Reading a Software Assigned Pin Value

SYSTEM CLK

0xFF

r16

out PORTx, r16

nop

in r17, PINx

INSTRUCTIONS

SYNC LATCH

PINxn

0x00

t_pd

0xFF

r17

52

ATmega8(L)

2486M–AVR–12/03

ATmega8(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 r16,(1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0)

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

out PORTB,r16

out 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 As shown in Figure 22, the digital input signal can be clamped to ground at the input of

Modes

the Schmitt-trigger. The signal denoted SLEEP in the figure, is set by the MCU Sleep

Controller in Power-down mode, Power-save mode, and 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 various other alternate functions as described in “Alternate Port Func-

tions” on page 54.

If a logic high level (“one”) is present on an Asynchronous External Interrupt pin config-

ured as “Interrupt on Rising Edge, Falling Edge, or Any Logic Change on Pin” while the

external interrupt 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.

53

2486M–AVR–12/03

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, floating 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

25 shows how the port pin control signals from the simplified Figure 22 can be overrid-

den by alternate 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 micro-

controller family.

Figure 25. 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.

54

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Table 21 summarizes the function of the overriding signals. The pin and port indexes

from Figure 25 are not shown in the succeeding tables. The overriding signals are gen-

erated 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

If PVOE is set, the port value is set to PVOV,

Override Value

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.

55

2486M–AVR–12/03

Special Function IO Register –

SFIOR

Bit

7

6

5

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

R

0

R

0

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 “Configuring the Pin” on page 50 for more details about this feature.

Alternate Functions of Port B The Port B pins with alternate functions are shown in Table 22.

Table 22. Port B Pins Alternate Functions

Port Pin

Alternate Functions

XTAL2 (Chip Clock Oscillator pin 2)

TOSC2 (Timer Oscillator pin 2)

PB7

XTAL1 (Chip Clock Oscillator pin 1 or External clock input)

TOSC1 (Timer Oscillator pin 1)

PB6

PB5

PB4

SCK (SPI Bus Master clock Input)

MISO (SPI Bus Master Input/Slave Output)

MOSI (SPI Bus Master Output/Slave Input)

OC2 (Timer/Counter2 Output Compare Match Output)

PB3

PB2

SS (SPI Bus Master Slave select)

OC1B (Timer/Counter1 Output Compare Match B Output)

PB1

PB0

OC1A (Timer/Counter1 Output Compare Match A Output)

ICP1 (Timer/Counter1 Input Capture Pin)

The alternate pin configuration is as follows:

• XTAL2/TOSC2 – Port B, Bit 7

XTAL2: Chip clock Oscillator pin 2. Used as clock pin for crystal Oscillator or Low-fre-

quency crystal Oscillator. When used as a clock pin, the pin can not be used as an I/O

pin.

TOSC2: Timer Oscillator pin 2. Used only if internal calibrated RC Oscillator is selected

as chip clock source, and the asynchronous timer is enabled by the correct setting in

ASSR. When the AS2 bit in ASSR is set (one) to enable asynchronous clocking of

Timer/Counter2, pin PB7 is disconnected from the port, and becomes the inverting out-

put of the Oscillator amplifier. In this mode, a crystal Oscillator is connected to this pin,

and the pin cannot be used as an I/O pin.

If PB7 is used as a clock pin, DDB7, PORTB7 and PINB7 will all read 0.

• XTAL1/TOSC1 – Port B, Bit 6

XTAL1: Chip clock Oscillator pin 1. Used for all chip clock sources except internal cali-

brated RC Oscillator. When used as a clock pin, the pin can not be used as an I/O pin.

TOSC1: Timer Oscillator pin 1. Used only if internal calibrated RC Oscillator is selected

as chip clock source, and the asynchronous timer is enabled by the correct setting in

ASSR. When the AS2 bit in ASSR is set (one) to enable asynchronous clocking of

Timer/Counter2, pin PB6 is disconnected from the port, and becomes the input of the

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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.

If PB6 is used as a clock pin, DDB6, PORTB6 and PINB6 will all read 0.

• SCK – Port B, Bit 5

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 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 con-

trolled by the PORTB5 bit.

• MISO – Port B, Bit 4

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

DDB4. When the SPI is enabled as a Slave, 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 con-

trolled by the PORTB4 bit.

• MOSI/OC2 – Port B, Bit 3

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 DDB3.

When the SPI is enabled as a Master, the data direction of this pin is controlled by

DDB3. When the pin is forced by the SPI to be an input, the pull-up can still be con-

trolled by the PORTB3 bit.

OC2, Output Compare Match Output: The PB3 pin can serve as an external output for

the Timer/Counter2 Compare Match. The PB3 pin has to be configured as an output

(DDB3 set (one)) to serve this function. The OC2 pin is also the output pin for the PWM

mode timer function.

• SS/OC1B – Port B, Bit 2

SS: Slave Select input. When the SPI is enabled as a Slave, this pin is configured as an

input regardless of the setting of DDB2. 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 con-

trolled by DDB2. When the pin is forced by the SPI to be an input, the pull-up can still be

controlled by the PORTB2 bit.

OC1B, Output Compare Match output: The PB2 pin can serve as an external output for

the Timer/Counter1 Compare Match B. The PB2 pin has to be configured as an output

(DDB2 set (one)) to serve this function. The OC1B pin is also the output pin for the PWM

mode timer function.

• OC1A – Port B, Bit 1

OC1A, Output Compare Match output: The PB1 pin can serve as an external output for

the Timer/Counter1 Compare Match A. The PB1 pin has to be configured as an output

(DDB1 set (one)) to serve this function. The OC1A pin is also the output pin for the PWM

mode timer function.

• ICP1 – Port B, Bit 0

ICP1 – Input Capture Pin: The PB0 pin can act as an Input Capture Pin for

Timer/Counter1.

Table 23 and Table 24 relate the alternate functions of Port B to the overriding signals

shown in Figure 25 on page 54. 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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2486M–AVR–12/03

Table 23. Overriding Signals for Alternate Functions in PB7..PB4

Signal

Name

PB7/XTAL2/

TOSC2⁽¹⁾⁽²⁾

PB6/XTAL1/

TOSC1⁽¹⁾

PB5/SCK

PB4/MISO

PUOE

EXT • (INTRC +

AS2)

INTRC + AS2

SPE • MSTR

PUO

0

PORTB5 • PUD

SPE • MSTR

PORTB4 • PUD

SPE • MSTR

DDOE

EXT • (INTRC +

AS2)

INTRC + AS2

DDOV

PVOE

PVOV

0

SPE • MSTR

SCK OUTPUT

0

SPE • MSTR

0

SPI SLAVE OUTPUT

0

DIEOE EXT • (INTRC +

AS2)

INTRC + AS2

DIEOV

DI

0

–

0

–

0

SCK INPUT

–

SPI MSTR INPUT

–

AIO

Oscillator Output Oscillator/Clock

Input

Notes: 1. INTRC means that the internal RC Oscillator is selected (by the CKSEL Fuse).

2. EXT means that the external RC Oscillator or an external clock is selected (by the

CKSEL Fuse).

Table 24. Overriding Signals for Alternate Functions in PB3..PB0

Signal

Name

PUOE

PUO

PB3/MOSI/OC2

SPE • MSTR

PORTB3 • PUD

SPE • MSTR

0

PB2/SS/OC1B

SPE • MSTR

PORTB2 • PUD

SPE • MSTR

0

PB1/OC1A

PB0/ICP1

0

DDOE

DDOV

PVOE

0

SPE • MSTR +

OC2 ENABLE

OC1B ENABLE

OC1A ENABLE

PVOV

DIEOE

DIEOV

DI

SPI MSTR OUTPUT + OC2 OC1B

OC1A

0

–

0

SPI SLAVE INPUT

–

SPI SS

–

ICP1 INPUT

–

AIO

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Alternate Functions of Port C The Port C pins with alternate functions are shown in Table 25.

Table 25. Port C Pins Alternate Functions

Port Pin

Alternate Function

PC6

RESET (Reset pin)

ADC5 (ADC Input Channel 5)

SCL (Two-wire Serial Bus Clock Line)

PC5

PC4

ADC4 (ADC Input Channel 4)

SDA (Two-wire Serial Bus Data Input/Output Line)

PC3

PC2

PC1

PC0

ADC3 (ADC Input Channel 3)

ADC2 (ADC Input Channel 2)

ADC1 (ADC Input Channel 1)

ADC0 (ADC Input Channel 0)

The alternate pin configuration is as follows:

• RESET – Port C, Bit 6

RESET, Reset pin: When the RSTDISBL Fuse is programmed, this pin functions as a

normal I/O pin, and the part will have to rely on Power-on Reset and Brown-out Reset as

its reset sources. When the RSTDISBL Fuse is unprogrammed, the reset circuitry is

connected to the pin, and the pin can not be used as an I/O pin.

If PC6 is used as a reset pin, DDC6, PORTC6 and PINC6 will all read 0.

• SCL/ADC5 – Port C, Bit 5

SCL, Two-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to

enable the Two-wire Serial Interface, pin PC5 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 suppress spikes shorter than 50 ns on the input signal, and the

pin is driven by an open drain driver with slew-rate limitation.

PC5 can also be used as ADC input Channel 5. Note that ADC input channel 5 uses dig-

ital power.

• SDA/ADC4 – Port C, Bit 4

SDA, Two-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to

enable the Two-wire Serial Interface, pin PC4 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 suppress spikes shorter than 50 ns on the input signal, and the

pin is driven by an open drain driver with slew-rate limitation.

PC4 can also be used as ADC input Channel 4. Note that ADC input channel 4 uses dig-

ital power.

• ADC3 – Port C, Bit 3

PC3 can also be used as ADC input Channel 3. Note that ADC input channel 3 uses

analog power.

• ADC2 – Port C, Bit 2

PC2 can also be used as ADC input Channel 2. Note that ADC input channel 2 uses

analog power.

• ADC1 – Port C, Bit 1

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2486M–AVR–12/03

PC1 can also be used as ADC input Channel 1. Note that ADC input channel 1 uses

analog power.

• ADC0 – Port C, Bit 0

PC0 can also be used as ADC input Channel 0. Note that ADC input channel 0 uses

analog power.

Table 26 and Table 27 relate the alternate functions of Port C to the overriding signals

shown in Figure 25 on page 54.

Table 26. Overriding Signals for Alternate Functions in PC6..PC4

Signal Name

PUOE

PUOV

DDOE

DDOV

PVOE

PVOV

DIEOE

DIEOV

DI

PC6/RESET

PC5/SCL/ADC5

PC4/SDA/ADC4

RSTDISBL

TWEN

1

PORTC5 • PUD

PORTC4 • PUD

RSTDISBL

TWEN

0

SCL_OUT

SDA_OUT

0

TWEN

0

RSTDISBL

0

–

AIO

RESET INPUT

ADC5 INPUT / SCL INPUT

ADC4 INPUT / SDA INPUT

Table 27. Overriding Signals for Alternate Functions in PC3..PC0⁽¹⁾

Signal Name

PUOE

PUOV

DDOE

DDOV

PVOE

PVOV

DIEOE

DIEOV

DI

PC3/ADC3

PC2/ADC2

PC1/ADC1

PC0/ADC0

0

–

AIO

ADC3 INPUT

ADC2 INPUT

ADC1 INPUT

ADC0 INPUT

Note:

1. When enabled, the Two-wire Serial Interface enables slew-rate controls on the output

pins PC4 and PC5. This is not shown in the figure. In addition, spike filters are con-

nected between the AIO outputs shown in the port figure and the digital logic of the

TWI module.

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Alternate Functions of Port D The Port D pins with alternate functions are shown in Table 28.

Table 28. Port D Pins Alternate Functions

Port Pin

PD7

Alternate Function

AIN1 (Analog Comparator Negative Input)

AIN0 (Analog Comparator Positive Input)

T1 (Timer/Counter 1 External Counter Input)

PD6

PD5

XCK (USART External Clock Input/Output)

T0 (Timer/Counter 0 External Counter Input)

PD4

PD3

PD2

PD1

PD0

INT1 (External Interrupt 1 Input)

INT0 (External Interrupt 0 Input)

TXD (USART Output Pin)

RXD (USART Input Pin)

The alternate pin configuration is as follows:

• AIN1 – Port D, Bit 7

AIN1, Analog Comparator Negative Input. Configure the port pin as input with the inter-

nal pull-up switched off to avoid the digital port function from interfering with the function

of the Analog Comparator.

• AIN0 – Port D, Bit 6

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.

• T1 – Port D, Bit 5

T1, Timer/Counter1 counter source.

• XCK/T0 – Port D, Bit 4

XCK, USART external clock.

T0, Timer/Counter0 counter source.

• 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.

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Table 29 and Table 30 relate the alternate functions of Port D to the overriding signals

shown in Figure 25 on page 54.

Table 29. Overriding Signals for Alternate Functions PD7..PD4

Signal Name

PUOE

PUO

PD7/AIN1

PD6/AIN0

PD5/T1

PD4/XCK/T0

0

OOE

0

OO

0

PVOE

PVO

0

UMSEL

0

XCK OUTPUT

DIEOE

DIEO

DI

0

–

T1 INPUT

–

XCK INPUT / T0 INPUT

–

AIO

AIN1 INPUT

AIN0 INPUT

Table 30. Overriding Signals for Alternate Functions in PD3..PD0

Signal Name

PUOE

PUO

PD3/INT1

PD2/INT0

PD1/TXD

PD0/RXD

0

TXEN

RXEN

0

PORTD0 • PUD

OOE

0

TXEN

RXEN

OO

0

1

0

PVOE

PVO

0

TXEN

0

TXD

0

DIEOE

DIEO

DI

INT1 ENABLE

INT0 ENABLE

0

–

0

1

0

INT1 INPUT

–

INT0 INPUT

–

RXD

–

AIO

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Register Description for I/O Ports

The 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

2

PORTB2

R/W

0

1

PORTB1

R/W

0

PORTB0

R/W

PORTB

DDRB

PINB

Read/Write

Initial Value

0

The 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

The 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

The Port C Data Register –

PORTC

Bit

7

–

6

PORTC6

R/W

0

5

PORTC5

R/W

0

4

PORTC4

R/W

0

3

PORTC3

R/W

2

PORTC2

R/W

0

1

PORTC1

R/W

0

PORTC0

R/W

PORTC

DDRC

PINC

Read/Write

Initial Value

R

0

The Port C Data Direction

Register – DDRC

Bit

7

–

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

R

0

The Port C Input Pins Address

– PINC

Bit

7

–

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

R

0

N/A

The 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

2

PORTD2

R/W

0

1

PORTD1

R/W

0

PORTD0

R/W

PORTD

DDRD

PIND

Read/Write

Initial Value

0

The 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

The 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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2486M–AVR–12/03

External Interrupts

The external interrupts are triggered by the INT0, and INT1 pins. Observe that, if

enabled, the interrupts will trigger even if the INT0..1 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. This is set up as indicated in

the specification for the MCU Control Register – MCUCR. When the external interrupt is

enabled and is configured as level triggered, 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 Distribu-

tion” on page 23. Low level interrupts on INT0/INT1 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 Oscillator is voltage dependent as shown in “Electrical Char-

acteristics” on page 237. 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 23. 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 –

MCUCR

The MCU Control Register contains control bits for interrupt sense control and general

MCU functions.

Bit

7

SE

R/W

0

6

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

corresponding interrupt mask in the GICR are set. The level and edges on the external

INT1 pin that activate the interrupt are defined in Table 31. 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 guaran-

teed 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 31. 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

corresponding interrupt mask are set. The level and edges on the external INT0 pin that

activate the interrupt are defined in Table 32. 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 32. 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.

General Interrupt Control

Register – GICR

Bit

7

INT1

R/W

0

6

5

–

4

–

3

–

2

–

1

IVSEL

R/W

0

IVCE

R/W

0

INT0

R/W

0

GICR

Read/Write

Initial Value

R

0

R

0

R

0

R

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 external 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 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 Inter-

rupt 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 external 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 Inter-

rupt Vector.

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2486M–AVR–12/03

General Interrupt Flag

Register – GIFR

Bit

7

INTF1

R/W

0

6

INTF0

R/W

0

5

–

4

–

3

–

2

–

1

–

0

–

GIFR

Read/Write

Initial Value

R

0

R

0

R

0

R

0

R

0

R

0

• Bit 7 – INTF1: External Interrupt Flag 1

When an event 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 cor-

responding 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 event 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 cor-

responding 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.

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ATmega8(L)

8-bit Timer/Counter0 Timer/Counter0 is a general purpose, single channel, 8-bit Timer/Counter module. The

main features are:

• Single Channel Counter

• Frequency Generator

• External Event Counter

• 10-bit Clock Prescaler

Overview

A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 26. For the

actual placement of I/O pins, refer to “Pin Configurations” 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 70.

Figure 26. 8-bit Timer/Counter Block Diagram

TCCRn

TOVn

(Int.Req.)

count

Control Logic

Clock Select

clk_Tn

Edge

Detector

Tn

Timer/Counter

TCNTn

( From Prescaler )

=

0xFF

Registers

The Timer/Counter (TCNT0) is an 8-bit register. 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 or 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 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).

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 i.e. TCNT0 for

accessing Timer/Counter0 counter value and so on.

The definitions in Table 33 are also used extensively throughout this datasheet.

Table 33. Definitions

BOTTOM The counter reaches the BOTTOM when it becomes 0x00

MAX

The counter reaches its MAXimum when it becomes 0xFF (decimal 255)

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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 72.

Counter Unit

The main part of the 8-bit Timer/Counter is the programmable counter unit. Figure 27

shows a block diagram of the counter and its surroundings.

Figure 27. Counter Unit Block Diagram

TOVn

(Int. Req.)

DATA BUS

Clock Select

count

Edge

Detector

TCNTn

Control Logic

Tn

clk_Tn

( From Prescaler )

max

Signal description (internal signals):

count

clk_Tn

max

Increment TCNT0 by 1.

Timer/Counter clock, referred to as clk_T0in the following.

Signalize that TCNT0 has reached maximum value.

The counter is incremented 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 whether clk_T0is present or not. A CPU write

overrides (has priority over) all counter clear or count operations.

Operation

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 (MAX = 0xFF) and

then restarts from the bottom (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. A new counter value can be

written anytime.

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ATmega8(L)

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 28 contains timing data for basic Timer/Counter

operation. The figure shows the count sequence close to the MAX value.

Figure 28. Timer/Counter Timing Diagram, No Prescaling

clk_I/O

clk_Tn

(clk_I/O/1)

TCNTn

TOVn

MAX - 1

MAX

BOTTOM

BOTTOM + 1

Figure 29 shows the same timing data, but with the prescaler enabled.

Figure 29. 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

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2486M–AVR–12/03

8-bit Timer/Counter

Register Description

Timer/Counter Control

Register – TCCR0

Bit

7

–

6

–

5

–

4

–

3

–

2

CS02

R/W

0

1

CS01

R/W

0

CS00

R/W

0

TCCR0

Read/Write

Initial Value

R

0

R

0

R

0

R

0

R

0

• Bit 2:0 – CS02:0: Clock Select

The three clock select bits select the clock source to be used by the Timer/Counter.

Table 34. 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.

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

–

0

TOIE0

R/W

0

TIMSK

Read/Write

Initial Value

R/W

0

• 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, i.e., when the TOV0 bit is set in the

Timer/Counter Interrupt Flag Register – TIFR.

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ATmega8(L)

Timer/Counter Interrupt Flag

Register – TIFR

Bit

7

OCF2

R/W

0

6

TOV2

R/W

0

5

ICF1

R/W

0

4

OCF1A

R/W

0

3

OCF1B

R/W

0

2

TOV1

R/W

0

1

–

0

TOV0

R/W

0

TIFR

Read/Write

Initial Value

R/W

0

• Bit 0 – TOV0: Timer/Counter0 Overflow Flag

The bit TOV0 is set (one) when an overflow occurs in Timer/Counter0. TOV0 is cleared

by hardware 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 Interrupt Enable), and TOV0 are set (one), the

Timer/Counter0 Overflow interrupt is executed.

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Timer/Counter0 and

Timer/Counter1

Prescalers

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.

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 pres-

caler 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 (i.e., operates independently of the clock select logic of the

Timer/Counter) and it is shared by Timer/Counter1 and Timer/Counter0. Since the pres-

caler 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 pres-

caling 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

execution. 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 syn-

chronization logic. The synchronized (sampled) signal is then passed through the edge

detector. Figure 30 shows a functional equivalent block diagram of the T1/T0 synchroni-

zation 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_T₀pulse for each positive (CSn2:0 = 7) or neg-

ative (CSn2:0 = 6) edge it detects.

Figure 30. T1/T0 Pin Sampling

Tn_sync

(To Clock

Select Logic)

Tn

D Q

LE

D

Q

D Q

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.

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ATmega8(L)

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 system clock frequency (f_ExtClk< f_{clk_I/O}/2) given a 50/50% duty cycle. Since

the edge detector uses sampling, the maximum frequency of an external clock it can

detect is half the sampling frequency (Nyquist sampling theorem). However, due to vari-

ation 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 31. 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 30.

Special Function IO Register –

SFIOR

Bit

7

–

6

–

5

–

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

R

0

R

0

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 man-

agement), wave generation, and signal timing measurement. The main features are:

• True 16-bit Design (i.e., 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 Com-

pare unit channel. However, when using the register or bit defines in a program, the

precise form must be used i.e., TCNT1 for accessing Timer/Counter1 counter value and

so on.

A simplified block diagram of the 16-bit Timer/Counter is shown in Figure 32. For the

actual placement of I/O pins, refer to “Pin Configurations” 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 95.

