ATMEGA3250P-20AU-SL383 [ATMEL]
Microcontroller;
| 型号： | ATMEGA3250P-20AU-SL383 |
| 厂家： | 
ATMEL |
| 描述： | Microcontroller 微控制器 |
| 文件： | 总364页 (文件大小：5293K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
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 20 MIPS Throughput at 20 MHz
– On-Chip 2-cycle Multiplier
• High Endurance Non-volatile Memory segments
– 32K Bytes of In-System Self-programmable Flash program memory
– 1K Bytes EEPROM
– 2K Bytes Internal SRAM
8-bit
– Write/Erase cyles: 10,000 Flash/100,000 EEPROM
– Data retention: 20 years at 85°C/100 years at 25°C(1)
– Optional Boot Code Section with Independent Lock Bits
In-System Programming by On-chip Boot Program
True Read-While-Write Operation
Microcontroller
with 32K Bytes
In-System
Programmable
Flash
– Programming Lock for Software Security
• JTAG (IEEE std. 1149.1 compliant) Interface
– Boundary-scan Capabilities According to the JTAG Standard
– Extensive On-chip Debug Support
– Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface
• Peripheral Features
– Two 8-bit Timer/Counters with Separate Prescaler and Compare Mode
– One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture
Mode
– Real Time Counter with Separate Oscillator
– Four PWM Channels
– 8-channel, 10-bit ADC
ATmega325P/V
ATmega3250P/V
– Programmable Serial USART
– Master/Slave SPI Serial Interface
– Universal Serial Interface with Start Condition Detector
– Programmable Watchdog Timer with Separate On-chip Oscillator
– On-chip Analog Comparator
– Interrupt and Wake-up on Pin Change
• Special Microcontroller Features
– Power-on Reset and Programmable Brown-out Detection
– Internal Calibrated Oscillator
Preliminary
– External and Internal Interrupt Sources
– Five Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, and
Standby
• I/O and Packages
– 54/69 Programmable I/O Lines
– 64-lead TQFP, 64-pad QFN/MLF, and 100-lead TQFP
• Speed Grade:
– ATmega325PV/ATmega3250PV:
0 - 4 MHz @ 1.8 - 5.5V, 0 - 10 MHz @ 2.7 - 5.5V
– ATmega325P/3250P:
0 - 10 MHz @ 2.7 - 5.5V, 0 - 20 MHz @ 4.5 - 5.5V
• Temperature range:
– -40°C to 85°C Industrial
• Ultra-Low Power Consumption
– Active Mode:
420 µA at 1 MHz, 1.8V
– Power-down Mode:
40 nA at 1.8V
– Power-save Mode:
750 nA at 1.8V
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1. Pin Configurations
Figure 1-1. Pinout ATmega3250P
TQFP
1
75
74
73
72
71
70
69
68
67
66
65
64
63
62
61
60
59
58
57
56
55
54
53
52
51
DNC
(RXD/PCINT0) PE0
(TXD/PCINT1) PE1
(XCK/AIN0/PCINT2) PE2
(AIN1/PCINT3) PE3
(USCK/SCL/PCINT4) PE4
(DI/SDA/PCINT5) PE5
(DO/PCINT6) PE6
(CLKO/PCINT7) PE7
VCC
PA3
2
PA4
INDEX CORNER
3
PA5
4
PA6
5
PA7
6
PG2
7
PC7
8
PC6
9
DNC
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
PH3 (PCINT19)
GND
PH2 (PCINT18)
DNC
PH1 (PCINT17)
PH0 (PCINT16)
DNC
(PCINT24) PJ0
(PCINT25) PJ1
DNC
ATmega3250
DNC
DNC
DNC
DNC
DNC
DNC
PC5
(SS/PCINT8) PB0
(SCK/PCINT9) PB1
(MOSI/PCINT10) PB2
(MISO/PCINT11) PB3
(OC0A/PCINT12) PB4
(OC1A/PCINT13) PB5
(OC1B/PCINT14) PB6
PC4
PC3
PC2
PC1
PC0
PG1
PG0
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ATmega325P/3250P
Figure 1-2. Pinout ATmega325P
DNC
(RXD/PCINT0) PE0
(TXD/PCINT1) PE1
1
2
3
PA3
PA4
PA5
PA6
48
47
46
INDEX CORNER
(XCK/AIN0/PCINT2) PE2
4
45
(AIN1/PCINT3) PE3
5
6
PA7
PG2
PC7
PC6
PC5
44
43
(USCK/SCL/PCINT4) PE4
(DI/SDA/PCINT5) PE5
(DO/PCINT6) PE6
7
8
9
42
41
40
ATmega325
(CLKO/PCINT7) PE7
(SS/PCINT8) PB0 10
(SCK/PCINT9) PB1 11
(MOSI/PCINT10) PB2 12
(MISO/PCINT11) PB3 13
39 PC4
PC3
PC2
PC1
PC0
38
37
36
(OC0A/PCINT12) PB4 14
(OC1A/PCINT13) PB5 15
(OC1B/PCINT14) PB6 16
35
34
33
PG1
PG0
Note:
The large center pad underneath the QFN/MLF packages is made of metal and internally con-
nected to GND. It should be soldered or glued to the board to ensure good mechanical stability. If
the center pad is left unconnected, the package might loosen from the board.
1.1
Disclaimer
Typical values contained in this datasheet are based on simulations and characterization of
other AVR microcontrollers manufactured on the same process technology. Min and Max values
will be available after the device is characterized.
2. Overview
The ATmega325P/3250P is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC architecture. By
executing powerful instructions in a single clock cycle, the ATmega325P/3250P achieves throughputs approaching 1 MIPS
per MHz allowing the system designer to optimize power consumption versus processing speed.
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2.1
Block Diagram
Figure 2-1. Block Diagram
PF0 - PF7
PA0 - PA7
PC0 - PC7
GND
VCC
PORTA DRIVERS
PORTF DRIVERS
PORTC DRIVERS
DATA REGISTER
PORTF
DATA DIR.
REG. PORTF
DATA REGISTER
PORTA
DATA DIR.
REG. PORTA
DATA REGISTER
PORTC
DATA DIR.
REG. PORTC
8-BIT DATA BUS
AVCC
CALIB. OSC
AGND
ADC
INTERNAL
OSCILLATOR
AREF
OSCILLATOR
PROGRAM
COUNTER
STACK
POINTER
WATCHDOG
TIMER
JTAG TAP
TIMING AND
CONTROL
PROGRAM
FLASH
MCU CONTROL
REGISTER
SRAM
ON-CHIP DEBUG
BOUNDARY-
SCAN
INSTRUCTION
REGISTER
TIMER/
COUNTERS
GENERAL
PURPOSE
REGISTERS
X
Y
Z
PROGRAMMING
INSTRUCTION
DECODER
INTERRUPT
UNIT
LOGIC
CONTROL
LINES
ALU
EEPROM
STATUS
REGISTER
AVR CPU
UNIVERSAL
SERIAL INTERFACE
SPI
USART
DATA REGISTER
PORTE
DATA DIR.
REG. PORTE
DATA REGISTER
PORTB
DATA DIR.
REG. PORTB
DATA REGISTER
PORTD
DATA DIR.
REG. PORTD
DATA REG. DATA DIR.
PORTG
REG. PORTG
PORTB DRIVERS
PORTD DRIVERS
PORTG DRIVERS
PORTE DRIVERS
PE0 - PE7
PB0 - PB7
PD0 - PD7
PG0 - PG4
The AVR core combines a rich instruction set with 32 general purpose working registers. All the
32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent
registers to be accessed in one single instruction executed in one clock cycle. The resulting
architecture is more code efficient while achieving throughputs up to ten times faster than con-
ventional CISC microcontrollers.
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ATmega325P/3250P
The ATmega325P/3250P provides the following features: 32K bytes of In-System Programma-
ble Flash with Read-While-Write capabilities, 1K bytes EEPROM, 2K byte SRAM, 54/69 general
purpose I/O lines, 32 general purpose working registers, a JTAG interface for Boundary-scan,
On-chip Debugging support and programming, three flexible Timer/Counters with compare
modes, internal and external interrupts, a serial programmable USART, Universal Serial Inter-
face with Start Condition Detector, an 8-channel, 10-bit ADC, 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 sys-
tem 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 hardware reset. In Power-
save mode, the asynchronous timer, allowing the user to maintain a timer base while the rest of
the device is sleeping. The ADC Noise Reduction mode stops the CPU and all I/O modules
except asynchronous timer and ADC, to minimize switching noise during ADC conversions. In
Standby mode, the crystal/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
On-chip In-System re-Programmable (ISP) Flash allows the program memory to be repro-
grammed 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. Soft-
ware 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 ATmega325P/3250P is a powerful
microcontroller that provides a highly flexible and cost effective solution to many embedded con-
trol applications.
The ATmega325P/3250P AVR is supported with a full suite of program and system develop-
ment tools including: C Compilers, Macro Assemblers, Program Debugger/Simulators, In-Circuit
Emulators, and Evaluation kits.
2.2
Comparison between ATmega325P and ATmega3250P
The ATmega325P and ATmega3250P differs only in memory sizes, pin count and pinout. Table
2-1 on page 5 summarizes the different configurations for the four devices.
Table 2-1.
Configuration Summary
General Purpose
I/O Pins
Device
Flash
EEPROM
RAM
ATmega325P
ATmega3250P
32K bytes
32K bytes
1K bytes
1K bytes
2K bytes
2K bytes
54
69
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2.3
Pin Descriptions
The following section describes the I/O-pin special functions.
2.3.1
2.3.2
2.3.3
VCC
Digital supply voltage.
Ground.
GND
Port A (PA7..PA0)
Port A is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port A output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port A pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port A pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
2.3.4
Port B (PB7..PB0)
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port B output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port B pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
Port B has better driving capabilities than the other ports.
Port B also serves the functions of various special features of the ATmega325P/3250P as listed
on page 71.
2.3.5
2.3.6
Port C (PC7..PC0)
Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port C output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port C pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port C pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
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 ATmega325P/3250P as listed
on page 74.
2.3.7
Port E (PE7..PE0)
Port E is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port E output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port E pins that are externally pulled low will source current if the pull-up
6
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ATmega325P/3250P
resistors are activated. The Port E pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
Port E also serves the functions of various special features of the ATmega325P/3250P as listed
on page 75.
2.3.8
Port F (PF7..PF0)
Port F serves as the analog inputs to the A/D Converter.
Port F also serves as an 8-bit bi-directional I/O port, if the A/D Converter is not used. Port pins
can provide internal pull-up resistors (selected for each bit). The Port F output buffers have sym-
metrical drive characteristics with both high sink and source capability. As inputs, Port F pins
that are externally pulled low will source current if the pull-up resistors are activated. The Port F
pins are tri-stated when a reset condition becomes active, even if the clock is not running. If the
JTAG interface is enabled, the pull-up resistors on pins PF7(TDI), PF5(TMS), and PF4(TCK) will
be activated even if a reset occurs.
Port F also serves the functions of the JTAG interface.
2.3.9
Port G (PG5..PG0)
Port G is a 6-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port G output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port G pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port G pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
Port G also serves the functions of various special features of the ATmega325P/3250P as listed
on page 75.
2.3.10
Port H (PH7..PH0)
Port H is a 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port H output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port H pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port H pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
Port H also serves the functions of various special features of the ATmega3250P as listed on
page 75.
2.3.11
Port J (PJ6..PJ0)
Port J is a 7-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port J output buffers have symmetrical drive characteristics with both high sink and source capa-
bility. As inputs, Port J pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port J pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
Port J also serves the functions of various special features of the ATmega3250P as listed on
page 75.
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2.3.12
RESET
Reset input. A low level on this pin for longer than the minimum pulse length will generate a
reset, even if the clock is not running. The minimum pulse length is given in ”System and Reset
Characterizations” on page 308. Shorter pulses are not guaranteed to generate a reset.
2.3.13
2.3.14
2.3.15
XTAL1
XTAL2
AVCC
Input to the inverting Oscillator amplifier and input to the internal clock operating circuit.
Output from the inverting Oscillator amplifier.
AVCC is the supply voltage pin for Port F and the A/D Converter. It should be externally con-
nected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC
through a low-pass filter.
2.3.16
AREF
This is the analog reference pin for the A/D Converter.
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8023F–AVR–07/09
ATmega325P/3250P
3. Resources
A comprehensive set of development tools, application notes and datasheets are available for
download on http://www.atmel.com/avr.
9
8023F–AVR–07/09
Note:
1.
4. Data Retention
Reliability Qualification results show that the projected data retention failure rate is much less
than 1 PPM over 20 years at 85°C or 100 years at 25°C.
5. About Code Examples
This documentation contains simple code examples that briefly show how to use various parts of
the device. These code examples assume that the part specific header file is included before
compilation. Be aware that not all C compiler vendors include bit definitions in the header files
and interrupt handling in C is compiler dependent. Please confirm with the C compiler documen-
tation for more details.
For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”
instructions must be replaced with instructions that allow access to extended I/O. Typically
“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.
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ATmega325P/3250P
6. AVR CPU Core
6.1
Overview
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.
6.2
Architectural Overview
Figure 6-1. Block Diagram of the AVR Architecture
Data Bus 8-bit
Program
Counter
Status
and Control
Flash
Program
Memory
Interrupt
Unit
32 x 8
General
Purpose
Registrers
Instruction
Register
SPI
Unit
Instruction
Decoder
Watchdog
Timer
ALU
Analog
Comparator
Control Lines
I/O Module1
I/O Module 2
I/O Module n
Data
SRAM
EEPROM
I/O Lines
In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with
separate memories and buses for program and data. Instructions in the program memory are
executed with a single level pipelining. While one instruction is being executed, the next instruc-
tion is pre-fetched from the program memory. This concept enables instructions to be executed
in every clock cycle. The program memory is In-System Reprogrammable Flash memory.
The fast-access Register File contains 32 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 typ-
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8023F–AVR–07/09
ical ALU operation, two operands are output from the Register File, the operation is executed,
and the result is stored back in the Register File – in one clock cycle.
Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data
Space addressing – enabling efficient address calculations. One of the these address pointers
can also be used as an address pointer for look up tables in Flash program memory. These
added function registers are the 16-bit X-, Y-, and Z-register, described later in this section.
The ALU supports arithmetic and logic operations between registers or between a constant and
a register. Single register operations can also be executed in the ALU. After an arithmetic opera-
tion, the Status Register is updated to reflect information about the result of the operation.
Program flow is provided by conditional and unconditional jump and call instructions, able to
directly address the whole address space. Most AVR instructions have a single 16-bit word for-
mat. Every program memory address contains a 16- or 32-bit instruction.
Program Flash memory space is divided in two sections, the Boot Program section and the
Application Program section. Both sections have dedicated Lock bits for write and read/write
protection. The SPM instruction that writes into the Application Flash memory section must
reside in the Boot Program section.
During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the
Stack. The Stack is effectively allocated in the general data SRAM, and consequently the Stack
size is only limited by the total SRAM size and the usage of the SRAM. All user programs must
initialize the SP in the Reset routine (before subroutines or interrupts are executed). The Stack
Pointer (SP) is read/write accessible in the I/O space. The data SRAM can easily be accessed
through the five different addressing modes supported in the AVR architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional Global
Interrupt Enable bit in the Status Register. All interrupts have a separate Interrupt Vector in the
Interrupt Vector table. The interrupts have priority in accordance with their Interrupt Vector posi-
tion. The lower the Interrupt Vector address, the higher the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as Control Regis-
ters, SPI, and other I/O functions. The I/O Memory can be accessed directly, or as the Data
Space locations following those of the Register File, 0x20 - 0x5F. In addition, the
ATmega325P/3250P has Extended I/O space from 0x60 - 0xFF in SRAM where only the
ST/STS/STD and LD/LDS/LDD instructions can be used.
6.3
ALU – Arithmetic Logic Unit
The high-performance AVR ALU operates in direct connection with all the 32 general purpose
working registers. Within a single clock cycle, arithmetic operations between general purpose
registers or between a register and an immediate are executed. The ALU operations are divided
into three main categories – arithmetic, logical, and bit-functions. Some implementations of the
architecture also provide a powerful multiplier supporting both signed/unsigned multiplication
and fractional format. See the “Instruction Set” section for a detailed description.
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6.4
AVR Status Register
The Status Register contains information about the result of the most recently executed arithme-
tic instruction. This information can be used for altering program flow in order to perform
conditional operations. Note that the Status Register is updated after all ALU operations, as
specified in the Instruction Set Reference. This will in many cases remove the need for using the
dedicated compare instructions, resulting in faster and more compact code.
The Status Register is not automatically stored when entering an interrupt routine and restored
when returning from an interrupt. This must be handled by software.
6.4.1
SREG – AVR Status Register
Bit
7
I
6
T
5
H
4
S
3
V
2
N
1
Z
0
C
0x3F (0x5F)
Read/Write
Initial Value
SREG
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
• Bit 7 – I: Global Interrupt Enable
The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual inter-
rupt enable control is then performed in separate control registers. If the Global Interrupt Enable
Register is cleared, none of the interrupts are enabled independent of the individual interrupt
enable settings. The I-bit is cleared by hardware after an interrupt has occurred, and is set by
the RETI instruction to enable subsequent interrupts. The I-bit can also be set and cleared by
the application with the SEI and CLI instructions, as described in the instruction set reference.
• Bit 6 – T: Bit Copy Storage
The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or desti-
nation for the operated bit. A bit from a register in the Register File can be copied into T by the
BST instruction, and a bit in T can be copied into a bit in a register in the Register File by the
BLD instruction.
• Bit 5 – H: Half Carry Flag
The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry Is useful
in BCD arithmetic. See the “Instruction Set Description” for detailed information.
• Bit 4 – S: Sign Bit, S = N ⊕ V
The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement
Overflow Flag V. See the “Instruction Set Description” for detailed information.
• Bit 3 – V: Two’s Complement Overflow Flag
The Two’s Complement Overflow Flag V supports two’s complement arithmetics. See the
“Instruction Set Description” for detailed information.
• Bit 2 – N: Negative Flag
The Negative Flag N indicates a negative result in an arithmetic or logic operation. See the
“Instruction Set Description” for detailed information.
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• Bit 1 – Z: Zero Flag
The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the “Instruction
Set Description” for detailed information.
• Bit 0 – C: Carry Flag
The Carry Flag C indicates a carry in an arithmetic or logic operation. See the “Instruction Set
Description” for detailed information.
6.5
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 6-2 shows the structure of the 32 general purpose working registers in the CPU.
Figure 6-2. 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 6-2, each register is also assigned a data memory address, mapping them
directly into the first 32 locations of the user Data Space. Although not being physically imple-
mented as SRAM locations, this memory organization provides great flexibility in access of the
registers, as the X-, Y- and Z-pointer registers can be set to index any register in the file.
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ATmega325P/3250P
6.5.1
The X-register, Y-register, and Z-register
The registers R26..R31 have some added functions to their general purpose usage. These reg-
isters are 16-bit address pointers for indirect addressing of the data space. The three indirect
address registers X, Y, and Z are defined as described in Figure .
The X-, Y-, and Z-registers
15
XH
YH
XL
0
0
X-register
7
0
0
7
R27 (0x1B)
R26 (0x1A)
15
YL
ZL
0
0
Y-register
Z-register
7
7
R29 (0x1D)
R28 (0x1C)
15
ZH
0
0
7
7
0
R31 (0x1F)
R30 (0x1E)
In the different addressing modes these address registers have functions as fixed displacement,
automatic increment, and automatic decrement (see the instruction set reference for details).
6.6
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. Note that the Stack is implemented as
growing from higher to lower memory locations. The Stack Pointer Register always points to the
top of the Stack. The Stack Pointer points to the data SRAM Stack area where the Subroutine
and Interrupt Stacks are located. A Stack PUSH command will decrease the Stack Pointer.
The Stack in the data SRAM must be defined by the program before any subroutine calls are
executed or interrupts are enabled. Initial Stack Pointer value equals the last address of the
internal SRAM and the Stack Pointer must be set to point above start of the SRAM, see Figure
7-2 on page 21.
See Table 6-1 for Stack Pointer details.
Table 6-1.
Stack Pointer instructions
Description
Decremented by 1 Data is pushed onto the stack
Return address is pushed onto the stack with a subroutine call or
Instruction Stack pointer
PUSH
CALL
ICALL
RCALL
Decremented by 2 interrupt
POP
Incremented by 1
Incremented by 2
Data is popped from the stack
RET
RETI
Return address is popped from the stack with return from
subroutine or return from interrupt
The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of
bits actually used is implementation dependent. Note that the data space in some implementa-
tions of the AVR architecture is so small that only SPL is needed. In this case, the SPH Register
will not be present.
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6.6.1
SPH and SPL – Stack Pointer High and Stack Pointer Low
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
0x3E (0x5E)
0x3D (0x5D)
SPH
SPL
Read/Write
Initial Value
R/W
R/W
0
R/W
R/W
0
R/W
R/W
0
R/W
R/W
0
R/W
R/W
0
R/W
R/W
0
R/W
R/W
0
R/W
R/W
0
0
0
0
0
0
0
0
0
6.7
Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR
CPU is driven by the CPU clock clkCPU, directly generated from the selected clock source for the
chip. No internal clock division is used.
Figure 1 shows the parallel instruction fetches and instruction executions enabled by the Har-
vard architecture and the fast-access Register File concept. This is the basic pipelining concept
to obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost,
functions per clocks, and functions per power-unit.
Figure 1. The Parallel Instruction Fetches and Instruction Executions
T1
T2
T3
T4
clkCPU
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
Figure 2 shows the internal timing concept for the Register File. In a single clock cycle an ALU
operation using two register operands is executed, and the result is stored back to the destina-
tion register.
Figure 2. Single Cycle ALU Operation
T1
T2
T3
T4
clkCPU
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
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6.8
Reset and Interrupt Handling
The AVR provides several different interrupt sources. These interrupts and the separate Reset
Vector each have a separate program vector in the program memory space. All interrupts are
assigned individual enable bits which must be written logic one together with the Global Interrupt
Enable bit in the Status Register in order to enable the interrupt. Depending on the Program
Counter value, interrupts may be automatically disabled when Boot Lock bits BLB02 or BLB12
are programmed. This feature improves software security. See the section ”Memory Program-
ming” on page 271 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 53. 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 of the Boot Flash section by setting the IVSEL
bit in the MCU Control Register (MCUCR). Refer to ”Interrupts” on page 53 for more information.
The Reset Vector can also be moved to the start of the Boot Flash section by programming the
BOOTRST Fuse, see ”Boot Loader Support – Read-While-Write Self-Programming” on page
256.
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are dis-
abled. The user software can write logic one to the I-bit to enable nested interrupts. All enabled
interrupts can then interrupt the current interrupt routine. The I-bit is automatically set when a
Return from Interrupt instruction – RETI – is executed.
There are basically two types of interrupts. The first type is triggered by an event that sets the
Interrupt Flag. For these interrupts, the Program Counter is vectored to the actual Interrupt Vec-
tor in order to execute the interrupt handling routine, and hardware clears the corresponding
Interrupt Flag. Interrupt Flags can also be cleared by writing a logic one to the flag bit position(s)
to be cleared. If an interrupt condition occurs while the corresponding interrupt enable bit is
cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled, or the flag is
cleared by software. Similarly, if one or more interrupt conditions occur while the Global Interrupt
Enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the
Global Interrupt Enable bit is set, and will then be executed by order of priority.
The second type of interrupts will trigger as long as the interrupt condition is present. These
interrupts do not necessarily have Interrupt Flags. If the interrupt condition disappears before the
interrupt is enabled, the interrupt will not be triggered.
When the AVR exits from an interrupt, it will always return to the main program and execute one
more instruction before any pending interrupt is served.
Note that the Status Register is not automatically stored when entering an interrupt routine, nor
restored when returning from an interrupt routine. This must be handled by software.
When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled.
No interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the
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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 */
__disable_interrupt();
EECR |= (1<<EEMWE); /* start EEPROM write */
EECR |= (1<<EEWE);
SREG = cSREG; /* restore SREG value (I-bit) */
When using the SEI instruction to enable interrupts, the instruction following SEI will be exe-
cuted before any pending interrupts, as shown in this example.
Assembly Code Example
sei ; set Global Interrupt Enable
sleep; enter sleep, waiting for interrupt
; note: will enter sleep before any pending
; interrupt(s)
C Code Example
__enable_interrupt(); /* set Global Interrupt Enable */
__sleep(); /* enter sleep, waiting for interrupt */
/* note: will enter sleep before any pending interrupt(s) */
6.8.1
Interrupt Response Time
The interrupt execution response for all the enabled AVR interrupts is four clock cycles mini-
mum. After four clock cycles the program vector address for the actual interrupt handling routine
is executed. During this four clock cycle period, the Program Counter is pushed onto the Stack.