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ATmega8(L)

Figure 32. 16-bit Timer/Counter Block Diagram⁽¹⁾

Count

TOVn

(Int. Req.)

Clear

Control Logic

Clock Select

Direction

clk_Tn

Edge

Detector

Tn

TOP

BOTTOM

( From Prescaler )

Timer/Counter

TCNTn

=

= 0

OCFnA

(Int. Req.)

Waveform

Generation

OCnA

=

OCRnA

OCFnB

(Int.Req.)

Fixed

TOP

Values

Waveform

OCnB

=

Generation

OCRnB

( From Analog

Comparator Ouput )

ICFn (Int.Req.)

Edge

Detector

Noise

Canceler

ICRn

ICPn

TCCRnA

TCCRnB

Note:

1. Refer to “Pin Configurations” on page 2, Table 22 on page 56, and Table 28 on page

61 for Timer/Counter1 pin placement and description.

Registers

The Timer/Counter (TCNT1), Output Compare Registers (OCR1A/B), and Input Capture

Register (ICR1) are all 16-bit registers. Special procedures must be followed when

accessing the 16-bit registers. These procedures are described in the section “Access-

ing 16-bit Registers” on page 77. 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 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 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/Counter 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 “Output Compare Units” on page 83. The Compare Match event will also

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2486M–AVR–12/03

set the Compare Match Flag (OCF1A/B) which can be used to generate an Output

Compare interrupt request.

The Input Capture Register can capture the Timer/Counter value at a given external

(edge triggered) event on either the Input Capture Pin (ICP1) or on the Analog Compar-

ator pins (see “Analog Comparator” on page 190). 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 35. Definitions

BOTTOM The counter reaches the BOTTOM when it becomes 0x0000.

MAX

The counter reaches its MAXimum when it becomes 0xFFFF (decimal

65535).

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 one

of the fixed values: 0x00FF, 0x01FF, or 0x03FF, or to the value stored in

the OCR1A or ICR1 Register. The assignment is dependent of the mode

of operation.

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. The 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 the 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 temporary 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

ldir17,0x01

ldir16,0xFF

outTCNT1H,r17

outTCNT1L,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. The example code assumes that the part specific header file is included.

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 inter-

rupt 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 Registers, 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 regis-

ter, the main code must disable the interrupts during the 16-bit access.

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2486M–AVR–12/03

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

outSREG,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. The example code assumes that the part specific header file is included.

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

outTCNT1H,r17

outTCNT1L,r16

; Restore Global Interrupt Flag

outSREG,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. The example code assumes that the part specific header file is included.

The assembly code example requires that the r17:r16 Register pair contains the value to

be written 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 regis-

ters written, 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.

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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 72.

Counter Unit

The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional

counter unit. Figure 33 shows a block diagram of the counter and its surroundings.

Figure 33. Counter Unit Block Diagram

DATA BUS (8-bit)

TOVn

(Int. Req.)

TEMP (8-bit)

Clock Select

Edge

count

Tn

Detector

TCNTnH (8-bit) TCNTnL (8-bit)

TCNTn (16-bit Counter)

clear

clk_Tn

Control Logic

direction

( 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) containing the upper eight bits of the counter, and Counter Low (TCNT1L)

containing the lower eight 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 impor-

tant 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 dec-

remented at each timer clock (clk ). The clk can be generated from an external or

1

T

internal clock source, 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_T₁is present or not. A CPU write over-

rides (has priority over) all counter clear or count operations.

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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 Opera-

tion” on page 86.

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.

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 multiple 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 34. The ele-

ments 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 34. 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.

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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 Generation 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 writ-

ten to the ICR1H I/O location before the Low byte is written to ICR1L.

For more information on how to access the 16-bit registers refer to “Accessing 16-bit

Registers” on page 77.

Input Capture Trigger Source The main trigger source for the Input Capture unit is the Input Capture Pin (ICP1).

Timer/Counter 1 can alternatively use the Analog Comparator Output as trigger source

for the Input Capture unit. The Analog Comparator is selected as trigger source by set-

ting 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 cap-

ture. 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 30 on page 72). 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 Waveform 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 intro-

duces additional 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 there-

fore 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 cap-

ture will be incorrect.

When using the Input Capture interrupt, the ICR1 Register should be read as early in the

interrupt handler routine as possible. Even though the Input Capture interrupt has rela-

tively 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

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(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).

Output Compare Units

The 16-bit comparator continuously compares TCNT1 with the Output Compare Regis-

ter (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 Compare 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 writing 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 86.)

A special feature of Output Compare unit A allows it to define the Timer/Counter TOP

value (i.e. counter resolution). In addition to the counter resolution, the TOP value

defines the period time for waveforms generated by the waveform generator.

Figure 35 shows a block diagram of the Output Compare unit. The small “n” in the regis-

ter and bit names indicates the device number (n = 1 for Timer/Counter 1), and the “x”

indicates Output Compare unit (A/B). The elements of the block diagram that are not

directly a part of the Output Compare unit are gray shaded.

Figure 35. 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 Mod-

ulation (PWM) modes. For the normal and Clear Timer on Compare (CTC) modes of

operation, the double buffering is disabled. The double buffering synchronizes the

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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 disabled 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 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 Register since the compare of all 16-bit 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 writ-

ten. 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 Compare Reg-

ister 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 77.

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 channels, 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 genera-

tor 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. Fig-

ure 36 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 36. 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 output) 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 visible on the pin. The port override function is generally

independent of the Waveform Generation mode, but there are some exceptions. Refer

to Table 36, Table 37 and Table 38 for details.

The design of the Output Compare Pin logic allows initialization of the OC1x state

before the output 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

95.

The COM1x1:0 bits have no effect on the Input Capture unit.

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Compare Output Mode and

Waveform Generation

The waveform generator uses the COM1x1:0 bits differently in normal, CTC, and PWM

modes. 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 com-

pare output actions in the non-PWM modes refer to Table 36 on page 95. For fast PWM

mode refer to Table 37 on page 96, and for phase correct and phase and frequency cor-

rect PWM refer to Table 38 on page 96.

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.

Modes of Operation

The mode of operation (i.e., 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 Output 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 output generated should be inverted or not (inverted or

non-inverted PWM). For non-PWM modes the COM1x1:0 bits control whether the out-

put should be set, cleared or toggle at a Compare Match. See “Compare Match Output

Unit” on page 85.

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 93.

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 Over-

flow 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 software. 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 maxi-

mum 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 Output Compare to generate waveforms in Normal mode is not recommended,

since this will occupy too much of the CPU time.

Clear Timer on Compare

Match (CTC) Mode

In Clear Timer on Compare or CTC mode (WGM13:0 = 4 or 12), the OCR1A or ICR1

Register are used to manipulate the counter resolution. In CTC mode the counter is

cleared to zero when 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 operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 37. The counter value

(TCNT1) increases until a Compare Match occurs with either OCR1A or ICR1, and then

counter (TCNT1) is cleared.

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Figure 37. 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

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. However, 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 buffering 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 maximum 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 frequency of f_{OC A}= f_{clk_I/O}/2 when OCR1A is set to zero (0x0000).

1

The waveform frequency is 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) pro-

vides 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 set on the Compare Match between TCNT1 and OCR1x, and

cleared at TOP. In inverting Compare Output mode output is cleared on Compare Match

and set at TOP. Due to the single-slope operation, the operating frequency of the fast

PWM mode can be twice as high as the phase correct 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 fre-

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quency allows physically small sized external components (coils, capacitors), hence

reduces total system cost.

The PWM resolution for fast PWM can be fixed to 8-, 9-, 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 38. 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 illus-

trating 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 Com-

pare Match occurs.

Figure 38. Fast PWM Mode, Timing Diagram

OCRnx / TOP Update

and TOVn Interrupt Flag

Set and OCnA Interrupt

Flag Set or ICFn

Interrupt Flag Set

(Interrupt on TOP)

TCNTn

(COMnx1:0 = 2)

OCnx

(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 OCF1A 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 handler routine can be used for updating the TOP and com-

pare 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 start-

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ing 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 37 on

page 96. 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 set-

ting (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).

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 output 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 polar-

ity 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 setting 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 wave-

form generated will have a maximum frequency of f_{OC A}= f_{clk_I/O}/2 when OCR1A is set to

1

zero (0x0000). This feature is similar to the OC1A toggle in CTC mode, except the dou-

ble 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. However, 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-, 9-, or 10-bit, or

defined by either ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or

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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

= ----------------------------------

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 39. 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 repre-

sent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be

set when a Compare Match occurs.

Figure 39. 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)

OCnx

(COMnx1:0 = 3)

OCnx

1

2

3

4

Period

The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches BOT-

TOM. When either OCR1A or ICR1 is used for defining the TOP value, the OC1A or

ICF1 Flag is set accordingly 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 gen-

erate 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 39 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 Register. Since the OCR1x update occurs

at TOP, the PWM period starts and ends at TOP. This implies that the length of the fall-

ing 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

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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 38 on

page 96. 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 set-

ting (or clearing) the OC1x Register 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 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 (WMG13:0 = 11) and COM1A1:0 = 1, the

OC1A output will toggle with a 50% duty cycle.

Phase and Frequency Correct The phase and frequency correct Pulse Width Modulation, or phase and frequency cor-

PWM Mode

rect PWM mode (WGM13:0 = 8 or 9) provides a high resolution phase and frequency

correct PWM waveform 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 BOT-

TOM. 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 frequency com-

pared 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 39 and Figure 40).

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

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(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 40.

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 represent compare

matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a

Compare Match occurs.

Figure 40. 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)

OCnx

(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 40 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 frequency 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 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

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to 3. See Table 38 on page 96. 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 increments, 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 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 41 shows a timing

diagram for the setting of OCF1x.

Figure 41. 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 42 shows the same timing data, but with the prescaler enabled.

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Figure 42. 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 43 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 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 43. 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 44 shows the same timing data, but with the prescaler enabled.

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Figure 44. 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

16-bit Timer/Counter

Register Description

Timer/Counter 1 Control

Register A – TCCR1A

Bit

7

COM1A1

R/W

6

5

COM1B1

R/W

4

3

FOC1A

W

2

FOC1B

W

1

WGM11

R/W

0

WGM10

R/W

0

COM1A0

R/W

0

COM1B0

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

respectively) 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 Reg-

ister (DDR) bit corresponding 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

dependent of the WGM13:0 bits setting. Table 36 shows the COM1x1:0 bit functionality

when the WGM13:0 bits are set to a normal or a CTC mode (non-PWM).

Table 36. Compare Output Mode, Non-PWM

COM1A1/

COM1B1

COM1A0/

COM1B0

Description

0

1

0

1

0

1

Normal port operation, OC1A/OC1B disconnected.

Toggle OC1A/OC1B on Compare Match

Clear OC1A/OC1B on Compare Match (Set output to low level)

Set OC1A/OC1B on Compare Match (Set output to high level)

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Table 37 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the

fast PWM mode.

Table 37. Compare Output Mode, Fast PWM⁽¹⁾

COM1A1/

COM1B1

COM1A0/

COM1B0

Description

0

1

Normal port operation, OC1A/OC1B disconnected.

WGM13:0 = 15: Toggle OC1A on Compare Match, OC1B

disconnected (normal port operation). For all other WGM1

settings, normal port operation, OC1A/OC1B disconnected.

1

0

1

Clear OC1A/OC1B on Compare Match, set OC1A/OC1B at TOP

Set OC1A/OC1B on Compare Match, clear OC1A/OC1B at TOP

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 TOP.

See “Fast PWM Mode” on page 87. for more details.

Table 38 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the

phase correct or the phase and frequency correct, PWM mode.

Table 38. Compare Output Mode, Phase Correct and Phase and Frequency Correct

PWM⁽¹⁾

COM1A1/

COM1B1

COM1A0/

COM1B0

Description

0

1

Normal port operation, OC1A/OC1B disconnected.

WGM13:0 = 9 or 14: Toggle OC1A on Compare Match, OC1B

disconnected (normal port operation). For all other WGM1

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 89. 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 set-

ting. 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.

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

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what type of waveform generation to be used, see Table 39. Modes of operation sup-

ported 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 86.)

Table 39. Waveform Generation Mode Bit Description

WGM12

(CTC1)

WGM11

WGM10

Timer/Counter Mode of

Update of

OCR1x

TOV1 Flag

Set on

Mode WGM13

(PWM11) (PWM10) Operation⁽¹⁾

TOP

0

1

2

3

4

5

6

7

8

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

Normal

0xFFFF Immediate

MAX

PWM, Phase Correct, 8-bit

PWM, Phase Correct, 9-bit

PWM, Phase Correct, 10-bit

CTC

0x00FF

0x01FF

0x03FF

TOP

BOTTOM

MAX

OCR1A Immediate

Fast PWM, 8-bit

0x00FF

0x01FF

0x03FF

ICR1

TOP

Fast PWM, 9-bit

TOP

Fast PWM, 10-bit

TOP

PWM, Phase and Frequency

Correct

BOTTOM

9

1

0

1

PWM, Phase and Frequency

Correct

OCR1A BOTTOM

BOTTOM

10

11

12

13

14

1

0

1

0

1

0

1

0

1

0

1

PWM, Phase Correct

CTC

ICR1

TOP

BOTTOM

MAX

OCR1A TOP

ICR1

–

Immediate

(Reserved)

–

Fast PWM

ICR1

TOP

15

Fast PWM

OCR1A TOP

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/Counter 1 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 can-

celer 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 cap-

ture 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 Capture 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 41 and Figure 42.

Table 40. 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/Counter 1 – 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 simulta-

neously 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 77.

Modifying the counter (TCNT1) while the counter is running introduces a risk of missing

a Compare 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

– OCR1AH and OCR1AL

Bit

7

6

5

4

3

2

1

0

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

– OCR1BH and OCR1BL

Bit

7

6

5

4

3

2

1

0

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 77.

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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

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 per-

formed 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 77.

Timer/Counter Interrupt Mask

Register – TIMSK⁽¹⁾

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

–

0

TOIE0

R/W

0

TIMSK

Read/Write

Initial Value

R

0

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 glo-

bally enabled), the Timer/Counter1 Input Capture Interrupt is enabled. The

corresponding Interrupt Vector (see “Interrupts” on page 44) 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 glo-

bally enabled), the Timer/Counter1 Output Compare A match interrupt is enabled. The

corresponding Interrupt Vector (see “Interrupts” on page 44) 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 glo-

bally enabled), the Timer/Counter1 Output Compare B match interrupt is enabled. The

corresponding Interrupt Vector (see “Interrupts” on page 44) 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 glo-

bally enabled), the Timer/Counter1 Overflow Interrupt is enabled. The corresponding

Interrupt Vector (see “Interrupts” on page 44) is executed when the TOV1 Flag, located

in TIFR, is set.

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Timer/Counter Interrupt Flag

Register – TIFR⁽¹⁾

Bit

7

OCF2

R/W

0

6

TOV2

R/W

0

5

ICF1

R/W

0

4

OCF1A

R/W

0

3

OCF1B

R/W

0

2

TOV1

R/W

0

1

–

0

TOV0

R/W

0

TIFR

Read/Write

Initial Value

R

0

Note:

1. 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.

• 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 counter reaches the TOP value.

ICF1 is automatically cleared when the Input Capture Interrupt Vector is executed. Alter-

natively, 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 Out-

put 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

executed. 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 Out-

put 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

executed. 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 39 on page 97 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 Timer/Counter2 is a general purpose, single channel, 8-bit Timer/Counter module. The

main features are:

• Single Channel Counter

with PWM and

Asynchronous

Operation

• 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

Overview

A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 45. For the

actual placement of I/O pins, refer to “Pin Configurations” 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 115.

Figure 45. 8-bit Timer/Counter Block Diagram

TCCRn

count

TOVn

(Int. Req.)

clear

direction

Control Logic

clk_Tn

TOSC1

TOSC2

BOTTOM

TOP

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)

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ATmega8(L)

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 decre-

ment) 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_T2).

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 wave-

form generator to generate a PWM or variable frequency output on the Output Compare

Pin (OC2). For details, see “Output Compare Unit” on page 105. 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 (i.e., TCNT2 for

accessing Timer/Counter2 counter value and so on).

The definitions in Table 41 are also used extensively throughout the document.

Table 41. Definitions

BOTTOM The counter reaches the BOTTOM when it becomes zero (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 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 asynchro-

nous 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 “Asynchronous Status Register – ASSR” on page 117. For

details on clock sources and prescaler, see “Timer/Counter Prescaler” on page 121.

.

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Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.

Figure 46 shows a block diagram of the counter and its surrounding environment.

Figure 46. Counter Unit Block Diagram

TOVn

(Int. Req.)

DATA BUS

TOSC1

count

T/C

Oscillator

clk _Tn

clear

TCNTn

Control Logic

Prescaler

direction

TOSC2

BOTTOM

TOP

clk

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.

BOTTOM Signalizes that TCNT2 has reached minimum value (zero).

Depending on the mode of operation used, the counter is cleared, incremented, or dec-

remented 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 108.

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.

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ATmega8(L)

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 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 WGM21:0 bits and Compare Output mode (COM21:0) bits. The max and bottom sig-

nals 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 108).

Figure 47 shows a block diagram of the Output Compare unit.

Figure 47. 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 buffering is disabled. The double buffering synchronizes the update of the

OCR2 Compare Register to either top or bottom of the counting sequence. The synchro-

nization 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.

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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 channel, independently 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 Out-

put 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 genera-

tor 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 48

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 figure 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 48. 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 direc-

tion (input or output) is still controlled by the Data Direction Register (DDR) for the port

pin. The Data Direction Register 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 output 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 115.

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Compare Output Mode and

Waveform Generation

The Waveform Generator uses the COM21:0 bits differently in normal, CTC, and PWM

modes. 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 com-

pare output actions in the non-PWM modes refer to Table 43 on page 116. For fast

PWM mode, refer to Table 44 on page 116, and for phase correct PWM refer to Table

45 on page 116.

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.

Modes of Operation

The mode of operation (i.e., 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 Output 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 output 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 107).

For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 113.

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 bottom (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 Output Compare to generate waveforms in Normal mode is not recommended,

since this will occupy too much of the CPU time.

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Clear Timer on Compare

Match (CTC) Mode

In Clear Timer on Compare or CTC mode (WGM21:0 = 2), the OCR2 Register is used to

manipulate the counter resolution. In CTC mode the counter is cleared to zero when the

counter value (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 simplifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 49. The counter value

(TCNT2) increases until a Compare Match occurs between TCNT2 and OCR2, and then

counter (TCNT2) is cleared.

Figure 49. 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 BOT-

TOM 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 buffering feature. If the new value

written to OCR2 is lower than the current 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 fre-

quency 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.

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Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGM21:0 = 3) provides a high fre-

quency PWM waveform generation option. The fast PWM differs from the other PWM

option by its single-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 BOT-

TOM. 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 50. The TCNT2 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 TCNT2

slopes represent compare matches between OCR2 and TCNT2.

Figure 50. 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 interrupt is enabled, the interrupt handler routine can be used for updating the com-

pare value.

In fast 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 and an inverted

PWM output can be generated by setting the COM21:0 to 3 (see Table 44 on page 116).

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 counter is cleared (changes from MAX to

BOTTOM).

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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 out-

put set by the COM21:0 bits.)

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved

by setting 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 feature 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 downcounting. 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 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 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 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 51. 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 repre-

sent compare matches between OCR2 and TCNT2.

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Figure 51. 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 BOT-

TOM. 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 output can be generated by setting the COM21:0 to 3 (see Table 45 on

page 116). 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 following 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 output 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 51 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:

•

OCR2A changes its value from MAX, like in Figure 51. 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

correspond to the result of an up-counting Compare Match.

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•

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 change that would have

happened on the way up.

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 52 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 52. Timer/Counter Timing Diagram, no Prescaling

clk_I/O

clk_Tn

(clk_I/O/1)

TCNTn

TOVn

MAX - 1

MAX

BOTTOM

BOTTOM + 1

Figure 53 shows the same timing data, but with the prescaler enabled.

Figure 53. 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 54 shows the setting of OCF2 in all modes except CTC mode.

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Figure 54. 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 55 shows the setting of OCF2 and the clearing of TCNT2 in CTC mode.

Figure 55. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with

Prescaler (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

ensuring 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 6,3 – 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 oper-

ation 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

42 and “Modes of Operation” on page 108.

Table 42. Waveform Generation Mode Bit Description

WGM21 WGM20 Timer/Counter Mode

(CTC2)

Update of TOV2 Flag

OCR2 Set

Mode

(PWM2) of Operation⁽¹⁾

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 TOP MAX

Fast PWM

Note:

1. The CTC2 and PWM2 bit definition names are now obsolete. Use the WGM21:0 def-

initions. 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 corre-

sponding to OC2 pin must be set in order to enable the output driver.

When OC2 is connected to the pin, the function of the COM21:0 bits depends on the

WGM21:0 bit setting. Table 43 shows the COM21:0 bit functionality when the WGM21:0

bits are set to a normal or CTC mode (non-PWM).

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Table 43. 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 44 shows the COM21:0 bit functionality when the WGM21:0 bits are set to fast

PWM mode.

Table 44. Compare Output Mode, Fast PWM Mode⁽¹⁾

COM21

COM20

Description

0

1

0

1

0

1

Normal port operation, OC2 disconnected.

Reserved

Clear OC2 on Compare Match, set OC2 at TOP

Set OC2 on Compare Match, clear OC2 at TOP

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 “Fast PWM

Mode” on page 110 for more details.