The vector is normally a jump to the interrupt routine, and this jump takes three clock cycles. If
an interrupt occurs during execution of a multi-cycle instruction, this instruction is completed
before the interrupt is served. If an interrupt occurs when the MCU is in sleep mode, the interrupt
execution response time is increased by four clock cycles. This increase comes in addition to the
start-up time from the selected sleep mode.
A return from an interrupt handling routine takes four clock cycles. During these four clock
cycles, the Program Counter (two bytes) is popped back from the Stack, the Stack Pointer is
incremented by two, and the I-bit in SREG is set.
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7. AVR Memories
7.1
Overview
This section describes the different memories in the ATmega325P/3250P. The AVR architecture
has two main memory spaces, the Data Memory and the Program Memory space. In addition,
the ATmega325P/3250P features an EEPROM Memory for data storage. All three memory
spaces are linear.
7.2
In-System Reprogrammable Flash Program Memory
The ATmega325P/3250P contains 32K bytes On-chip In-System Reprogrammable Flash mem-
ory for program storage. Since all AVR instructions are 16 or 32 bits wide, the Flash is organized
as 16K x 16. For software security, the Flash Program memory space is divided into two sec-
tions, Boot Program section and Application Program section.
The Flash memory has an endurance of at least 10,000 write/erase cycles. The
ATmega325P/3250P Program Counter (PC) is 14 bits wide, thus addressing the 16K program
memory locations. The operation of Boot Program section and associated Boot Lock bits for
software protection are described in detail in ”Boot Loader Support – Read-While-Write Self-Pro-
gramming” on page 256. ”Memory Programming” on page 271 contains a detailed description
on Flash data serial downloading using the SPI pins or the JTAG interface.
Constant tables can be allocated within the entire program memory address space (see the LPM
– Load Program Memory instruction description).
Timing diagrams for instruction fetch and execution are presented in ”Instruction Execution Tim-
ing” on page 16.
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Figure 7-1. Program Memory Map
Program Memory
0x0000
Application Flash Section
Boot Flash Section
0x3FFF
7.3
SRAM Data Memory
Figure 7-2 shows how the ATmega325P/3250P SRAM Memory is organized.
The ATmega325P/3250P is a complex microcontroller with more peripheral units than can be
supported within the 64 locations reserved in the Opcode for the IN and OUT instructions. For
the Extended I/O space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD
instructions can be used.
The lower 2304 data memory locations address both the Register File, the I/O memory,
Extended I/O memory, and the internal data SRAM. The first 32 locations address the Register
File, the next 64 location the standard I/O memory, then 160 locations of Extended I/O memory,
and the next 2048 locations address the internal data SRAM.
The five different addressing modes for the data memory cover: Direct, Indirect with Displace-
ment, Indirect, Indirect with Pre-decrement, and Indirect with Post-increment. In the Register
File, registers R26 to R31 feature the indirect addressing pointer registers.
The direct addressing reaches the entire data space.
The Indirect with Displacement mode reaches 63 address locations from the base address given
by the Y- or Z-register.
When using register indirect addressing modes with automatic pre-decrement and post-incre-
ment, the address registers X, Y, and Z are decremented or incremented.
The 32 general purpose working registers, 64 I/O Registers, 160 Extended I/O Registers, and
the 2,048 bytes of internal data SRAM in the ATmega325P/3250P are all accessible through all
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ATmega325P/3250P
these addressing modes. The Register File is described in ”General Purpose Register File” on
page 14.
Figure 7-2. Data Memory Map
Data Memory
0x0000 - 0x001F
0x0020 - 0x005F
0x0060 - 0x00FF
32 Registers
64 I/O Registers
160 Ext I/O Reg.
0x0100
Internal SRAM
(2048 x 8)
0x08FF
7.3.1
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 clkCPU cycles as described in Figure 7-3.
Figure 7-3. On-chip Data SRAM Access Cycles
T1
T2
T3
clkCPU
Address valid
Compute Address
Address
Data
WR
Data
RD
Memory Access Instruction
Next Instruction
7.4
EEPROM Data Memory
The ATmega325P/3250P contains 1K bytes of data EEPROM memory. It is organized as a sep-
arate data space, in which single bytes can be read and written. The EEPROM has an
endurance of at least 100,000 write/erase cycles. The access between the EEPROM and the
CPU is described in the following, specifying the EEPROM Address Registers, the EEPROM
Data Register, and the EEPROM Control Register.
For a detailed description of SPI, JTAG and Parallel data downloading to the EEPROM, see
page 285, page 291, and page 274 respectively.
7.4.1
EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space.
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The write access time for the EEPROM is given in Table 7-1. A self-timing function, however,
lets the user software detect when the next byte can be written. If the user code contains instruc-
tions that write the EEPROM, some precautions must be taken. In heavily filtered power
supplies, VCC is likely to rise or fall slowly on power-up/down. This causes the device for some
period of time to run at a voltage lower than specified as minimum for the clock frequency used.
See Section “7.4.3” on page 22. for details on how to avoid problems in these situations.
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed.
Refer to the description of the EEPROM Control Register for details on this.
When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is
executed. When the EEPROM is written, the CPU is halted for two clock cycles before the next
instruction is executed.
7.4.2
7.4.3
EEPROM Write During Power-down Sleep Mode
When entering Power-down sleep mode while an EEPROM write operation is active, the
EEPROM write operation will continue, and will complete before the Write Access time has
passed. However, when the write operation is completed, the clock 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 VCC, the EEPROM data can be corrupted because the supply voltage is
too low for the CPU and the EEPROM to operate properly. These issues are the same as for
board level systems using EEPROM, and the same design solutions should be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too low. First,
a regular write sequence to the EEPROM requires a minimum voltage to operate correctly. Sec-
ondly, the CPU itself can execute instructions incorrectly, if the supply voltage is too low.
EEPROM data corruption can easily be avoided by following this design recommendation:
Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can
be done by enabling the internal Brown-out Detector (BOD). If the detection level of the internal
BOD does not match the needed detection level, an external low VCC reset Protection circuit can
be used. If a reset occurs while a write operation is in progress, the write operation will be com-
pleted provided that the power supply voltage is sufficient.
7.5
I/O Memory
The I/O space definition of the ATmega325P/3250P is shown in ”Register Summary” on page
342.
All ATmega325P/3250P I/Os and peripherals are placed in the I/O space. All I/O locations may
be accessed by the LD/LDS/LDD and ST/STS/STD instructions, transferring data between the
32 general purpose 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. The ATmega325P/3250P is a
complex microcontroller with more peripheral units than can be supported within the 64 location
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ATmega325P/3250P
reserved in Opcode for the IN and OUT instructions. For the Extended I/O space from 0x60 -
0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
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, unlike most
other AVRs, the CBI and SBI instructions will only operate on the specified bit, and can therefore
be used on registers containing such Status Flags. 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.
7.6
Register Description
7.6.1
EEARH and EEARL – The EEPROM Address Register
Bit
15
14
13
12
11
10
EEAR10
EEAR2
2
9
EEAR9
EEAR1
1
8
EEAR8
EEAR0
0
0x22 (0x42)
0x21 (0x41)
–
–
–
–
–
EEARH
EEARL
EEAR7
EEAR6
EEAR5
EEAR4
EEAR3
7
R
6
R
5
R
4
R
3
R
Read/Write
Initial Value
R/W
R/W
X
R/W
R/W
X
R/W
R/W
X
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
X
X
X
X
X
X
X
X
• Bits 15:11 – Res: Reserved Bits
These bits are reserved and will always read as zero.
• Bits 10:0 – EEAR10:0: EEPROM Address
The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address in the 1K
bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and 1023.
The initial value of EEAR is undefined. A proper value must be written before the EEPROM may
be accessed.
Note:
EEAR10 is only valid for ATmega645P and ATmega6450P.
7.6.2
EEDR – The EEPROM Data Register
Bit
7
6
5
4
3
2
1
0
0x20 (0x40)
Read/Write
Initial Value
MSB
R/W
0
LSB
R/W
0
EEDR
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
• Bits 7:0 – EEDR7:0: EEPROM Data
For the EEPROM write operation, the EEDR Register contains the data to be written to the
EEPROM in the address given by the EEAR Register. For the EEPROM read operation, the
EEDR contains the data read out from the EEPROM at the address given by EEAR.
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7.6.3
EECR – The EEPROM Control Register
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
0x1F (0x3F)
Read/Write
Initial Value
EECR
R
0
R
0
R
0
R
0
• Bits 7:4 – Res: Reserved Bits
These bits are reserved and will always read as zero.
• Bit 3 – EERIE: EEPROM Ready Interrupt Enable
Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set. Writing
EERIE to zero disables the interrupt. The EEPROM Ready interrupt generates a constant inter-
rupt when EEWE is cleared.
• Bit 2 – EEMWE: EEPROM Master Write Enable
The EEMWE bit determines whether setting EEWE to one causes the EEPROM to be written.
When EEMWE is set, setting EEWE within four clock cycles will write data to the EEPROM at
the selected address If EEMWE is zero, setting EEWE will have no effect. When EEMWE has
been written to one by software, hardware clears the bit to zero after four clock cycles. See the
description of the EEWE bit for an EEPROM write procedure.
• Bit 1 – EEWE: EEPROM Write Enable
The EEPROM Write Enable Signal EEWE is the write strobe to the EEPROM. When address
and data are correctly set up, the EEWE bit must be written to one to write the value into the
EEPROM. The EEMWE bit must be written to one before a logical one is written to EEWE, oth-
erwise no EEPROM write takes place. The following procedure should be followed when writing
the EEPROM (the order of steps 3 and 4 is not essential):
1. Wait until EEWE becomes zero.
2. Wait until SPMEN in SPMCSR 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 256 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.
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When the write access time has elapsed, the EEWE bit is cleared by hardware. The user soft-
ware can poll this bit and wait for a zero before writing the next byte. When EEWE has been set,
the CPU is halted for two cycles before the next instruction is executed.
• Bit 0 – EERE: EEPROM Read Enable
The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the 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 7-1 lists the typical pro-
gramming time for EEPROM access from the CPU.
Table 7-1.
EEPROM Programming Time
Number of Calibrated
Symbol
RC Oscillator Cycles
Typical Programming Time
EEPROM write (from CPU)
27,072
3.4 ms
The following code examples show one assembly and one C function for writing to the
EEPROM. The examples assume that interrupts are controlled (e.g. by disabling interrupts glob-
ally) so that no interrupts will occur during execution of these functions. The examples also
assume that no Flash Boot Loader is present in the software. If such code is present, the
EEPROM write function must also wait for any ongoing SPM command to finish.
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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);
}
The next code examples show assembly and C functions for reading the EEPROM. The exam-
ples assume that interrupts are controlled so that no interrupts will occur during execution of
these functions.
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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;
}
7.6.4
General Purpose I/O Registers
The ATmega325P/3250P contains three General Purpose I/O Registers. These registers can be
used for storing any information, and they are particularly useful for storing global variables and
Status Flags. General Purpose I/O Registers within the address range 0x00 - 0x1F are directly
bit-accessible using the SBI, CBI, SBIS, and SBIC instructions.
7.6.5
7.6.6
GPIOR2 – General Purpose I/O Register 2
Bit
0x2B (0x4B)
7
6
5
4
3
2
1
0
MSB
LSB
R/W
0
GPIOR2
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
GPIOR1 – General Purpose I/O Register 1
Bit
7
6
5
4
3
2
1
0
0x2A (0x4A)
Read/Write
Initial Value
MSB
R/W
0
LSB
R/W
0
GPIOR1
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
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7.6.7
GPIOR0 – General Purpose I/O Register 0
Bit
7
6
5
4
3
2
1
0
0x1E (0x3E)
Read/Write
Initial Value
MSB
R/W
0
LSB
R/W
0
GPIOR0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
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8. System Clock and Clock Options
8.1
Clock Systems and their Distribution
Figure 8-1 presents the principal clock systems in the AVR and their distribution. All of the clocks
need not be active at a given time. In order to reduce power consumption, the clocks to modules
not being used can be halted by using different sleep modes, as described in ”Power Manage-
ment and Sleep Modes” on page 38. The clock systems are detailed below.
Figure 8-1. Clock Distribution
Asynchronous
Timer/Counter
General I/O
Modules
Flash and
EEPROM
CPU Core
RAM
clkI/O
clkCPU
AVR Clock
Control Unit
clkASY
clkFLASH
Reset Logic
Watchdog Timer
Source clock
Watchdog clock
Clock
Multiplexer
Watchdog
Oscillator
Timer/Counter
Oscillator
Crystal
Oscillator
Low-frequency
Crystal Oscillator
Calibrated RC
Oscillator
External Clock
8.1.1
8.1.2
CPU Clock – clkCPU
The CPU clock is routed to parts of the system concerned with operation of the AVR core.
Examples of such modules are the General Purpose Register File, the Status Register and the
data memory holding the Stack Pointer. Halting the CPU clock inhibits the core from performing
general operations and calculations.
I/O Clock – clkI/O
The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART.
The I/O clock is also used by the External Interrupt module, but note that some external inter-
rupts are detected by asynchronous logic, allowing such interrupts to be detected even if the I/O
clock is halted. Also note that start condition detection in the USI module is carried out asynchro-
nously when clkI/O is halted, enabling USI start condition detection in all sleep modes.
8.1.3
Flash Clock – clkFLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually active simul-
taneously with the CPU clock.
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8.1.4
8.1.5
8.2
Asynchronous Timer Clock – clkASY
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly
from an external clock or 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.
ADC Clock – clkADC
The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks
in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion
results.
Clock Sources
The device has the following clock source options, selectable by Flash Fuse bits as shown
below. The clock from the selected source is input to the AVR clock generator, and routed to the
appropriate modules.
Table 8-1.
Device Clocking Options Select(1)
Device Clocking Option
External Crystal/Ceramic Resonator
External Low-frequency Crystal
Calibrated Internal RC Oscillator
External Clock
CKSEL3..0
1111 - 1000
0111 - 0110
0010
0000
Reserved
0011, 0001, 0101, 0100
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 an additional delay allowing the power to reach a stable level before com-
mencing 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 8-
2. The frequency of the Watchdog Oscillator is voltage dependent as shown in ”Typical Charac-
teristics” on page 314.
Table 8-2.
Typ Time-out (VCC = 5.0V)
4.1 ms
65 ms
Number of Watchdog Oscillator Cycles
Typ Time-out (VCC = 3.0V)
4.3 ms
69 ms
Number of Cycles
4K (4,096)
64K (65,536)
8.3
Default Clock Source
The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default
clock source setting is the Internal RC Oscillator with longest start-up time and an initial system
clock prescaling of 8, resulting in 1.0 MHz system clock. This default setting ensures that all
users can make their desired clock source setting using an In-System or Parallel programmer.
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8.4
Crystal Oscillator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be con-
figured for use as an On-chip Oscillator, as shown in Figure 8-2. Either a quartz crystal or a
ceramic resonator may be used.
C1 and C2 should always be equal for both crystals and resonators. The optimal value of the
capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the
electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for
use with crystals are given in Table 8-3. For ceramic resonators, the capacitor values given by
the manufacturer should be used.
Figure 8-2. Crystal Oscillator Connections
C2
XTAL2
C1
XTAL1
GND
The Oscillator can operate in three different modes, each optimized for a specific frequency
range. The operating mode is selected by the fuses CKSEL3..1 as shown in Table 8-3.
Table 8-3.
Crystal Oscillator Operating Modes
Frequency Range
Recommended Range for Capacitors C1 and
C2 for Use with Crystals (pF)
CKSEL3..1
100(1)
(MHz)
0.4 - 0.9
0.9 - 3.0
3.0 - 8.0
8.0 -
–
101
110
111
12 - 22
12 - 22
12 - 22
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
8-4.
Table 8-4.
Start-up Times for the Crystal Oscillator Clock Selection
Start-up Time from
Power-down and
Power-save
AdditionalDelayfrom
Reset
CKSEL0
SUT1..0
(VCC = 5.0V)
Recommended Usage
Ceramic resonator, fast
rising power
0
00
258 CK(1)
258 CK(1)
1K CK(2)
14CK + 4.1 ms
14CK + 65 ms
14CK
Ceramic resonator,
slowly rising power
0
0
01
10
Ceramic resonator,
BOD enabled
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Table 8-4.
Start-up Times for the Crystal Oscillator Clock Selection (Continued)
Start-up Time from
Power-down and
Power-save
AdditionalDelayfrom
Reset
CKSEL0
SUT1..0
(VCC = 5.0V)
Recommended Usage
0
11
1K CK(2)
1K CK(2)
16K CK
16K CK
16K CK
14CK + 4.1 ms
14CK + 65 ms
14CK
Ceramic resonator, fast
rising power
1
00
01
10
11
Ceramic resonator,
slowly rising power
Crystal Oscillator, BOD
enabled
1
1
1
14CK + 4.1 ms
14CK + 65 ms
Crystal Oscillator, fast
rising power
Crystal Oscillator, slowly
rising power
Notes: 1. These options should only be used when not operating close to the maximum frequency of the
device, and only if frequency stability at start-up is not important for the application. These
options are not suitable for crystals.
2. These options are intended for use with ceramic resonators and will ensure frequency stability
at start-up. They can also be used with crystals when not operating close to the maximum fre-
quency of the device, and if frequency stability at start-up is not important for the application.
8.5
Low-frequency Crystal Oscillator
The Low-frequency Crystal Oscillator is optimized for use with a 32.768 kHz watch crystal.
When selecting crystals, load capasitance and crystal’s Equivalent Series Resistance, ESR
must be taken into consideration. Both values are specified by the crystal vendor.
ATmega325P/3250P oscillator is optimized for very low power consumption, and thus when
selecting crystals, see Table 8-5 on page 32 for maximum ESR recommendations on 6.5 pF,
9.0 pF and 12.5 pF crystals
Table 8-5.
Maximum ESR Recommendation for 32.768 kHz Watch Crystal
Crystal CL (pF)
Max ESR [kΩ](1)
6.5
9.0
75
65
30
12.5
Note:
1. Maximum ESR is typical value based on characterization
The Low-frequency Crystal Oscillator provides an internal load capacitance of typical 6 pF at
each TOSC pin. The external capacitance (C) needed at each TOSC pin can be calculated by
using:
C = 2 ⋅ CL – Cs
where CL is the load capacitance for a 32.768 kHz crystal specified by the crystal vendor and CS
is the total stray capacitance for one TOSC pin.
Crystals specifying load capacitance (CL) higher than 6 pF, require external capacitors applied
as described in Figure 8-2 on page 31.
To find suitable load capacitance for a 32.768 kHz crystal, please consult the crystal datasheet.
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The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or
“0111” as shown in Table 8-7. Start-up times are determined by the SUT Fuses as shown in
Table 8-6.
Table 8-6.
Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0
00
Additional Delay from Reset (VCC = 5.0V) Recommended Usage
4 CK
Fast rising power or BOD enabled
Slowly rising power
01
4 CK + 4.1 ms
4 CK + 65 ms
10
Stable frequency at start-up
11
Reserved
Table 8-7.
Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
Start-up Time from
CKSEL3..0
0110(1)
Power-down and Power-save
Recommended Usage
1K CK
0111
32K CK
Stable frequency at start-up
Note:
1. This option should only be used if frequency stability at start-up is not important for the
application
8.6
Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8 MHz clock. Though voltage and
temperature dependent, this clock can be very accurately calibrated by the user. See Table 26-1
on page 307 for more details. The device is shipped with the CKDIV8 Fuse programmed. See
”System Clock Prescaler” on page 35 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 8-8 on page 33. If selected, it will operate with no external components. During reset,
hardware loads the pre-programmed calibration value into the OSCCAL Register and thereby
automatically calibrates the RC Oscillator. The accuracy of this calibration is shown as Factory
calibration in Table 26-1 on page 307.
By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on
page 36, it is possible to get a higher calibration accuracy than by using the factory calibration.
The accuracy of this calibration is shown as User calibration in Table 26-1 on page 307.
When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the
Watchdog Timer and for the Reset Time-out. For more information on the pre-programmed cali-
bration value, see the section ”Calibration Byte” on page 274.
Table 8-8.
Internal Calibrated RC Oscillator Operating Modes(1)(3)
Frequency Range(2) (MHz)
CKSEL3..0
7.3 - 8.1
0010
Notes: 1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8.
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When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-9 on page 34.
Table 8-9.
Start-up times for the internal calibrated RC Oscillator clock selection
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
Power Conditions
BOD enabled
SUT1..0
00
6 CK
6 CK
14CK
Fast rising power
Slowly rising power
14CK + 4.1 ms
14CK + 65 ms(1)
01
6 CK
10
Reserved
11
Note:
1. The device is shipped with this option selected.
8.7
External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
3. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Figure 3. External Clock Drive Configuration
NC
XTAL2
EXTERNAL
CLOCK
XTAL1
GND
SIGNAL
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 2.
Table 1. Crystal Oscillator Clock Frequency
CKSEL3..0
Frequency Range
0000
0 - 16 MHz
Table 2. Start-up Times for the External Clock Selection
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
SUT1..0
00
Recommended Usage
BOD enabled
6 CK
6 CK
6 CK
14CK
01
14CK + 4.1 ms
14CK + 65 ms
Reserved
Fast rising power
Slowly rising power
10
11
When applying an external clock, it is required to avoid sudden changes in the applied clock fre-
quency to ensure stable operation of the MCU. A variation in frequency of more than 2% from
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ATmega325P/3250P
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.
Note that the System Clock Prescaler can be used to implement run-time changes of the internal
clock frequency while still ensuring stable operation. Refer to ”System Clock Prescaler” on page
35 for details.
8.8
8.9
Clock Output Buffer
When the CKOUT Fuse is programmed, the system Clock will be output on CLKO. This mode is
suitable when the chip clock is used to drive other circuits on the system. The clock will be out-
put also during reset and the normal operation of I/O pin will be overridden when the fuse is
programmed. Any clock source, including internal RC Oscillator, can be selected when CLKO
serves as clock output. If the System Clock Prescaler is used, it is the divided system clock that
is output when the CKOUT Fuse is programmed.
Timer/Counter Oscillator
ATmega325P/3250P uses the same crystal oscillator for Low-frequency Oscillator and
Timer/Counter Oscillator. See ”Low-frequency Crystal Oscillator” on page 32 for details on the
oscillator and crystal requirements.
ATmega325P/3250P share the Timer/Counter Oscillator Pins (TOSC1 and TOSC2) with XTAL1
and XTAL2. When using the Timer/Counter Oscillator, the system clock needs to be four times
the oscillator frequency. Due to this and the pin sharing, the Timer/Counter Oscillator can only
be used when the Calibrated Internal RC Oscillator is selected as system clock source.
Applying an external clock source to TOSC1 can be done if EXTCLK in the ASSR Register is
written to logic one. See ”Timer/Counter Timing Diagram, Clear Timer on Compare Match mode,
with Prescaler (fclk_I/O/8)” on page 146 for further description on selecting external clock as
input instead of a 32.768 kHz watch crystal.
8.10 System Clock Prescaler
The ATmega325P/3250P system clock can be divided by setting the Clock Prescale Register –
CLKPR. This feature can be used to decrease power consumption when the requirement for
processing power is low. This can be used with all clock source options, and it will affect the
clock frequency of the CPU and all synchronous peripherals. clkI/O, clkADC, clkCPU, and clkFLASH
are divided by a factor as shown in Table 8-10.
8.10.1
Switching Time
When switching between prescaler settings, the System Clock Prescaler ensures that no
glitches occur in the clock system and that no intermediate frequency is higher than neither the
clock frequency corresponding to the previous setting, nor the clock frequency corresponding to
the new setting.
The ripple counter that implements the prescaler runs at the frequency of the undivided clock,
which may be faster than the CPU’s clock frequency. Hence, it is not possible to determine the
state of the prescaler – even if it were readable, and the exact time it takes to switch from one
clock division to another cannot be exactly predicted.
From the time the CLKPS values are written, it takes between T1 + T2 and T1 + 2*T2 before the
new clock frequency is active. In this interval, 2 active clock edges are produced. Here, T1 is the
previous clock period, and T2 is the period corresponding to the new prescaler setting.
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8.11 Register Description
8.11.1
OSCCAL – Oscillator Calibration Register
Bit
7
6
5
4
3
2
1
0
(0x66)
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
The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to
remove process variations from the oscillator frequency. A pre-programmed calibration value is
automatically written to this register during chip reset, giving the Factory calibrated frequency as
specified in Table 26-1 on page 307. The application software can write this register to change
the oscillator frequency. The oscillator can be calibrated to frequencies as specified in Table 26-
1 on page 307. Calibration outside that range is not guaranteed.