Table 45 shows the COM21:0 bit functionality when the WGM21:0 bits are set to phase

correct PWM mode.

Table 45. Compare Output Mode, Phase Correct PWM Mode⁽¹⁾

COM21 COM20 Description

0

1

Normal port operation, OC2 disconnected.

Reserved

Clear OC2 on Compare Match when up-counting. Set OC2 on Compare

Match when downcounting.

1

0

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 111 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 46.

Table 46. 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.

AsynchronousOperation

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/Counter 2 is clocked from a crystal Oscillator connected to

the Timer Oscillator 1 (TOSC1) pin. When the value of AS2 is changed, the contents of

TCNT2, OCR2, and TCCR2 might be corrupted.

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• 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 hardware. 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 hardware. 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 hardware. 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 inter-

rupt 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 storage 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.

•

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 that e.g. 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 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

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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 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:

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.

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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

–

0

TOIE0

R/W

0

TIMSK

Read/Write

Initial Value

R

0

• 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 (i.e., when the OCF2 bit is set in

the Timer/Counter 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 (i.e., when the TOV2 bit is set in the

Timer/Counter Interrupt Flag Register – TIFR).

Timer/Counter Interrupt Flag

Register – TIFR

Bit

7

OCF2

R/W

0

6

TOV2

R/W

0

5

ICF1

R/W

0

4

OCF1A

R/W

0

3

OCF1B

R/W

0

2

TOV1

R/W

0

1

–

0

TOV0

R/W

0

TIFR

Read/Write

Initial Value

R

0

• 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 Com-

pare 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 hardware 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 Interrupt 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 0x00.

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Timer/Counter Prescaler Figure 56. 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_I/O. 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 B. 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. Applying 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 predictable prescaler.

Special Function IO Register –

SFIOR

Bit

7

–

6

–

5

–

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

R

0

R

0

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 ATmega8 and peripheral devices or between several AVR devices. The

ATmega8 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 57. SPI Block Diagram⁽¹⁾

DIVIDER

/2/4/8/16/32/64/128

Note:

1. Refer to “Pin Configurations” on page 2, and Table 22 on page 56 for SPI pin

placement.

The interconnection between Master and Slave CPUs with SPI is shown in Figure 58.

The system consists of two Shift Registers, and a Master clock generator. The SPI Mas-

ter 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 inter-

change data. Data is always shifted from Master 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.

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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 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 gener-

ator 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 58. SPI Master-Slave Interconnection

MSB

MASTER

LSB

MSB

SLAVE

LSB

MISO

MOSI

MISO

MOSI

8 BIT SHIFT REGISTER

SHIFT

ENABLE

SPI

SCK

SS

SCK

CLOCK GENERATOR

SS

V_CC

The system is single buffered in the transmit direction and double buffered in the receive

direction. 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. Otherwise, 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 frequency of the SPI clock should never

exceed f_osc/4.

When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is

overridden according to Table 47. For more details on automatic port overrides, refer to

“Alternate Port Functions” on page 54.

Table 47. SPI Pin Overrides⁽¹⁾

MOSI

MISO

SCK

SS

Direction, Master SPI

User Defined

Input

Direction, Slave SPI

Input

User Defined

Input

User Defined

Input

Note:

1. See “Port B Pins Alternate Functions” on page 56 for a detailed description of how to

define the direction of the user defined SPI pins.

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The following code examples show how to initialize the SPI as a Master and how to per-

form 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. E.g. if MOSI is placed

on pin PB5, replace DD_MOSI with DDB5 and DDR_SPI with DDRB.

Assembly Code Example⁽¹⁾

SPI_MasterInit:

; Set MOSI and SCK output, all others input

ldi r17,(1<<DD_MOSI)|(1<<DD_SCK)

out DDR_SPI,r17

; Enable SPI, Master, set clock rate fck/16

ldi r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0)

out SPCR,r17

ret

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. The example code assumes that the part specific header file is included.

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The following code examples show how to initialize the SPI as a Slave and how to per-

form a simple reception.

Assembly Code Example⁽¹⁾

SPI_SlaveInit:

; Set MISO output, all others input

ldi r17,(1<<DD_MISO)

out DDR_SPI,r17

; Enable SPI

ldi r17,(1<<SPE)

out SPCR,r17

ret

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. The example code assumes that the part specific header file is included.

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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, 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 syn-

chronous 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

possibility 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.

• 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

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be cleared, and SPIF in SPSR will become set. The user will then have to set MSTR to

re-enable SPI Master 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 59 and Figure 60 for an example. The CPOL func-

tionality is summarized below:

Table 48. 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 59 and Figure 60 for an example.

The CPHA functionality is summarized below:

Table 49. 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 50. 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

SPI2X

R/W

0

WCOL

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 ATmega8 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 50). This means that the minimum SCK period will

be 2 CPU clock periods. When the SPI is configured as Slave, the SPI is only guaran-

teed to work at f_osc/4 or lower.

The SPI interface on the ATmega8 is also used for Program memory and EEPROM

downloading or uploading. See page 232 for 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 Reg-

ister File and the SPI Shift Register. Writing to the register initiates data transmission.

Reading the register 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 59 and Figure 60. Data bits are shifted out and latched in on oppo-

site edges of the SCK signal, ensuring sufficient time for data signals to stabilize. This is

clearly seen by summarizing Table 48 and Table 49, as done below:

Table 51. 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 59. 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)

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

Figure 60. 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 Databits 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 61. CPU acces-

sible I/O Registers and I/O pins are shown in bold.

Figure 61. 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 “Pin Configurations” on page 2, Table 30 on page 62, and Table 29 on page

62 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 exter-

nal clock input used by synchronous 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 ninth

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 61) 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 asynchronous, Master synchronous and Slave Synchronous mode. The UMSEL

bit in USART Control and Status Register C (UCSRC) selects between asynchronous

and synchronous operation. 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 62 shows a block diagram of the clock generation logic.

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Figure 62. Clock Generation Logic, Block Diagram

UBRR

U2X

fosc

UBRR+1

Prescaling

Down-Counter

/ 2

/ 4

/ 2

0

1

0

1

OSC

txclk

UMSEL

rxclk

DDR_XCK

Sync

Register

Edge

Detector

xcki

0

1

XCK

xcko

DDR_XCK

UCPOL

1

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 62.

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 sys-

tem 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 output 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 52 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 52. Equations for Calculating Baud Rate Register Setting

Equation for Calculating

Baud Rate⁽¹⁾

Equation for Calculating

UBRR Value

Operating Mode

Asynchronous Normal mode

(U2X = 0)

f

OSC

BAUD = --------------------------------------

16(UBRR + 1)

UBRR = ----------------------- – 1

16BAUD

Asynchronous Double Speed

Mode (U2X = 1)

f

OSC

BAUD = -----------------------------------

8(UBRR + 1)

UBRR = -------------------- – 1

8BAUD

Synchronous Master Mode

f

OSC

BAUD = -----------------------------------

2(UBRR + 1)

UBRR = -------------------- – 1

2BAUD

Note:

BAUD Baud rate (in bits per second, bps)

f_OSCSystem Oscillator clock frequency

1. The baud rate is defined to be the transfer rate in bit per second (bps).

UBRR Contents of the UBRRH and UBRRL Registers, (0 - 4095)

Some examples of UBRR values for some system clock frequencies are found in Table

60 (see page 156).

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 descrip-

tion in this section refers to Figure 62 for details.

External clock input from the XCK pin is sampled by a synchronization register to mini-

mize 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 max-

imum external XCK clock frequency is limited by the following equation:

f

OSC

f

<

-----------

XCK

4

Note that f_oscdepends on the stability of the system clock source. It is therefore recom-

mended 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 63. 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 63 shows, when UCPOL is zero the data will

be changed at rising 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 com-

plete 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 64 illustrates the possible

combinations of the frame formats. Bits inside brackets are optional.

Figure 64. 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.

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ATmega8(L)

Sp

Stop bit, always high.

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 selec-

tion 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

0

1

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 initial-

ization process 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 function that are equal in functionality. The examples assume asynchronous opera-

tion 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 Registers. 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

out UBRRL, r16

; Enable Receiver and Transmitter

ldi r16, (1<<RXEN)|(1<<TXEN)

out UCSRB,r16

; Set frame format: 8data, 2stop bit

ldi r16, (1<<URSEL)|(1<<USBS)|(3<<UCSZ0)

out UCSRC,r16

ret

C Code Example⁽¹⁾

void USART_Init( unsigned int baud )

{

/* Set baud rate */

UBRRH = (unsigned char)(baud>>8);

UBRRL = (unsigned char)baud;

/* Enable Receiver and Transmitter */

UCSRB = (1<<RXEN)|(1<<TXEN);

/* Set frame format: 8data, 2stop bit */

UCSRC = (1<<URSEL)|(1<<USBS)|(3<<UCSZ0);

}

Note:

1. The example codes assume that the part specific header file is included.

More advanced initialization routines can be made that include frame format as parame-

ters, disable 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.

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Data Transmission – The 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

USART Transmitter

TxD pin is overridden 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 synchronous operation is used, the clock on the XCK pin will

be overridden and used as transmission clock.

Sending Frames with 5 to 8

Data Bits

A data transmission is initiated by loading the transmit buffer with the data to be trans-

mitted. 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 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 previ-

ous 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 depend-

ing 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 significant 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 UDR,r16

ret

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. The example codes assumes that the part specific header file is included.

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 Inter-

rupt is utilized, the interrupt routine writes the data into the buffer.

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Sending Frames with 9 Data

Bits

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 ninth bit from r17 to TXB8

cbi UCSRB,TXB8

sbrc r17,0

sbi UCSRB,TXB8

; Put LSB data (r16) into buffer, sends the data

out UDR,r16

ret

C Code Example⁽¹⁾

void USART_Transmit( unsigned int data )

{

/* Wait for empty transmit buffer */

while ( !( UCSRA & (1<<UDRE)) )

;

/* Copy ninth 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 contents of the UCSRB is static. I.e. 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

communication mode or for other protocol handling as for example synchronization.

Transmitter Flags and

Interrupts

The USART Transmitter has two flags that indicate its state: USART Data Register

Empty (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 compatibility 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 (pro-

vided that global interrupts are enabled). UDRE is cleared by writing UDR. When

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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 interrupt will occur once the interrupt routine

terminates.

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 (pro-

vided that global interrupts are enabled). When the transmit complete interrupt is used,

the interrupt handling routine does not have to clear the TXC Flag, this is done automat-

ically 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 (i.e., when the Transmit Shift

Register and Transmit Buffer Register do not contain data to be transmitted). When dis-

abled, the Transmitter will no longer 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 Register 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 operation 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 5 to 8

Data Bits

The Receiver starts data reception when it detects a valid start bit. Each bit that follows

the start 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 (i.e., a complete serial frame

is present in the Receive Shift Register), 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. The example code assumes that the part specific header file is included.

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 9 Data If 9-bit characters are used (UCSZ=7) the ninth bit must be read from the RXB8 bit in

Bits

UCSRB 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.

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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 ninth 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)

ldi r16, LOW(-1)

USART_ReceiveNoError:

; Filter the ninth 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 ninth 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 ninth bit, then return */

resh = (resh >> 1) & 0x01;

return ((resh << 8) | resl);

}

Note:

1. The example code assumes that the part specific header file is included.

The receive function example reads all the I/O Registers into the Register File before

any computation 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.

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Receive Compete Flag and

Interrupt

The USART Receiver has one flag that indicates the Receiver state.

The Receive Complete (RXC) Flag indicates if there are unread data present in the

receive buffer. This flag is one when unread data exist in the receive buffer, and zero

when the receive buffer is empty (i.e., 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 interrupts 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. However, 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 condi-

tion. 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 Register 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 compatibility with future devices, always set this bit to zero when writing to UCSRA.

For more details see “Parity Bit Calculation” on page 135 and “Parity Checker” on page

144.

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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 par-

ity 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.

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 (i.e., 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 (i.e., 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 loca-

tion 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

rjmp USART_Flush

C Code Example⁽¹⁾

r16, UDR

void USART_Flush( void )

{

unsigned char dummy;

while ( UCSRA & (1<<RXC) ) dummy = UDR;

}

Note:

1. The example code assumes that the part specific header file is included.

Asynchronous Data

Reception

The USART includes a clock recovery and a data recovery unit for handling asynchro-

nous 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 samples and low pass filters each incoming bit, thereby improv-

ing 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. Fig-

ure 65 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 eight times the baud rate for Double

Speed mode. The horizontal arrows illustrate the synchronization variation due to the

sampling process. Note the larger time variation when using the Double Speed mode

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(U2X = 1) of operation. Samples denoted zero are samples done when the RxD line is

idle (i.e., no communication activity).

Figure 65. 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 samples 4, 5 and 6 for Double Speed mode (indicated with

sample numbers inside boxes on the 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 recovery 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 eight states for each bit in Double Speed mode. Figure 66 shows the sam-

pling 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 66. 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 67 shows the sampling of the stop bit and the earliest possible beginning of the

start bit of the next frame.

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Figure 67. 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 67. 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 detec-

tion influences the operational range of the Receiver.

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 53) 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 + 2)S

(D + 1)S + S

(D + 1)S

S – 1 + D S + S

R

= -----------------------------------

R

= ------------------------------------------

fast

slow

M

F

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 53 and Table 54 list the maximum Receiver baud rate error that can be tolerated.

Note that Normal Speed mode has higher toleration of baud rate variations.

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Table 53. 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(%) R_fast(%)

5

6

93,20

94,12

94,81

95,36

95,81

96,17

106,67

105,79

105,11

104,58

104,14

103,78

+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

± 2.0

± 1.5

7

8

9

10

Table 54. 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

94,12

94,92

95,52

96,00

96,39

96,70

105,66

104,92

104,35

103,90

103,53

103,23

+5.66/-5.88

+4.92/-5.08

+4.35/-4.48

+3.90/-4.00

+3.53/-3.61

+3.23/-3.30

± 2.5

± 2.0

± 1.5

± 1.0

7

8

9

10

The recommendations of the maximum Receiver baud rate error was made under the

assumption 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 sys-

tem clock (XTAL) will always have some minor instability over the supply voltage range

and the temperature 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% depend-

ing 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 fil-

tering 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 Trans-

mitter 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 indicates 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 Communi-

cation 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- 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 charac-

ter size setting. If 5- 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 Register 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 Examples⁽¹⁾

...

; Set UBRRH to 2

ldir16,0x02

outUBRRH,r16

...

; Set the USBS and the UCSZ1 bit to one, and

; the remaining bits to zero.

ldir16,(1<<URSEL)|(1<<USBS)|(1<<UCSZ1)

outUCSRC,r16

...

C Code Examples⁽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. The example code assumes that the part specific header file is included.

As the code examples illustrate, write accesses of the two registers are relatively unaf-

fected of the sharing of I/O location.

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Read Access

Doing a read access to the UBRRH or the UCSRC Register is a more complex opera-

tion. However, 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 sys-

tem clock cycle, reading 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 (e.g., by disabling interrupts globally) during the

read operation.

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. The example code assumes that the part specific header file is included.

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 ordinary 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 Register (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-, 6-, or 7-bit characters the upper unused bits will be ignored by the Transmitter

and set to zero by the Receiver.

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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 serially 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.

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

MPCM

R/W

0

TXC

R/W

0

U2X

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 (i.e. does not contain any unread data). If the Receiver is dis-

abled, 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 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 can generate a Transmit

Complete interrupt (see description 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

(i.e., 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

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This bit only has effect for the asynchronous operation. Write this bit to zero when using

synchronous operation.

Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effec-

tively doubling the transfer rate for asynchronous communication.

• Bit 0 – MPCM: Multi-processor Communication Mode

This bit enables the Multi-processor Communication mode. When the MPCM bit is writ-

ten to one, all the incoming frames received by the USART Receiver that do not contain

address information will be ignored. The Transmitter is unaffected by the MPCM setting.

For more detailed information see “Multi-processor Communication Mode” on page 148.

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 written 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 written 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 inter-

rupt 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 operation 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 nor-

mal 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 transmis-

sions are completed (i.e., 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

(Character 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.

• Bit 0 – TXB8: Transmit Data Bit 8

152

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ATmega8(L)

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 149 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 55. UMSEL Bit Settings

UMSEL

Mode

0

1

Asynchronous Operation

Synchronous Operation

153

2486M–AVR–12/03

• Bit 5:4 – UPM1:0: Parity Mode

These bits enable and set type of Parity Generation and Check. If enabled, the Trans-

mitter 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 com-

pare it to the UPM0 setting. If a mismatch is detected, the PE Flag in UCSRA will be set.

Table 56. 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 57. USBS Bit Settings

USBS

Stop Bit(s)

1-bit

0

1

2-bit

• Bit 2:1 – UCSZ1:0: Character Size

The UCSZ1:0 bits combined with the UCSZ2 bit in UCSRB sets the number of data bits

(Character Size) in a frame the Receiver and Transmitter use.

Table 58. 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

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ATmega8(L)

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 59. 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 UBRRHs

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 149 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.

• 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 eight least significant bits of the

USART baud rate. Ongoing transmissions by the Transmitter and Receiver will be cor-

rupted if the baud rate is changed. Writing UBRRL will trigger an immediate update of

the baud rate prescaler.

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2486M–AVR–12/03

Examples of Baud Rate

Setting

For standard crystal and resonator frequencies, the most commonly used baud rates for

asynchronous operation can be generated by using the UBRR settings in Table 60.

UBRR values 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 146). The error values are cal-

culated using the following equation:

BaudRate_{Closest Match}

Error[%] = ------------------------------------------------------- – 1 • 100%

BaudRate

Table 60. 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 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%

–

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.

156

ATmega8(L)

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ATmega8(L)

Table 61. 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

1M

–

0

–

0

1

–

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.

157

2486M–AVR–12/03

Table 62. Examples of UBRR Settings for Commonly Used Oscillator Frequencies (Continued)

f_osc= 8.0000 MHz f_osc= 11.0592 MHz f_osc= 14.7456 MHz

U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 1

UBRR UBRR UBRR UBRR UBRR UBRR

Baud

Rate

(bps)

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%

–

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

416

207

103

68

287

575

287

143

95

71

47

35

23

17

11

5

383

767

383

191

127

95

63

47

31

23

15

7

103

143

191

51

71

95

34

47

63

25

51

35

47

16

34

23

31

12

25

17

23

8

16

11

15

6

12

8

11

3

8

5

7

1

3

2

3

1

3

2

5

3

6

0.5M

1M

0

1

–

2

1

3

–

0

–

0

1

Max ⁽¹⁾

0.5 Mbps

1 Mbps

691.2 kbps

1.3824 Mbps

921.6 kbps

1.8432 Mbps

1.

UBRR = 0, Error = 0.0%

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ATmega8(L)

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ATmega8(L)

Table 63. 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

1M

1

3

–

4

–

4

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.

159

2486M–AVR–12/03

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 The Two-wire Serial Interface (TWI) is ideally suited for typical microcontroller applica-

tions. The TWI protocol allows the systems designer to interconnect up to 128 different

Bus Definition

devices using only two bi-directional bus lines, one for clock (SCL) and one for data

(SDA). The only external hardware 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 68. 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 64. 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.

Transmitter The device placing data on the bus.

Receiver The device reading data from the bus.

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Electrical Interconnection

As depicted in Figure 68, 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 opera-

tion 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 characteristics of the TWI is given in “Two-wire Serial Interface Character-

istics” on page 240. 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 69. 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 con-

dition. This is referred to as a REPEATED START condition, and is used when the

Master wishes to initiate a new transfer without relinquishing 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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2486M–AVR–12/03

Figure 70. 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 9 bits long, consisting of 7 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 per-

formed. 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 Master’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 consist-

ing 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 allo-

cated 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 mes-

sage 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 71. Address Packet Format

Addr MSB

Addr LSB

R/W

ACK

SDA

SCL

1

2

7

8

9

START

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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 sig-

nalled. 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 72. Data Packet Format

Data MSB

Data LSB

ACK

Aggregate

SDA

SDA from

Transmitter

SDA from

Receiver

SCL from

Master

1

2

7

8

9

STOP, REPEATED

START or Next

Data Byte

SLA+R/W

Data Byte

Combining Address and Data A transmission basically consists of a START condition, a SLA+R/W, one or more data

Packets into a Transmission

packets and a STOP condition. An empty message, consisting of a START followed by

a STOP condition, 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 73 shows a typical data transmission. Note that several data bytes can be trans-

mitted between the SLA+R/W and the STOP condition, depending on the software

protocol implemented by the application software.

Figure 73. 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

163

2486M–AVR–12/03

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-mas-

ter 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, i.e. 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 74. 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 out-

putting 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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ATmega8(L)

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ATmega8(L)

Figure 75. 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 composition 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.

165

2486M–AVR–12/03

Overview of the TWI

Module

The TWI module is comprised of several submodules, as shown in Figure 76. All regis-

ters drawn in a thick line are accessible through the AVR data bus.

Figure 76. 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

(TWBR)

Arbitration detection

Ack

Address Match Unit

Control Unit

Address Register

(TWAR)

Status Register

(TWSR)

Control Register

(TWCR)

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 inter-

nal 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.

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Bit Rate Generator Unit

This unit controls the period of SCL when operating in a Master mode. The SCL period

is controlled 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 Pres-

caler 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:

TWBR should be 10 or higher if the TWI operates in Master mode. If TWBR is lower than

10, the Master may produce an incorrect output on SDA and SCL for the reminder of the

byte. The problem occurs when operating the TWI in Master mode, sending Start + SLA

+ R/W to a Slave (a Slave does not need to be connected to the bus for the condition to

happen).

Bus Interface Unit

This unit contains the Data and Address Shift Register (TWDR), a START/STOP Con-

troller and Arbitration detection hardware. The TWDR contains the address or data

bytes to be transmitted, 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 Register is not directly accessible by the applica-

tion 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

continuously 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 seven-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 com-

pared 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 com-

pare addresses even when the AVR MCU is in sleep mode, enabling the MCU to wake

up if addressed by a Master. If another interrupt (e.g., INT0) occurs during TWI Power-

down address match and wakes up the CPU, the TWI aborts operation and return to it’s

idle state. If this cause any problems, ensure that TWI Address Match is the only

enabled interrupt when entering Power-down.

Control Unit

The Control unit monitors the TWI bus and generates responses corresponding to set-

tings 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 Status Register (TWSR) is updated with a status code identify-

ing the event. The TWSR only contains relevant status information when the TWI

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Interrupt Flag is asserted. At all other times, the TWSR contains a special status code

indicating that no relevant status information is available. 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.

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 167 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 appli-

cation 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 automatically cleared by hardware when executing the interrupt rou-

tine. Also note that clearing this flag starts the operation of the TWI, so all accesses to

the TWI Address Register (TWAR), TWI Status Register (TWSR), and TWI Data Regis-

ter (TWDR) must be complete before clearing this flag.

• Bit 6 – TWEA: TWI Enable Acknowledge Bit

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The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is writ-

ten 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 condition on the bus if it is free. However, if the bus is not free, the TWI waits

until a STOP condition 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 automatically. 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 activated for as long as the TWINT Flag is high.

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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 5 bits reflect the status of the TWI logic and the Two-wire Serial Bus. The differ-

ent status 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.

• Bits 1..0 – TWPS: TWI Prescaler Bits

These bits can be read and written, and control the bit rate prescaler.

Table 65. 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 167. 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 Register 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 constitute 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

6

5

4

3

2

1

0

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TWA6

R/W

1

TWA5

R/W

1

TWA4

R/W

1

TWA3

R/W

1

TWA2

R/W

1

TWA1

R/W

1

TWA0

R/W

1

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, and not needed in the Master modes. In multimaster 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 (0x00).

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.

• 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 generate 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 appli-

cation 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 77 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 descrip-

tion is quite abstract, a more detailed explanation follows later in this section. A simple

code example implementing the desired behavior is also presented.

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Figure 77. 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

signals into TWCR, makin sure that

TWINT is written to one,

5. Check TWSR to see if SLA+W was

sent and ACK received.

Application loads data into TWDR, and

loads appropriate control signals into

TWCR, making sure that TWINT is

written to one

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

and TWSTA is written to zero.

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.

Status code indicates

START condition sent

6. TWINT set.

Status code indicates

data sent, ACK received

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 condi-

tion has successfully 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 other-

wise, 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 SLA+W into TWDR. Remember that TWDR is used both for address and

data. After TWDR has been loaded with the 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

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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 opera-

tion 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 suc-

cessfully 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 appli-

cation has cleared TWINT, the TWI will initiate transmission of the STOP

condition. 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 transmis-

sions. 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

TWCR = (1<<TWINT)|(1<<TWSTA)|

Comments

ldi r16, (1<<TWINT)|(1<<TWSTA)|

1

2

Send START condition

(1<<TWEN)

out TWCR, r16

wait1:

(1<<TWEN)

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

r16,TWSR

if ((TWSR & 0xF8) != START)

3

Check value of TWI Status

Register. Mask prescaler bits. If

status different from START go to

ERROR

andi r16, 0xF8

cpi r16, START

brne ERROR

ERROR();

ldi r16, SLA_W

out TWDR, r16

ldi r16, (1<<TWINT) | (1<<TWEN)

out TWCR, r16

wait2:

TWDR = SLA_W;

Load SLA_W into TWDR Register.

Clear TWINT bit in TWCR to start

transmission of address

TWCR = (1<<TWINT) | (1<<TWEN);

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 r16, DATA

TWDR = DATA;

Load DATA into TWDR Register.

Clear TWINT bit in TWCR to start

transmission of data

out TWDR, r16

TWCR = (1<<TWINT) | (1<<TWEN);

ldi r16, (1<<TWINT) | (1<<TWEN)

out 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

andi r16, 0xF8

ERROR();

cpi r16, MT_DATA_ACK

brne ERROR

MT_DATA_ACK go to ERROR

ldi r16, (1<<TWINT)|(1<<TWEN)|

(1<<TWSTO)

TWCR = (1<<TWINT)|(1<<TWEN)|

(1<<TWSTO);

Transmit STOP condition

out 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 fig-

ures 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 79 to Figure 85, 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 appro-

priate software action. For each status code, the required software action and details of

the following serial transfer are given in Table 66 to Table 69. Note that the prescaler

bits are masked to zero in these tables.

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Master Transmitter Mode

In the Master Transmitter mode, a number of data bytes are transmitted to a Slave

Receiver (see Figure 78). 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 transmitted, 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 78. Data Transfer in Master Transmitter Mode

V_CC

Device 1

MASTER

TRANSMITTER

Device 2

SLAVE

RECEIVER

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 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 transmit-

ted, the TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (see

Table 66). In order to enter MT mode, SLA+W must be transmitted. This is done by writ-

ing 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 sta-

tus codes in Master mode are 0x18, 0x20, or 0x38. The appropriate action to be taken

for each of these status codes is detailed in Table 66.

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 Register. 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

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This scheme is repeated until the last byte has been sent and the transfer is ended by

generating a STOP condition or a repeated START condition. A STOP condition is gen-

erated 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 0x10) 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 66. Status codes for Master Transmitter Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Application Software Response

To/from TWDR To TWCR

STO TWINT

Status of the Two-wire Serial

Bus and Two-wire Serial Inter-

face Hardware

STA

0

TWEA

X

Next Action Taken by TWI Hardware

0x08

A START condition has been Load SLA+W

transmitted

0

1

SLA+W will be transmitted;

ACK or NOT ACK will be received

0x10

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

0x18

0x20

0x28

0x30

0x38

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

177

2486M–AVR–12/03

Figure 79. 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

178

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Master Receiver Mode

In the Master Receiver mode, a number of data bytes are received from a Slave Trans-

mitter (see Figure 80). 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 transmitted, 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 80. 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 hardware, and the status code in TWSR will be 0x08 (See Table

66). 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 sta-

tus codes in Master mode are 0x38, 0x40, or 0x48. The appropriate action to be taken

for each of these status codes is detailed in Table 67. 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

179

2486M–AVR–12/03

After a repeated START condition (state 0x10) 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 67. 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

0

STO

0

TWINT

1

TWEA

X

Next Action Taken by TWI Hardware

0x08

A START condition has been Load SLA+R

transmitted

SLA+R will be transmitted

ACK or NOT ACK will be received

0x10

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

0x38

0x40

0x48

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

0x50

0x58

Data byte has been received;

ACK has been returned

Read data byte or

Read data byte

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

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

180

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 81. 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

Slave Receiver Mode

In the Slave Receiver mode, a number of data bytes are received from a Master Trans-

mitter (see Figure 82). All the status codes mentioned in this section assume that the

prescaler bits are zero or are masked to zero.

Figure 82. 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

181

2486M–AVR–12/03

The upper 7 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 (0x00), 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 software action. The appropriate action to be taken for each

status code is detailed in Table 68. The Slave Receiver mode may also be entered if

arbitration is lost while the TWI is in the Master mode (see states 0x68 and 0x78).

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 car-

ried 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.

182

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Table 68. 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

0x60

0x68

0x70

0x78

Own SLA+W 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; 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

0x80

0x88

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

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

0x90

0x98

Previously addressed with

general call; data has been re-

ceived; ACK has been returned

Read data byte or

X

0

1

0

Data byte will be received and NOT ACK will be

returned

Data byte will be received and ACK will be returned

Read data byte

X

0

1

0

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

0xA0

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

183

2486M–AVR–12/03

Figure 83. 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

184

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Slave Transmitter Mode

In the Slave Transmitter mode, a number of data bytes are transmitted to a Master

Receiver (see Figure 84). All the status codes mentioned in this section assume that the

prescaler bits are zero or are masked to zero.

Figure 84. 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

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 (0x00), 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 software action. The appropriate action to be taken for each

status code is detailed in Table 69. The Slave Transmitter mode may also be entered if

arbitration is lost while the TWI is in the Master mode (see state 0xB0).

If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of

the transfer. State 0xC0 or state 0xC8 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 0xC8 is entered if the Master

demands additional data bytes (by transmitting ACK), even though the Slave has trans-

mitted 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.

185

2486M–AVR–12/03

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.

Table 69. Status Codes for Slave 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 Interface

Hardware

To/from TWDR

Load data byte or

Load data byte

STA

X

STO

0

TWINT

1

TWEA

0

Next Action Taken by TWI Hardware

0xA8

0xB0

0xB8

0xC0

Own SLA+R has been received;

ACK has been returned

Last data byte will be transmitted and NOT ACK should

be received

X

0

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

0xC8

Last data byte in TWDR has been 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

transmitted (TWEA

has been received

= “0”); ACK

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

186

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 85. 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 70.

Status 0xF8 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 0x00 indicates that a bus error has occurred during a Two-wire Serial Bus trans-

fer. 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 70. Miscellaneous States

Status Code

Application Software Response

(TWSR)

Prescaler Bits

are 0

Status of the Two-wire Serial

Bus and Two-wire Serial Inter-

face Hardware

To TWCR

STO TWINT

No TWCR action

To/from TWDR

STA

0

TWEA

X

Next Action Taken by TWI Hardware

Wait or proceed current transfer

0xF8

No relevant state information No TWDR action

available; TWINT = “0”

0x00

Bus error due to an illegal No TWDR action

START or STOP condition

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. Sub-

sequently, 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 prin-

ciple is violated in a multimaster system, 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 86. 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 simul-

taneously 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 trans-

fer, 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 87. An Arbitration Example

V_CC

Device 1

MASTER

TRANSMITTER

Device 3

SLAVE

RECEIVER

Device 2

MASTER

TRANSMITTER

Device n

R1

R2

........

SDA

SCL

Several different scenarios may arise during arbitration, as described below:

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ATmega8(L)

•

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 88. Possible status values are given in circles.

Figure 88. 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 nega-

tive 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 comparator output rise, fall or toggle. A block dia-

gram of the comparator and its surrounding logic is shown in Figure 89.

Figure 89. Analog Comparator Block Diagram⁽²⁾

BANDGAP

REFERENCE

ACBG

ACME

ADEN

ADC MULTIPLEXER

OUTPUT⁽¹⁾

Notes: 1. See Table 72 on page 192.

2. Refer to “Pin Configurations” on page 2 and Table 28 on page 61 for Analog Compar-

ator pin placement.

Special Function IO Register –

SFIOR

Bit

7

–

6

–

5

–

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

R

0

R

0

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 Compar-

ator. For a detailed description of this bit, see “Analog Comparator Multiplexed Input” on

page 192.

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ATmega8(L)

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

ACIS0

R/W

0

ACD

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 Compar-

ator 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 Comparator. See “Internal Voltage Reference” on page 40.

• 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 execut-

ing the corresponding interrupt 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 Ana-

log Comparator 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 triggered 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 inter-

rupt, 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 inter-

rupt. The different settings are shown in Table 71.

Table 71. 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

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When changing the ACIS1/ACIS0 bits, the Analog Comparator Interrupt must be dis-

abled 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⁽¹⁾pins to replace the negative input to the

Analog Comparator. 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 Multi-

plexer 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 72. If ACME is cleared or ADEN is set,

AIN1 is applied to the negative input to the Analog Comparator.

Table 72. Analog Comparator Multiplexed Input⁽¹⁾

ACME

ADEN

MUX2..0

xxx

Analog Comparator Negative Input

0

1

x

1

0

AIN1

xxx

AIN1

000

001

010

011

100

101

110

111

ADC0

ADC1

ADC2

ADC3

ADC4

ADC5

ADC6

ADC7

Note:

1. ADC7..6 are only available in TQFP and MLF Package.

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ATmega8(L)

Analog-to-Digital

Converter

Features

• 10-bit Resolution (8-bit Accuracy on ADC4 and ADC5)

• 0.5 LSB Integral Non-linearity

• ± 2 LSB Absolute Accuracy

• 65 - 260 µs Conversion Time

• Up to 15 kSPS at Maximum Resolution

• 6 Multiplexed Single Ended Input Channels

• 2 Additional Multiplexed Single Ended Input Channels (TQFP and MLF Package only)

• 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

• Interrupt on ADC Conversion Complete

• Sleep Mode Noise Canceler

The ATmega8 features a 10-bit successive approximation ADC. The ADC is connected

to an 8-channel Analog Multiplexer which allows eight single-ended voltage inputs con-

structed from the pins of Port C. The single-ended voltage inputs refer to 0V (GND).

Note that ADC channels ADC4 and ADC5 are limited to 8-bit accuracy. Channels

ADC[3:0] and ADC[7:6] offer full 10-bit accuracy.

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 90.

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 198 on how to

connect this pin.

Internal reference voltages of nominally 2.56V or AV_CCare provided On-chip. The volt-

age reference may be externally decoupled at the AREF pin by a capacitor for better

noise performance.

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Figure 90. Analog to Digital Converter Block Schematic Operation

ADC CONVERSION

COMPLETE IRQ

8-BIT DATA BUS

15

0

ADC MULTIPLEXER

SELECT (ADMUX)

ADC DATA REGISTER

(ADCH/ADCL)

ADC CTRL. & STATUS

REGISTER (ADCSRA)

MUX DECODER

PRESCALER

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

ADC MULTIPLEXER

OUTPUT

INPUT

MUX

The ADC converts an analog input voltage to a 10-bit digital value through successive

approximation. 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 refer-

ence 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 is 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. 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.

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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 con-

version 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.

In Free Running mode, the ADC is constantly sampling and updating the ADC Data

Register. Free Running mode is selected by writing the ADFR bit in ADCSRA to one.

The first conversion must be started by writing a logical one to the ADSC bit in ADC-

SRA. In this mode the ADC will perform successive conversions independently of

whether the ADC Interrupt Flag, ADIF is cleared or not.

Prescaling and

Figure 91. 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 fre-

quency 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 con-

version starts at the following rising edge of the ADC clock cycle. A normal conversion

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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

conversion and 13.5 ADC clock cycles after the start of an first conversion. When a con-

version 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.

In Free Running mode, a new conversion will be started immediately after the conver-

sion completes, while ADSC remains high. For a summary of conversion times, see

Table 73.

Figure 92. 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

MUX and REFS

Update

Sample & Hold

Figure 93. 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

Complete

MUX and REFS

Update

MUX and REFS

Update

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Figure 94. 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 73. ADC Conversion Time

Sample & Hold (Cycles

from Start of Conversion)

Conversion Time

(Cycles)

Condition

Extended conversion

Normal conversions, single ended

13.5

1.5

25

13

Changing Channel or

Reference Selection

The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a tem-

porary 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 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. Continuous 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 both ADFR 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 ADFR 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.

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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 channel 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 channel 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 automati-

cally, the next result will reflect the previous channel selection. Subsequent conversions

will reflect the new channel selection.

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 generated 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 AV_CCand 2.56V as reference selection. The first ADC conversion result after

switching reference voltage source may be inaccurate, and the user is advised to dis-

card this result.

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 Con-

version 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 con-

version 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 con-

version 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 entering such sleep modes to avoid excessive power consumption.

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Analog Input Circuitry

The analog input circuitry for single ended channels is illustrated in Figure 95. An analog

source applied to ADCn is subjected to the pin capacitance and input leakage of that

pin, regardless of whether that channel is selected as input for the ADC. When the chan-

nel 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 impedance 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 recom-

mended to only use low impedant sources with slowly varying signals, since this

minimizes the required charge transfer to the S/H capacitor.

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 95. Analog Input Circuitry

I_IH

ADCn

1..100 kΩ

C_S/H= 14 pF

I_IL

V_CC/2

Analog Noise Canceling

Techniques

Digital circuitry inside and outside the device generates EMI which might affect the

accuracy of 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. Make sure analog tracks run

over the analog ground plane, and 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 96.

3. Use the ADC noise canceler function to reduce induced noise from the CPU.

4. If any ADC [3..0] port pins are used as digital outputs, it is essential that

these do not switch while a conversion is in progress. However, using the

Two-wire Interface (ADC4 and ADC5) will only affect the conversion on

ADC4 and ADC5 and not the other ADC channels.

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Figure 96. ADC Power Connections

PC1 (ADC1)

PC0 (ADC0)

ADC7

GND

AREF

ADC6

AVCC

PB5

ADC Accuracy Definitions

An n-bit single-ended ADC converts a voltage linearly between GND and V_REFin 2ⁿ

steps (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.

Figure 97. Offset Error

Output Code

Ideal ADC

Actual ADC

Offset

Error

V_REF

Input Voltage

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ATmega8(L)

•

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 98. Gain Error

Gain

Error

Output Code

Ideal ADC

Actual ADC

V

Input Voltage

REF

•

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.

Figure 99. 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.

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Figure 100. 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.

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 refer-

ence (see Table 74 on page 203 and Table 75 on page 203). 0x000 represents analog

ground, and 0x3FF represents the selected reference voltage minus one LSB.

ADC Multiplexer Selection

Register – ADMUX

Bit

7

REFS1

R/W

0

6

REFS0

R/W

0

5

ADLAR

R/W

0

4

–

3

MUX3

R/W

0

2

MUX2

R/W

0

1

MUX1

R/W

0

MUX0

R/W

0

ADMUX

Read/Write

Initial Value

R

0

• Bit 7:6 – REFS1:0: Reference Selection Bits

These bits select the voltage reference for the ADC, as shown in Table 74. 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.

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Table 74. Voltage Reference Selections for ADC

REFS1

REFS0 Voltage Reference Selection

0

1

0

1

0

1

AREF, Internal V_refturned 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 conversions. For a complete description of this bit, see “The

ADC Data Register – ADCL and ADCH” on page 205.

• Bits 3:0 – MUX3:0: Analog Channel Selection Bits

The value of these bits selects which analog inputs are connected to the ADC. See

Table 75 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 75. Input Channel Selections

MUX3..0

0000

0001

0010

0011

0100

0101

0110

0111

1000

1001

1010

1011

1100

1101

1110

1111

Single Ended Input

ADC0

ADC1

ADC2

ADC3

ADC4

ADC5

ADC6

ADC7

1.23V (V_BG

0V (GND)

)

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ADC Control and Status

Register A – ADCSRA

Bit

7

ADEN

R/W

0

6

ADSC

R/W

0

5

ADFR

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

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. Turn-

ing 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 Run-

ning 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 initialization 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 – ADFR: ADC Free Running Select

When this bit is set (one) the ADC operates in Free Running mode. In this mode, the

ADC samples and updates the Data Registers continuously. Clearing this bit (zero) will

terminate Free Running mode.

• 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. Alternatively, 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 dis-

abled. 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 Com-

plete Interrupt is activated.

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• 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.

Table 76. 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

ADCL

ADC7

ADC6

ADC5

ADC4

ADC3

ADC2

ADC1

ADC0

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

ADCL

ADC1

ADC0

–

5

–

4

–

3

–

2

–

1

–

0

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.

When ADCL is read, the ADC Data Register is not updated until ADCH is read. Conse-

quently, 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 202.

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Boot Loader Support The Boot Loader Support provides a real Read-While-Write Self-Programming mecha-

nism for downloading and uploading program code by the MCU itself. This feature

allows flexible application software updates controlled by the MCU using a Flash-resi-

dent Boot Loader program. The Boot Loader program can use any available data

– Read-While-Write

Self-Programming

interface and associated protocol to read code and write (program) that code into the

Flash memory, or read the code from the Program memory. 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 different levels of protection.

Boot Loader 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 93 on page

224) 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 102). The size of the different sections is configured by

the BOOTSZ Fuses as shown in Table 82 on page 217 and Figure 102. These two sec-

tions can have different 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 78 on page 209. The application section

can never store any Boot Loader code since the SPM instruction is disabled when exe-

cuted from the application section.

BLS – Boot Loader Section

While the application section is used for storing the application code, the The Boot

Loader software must be located in the BLS since the SPM instruction can initiate a pro-

gramming 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 79 on page

209.

Read-While-Write and No Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot

Loader software 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 83 on page 218 and Figure 102 on page 208. The

main difference between the two sections is:

Read-While-Write Flash

Sections

•

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.

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Note that the user software can never read any code that is located inside the RWW

section during a Boot Loader software operation. The syntax “Read-While-Write sec-

tion” 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.

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 sec-

tion. 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 (i.e. by a call/rjmp/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 Section. 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 210. for details on how to clear RWWSB.

NRWW – No Read-While-Write The code located in the NRWW section can be read when the Boot Loader software is

Section

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 77. 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 101. Read-While-Write vs. No Read-While-Write

Read-While-Write

(RWW) Section

Z-pointer

Addresses NRWW

section

Z-pointer

No Read-While-Write

(NRWW) Section

Addresses RWW

section

CPU is Halted

during the Operation

Code Located in

NRWW Section

Can be Read during

the Operation

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Figure 102. 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

$0000

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 are given in Table 82 on page 217.

Boot Loader Lock Bits

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 indepen-

dently. This gives the user a unique flexibility to select different levels of protection.

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 78 and Table 79 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 program-

ming of the Flash memory 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 78. 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 79. 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

1

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.