Note that this oscillator is used to time EEPROM and Flash write accesses, and these write
times will be affected accordingly. If the EEPROM or Flash are written, do not calibrate to more
than 8.8 MHz. Otherwise, the EEPROM or Flash write may fail.
The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the
lowest frequency range, setting this bit to 1 gives the highest frequency range. The two fre-
quency ranges are overlapping, in other words a setting of OSCCAL = 0x7F gives a higher
frequency than OSCCAL = 0x80.
The CAL6..0 bits are used to tune the frequency within the selected range. A setting of 0x00
gives the lowest frequency in that range, and a setting of 0x7F gives the highest frequency in the
range.
8.11.2
CLKPR – Clock Prescale Register
Bit
7
6
–
5
–
4
–
3
2
1
0
CLKPCE
R/W
0
CLKPS3
R/W
CLKPS2
R/W
CLKPS1
R/W
CLKPS0
R/W
CLKPR
(0x61)
Read/Write
Initial Value
R
0
R
0
R
0
See Bit Description
• Bit 7 – CLKPCE: Clock Prescaler Change Enable
The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE
bit is only updated when the other bits in CLKPR are simultaneously written to zero. CLKPCE is
cleared by hardware four cycles after it is written or when CLKPS bits are written. Rewriting the
CLKPCE bit within this time-out period does neither extend the time-out period, nor clear the
CLKPCE bit.
• Bits 3:0 – CLKPS3:0: Clock Prescaler Select Bits 3 - 0
These bits define the division factor between the selected clock source and the internal system
clock. These bits can be written run-time to vary the clock frequency to suit the application
requirements. As the divider divides the master clock input to the MCU, the speed of all synchro-
nous peripherals is reduced when a division factor is used. The division factors are given in
Table 8-10.
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To avoid unintentional changes of clock frequency, a special write procedure must be followed
to change the CLKPS bits:
1. Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in
CLKPR to zero.
2. Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is
not interrupted.
The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed,
the CLKPS bits will be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to
“0011”, giving a division factor of 8 at start up. This feature should be used if the selected clock
source has a higher frequency than the maximum frequency of the device at the present operat-
ing conditions. Note that any value can be written to the CLKPS bits regardless of the CKDIV8
Fuse setting. The Application software must ensure that a sufficient division factor is chosen if
the selected clock source has a higher frequency than the maximum frequency of the device at
the present operating conditions. The device is shipped with the CKDIV8 Fuse programmed.
Table 8-10. Clock Prescaler Select
CLKPS3
CLKPS2
CLKPS1
CLKPS0
Clock Division Factor
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
1
2
4
8
16
32
64
128
256
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
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9. Power Management and Sleep Modes
9.1
Overview
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.
When enabled, the Brown-out Detector (BOD) actively monitors the power supply voltage during
the sleep periods. To further save power, it is possible to disable the BOD in some sleep modes.
See ”BOD Disable” on page 39 for more details.
9.2
Sleep Modes
Figure 8-1 on page 29 presents the different clock systems in the ATmega325P/3250P, and
their distribution. The figure is helpful in selecting an appropriate sleep mode. Table 9-1 shows
the different sleep modes, their wake up sources and BOD disable ability.
Table 9-1.
Active Clock Domains and Wake-up Sources in the Different Sleep Modes.
Active Clock Domains Oscillators Wake-up Sources
Sleep Mode
Idle
X
X
X
X
X
X
X
X(2)
X(2)
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
ADCNRM
Power-down
Power-save
Standby(1)
X(3)
X(3)
X(3)
X(3)
X(2)
X
X
X
X
X(2)
X
X
Notes: 1. Only recommended with external crystal or resonator selected as clock source.
2. If Timer/Counter2 is running in asynchronous mode.
3. For INT0, only level interrupt.
To enter any of the sleep modes, the SE bit in SMCR must be written to logic one and a SLEEP
instruction must be executed. The SM2, SM1, and SM0 bits in the SMCR Register select which
sleep mode will be activated by the SLEEP instruction. See Table 9-2 on page 43 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, executes the interrupt routine, and
resumes execution from the instruction following SLEEP. The contents of the Register File and
SRAM are unaltered when the device wakes up from sleep. If a reset occurs during sleep mode,
the MCU wakes up and executes from the Reset Vector.
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9.3
BOD Disable
When the Brown-out Detector (BOD) is enabled by BODLEVEL fuses, Table 25-3 on page 272,
the BOD is actively monitoring the power supply voltage during a sleep period. To save power, it
is possible to disable the BOD by software for some of the sleep modes, see Table 9-1 on page
38. The sleep mode power consumption will then be at the same level as when BOD is globally
disabled by fuses. If BOD is disabled in software, the BOD function is turned off immediately
after entering the sleep mode. Upon wake-up from sleep, BOD is automatically enabled again.
This ensures safe operation in case the VCC level has dropped during the sleep period.
When the BOD has been disabled, the wake-up time from sleep mode will be approximately 60
µs to ensure that the BOD is working correctly before the MCU continues executing code.
BOD disable is controlled by bit 6, BODS (BOD Sleep) in the control register MCUCR, see
”MCUCR – MCU Control Register” on page 43. Writing this bit to one turns off the BOD in rele-
vant sleep modes, while a zero in this bit keeps BOD active. Default setting keeps BOD active,
i.e. BODS set to zero.
Writing to the BODS bit is controlled by a timed sequence and an enable bit, see ”MCUCR –
MCU Control Register” on page 43.
9.4
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 the SPI, USART, Analog Comparator, ADC, USI, Timer/Coun-
ters, Watchdog, and the interrupt system to continue operating. This sleep mode basically halts
clkCPU and clkFLASH, while allowing the other clocks to run.
Idle mode enables the MCU to wake up from external triggered interrupts as well as internal
ones like the Timer Overflow and USART Transmit Complete interrupts. If wake-up from the
Analog Comparator interrupt is not required, the Analog Comparator can be powered down by
setting the ACD bit in the Analog Comparator Control and Status Register – ACSR. This will
reduce power consumption in Idle mode. If the ADC is enabled, a conversion starts automati-
cally when this mode is entered.
9.5
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 USI
start condition detection, Timer/Counter2 and the Watchdog to continue operating (if enabled).
This sleep mode basically halts clkI/O, clkCPU, and clkFLASH, while allowing the other clocks to run.
This improves the noise environment for the ADC, enabling higher resolution measurements. If
the ADC is enabled, a conversion starts automatically when this mode is entered. Apart form the
ADC Conversion Complete interrupt, only an External Reset, a Watchdog Reset, a Brown-out
Reset, USI start condition interrupt, a Timer/Counter2 interrupt, an SPM/EEPROM ready inter-
rupt, an external level interrupt on INT0 or a pin change interrupt can wake up the MCU from
ADC Noise Reduction mode.
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9.6
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
USI start condition detection, and the Watchdog continue operating (if enabled). Only an Exter-
nal Reset, a Watchdog Reset, a Brown-out Reset, USI start condition interrupt, an external level
interrupt on INT0, or a pin change interrupt 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 Interrupts” on page 58
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 30.
9.7
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 enabled, it will keep running 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 TIMSK2, and the Global Interrupt Enable bit in
SREG is set.
If Timer/Counter2 is running, Power-down mode is recommended instead of Power-save mode.
The Timer/Counter2 can be clocked both synchronously and asynchronously in Power-save
mode. The clock source for the two modules can be selected independent of each other. If the
Timer/Counter2 is using the asynchronous clock, the Timer/Counter Oscillator is stopped during
sleep. If the Timer/Counter2 is using the synchronous clock, the clock source is stopped during
sleep. Note that even if the synchronous clock is running in Power-save, this clock is only avail-
able for the Timer/Counter2.
9.8
9.9
Standby Mode
When the SM2:0 bits are 110 and an external crystal/resonator clock option is selected, the
SLEEP instruction makes the MCU enter Standby mode. This mode is identical to Power-down
with the exception that the Oscillator is kept running. From Standby mode, the device wakes up
in six clock cycles.
Power Reduction Register
The Power Reduction Register (PRR), see ”PRR – Power Reduction Register” on page 44, pro-
vides a method to stop the clock to individual peripherals to reduce power consumption. The
current state of the peripheral is frozen and the I/O registers inaccessible. Resources used by
the peripheral when stopping the clock will remain occupied so the peripheral should be disabled
before stopping the clock. Waking up a module, which is done by clearing the bit in PRR, puts
the module in the same state as before shutdown.
Module shutdown can be used in IDLE mode and active mode to reduce the overall power con-
sumption. In all other sleep modes, the clock is already stopped.
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9.10 Minimizing Power Consumption
There are several possibilities to consider when trying to minimize the power consumption in an
AVR controlled system. In general, sleep modes should be used as much as possible, and the
sleep mode should be selected so that as few as possible of the device’s functions are operat-
ing. 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.
9.10.1
9.10.2
Analog to Digital Converter
If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be dis-
abled before entering any sleep mode. When the ADC is turned off and on again, the next
conversion will be an extended conversion. Refer to ”Analog to Digital Converter” on page 206
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 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 Reference will be enabled, independent of sleep
mode. Refer to ”Analog Comparator” on page 203 for details on how to configure the Analog
Comparator.
9.10.3
9.10.4
Brown-out Detector
If the Brown-out Detector is not needed by the application, this module should be turned off. If
the Brown-out Detector is enabled by the BODLEVEL Fuses, it will be enabled in all sleep
modes, and hence, always consume power. In the deeper sleep modes, this will contribute sig-
nificantly to the total current consumption. Refer to ”Brown-out Detection” on page 47 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 Detection, the
Analog Comparator or the ADC. If these modules are disabled as described in the sections
above, the internal voltage reference will be disabled and it will not be consuming power. When
turned on again, the user must allow the reference to start up before the output is used. If the
reference is kept on in sleep mode, the output can be used immediately. Refer to ”Internal Volt-
age Reference” on page 48 for details on the start-up time.
9.10.5
9.10.6
Watchdog Timer
If the Watchdog Timer is not needed in the application, the module should be turned off. If the
Watchdog Timer is enabled, it will be enabled in all sleep modes, and hence, always consume
power. In the deeper sleep modes, this will contribute significantly to the total current consump-
tion. Refer to ”Watchdog Timer” on page 49 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 is then to ensure that no pins drive resistive loads. In sleep modes where both
the I/O clock (clkI/O) and the ADC clock (clkADC) 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
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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 67 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 VCC/2, the input buffer will use excessive power.
For analog input pins, the digital input buffer should be disabled at all times. An analog signal
level close to VCC/2 on an input pin can cause significant current even in active mode. Digital
input buffers can be disabled by writing to the Digital Input Disable Registers (DIDR1 and
DIDR0). Refer to ”DIDR1 – Digital Input Disable Register 1” on page 205 and ”DIDR0 – Digital
Input Disable Register 0” on page 222 for details.
9.10.7
JTAG Interface and On-chip Debug System
If the On-chip debug system is enabled by the OCDEN Fuse and the chip enter Power down or
Power save sleep mode, the main clock source remains enabled. In these sleep modes, this will
contribute significantly to the total current consumption. There are three alternative ways to
avoid this:
• Disable OCDEN Fuse.
• Disable JTAGEN Fuse.
• Write one to the JTD bit in MCUCR.
The TDO pin is left floating when the JTAG interface is enabled while the JTAG TAP controller is
not shifting data. If the hardware connected to the TDO pin does not pull up the logic level,
power consumption will increase. Note that the TDI pin for the next device in the scan chain con-
tains a pull-up that avoids this problem. Writing the JTD bit in the MCUCR register to one or
leaving the JTAG fuse unprogrammed disables the JTAG interface.
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9.11 Register Description
9.11.1
SMCR – Sleep Mode Control Register
The Sleep Mode Control Register contains control bits for power management.
Bit
7
–
6
–
5
–
4
–
3
2
1
0
SE
R/W
0
0x33 (0x53)
Read/Write
Initial Value
SM2
R/W
0
SM1
R/W
0
SM0
R/W
0
SMCR
R
0
R
0
R
0
R
0
• Bits 3, 2, 1 – SM2:0: Sleep Mode Select Bits 2, 1, and 0
These bits select between the five available sleep modes as shown in Table 9-2.
Table 9-2.
Sleep Mode Select
SM2
SM1
SM0
Sleep Mode
Idle
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
ADC Noise Reduction
Power-down
Power-save
Reserved
Reserved
Standby(1)
Reserved
Note:
1. Standby mode is only recommended for use with external crystals or resonators.
• Bit 1 – 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 write the Sleep Enable (SE) bit to one just before the execution of
the SLEEP instruction and to clear it immediately after waking up.
9.11.2
MCUCR – MCU Control Register
Bit
0x35 (0x55)
7
6
BODS
R
5
4
3
–
2
–
1
IVSEL
R/W
0
0
IVCE
R/W
0
JTD
BODSE
PUD
R/W
0
MCUCR
Read/Write
Initial Value
R/W
0
R
0
R
0
R
0
0
• Bit 6 – BODS: BOD Sleep
The BODS bit must be written to logic one in order to turn off BOD during sleep, see Table 9-1
on page 38. Writing to the BODS bit is controlled by a timed sequence and an enable bit,
BODSE in MCUCR. To disable BOD in relevant sleep modes, both BODS and BODSE must first
be set to one. Then, to set the BODS bit, BODS must be set to one and BODSE must be set to
zero within four clock cycles.
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The BODS bit is active three clock cycles after it is set. A sleep instruction must be executed
while BODS is active in order to turn off the BOD for the actual sleep mode. The BODS bit is
automatically cleared after three clock cycles.
• Bit 5 – BODSE: BOD Sleep Enable
BODSE enables setting of BODS control bit, as explained in BODS bit description. BOD disable
is controlled by a timed sequence.
9.11.3
PRR – Power Reduction Register
Bit
(0x64)
7
6
–
5
–
4
–
3
PRTIM1
R/W
0
2
PRSPI
R/W
0
1
PRUSART0
R/W
0
PRADC
R/W
0
–
PRR
Read/Write
Initial Value
R
0
R
0
R
0
R/W
0
0
• Bits 7:4- Res: Reserved bits
These bits are reserved and will always read as zero.
• Bit 3 - PRTIM1: Power Reduction Timer/Counter1
Writing logic one to this bit shuts down the Timer/Counter1 module. When Timer/Counter1 is
enabled, operation will continue like before the shutdown.
• Bit 2 - PRSPI: Power Reduction Serial Peripheral Interface
Writing logic one to this bit shuts down the Serial Peripheral Interface by stopping the clock to
the module. When waking up the SPI again, the SPI should be re-initialized to ensure proper
operation.
• Bit 1 - PRUSART: Power Reduction USART
Writing logic one to this bit shuts down the USART by stopping the clock to the module. When
waking up the USART again, the USART should be re-initialized to ensure proper operation.
• Bit 0 - PRADC: Power Reduction ADC
Writing logic one to this bit shuts down the ADC. The ADC must be disabled before shut down.
The analog comparator cannot use the ADC input MUX when the ADC is shut down.
Note:
The Analog Comparator is disabled using the ACD-bit in the ”ACSR – Analog Comparator Control
and Status Register” on page 204.
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ATmega325P/3250P
10. System Control and Reset
10.1 Resetting the AVR
During reset, all I/O Registers are set to their initial values, and the program starts execution
from the Reset Vector. The instruction placed at the Reset Vector must be a JMP – Absolute
Jump – instruction to the reset handling routine. If the program never enables an interrupt
source, the Interrupt Vectors are not used, and regular program code can be placed at these
locations. This is also the case if the Reset Vector is in the Application section while the Interrupt
Vectors are in the Boot section or vice versa. The circuit diagram in Figure 10-1 on page 46
shows the reset logic. ”System and Reset Characterizations” on page 308 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 SUT and CKSEL Fuses. The dif-
ferent selections for the delay period are presented in ”Clock Sources” on page 30.
10.2 Reset Sources
The ATmega325P/3250P has five sources of reset:
• Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset
threshold (VPOT).
• 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 VCC is below the Brown-out Reset
threshold (VBOT) and the Brown-out Detector is enabled.
• JTAG AVR Reset. The MCU is reset as long as there is a logic one in the Reset Register, one
of the scan chains of the JTAG system. Refer to the section ”IEEE 1149.1 (JTAG) Boundary-
scan” on page 229 for details.
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Figure 10-1. Reset Logic
DATA BUS
MCU Status
Register (MCUSR)
Power-on Reset
Circuit
Brown-out
Reset Circuit
BODLEVEL [1..0]
Pull-up Resistor
SPIKE
FILTER
JTAG Reset
Register
Watchdog
Oscillator
Delay Counters
Clock
CK
Generator
TIMEOUT
CKSEL[3:0]
SUT[1:0]
10.2.1
Power-on Reset
A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level
is defined in ”System and Reset Characterizations” on page 308. The POR is activated when-
ever VCC is below the detection level. The POR circuit can be used to trigger the start-up Reset,
as well as to detect a failure in supply voltage.
A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the
Power-on Reset threshold voltage invokes the delay counter, which determines how long the
device is kept in RESET after VCC rise. The RESET signal is activated again, without any delay,
when VCC decreases below the detection level.
Figure 10-2. MCU Start-up, RESET Tied to VCC
VPOT
VCC
VRST
RESET
tTOUT
TIME-OUT
INTERNAL
RESET
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Figure 10-3. MCU Start-up, RESET Extended Externally
VPOT
VCC
VRST
RESET
tTOUT
TIME-OUT
INTERNAL
RESET
10.2.2
External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the
minimum pulse width (see ”System and Reset Characterizations” on page 308) 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 – VRST – on its positive edge, the
delay counter starts the MCU after the Time-out period – tTOUT – has expired.
Figure 10-4. External Reset During Operation
CC
10.2.3
Brown-out Detection
ATmega325P/3250P has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC
level during operation by comparing it to a fixed trigger level. The trigger level for the BOD can
be selected by the BODLEVEL Fuses. The trigger level has a hysteresis to ensure spike free
Brown-out Detection. The hysteresis on the detection level should be interpreted as VBOT+
=
V
BOT + VHYST/2 and VBOT- = VBOT - VHYST/2.When the BOD is enabled, and VCC decreases to a
value below the trigger level (VBOT- in Figure 10-5), the Brown-out Reset is immediately acti-
vated. When VCC increases above the trigger level (VBOT+ in Figure 10-5), the delay counter
starts the MCU after the Time-out period tTOUT has expired.
The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for lon-
ger than tBOD given in ”System and Reset Characterizations” on page 308.
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Figure 10-5. Brown-out Reset During Operation
VBOT+
VCC
VBOT-
RESET
t
TOUT
TIME-OUT
INTERNAL
RESET
10.2.4
Watchdog Reset
When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On
the falling edge of this pulse, the delay timer starts counting the Time-out period tTOUT. Refer to
page 49 for details on operation of the Watchdog Timer.
Figure 10-6. Watchdog Reset During Operation
CC
CK
10.3 Internal Voltage Reference
ATmega325P/3250P 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.
10.3.1
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 ”System and Reset Characterizations” on page 308. 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 BODLEVEL [1:0] Fuse).
2. When the bandgap reference is connected to the Analog Comparator (by setting the
ACBG bit in ACSR).
3. When the ADC is enabled.
Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user
must always allow the reference to start up before the output from the Analog Comparator or
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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.
10.4 Watchdog Timer
The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1 MHz. This is
the typical value at VCC = 5V. See characterization data for typical values at other VCC levels. By
controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as
shown in Table 10-2 on page 51. The WDR – Watchdog Reset – instruction resets the Watch-
dog 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 ATmega325P/3250P resets and executes
from the Reset Vector. For timing details on the Watchdog Reset, refer to Table 10-2 on page
51.
To prevent unintentional disabling of the Watchdog or unintentional change of time-out period,
two different safety levels are selected by the fuse WDTON as shown in Figure 10-1 Refer to
”Timed Sequences for Changing the Configuration of the Watchdog Timer” on page 49 for
details.
Table 10-1. WDT Configuration as a Function of the Fuse Settings of WDTON
Safety
Level
WDT Initial
State
How to Disable the
WDT
How to Change Time-
out
WDTON
Unprogrammed
Programmed
1
2
Disabled
Enabled
Timed sequence
Always enabled
Timed sequence
Timed sequence
Figure 10-7. Watchdog Timer
WATCHDOG
OSCILLATOR
10.4.1
10.4.2
Timed Sequences for Changing the Configuration of the Watchdog Timer
The sequence for changing configuration differs slightly between the two safety levels. Separate
procedures are described for each level.
Safety Level 1
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 procedure must be followed:
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8023F–AVR–07/09
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.
10.4.3
Safety Level 2
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.
10.5 Register Description
10.5.1
MCUSR – MCU Status Register
The MCU Status Register provides information on which reset source caused an MCU reset.
Bit
7
–
6
–
5
–
4
3
2
BORF
1
0
0x35 (0x55)
Read/Write
Initial Value
JTRF
R/W
WDRF
R/W
EXTRF
R/W
PORF
R/W
MCUSR
R
0
R
0
R
0
R/W
See Bit Description
• Bit 4 – JTRF: JTAG Reset Flag
This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by
the JTAG instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic
zero to the flag.
• Bit 3 – WDRF: Watchdog Reset Flag
This bit is set if a Watchdog Reset occurs. The bit is reset by a Power-on Reset, or by writing a
logic zero to the flag.
• Bit 2 – BORF: Brown-out Reset Flag
This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a
logic zero to the flag.
• Bit 1 – EXTRF: External Reset Flag
This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a
logic zero to the flag.
• Bit 0 – PORF: Power-on Reset Flag
This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag.
To make use of the Reset Flags to identify a reset condition, the user should read and then
Reset the MCUSR as early as possible in the program. If the register is cleared before another
reset occurs, the source of the reset can be found by examining the Reset Flags.
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10.5.2
WDTCR – Watchdog Timer Control Register
Bit
7
–
6
–
5
–
4
WDCE
R/W
0
3
WDE
R/W
0
2
WDP2
R/W
0
1
WDP1
R/W
0
0
WDP0
R/W
0
(0x60)
WDTCR
Read/Write
Initial Value
R
0
R
0
R
0
• Bits 7:5 – Res: Reserved Bits
These bits are reserved 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. This bit must also be set when
changing the prescaler bits. See ”Timed Sequences for Changing the Configuration of the
Watchdog Timer” on page 49.
• 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 following 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.
In safety level 2, it is not possible to disable the Watchdog Timer, even with the algorithm
described above. See Section “10.4.1” on page 49.
• Bits 2:0 – WDP2, WDP1, WDP0: Watchdog Timer Prescaler 2, 1, and 0
The WDP2, WDP1, and WDP0 bits determine the Watchdog Timer prescaling when the Watch-
dog Timer is enabled. The different prescaling values and their corresponding Time-out Periods
are shown in Table 10-2 on page 51.
Table 10-2. Watchdog Timer Prescale Select
Number of WDT
Oscillator Cycles
Typical Time-out at
VCC = 3.0V
Typical Time-out at
VCC = 5.0V
WDP2
WDP1
WDP0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
16K cycles
32K cycles
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 cycles
128K cycles
256K cycles
512K cycles
1,024K cycles
2,048K cycles
2.2 s
2.1 s
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The following code example shows one assembly and one C function for turning off the WDT.
The example assumes that interrupts are controlled (e.g. by disabling interrupts globally) so that
no interrupts will occur during execution of these functions.
Assembly Code Example(1)
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(1)
void WDT_off(void)
{
/* Reset WDT */
__watchdog_reset();
/* Write logical one to WDCE and WDE */
WDTCR |= (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCR = 0x00;
}
Note:
1. See Section “5.” on page 10.
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11. Interrupts
11.1 Overview
This section describes the specifics of the interrupt handling as performed in
ATmega325P/3250P. For a general explanation of the AVR interrupt handling, refer to ”Reset
and Interrupt Handling” on page 17.
11.2 Interrupt Vectors
Table 11-1. Reset and Interrupt Vectors
Vector
No.