Note:

1. “1” means unprogrammed, “0” means programmed

Entering the Boot Loader 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 inter-

Program

face. Alternatively, 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 application code is loaded, the program can start execut-

ing the application code. Note that the fuses cannot be changed by the MCU itself. This

means that once the Boot Reset Fuse is programmed, 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 80. Boot Reset Fuse⁽¹⁾

BOOTRST

Reset Address

1

0

Reset Vector = Application Reset (address 0x0000)

Reset Vector = Boot Loader Reset (see Table 82 on page 217)

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 cannot 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 ATmega8 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 written 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

Register, 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 Soft-

ware” on page 214 for details.

• 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

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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 special 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 93 on page 224), the Program Counter

can be treated as having two different sections. One section, consisting of the least sig-

nificant bits, is addressing the words within a page, while the most significant bits are

addressing the pages. This is shown in Figure 103. 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 103. Addressing the Flash during SPM⁽¹⁾

BIT 15

Z - REGISTER

ZPCMSB

ZPAGEMSB

1

0

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 the figure are listed in Table 84 on page 218.

2. PCPAGE and PCWORD are listed in Table 93 on page 224.

Self-Programming the

Flash

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 buffer 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:

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 alternative 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

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Assembly Code Example for a Boot Loader” on page 216 for an assembly code

example.

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 will be ignored 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 Buffer

(Page Loading)

To write an instruction word, set up the address in the Z-pointer and data in R1:R0, write

“00000001” to SPMCR and execute SPM within four clock cycles after writing SPMCR.

The content 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 pos-

sible 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 to 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 44.

Consideration While Updating Special care must be taken if the user allows the Boot Loader section to be updated by

BLS

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 44, 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 216 for an example.

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Setting the Boot Loader Lock To set the Boot Loader Lock Bits, write the desired data to R0, write “X0001001” to

Bits by SPM

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 78 and Table 79 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 0x0001 (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 programming 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 Lock

Bits from Software

It is possible to read both the Fuse and Lock Bits from software. To read the Lock Bits,

load the Z-pointer with 0x0001 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 regis-

ter. 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 BLBSET and SPMEN are cleared, LPM will

work as described in the Instruction set Manual.

Bit

7

6

5

4

3

2

1

0

Rd

–

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 0x0000 and set

the BLBSET and SPMEN bits in SPMCR. When an LPM instruction is executed within

three cycles after the BLBSET 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 88 on page 221 for a detailed description and mapping of the fuse low bits.

Bit

7

6

5

4

3

2

1

0

Rd

FLB7

FLB6

FLB5

FLB4

FLB3

FLB2

FLB1

FLB0

Similarly, when reading the Fuse High bits, load 0x0003 in the Z-pointer. When an LPM

instruction 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 reg-

ister as shown below. Refer to Table 87 on page 220 for detailed description and

mapping of the fuse high bits.

Bit

7

6

5

4

3

2

1

0

Rd

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.

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ATmega8(L)

Preventing Flash Corruption

During periods of low V_CC,the Flash program can be corrupted because the supply volt-

age 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.

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 cor-

rectly. 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 voltage matches the detection level. If not, an external low V_CC

Reset 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 prevent the CPU from attempting to decode and execute instructions, effec-

tively protecting the SPMCR Register and thus the Flash from unintentional

writes.

Programming Time for Flash

when using SPM

The calibrated RC Oscillator is used to time Flash accesses. Table 81 shows the typical

programming time for Flash accesses from the CPU.

Table 81. SPM Programming Time

Symbol

Min Programming Time Max Programming Time

3.7 ms 4.5 ms

Flash write (page erase, page write,

and write Lock Bits by SPM)

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2486M–AVR–12/03

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

words

;PAGESIZEB is page size in BYTES, not

.org SMALLBOOTSTART

Write_page:

; page erase

ldi spmcrval, (1<<PGERS) | (1<<SPMEN)

rcallDo_spm

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)

rcallDo_spm

; transfer data from RAM to Flash page buffer

ldi looplo, low(PAGESIZEB)

;init loop variable

ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256

Wrloop:

ld

r0, Y+

r1, Y+

ldi spmcrval, (1<<SPMEN)

rcallDo_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)

rcallDo_spm

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)

rcallDo_spm

; read back and check, optional

ldi looplo, low(PAGESIZEB)

;init loop variable

ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256

subi YL, low(PAGESIZEB)

sbci YH, high(PAGESIZEB)

Rdloop:

;restore pointer

lpm r0, Z+

ld

r1, Y+

cpse r0, r1

rjmp Error

216

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

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

sbrs temp1, RWWSB

not ready yet

; If RWWSB is set, the RWW section is

ret

; re-enable the RWW section

ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)

rcallDo_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 SPMCR, spmcrval

spm

; restore SREG (to enable interrupts if originally enabled)

out SREG, temp2

ret

ATmega8 Boot Loader

Parameters

In Table 82 through Table 84, the parameters used in the description of the self pro-

gramming are given.

Table 82. Boot Size Configuration

Boot Reset

Boot

Loader

Flash

Address

(Start Boot

Loader

Application

Flash

Section

End

Application

Section

Boot

Size

BOOTSZ1

BOOTSZ0

Pages

Section

Section)

128

words

0x000 -

0xF7F

0xF80 -

0xFFF

1

4

0xF7F

0xEFF

0xDFF

0xBFF

0xF80

0xF00

0xE00

0xC00

256

words

0x000 -

0xEFF

0xF00 -

0xFFF

1

0

1

0

8

512

words

0x000 -

0xDFF

0xE00 -

0xFFF

16

32

1024

words

0x000 -

0xBFF

0xC00 -

0xFFF

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2486M–AVR–12/03

Note:

The different BOOTSZ Fuse configurations are shown in Figure 102.

Table 83. Read-While-Write Limit

Section

Pages

96

Address

Read-While-Write section (RWW)

No Read-While-Write section (NRWW)

0x000 - 0xBFF

0xC00 - 0xFFF

32

For details about these two section, see “NRWW – No Read-While-Write Section” on

page 207 and “RWW – Read-While-Write Section” on page 207

Table 84. Explanation of Different Variables used in Figure 103 and the Mapping to the

Z-pointer

Corresponding

Variable

Z-value⁽¹⁾

Description

PCMSB

11

4

Most significant bit in the Program Counter.

(The Program Counter is 12 bits PC[11:0])

PAGEMSB

ZPCMSB

Most significant bit which is used to address the

words within one page (32 words in a page

requires 5 bits PC [4:0]).

Z12

Z5

Bit in Z-register that is mapped to PCMSB.

Because Z0 is not used, the ZPCMSB equals

PCMSB + 1.

ZPAGEMSB

Bit in Z-register that is mapped to PAGEMSB.

Because Z0 is not used, the ZPAGEMSB

equals PAGEMSB + 1.

PCPAGE

PCWORD

PC[11:5]

PC[4:0]

Z12:Z6

Z5: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)

Note:

1. Z15:Z13: 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 211 for details about

the use of Z-pointer during Self-Programming.

218

ATmega8(L)

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ATmega8(L)

Memory

Programming

Program And Data

Memory Lock Bits

The ATmega8 provides six Lock Bits which can be left unprogrammed (“1”) or can be

programmed (“0”) to obtain the additional features listed in Table 86. The Lock Bits can

only be erased to “1” with the Chip Erase command.

Table 85. Lock Bit Byte

Lock Bit Byte

Bit No.

Description

–

Default Value⁽¹⁾

7

6

5

4

3

2

1

0

1 (unprogrammed)

–

BLB12

Boot lock bit

Lock bit

BLB11

BLB02

BLB01

LB2

LB1

Lock bit

Note:

Table 86. Lock Bit Protection Modes⁽²⁾

Memory Lock Bits Protection Type

1. “1” means unprogrammed, “0” means programmed

LB Mode

LB2

LB1

1

No memory lock features enabled.

Further programming of the Flash and EEPROM is

disabled in Parallel and 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 Serial Programming

mode. The Fuse Bits are locked in both Serial and Parallel

Programming modes.⁽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.

BLB1 Mode BLB12 BLB11

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2486M–AVR–12/03

Table 86. Lock Bit Protection Modes⁽²⁾(Continued)

Memory Lock Bits

Protection Type

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 ATmega8 has two fuse bytes. Table 87 and Table 88 describe briefly the functional-

ity of all the fuses and how they are mapped into the fuse bytes. Note that the fuses are

read as logical zero, “0”, if they are programmed.

Table 87. Fuse High Byte

Fuse High

Byte

Bit

No.

Description

Default Value

1 (unprogrammed, PC6 is

Select if PC6 is I/O pin or RESET pin RESET-pin)

RSTDISBL⁽⁴⁾

7

6

1 (unprogrammed, WDT

enabled by WDTCR)

WDTON

WDT always on

Enable Serial Program and Data

Downloading

0 (programmed, SPI prog.

enabled)

SPIEN⁽¹⁾

CKOPT⁽²⁾

EESAVE

5

4

3

Oscillator options

1 (unprogrammed)

EEPROM memory is preserved

through the Chip Erase

1 (unprogrammed,

EEPROM not preserved)

Select Boot Size (see Table 82 for

details)

BOOTSZ1

2

0 (programmed)⁽³⁾

Select Boot Size (see Table 82 for

details)

BOOTSZ0

BOOTRST

1

0

0 (programmed)⁽³⁾

1 (unprogrammed)

Select Reset Vector

Notes: 1. The SPIEN Fuse is not accessible in Serial Programming mode.

2. The CKOPT Fuse functionality depends on the setting of the CKSEL bits, see “Clock

Sources” on page 24 for details.

3. The default value of BOOTSZ1..0 results in maximum Boot Size. See Table 82 on

page 217.

4. When programming the RSTDISBL Fuse Parallel Programming has to be used to

change fuses or perform further programming.

220

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Table 88. 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

Brown out detector enable

Select start-up time

Select Clock source

1 (unprogrammed)

1 (unprogrammed, BOD disabled)

1 (unprogrammed)⁽¹⁾

0 (programmed)⁽¹⁾

SUT0

CKSEL3

CKSEL2

CKSEL1

CKSEL0

0 (programmed)⁽²⁾

1 (unprogrammed)⁽²⁾

Notes: 1. The default value of SUT1..0 results in maximum start-up time. SeeTable 10 on page

28 for details.

2. The default setting of CKSEL3..0 results in internal RC Oscillator @ 1MHz. See

Table 2 on page 24 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 3-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 ATmega8 the signature bytes are:

1. 0x000: 0x1E (indicates manufactured by Atmel).

2. 0x001: 0x93 (indicates 8KB Flash memory).

3. 0x002: 0x07 (indicates ATmega8 device).

Calibration Byte

The ATmega8 stores four different calibration values for the internal RC Oscillator.

These bytes resides in the signature row High byte of the addresses 0x000, 0x0001,

0x0002, and 0x0003 for 1, 2, 4, and 8 Mhz respectively. During Reset, the 1 MHz value

is automatically loaded into the OSCCAL Register. If other frequencies are used, the

calibration value has to be loaded manually, see “Oscillator Calibration Register – OSC-

CAL” on page 29 for details.

221

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Parallel Programming

Parameters, Pin

Mapping, and

This section describes how to parallel program and verify Flash Program memory,

EEPROM Data memory, Memory Lock Bits, and Fuse Bits in the ATmega8. Pulses are

assumed to be at least 250 ns unless otherwise noted.

Commands

Signal Names

In this section, some pins of the ATmega8 are referenced by signal names describing

their functionality during parallel programming, see Figure 104 and Table 89. 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 posi-

tive pulse. The bit coding is shown in Table 91.

When pulsing WR or OE, the command loaded determines the action executed. The dif-

ferent Commands are shown in Table 92.

Figure 104. Parallel Programming

+5V

RDY/BSY

OE

PD1

PD2

PD3

PD4

PD5

PD6

PD7

VCC

+5V

AVCC

WR

BS1

PC[1:0]:PB[5:0]

DATA

XA0

XA1

PAGEL

+12 V

BS2

RESET

PC2

XTAL1

GND

Table 89. 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

XA1

PD5

PD6

I

XTAL Action Bit 0

XTAL Action Bit 1

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ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Table 89. Pin Name Mapping (Continued)

Signal Name in

Programming Mode

Pin Name

I/O Function

Program memory and EEPROM Data

Page Load

PAGEL

PD7

I

Byte Select 2 (“0” selects Low byte, “1”

selects 2’nd High byte)

BS2

PC2

Bi-directional Data bus (Output when OE is

low)

DATA

{PC[1:0]: PB[5:0]} I/O

Table 90. 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 91. XA1 and XA0 Coding

XA1 XA0 Action when XTAL1 is Pulsed

Load Flash or EEPROM Address (High or low address byte determined by

BS1)

0

1

0

1

Load Data (High or Low data byte for Flash determined by BS1)

Load Command

No Action, Idle

Table 92. Command Byte Bit Coding

Command Byte

1000 0000

0100 0000

0010 0000

0001 0000

0001 0001

0000 1000

0000 0100

0000 0010

0000 0011

Command Executed

Chip Erase

Write Fuse Bits

Write Lock Bits

Write Flash

Write EEPROM

Read Signature Bytes and Calibration byte

Read Fuse and Lock Bits

Read Flash

Read EEPROM

223

2486M–AVR–12/03

Table 93. No. of Words in a Page and no. of Pages in the Flash

Flash Size

Page Size

PCWORD

No. of Pages

PCPAGE

PCMSB

4K words (8K bytes)

32 words

PC[4:0]

128

PC[11:5]

11

Table 94. 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 Programming

Enter Programming Mode

The following algorithm puts the device in Parallel Programming mode:

1. Apply 4.5 - 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 90 on page 223 to “0000” and wait at

least 100 ns.

4. Apply 11.5 - 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 Pro-

gramming mode.

Note, if the RESET pin is disabled by programming the RSTDISBL Fuse, it may not be

possible to follow the proposed algorithm above. The same may apply when External

Crystal or External RC configuration is selected because it is not possible to apply qual-

ified XTAL1 pulses. In such cases, the following algorithm should be followed:

1. Set Prog_enable pins listed in Table 90 on page 223 to “0000”.

2. Apply 4.5 - 5.5V between V_CCand GND simultaneously as 11.5 - 12.5V is

applied to RESET.

3. Wait 100 ns.

4. Re-program the fuses to ensure that External Clock is selected as clock source

(CKSEL3:0 = 0’b0000) and RESET pin is activated (RSTDISBL) unpro-

grammed). 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

0’b0.

6. Entering Programming mode with the original algorithm, as described above.

Considerations for Efficient

Programming

The loaded command and address are retained in the device during programming. For

efficient 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 0xFF, 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.

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ATmega8(L)

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.

Programming the Flash

The Flash is organized in pages, see Table 93 on page 224. When programming the

Flash, the program data is latched into a page buffer. This allows one page of program

data to be programmed simultaneously. The following procedure describes how to pro-

gram 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 (0x00 - 0xFF).

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 (0x00 - 0xFF).

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 (0x00 - 0xFF).

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 106 for

signal waveforms)

F. Repeat B through E until the entire buffer is filled or until all data within the page is

loaded.

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2486M–AVR–12/03

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 105 on page 226. Note

that if less than eight 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 (0x00 - 0xFF).

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 106 for signal waveforms)

I. Repeat B through H until the entire Flash is programmed or until all data has been

programmed.

J. End Page Programming

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 sig-

nals are reset.

Figure 105. Addressing the Flash which is Organized in Pages⁽¹⁾

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 93 on page 224.

226

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 106. Programming the Flash Waveforms⁽¹⁾

F

A

B

C

D

E

B

C

D

E

G

H

0x10

ADDR. LOW DATA LOW DATA HIGH

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 94 on page 224. 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 225 for details on

Command, Address and Data loading):

1. A: Load Command “0001 0001”.

2. G: Load Address High byte (0x00 - 0xFF).

3. B: Load Address Low byte (0x00 - 0xFF).

4. C: Load Data (0x00 - 0xFF).

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 107 for signal waveforms).

227

2486M–AVR–12/03

Figure 107. 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 225 for details on Command and Address loading):

1. A: Load Command “0000 0010”.

2. G: Load Address High byte (0x00 - 0xFF).

3. B: Load Address Low byte (0x00 - 0xFF).

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 225 for details on Command and Address loading):

1. A: Load Command “0000 0011”.

2. G: Load Address High byte (0x00 - 0xFF).

3. B: Load Address Low byte (0x00 - 0xFF).

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 225 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 and BS2 to “0”.

4. Give WR a negative pulse and wait for RDY/BSY to go high.

228

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Programming the Fuse High

Bits

The algorithm for programming the Fuse high bits is as follows (refer to “Programming

the Flash” on page 225 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.

Programming the Lock Bits

The algorithm for programming the Lock Bits is as follows (refer to “Programming the

Flash” on page 225 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 Lock

Bits

The algorithm for reading the Fuse and Lock Bits is as follows (refer to “Programming

the Flash” on page 225 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”.

Figure 108. Mapping Between BS1, BS2 and the Fuse- and Lock Bits During Read

Fuse low byte

Lock bits

0

DATA

0

1

Fuse high byte

1

BS1

BS2

229

2486M–AVR–12/03

Reading the Signature Bytes

The algorithm for reading the Signature bytes is as follows (refer to “Programming the

Flash” on page 225 for details on Command and Address loading):

1. A: Load Command “0000 1000”.

2. B: Load Address Low byte (0x00 - 0x02).

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 bytes is as follows (refer to “Programming the

Flash” on page 225 for details on Command and Address loading):

1. A: Load Command “0000 1000”.

2. B: Load Address Low byte, (0x00 - 0x03).

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

Characteristics

Figure 109. Parallel Programming Timing, Including some General Timing

Requirements

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

Figure 110. 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 109 (i.e., t_DVXH, t_XHXL, and t_XLDX) also apply

to loading operation.

230

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 111. 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 109 (i.e., t_DVXH, t_XHXL, and t_XLDX) also apply

to reading operation.

Table 95. 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

Data and Control Valid before XTAL1 High

XTAL1 Low to XTAL1 High

XTAL1 Pulse Width High

11.5

I_PP

µA

ns

µs

ms

ns

t_DVXH

t_XLXH

t_XHXL

t_XLDX

t_XLWL

t_XLPH

t_PLXH

t_BVPH

t_PHPL

t_PLBX

t_WLBX

t_PLWL

t_BVWL

t_WLWH

t_WLRL

t_WLRH

t_{WLRH_CE}

t_XLOL

67

200

150

67

0

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

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

3.7

7.5

0

231

2486M–AVR–12/03

Table 95. Parallel Programming Characteristics, V_CC= 5V ± 10% (Continued)

Symbol

t_BVDV

Parameter

Min

Typ

Max

250

Units

ns

BS1 Valid to DATA valid

OE Low to DATA Valid

OE High to DATA Tri-stated

0

t_OLDV

ns

t_OHDZ

ns

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 96 on page 232, the pin mapping for SPI programming is listed. Not all parts use

the SPI pins dedicated for the internal SPI interface.

Serial Programming Pin

Mapping

Table 96. Pin Mapping Serial Programming

Symbol

MOSI

MISO

SCK

Pins

PB3

PB4

PB5

I/O

Description

Serial data in

Serial data out

Serial clock

I

O

I

Figure 112. Serial Programming and Verify⁽¹⁾

+2.7 - 5.5V

VCC

+2.7 - 5.5V ⁽²⁾

MOSI

MISO

PB3

PB4

PB5

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- 0.3 < AVCC < V_CC+ 0.3, however, AVCC should always be within 2.7 - 5.5V.

When programming the EEPROM, an auto-erase cycle is built into the self-timed pro-

gramming operation (in the Serial mode ONLY) and there is no need to first execute the

Chip Erase instruction. The Chip Erase operation turns the content of every memory

location in both the Program and EEPROM arrays into 0xFF.

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:

232

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

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

Serial Programming

Algorithm

When writing serial data to the ATmega8, data is clocked on the rising edge of SCK.

When reading data from the ATmega8, data is clocked on the falling edge of SCK. See

Figure 113 for timing details.

To program and verify the ATmega8 in the Serial Programming mode, the following

sequence is recommended (See four byte instruction formats in Table 98):

1. Power-up sequence:

Apply power between V_CCand GND while RESET and SCK are set to “0”. In

some systems, 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 Serial Programming by sending the Program-

ming Enable serial instruction to pin MOSI.

3. The Serial Programming instructions will not work if the communication is out of

synchronization. When in sync. the second byte (0x53), 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 0x53

did not echo back, give RESET a positive pulse and issue a new Programming

Enable command.

4. The Flash is programmed one page at a time. The page size is found in Table 93

on page 224. The memory page is loaded one byte at a time by supplying the 5

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 mem-

ory 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 97).

Note: If other commands than polling (read) are applied before any write operation

(FLASH, EEPROM, Lock Bits, Fuses) is completed, it may 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 97). In a chip erased device, no 0xFFs in the data file(s) need to be

programmed.

6. Any memory location can be verified by using the Read instruction which returns

the content 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

233

2486M–AVR–12/03

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 0xFF. 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 written. 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 0xFF, 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

0xFF in all locations, programming of addresses that are meant to contain 0xFF, can be

skipped. See Table 97 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 0xFF. At the time the device is

ready for a new byte, the programmed value will read correctly. This is used to deter-

mine when the next byte can be written. This will not work for the value 0xFF, but the

user should have the following in mind: As a chip-erased device contains 0xFF in all

locations, programming of addresses that are meant to contain 0xFF, 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 0xFF, and the user will have to

wait at least t_{WD_EEPROM}before programming the next byte. See Table 97 for t_{WD_EEPROM}

value.