Program
Address(2)
Source
Interrupt Definition
External Pin, Power-on Reset, Brown-out Reset,
Watchdog Reset, and JTAG AVR Reset
1
0x0000(1)
RESET
2
3
0x0002
0x0004
0x0006
0x0008
0x000A
0x000C
0x000E
0x0010
0x0012
0x0014
0x0016
0x0018
0x001A
0x001C
0x001E
0x0020
0x0022
0x0024
0x0026
0x0028
0x002A
0x002C
0x002E
0x0030
INT0
External Interrupt Request 0
Pin Change Interrupt Request 0
Pin Change Interrupt Request 1
Timer/Counter2 Compare Match
Timer/Counter2 Overflow
Timer/Counter1 Capture Event
Timer/Counter1 Compare Match A
Timer/Counter1 Compare Match B
Timer/Counter1 Overflow
Timer/Counter0 Compare Match
Timer/Counter0 Overflow
SPI Serial Transfer Complete
USART0, Rx Complete
PCINT0
4
PCINT1
5
TIMER2 COMP
TIMER2 OVF
TIMER1 CAPT
TIMER1 COMPA
TIMER1 COMPB
TIMER1 OVF
TIMER0 COMP
TIMER0 OVF
SPI, STC
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24(3)
25(3)
USART, RX
USART, UDRE
USART, TX
USI START
USI OVERFLOW
ANALOG COMP
ADC
USART0 Data Register Empty
USART0, Tx Complete
USI Start Condition
USI Overflow
Analog Comparator
ADC Conversion Complete
EEPROM Ready
EE READY
SPM READY
NOT_USED
PCINT2
Store Program Memory Ready
RESERVED
Pin Change Interrupt Request 2
Pin Change Interrupt Request 3
PCINT3
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 256.
53
8023F–AVR–07/09
2. When the IVSEL bit in MCUCR 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.
3. PCINT2 and PCINT3 are only present in ATmega3250P and ATmega6450P.
Table 11-2 shows reset and Interrupt Vectors placement for the various combinations of
BOOTRST and IVSEL settings. If the program never enables an interrupt source, the Interrupt
Vectors are not used, and regular program code can be placed at these locations. This is also
the case if the Reset Vector is in the Application section while the Interrupt Vectors are in the
Boot section or vice versa.
Table 11-2. Reset and Interrupt Vectors Placement(1)
BOOTRST
IVSEL
Reset Address
0x0000
Interrupt Vectors Start Address
0x0002
1
1
0
0
0
1
0
1
0x0000
Boot Reset Address + 0x0002
0x0002
Boot Reset Address
Boot Reset Address
Boot Reset Address + 0x0002
Note:
1. The Boot Reset Address is shown in Table 24-6 on page 268. 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
ATmega325P/3250P is:
Address
0x0000
0x0002
0x0004
0x0006
0x0008
0x000A
0x000C
0x000E
0x0010
0x0012
0x0014
0x0016
0X0018
0x001A
0x001C
0x001E
0x0020
0x0022
0x0024
0x0026
0x0028
0x002A
0x002C
0x002E
0x0030
;
Labels
Code
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
jmp
Comments
RESET
; Reset Handler
EXT_INT0
PCINT0
; IRQ0 Handler
; PCINT0 Handler
PCINT1
; PCINT1 Handler
TIM2_COMP
TIM2_OVF
TIM1_CAPT
TIM1_COMPA
TIM1_COMPB
TIM1_OVF
TIM0_COMP
TIM0_OVF
SPI_STC
USART_RXC
USART_UDRE
USART_TXC
USI_STRT
USI_OVF
ANA_COMP
ADC
; Timer2 Compare Handler
; Timer2 Overflow Handler
; Timer1 Capture Handler
; Timer1 CompareA Handler
; Timer1 CompareB Handler
; Timer1 Overflow Handler
; Timer0 Compare Handler
; Timer0 Overflow Handler
; SPI Transfer Complete Handler
; USART0 RX Complete Handler
; USART0,UDR0 Empty Handler
; USART0 TX Complete Handler
; USI Start Condition Handler
; USI Overflow Handler
; Analog Comparator Handler
; ADC Conversion Complete Handler
; EEPROM Ready Handler
; SPM Ready Handler
EE_RDY
SPM_RDY
NOT_USED
PCINT2
; RESERVED
; PCINT2 Handler
PCINT3
; PCINT3 Handler
54
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
0x0032
0x0033
0x0034
0x0035
0x0036
0x0037
RESET:
ldi
out
ldi
out
sei
<inst
r>
r16, high(RAMEND)
SPH,r16
; Main program start
; Set Stack Pointer to top of RAM
r16, low(RAMEND)
SPL,r16
; Enable interrupts
xxx
...
...
...
When the BOOTRST Fuse is unprogrammed, the Boot section size set to 4K bytes and the
IVSEL bit in the MCUCR Register is set before any interrupts are enabled, the most typical and
general program setup for the Reset and Interrupt Vector Addresses is:
Address Labels Code
0x0000 RESET: ldi
Comments
r16,high(RAMEND); Main program start
0x0001
0x0002
out
ldi
SPH,r16
; Set Stack Pointer to top of RAM
r16,low(RAMEND)
SPL,r16
0x0003
0x0004
out
sei
; Enable interrupts
0x0005
;
<instr> xxx
.org 0x3802/0x7802
0x3804/0x7804 jmp
0x3806/0x7806 jmp
EXT_INT0
PCINT0
...
; IRQ0 Handler
; PCINT0 Handler
...
...
jmp
;
0x1C2C
SPM_RDY
; Store Program Memory Ready Handler
When the BOOTRST Fuse is programmed and the Boot section size set to 4K bytes, the most
typical and general program setup for the Reset and Interrupt Vector Addresses is:
Address Labels Code
.org 0x0002
Comments
0x0002
0x0004
...
jmp
jmp
...
jmp
EXT_INT0
PCINT0
...
; IRQ0 Handler
; PCINT0 Handler
;
0x002C
;
SPM_RDY
; Store Program Memory Ready Handler
.org 0x3800/0x7800
0x3800/0x7801RESET:ldir16,high(RAMEND); Main program start
0x3801/0x7801 out
0x3802/0x7802 ldi
SPH,r16
; Set Stack Pointer to top of RAM
r16,low(RAMEND)
SPL,r16
0x3803/0x7803 out
0x3804/0x7804 sei
; Enable interrupts
0x3805/0x7805 <instr> xxx
When the BOOTRST Fuse is programmed, the Boot section size set to 4K bytes and the IVSEL
bit in the MCUCR Register is set before any interrupts are enabled, the most typical and general
program setup for the Reset and Interrupt Vector Addresses is:
55
8023F–AVR–07/09
Address Labels Code
;
Comments
.org 0x3800/0x7800
0x3800/0x7800 jmp
0x3802/0x7802 jmp
RESET
EXT_INT0
; Reset handler
; IRQ0 Handler
0x3804/0x7804 jmp
PCINT0
...
; PCINT0 Handler
...
...
;
0x382C/0x782C jmp
;
SPM_RDY
; Store Program Memory Ready Handler
0x382E/0x782ERESET:ldir16,high(RAMEND); Main program start
0x382F/0x782F out
0x3830/0x7830 ldi
SPH,r16
; Set Stack Pointer to top of RAM
r16,low(RAMEND)
SPL,r16
0x3831/0x7831 out
0x3832/0x7832 sei
; Enable interrupts
0x3833/0x7833 <instr> xxx
11.2.1
Moving Interrupts Between Application and Boot Space
The MCU Control Register controls the placement of the Interrupt Vector table.
11.3 Register Description
11.3.1
MCUCR – MCU Control Register
Bit
7
6
BODS
R
5
4
3
–
2
–
1
IVSEL
R/W
0
0
IVCE
R/W
0
0x35 (0x55)
Read/Write
Initial Value
JTD
R/W
0
BODSE
PUD
R/W
0
MCUCR
R
0
R
0
R
0
0
• Bit 1 – IVSEL: Interrupt Vector Select
When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash
memory. When this bit is set (one), the Interrupt Vectors are moved to the beginning of the Boot
Loader section of the Flash. The actual address of the start of the Boot Flash Section is deter-
mined by the BOOTSZ Fuses. Refer to the section ”Boot Loader Support – Read-While-Write
Self-Programming” on page 256 for details. To avoid unintentional changes of Interrupt Vector
tables, a special write procedure must be followed to change the IVSEL bit:
1. Write the Interrupt Vector Change Enable (IVCE) bit to one.
2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.
Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled
in the cycle IVCE is set, and they remain disabled until after the instruction following the write to
IVSEL. If IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status
Register is unaffected by the automatic disabling.
Note:
If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is programmed,
interrupts are disabled while executing from the Application section. If Interrupt Vectors are placed
in the Application section and Boot Lock bit BLB12 is programed, interrupts are disabled while
executing from the Boot Loader section. Refer to the section ”Boot Loader Support – Read-While-
Write Self-Programming” on page 256 for details on Boot Lock bits.
56
ATmega325P/3250P
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ATmega325P/3250P
• 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:
;Get MCUCR
in r16, MCUCR
mov r17, r16
; Enable change of Interrupt Vectors
ori r16, (1<<IVCE)
out MCUCR, r16
; Move interrupts to Boot Flash section
ori r17, (1<<IVSEL)
out MCUCR, r17
ret
C Code Example
void Move_interrupts(void)
{
/* Enable change of Interrupt Vectors */
MCUCR |= (1<<IVCE);
/* Move interrupts to Boot Flash section */
MCUCR |= (1<<IVSEL);
}
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8023F–AVR–07/09
ATmega325P/3250P
12. External Interrupts
12.1 Overview
The External Interrupts are triggered by the INT0 pin or any of the PCINT30:0 pins(2). Observe
that, if enabled, the interrupts will trigger even if the INT0 or PCINT30:0 pins are configured as
outputs. This feature provides a way of generating a software interrupt. The pin change interrupt
PCI1 will trigger if any enabled PCINT15:8 pin toggles. Pin change interrupts PCI0 will trigger if
any enabled PCINT7:0 pin toggles. The PCMSK3(1), PCMSK2(1), PCMSK1, and PCMSK0 Reg-
isters control which pins contribute to the pin change interrupts. Pin change interrupts on
PCINT30:0 are detected asynchronously. This implies that these interrupts can be used for wak-
ing the part also from sleep modes other than Idle mode.
The INT0 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 ”EICRA – External Interrupt Control Register A” on page 59.
When the INT0 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
requires the presence of an I/O clock, described in ”Clock Systems and their Distribution” on
page 29. Low level interrupt on INT0 is detected asynchronously. This implies that this interrupt
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, the required level
must be held long enough for the MCU to complete the wake-up to trigger the level interrupt. If
the level disappears before the end of the Start-up Time, the MCU will still wake up, but no inter-
rupt will be generated. The start-up time is defined by the SUT and CKSEL Fuses as described
in ”System Clock and Clock Options” on page 29.
Notes: 1. PCMSK3 and PCMSK2 are only present in ATmega3250P.
2. PCINT30:16 are only present in ATmega3250P. Only PCINT15:0 are present in ATmega325P.
See ”Pin Configurations” on page 2 and ”Register Description” on page 59 for details.
58
8023F–AVR–07/09
12.2 Pin Change Interrupt Timing
An example of timing of a pin change interrupt is shown in Figure 12-1.
Figure 12-1. Pin Change Interrupt
pin_lat
pcint_in_(0)
PCINT(0)
0
x
D
Q
pcint_syn
pcint_setflag
PCIF
pin_sync
PCINT(0) in PCMSK(x)
LE
clk
clk
clk
PCINT(n)
pin_lat
pin_sync
pcint_in_(n)
pcint_syn
pcint_setflag
PCIF
12.3 Register Description
12.3.1
EICRA – External Interrupt Control Register A
The External Interrupt Control Register A contains control bits for interrupt sense control.
Bit
7
–
6
–
5
–
4
–
3
–
2
–
1
ISC01
R/W
0
0
ISC00
R/W
0
(0x69)
EICRA
Read/Write
Initial Value
R
0
R
0
R
0
R
0
R
0
R
0
• Bit 1, 0 – ISC01, ISC00: Interrupt Sense Control 0 Bit 1 and Bit 0
The External Interrupt 0 is activated by the external pin INT0 if the SREG I-flag and the corre-
sponding interrupt mask are set. The level and edges on the external INT0 pin that activate the
interrupt are defined in Table 12-1. 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.
59
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Table 12-1. Interrupt 0 Sense Control
ISC01
ISC00
Description
0
0
1
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.
12.3.2
EIMSK – External Interrupt Mask Register
Bit
7
PCIE3
R/W
0
6
PCIE2
R/W
0
5
PCIE1
R/W
0
4
PCIE0
R/W
0
3
–
2
–
1
–
0
(0xD1)
INT0
R/W
0
EIMSK
Read/Write
Initial Value
R
0
R
0
R
0
• Bit 7 – PCIE3: Pin Change Interrupt Enable 3
When the PCIE3 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin
change interrupt 3 is enabled. Any change on any enabled PCINT30:24 pin will cause an inter-
rupt. The corresponding interrupt of Pin Change Interrupt Request is executed from the PCINT3
Interrupt Vector. PCINT30:24 pins are enabled individually by the PCMSK3 Register.
This bit is reserved bit in ATmega325P and should always be written to zero.
• Bit 6 – PCIE2: Pin Change Interrupt Enable 2
When the PCIE2 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin
change interrupt 2 is enabled. Any change on any enabled PCINT23:16 pin will cause an inter-
rupt. The corresponding interrupt of Pin Change Interrupt Request is executed from the PCINT2
Interrupt Vector. PCINT23:16 pins are enabled individually by the PCMSK2 Register.
This bit is reserved bit in ATmega325P and should always be written to zero.
• Bit 5 – PCIE1: Pin Change Interrupt Enable 1
When the PCIE1 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin
change interrupt 1 is enabled. Any change on any enabled PCINT15:8 pin will cause an inter-
rupt. The corresponding interrupt of Pin Change Interrupt Request is executed from the PCINT1
Interrupt Vector. PCINT15:8 pins are enabled individually by the PCMSK1 Register.
• Bit 4 – PCIE0: Pin Change Interrupt Enable 0
When the PCIE0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin
change interrupt 0 is enabled. Any change on any enabled PCINT7:0 pin will cause an interrupt.
The corresponding interrupt of Pin Change Interrupt Request is executed from the PCINT0 Inter-
rupt Vector. PCINT7:0 pins are enabled individually by the PCMSK0 Register.
• Bit 0 – INT0: External Interrupt Request 0 Enable
When the INT0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the exter-
nal pin interrupt is enabled. The Interrupt Sense Control0 bits 1/0 (ISC01 and ISC00) in the
External Interrupt Control Register A (EICRA) define whether the external interrupt is activated
on rising and/or falling edge of the INT0 pin or level sensed. Activity on the pin will cause an
interrupt request even if INT0 is configured as an output. The corresponding interrupt of External
Interrupt Request 0 is executed from the INT0 Interrupt Vector.
60
8023F–AVR–07/09
12.3.3
EIFR – External Interrupt Flag Register
Bit
7
PCIF3
R/W
0
6
PCIF2
R/W
0
5
PCIF1
R/W
0
4
PCIF0
R/W
0
3
–
2
–
1
–
0
INTF0
R/W
0
0x1C (0x3C)
Read/Write
Initial Value
EIFR
R
0
R
0
R
0
• Bit 7 – PCIF3: Pin Change Interrupt Flag 3
When a logic change on any PCINT30:24 pin triggers an interrupt request, PCIF3 becomes set
(one). If the I-bit in SREG and the PCIE3 bit in EIMSK are set (one), the MCU will jump to the
corresponding Interrupt Vector. The flag is cleared when the interrupt routine is executed. Alter-
natively, the flag can be cleared by writing a logical one to it.
This bit is reserved bit in ATmega325P and will always be read as zero.
• Bit 6 – PCIF2: Pin Change Interrupt Flag 2
When a logic change on any PCINT24:16 pin triggers an interrupt request, PCIF2 becomes set
(one). If the I-bit in SREG and the PCIE2 bit in EIMSK are set (one), the MCU will jump to the
corresponding Interrupt Vector. The flag is cleared when the interrupt routine is executed. Alter-
natively, the flag can be cleared by writing a logical one to it.
This bit is reserved bit in ATmega325P and will always be read as zero.
• Bit 5 – PCIF1: Pin Change Interrupt Flag 1
When a logic change on any PCINT15:8 pin triggers an interrupt request, PCIF1 becomes set
(one). If the I-bit in SREG and the PCIE1 bit in EIMSK are set (one), the MCU will jump to the
corresponding Interrupt Vector. The flag is cleared when the interrupt routine is executed. Alter-
natively, the flag can be cleared by writing a logical one to it.
• Bit 4 – PCIF0: Pin Change Interrupt Flag 0
When a logic change on any PCINT7:0 pin triggers an interrupt request, PCIF0 becomes set
(one). If the I-bit in SREG and the PCIE0 bit in EIMSK are set (one), the MCU will jump to the
corresponding Interrupt Vector. The flag is cleared when the interrupt routine is executed. Alter-
natively, the flag can be cleared by writing a logical one to it.
• Bit 0 – INTF0: External Interrupt Flag 0
When an edge or logic change on the INT0 pin triggers an interrupt request, INTF0 becomes set
(one). If the I-bit in SREG and the INT0 bit in EIMSK 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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ATmega325P/3250P
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ATmega325P/3250P
12.3.4
PCMSK3 – Pin Change Mask Register 3(1)
Bit
7
–
6
PCINT30
R/W
0
5
PCINT29
R/W
0
4
PCINT28
R/W
0
3
PCINT27
R/W
0
2
PCINT26
R/W
0
1
PCINT25
R/W
0
0
PCINT24
R/W
0
PCMSK3
(0x73)
Read/Write
Initial Value
R
0
• Bit 6:0 – PCINT30:24: Pin Change Enable Mask 30:24
Each PCINT30:24-bit selects whether pin change interrupt is enabled on the corresponding I/O
pin. If PCINT30:24 is set and the PCIE3 bit in EIMSK is set, pin change interrupt is enabled on
the corresponding I/O pin. If PCINT30:24 is cleared, pin change interrupt on the corresponding
I/O pin is disabled.
12.3.5
PCMSK2 – Pin Change Mask Register 2(1)
Bit
(0x6D)
7
6
5
PCINT21
R/W
4
PCINT20
R/W
3
PCINT19
R/W
2
PCINT18
R/W
1
PCINT17
R/W
0
PCINT16
R/W
PCINT23
PCINT22
PCMSK2
Read/Write
R/W
0
R/W
0
Initial
Value
0
0
0
0
0
0
• Bit 7:0 – PCINT23:16: Pin Change Enable Mask 23:16
Each PCINT23:16 bit selects whether pin change interrupt is enabled on the corresponding I/O
pin. If PCINT23:16 is set and the PCIE2 bit in EIMSK is set, pin change interrupt is enabled on
the corresponding I/O pin. If PCINT23:16 is cleared, pin change interrupt on the corresponding
I/O pin is disabled.
Note:
1. PCMSK3 and PCMSK2 are only present in ATmega3250P.
12.3.6
PCMSK1 – Pin Change Mask Register 1
Bit
7
PCINT15
R/W
0
6
PCINT14
R/W
0
5
PCINT13
R/W
0
4
PCINT12
R/W
0
3
PCINT11
R/W
0
2
PCINT10
R/W
0
1
PCINT9
R/W
0
0
PCINT8
R/W
0
PCMSK1
(0x6C)
Read/Write
Initial Value
• Bit 7:0 – PCINT15:8: Pin Change Enable Mask 15:8
Each PCINT15:8-bit selects whether pin change interrupt is enabled on the corresponding I/O
pin. If PCINT15:8 is set and the PCIE1 bit in EIMSK is set, pin change interrupt is enabled on the
corresponding I/O pin. If PCINT15:8 is cleared, pin change interrupt on the corresponding I/O
pin is disabled.
12.3.7
PCMSK0 – Pin Change Mask Register 0
Bit
(0x6B)
7
6
5
4
3
2
1
0
PCINT7
PCINT6
PCINT5
R/W
0
PCINT4
R/W
0
PCINT3
R/W
0
PCINT2
R/W
0
PCINT1
R/W
0
PCINT0
R/W
0
PCMSK0
Read/Write
Initial Value
R/W
0
R/W
0
• Bit 7:0 – PCINT7:0: Pin Change Enable Mask 7:0
Each PCINT7:0 bit selects whether pin change interrupt is enabled on the corresponding I/O pin.
If PCINT7:0 is set and the PCIE0 bit in EIMSK is set, pin change interrupt is enabled on the cor-
responding I/O pin. If PCINT7:0 is cleared, pin change interrupt on the corresponding I/O pin is
disabled.
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ATmega325P/3250P
13. I/O-Ports
13.1 Overview
All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports.
This means that the direction of one port pin can be changed without unintentionally changing
the direction of any other pin with the SBI and CBI instructions. The same applies when chang-
ing drive value (if configured as output) or enabling/disabling of pull-up resistors (if configured as
input). Each output buffer has symmetrical drive characteristics with both high sink and source
capability. Port B has a higher pin driver strength than the other ports, but all the pin drivers are
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
CC and Ground as indicated in Figure 13-1. Refer to ”Electrical Characteristics” on page 304 for
a complete list of parameters. If exceeding the pin voltage “Absolute Maximum Ratings”, result-
ing currents can harm the device if not limited accordingly.
Figure 13-1. I/O Pin Equivalent Schematic
Rpu
Pxn
Logic
Cpin
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” repre-
sents the numbering letter for the port, and a lower case “n” represents the bit number. However,
when using the register or bit defines in a program, the precise form must be used. For example,
PORTB3 for bit no. 3 in Port B, here documented generally as PORTxn. The physical I/O Regis-
ters and bit locations are listed in ”Register Description” on page 85.
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.
However, writing a logic one to a bit in the PINx Register, will result in a toggle in the correspond-
ing bit in the Data Register. In addition, the Pull-up Disable – PUD bit in MCUCR 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
64. Most port pins are multiplexed with alternate functions for the peripheral features on the
device. How each alternate function interferes with the port pin is described in ”Alternate Port
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8023F–AVR–07/09
Functions” on page 69. Refer to the individual module sections for a full description of the alter-
nate functions.
Note that enabling the alternate function of some of the port pins does not affect the use of the
other pins in the port as general digital I/O.
13.2 Ports as General Digital I/O
The ports are bi-directional I/O ports with optional internal pull-ups. Figure 13-2 shows a func-
tional description of one I/O-port pin, here generically called Pxn.
Figure 13-2. General Digital I/O(1)
PUD
Q
D
DDxn
Q CLR
WDx
RDx
RESET
1
0
Q
D
Pxn
PORTxn
Q CLR
RESET
WPx
WRx
SLEEP
RRx
SYNCHRONIZER
RPx
D
Q
D
L
Q
Q
PINxn
Q
clk I/O
WDx:
RDx:
WRx:
RRx:
RPx:
WPx:
WRITE DDRx
READ DDRx
WRITE PORTx
PUD:
PULLUP DISABLE
SLEEP CONTROL
I/O CLOCK
SLEEP:
clkI/O
:
READ PORTx REGISTER
READ PORTx PIN
WRITE PINx REGISTER
Note:
1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O
,
SLEEP, and PUD are common to all ports.
13.2.1
Configuring the Pin
Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in ”Register
Description” on page 85, the DDxn bits are accessed at the DDRx I/O address, the PORTxn bits
at the PORTx I/O address, and the PINxn bits at the PINx I/O address.
The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one,
Pxn is configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input
pin.
If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is
activated. To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to
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be configured as an output pin. The port pins are tri-stated when reset condition becomes active,
even if no clocks are running.
If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven
high (one). If PORTxn is written logic zero when the pin is configured as an output pin, the port
pin is driven low (zero).
13.2.2
13.2.3
Toggling the Pin
Writing a logic one to PINxn toggles the value of PORTxn, independent on the value of DDRxn.
Note that the SBI instruction can be used to toggle one single bit in a port.
Switching Between Input and Output
When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn}
= 0b11), an intermediate state with either pull-up enabled {DDxn, PORTxn} = 0b01) or output
low ({DDxn, PORTxn} = 0b10) must occur. Normally, the pull-up enabled state is fully accept-
able, as a high-impedant environment will not notice the difference between a strong high driver
and a pull-up. If this is not the case, the PUD bit in the MCUCR 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 13-1 summarizes the control signals for the pin value.
Table 13-1. Port Pin Configurations
PUD
DDxn
PORTxn
(in MCUCR)
I/O
Input
Pull-up
No
Comment
0
0
0
1
1
0
1
1
0
1
X
0
Tri-state (Hi-Z)
Input
Yes
No
Pxn will source current if ext. pulled low.
Tri-state (Hi-Z)
1
Input
X
X
Output
Output
No
Output Low (Sink)
Output High (Source)
No
13.2.4
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 13-2, the PINxn Register bit and the preceding latch con-
stitute 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 13-3 shows a timing dia-
gram of the synchronization when reading an externally applied pin value. The maximum and
minimum propagation delays are denoted tpd,max and tpd,min respectively.
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Figure 13-3. Synchronization when Reading an Externally Applied Pin value
SYSTEM CLK
XXX
XXX
in r17, PINx
INSTRUCTIONS
SYNC LATCH
PINxn
r17
0x00
tpd, max
0xFF
tpd, min
Consider the clock period starting shortly after the first falling edge of the system clock. The latch
is closed when the clock is low, and goes transparent when the clock is high, as indicated by the
shaded region of the “SYNC LATCH” signal. The signal value is latched when the system clock
goes low. It is clocked into the PINxn Register at the succeeding positive clock edge. As indi-
cated by the two arrows tpd,max and tpd,min, a single signal transition on the pin will be delayed
between ½ and 1½ system clock period depending upon the time of assertion.
When reading back a software assigned pin value, a nop instruction must be inserted as indi-
cated in Figure 13-4. The out instruction sets the “SYNC LATCH” signal at the positive edge of
the clock. In this case, the delay tpd through the synchronizer is 1 system clock period.
Figure 13-4. Synchronization when Reading a Software Assigned Pin Value
SYSTEM CLK
0xFF
r16
out PORTx, r16
nop
in r17, PINx
INSTRUCTIONS
SYNC LATCH
PINxn
r17
0x00
tpd
0xFF
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.
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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
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*/
__no_operation();
/* 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.
13.2.5
Digital Input Enable and Sleep Modes
As shown in Figure 13-2, the digital input signal can be clamped to ground at the input of the
Schmitt-trigger. The signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in
Power-down mode, Power-save mode, and Standby mode to avoid high power consumption if
some input signals are left floating, or have an analog signal level close to VCC/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 Functions” on page 69.
If a logic high level (“one”) is present on an asynchronous external interrupt pin configured as
“Interrupt on Rising Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt
is not enabled, the corresponding External Interrupt Flag will be set when resuming from the
above mentioned Sleep mode, as the clamping in these sleep mode produces the requested
logic change.
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13.2.6
Unconnected Pins
If some pins are unused, it is recommended to ensure that these pins have a defined level. Even
though most of the digital inputs are disabled in the deep sleep modes as described above, float-
ing inputs should be avoided to reduce current consumption in all other modes where the digital
inputs are enabled (Reset, Active mode and Idle mode).
The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up.
In this case, the pull-up will be disabled during reset. If low power consumption during reset is
important, it is recommended to use an external pull-up or pull-down. Connecting unused pins
directly to VCC or GND is not recommended, since this may cause excessive currents if the pin is
accidentally configured as an output.
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13.3 Alternate Port Functions
Most port pins have alternate functions in addition to being general digital I/Os. Figure 13-5
shows how the port pin control signals from the simplified Figure 13-2 can be overridden 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 microcontroller family.
Figure 13-5. 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
1
0
Pxn
Q
D
PORTxn
PTOExn
WPx
Q CLR
DIEOExn
DIEOVxn
SLEEP
RESET
WRx
1
0
RRx
RPx
SYNCHRONIZER
SET
D
Q
D
L
Q
Q
PINxn
CLR Q
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:
WRx:
RPx:
WPx:
PULLUP DISABLE
WRITE DDRx
READ DDRx
READ PORTx REGISTER
WRITE PORTx
READ PORTx PIN
WRITE PINx
I/O CLOCK
DIGITAL INPUT PIN n ON PORTx
ANALOG INPUT/OUTPUT PIN n ON PORTx
DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE
DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE
clkI/O
DIxn:
AIOxn:
:
SLEEP:
SLEEP CONTROL
PTOExn: Pxn, PORT TOGGLE OVERRIDE ENABLE
Note:
1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O
,
SLEEP, and PUD are common to all ports. All other signals are unique for each pin.
Table 13-2 summarizes the function of the overriding signals. The pin and port indexes from Fig-
ure 13-5 are not shown in the succeeding tables. The overriding signals are generated internally
in the modules having the alternate function.
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Table 13-2. Generic Description of Overriding Signals for Alternate Functions
Signal Name
Full Name
Description
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
Enable
PUOE
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.
Pull-up Override
Value
PUOV
DDOE
DDOV
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 Enable
If DDOE is set, the Output Driver is enabled/disabled when
DDOV is set/cleared, regardless of the setting of the DDxn
Register bit.
Data Direction
Override Value
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.
Port Value
Override Enable
PVOE
Port Value
Override Value
If PVOE is set, the port value is set to PVOV, regardless of the
setting of the PORTxn Register bit.
PVOV
PTOE
Port Toggle
Override Enable
If PTOE is set, the PORTxn Register bit is inverted.
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 mode).
DIEOE
DIEOV
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 mode).
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.
DI
Digital Input
This is the Analog Input/output to/from alternate functions. The
signal is connected directly to the pad, and can be used bi-
directionally.
Analog
Input/Output
AIO
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.
Some pins are connected to different LCS segments on ATmega325P and ATmega3250P. See
pinout on ”Pinout ATmega3250P” on page 2 and ”Pinout ATmega325P” on page 3 for details.
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13.3.1
Alternate Functions of Port B
The Port B pins with alternate functions are shown in Table 13-3.
Table 13-3. Port B Pins Alternate Functions
Port Pin
Alternate Functions
OC2A/PCINT15 (Output Compare and PWM Output A for Timer/Counter2 or Pin
Change Interrupt15).
PB7
OC1B/PCINT14 (Output Compare and PWM Output B for Timer/Counter1 or Pin
Change Interrupt14).
PB6
PB5
PB4
OC1A/PCINT13 (Output Compare and PWM Output A for Timer/Counter1 or Pin
Change Interrupt13).
OC0A/PCINT12 (Output Compare and PWM Output A for Timer/Counter0 or Pin
Change Interrupt12).
PB3
PB2
PB1
PB0
MISO/PCINT11 (SPI Bus Master Input/Slave Output or Pin Change Interrupt11).
MOSI/PCINT10 (SPI Bus Master Output/Slave Input or Pin Change Interrupt10).
SCK/PCINT9 (SPI Bus Serial Clock or Pin Change Interrupt9).
SS/PCINT8 (SPI Slave Select input or Pin Change Interrupt8).
The alternate pin configuration is as follows:
• OC2A/PCINT15, Bit 7
OC2, Output Compare Match A output: The PB7 pin can serve as an external output for the
Timer/Counter2 Output Compare A. The pin has to be configured as an output (DDB7 set (one))
to serve this function. The OC2A pin is also the output pin for the PWM mode timer function.
PCINT15, Pin Change Interrupt source 15: The PB7 pin can serve as an external interrupt
source.
• OC1B/PCINT14, Bit 6
OC1B, Output Compare Match B output: The PB6 pin can serve as an external output for the
Timer/Counter1 Output Compare B. The pin has to be configured as an output (DDB6 set (one))
to serve this function. The OC1B pin is also the output pin for the PWM mode timer function.
PCINT14, Pin Change Interrupt Source 14: The PB6 pin can serve as an external interrupt
source.
• OC1A/PCINT13, Bit 5
OC1A, Output Compare Match A output: The PB5 pin can serve as an external output for the
Timer/Counter1 Output Compare A. The pin has to be configured as an output (DDB5 set (one))
to serve this function. The OC1A pin is also the output pin for the PWM mode timer function.
PCINT13, Pin Change Interrupt Source 13: The PB5 pin can serve as an external interrupt
source.
• OC0A/PCINT12, Bit 4
OC0A, Output Compare Match A output: The PB4 pin can serve as an external output for the
Timer/Counter0 Output Compare A. The pin has to be configured as an output (DDB4 set (one))
to serve this function. The OC0A pin is also the output pin for the PWM mode timer function.
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PCINT12, Pin Change Interrupt Source 12: The PB4 pin can serve as an external interrupt
source.
• MISO/PCINT11 – Port B, Bit 3
MISO: Master Data input, Slave Data output pin for SPI. When the SPI is enabled as a Master,
this pin is configured as an input regardless of the setting of DDB3. When the SPI is enabled as
a Slave, the data direction of this pin is controlled by DDB3. When the pin is forced to be an
input, the pull-up can still be controlled by the PORTB3 bit.
PCINT11, Pin Change Interrupt Source 11: The PB3 pin can serve as an external interrupt
source.
• MOSI/PCINT10 – Port B, Bit 2
MOSI: SPI Master Data output, Slave Data input for SPI. When the SPI is enabled as a Slave,
this pin is configured as an input regardless of the setting of DDB2. When the SPI is enabled as
a Master, the data direction of this pin is controlled by DDB2. When the pin is forced to be an
input, the pull-up can still be controlled by the PORTB2 bit.
PCINT10, Pin Change Interrupt Source 10: The PB2 pin can serve as an external interrupt
source.
• SCK/PCINT9 – Port B, Bit 1
SCK: Master Clock output, Slave Clock input pin for SPI. When the SPI is enabled as a Slave,
this pin is configured as an input regardless of the setting of DDB1. When the SPI is enabled as
a Master, the data direction of this pin is controlled by DDB1. When the pin is forced to be an
input, the pull-up can still be controlled by the PORTB1 bit.
PCINT9, Pin Change Interrupt Source 9: The PB1 pin can serve as an external interrupt source.
• SS/PCINT8 – Port B, Bit 0
SS: Slave Port Select input. When the SPI is enabled as a Slave, this pin is configured as an
input regardless of the setting of DDB0. As a Slave, the SPI is activated when this pin is driven
low. When the SPI is enabled as a Master, the data direction of this pin is controlled by DDB0.
When the pin is forced to be an input, the pull-up can still be controlled by the PORTB0 bit
PCINT8, Pin Change Interrupt Source 8: The PB0 pin can serve as an external interrupt source.
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Table 13-4 and Table 13-5 relate the alternate functions of Port B to the overriding signals
shown in Figure 13-5 on page 69. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the
MISO signal, while MOSI is divided into SPI MSTR OUTPUT and SPI SLAVE INPUT.
Table 13-4. Overriding Signals for Alternate Functions in PB7:PB4
PB7/OC2A/
PCINT15
PB6/OC1B/
PCINT14
PB5/OC1A/
PCINT13
PB4/OC0A/
PCINT12
Signal Name
PUOE
PUOV
DDOE
DDOV
PVOE
PVOV
PTOE
DIEOE
DIEOV
DI
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
OC2A ENABLE
OC1B ENABLE
OC1A ENABLE
OC0A ENABLE
OC2A
OC1B
OC1A
OC0A
–
–
–
–
PCINT15 • PCIE1
PCINT14 • PCIE1
PCINT13 • PCIE1
PCINT12 • PCIE1
1
1
1
1
PCINT15 INPUT
–
PCINT14 INPUT
–
PCINT13 INPUT
–
PCINT12 INPUT
–
AIO
Table 13-5. Overriding Signals for Alternate Functions in PB3:PB0
PB3/MISO/
PCINT11
PB2/MOSI/
PCINT10
PB1/SCK/
PCINT9
PB0/SS/
PCINT8
Signal Name
PUOE
SPE • MSTR
PORTB3 • PUD
SPE • MSTR
0
SPE • MSTR
PORTB2 • PUD
SPE • MSTR
0
SPE • MSTR
PORTB1 • PUD
SPE • MSTR
0
SPE • MSTR
PUOV
PORTB0 • PUD
DDOE
SPE • MSTR
DDOV
0
0
PVOE
SPE • MSTR
SPE • MSTR
SPE • MSTR
SPI SLAVE
OUTPUT
SPI MSTR
OUTPUT
PVOV
SCK OUTPUT
0
PTOE
–
–
–
–
DIEOE
DIEOV
PCINT11 • PCIE1
1
PCINT10 • PCIE1
1
PCINT9 • PCIE1
1
PCINT8 • PCIE1
1
PCINT11 INPUT
SPI MSTR INPUT
PCINT10 INPUT
PCINT9 INPUT
SCK INPUT
PCINT8 INPUT
SPI SS
DI
SPI SLAVE INPUT
AIO
–
–
–
–
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8023F–AVR–07/09
13.3.2
Alternate Functions of Port D
The Port D pins with alternate functions are shown in Table 13-6.
Table 13-6. Port D Pins Alternate Functions (SEG refers to 100-pin/64-pin pinout)
Port Pin
PD7
Alternate Function
PD6
PD5
PD4
PD3
PD2
PD1
INT0 (External Interrupt0 Input)
PD0
ICP1 (Timer/Counter1 Input Capture pin)
The alternate pin configuration is as follows:
• INT0 – Port D, Bit 1
INT0, External Interrupt Source 0. The PD1 pin can serve as an external interrupt source to the
MCU.
• ICP1 – Port D, Bit 0
ICP1 – Input Capture pin1: The PD0 pin can act as an Input Capture pin for Timer/Counter1.
Table 13-7 relates the alternate functions of Port D to the overriding signals shown in Figure 13-
5 on page 69.
Table 13-7. Overriding Signals for Alternate Functions in PD3:PD0
Signal Name
PUOE
PUOV
DDOE
DDOV
PVOE
PVOV
PTOE
DIEOE
DIEOV
DI
PD3
0
PD2
0
PD1/INT0
PD0/ICP1
0)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
–
–
–
–
0
0
INT0 ENABLE
INT0 ENABLE
INT0 INPUT
–
0
0
0
0
–
–
ICP1 INPUT
–
AIO
–
–
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13.3.3
Alternate Functions of Port E
The Port E pins with alternate functions are shown in Table 13-8.
Table 13-8. Port E Pins Alternate Functions
Port Pin
Alternate Function
PCINT7 (Pin Change Interrupt7)
CLKO (Divided System Clock)
PE7
PE6
PE5
DO/PCINT6 (USI Data Output or Pin Change Interrupt6)
DI/SDA/PCINT5 (USI Data Input or TWI Serial DAta or Pin Change Interrupt5)
USCK/SCL/PCINT4 (USART0 External Clock Input/Output or TWI Serial Clock or Pin
Change Interrupt4)
PE4
PE3
PE2
AIN1/PCINT3 (Analog Comparator Negative Input or Pin Change Interrupt3)
XCK/AIN0/ PCINT2 (USART0 External Clock or Analog Comparator Positive Input or
Pin Change Interrupt2)
PE1
PE0
TXD/PCINT1 (USART0 Transmit Pin or Pin Change Interrupt1)
RXD/PCINT0 (USART0 Receive Pin or Pin Change Interrupt0)
• PCINT7 – Port E, Bit 7
PCINT7, Pin Change Interrupt Source 7: The PE7 pin can serve as an external interrupt source.
CLKO, Divided System Clock: The divided system clock can be output on the PE7 pin. The
divided system clock will be output if the CKOUT Fuse is programmed, regardless of the
PORTE7 and DDE7 settings. It will also be output during reset.
• DO/PCINT6 – Port E, Bit 6
DO, Universal Serial Interface Data output.
PCINT6, Pin Change Interrupt Source 6: The PE6 pin can serve as an external interrupt source.
• DI/SDA/PCINT5 – Port E, Bit 5
DI, Universal Serial Interface Data input.
SDA, Two-wire Serial Interface Data:
PCINT5, Pin Change Interrupt Source 5: The PE5 pin can serve as an external interrupt source.
• USCK/SCL/PCINT4 – Port E, Bit 4
USCK, Universal Serial Interface Clock.
SCL, Two-wire Serial Interface Clock.
PCINT4, Pin Change Interrupt Source 4: The PE4 pin can serve as an external interrupt source.
• AIN1/PCINT3 – Port E, Bit 3
AIN1 – Analog Comparator Negative input. This pin is directly connected to the negative input of
the Analog Comparator.
PCINT3, Pin Change Interrupt Source 3: The PE3 pin can serve as an external interrupt source.
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8023F–AVR–07/09
• XCK/AIN0/PCINT2 – Port E, Bit 2
XCK, USART0 External Clock. The Data Direction Register (DDE2) controls whether the clock is
output (DDE2 set) or input (DDE2 cleared). The XCK pin is active only when the USART0 oper-
ates in synchronous mode.
AIN0 – Analog Comparator Positive input. This pin is directly connected to the positive input of
the Analog Comparator.
PCINT2, Pin Change Interrupt Source 2: The PE2 pin can serve as an external interrupt source.
• TXD/PCINT1 – Port E, Bit 1
TXD0, UART0 Transmit pin.
PCINT1, Pin Change Interrupt Source 1: The PE1 pin can serve as an external interrupt source.
• RXD/PCINT0 – Port E, Bit 0
RXD, USART0 Receive pin. Receive Data (Data input pin for the USART0). When the USART0
Receiver is enabled this pin is configured as an input regardless of the value of DDE0. When the
USART0 forces this pin to be an input, a logical one in PORTE0 will turn on the internal pull-up.
PCINT0, Pin Change Interrupt Source 0: The PE0 pin can serve as an external interrupt source.
Table 13-9 and Table 13-10 relates the alternate functions of Port E to the overriding signals
shown in Figure 13-5 on page 69.
Table 13-9. Overriding Signals for Alternate Functions PE7:PE4
PE6/DO/
PCINT6
PE5/DI/SDA/
PCINT5
PE4/USCK/SCL/
PCINT4
Signal Name
PUOE
PE7/PCINT7
0
0
0
0
USI_TWO-WIRE
0
USI_TWO-WIRE
0
PUOV
0
DDOE
CKOUT(1)
USI_TWO-WIRE
USI_TWO-WIRE
(USI_SCL_HOLD
+ PORTE4) •
DDE4
(SDA + PORTE5) •
DDE5
DDOV
PVOE
1
0
USI_THREE-
WIRE
USI_TWO-WIRE •
DDE5
USI_TWO-WIRE •
DDE4
CKOUT(1)
PVOV
PTOE
clkI/O
–
DO
–
0
0
0
USITC
(PCINT5 • PCIE0)
+ USISIE
(PCINT4 • PCIE0)
+ USISIE
DIEOE
DIEOV
PCINT7 • PCIE0
1
PCINT6 • PCIE0
1
1
1
USCKL/SCL
INPUT
DI/SDA INPUT
PCINT5 INPUT
DI
PCINT7 INPUT
–
PCINT6 INPUT
–
PCINT4 INPUT
AIO
–
–
Note:
1. CKOUT is one if the CKOUT Fuse is programmed
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Table 13-10. Overriding Signals for Alternate Functions in PE3:PE0
PE3/AIN1/
PCINT3
PE2/XCK/AIN0/
PCINT2
PE1/TXD/
PCINT1
PE0/RXD/
PCINT0
Signal Name
XCK OUTPUT
ENABLE
PUOE
0
TXEN
RXEN
PUOV
DDOE
DDOV
PVOE
PVOV
PTOE
0
0
0
0
0
–
XCK
0
PORTE0 • PUD
0
0
0
0
–
TXEN
1
RXEN
0
0
0
–
TXEN
TXD
–
(PCINT3 • PCIE0)
+ AIN1D(1)
(PCINT2 • PCIE0)
+ AIN0D(1)
DIEOE
DIEOV
DI
PCINT1 • PCIE0
PCINT0 • PCIE0
1
PCINT3 • PCIE0
PCINT3 INPUT
AIN1 INPUT
PCINT2 • PCIE0
1
XCK/PCINT2
INPUT
RXD/PCINT0
INPUT
PCINT1 INPUT
–
AIO
AIN0 INPUT
–
Note:
1. AIN0D and AIN1D is described in ”DIDR1 – Digital Input Disable Register 1” on page 205.
13.3.4
Alternate Functions of Port F
The Port F has an alternate function as analog input for the ADC as shown in Table 13-11. If
some Port F pins are configured as outputs, it is essential that these do not switch when a con-
version is in progress. This might corrupt the result of the conversion. If the JTAG interface is
enabled, the pull-up resistors on pins PF7(TDI), PF5(TMS) and PF4(TCK) will be activated even
if a reset occurs.
Table 13-11. Port F Pins Alternate Functions
Port Pin
PF7
Alternate Function
ADC7/TDI (ADC input channel 7 or JTAG Test Data Input)
ADC6/TDO (ADC input channel 6 or JTAG Test Data Output)
ADC5/TMS (ADC input channel 5 or JTAG Test mode Select)
ADC4/TCK (ADC input channel 4 or JTAG Test ClocK)
ADC3 (ADC input channel 3)
PF6
PF5
PF4
PF3
PF2
ADC2 (ADC input channel 2)
PF1
ADC1 (ADC input channel 1)
PF0
ADC0 (ADC input channel 0)
• TDI, ADC7 – Port F, Bit 7
ADC7, Analog to Digital Converter, Channel 7.
TDI, JTAG Test Data In: Serial input data to be shifted in to the Instruction Register or Data Reg-
ister (scan chains). When the JTAG interface is enabled, this pin can not be used as an I/O pin.
77
8023F–AVR–07/09
• TDO, ADC6 – Port F, Bit 6
ADC6, Analog to Digital Converter, Channel 6.
TDO, JTAG Test Data Out: Serial output data from Instruction Register or Data Register. When
the JTAG interface is enabled, this pin can not be used as an I/O pin. In TAP states that shift out
data, the TDO pin drives actively. In other states the pin is pulled high.
• TMS, ADC5 – Port F, Bit 5
ADC5, Analog to Digital Converter, Channel 5.
TMS, JTAG Test mode Select: This pin is used for navigating through the TAP-controller state
machine. When the JTAG interface is enabled, this pin can not be used as an I/O pin.
• TCK, ADC4 – Port F, Bit 4
ADC4, Analog to Digital Converter, Channel 4.
TCK, JTAG Test Clock: JTAG operation is synchronous to TCK. When the JTAG interface is
enabled, this pin can not be used as an I/O pin.
• ADC3 - ADC0 – Port F, Bit 3:0
Analog to Digital Converter, Channel 3-0.
Table 13-12. Overriding Signals for Alternate Functions in PF7:PF4
Signal Name
PUOE
PF7/ADC7/TDI
JTAGEN
1
PF6/ADC6/TDO
JTAGEN
1
PF5/ADC5/TMS
JTAGEN
1
PF4/ADC4/TCK
JTAGEN
1
PUOV
DDOE
JTAGEN
JTAGEN
JTAGEN
JTAGEN
SHIFT_IR +
SHIFT_DR
DDOV
0
0
0
PVOE
PVOV
PTOE
DIEOE
DIEOV
DI
0
JTAGEN
0
0
0
TDO
0
0
–
–
–
–
JTAGEN
JTAGEN
JTAGEN
JTAGEN
0
–
0
–
0
–
1
–
TDI
ADC7 INPUT
TMS
ADC5 INPUT
TCK
ADC4 INPUT
AIO
ADC6 INPUT
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Table 13-13. Overriding Signals for Alternate Functions in PF3:PF0
Signal Name
PF3/ADC3
PF2/ADC2
PF1/ADC1
PF0/ADC0
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
0
0
DDOV
0
0
0
0
PVOE
0
0
0
0
PVOV
0
0
0
0
PTOE
–
–
–
–
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
AIO
–
–
–
–
ADC3 INPUT
ADC2 INPUT
ADC1 INPUT
ADC0 INPUT
13.3.5
Alternate Functions of Port G
The alternate pin configuration is as follows:
Table 13-14. Port G Pins Alternate Functions (SEG refers to 100-pin/64-pin pinout)
Port Pin
PG5
Alternate Function
RESET(1)
PG4
T0(Timer/Counter0 Clock Input)
PG3
T1(Timer/Counter1 Clock Input)
PG2
–
–
–
PG1
PG0
Note:
1. Port G, PG5 is input only. Pull-up is always on.
See Table 25-3 on page 272 for RSTDISBL fuse.
The alternate pin configuration is as follows:
• RESET – Port G, Bit 5
RESET: External Reset input. When the RSTDISBL Fuse is programmed (‘0’), PG5 will function
as input with pull-up always on.
• T0 – Port G, Bit 4
T0, Timer/Counter0 Counter Source.
• T1 – Port G, Bit 3
T1, Timer/Counter1 Counter Source.
Table 13-14 and Table 13-15 relates the alternate functions of Port G to the overriding signals
shown in Figure 13-5 on page 69.
79
8023F–AVR–07/09
Table 13-15. Overriding Signals for Alternate Functions in PG4 and PG3
Signal Name
PUOE
PUOV
DDOE
DDOV
PVOE
PVOV
PTOE
DIEOE
DIEOV
DI
PG4/T0
PG3/T1
0
0
0
0
0
0
0
0
0
0
0
0
–
–
0
0
0
0
T0 INPUT
–
T1 INPUT
–
AIO
13.3.6
Alternate Functions of Port H
Port H is only present in ATmega3250P. The alternate pin configuration is as follows:
Table 13-16. Port H Pins Alternate Functions
Port Pin
PH7
Alternate Function
PCINT23(Pin Change Interrupt23)
PCINT22 (Pin Change Interrupt22)
PCINT21(Pin Change Interrupt21)
PCINT20(Pin Change Interrupt20)
PCINT19 (Pin Change Interrupt19)
PCINT18(Pin Change Interrupt18)
PCINT17(Pin Change Interrupt17)
PCINT16(Pin Change Interrupt16)
PH6
PH5
PH4
PH3
PH2
PH1
PH0
The alternate pin configuration is as follows:
• PCINT23 – Port H, Bit 7
PCINT23, Pin Change Interrupt Source 23: The PH7 pin can serve as an external interrupt
source.