Table 97. 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

Figure 113. Serial Programming Waveforms

SERIAL DATA INPUT

MSB

LSB

(MOSI)

SERIAL DATA OUTPUT

MSB

(MISO)

SERIAL CLOCK INPUT

(SCK)

SAMPLE

234

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Table 98. Serial Programming Instruction Set

Instruction Format

Instruction

Byte 1

Byte 2

Byte 3

Byte4

Operation

Programming Enable

1010 1100

0101 0011

xxxx xxxx

Enable Serial Programming after

RESET goes low.

Chip Erase

1010 1100

100x xxxx

xxxx xxxx

Chip Erase EEPROM and Flash.

Read Program Memory

0010 H000

0000 aaaa

bbbb bbbb

oooo oooo

Read H (high or low) data o from

Program memory at word address

a:b.

Load Program Memory

Page

0100 H000

0000 xxxx

xxxb bbbb

iiii iiii

Write H (high or low) data i to

Program memory page at word

address b. Data Low byte must be

loaded before Data High byte is

applied within the same address.

Write Program Memory

Page

0100 1100

1010 0000

1100 0000

0101 1000

0000 aaaa

00xx xxxa

0000 0000

bbbx xxxx

bbbb bbbb

xxxx xxxx

oooo oooo

iiii iiii

xxoo oooo

Write Program memory Page at

address a:b.

Read EEPROM Memory

Write EEPROM Memory

Read Lock Bits

Read data o from EEPROM

memory at address a:b.

Write data i to EEPROM memory at

address a:b.

Read Lock Bits. “0” = programmed,

“1” = unprogrammed. See Table

85 on page 219 for details.

Write Lock Bits

1010 1100

111x xxxx

xxxx xxxx

11ii iiii

Write Lock Bits. Set bits = “0” to

program Lock Bits. See Table 85

on page 219 for details.

Read Signature Byte

Write Fuse Bits

0011 0000

1010 1100

00xx xxxx

1010 0000

xxxx xxbb

oooo oooo

iiii iiii

Read Signature Byte o at address

b.

xxxx xxxx

Set bits = “0” to program, “1” to

unprogram. See Table 88 on

page 221 for details.

Write Fuse High Bits

Read Fuse Bits

1010 1100

0101 0000

0101 1000

1010 1000

0000 0000

0000 1000

xxxx xxxx

iiii iiii

oooo oooo

Set bits = “0” to program, “1” to

unprogram. See Table 87 on

page 220 for details.

Read Fuse Bits. “0” = programmed,

“1” = unprogrammed. See Table

88 on page 221 for details.

Read Fuse High Bits

Read Fuse high bits. “0” = pro-

grammed, “1” = unprogrammed.

See Table 87 on page 220 for

details.

Read Calibration Byte

0011 1000

00xx xxxx

0000 00bb

oooo oooo

Read Calibration Byte

Note:

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

235

2486M–AVR–12/03

SPI Serial Programming

Characteristics

For characteristics of the SPI module, see “SPI Timing Characteristics” on page 241.

236

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Electrical Characteristics

Note:

Typical values contained in this datasheet are based on simulations and characterization of other AVR microcontrollers manu-

factured on the same process technology. Min and Max values will be available after the device is characterized.

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

DC Characteristics

T_A= -40°C to 85°C, V_CC= 2.7V to 5.5V (unless otherwise noted)

Symbol

V_IL

Parameter

Condition

Min

Typ

Max

Units

(1)

Input Low Voltage

Input High Voltage

Except XTAL1 pin

-0.5

0.2 V_CC

0.1 V_CC

V

V_IL1

XTAL1 pin, External Clock Selected

Except XTAL1 and RESET pins

XTAL1 pin, External Clock Selected

RESET pin

-0.5

(2)

V_IH

0.6 V_CC

0.8 V_CC

0.9 V_CC

V_CC+ 0.5

V_IH1

V_IH2

Output Low Voltage⁽³⁾

(Ports A,B,C,D)

I_OL= 20 mA, V_CC= 5V

I_OL= 10 mA, V_CC= 3V

0.7

0.5

V

V_OL

V_OH

I_IL

Output High Voltage⁽⁴⁾

(Ports A,B,C,D)

I_OH= -20 mA, V_CC= 5V

I_OH= -10 mA, V_CC= 3V

4.2

2.2

V

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

80

50

kΩ

237

2486M–AVR–12/03

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 4 MHz, V_CC= 3V

(ATmega8L)

5

mA

Active 8 MHz, V_CC= 5V

(ATmega8)

15

2

mA

Power Supply Current

Idle 4 MHz, V_CC= 3V

(ATmega8L)

I_CC

Idle 8 MHz, V_CC= 5V

(ATmega8)

7

WDT enabled, V_CC= 3V

WDT disabled, V_CC= 3V

V_CC= 5V

25

2

µA

Power-down mode⁽⁵⁾

Analog Comparator

Input Offset Voltage

V_ACIO

I_ACLK

t_ACID

20

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 (20mA at Vcc = 5V, 10mA 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 400 mA.

2] The sum of all IOL, for ports C0 - C5 should not exceed 200 mA.

3] The sum of all IOL, for ports B0 - B7, C6, D0 - D7 and XTAL2, should not exceed 100 mA.

TQFP and MLF Package:

1] The sum of all IOL, for all ports, should not exceed 400 mA.

2] The sum of all IOL, for ports C0 - C5, should not exceed 200 mA.

3] The sum of all IOL, for ports C6, D0 - D4, should not exceed 300 mA.

4] The sum of all IOL, for ports B0 - B7, D5 - D7, should not exceed 300 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 (20mA at Vcc = 5V, 10mA 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 400 mA.

2] The sum of all IOH, for port C0 - C5, should not exceed 100 mA.

3] The sum of all IOH, for ports B0 - B7, C6, D0 - D7 and XTAL2, should not exceed 100 mA.

TQFP and MLF Package:

1] The sum of all IOH, for all ports, should not exceed 400 mA.

2] The sum of all IOH, for ports C0 - C5, should not exceed 200 mA.

3] The sum of all IOH, for ports C6, D0 - D4, should not exceed 300 mA.

4] The sum of all IOH, for ports B0 - B7, D5 - D7, should not exceed 300 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.

238

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

External Clock Drive

Waveforms

Figure 114. External Clock Drive Waveforms

V_IH1

V_IL1

External Clock Drive

Table 99. External Clock Drive

V_CC= 2.7V to 5.5V V_CC= 4.5V to 5.5V

Symbol Parameter

Min

0

Max

Min

0

Max

Units

MHz

ns

1/t_CLCL

t_CLCL

t_CHCX

t_CLCX

t_CLCH

t_CHCL

Oscillator Frequency

8

16

Clock Period

High Time

Low Time

Rise Time

Fall Time

125

50

62.5

25

ns

50

25

ns

1.6

0.5

µs

Change in period from one

clock cycle to the next

2

%

∆t_CLCL

Table 100. External RC Oscillator, Typical Frequencies

R [kΩ]⁽¹⁾

C [pF]

47

f⁽²⁾

100

87 kHz

650 kHz

2.0 MHz

33

22

10

22

Notes: 1. R should be in the range 3 kΩ - 100 kΩ, and C should be at least 20 pF. The C values

given in the table includes pin capacitance. This will vary with package type.

2. The frequency will vary with package type and board layout.

239

2486M–AVR–12/03

-----------------

-

Two-wire Serial Interface Characteristics

Table 101 describes the requirements for devices connected to the Two-wire Serial Bus. The ATmega8 Two-wire Serial

Interface meets or exceeds these requirements under the noted conditions.

Timing symbols refer to Figure 115.

Table 101. Two-wire Serial Bus Requirements

Symbol Parameter

Condition

Min

-0.5

Max

0.3 V_CC

V_CC+ 0.5

–

Units

V

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

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

V

(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

ns

µA

pF

kHz

(1)

t_of

20 + 0.1C_b

250

(1)

t_SP

I_i

C_i⁽¹⁾

f_SCL

0

-10

–

50⁽²⁾

10

f_CK⁽⁴⁾> max(16f_SCL, 250kHz)⁽⁵⁾

0

400

V_CC– 0,4V

f

_SCL≤ 100 kHz

1000ns

C_b

Ω

----------------------------

3mA

Rp

Value of Pull-up resistor

V_CC– 0,4V

f_SCL> 100 kHz

300ns

---------------

C_b

----------------------------

3mA

f

_SCL≤ 100 kHz

4.0

0.6

4.7

1.3

4.0

0.6

4.7

0.6

0

–

µs

ns

µs

t_HD;STA

Hold Time (repeated) START Condition

Low Period of the SCL Clock

High period of the SCL clock

Set-up time for a repeated START condition

Data hold time

f_SCL> 100 kHz

f

_SCL≤ 100 kHz⁽⁶⁾

–

t_LOW

f_SCL> 100 kHz⁽⁷⁾

_SCL≤ 100 kHz

f_SCL> 100 kHz

_SCL≤ 100 kHz

f_SCL> 100 kHz

_SCL≤ 100 kHz

f_SCL> 100 kHz

_SCL≤ 100 kHz

f_SCL> 100 kHz

_SCL≤ 100 kHz

f_SCL> 100 kHz

_SCL≤ 100 kHz

f_SCL> 100 kHz

–

f

–

t_HIGH

–

f

–

t_SU;STA

t_HD;DAT

t_SU;DAT

t_SU;STO

t_BUF

–

f

3.45

0.9

–

0

f

250

100

4.0

0.6

4.7

1.3

Data setup time

–

f

–

Setup time for STOP condition

–

f

–

Bus free time between a STOP and START

condition

–

Notes: 1. In ATmega8, 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

240

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

5. This requirement applies to all ATmega8 Two-wire Serial Interface operation. Other

devices connected to the Two-wire Serial Bus need only obey the general f_SCL

requirement.

6. The actual low period generated by the ATmega8 Two-wire Serial Interface is (1/f_SCL

- 2/f_CK), thus f_CKmust be greater than 6 MHz for the low time requirement to be

strictly met at f_SCL= 100 kHz.

7. The actual low period generated by the ATmega8 Two-wire Serial Interface is (1/f_SCL

- 2/f_CK), thus the low time requirement will not be strictly met for f_SCL> 308 kHz when

f_CK= 8 MHz. Still, ATmega8 devices connected to the bus may communicate at full

speed (400 kHz) with other ATmega8 devices, as well as any other device with a

proper t_LOWacceptance margin.

Figure 115. 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 116 and Figure 117 for details.

Characteristics

Table 102. SPI Timing Parameters

Description

SCK period

SCK high/low

Rise/Fall time

Setup

Mode

Master

Slave

Min

Typ

Max

1

2

See Table 50

50% duty cycle

3

3.6

10

4

5

Hold

10

6

Out to SCK

SCK to out

SCK to out high

SS low to out

SCK period

SCK high/low⁽¹⁾

Rise/Fall time

Setup

0.5 • t_SCK

10

7

8

10

9

15

ns

10

11

12

13

14

15

16

17

18

Slave

4 • t_ck

2 • t_ck

Slave

1.6

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

Salve

2 • t_ck

Note:

1. In SPI Programming mode the minimum SCK high/low period is:

- 2t_CLCLfor f_CK< 12 MHz

- 3t_CLCLfor f_CK> 12 MHz

241

2486M–AVR–12/03

Figure 116. 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 117. 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

242

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

ADC Characteristics

Table 103. ADC Characteristics

Symbol

Parameter

Condition

Min⁽¹⁾

Typ⁽¹⁾

Max⁽¹⁾

Units

Resolution

Single Ended Conversion

10

Bits

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 200 kHz

1.75

3

LSB

Absolute accuracy

(Including INL, DNL,

Quantization Error, Gain,

and Offset Error)

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 1 MHz

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 200 kHz

Integral Non-linearity (INL)

0.75

LSB

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 200 kHz

Differential Non-linearity

(DNL)

0.5

1

LSB

Gain Error

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 200 kHz

Offset Error

Single Ended Conversion

V_REF= 4V, V_CC= 4V

ADC clock = 200 kHz

1

LSB

Conversion Time

Free Running Conversion

µs

kHz

V

Clock Frequency

50

V_CC- 0.3⁽²⁾

2.0

1000

V_CC+ 0.3⁽³⁾

AVCC

V_REF

V_IN

Analog Supply Voltage

Reference Voltage

Input voltage

V

GND

V_REF

V

Input bandwidth

38.5

2.56

32

kHz

V

V_INT

R_REF

R_AIN

Internal Voltage Reference

Reference Input Resistance

Analog Input Resistance

2.3

55

2.7

kΩ

MΩ

100

Notes: 1. Values are guidelines only.

2. Minimum for AV_CCis 2.7V.

3. Maximum for AV_CCis 5.5V.

243

2486M–AVR–12/03

ATmega8 Typical

Characteristics

The following charts show typical behavior. These figures are not tested during manu-

facturing. 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 temperature. 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 switch-

ing frequency of I/O pin.

The parts are characterized at frequencies higher than test limits. Parts are not guaran-

teed 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 dif-

ferential current drawn by the Watchdog Timer.

Active Supply Current

Figure 118. Active Supply Current vs. Frequency (0.1 - 1.0 MHz)

ACTIVE SUPPLY CURRENT vs. FREQUENCY

0.1 - 1.0 MHz

3

5.5V

5.0V

2.5

4.5V

4.0V

3.3V

3.0V

2.7V

2

1.5

1

0.5

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Frequency (MHz)

244

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 119. Active Supply Current vs. Frequency (1 - 20 MHz)

ACTIVE SUPPLY CURRENT vs. FREQUENCY

1 - 20 MHz

30

5.5V

5.0V

4.5V

25

20

15

10

3.3V

3.0V

5

2.7V

0

2

4

6

8

10

12

14

16

18

20

Frequency (MHz)

Figure 120. Active Supply Current vs. V_CC(Internal RC Oscillator, 8 MHz)

ACTIVE SUPPLY CURRENT vs. V_CC

INTERNAL RC OSCILLATOR, 8 MHz

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)

245

2486M–AVR–12/03

Figure 121. Active Supply Current vs. V_CC(Internal RC Oscillator, 4 MHz)

ACTIVE SUPPLY CURRENT vs. V_CC

INTERNAL RC OSCILLATOR, 4 MHz

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 122. Active Supply Current vs. V_CC(Internal RC Oscillator, 2 MHz)

ACTIVE SUPPLY CURRENT vs. V_CC

INTERNAL RC OSCILLATOR, 2 MHz

6

25°C

-40°C

85°C

5

4

3

2

1

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

246

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 123. Active Supply Current vs. V_CC(Internal RC Oscillator, 1 MHz)

ACTIVE SUPPLY CURRENT vs. V_CC

INTERNAL RC OSCILLATOR, 1 MHz

3.5

3

85°C

25°C

2.5

2

-40°C

1.5

1

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 124. Active Supply Current vs. V_CC(32 kHz External Oscillator)

ACTIVE SUPPLY CURRENT vs. V_CC

32kHz EXTERNAL OSCILLATOR

120

100

80

60

40

20

0

25°C

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

247

2486M–AVR–12/03

Idle Supply Current

Figure 125. Idle Supply Current vs. Frequency (0.1 - 1.0 MHz)

IDLE SUPPLY CURRENT vs. FREQUENCY

0.1 - 1.0 MHz

0.7

5.5V

0.6

0.5

0.4

0.3

0.2

0.1

0

5.0V

4.5V

4.0V

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 126. Idle Supply Current vs. Frequency (1 - 20 MHz)

IDLE SUPPLY CURRENT vs. FREQUENCY

1 - 20 MHz

14

5.5V

12

10

8

5.0V

4.5V

4.0V

6

3.3V

4

3.0V

2

2.7V

0

2

4

6

8

10

12

14

16

18

20

Frequency (MHz)

248

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 127. Idle Supply Current vs. V_CC(Internal RC Oscillator, 8 MHz)

IDLE SUPPLY CURRENT vs. V_CC

INTERNAL RC OSCILLATOR, 8 MHz

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)

Figure 128. Idle Supply Current vs. V_CC(Internal RC Oscillator, 4 MHz)

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)

249

2486M–AVR–12/03

Figure 129. Idle Supply Current vs. V_CC(Internal RC Oscillator, 2 MHz)

IDLE SUPPLY CURRENT vs. V_CC

INTERNAL RC OSCILLATOR, 2 MHz

1.8

-40°C

85°C

25°C

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 130. Idle Supply Current vs. V_CC(Internal RC Oscillator, 1 MHz)

IDLE SUPPLY CURRENT vs. V_CC

INTERNAL RC OSCILLATOR, 1 MHz

1

85°C

25°C

-40°C

0.9

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)

250

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 131. Idle Supply Current vs. V_CC(32 kHz External Oscillator)

IDLE SUPPLY CURRENT vs. V_CC

32kHz EXTERNAL OSCILLATOR

40

35

30

25

20

15

10

5

25°C

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

Power-down Supply Current

Figure 132. Power-down Supply Current vs. V_CC(Watchdog Timer Disabled)

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)

251

2486M–AVR–12/03

Figure 133. Power-down Supply Current vs. V_CC(Watchdog Timer Enabled)

POWER-DOWN SUPPLY CURRENT vs. V_CC

WATCHDOG TIMER ENABLED

80

85°C

25°C

-40°C

70

60

50

40

30

20

10

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Power-save Supply Current

Figure 134. Power-save Supply Current vs. V_CC(Watchdog Timer Disabled)

POWER-SAVE SUPPLY CURRENT vs. V_CC

WATCHDOG TIMER DISABLED

25

20

15

10

5

25°C

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

252

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Standby Supply Current

Figure 135. Standby Supply Current vs. V_CC(455 kHz Resonator, Watchdog Timer

Disabled)

STANDBY SUPPLY CURRENT vs. V_CC

455 kHz 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 136. Standby Supply Current vs. V_CC(1 MHz Resonator, Watchdog Timer

Disabled)

STANDBY SUPPLY CURRENT vs. V_CC

1 MHz RESONATOR, WATCHDOG TIMER DISABLED

70

60

50

40

30

20

10

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

253

2486M–AVR–12/03

Figure 137. Standby Supply Current vs. V_CC(2 MHz Resonator, Watchdog Timer

Disabled)

STANDBY SUPPLY CURRENT vs. V_CC

2 MHz RESONATOR, WATCHDOG TIMER DISABLED

90

80

70

60

50

40

30

20

10

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 138. Standby Supply Current vs. V_CC(2 MHz Xtal, Watchdog Timer Disabled)

STANDBY SUPPLY CURRENT vs. V_CC

2 MHz XTAL, WATCHDOG TIMER DISABLED

90

80

70

60

50

40

30

20

10

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

254

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 139. Standby Supply Current vs. V_CC(4 MHz Resonator, Watchdog Timer

Disabled)

STANDBY SUPPLY CURRENT vs. V_CC

4 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 140. Standby Supply Current vs. V_CC(4 MHz Xtal, Watchdog Timer Disabled)

STANDBY SUPPLY CURRENT vs. V_CC

4 MHz XTAL, 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)

255

2486M–AVR–12/03

Figure 141. Standby Supply Current vs. V_CC(6 MHz Resonator, Watchdog Timer

Disabled)

STANDBY SUPPLY CURRENT vs. V_CC

6 MHz RESONATOR, WATCHDOG TIMER DISABLED

160

140

120

100

80

60

40

20

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

Figure 142. Standby Supply Current vs. V_CC(6 MHz Xtal, Watchdog Timer Disabled)

STANDBY SUPPLY CURRENT vs. V_CC

6 MHz XTAL, WATCHDOG TIMER DISABLED

200

180

160

140

120

100

80

60

40

20

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

256

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Pin Pull-up

Figure 143. I/O Pin Pull-up Resistor Current vs. Input Voltage (V_CC= 5V)

I/O PIN PULL-UP RESISTOR CURRENT vs. INPUT VOLTAGE

Vcc = 5V

160

85°C

140

120

100

80

25°C

-40°C

60

40

20

0

1

2

3

4

5

6

V

_OP(V)

Figure 144. I/O Pin Pull-up Resistor Current vs. Input Voltage (V_CC= 2.7V)

I/O PIN PULL-UP RESISTOR CURRENT vs. INPUT VOLTAGE

Vcc = 2.7V

90

80

85°C

25°C

-40°C

70

60

50

40

30

20

10

0

0.5

1

1.5

2

2.5

3

V_OP(V)

257

2486M–AVR–12/03

Figure 145. Reset Pull-up Resistor Current vs. Reset Pin Voltage (V_CC= 5V)

RESET PULL-UP RESISTOR CURRENT vs. RESET PIN VOLTAGE

Vcc = 5V

100

- 40°C

25°C

80

85°C

60

40

20

0

1

2

V_RESET(V)

Figure 146. Reset Pull-up Resistor Current vs. Reset Pin Voltage (V_CC= 2.7V)

RESET PULL-UP RESISTOR CURRENT vs. RESET PIN VOLTAGE

Vcc = 2.7V

45

-40°C

40

25°C

35

85°C

30

25

20

15

10

5

0

0.5

1

1.5

2

2.5

V_RESET(V)

258

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Pin Driver Strength

Figure 147. I/O Pin Source Current vs. Output Voltage (V_CC= 5V)

I/O PIN SOURCE CURRENT vs. OUTPUT VOLTAGE

Vcc = 5V

80

70

60

50

40

30

20

10

0

-40°C

25°C

85°C

V

_OH(V)

Figure 148. I/O Pin Source Current vs. Output Voltage (V_CC= 2.7V)

I/O PIN SOURCE CURRENT vs. OUTPUT VOLTAGE

Vcc = 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)

259

2486M–AVR–12/03

Figure 149. I/O Pin Sink Current vs. Output Voltage (V_CC= 5V)