• PCINT22 – Port H, Bit 6
PCINT22, Pin Change Interrupt Source 22: The PH6 pin can serve as an external interrupt
source.
• PCINT21 – Port H, Bit 5
PCINT21, Pin Change Interrupt Source 21: The PH5 pin can serve as an external interrupt
source.
80
ATmega325P/3250P
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ATmega325P/3250P
• PCINT20 – Port H, Bit 4
PCINT20, Pin Change Interrupt Source 20: The PH4 pin can serve as an external interrupt
source.
• PCINT19– Port H, Bit 3
PCINT19, Pin Change Interrupt Source 19: The PH3 pin can serve as an external interrupt
source.
• PCINT18 – Port H, Bit 2
PCINT18, Pin Change Interrupt Source 18: The PH2 pin can serve as an external interrupt
source.
• PCINT17/SEG – Port H, Bit 1
PCINT17, Pin Change Interrupt Source 17: The P1 pin can serve as an external interrupt
source.
SEG, LCD front plane 9.
• PCINT16 – Port H, Bit 0
PCINT16, Pin Change Interrupt Source 16: The PH0 pin can serve as an external interrupt
source.
Table 13-17 and Table 13-18 relates the alternate functions of Port H to the overriding signals
shown in Figure 13-5 on page 69.
Table 13-17. Overriding Signals for Alternate Functions in PH7:4
Signal Name
PUOE
PUOV
DDOE
DDOV
PVOE
PVOV
PTOE
DIEOE
DIEOV
DI
PH7/PCINT23
PH6/PCINT22
PH5/PCINT21
PH4/PCINT20
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
–
–
–
–
PCINT23 • PCIE0
PCINT22 • PCIE0
PCINT21 • PCIE0
PCINT20 • PCIE0
0
0
0
0
PCINT23 INPUT
–
PCINT22 INPUT
–
PCINT21 INPUT
–
PCINT20 INPUT
–
AIO
81
8023F–AVR–07/09
Table 13-18. Overriding Signals for Alternate Functions in PH3:0
Signal Name
PUOE
PUOV
DDOE
DDOV
PVOE
PVOV
PTOE
DIEOE
DIEOV
DI
PH3/PCINT19
PH2/PCINT18
PH1/PCINT17
PH0/PCINT16
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
–
–
–
–
PCINT19 • PCIE0
PCINT18 • PCIE0
PCINT17 • PCIE0
PCINT16 • PCIE0
0
0
0
0
PCINT19 INPUT
–
PCINT18 INPUT
–
PCINT17 INPUT
–
PCINT16 INPUT
–
AIO
13.3.7
Alternate Functions of Port J
Port J is only present in ATmega3250P. The alternate pin configuration is as follows:
Table 13-19. Port J Pins Alternate Functions
Port Pin
PJ6
Alternate Function
PCINT30(Pin Change Interrupt30)
PCINT29(Pin Change Interrupt29)
PCINT28(Pin Change Interrupt28)
PCINT27(Pin Change Interrupt27)
PCINT26(Pin Change Interrupt26)
PCINT25(Pin Change Interrupt25)
PCINT24(Pin Change Interrupt26)
PJ5
PJ4
PJ3
PJ2
PJ1
PJ0
The alternate pin configuration is as follows:
• PCINT30 – Port J, Bit 6
PCINT30, Pin Change Interrupt Source 30: The PE30 pin can serve as an external interrupt
source.
• PCINT29 – Port J, Bit 5
PCINT29, Pin Change Interrupt Source 29: The PE29 pin can serve as an external interrupt
source.
• PCINT28 – Port J, Bit 4
PCINT28, Pin Change Interrupt Source 28: The PE28 pin can serve as an external interrupt
source.
82
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ATmega325P/3250P
• PCINT27 – Port J, Bit 3
PCINT27, Pin Change Interrupt Source 27: The PE27 pin can serve as an external interrupt
source.
• PCINT26 – Port J, Bit 2
PCINT26, Pin Change Interrupt Source 26: The PE26 pin can serve as an external interrupt
source.
• PCINT25 – Port J, Bit 1
PCINT25, Pin Change Interrupt Source 25: The PE25 pin can serve as an external interrupt
source.
• PCINT24 – Port J, Bit 0
PCINT24, Pin Change Interrupt Source 24: The PE24 pin can serve as an external interrupt
source.
Table 13-20 and Table 13-21 relates the alternate functions of Port J to the overriding signals
shown in Figure 13-5 on page 69.
Table 13-20. Overriding Signals for Alternate Functions in PJ7:4
Signal Name
PUOE
PUOV
DDOE
DDOV
PVOE
PVOV
PTOE
DIEOE
DIEOV
DI
PJ7
0
PJ6/PCINT30
PJ5/PCINT29
PJ4/PCINT28
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
–
–
–
–
0
PCINT30 • PCIE0
PCINT29 • PCIE0
PCINT28 • PCIE0
0
0
–
–
0
–
–
0
–
–
–
AIO
–
83
8023F–AVR–07/09
Table 13-21. Overriding Signals for Alternate Functions in PH3:0
Signal Name
PUOE
PUOV
DDOE
DDOV
PVOE
PVOV
PTOE
DIEOE
DIEOV
DI
PJ3/PCINT27
PJ2/PCINT26
PJ1/PCINT25
PJ0/PCINT24
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
–
–
–
–
PCINT27 • PCIE0
PCINT26 • PCIE0
PCINT25 • PCIE0
PCINT24 • PCIE0
0
–
–
0
–
–
0
–
–
0
–
–
AIO
84
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ATmega325P/3250P
13.4 Register Description
13.4.1
MCUCR – MCU Control Register
Bit
7
6
BODS
R
5
4
3
–
2
–
1
IVSEL
R/W
0
0
IVCE
R/W
0
0x35 (0x55)
Read/Write
Initial Value
JTD
R/W
0
BODSE
PUD
R/W
0
MCUCR
R
0
R
0
R
0
0
• Bit 4 – PUD: Pull-up Disable
When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and
PORTxn Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See ”Con-
figuring the Pin” on page 64 for more details about this feature.
13.4.2
13.4.3
13.4.4
13.4.5
13.4.6
13.4.7
PORTA – Port A Data Register
Bit
7
6
PORTA6
R/W
0
5
PORTA5
R/W
0
4
PORTA4
R/W
0
3
PORTA3
R/W
0
2
PORTA2
R/W
0
1
PORTA1
R/W
0
0
PORTA0
R/W
0
PORTA7
R/W
0
0x02 (0x22)
Read/Write
Initial Value
PORTA
DDRA
PINA
DDRA – Port A Data Direction Register
Bit
7
DDA7
R/W
0
6
DDA6
R/W
0
5
DDA5
R/W
0
4
DDA4
R/W
0
3
DDA3
R/W
0
2
DDA2
R/W
0
1
DDA1
R/W
0
0
DDA0
R/W
0
0x01 (0x21)
Read/Write
Initial Value
PINA – Port A Input Pins Address
Bit
7
6
5
4
3
2
1
0
0x00 (0x20)
Read/Write
Initial Value
PINA7
R/W
N/A
PINA6
R/W
N/A
PINA5
R/W
N/A
PINA4
R/W
N/A
PINA3
R/W
N/A
PINA2
R/W
N/A
PINA1
R/W
N/A
PINA0
R/W
N/A
PORTB – Port B Data Register
Bit
7
6
PORTB6
R/W
0
5
PORTB5
R/W
0
4
PORTB4
R/W
0
3
PORTB3
R/W
0
2
PORTB2
R/W
0
1
PORTB1
R/W
0
0
PORTB0
R/W
0
PORTB7
R/W
0
PORTB
DDRB
PINB
0x05 (0x25)
Read/Write
Initial Value
DDRB – Port B Data Direction Register
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
0
DDB0
R/W
0
0x04 (0x24)
Read/Write
Initial Value
PINB – Port B Input Pins Address
Bit
7
6
5
4
3
2
1
0
0x03 (0x23)
Read/Write
Initial Value
PINB7
R/W
N/A
PINB6
R/W
N/A
PINB5
R/W
N/A
PINB4
R/W
N/A
PINB3
R/W
N/A
PINB2
R/W
N/A
PINB1
R/W
N/A
PINB0
R/W
N/A
85
8023F–AVR–07/09
13.4.8
13.4.9
PORTC – Port C Data Register
Bit
7
6
PORTC6
R/W
0
5
PORTC5
R/W
0
4
PORTC4
R/W
0
3
PORTC3
R/W
0
2
PORTC2
R/W
0
1
PORTC1
R/W
0
0
PORTC0
R/W
0
PORTC7
R/W
0
PORTC
DDRC
PINC
0x08 (0x28)
Read/Write
Initial Value
DDRC – Port C Data Direction Register
Bit
7
DDC7
R/W
0
6
DDC6
R/W
0
5
DDC5
R/W
0
4
DDC4
R/W
0
3
DDC3
R/W
0
2
DDC2
R/W
0
1
DDC1
R/W
0
0
DDC0
R/W
0
0x07 (0x27)
Read/Write
Initial Value
13.4.10 PINC – Port C Input Pins Address
Bit
7
6
5
4
3
2
1
0
0x06 (0x26)
Read/Write
Initial Value
PINC7
R/W
N/A
PINC6
R/W
N/A
PINC5
R/W
N/A
PINC4
R/W
N/A
PINC3
R/W
N/A
PINC2
R/W
N/A
PINC1
R/W
N/A
PINC0
R/W
N/A
13.4.11 PORTD – Port D Data Register
Bit
7
6
PORTD6
R/W
0
5
PORTD5
R/W
0
4
PORTD4
R/W
0
3
PORTD3
R/W
0
2
PORTD2
R/W
0
1
PORTD1
R/W
0
0
PORTD0
R/W
0
PORTD7
R/W
0
PORTD
DDRD
PIND
0x0B (0x2B)
Read/Write
Initial Value
13.4.12 DDRD – Port D Data Direction Register
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
0
DDD0
R/W
0
0x0A (0x2A)
Read/Write
Initial Value
13.4.13 PIND – Port D Input Pins Address
Bit
7
6
5
4
3
2
1
0
0x09 (0x29)
Read/Write
Initial Value
PIND7
R/W
N/A
PIND6
R/W
N/A
PIND5
R/W
N/A
PIND4
R/W
N/A
PIND3
R/W
N/A
PIND2
R/W
N/A
PIND1
R/W
N/A
PIND0
R/W
N/A
13.4.14 PORTE – Port E Data Register
Bit
7
6
PORTE6
R/W
0
5
PORTE5
R/W
0
4
PORTE4
R/W
0
3
PORTE3
R/W
0
2
PORTE2
R/W
0
1
PORTE1
R/W
0
0
PORTE0
R/W
0
PORTE7
R/W
0
PORTE
0x0E (0x2E)
Read/Write
Initial Value
13.4.15 DDRE – Port E Data Direction Register
Bit
7
DDE7
R/W
0
6
DDE6
R/W
0
5
DDE5
R/W
0
4
DDE4
R/W
0
3
DDE3
R/W
0
2
DDE2
R/W
0
1
DDE1
R/W
0
0
DDE0
R/W
0
0x0D (0x2D)
Read/Write
Initial Value
DDRE
86
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
13.4.16 PINE – Port E Input Pins Address
Bit
7
6
5
4
3
2
1
0
0x0C (0x2C)
Read/Write
Initial Value
PINE7
R/W
N/A
PINE6
R/W
N/A
PINE5
R/W
N/A
PINE4
R/W
N/A
PINE3
R/W
N/A
PINE2
R/W
N/A
PINE1
R/W
N/A
PINE0
R/W
N/A
PINE
PORTF
DDRF
PINF
13.4.17 PORTF – Port F Data Register
Bit
7
6
PORTF6
R/W
0
5
PORTF5
R/W
0
4
PORTF4
R/W
0
3
PORTF3
R/W
0
2
PORTF2
R/W
0
1
PORTF1
R/W
0
0
PORTF0
R/W
0
PORTF7
R/W
0
0x11 (0x31)
Read/Write
Initial Value
13.4.18 DDRF – Port F Data Direction Register
Bit
7
DDF7
R/W
0
6
DDF6
R/W
0
5
DDF5
R/W
0
4
DDF4
R/W
0
3
DDF3
R/W
0
2
DDF2
R/W
0
1
DDF1
R/W
0
0
DDF0
R/W
0
0x10 (0x30)
Read/Write
Initial Value
13.4.19 PINF – Port F Input Pins Address
Bit
7
6
5
4
3
2
1
0
0x0F (0x2F)
Read/Write
Initial Value
PINF7
R/W
N/A
PINF6
R/W
N/A
PINF5
R/W
N/A
PINF4
R/W
N/A
PINF3
R/W
N/A
PINF2
R/W
N/A
PINF1
R/W
N/A
PINF0
R/W
N/A
13.4.20 PORTG – Port G Data Register
Bit
7
6
–
5
–
4
PORTG4
R/W
0
3
PORTG3
R/W
0
2
PORTG2
R/W
0
1
PORTG1
R/W
0
0
PORTG0
R/W
0
PORTG
DDRG
PING
0x14 (0x34)
Read/Write
Initial Value
–
R
0
R
0
R
0
13.4.21 DDRG – Port G Data Direction Register
Bit
7
–
6
–
5
–
4
DDG4
R/W
0
3
DDG3
R/W
0
2
DDG2
R/W
0
1
DDG1
R/W
0
0
DDG0
R/W
0
0x13 (0x33)
Read/Write
Initial Value
R
0
R
0
R
0
13.4.22 PING – Port G Input Pins Address
Bit
7
–
6
–
5
PING5
R
4
3
2
1
0
0x12 (0x32)
Read/Write
Initial Value
PING4
R/W
N/A
PING3
R/W
N/A
PING2
R/W
N/A
PING1
R/W
N/A
PING0
R/W
N/A
R
0
R
0
0
13.4.23 PORTH – Port H Data Register(1)
Bit
7
PORTH7
R/W
6
5
PORTH5
R/W
0
4
PORTH4
R/W
0
3
PORTH3
R/W
0
2
PORTH2
R/W
0
1
PORTH1
R/W
0
0
PORTH0
R/W
0
PORTH6
R/W
0
PORTH
(0xDA)
Read/Write
Initial Value
0
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13.4.24 DDRH – Port H Data Direction Register(1)
Bit
7
DDH7
R/W
0
6
DDH6
R/W
0
5
DDH5
R/W
0
4
DDH4
R/W
0
3
DDH3
R/W
0
2
DDH2
R/W
0
1
DDH1
R/W
0
0
DDH0
R/W
0
(0xD9)
DDRH
Read/Write
Initial Value
13.4.25 PINH – Port H Input Pins Address(1)
Bit
7
6
5
4
3
2
1
0
(0xD8)
PINH7
R/W
N/A
PINH6
R/W
N/A
PINH5
R/W
N/A
PINH4
R/W
N/A
PINH3
R/W
N/A
PINH2
R/W
N/A
PINH1
R/W
N/A
PINH0
R/W
N/A
PINH
Read/Write
Initial Value
13.4.26 PORTJ – Port J Data Register(1)
Bit
7
–
6
PORTJ6
R/W
0
5
PORTJ5
R/W
0
4
PORTJ4
R/W
0
3
PORTJ3
R/W
0
2
PORTJ2
R/W
0
1
PORTJ1
R/W
0
0
PORTJ0
R/W
0
PORTJ
DDRJ
PINJ
(0xDD)
Read/Write
Initial Value
R
0
13.4.27 DDRJ – Port J Data Direction Register(1)
Bit
7
–
6
DDJ6
R/W
0
5
DDJ5
R/W
0
4
DDJ4
R/W
0
3
DDJ3
R/W
0
2
DDJ2
R/W
0
1
DDJ1
R/W
0
0
DDJ0
R/W
0
(0xDC)
Read/Write
Initial Value
R
0
13.4.28 PINJ – Port J Input Pins Address(1)
Bit
7
–
6
5
4
3
2
1
0
(0xDB)
PINJ6
R/W
N/A
PINJ5
R/W
N/A
PINJ4
R/W
N/A
PINJ3
R/W
N/A
PINJ2
R/W
N/A
PINJ1
R/W
N/A
PINJ0
R/W
N/A
Read/Write
Initial Value
R
0
Note:
1. Register only available in ATmega3250P.
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14. 8-bit Timer/Counter0 with PWM
14.1 Features
• Single Compare Unit Counter
• Clear Timer on Compare Match (Auto Reload)
• Glitch-free, Phase Correct Pulse Width Modulator (PWM)
• Frequency Generator
• External Event Counter
• 10-bit Clock Prescaler
• Overflow and Compare Match Interrupt Sources (TOV0 and OCF0A)
14.2 Overview
Timer/Counter0 is a general purpose, single compare unit, 8-bit Timer/Counter module. A simpli-
fied block diagram of the 8-bit Timer/Counter is shown in Figure 14-1. For the actual placement
of I/O pins, refer to ”Pinout ATmega3250P” on page 2 and ”Pinout ATmega325P” on page 3.
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 ”Register Description” on page 102.
Figure 14-1. 8-bit Timer/Counter Block Diagram
TCCRn
count
TOVn
(Int.Req.)
clear
Control Logic
TOP
Clock Select
direction
clk
Tn
Edge
Detector
Tn
BOTTOM
( From Prescaler )
Timer/Counter
TCNTn
= 0
= 0xFF
OCn
(Int.Req.)
Waveform
Generation
OCn
=
OCRn
14.2.1
Registers
The Timer/Counter (TCNT0) and Output Compare Register (OCR0A) are 8-bit registers. Inter-
rupt request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt
Flag Register (TIFR0). All interrupts are individually masked with the Timer Interrupt Mask Reg-
ister (TIMSK0). TIFR0 and TIMSK0 are not shown in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on
the T0 pin. The Clock Select logic block controls which clock source and edge the Timer/Counter
uses to increment (or decrement) its value. The Timer/Counter is inactive when no clock source
is selected. The output from the Clock Select logic is referred to as the timer clock (clkT0).
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The double buffered Output Compare Register (OCR0A) is compared with the Timer/Counter
value at all times. The result of the compare can be used by the Waveform Generator to gener-
ate a PWM or variable frequency output on the Output Compare pin (OC0A). See Section “14.5”
on page 92. for details. The compare match event will also set the Compare Flag (OCF0A)
which can be used to generate an Output Compare interrupt request.
14.2.2
Definitions
Many register and bit references in this section are written in general form. A lower case “n”
replaces the Timer/Counter number, in this case 0. A lower case “x” replaces the Output Com-
pare unit number, in this case unit A. 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 14-1 are also used extensively throughout the document.
Table 14-1. Definitions of Timer/Counter values.
BOTTOM
MAX
TOP
The counter reaches the BOTTOM when it becomes 0x00.
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 OCR0A Register. The assignment is depen-
dent on the mode of operation.
14.3 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 (TCCR0A). For details on clock sources and pres-
caler, see ”Timer/Counter0 and Timer/Counter1 Prescalers” on page 100.
14.4 Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
14-2 shows a block diagram of the counter and its surroundings.
Figure 14-2. Counter Unit Block Diagram
TOVn
(Int.Req.)
DATA BUS
Clock Select
count
clear
Edge
Detector
Tn
clkTn
TCNTn
Control Logic
direction
( From Prescaler )
bottom
top
Signal description (internal signals):
count
Increment or decrement TCNT0 by 1.
Select between increment and decrement.
Clear TCNT0 (set all bits to zero).
direction
clear
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clkTn
top
Timer/Counter clock, referred to as clkT0 in the following.
Signalize that TCNT0 has reached maximum value.
Signalize that TCNT0 has reached minimum value (zero).
bottom
Depending of the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clkT0). clkT0 can 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 clkT0 is present or not. A CPU write overrides (has priority over) all counter clear or
count operations.
The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in
the Timer/Counter Control Register (TCCR0A). There are close connections between how the
counter behaves (counts) and how waveforms are generated on the Output Compare output
OC0A. For more details about advanced counting sequences and waveform generation, see
”Modes of Operation” on page 94.
The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by
the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.
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14.5 Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Register
(OCR0A). Whenever TCNT0 equals OCR0A, the comparator signals a match. A match will set
the Output Compare Flag (OCF0A) at the next timer clock cycle. If enabled (OCIE0A = 1 and
Global Interrupt Flag in SREG is set), the Output Compare Flag generates an Output Compare
interrupt. The OCF0A Flag is automatically cleared when the interrupt is executed. Alternatively,
the OCF0A 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 WGM01:0 bits and Compare Output mode (COM0A1:0) 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 Section “14.7” on page 94.).
Figure 14-3 shows a block diagram of the Output Compare unit.
Figure 14-3. Output Compare Unit, Block Diagram
DATA BUS
OCRnx
TCNTn
= (8-bit Comparator )
OCFnx (Int.Req.)
top
bottom
FOCn
Waveform Generator
OCnx
WGMn1:0
COMnX1:0
The OCR0A Register is double buffered when using any of the Pulse Width Modulation (PWM)
modes. For the normal and Clear Timer on Compare (CTC) modes of operation, the double buff-
ering is disabled. The double buffering synchronizes the update of the OCR0 Compare Register
to either top or bottom of the counting sequence. The synchronization prevents the occurrence
of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free.
The OCR0A Register access may seem complex, but this is not case. When the double buffer-
ing is enabled, the CPU has access to the OCR0A Buffer Register, and if double buffering is
disabled the CPU will access the OCR0A directly.
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14.5.1
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 (FOC0A) bit. Forcing compare match will not set the
OCF0A Flag or reload/clear the timer, but the OC0A pin will be updated as if a real compare
match had occurred (the COM0A1:0 bits settings define whether the OC0A pin is set, cleared or
toggled).
14.5.2
14.5.3
Compare Match Blocking by TCNT0 Write
All CPU write operations to the TCNT0 Register will block any compare match that occur in the
next timer clock cycle, even when the timer is stopped. This feature allows OCR0A to be initial-
ized to the same value as TCNT0 without triggering an interrupt when the Timer/Counter clock is
enabled.
Using the Output Compare Unit
Since writing TCNT0 in any mode of operation will block all compare matches for one timer clock
cycle, there are risks involved when changing TCNT0 when using the Output Compare unit,
independently of whether the Timer/Counter is running or not. If the value written to TCNT0
equals the OCR0A value, the compare match will be missed, resulting in incorrect waveform
generation. Similarly, do not write the TCNT0 value equal to BOTTOM when the counter is
counting down.
The setup of the OC0A should be performed before setting the Data Direction Register for the
port pin to output. The easiest way of setting the OC0A value is to use the Force Output Com-
pare (FOC0A) strobe bits in Normal mode. The OC0A Register keeps its value even when
changing between Waveform Generation modes.
Be aware that the COM0A1:0 bits are not double buffered together with the compare value.
Changing the COM0A1:0 bits will take effect immediately.
14.6 Compare Match Output Unit
The Compare Output mode (COM0A1:0) bits have two functions. The Waveform Generator
uses the COM0A1:0 bits for defining the Output Compare (OC0A) state at the next compare
match. Also, the COM0A1:0 bits control the OC0A pin output source. Figure 14-4 shows a sim-
plified schematic of the logic affected by the COM0A1:0 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 regis-
ters (DDR and PORT) that are affected by the COM0A1:0 bits are shown. When referring to the
OC0A state, the reference is for the internal OC0A Register, not the OC0A pin. If a System
Reset occur, the OC0A Register is reset to “0”.
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Figure 14-4. Compare Match Output Unit, Schematic
COMnx1
Waveform
Generator
COMnx0
FOCn
D
Q
1
0
OCn
Pin
OCnx
D
Q
PORT
D
Q
DDR
clkI/O
The general I/O port function is overridden by the Output Compare (OC0A) from the Waveform
Generator if either of the COM0A1:0 bits are set. However, the OC0A pin direction (input or out-
put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction
Register bit for the OC0A pin (DDR_OC0A) must be set as output before the OC0A value is vis-
ible 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 OC0A state before the
output is enabled. Note that some COM0A1:0 bit settings are reserved for certain modes of
operation. See ”Register Description” on page 102.
14.6.1
Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM0A1:0 bits differently in Normal, CTC, and PWM
modes. For all modes, setting the COM0A1:0 = 0 tells the Waveform Generator that no action on
the OC0A Register is to be performed on the next compare match. For compare output actions
in the non-PWM modes refer to Table 14-3 on page 103. For fast PWM mode, refer to Table 14-
4 on page 103, and for phase correct PWM refer to Table 14-5 on page 103.
A change of the COM0A1:0 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
FOC0A strobe bits.
14.7 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 (WGM01:0) and Compare Output
mode (COM0A1:0) bits. The Compare Output mode bits do not affect the counting sequence,
while the Waveform Generation mode bits do. The COM0A1:0 bits control whether the PWM
output generated should be inverted or not (inverted or non-inverted PWM). For non-PWM
modes the COM0A1:0 bits control whether the output should be set, cleared, or toggled at a
compare match (See Section “14.6” on page 93.).