I/O PIN SINK CURRENT vs. OUTPUT VOLTAGE

Vcc = 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 150. I/O Pin Sink Current vs. Output Voltage (V_CC= 2.7V)

I/O PIN SINK CURRENT vs. OUTPUT VOLTAGE

Vcc = 2.7V

35

-40°C

30

25

20

15

10

5

25°C

85°C

0

0.5

1

1.5

2

2.5

V_OL(V)

260

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 151. Reset Pin as I/O – Pin Source Current vs. Output Voltage (V_CC= 5V)

RESET PIN AS I/O - SOURCE CURRENT vs. OUTPUT VOLTAGE

Vcc = 5V

4

3.5

-40°C

3

25°C

2.5

85°C

2

1.5

1

0.5

0

2

2.5

3

3.5

4

4.5

V

_OH(V)

Figure 152. Reset Pin as I/O – Pin Source Current vs. Output Voltage (V_CC= 2.7V)

RESET PIN AS I/O - SOURCE CURRENT vs. OUTPUT VOLTAGE

Vcc = 2.7V

5

4.5

-40°C

25°C

4

3.5

3

2.5

2

85°C

1.5

1

0.5

0

0.5

1

1.5

2

2.5

V_OH(V)

261

2486M–AVR–12/03

Figure 153. Reset Pin as I/O – Pin Sink Current vs. Output Voltage (V_CC= 5V)

RESET PIN AS I/O - SINK CURRENT vs. OUTPUT VOLTAGE

Vcc = 5V

14

-40°C

12

25°C

10

85°C

8

6

4

2

0

0.5

1

1.5

2

2.5

V_OL(V)

Figure 154. Reset Pin as I/O – Pin Sink Current vs. Output Voltage (V_CC= 2.7V)

RESET PIN AS I/O - SINK CURRENT vs. OUTPUT VOLTAGE

Vcc = 2.7V

4.5

4

-40°C

3.5

25°C

3

85°C

2.5

2

1.5

1

0.5

0

0.5

1

1.5

2

2.5

V_OL(V)

262

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Pin Thresholds and

Hysteresis

Figure 155. I/O Pin Input Threshold Voltage vs. V_CC(V_IH, I/O Pin Read as “1”)

I/O PIN INPUT THRESHOLD VOLTAGE vs. V_CC

VIH, IO 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 156. I/O Pin Input Threshold Voltage vs. V_CC(V_IL, I/O Pin Read as “0”)

I/O PIN INPUT THRESHOLD VOLTAGE vs. V_CC

VIL, IO PIN READ AS '0'

2

-40°C

25°C

85°C

1.5

1

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

263

2486M–AVR–12/03

Figure 157. I/O Pin Input Hysteresis vs. V_CC

I/O PIN INPUT HYSTERESIS vs. V_CC

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

85°C

-40°C

25°C

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 158. Reset Pin as I/O – Input Threshold Voltage vs. V_CC(V_IH, I/O Pin Read as

“1”)

RESET PIN AS I/O - INPUT THRESHOLD VOLTAGE vs. V_CC

VIH, RESET PIN READ AS '1'

4

-40°C

85°C

25°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)

264

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 159. Reset Pin as I/O – Input Threshold Voltage vs. V_CC(V_IL, I/O Pin Read as

“0”)

RESET PIN AS I/O - INPUT THRESHOLD VOLTAGE vs. V_CC

VIL, RESET PIN READ AS '0'

2.5

85°C

25°C

-40°C

2

1.5

1

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 160. Reset Pin as I/O – Pin Hysteresis vs. V_CC

RESET PIN AS I/O - PIN HYSTERESIS vs. V_CC

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)

265

2486M–AVR–12/03

Figure 161. Reset Input Threshold Voltage vs. V_CC(V_IH, Reset Pin Read as “1”)

RESET INPUT THRESHOLD VOLTAGE vs. V_CC

VIH, RESET PIN READ AS '1'

2.5

-40°C

25°C

85°C

2

1.5

1

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 162. Reset Input Threshold Voltage vs. V_CC(V_IL, Reset Pin Read as “0”)

RESET INPUT THRESHOLD VOLTAGE vs. V_CC

VIL, RESET PIN READ AS '0'

2.5

2

85°C

25°C

-40°C

1.5

1

0.5

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

266

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 163. Reset Input Pin Hysteresis vs. V_CC

RESET INPUT PIN HYSTERESIS vs. V_CC

1

0.8

0.6

0.4

0.2

0

-40°C

25°C

85°C

2

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Bod Thresholds and Analog

Comparator Offset

Figure 164. BOD Thresholds vs. Temperature (BOD Level is 4.0V)

BOD THRESHOLDS vs. TEMPERATURE

BODLEVEL IS 4.0V

4.3

4.2

Rising V_CC

4.1

4

Falling V_CC

3.9

3.8

3.7

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

80

90

100

Temperature (˚C)

267

2486M–AVR–12/03

Figure 165. BOD Thresholds vs. Temperature (BOD Level is 2.7v)

BOD THRESHOLDS vs. TEMPERATURE

BODLEVEL IS 2.7V

2.8

2.7

Rising V_CC

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)

Figure 166. Bandgap Voltage vs. V_CC

BANDGAP VOLTAGE vs. V_CC

1.315

1.31

1.305

1.3

-40°

85°

25°

1.295

1.29

2.5

3

3.5

4

4.5

5

5.5

Vcc (V)

268

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 167. Analog Comparator Offset Voltage vs. Common Mode Voltage (V_CC= 5V)

ANALOG COMPARATOR OFFSET VOLTAGE vs. COMMON MODE VOLTAGE

V_CC= 5V

0.003

0.002

0.001

0

-0.001

85°

-0.002

-0.003

25°C

-0.004

-0.005

-40°

-0.006

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Common Mode Voltage (V)

Figure 168. Analog Comparator Offset Voltage vs. Common Mode Voltage (V_CC

=

2.7V)

ANALOG COMPARATOR OFFSET VOLTAGE vs. COMMON MODE VOLTAGE

V_CC= 2.7V

0.003

0.002

0.001

0

-0.001

85°

-0.002

25°

-0.003

-0.004

-40°

-0.005

0

0.5

1

1.5

2

2.5

3

Common Mode Voltage (V)

269

2486M–AVR–12/03

Internal Oscillator Speed

Figure 169. Watchdog Oscillator Frequency vs. V_CC

WATCHDOG OSCILLATOR FREQUENCY vs. V_CC

1260

1240

1220

1200

1180

1160

1140

1120

1100

-40°C

25°C

85°C

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

Figure 170. Calibrated 8 MHz RC Oscillator Frequency vs. Temperature

CALIBRATED 8MHz RC OSCILLATOR FREQUENCY vs. TEMPERATURE

8.5

5.5V

8.3

8.1

4.0V

7.9

7.7

7.5

2.7V

7.3

7.1

6.9

6.7

6.5

-60

-40

-20

0

20

40

60

80

100

Temperature (˚C)

270

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 171. Calibrated 8 MHz RC Oscillator Frequency vs. V_CC

CALIBRATED 8MHz 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)

Figure 172. Calibrated 8 MHz RC Oscillator Frequency vs. Osccal Value

CALIBRATED 8MHz RC OSCILLATOR FREQUENCY vs. OSCCAL VALUE

16

14

12

10

8

6

4

0

16

32

48

64

80

96

112 128 144 160 176 192 208 224 240

OSCCAL VALUE

271

2486M–AVR–12/03

Figure 173. Calibrated 4 MHz RC Oscillator Frequency vs. Temperature

CALIBRATED 4MHz RC OSCILLATOR FREQUENCY vs. TEMPERATURE

4.2

5.5V

4.1

4.0V

4

3.9

2.7V

3.8

3.7

3.6

3.5

-60

-40

-20

0

20

40

60

80

100

Temperature (˚C)

Figure 174. Calibrated 4 MHz RC Oscillator Frequency vs. V_CC

CALIBRATED 4MHz RC OSCILLATOR FREQUENCY vs. V_CC

4.2

-40°C

4.1

4

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)

272

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 175. Calibrated 4 MHz RC Oscillator Frequency vs. Osccal Value

CALIBRATED 4MHz RC OSCILLATOR FREQUENCY vs. OSCCAL VALUE

8

7

6

5

4

3

2

0

16

32

48

64

80

96

112 128 144 160 176 192 208 224 240

OSCCAL VALUE

Figure 176. Calibrated 2 MHz RC Oscillator Frequency vs. Temperature

CALIBRATED 2MHz RC OSCILLATOR FREQUENCY vs. TEMPERATURE

2.1

5.5V

2.05

4.0V

2

1.95

2.7V

1.9

1.85

1.8

-60

-40

-20

0

20

40

60

80

100

Temperature (˚C)

273

2486M–AVR–12/03

Figure 177. Calibrated 2 MHz RC Oscillator Frequency vs. V_CC

CALIBRATED 2MHz 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)

Figure 178. Calibrated 2 MHz RC Oscillator Frequency vs. Osccal Value

CALIBRATED 2MHz RC OSCILLATOR FREQUENCY vs. OSCCAL VALUE

3.8

3.3

2.8

2.3

1.8

1.3

0.8

0

16

32

48

64

80

96

112 128 144 160 176 192 208 224 240

OSCCAL VALUE

274

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 179. Calibrated 1 MHz RC Oscillator Frequency vs. Temperature

CALIBRATED 1MHz RC OSCILLATOR FREQUENCY vs. TEMPERATURE

1.04

1.02

1

5.5V

4.0V

0.98

0.96

0.94

0.92

0.9

2.7V

-60

-40

-20

0

20

40

60

80

100

Temperature (˚C)

Figure 180. Calibrated 1 MHz RC Oscillator Frequency vs. V_CC

CALIBRATED 1MHz RC OSCILLATOR FREQUENCY vs. V_CC

1.1

1.05

-40°C

25°C

85°C

1

0.95

0.9

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

275

2486M–AVR–12/03

Figure 181. Calibrated 1 MHz RC Oscillator Frequency vs. Osccal Value

CALIBRATED 1MHz RC OSCILLATOR FREQUENCY vs. OSCCAL VALUE

1.9

1.7

1.5

1.3

1.1

0.9

0.7

0.5

0

16

32

48

64

80

96

112 128 144 160 176 192 208 224 240

OSCCAL VALUE

Current Consumption of

Peripheral Units

Figure 182. Brown-out Detector Current vs. V_CC

BROWN-OUT DETECTOR CURRENT vs. V_CC

30

25

20

15

10

5

-40°C

25°C

85°C

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

276

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 183. ADC Current vs. V_CC(AREF = AVCC)

ADC CURRENT vs. V_CC

AREF = AVCC

450

400

350

300

250

200

150

100

50

25°C

-40°C

85°C

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 184. AREF External Reference Current vs. V_CC

AREF EXTERNAL REFERENCE CURRENT vs. V_CC

250

200

150

100

50

85°C

25°C

-40°C

0

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

277

2486M–AVR–12/03

Figure 185. 32 kHz TOSC Current vs. V_CC(Watchdog Timer Disabled)

32 kHz TOSC CURRENT vs. V_CC

WATCHDOG TIMER DISABLED

25

20

15

10

5

25°C

0

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

Figure 186. Watchdog Timer Current vs. V_CC

WATCHDOG TIMER CURRENT vs. V_CC

80

70

60

50

40

30

20

10

0

85°C

25°C

-40°C

2

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

278

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 187. Analog Comparator Current vs. V_CC

ANALOG COMPARATOR CURRENT vs. V_CC

100

90

80

70

60

50

40

30

20

10

0

85°C

25°C

-40°C

2.5

3

3.5

4

4.5

5

5.5

V

_CC(V)

Figure 188. Programming Current vs. V_CC

PROGRAMMING CURRENT vs. V_CC

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)

279

2486M–AVR–12/03

Current Consumption in

Figure 189. Reset Supply Current vs. V_CC(0.1 - 1.0 MHz, Excluding Current Through

Reset and Reset Pulsewidth

The Reset Pull-up)

RESET SUPPLY CURRENT vs. V_CC

0.1 - 1 MHz, EXCLUDING CURRENT THROUGH THE RESET PULL-UP

4

5.5V

3.5

5.0V

3

4.5V

2.5

4.0V

2

3.3V

3.0V

2.7V

1.5

1

0.5

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Frequency (MHz)

Figure 190. Reset Supply Current vs. V_CC(1 - 20 MHz, Excluding Current Through The

Reset Pull-up)

RESET SUPPLY CURRENT vs. V_CC

1 - 20 MHz, EXCLUDING CURRENT THROUGH THE RESET PULL-UP

25

5.5V

20

5.0V

4.5V

15

10

3.3V

5

3.0V

2.7V

0

2

4

6

8

10

12

14

16

18

20

Frequency (MHz)

280

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Figure 191. Reset Pulse Width vs. V_CC

RESET PULSE WIDTH vs. V_CC

1400

1200

1000

800

600

400

200

0

85°C

25°C

-40°C

2.5

3

3.5

4

4.5

5

5.5

V_CC(V)

281

2486M–AVR–12/03

Register Summary

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Page

0x3F (0x5F)

0x3E (0x5E)

0x3D (0x5D)

0x3C (0x5C)

0x3B (0x5B)

0x3A (0x5A)

0x39 (0x59)

0x38 (0x58)

0x37 (0x57)

0x36 (0x56)

0x35 (0x55)

0x34 (0x54)

0x33 (0x53)

0x32 (0x52)

0x31 (0x51)

0x30 (0x50)

0x2F (0x4F)

0x2E (0x4E)

0x2D (0x4D)

0x2C (0x4C)

0x2B (0x4B)

0x2A (0x4A)

0x29 (0x49)

0x28 (0x48)

0x27 (0x47)

0x26 (0x46)

0x25 (0x45)

0x24 (0x44)

0x23 (0x43)

0x22 (0x42)

0x21 (0x41)

SREG

SPH

I

–

T

–

H

–

S

–

V

–

N

Z

C

9

SP10

SP2

SP9

SP1

SP8

SP0

11

SPL

SP7

SP6

SP5

SP4

SP3

Reserved

GICR

INT1

INTF1

OCIE2

OCF2

SPMIE

TWINT

SE

INT0

INTF0

TOIE2

TOV2

RWWSB

TWEA

SM2

–

IVSEL

IVCE

–

47, 65

GIFR

–

TICIE1

ICF1

–

66

TIMSK

TIFR

OCIE1A

OCF1A

RWWSRE

TWSTO

SM0

OCIE1B

OCF1B

BLBSET

TWWC

ISC11

WDRF

–

TOIE1

TOV1

PGWRT

TWEN

ISC10

BORF

CS02

TOIE0

TOV0

SPMEN

TWIE

ISC00

PORF

CS00

70, 100, 120

–

71, 101, 120

SPMCR

TWCR

MCUCR

MCUCSR

TCCR0

TCNT0

OSCCAL

SFIOR

TCCR1A

TCCR1B

TCNT1H

TCNT1L

OCR1AH

OCR1AL

OCR1BH

OCR1BL

ICR1H

ICR1L

PGERS

–

210

TWSTA

SM1

–

168

ISC01

EXTRF

CS01

31, 64

–

39

–

70

Timer/Counter0 (8 Bits)

70

Oscillator Calibration Register

29

–

COM1B1

–

ACME

FOC1A

WGM12

PUD

FOC1B

CS12

PSR2

WGM11

CS11

PSR10

WGM10

CS10

56, 73, 121, 190

COM1A1

ICNC1

COM1A0

ICES1

COM1B0

WGM13

95

98

Timer/Counter1 – Counter Register High byte

Timer/Counter1 – Counter Register Low byte

99

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

99

100

115

117

41

TCCR2

TCNT2

OCR2

FOC2

WGM20

COM21

COM20

WGM21

CS22

CS21

CS20

Timer/Counter2 (8 Bits)

Timer/Counter2 Output Compare Register

ASSR

–

WDCE

–

AS2

TCN2UB

WDP2

OCR2UB

WDP1

TCR2UB

WDP0

WDTCR

UBRRH

UCSRC

EEARH

EEARL

EEDR

–

WDE

URSEL

–

UBRR[11:8]

155

153

18

0x20⁽¹⁾(0x40)⁽¹⁾

UMSEL

–

UPM1

–

UPM0

–

USBS

–

UCSZ1

–

UCSZ0

–

UCPOL

EEAR8

EEAR0

0x1F (0x3F)

0x1E (0x3E)

0x1D (0x3D)

0x1C (0x3C)

0x1B (0x3B)

0x1A (0x3A)

0x19 (0x39)

0x18 (0x38)

0x17 (0x37)

0x16 (0x36)

0x15 (0x35)

0x14 (0x34)

0x13 (0x33)

0x12 (0x32)

0x11 (0x31)

0x10 (0x30)

0x0F (0x2F)

0x0E (0x2E)

0x0D (0x2D)

0x0C (0x2C)

0x0B (0x2B)

0x0A (0x2A)

0x09 (0x29)

0x08 (0x28)

0x07 (0x27)

0x06 (0x26)

0x05 (0x25)

0x04 (0x24)

0x03 (0x23)

0x02 (0x22)

EEAR7

EEAR6

EEAR5

EEAR4

EEAR3

EEAR2

EEAR1

18

EEPROM Data Register

18

EECR

–

EERIE

EEMWE

EEWE

EERE

18

Reserved

PORTB

DDRB

PORTB7

DDB7

PINB7

–

PORTB6

DDB6

PORTB5

DDB5

PORTB4

DDB4

PORTB3

DDB3

PORTB2

DDB2

PORTB1

DDB1

PORTB0

DDB0

63

PINB

PINB6

PINB5

PINB4

PINB3

PINB2

PINB1

PINB0

63

PORTC

DDRC

PORTC6

DDC6

PORTC5

DDC5

PORTC4

DDC4

PORTC3

DDC3

PORTC2

DDC2

PORTC1

DDC1

PORTC0

DDC0

63

–

63

PINC

–

PINC6

PORTD6

DDD6

PINC5

PINC4

PINC3

PORTD3

DDD3

PINC2

PORTD2

DDD2

PINC1

PINC0

PORTD0

DDD0

63

PORTD

DDRD

PORTD7

DDD7

PIND7

PORTD5

DDD5

PORTD4

DDD4

PORTD1

DDD1

63

PIND

PIND6

PIND5

PIND4

PIND3

PIND2

PIND1

PIND0

63

SPDR

SPI Data Register

128

126

150

151

152

155

191

202

204

205

170

SPSR

SPIF

SPIE

WCOL

SPE

–

SPI2X

SPR0

SPCR

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

ADLAR

ADFR

ACI

–

ACIE

MUX3

ADIE

ACIC

MUX2

ADPS2

ACIS1

MUX1

ADPS1

ACIS0

MUX0

ADPS0

ADMUX

ADCSRA

ADCH

ADIF

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

282

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Register Summary (Continued)

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

0x01 (0x21)

0x00 (0x20)

TWSR

TWBR

TWS7

–

170

168

Two-wire Serial Interface Bit Rate Register

Notes: 1. Refer to the USART description for details on how to access UBRRH and UCSRC.

2. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory addresses

should never be written.

3. 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 0x00 to 0x1F only.

283

2486M–AVR–12/03

Instruction Set Summary

Mnemonics

Operands

Description

Operation

Flags

#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

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

ADIW

SUB

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 ← 0xFF − Rd

Rd ← 0x00 − 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 • (0xFF - 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

Rd ← Rd • Rd

Z,N,V

CLR

Rd

Clear Register

Rd ← Rd Rd

Rd ← 0xFF

Z,N,V

SER

Rd

Set Register

None

MUL

MULS

MULSU

FMUL

FMULS

FMULSU

Rd, Rr

Multiply Unsigned

R1:R0 ← Rd x Rr

R1:R0 ← (Rd x Rr) << 1

Z,C

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

RCALL

ICALL

RET

Relative Subroutine Call

Indirect Call to (Z)

PC ← PC + k + 1

3

PC ← Z

3

Subroutine Return

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

Mnemonics

Operands

Description

Operation

Flags

#Clocks

284

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Instruction Set Summary (Continued)

BRIE

k

Branch if Interrupt Enabled

if ( I = 1) then PC ← PC + k + 1

if ( I = 0) then PC ← PC + k + 1

None

1 / 2

BRID

k

Branch if Interrupt Disabled

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

S ← 1

S

V

T

S ← 0

V ← 1

V ← 0

T ← 1

Mnemonics

Operands

Description

Operation

Flags

#Clocks

285

2486M–AVR–12/03

Instruction Set Summary (Continued)

CLT

SEH

CLH

Clear T in SREG

T ← 0

H ← 1

H ← 0

T

1

Set Half Carry Flag in SREG

Clear Half Carry Flag in SREG

H

MCU CONTROL INSTRUCTIONS

NOP

No Operation

Sleep

None

1

SLEEP

WDR

(see specific descr. for Sleep function)

(see specific descr. for WDR/timer)

Watchdog Reset

286

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Ordering Information

Speed (MHz)

Power Supply

Ordering Code

Package

Operation Range

8

2.7 - 5.5

ATmega8L-8AC

ATmega8L-8PC

ATmega8L-8MC

32A

Commercial

28P3

32M1-A

(0°C to 70°C)

ATmega8L-8AI

ATmega8L-8PI

ATmega8L-8MI

32A

Industrial

28P3

32M1-A

(-40°C to 85°C)

16

4.5 - 5.5

ATmega8-16AC

ATmega8-16PC

ATmega8-16MC

32A

Commercial

28P3

32M1-A

(0°C to 70°C)

ATmega8-16AI

ATmega8-16PI

ATmega8-16MI

32A

Industrial

28P3

32M1-A

(-40°C to 85°C)

Note:

This device can also be supplied in wafer form. Please contact your local Atmel sales office for detailed ordering information and

minimum quantities.