For detailed timing information refer to Figure 4, Figure 5, Figure 6 and Figure 7 in ”Timer/Coun-
ter Timing Diagrams” on page 99.
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14.7.1
Normal Mode
The simplest mode of operation is the Normal mode (WGM01:0 = 0). In this mode the counting
direction is always up (incrementing), and no counter clear is performed. The counter simply
overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bot-
tom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV0) will be set in the same
timer clock cycle as the TCNT0 becomes zero. The TOV0 Flag in this case behaves like a ninth
bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt
that automatically clears the TOV0 Flag, the timer resolution can be increased by software.
There are no special cases to consider in the Normal mode, a new counter value can be written
anytime.
The Output Compare unit can be used to generate interrupts at some given time. Using the Out-
put Compare to generate waveforms in Normal mode is not recommended, since this will
occupy too much of the CPU time.
14.7.2
Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM01:0 = 2), the OCR0A Register is used to
manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter
value (TCNT0) matches the OCR0A. The OCR0A 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 14-5. The counter value (TCNT0)
increases until a compare match occurs between TCNT0 and OCR0A, and then counter
(TCNT0) is cleared.
Figure 14-5. CTC Mode, Timing Diagram
OCnx Interrupt Flag Set
TCNTn
OCn
(Toggle)
(COMnx1:0 = 1)
1
2
3
4
Period
An interrupt can be generated each time the counter value reaches the TOP value by using the
OCF0A Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating
the TOP value. However, changing TOP to a value close to BOTTOM when the counter is run-
ning with none or a low prescaler value must be done with care since the CTC mode does not
have the double buffering feature. If the new value written to OCR0A is lower than the current
value of TCNT0, 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 OC0A output can be set to toggle its logical
level on each compare match by setting the Compare Output mode bits to toggle mode
(COM0A1:0 = 1). The OC0A value will not be visible on the port pin unless the data direction for
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the pin is set to output. The waveform generated will have a maximum frequency of fOC0
clk_I/O/2 when OCR0A is set to zero (0x00). The waveform frequency is defined by the following
equation:
=
f
f
clk_I/O
f
= -------------------------------------------------
OCnx
2 ⋅ N ⋅ (1 + OCRnx)
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle that the
counter counts from MAX to 0x00.
14.7.3
Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM01:0 = 3) provides a high frequency
PWM waveform generation option. The fast PWM differs from the other PWM option by its sin-
gle-slope operation. The counter counts from BOTTOM to MAX then restarts from BOTTOM. In
non-inverting Compare Output mode, the Output Compare (OC0A) is cleared on the compare
match between TCNT0 and OCR0A, and set at BOTTOM. In inverting Compare Output mode,
the output is set on compare match and cleared at BOTTOM. Due to the single-slope operation,
the operating frequency of the fast PWM mode can be twice as high as the phase correct PWM
mode that use dual-slope operation. This high frequency makes the fast PWM mode well suited
for power regulation, rectification, and DAC applications. High frequency allows physically small
sized external components (coils, capacitors), and therefore reduces total system cost.
In fast PWM mode, the counter is incremented until the counter value matches the MAX value.
The counter is then cleared at the following timer clock cycle. The timing diagram for the fast
PWM mode is shown in Figure 14-6. The TCNT0 value is in the timing diagram shown as a his-
togram for illustrating the single-slope operation. The diagram includes non-inverted and
inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent compare
matches between OCR0A and TCNT0.
Figure 14-6. Fast PWM Mode, Timing Diagram
OCRnx Interrupt Flag Set
OCRnx Update and
TOVn Interrupt Flag Set
TCNTn
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCn
OCn
1
2
3
4
5
6
7
Period
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches MAX. If the inter-
rupt is enabled, the interrupt handler routine can be used for updating the compare value.
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In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0A pin.
Setting the COM0A1:0 bits to two will produce a non-inverted PWM and an inverted PWM output
can be generated by setting the COM0A1:0 to three (See Table 14-4 on page 103). The actual
OC0A value will only be visible on the port pin if the data direction for the port pin is set as out-
put. The PWM waveform is generated by setting (or clearing) the OC0A Register at the compare
match between OCR0A and TCNT0, and clearing (or setting) the OC0A Register at the timer
clock cycle the counter is cleared (changes from MAX to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
f
clk_I/O
f
= -----------------
OCnxPWM
N ⋅ 256
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represents special cases when generating a PWM
waveform output in the fast PWM mode. If the OCR0A is set equal to BOTTOM, the output will
be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR0A equal to MAX will result
in a constantly high or low output (depending on the polarity of the output set by the COM0A1:0
bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-
ting OC0A to toggle its logical level on each compare match (COM0A1:0 = 1). The waveform
generated will have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0A is set to zero. This
feature is similar to the OC0A toggle in CTC mode, except the double buffer feature of the Out-
put Compare unit is enabled in the fast PWM mode.
14.7.4
Phase Correct PWM Mode
The phase correct PWM mode (WGM01:0 = 1) provides a high resolution phase correct PWM
waveform generation option. The phase correct PWM mode is based on a dual-slope operation.
The counter counts repeatedly from BOTTOM to MAX and then from MAX to BOTTOM. In non-
inverting Compare Output mode, the Output Compare (OC0A) is cleared on the compare match
between TCNT0 and OCR0A while counting up, and set on the compare match while counting
down. 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 sym-
metric feature of the dual-slope PWM modes, these modes are preferred for motor control
applications.
The PWM resolution for the phase correct PWM mode is fixed to eight bits. In phase correct
PWM mode the counter is incremented until the counter value matches MAX. When the counter
reaches MAX, it changes the count direction. The TCNT0 value will be equal to MAX for one
timer clock cycle. The timing diagram for the phase correct PWM mode is shown on Figure 14-7.
The TCNT0 value is in the timing diagram shown as a histogram for illustrating the dual-slope
operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal
line marks on the TCNT0 slopes represent compare matches between OCR0A and TCNT0.
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Figure 14-7. Phase Correct PWM Mode, Timing Diagram
OCnx Interrupt Flag Set
OCRnx Update
TOVn Interrupt Flag Set
TCNTn
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCn
OCn
1
2
3
Period
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches BOTTOM. The
Interrupt Flag can be used to generate an interrupt each time the counter reaches the BOTTOM
value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the
OC0A pin. Setting the COM0A1:0 bits to two will produce a non-inverted PWM. An inverted
PWM output can be generated by setting the COM0A1:0 to three (See Table 14-5 on page 103).
The actual OC0A 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 OC0A Register at the
compare match between OCR0A and TCNT0 when the counter increments, and setting (or
clearing) the OC0A Register at compare match between OCR0A and TCNT0 when the counter
decrements. The PWM frequency for the output when using phase correct PWM can be calcu-
lated by the following equation:
f
clk_I/O
f
= -----------------
OCnxPCPWM
N ⋅ 510
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represent special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR0A 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 14-7 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 BOT-
TOM. There are two cases that give a transition without Compare Match.
• OCR0A changes its value from MAX, like in Figure 14-7. When the OCR0A value is MAX the
OCn pin value is the same as the result of a down-counting Compare Match. To ensure
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symmetry around BOTTOM the OCn value at MAX must correspond to the result of an up-
counting Compare Match.
• The timer starts counting from a value higher than the one in OCR0A, and for that reason
misses the Compare Match and hence the OCn change that would have happened on the way
up.
14.8 Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkT0) is therefore shown as a
clock enable signal in the following figures. The figures include information on when Interrupt
Flags are set. Figure 4 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 4. Timer/Counter Timing Diagram, no Prescaling
clkI/O
clkTn
(clkI/O/1)
TCNTn
TOVn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
Figure 5 shows the same timing data, but with the prescaler enabled.
Figure 5. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O/8)
TCNTn
TOVn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
Figure 6 shows the setting of OCF0A in all modes except CTC mode.
Figure 6. Timer/Counter Timing Diagram, Setting of OCF0A, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O/8)
TCNTn
OCRnx
OCFnx
OCRnx - 1
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
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Figure 7 shows the setting of OCF0A and the clearing of TCNT0 in CTC mode.
Figure 7. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Prescaler
(fclk_I/O/8)
clkI/O
clkTn
(clkI/O/8)
TCNTn
(CTC)
TOP - 1
TOP
BOTTOM
BOTTOM + 1
OCRnx
TOP
OCFnx
14.9 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.
14.9.1
14.9.2
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 (fCLK_I/O). Alternatively, one of four taps from the prescaler can be used as a
clock source. The prescaled clock has a frequency of either fCLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or
fCLK_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 prescaler is
not affected by the Timer/Counter’s clock select, the state of the prescaler will have implications
for situations where a prescaled clock is used. One example of prescaling artifacts occurs when
the timer is enabled and clocked by the prescaler (6 > CSn2:0 > 1). The number of system clock
cycles from when the timer is enabled to the first count occurs can be from 1 to N+1 system
clock cycles, where N equals the prescaler divisor (8, 64, 256, or 1024).
It is possible to use the prescaler reset for synchronizing the Timer/Counter to program execu-
tion. However, care must be taken if the other Timer/Counter that shares the same prescaler
also uses prescaling. A prescaler reset will affect the prescaler period for all Timer/Counters it is
connected to.
14.9.3
External Clock Source
An external clock source applied to the T1/T0 pin can be used as Timer/Counter clock
(clkT1/clkT0). The T1/T0 pin is sampled once every system clock cycle by the pin synchronization
logic. The synchronized (sampled) signal is then passed through the edge detector. Figure 14-8
shows a functional equivalent block diagram of the T1/T0 synchronization and edge detector
logic. The registers are clocked at the positive edge of the internal system clock (clkI/O). The latch
is transparent in the high period of the internal system clock.
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The edge detector generates one clkT1/clkT pulse for each positive (CSn2:0 = 7) or negative
0
(CSn2:0 = 6) edge it detects.
Figure 14-8. T1/T0 Pin Sampling
Tn_sync
(To Clock
Tn
D
Q
D
Q
D
Q
Select Logic)
LE
clkI/O
Synchronization
Edge Detector
The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles
from an edge has been applied to the T1/T0 pin to the counter is updated.
Enabling and disabling of the clock input must be done when T1/T0 has been stable for at least
one system clock cycle, otherwise it is a risk that a false Timer/Counter clock pulse is generated.
Each half period of the external clock applied must be longer than one system clock cycle to
ensure correct sampling. The external clock must be guaranteed to have less than half the sys-
tem clock frequency (fExtClk < fclk_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 fre-
quency (Nyquist sampling theorem). However, due to variation of the system clock frequency
and duty cycle caused by Oscillator source (crystal, resonator, and capacitors) tolerances, it is
recommended that maximum frequency of an external clock source is less than fclk_I/O/2.5.
An external clock source can not be prescaled.
Figure 14-9. Prescaler for Timer/Counter0 and Timer/Counter1(1)
clkI/O
Clear
PSR10
T0
Synchronization
T1
Synchronization
clkT1
clkT0
Note:
1. The synchronization logic on the input pins (T1/T0) is shown in Figure 14-8.
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14.10 Register Description
14.10.1 TCCR0A – Timer/Counter Control Register A
Bit
7
FOC0A
W
6
WGM00
R/W
0
5
COM0A1
R/W
4
COM0A0
R/W
3
WGM01
R/W
0
2
CS02
R/W
0
1
CS01
R/W
0
0
CS00
R/W
0
TCCR0A
0x24 (0x44)
Read/Write
Initial Value
0
0
0
• Bit 7 – FOC0A: Force Output Compare A
The FOC0A bit is only active when the WGM00 bit specifies a non-PWM mode. However, for
ensuring compatibility with future devices, this bit must be set to zero when TCCR0 is written
when operating in PWM mode. When writing a logical one to the FOC0A bit, an immediate com-
pare match is forced on the Waveform Generation unit. The OC0A output is changed according
to its COM0A1:0 bits setting. Note that the FOC0A bit is implemented as a strobe. Therefore it is
the value present in the COM0A1:0 bits that determines the effect of the forced compare.
A FOC0A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using
OCR0A as TOP.
The FOC0A bit is always read as zero.
• Bit 6, 3 – WGM01:0: Waveform Generation Mode
These bits control the counting sequence of the counter, the source for the maximum (TOP)
counter value, and what type of waveform generation to be used. Modes of operation 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 14-2 and ”Modes of Operation”
on page 94.
Table 14-2. Waveform Generation Mode Bit Description(1)
WGM01
(CTC0)
WGM00
(PWM0)
Timer/Counter Mode
of Operation
Update of
OCR0A at
TOV0 Flag Set
on
Mode
TOP
0
1
2
3
0
0
1
1
0
1
0
1
Normal
0xFF
0xFF
OCR0A
0xFF
Immediate
TOP
MAX
PWM, Phase Correct
CTC
BOTTOM
MAX
Immediate
BOTTOM
Fast PWM
MAX
Note:
1. The CTC0 and PWM0 bit definition names are now obsolete. Use the WGM01:0 definitions.
However, the functionality and location of these bits are compatible with previous versions of
the timer.
• Bit 5:4 – COM0A1:0: Compare Match Output Mode
These bits control the Output Compare pin (OC0A) behavior. If one or both of the COM0A1:0
bits are set, the OC0A output overrides the normal port functionality of the I/O pin it is connected
to. However, note that the Data Direction Register (DDR) bit corresponding to the OC0A pin
must be set in order to enable the output driver.
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When OC0A is connected to the pin, the function of the COM0A1:0 bits depends on the
WGM01:0 bit setting. Table 14-3 shows the COM0A1:0 bit functionality when the WGM01:0 bits
are set to a normal or CTC mode (non-PWM).
Table 14-3. Compare Output Mode, non-PWM Mode
COM0A1
COM0A0
Description
0
0
1
1
0
1
0
1
Normal port operation, OC0A disconnected.
Toggle OC0A on compare match
Clear OC0A on compare match
Set OC0A on compare match
Table 14-4 shows the COM0A1:0 bit functionality when the WGM01:0 bits are set to fast PWM
mode.
Table 14-4. Compare Output Mode, Fast PWM Mode(1)
COM0A1
COM0A0
Description
0
0
0
1
Normal port operation, OC0A disconnected.
Reserved
Clear OC0A on compare match, set OC0A at BOTTOM
(non-inverting mode)
1
1
0
1
Set OC0A on compare match, clear OC0A at BOTTOM
(inverting mode)
Note:
1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the com-
pare match is ignored, but the set or clear is done at BOTTOM. See ”Fast PWM Mode” on
page 96 for more details.
Table 14-5 shows the COM0A1:0 bit functionality when the WGM01:0 bits are set to phase cor-
rect PWM mode.
Table 14-5. Compare Output Mode, Phase Correct PWM Mode(1)
COM0A1
COM0A0
Description
0
0
0
1
Normal port operation, OC0A disconnected.
Reserved
Clear OC0A on compare match when up-counting. Set OC0A on
compare match when counting down.
1
1
0
1
Set OC0A on compare match when up-counting. Clear OC0A on
compare match when counting down.
Note:
1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the com-
pare match is ignored, but the set or clear is done at TOP. See ”Phase Correct PWM Mode” on
page 97 for more details.
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• Bit 2:0 – CS02:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter.
Table 14-6. Clock Select Bit Description
CS02
CS01
CS00
Description
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
No clock source (Timer/Counter stopped)
clkI/O/(No prescaling)
clkI/O/8 (From prescaler)
clkI/O/64 (From prescaler)
clkI/O/256 (From prescaler)
clkI/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.
14.10.2 TCNT0 – Timer/Counter Register
Bit
7
6
5
4
3
2
1
0
0x26 (0x46)
Read/Write
Initial Value
TCNT0[7:0]
TCNT0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
The Timer/Counter Register gives direct access, both for read and write operations, to the
Timer/Counter unit 8-bit counter. Writing to the TCNT0 Register blocks (removes) the compare
match on the following timer clock. Modifying the counter (TCNT0) while the counter is running,
introduces a risk of missing a compare match between TCNT0 and the OCR0A Register.
14.10.3 OCR0A – Output Compare Register A
Bit
7
6
5
4
3
2
1
0
0x27 (0x47)
Read/Write
Initial Value
OCR0A[7:0]
OCR0
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 A contains an 8-bit value that is continuously compared with the
counter value (TCNT0). A match can be used to generate an Output Compare interrupt, or to
generate a waveform output on the OC0A pin.
14.10.4 TIMSK0 – Timer/Counter 0 Interrupt Mask Register
Bit
(0x6E)
7
6
5
4
–
3
–
2
–
1
OCIE0A
R/W
0
0
TOIE0
R/W
0
–
–
–
TIMSK0
Read/Write
Initial Value
R
0
R
0
R
0
R
0
R
0
R
0
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• Bit 1 – OCIE0A: Timer/Counter0 Output Compare Match A Interrupt Enable
When the OCIE0A bit is written to one, and the I-bit in the Status Register is set (one), the
Timer/Counter0 Compare Match A interrupt is enabled. The corresponding interrupt is executed
if a compare match in Timer/Counter0 occurs, i.e., when the OCF0A bit is set in the ”TIFR0 –
Timer/Counter 0 Interrupt Flag Register” on page 105.
• 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 ”TIFR0 – Timer/Counter
0 Interrupt Flag Register” on page 105.
14.10.5 TIFR0 – Timer/Counter 0 Interrupt Flag Register
Bit
0x15 (0x35)
7
6
5
–
4
–
3
–
2
–
1
OCF0A
R/W
0
0
TOV0
R/W
0
–
–
TIFR0
Read/Write
Initial Value
R
0
R
0
R
0
R
0
R
0
R
0
• Bit 1 – OCF0A: Output Compare Flag 0 A
The OCF0A bit is set (one) when a compare match occurs between the Timer/Counter0 and the
data in OCR0A – Output Compare Register0. OCF0A is cleared by hardware when executing
the corresponding interrupt handling vector. Alternatively, OCF0A is cleared by writing a logic
one to the flag. When the I-bit in SREG, OCIE0A (Timer/Counter0 Compare match Interrupt
Enable), and OCF0A are set (one), the Timer/Counter0 Compare match Interrupt is executed.
• Bit 0 – TOV0: Timer/Counter0 Overflow Flag
The bit TOV0 is set (one) when an overflow occurs in Timer/Counter0. TOV0 is cleared by hard-
ware when executing the corresponding interrupt handling vector. Alternatively, TOV0 is cleared
by writing a logic one to the flag. When the SREG I-bit, TOIE0 (Timer/Counter0 Overflow Inter-
rupt Enable), and TOV0 are set (one), the Timer/Counter0 Overflow interrupt is executed. In
phase correct PWM mode, this bit is set when Timer/Counter0 changes counting direction at
0x00.
14.10.6 GTCCR – General Timer/Counter Control Register
Bit
0x23 (0x43)
7
6
5
4
–
3
–
2
–
1
PSR2
R/W
0
0
PSR10
R/W
0
TSM
–
–
GTCCR
Read/Write
Initial Value
R/W
0
R
0
R
0
R
0
R
0
R
0
• Bit 7 – TSM: Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the
value that is written to the PSR2 and PSR10 bits is kept, hence keeping the corresponding pres-
caler reset signals asserted. This ensures that the corresponding Timer/Counters are halted and
can be configured to the same value without the risk of one of them advancing during configura-
tion. When the TSM bit is written to zero, the PSR2 and PSR10 bits are cleared by hardware,
and the Timer/Counters start counting simultaneously.
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• Bit 0 – PSR10: Prescaler Reset Timer/Counter1 and Timer/Counter0
When this bit is one, Timer/Counter1 and Timer/Counter0 prescaler will be Reset. This bit is nor-
mally cleared immediately by hardware, except if the TSM bit is set. Note that Timer/Counter1
and Timer/Counter0 share the same prescaler and a reset of this prescaler will affect both
timers.
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15. 16-bit Timer/Counter1
15.1 Features
• 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)
15.2 Overview
The 16-bit Timer/Counter unit allows accurate program execution timing (event management),
wave generation, and signal timing measurement. Most register and bit references in this sec-
tion are written in general form. A lower case “n” replaces the Timer/Counter number, and a
lower case “x” replaces the Output Compare unit. However, when using the register or bit
defines in a program, the precise form must be used, 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 15-1. For the actual
placement of I/O pins, refer to ”Pinout ATmega3250P” 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 loca-
tions are listed in the ”Register Description” on page 128.
The PRTIM1 bit in ”PRR – Power Reduction Register” on page 44 must be written to zero to
enable the Timer/Counter1 module.
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Figure 15-1. 16-bit Timer/Counter Block Diagram(1)
Count
TOVn
(Int.Req.)
Clear
Control Logic
Clock Select
Direction
clkTn
Edge
Detector
Tn
TOP
BOTTOM
( From Prescaler )
Timer/Counter
TCNTn
=
= 0
OCnA
(Int.Req.)
Waveform
Generation
OCnA
OCnB
=
OCRnA
OCnB
(Int.Req.)
Fixed
TOP
Values
Waveform
Generation
=
OCRnB
( From Analog
Comparator Ouput )
ICFn (Int.Req.)
Edge
Detector
Noise
Canceler
ICRn
ICPn
TCCRnA
TCCRnB
Note:
1. Refer to Figure 1-1 on page 2, Table 13.3.2 on page 74, and Table 13.3.5 on page 79 for
Timer/Counter1 pin placement and description.
15.2.1
Registers
The Timer/Counter (TCNT1), Output Compare Registers (OCR1A/B), and Input Capture Regis-
ter (ICR1) are all 16-bit registers. Special procedures must be followed when accessing the 16-
bit registers. These procedures are described in the section ”Accessing 16-bit Registers” on
page 110. 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 (TIFR1). All interrupts are individually masked with
the Timer Interrupt Mask Register (TIMSK1). TIFR1 and TIMSK1 are not shown in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on
the T1 pin. The Clock Select logic block controls which clock source and edge the Timer/Counter
uses to increment (or decrement) its value. The Timer/Counter is inactive when no clock source
is selected. The output from the Clock Select logic is referred to as the timer clock (clk ).
1
T
The double buffered Output Compare Registers (OCR1A/B) are compared with the Timer/Coun-
ter value at all time. The result of the compare can be used by the Waveform Generator to
generate a PWM or variable frequency output on the Output Compare pin (OC1A/B). See Sec-
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tion “15.7” on page 116.. The compare match event will also 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 trig-
gered) event on either the Input Capture pin (ICP1) or on the Analog Comparator pins (”Analog
Comparator” on page 203) The Input Capture unit includes a digital filtering unit (Noise Can-
celer) 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.
15.2.2
Definitions
The following definitions are used extensively throughout the section:
Table 15-1. Definitions of Timer/Counter values.
BOTTOM
MAX
The counter reaches the BOTTOM when it becomes 0x0000.
The counter reaches its MAXimum when it becomes 0xFFFF (decimal 65535).
The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be one of the fixed values: 0x00FF, 0x01FF,
or 0x03FF, or to the value stored in the OCR1A or ICR1 Register. The assignment is
dependent of the mode of operation.
TOP
15.2.3
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 TCCR1C.
• 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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15.3 Accessing 16-bit Registers
The TCNT1, OCR1A/B, and ICR1 are 16-bit registers that can be accessed by the AVR CPU via
the 8-bit data bus. The 16-bit register must be byte accessed using two read or write operations.
Each 16-bit timer has a single 8-bit register for temporary storing of the high byte of the 16-bit
access. The same temporary register is shared between all 16-bit registers within each 16-bit
timer. Accessing the low byte triggers the 16-bit read or write operation. When the low byte of a
16-bit register is written by the CPU, the high byte stored in the temporary register, and the low
byte written are both copied into the 16-bit register in the same clock cycle. When the low byte of
a 16-bit register is read by the CPU, the high byte of the 16-bit register is copied into the tempo-
rary 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 Examples(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 Examples(1)
unsigned int i;
...
/* Set TCNT1 to 0x01FF */
TCNT1 = 0x1FF;
/* Read TCNT1 into i */
i = TCNT1;
...
Note:
1. See Section “5.” on page 10.
The assembly code example returns the TCNT1 value in the r17:r16 register pair.
It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt
occurs between the two instructions accessing the 16-bit register, and the interrupt code
updates the temporary register by accessing the same or any other of the 16-bit Timer Regis-
ters, then the result of the access outside the interrupt will be corrupted. Therefore, when both
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the main code and the interrupt code update the temporary register, the main code must disable
the interrupts during the 16-bit access.