Package Type

32-lead, Thin (1.0 mm) Plastic Quad Flat Package (TQFP)

28-lead, 0.300” Wide, Plastic Dual Inline Package (PDIP)

32A

28P3

32M1-A

32-pad, 5 x 5 x 1.0 body, Lead Pitch 0.50 mm Micro Lead Frame Package (MLF)

287

2486M–AVR–12/03

Packaging Information

32A

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

0.15

1.05

9.25

7.10

9.25

7.10

0.45

0.20

0.75

NOM

NOTE

SYMBOL

A

–

A1

A2

D

0.05

0.95

8.75

6.90

8.75

6.90

0.30

0.09

0.45

1.00

9.00

7.00

9.00

7.00

–

D1

E

Note 2

Notes:

1. This package conforms to JEDEC reference MS-026, Variation ABA.

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

C

–

3. Lead coplanarity is 0.10 mm maximum.

L

–

e

0.80 TYP

10/5/2001

TITLE

DRAWING NO. REV.

2325 Orchard Parkway

San Jose, CA 95131

32A, 32-lead, 7 x 7 mm Body Size, 1.0 mm Body Thickness,

0.8 mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP)

32A

B

R

288

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

28P3

D

1

E1

A

SEATING PLANE

A1

B2

(4 PLACES)

L

B

B1

e

E

COMMON DIMENSIONS

(Unit of Measure = mm)

0º ~ 15º REF

C

MIN

–

MAX

4.5724

–

NOM

NOTE

SYMBOL

A

–

eB

A1

D

0.508

34.544

7.620

7.112

0.381

1.143

0.762

3.175

0.203

–

34.798 Note 1

8.255

E

E1

B

7.493 Note 1

0.533

B1

B2

L

1.397

Note:

1. Dimensions D and E1 do not include mold Flash or Protrusion.

Mold Flash or Protrusion shall not exceed 0.25 mm (0.010").

1.143

3.429

C

0.356

eB

e

10.160

2.540 TYP

09/28/01

DRAWING NO. REV.

28P3

TITLE

2325 Orchard Parkway

San Jose, CA 95131

28P3, 28-lead (0.300"/7.62 mm Wide) Plastic Dual

Inline Package (PDIP)

B

R

289

2486M–AVR–12/03

32M1-A

D

D1

1

2

3

0

Pin 1 ID

SIDE VIEW

E1

E

TOP VIEW

A3

A1

A2

A

0.08

C

P

COMMON DIMENSIONS

D2

(Unit of Measure = mm)

Pin 1 ID

MIN

0.80

–

MAX

1.00

0.05

1.00

NOM

0.90

NOTE

SYMBOL

1

2

3

A

P

A1

A2

A3

b

0.02

–

0.65

E2

0.20 REF

0.23

0.18

2.95

0.30

3.25

D

5.00 BSC

4.75 BSC

3.10

D1

D2

E

e

b

L

5.00 BSC

4.75BSC

3.10

BOTTOM VIEW

E1

E2

e

0.50 BSC

0.40

L

0.30

–

0.50

0.60

P

–

Notes: 1. JEDEC Standard MO-220, Fig. 2 (Anvil Singulation), VHHD-2.

o

–

12

0

01/15/03

DRAWING NO. REV.

TITLE

2325 Orchard Parkway

San Jose, CA 95131

32M1-A, 32-pad, 5 x 5 x 1.0 mm Body, Lead Pitch 0.50 mm

Micro Lead Frame Package (MLF)

32M1-A

C

R

290

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Erratas

The revision letter in this section refers to the revision of the ATmega8 device.

ATmega8

Rev. D, E, F, and G

• CKOPT Does not Enable Internal Capacitors on XTALn/TOSCn Pins when 32 KHz

Oscillator is Used to Clock the Asynchronous Timer/Counter2

1. CKOPT Does not Enable Internal Capacitors on XTALn/TOSCn Pins when

32 KHz Oscillator is Used to Clock the Asynchronous Timer/Counter2

When the internal RC Oscillator is used as the main clock source, it is possible to

run the Timer/Counter2 asynchronously by connecting a 32 KHz Oscillator between

XTAL1/TOSC1 and XTAL2/TOSC2. But when the internal RC Oscillator is selected

as the main clock source, the CKOPT Fuse does not control the internal capacitors

on XTAL1/TOSC1 and XTAL2/TOSC2. As long as there are no capacitors con-

nected to XTAL1/TOSC1 and XTAL2/TOSC2, safe operation of the Oscillator is not

guaranteed.

Problem fix/Workaround

Use external capacitors in the range of 20 - 36 pF on XTAL1/TOSC1 and

XTAL2/TOSC2. This will be fixed in ATmega8 Rev. G where the CKOPT Fuse will

control internal capacitors also when internal RC Oscillator is selected as main clock

source. For ATmega8 Rev. G, CKOPT = 0 (programmed) will enable the internal

capacitors on XTAL1 and XTAL2. Customers who want compatibility between Rev.

G and older revisions, must ensure that CKOPT is unprogrammed (CKOPT = 1).

291

2486M–AVR–12/03

Datasheet Change

Log for ATmega8

This document contains a log on the changes made to the datasheet for ATmega8.

Changes from Rev.

2486K-08/03 to Rev.

2486L-10/03

All page numbers refers to this document.

1. Updated “Calibrated Internal RC Oscillator” on page 28.

Changes from Rev.

2486K-08/03 to Rev.

2486L-10/03

All page numbers refers to this document.

1. Removed “Preliminary” and TBDs from the datasheet.

2. Renamed ICP to ICP1 in the datasheet.

3. Removed instructions CALL and JMP from the datasheet.

4. Updated t_RSTin Table 15 on page 36, V_BGin Table 16 on page 40, Table 100 on

page 239 and Table 102 on page 241.

5. Replaced text “XTAL1 and XTAL2 should be left unconnected (NC)” after

Table 9 in “Calibrated Internal RC Oscillator” on page 28. Added text regard-

ing XTAL1/XTAL2 and CKOPT Fuse in “Timer/Counter Oscillator” on page 30.

6. Updated Watchdog Timer code examples in “Timed Sequences for Changing

the Configuration of the Watchdog Timer” on page 43.

7. Removed bit 4, ADHSM, from “Special Function IO Register – SFIOR” on page

56.

8. Added note 2 to Figure 103 on page 212.

9. Updated item 4 in the “Serial Programming Algorithm” on page 233.

10. Added t_{WD_FUSE}to Table 97 on page 234 and updated Read Calibration Byte,

Byte 3, in Table 98 on page 235.

11. Updated Absolute Maximum Ratings* and DC Characteristics in “Electrical

Characteristics” on page 237.

Changes from Rev.

2486J-02/03 to Rev.

2486K-08/03

All page numbers refers to this document.

1. Updated V_BOTvalues in Table 15 on page 36.

2. Updated “ADC Characteristics” on page 243.

3. Updated “ATmega8 Typical Characteristics” on page 244.

4. Updated “Erratas” on page 291.

Changes from Rev.

2486I-12/02 to Rev.

2486J-02/03

All page numbers refers to this document.

292

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

1. Improved the description of “Asynchronous Timer Clock – clkASY” on page

24.

2. Removed reference to the “Multipurpose Oscillator” application note and the

“32 kHz Crystal Oscillator” application note, which do not exist.

3. Corrected OCn waveforms in Figure 38 on page 88.

4. Various minor Timer 1 corrections.

5. Various minor TWI corrections.

6. Added note under “Filling the Temporary Buffer (Page Loading)” on page 213

about writing to the EEPROM during an SPM Page load.

7. Removed ADHSM completely.

8. Added section “EEPROM Write during Power-down Sleep Mode” on page 21.

9. Removed XTAL1 and XTAL2 description on page 5 because they were already

described as part of “Port B (PB7..PB0) XTAL1/ XTAL2/TOSC1/TOSC2” on

page 5.

10. Improved the table under “SPI Timing Characteristics” on page 241 and

removed the table under “SPI Serial Programming Characteristics” on page

236.

11. Corrected PC6 in “Alternate Functions of Port C” on page 59.

12. Corrected PB6 and PB7 in “Alternate Functions of Port B” on page 56.

13. Corrected 230.4 Mbps to 230.4 kbps under “Examples of Baud Rate Setting”

on page 156.

14. Added information about PWM symmetry for Timer 2 in “Phase Correct PWM

Mode” on page 111.

15. Added thick lines around accessible registers in Figure 76 on page 166.

16. Changed “will be ignored” to “must be written to zero” for unused Z-pointer

bits under “Performing a Page Write” on page 213.

17. Added note for RSTDISBL Fuse in Table 87 on page 220.

18.Updated drawings in “Packaging Information” on page 288.

Changes from Rev.

2486H-09/02 to Rev.

2486I-12/02

1.Added errata for Rev D, E, and F on page 291.

Changes from Rev.

2486G-09/02 to Rev.

2486H-09/02

1.Changed the Endurance on the Flash to 10,000 Write/Erase Cycles.

293

2486M–AVR–12/03

Changes from Rev.

2486F-07/02 to Rev.

2486G-09/02

All page numbers refers to this document.

1 Updated Table 103, “ADC Characteristics,” on page 243.

Changes from Rev.

2486E-06/02 to Rev.

2486F-07/02

All page numbers refers to this document.

1

2

3

Changes in “Digital Input Enable and Sleep Modes” on page 53.

Addition of OCS2 in “MOSI/OC2 – Port B, Bit 3” on page 57.

The following tables has been updated:

Table 51, “CPOL and CPHA Functionality,” on page 129, Table 59, “UCPOL Bit Set-

tings,” on page 155, Table 72, “Analog Comparator Multiplexed Input(1),” on

page 192, Table 73, “ADC Conversion Time,” on page 197, Table 75, “Input Chan-

nel Selections,” on page 203, and Table 84, “Explanation of Different Variables

used in Figure 103 and the Mapping to the Z-pointer,” on page 218.

5

Changes in “Reading the Calibration Byte” on page 230.

6 Corrected Errors in Cross References.

Changes from Rev.

2486D-03/02 to Rev.

2486E-06/02

All page numbers refers to this document.

1

Updated Some Preliminary Test Limits and Characterization Data

The following tables have been updated:

Table 15, “Reset Characteristics,” on page 36, Table 16, “Internal Voltage Refer-

ence Characteristics,” on page 40, DC Characteristics on page 237, Table , “ADC

Characteristics,” on page 243.

2

3

Changes in External Clock Frequency

Added the description at the end of “External Clock” on page 30.

Added period changing data in Table 99, “External Clock Drive,” on page 239.

Updated TWI Chapter

More details regarding use of the TWI bit rate prescaler and a Table 65, “TWI Bit

Rate Prescaler,” on page 170.

Changes from Rev.

2486C-03/02 to Rev.

2486D-03/02

All page numbers refers to this document.

1

Updated Typical Start-up Times.

The following tables has been updated:

Table 5, “Start-up Times for the Crystal Oscillator Clock Selection,” on page 26,

Table 6, “Start-up Times for the Low-frequency Crystal Oscillator Clock Selection,”

on page 26, Table 8, “Start-up Times for the External RC Oscillator Clock Selec-

tion,” on page 27, and Table 12, “Start-up Times for the External Clock Selection,”

on page 30.

2 Added “ATmega8 Typical Characteristics” on page 244.

294

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Changes from Rev.

2486B-12/01 to Rev.

2486C-03/02

All page numbers refers to this document.

1

Updated TWI Chapter.

More details regarding use of the TWI Power-down operation and using the TWI as

Master with low TWBRR values are added into the datasheet.

Added the note at the end of the “Bit Rate Generator Unit” on page 167.

Added the description at the end of “Address Match Unit” on page 167.

2

Updated Description of OSCCAL Calibration Byte.

In the datasheet, it was not explained how to take advantage of the calibration bytes

for 2, 4, and 8 MHz Oscillator selections. This is now added in the following

sections:

Improved description of “Oscillator Calibration Register – OSCCAL” on page 29 and

“Calibration Byte” on page 221.

3

Added Some Preliminary Test Limits and Characterization Data.

Removed some of the TBD’s in the following tables and pages:

Table 3 on page 24, Table 15 on page 36, Table 16 on page 40, Table 17 on page

42, “TA = -40×C to 85×C, VCC = 2.7V to 5.5V (unless otherwise noted)” on page

237, Table 99 on page 239, and Table 102 on page 241.

4

5

Updated Programming Figures.

Figure 104 on page 222 and Figure 112 on page 232 are updated to also reflect that

AVCC must be connected during Programming mode.

Added a Description on how to Enter Parallel Programming Mode if RESET

Pin is Disabled or if External Oscillators are Selected.

Added a note in section “Enter Programming Mode” on page 224.

295

2486M–AVR–12/03

296

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Table of Contents

Features................................................................................................ 1

Pin Configurations............................................................................... 2

Overview............................................................................................... 3

Block Diagram ...................................................................................................... 3

Disclaimer............................................................................................................. 4

Pin Descriptions.................................................................................................... 5

About Code Examples......................................................................... 6

AVR CPU Core ..................................................................................... 7

Introduction........................................................................................................... 7

Architectural Overview.......................................................................................... 7

Arithmetic Logic Unit – ALU.................................................................................. 9

Status Register ..................................................................................................... 9

General Purpose Register File ........................................................................... 10

Stack Pointer ...................................................................................................... 11

Instruction Execution Timing............................................................................... 12

Reset and Interrupt Handling.............................................................................. 12

AVR ATmega8 Memories .................................................................. 15

In-System Reprogrammable Flash Program Memory ........................................ 15

SRAM Data Memory........................................................................................... 16

Data Memory Access Times............................................................................... 17

EEPROM Data Memory...................................................................................... 17

I/O Memory......................................................................................................... 22

System Clock and Clock Options .................................................... 23

Clock Systems and their Distribution.................................................................. 23

Clock Sources..................................................................................................... 24

Crystal Oscillator................................................................................................. 25

Low-frequency Crystal Oscillator........................................................................ 26

External RC Oscillator ........................................................................................ 27

Calibrated Internal RC Oscillator ........................................................................ 28

External Clock..................................................................................................... 30

Timer/Counter Oscillator..................................................................................... 30

Power Management and Sleep Modes............................................. 31

Idle Mode............................................................................................................ 32

ADC Noise Reduction Mode............................................................................... 32

Power-down Mode.............................................................................................. 32

Power-save Mode............................................................................................... 32

Standby Mode..................................................................................................... 33

Minimizing Power Consumption ......................................................................... 33

1

2486M–AVR–12/03

System Control and Reset................................................................ 35

Internal Voltage Reference................................................................................. 40

Watchdog Timer ................................................................................................. 41

Timed Sequences for Changing the Configuration of the Watchdog Timer ....... 43

Interrupts............................................................................................ 44

Interrupt Vectors in ATmega8............................................................................. 44

I/O Ports.............................................................................................. 49

Introduction......................................................................................................... 49

Ports as General Digital I/O................................................................................ 50

Alternate Port Functions ..................................................................................... 54

Register Description for I/O Ports....................................................................... 63

External Interrupts............................................................................. 64

8-bit Timer/Counter0.......................................................................... 67

Overview............................................................................................................. 67

Timer/Counter Clock Sources............................................................................. 68

Counter Unit........................................................................................................ 68

Operation............................................................................................................ 68

Timer/Counter Timing Diagrams......................................................................... 69

8-bit Timer/Counter Register Description ........................................................... 70

Timer/Counter0 and Timer/Counter1 Prescalers............................ 72

16-bit Timer/Counter1........................................................................ 74

Overview............................................................................................................. 74

Accessing 16-bit Registers ................................................................................. 77

Timer/Counter Clock Sources............................................................................. 80

Counter Unit........................................................................................................ 80

Input Capture Unit............................................................................................... 81

Output Compare Units........................................................................................ 83

Compare Match Output Unit............................................................................... 85

Modes of Operation ............................................................................................ 86

Timer/Counter Timing Diagrams......................................................................... 93

16-bit Timer/Counter Register Description ......................................................... 95

8-bit Timer/Counter2 with PWM and Asynchronous Operation .. 102

Overview........................................................................................................... 102

Timer/Counter Clock Sources........................................................................... 103

Counter Unit...................................................................................................... 104

Output Compare Unit........................................................................................ 105

Compare Match Output Unit............................................................................. 107

Modes of Operation .......................................................................................... 108

Timer/Counter Timing Diagrams....................................................................... 113

2

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

8-bit Timer/Counter Register Description ......................................................... 115

Asynchronous Operation of the Timer/Counter ................................................ 117

Timer/Counter Prescaler................................................................................... 121

Serial Peripheral Interface – SPI..................................................... 122

SS Pin Functionality.......................................................................................... 126

Data Modes ...................................................................................................... 129

USART .............................................................................................. 130

Overview........................................................................................................... 130

Clock Generation.............................................................................................. 131

Frame Formats ................................................................................................. 134

USART Initialization.......................................................................................... 135

Data Transmission – The USART Transmitter ................................................. 137

Data Reception – The USART Receiver .......................................................... 140

Asynchronous Data Reception ......................................................................... 144

Multi-processor Communication Mode ............................................................. 148

Accessing UBRRH/UCSRC Registers.............................................................. 149

USART Register Description ............................................................................ 150

Examples of Baud Rate Setting........................................................................ 156

Two-wire Serial Interface ................................................................ 160

Features............................................................................................................ 160

Two-wire Serial Interface Bus Definition........................................................... 160

Data Transfer and Frame Format..................................................................... 161

Multi-master Bus Systems, Arbitration and Synchronization............................ 164

Overview of the TWI Module ............................................................................ 166

TWI Register Description.................................................................................. 168

Using the TWI................................................................................................... 171

Transmission Modes......................................................................................... 175

Multi-master Systems and Arbitration............................................................... 188

Analog Comparator ......................................................................... 190

Analog Comparator Multiplexed Input .............................................................. 192

Analog-to-Digital Converter............................................................ 193

Features............................................................................................................ 193

Starting a Conversion ....................................................................................... 195

Prescaling and Conversion Timing................................................................... 195

Changing Channel or Reference Selection ...................................................... 197

ADC Noise Canceler......................................................................................... 198

ADC Conversion Result.................................................................................... 202

3

2486M–AVR–12/03

Boot Loader Support – Read-While-Write Self-Programming..... 206

Boot Loader Features....................................................................................... 206

Application and Boot Loader Flash Sections.................................................... 206

Read-While-Write and No Read-While-Write Flash Sections........................... 206

Boot Loader Lock Bits....................................................................................... 208

Entering the Boot Loader Program................................................................... 209

Addressing the Flash During Self-Programming .............................................. 211

Self-Programming the Flash............................................................................. 212

Memory Programming..................................................................... 219

Program And Data Memory Lock Bits .............................................................. 219

Fuse Bits........................................................................................................... 220

Signature Bytes ................................................................................................ 221

Calibration Byte ................................................................................................ 221

Parallel Programming Parameters, Pin Mapping, and Commands .................. 222

Parallel Programming ....................................................................................... 224

Serial Downloading........................................................................................... 232

Serial Programming Pin Mapping..................................................................... 232

Electrical Characteristics................................................................ 237

Absolute Maximum Ratings*............................................................................. 237

DC Characteristics............................................................................................ 237

External Clock Drive Waveforms...................................................................... 239

External Clock Drive ......................................................................................... 239

Two-wire Serial Interface Characteristics ......................................................... 240

SPI Timing Characteristics ............................................................................... 241

ADC Characteristics ......................................................................................... 243

ATmega8 Typical Characteristics .................................................. 244

Register Summary........................................................................... 282

Instruction Set Summary ................................................................ 284

Ordering Information....................................................................... 287

Packaging Information.................................................................... 288

32A ................................................................................................................... 288

28P3 ................................................................................................................. 289

32M1-A ............................................................................................................. 290

Erratas .............................................................................................. 291

ATmega8

Rev. D, E, F, and G .......................................................................................... 291

4

ATmega8(L)

2486M–AVR–12/03

ATmega8(L)

Datasheet Change Log for ATmega8............................................. 292

Changes from Rev. 2486K-08/03 to Rev. 2486L-10/03.................................... 292

Changes from Rev. 2486J-02/03 to Rev. 2486K-08/03.................................... 292

Changes from Rev. 2486I-12/02 to Rev. 2486J-02/03 ..................................... 292

Changes from Rev. 2486H-09/02 to Rev. 2486I-12/02 .................................... 293

Changes from Rev. 2486G-09/02 to Rev. 2486H-09/02................................... 293

Changes from Rev. 2486F-07/02 to Rev. 2486G-09/02................................... 294

Changes from Rev. 2486E-06/02 to Rev. 2486F-07/02 ................................... 294

Changes from Rev. 2486D-03/02 to Rev. 2486E-06/02................................... 294

Changes from Rev. 2486C-03/02 to Rev. 2486D-03/02................................... 294

Changes from Rev. 2486B-12/01 to Rev. 2486C-03/02................................... 295

Table of Contents ................................................................................ 1

5

2486M–AVR–12/03

6

ATmega8(L)

2486M–AVR–12/03

Atmel Corporation

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Printed on recycled paper.

2486M–AVR–12/03


型号：	ATMEGA8-16PC
厂家：	ATMEL
描述：	-bit AVR Microcontroller with 8K Bytes In- System Programmable Flash 位的AVR微控制器8K字节在 - 系统内可编程Flash 微控制器
文件：	总303页 (文件大小：5122K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

ATMEGA8-16PC [ATMEL]

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