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 */
__disable_interrupt();
/* Read TCNT1 into i */
i = TCNT1;
/* Restore global interrupt flag */
SREG = sreg;
return i;
}
Note:
1. See Section “5.” on page 10.
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 */
__disable_interrupt();
/* Set TCNT1 to i */
TCNT1 = i;
/* Restore global interrupt flag */
SREG = sreg;
}
Note:
1. See Section “5.” on page 10.
The assembly code example requires that the r17:r16 register pair contains the value to be writ-
ten to TCNT1.
15.3.1
Reusing the Temporary High Byte Register
If writing to more than one 16-bit register where the high byte is the same for all registers 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.
15.4 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 ”TCCR1B – Timer/Counter1 Control Register B” on page 130. For details on clock
sources and prescaler, see ”Timer/Counter0 and Timer/Counter1 Prescalers” on page 100.
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15.5 Counter Unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit.
Figure 15-2 shows a block diagram of the counter and its surroundings.
Figure 15-2. Counter Unit Block Diagram
DATA BUS (8-bit)
TOVn
(Int.Req.)
TEMP (8-bit)
Clock Select
Count
Clear
Edge
Detector
Tn
TCNTnH (8-bit)
TCNTnL (8-bit)
clkTn
Control Logic
Direction
TCNTn (16-bit Counter)
( From Prescaler )
TOP
BOTTOM
Signal description (internal signals):
Count
Increment or decrement TCNT1 by 1.
Direction
Clear
Select between increment and decrement.
Clear TCNT1 (set all bits to zero).
clkT
Timer/Counter clock.
1
TOP
Signalize that TCNT1 has reached maximum value.
BOTTOM
Signalize that TCNT1 has reached minimum value (zero).
The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High (TCNT1H) con-
taining the upper eight bits of the counter, and Counter Low (TCNT1L) containing the lower 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 important to notice that there are special cases of writing to the TCNT1 Register when the
counter is counting that will give unpredictable results. The special cases are described in the
sections where they are of importance.
Depending on the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clk ). The clk 1 can be generated from an external or internal clock source,
1
T
T
selected by the Clock Select bits (CS12:0). When no clock source is selected (CS12:0 = 0) the
timer is stopped. However, the TCNT1 value can be accessed by the CPU, independent of
whether clkT is present or not. A CPU write overrides (has priority over) all counter clear or
1
count operations.
The counting sequence is determined by the setting of the Waveform Generation mode bits
(WGM13:0) located in the Timer/Counter Control Registers A and B (TCCR1A and TCCR1B).
There are close connections between how the counter behaves (counts) and how waveforms
are generated on the Output Compare outputs OC1x. For more details about advanced counting
sequences and waveform generation, see ”Modes of Operation” on page 119.
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The Timer/Counter Overflow Flag (TOV1) is set according to the mode of operation selected by
the WGM13:0 bits. TOV1 can be used for generating a CPU interrupt.
15.6 Input Capture Unit
The Timer/Counter incorporates an Input Capture unit that can capture external events and give
them a time-stamp indicating time of occurrence. The external signal indicating an event, or mul-
tiple events, can be applied via the ICP1 pin or alternatively, via the analog-comparator unit. The
time-stamps can then be used to calculate frequency, duty-cycle, and other features of the sig-
nal 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 15-3. The elements of
the block diagram that are not directly a part of the Input Capture unit are gray shaded. The
small “n” in register and bit names indicates the Timer/Counter number.
Figure 15-3. 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 (ICIE1 = 1),
the Input Capture Flag generates an Input Capture interrupt. The ICF1 Flag is automatically
cleared when the interrupt is executed. Alternatively the ICF1 Flag can be cleared by software
by writing a logical one to its I/O bit location.
Reading the 16-bit value in the Input Capture Register (ICR1) is done by first reading the low
byte (ICR1L) and then the high byte (ICR1H). When the low byte is read the high byte is copied
into the high byte temporary register (TEMP). When the CPU reads the ICR1H I/O location it will
access the TEMP Register.
The ICR1 Register can only be written when using a Waveform Generation mode that utilizes
the ICR1 Register for defining the counter’s TOP value. In these cases the Waveform Genera-
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tion mode (WGM13:0) bits must be set before the TOP value can be written to the ICR1
Register. When writing the ICR1 Register the high byte must be written to the ICR1H I/O 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 110.
15.6.1
Input Capture Trigger Source
The main trigger source for the Input Capture unit is the Input Capture pin (ICP1).
Timer/Counter1 can alternatively use the Analog Comparator output as trigger source for the
Input Capture unit. The Analog Comparator is selected as trigger source by setting the Analog
Comparator Input Capture (ACIC) bit in the Analog Comparator Control and Status Register
(ACSR). Be aware that changing trigger source can trigger a capture. The Input Capture Flag
must therefore be cleared after the change.
Both the Input Capture pin (ICP1) and the Analog Comparator output (ACO) inputs are sampled
using the same technique as for the T1 pin (Figure 14-8 on page 101). The edge detector is also
identical. However, when the noise canceler is enabled, additional logic is inserted before the
edge detector, which increases the delay by four system clock cycles. Note that the input of the
noise canceler and edge detector is always enabled unless the Timer/Counter is set in a Wave-
form Generation mode that uses ICR1 to define TOP.
An Input Capture can be triggered by software by controlling the port of the ICP1 pin.
15.6.2
Noise Canceler
The noise canceler improves noise immunity by using a simple digital filtering scheme. The
noise canceler input is monitored over four samples, and all four must be equal for changing the
output that in turn is used by the edge detector.
The noise canceler is enabled by setting the Input Capture Noise Canceler (ICNC1) bit in
Timer/Counter Control Register B (TCCR1B). When enabled the noise canceler introduces addi-
tional four system clock cycles of delay from a change applied to the input, to the update of the
ICR1 Register. The noise canceler uses the system clock and is therefore not affected by the
prescaler.
15.6.3
Using the Input Capture Unit
The main challenge when using the Input Capture unit is to assign enough processor capacity
for handling the incoming events. The time between two events is critical. If the processor has
not read the captured value in the ICR1 Register before the next event occurs, the ICR1 will be
overwritten with a new value. In this case the result of the capture will be incorrect.
When using the Input Capture interrupt, the ICR1 Register should be read as early in the inter-
rupt handler routine as possible. Even though the Input Capture interrupt has relatively high
priority, the maximum interrupt response time is dependent on the maximum number of clock
cycles it takes to handle any of the other interrupt requests.
Using the Input Capture unit in any mode of operation when the TOP value (resolution) is
actively changed during operation, is not recommended.
Measurement of an external signal’s duty cycle requires that the trigger edge is changed after
each capture. Changing the edge sensing must be done as early as possible after the ICR1
Register has been read. After a change of the edge, the Input Capture Flag (ICF1) must be
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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).
15.7 Output Compare Units
The 16-bit comparator continuously compares TCNT1 with the Output Compare Register
(OCR1x). If TCNT equals OCR1x the comparator signals a match. A match will set the Output
Compare Flag (OCF1x) at the next timer clock cycle. If enabled (OCIE1x = 1), the Output Com-
pare Flag generates an Output Compare interrupt. The OCF1x Flag is automatically cleared
when the interrupt is executed. Alternatively the OCF1x Flag can be cleared by software by writ-
ing a logical one to its I/O bit location. The Waveform Generator uses the match signal to
generate an output according to operating mode set by the Waveform Generation mode
(WGM13:0) bits and Compare Output mode (COM1x1:0) bits. The TOP and BOTTOM signals
are used by the Waveform Generator for handling the special cases of the extreme values in
some modes of operation (See Section “15.9” on page 119.)
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 15-4 shows a block diagram of the Output Compare unit. The small “n” in the register and
bit names indicates the device number (n = 1 for Timer/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 15-4. Output Compare Unit, Block Diagram
DATA BUS (8-bit)
TEMP (8-bit)
OCRnxH Buf. (8-bit)
OCRnxL Buf. (8-bit)
TCNTnH (8-bit)
TCNTnL (8-bit)
OCRnx Buffer (16-bit Register)
TCNTn (16-bit Counter)
OCRnxH (8-bit)
OCRnxL (8-bit)
OCRnx (16-bit Register)
=
(16-bit Comparator )
OCFnx (Int.Req.)
TOP
OCnx
Waveform Generator
BOTTOM
WGMn3:0
COMnx1:0
The OCR1x Register is double buffered when using any of the twelve Pulse Width Modulation
(PWM) modes. For the Normal and Clear Timer on Compare (CTC) modes of operation, the
double buffering is disabled. The double buffering synchronizes the update of the OCR1x Com-
pare Register to either TOP or BOTTOM of the counting sequence. The synchronization
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prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby making the out-
put glitch-free.
The OCR1x Register access may seem complex, but this is not case. When the double buffering
is enabled, the CPU has access to the OCR1x Buffer Register, and if double buffering is dis-
abled the CPU will access the OCR1x directly. The content of the OCR1x (Buffer or Compare)
Register is only changed by a write operation (the Timer/Counter does not update this register
automatically as the TCNT1 and ICR1 Register). Therefore OCR1x is not read via the high byte
temporary register (TEMP). However, it is a good practice to read the low byte first as when
accessing other 16-bit registers. Writing the OCR1x Registers must be done via the TEMP Reg-
ister since the compare of all 16 bits is done continuously. The high byte (OCR1xH) has to be
written first. When the high byte I/O location is written by the CPU, the TEMP Register will be
updated by the value written. Then when the low byte (OCR1xL) is written to the lower eight bits,
the high byte will be copied into the upper 8-bits of either the OCR1x buffer or OCR1x Compare
Register in the same system clock cycle.
For more information of how to access the 16-bit registers refer to ”Accessing 16-bit Registers”
on page 110.
15.7.1
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 COM11:0 bits settings define whether the OC1x pin is set, cleared or
toggled).
15.7.2
15.7.3
Compare Match Blocking by TCNT1 Write
All CPU writes to the TCNT1 Register will block any compare match that occurs in the next timer
clock cycle, even when the timer is stopped. This feature allows OCR1x to be initialized to the
same value as TCNT1 without triggering an interrupt when the Timer/Counter clock is enabled.
Using the Output Compare Unit
Since writing TCNT1 in any mode of operation will block all compare matches for one timer clock
cycle, there are risks involved when changing TCNT1 when using any of the Output Compare
units, independent of whether the Timer/Counter is running or not. If the value written to TCNT1
equals the OCR1x value, the compare match will be missed, resulting in incorrect waveform
generation. Do not write the TCNT1 equal to TOP in PWM modes with variable TOP values. The
compare match for the TOP will be ignored and the counter will continue to 0xFFFF. Similarly,
do not write the TCNT1 value equal to BOTTOM when the counter is counting down.
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 Com-
pare (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.
15.8 Compare Match Output Unit
The Compare Output mode (COM1x1:0) bits have two functions. The Waveform Generator uses
the COM1x1:0 bits for defining the Output Compare (OC1x) state at the next compare match.
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Secondly the COM1x1:0 bits control the OC1x pin output source. Figure 15-5 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 15-5. Compare Match Output Unit, Schematic
COMnx1
Waveform
Generator
COMnx0
FOCnx
D
Q
1
0
OCnx
Pin
OCnx
D
Q
PORT
D
Q
DDR
clkI/O
The general I/O port function is overridden by the Output Compare (OC1x) from the Waveform
Generator if either of the COM1x1:0 bits are set. However, the OC1x pin direction (input or out-
put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction
Register bit for the OC1x pin (DDR_OC1x) must be set as output before the OC1x value is visi-
ble on the pin. The port override function is generally independent of the Waveform Generation
mode, but there are some exceptions. Refer to Table 15-2, Table 15-3 and Table 15-4 for
details.
The design of the Output Compare pin logic allows initialization of the OC1x state before the out-
put is enabled. Note that some COM1x1:0 bit settings are reserved for certain modes of
operation. See Section “15.11” on page 128.
The COM1x1:0 bits have no effect on the Input Capture unit.
15.8.1
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 compare output actions in the
non-PWM modes refer to Table 15-2 on page 128. For fast PWM mode refer to Table 15-3 on
page 129, and for phase correct and phase and frequency correct PWM refer to Table 15-4 on
page 129.
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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.
15.9 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 out-
put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes
the COM1x1:0 bits control whether the output should be set, cleared or toggle at a compare
match (See Section “15.8” on page 117.)
For detailed timing information refer to ”Timer/Counter Timing Diagrams” on page 126.
15.9.1
Normal Mode
The simplest mode of operation is the Normal mode (WGM13:0 = 0). In this mode the counting
direction is always up (incrementing), and no counter clear is performed. The counter simply
overruns when it passes its maximum 16-bit value (MAX = 0xFFFF) and then restarts from the
BOTTOM (0x0000). In normal operation the Timer/Counter Overflow Flag (TOV1) will be set in
the same timer clock cycle as the TCNT1 becomes zero. The TOV1 Flag in this case behaves
like a 17th bit, except that it is only set, not cleared. However, combined with the timer overflow
interrupt that automatically clears the TOV1 Flag, the timer resolution can be increased by soft-
ware. There are no special cases to consider in the Normal mode, a new counter value can be
written anytime.
The Input Capture unit is easy to use in Normal mode. However, observe that the maximum
interval between the external events must not exceed the resolution of the counter. If the interval
between events are too long, the timer overflow interrupt or the prescaler must be used to
extend the resolution for the capture unit.
The Output Compare units can be used to generate interrupts at some given time. Using the
Output Compare to generate waveforms in Normal mode is not recommended, since this will
occupy too much of the CPU time.
15.9.2
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 opera-
tion of counting external events.
The timing diagram for the CTC mode is shown in Figure 15-6. 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 15-6. 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. How-
ever, changing the TOP to a value close to BOTTOM when the counter is running with none or a
low prescaler value must be done with care since the CTC mode does not have the double buff-
ering feature. If the new value written to OCR1A or ICR1 is lower than the current value of
TCNT1, the counter will miss the compare match. The counter will then have to count to its max-
imum value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur.
In many cases this feature is not desirable. An alternative will then be to use the fast PWM mode
using OCR1A for defining TOP (WGM13:0 = 15) since the OCR1A then will be double buffered.
For generating a waveform output in CTC mode, the OC1A output can be set to toggle its logical
level on each compare match by setting the Compare Output mode bits to toggle mode
(COM1A1:0 = 1). The OC1A value will not be visible on the port pin unless the data direction for
the pin is set to output (DDR_OC1A = 1). The waveform generated will have a maximum fre-
quency of fOC A = fclk_I/O/2 when OCR1A is set to zero (0x0000). The waveform frequency is
1
defined by the following equation:
f
clk_I/O
f
= --------------------------------------------------
OCnA
2 ⋅ N ⋅ (1 + OCRnA)
The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOV1 Flag is set in the same timer clock cycle that the
counter counts from MAX to 0x0000.
15.9.3
Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM13:0 = 5, 6, 7, 14, or 15) provides a
high frequency PWM waveform generation option. The fast PWM differs from the other PWM
options by its single-slope operation. The counter counts from BOTTOM to TOP then restarts
from BOTTOM. In non-inverting Compare Output mode, the Output Compare (OC1x) is cleared
on the compare match between TCNT1 and OCR1x, and set at BOTTOM. In inverting Compare
Output mode output is set on compare match and cleared at BOTTOM. Due to the single-slope
operation, the operating frequency of the fast PWM mode can be twice as high as the phase cor-
rect and phase and frequency correct PWM modes that use dual-slope operation. This high
frequency makes the fast PWM mode well suited for power regulation, rectification, and DAC
applications. High frequency allows physically small sized external components (coils, capaci-
tors), hence reduces total system cost.
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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 max-
imum 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 15-7. The figure
shows fast PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1 value is in the
timing diagram shown as a histogram for illustrating the single-slope operation. The diagram
includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT1
slopes represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will
be set when a compare match occurs.
Figure 15-7. 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 OC1A or ICF1 Flag is set at the same timer clock cycle as TOV1 is set when either OCR1A
or ICR1 is used for defining the TOP value. If one of the interrupts are enabled, the interrupt han-
dler routine can be used for updating the TOP and compare values.
When changing the TOP value the program must ensure that the new TOP value is higher or
equal to the value of all of the Compare Registers. If the TOP value is lower than any of the
Compare Registers, a compare match will never occur between the TCNT1 and the OCR1x.
Note that when using fixed TOP values the unused bits are masked to zero when any of the
OCR1x Registers are written.
The procedure for updating ICR1 differs from updating OCR1A when used for defining the TOP
value. The ICR1 Register is not double buffered. This means that if ICR1 is changed to a low
value when the counter is running with none or a low prescaler value, there is a risk that the new
ICR1 value written is lower than the current value of TCNT1. The result will then be that the
counter will miss the compare match at the TOP value. The counter will then have to count to the
MAX value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur.
The OCR1A Register however, is double buffered. This feature allows the OCR1A I/O location
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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 two will produce a non-inverted PWM and an inverted PWM output
can be generated by setting the COM1x1:0 to three (see Table 15-3 on page 129). The actual
OC1x value will only be visible on the port pin if the data direction for the port pin is set as output
(DDR_OC1x). The PWM waveform is generated by setting (or clearing) the OC1x Register at
the compare match between OCR1x and TCNT1, and clearing (or setting) the OC1x Register at
the timer clock cycle the counter is cleared (changes from TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
f
clk_I/O
f
= ----------------------------------
OCnxPWM
N ⋅ (1 + TOP)
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x Register represents special cases when generating a PWM
waveform output in the fast PWM mode. If the OCR1x is set equal to BOTTOM (0x0000) the out-
put will be a narrow spike for each TOP+1 timer clock cycle. Setting the OCR1x equal to TOP
will result in a constant high or low output (depending on the polarity of the output set by the
COM1x1:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-
ting OC1A to toggle its logical level on each compare match (COM1A1:0 = 1). This applies only
if OCR1A is used to define the TOP value (WGM13:0 = 15). The waveform generated will have
a maximum frequency of fOC A = fclk_I/O/2 when OCR1A is set to zero (0x0000). This feature is
1
similar to the OC1A toggle in CTC mode, except the double buffer feature of the Output Com-
pare unit is enabled in the fast PWM mode.
15.9.4
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 counting up, and set on the
compare match while counting down. 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 OCR1A set to
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0x0003), and the maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolu-
tion 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 15-8. The figure
shows phase correct PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1
value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The
diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on
the TCNT1 slopes represent compare matches between OCR1x and TCNT1. The OC1x Inter-
rupt Flag will be set when a compare match occurs.
Figure 15-8. 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 BOTTOM. When
either OCR1A or ICR1 is used for defining the TOP value, the OC1A or ICF1 Flag is set accord-
ingly at the same timer clock cycle as the OCR1x Registers are updated with the double buffer
value (at TOP). The Interrupt Flags can be used to generate an interrupt each time the counter
reaches the TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is higher or
equal to the value of all of the Compare Registers. If the TOP value is lower than any of the
Compare Registers, a compare match will never occur between the TCNT1 and the OCR1x.
Note that when using fixed TOP values, the unused bits are masked to zero when any of the
OCR1x Registers are written. As the third period shown in Figure 15-8 illustrates, changing the
TOP actively while the Timer/Counter is running in the phase correct mode can result in an
unsymmetrical output. The reason for this can be found in the time of update of the OCR1x Reg-
ister. Since the OCR1x update occurs at TOP, the PWM period starts and ends at TOP. This
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implies that the length of the falling slope is determined by the previous TOP value, while the
length of the rising slope is determined by the new TOP value. When these two values differ the
two slopes of the period will differ in length. The difference in length gives the unsymmetrical
result on the output.
It is recommended to use the phase and frequency correct mode instead of the phase correct
mode when changing the TOP value while the Timer/Counter is running. When using a static
TOP value there are practically no differences between the two modes of operation.
In phase correct PWM mode, the compare units allow generation of PWM waveforms on the
OC1x pins. Setting the COM1x1:0 bits to two will produce a non-inverted PWM and an inverted
PWM output can be generated by setting the COM1x1:0 to three (See Table 1 on page 129).
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 correct PWM can
be calculated by the following equation:
f
clk_I/O
f
= ---------------------------
OCnxPCPWM
2 ⋅ N ⋅ TOP
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x Register represent special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR1x is set equal to BOTTOM the
output will be continuously low and if set equal to TOP the output will be continuously high for
non-inverted PWM mode. For inverted PWM the output will have the opposite logic values. If
OCR1A is used to define the TOP value (WGM13:0 = 11) and COM1A1:0 = 1, the OC1A output
will toggle with a 50% duty cycle.
15.9.5
Phase and Frequency Correct PWM Mode
The phase and frequency correct Pulse Width Modulation, or phase and frequency correct PWM
mode (WGM13:0 = 8 or 9) provides a high resolution phase and frequency correct PWM wave-
form generation option. The phase and frequency correct PWM mode is, like the phase correct
PWM mode, based on a dual-slope operation. The counter counts repeatedly from BOTTOM
(0x0000) to TOP and then from TOP to BOTTOM. In non-inverting Compare Output mode, the
Output Compare (OC1x) is cleared on the compare match between TCNT1 and OCR1x while
counting up, and set on the compare match while counting down. In inverting Compare Output
mode, the operation is inverted. The dual-slope operation gives a lower maximum operation fre-
quency compared to the single-slope operation. However, due to the symmetric feature of the
dual-slope PWM modes, these modes are preferred for motor control applications.
The main difference between the phase correct, and the phase and frequency correct PWM
mode is the time the OCR1x Register is updated by the OCR1x Buffer Register, (see Figure 15-
8 and Figure 15-9).
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
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the maximum resolution is 16-bit (ICR1 or OCR1A set to MAX). The PWM resolution in bits can
be calculated using the following equation:
log(TOP + 1)
R
= ----------------------------------
PFCPWM
log(2)
In phase and frequency correct PWM mode the counter is incremented until the counter value
matches either the value in ICR1 (WGM13:0 = 8), or the value in OCR1A (WGM13:0 = 9). The
counter has then reached the TOP and changes the count direction. The TCNT1 value will be
equal to TOP for one timer clock cycle. The timing diagram for the phase correct and frequency
correct PWM mode is shown on Figure 15-9. 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 dia-
gram 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 15-9. Phase and Frequency Correct PWM Mode, Timing Diagram
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
OCRnx/TOP Updateand
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 15-9 shows the output generated is, in contrast to the phase correct mode, symmetri-
cal 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.
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Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using
ICR1, the OCR1A Register is free to be used for generating a PWM output on OC1A. However,
if the base PWM frequency is actively changed by changing the TOP value, using the OCR1A as
TOP is clearly a better choice due to its double buffer feature.
In phase and frequency correct PWM mode, the compare units allow generation of PWM wave-
forms on the OC1x pins. Setting the COM1x1:0 bits to two will produce a non-inverted PWM and
an inverted PWM output can be generated by setting the COM1x1:0 to three (See Table 1 on
page 129). The actual OC1x value will only be visible on the port pin if the data direction for the
port pin is set as output (DDR_OC1x). The PWM waveform is generated by setting (or clearing)
the OC1x Register at the compare match between OCR1x and TCNT1 when the counter incre-
ments, and clearing (or setting) the OC1x Register at compare match between OCR1x and
TCNT1 when the counter decrements. The PWM frequency for the output when using phase
and frequency correct PWM can be calculated by the following equation:
f
clk_I/O
f
= ---------------------------
OCnxPFCPWM
2 ⋅ N ⋅ TOP
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x Register represents special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR1x is set equal to BOTTOM the
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.
15.10 Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkT1) 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 15-10 shows a timing diagram for the setting of OCF1x.
Figure 15-10. Timer/Counter Timing Diagram, Setting of OCF1x, no Prescaling
clkI/O
clkTn
(clkI/O/1)
TCNTn
OCRnx
OCFnx
OCRnx - 1
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
Figure 15-11 shows the same timing data, but with the prescaler enabled.
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Figure 15-11. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O/8)
TCNTn
OCRnx
OCFnx
OCRnx - 1
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
Figure 15-12 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 15-12. Timer/Counter Timing Diagram, no Prescaling
clkI/O
clkTn
(clkI/O/1)
TCNTn
TOP - 1
TOP - 1
TOP
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)
New OCRnx Value
Old OCRnx Value
Figure 15-13 on page 128 shows the same timing data, but with the prescaler enabled.
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Figure 15-13. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
clk
I/O
clk
Tn
(clk /8)
I/O
TCNTn
TOP - 1
TOP - 1
TOP
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
15.11 Register Description
15.11.1 TCCR1A – Timer/Counter1 Control Register A
Bit
7
COM1A1
R/W
6
COM1A0
R/W
5
COM1B1
R/W
4
3
–
2
–
1
WGM11
R/W
0
0
WGM10
R/W
0
COM1B0
R/W
0
TCCR1A
(0x80)
Read/Write
Initial Value
R
0
R
0
0
0
0
• Bit 7:6 – COM1A1:0: Compare Output Mode for Unit A
• Bit 5:4 – COM1B1:0: Compare Output Mode for Unit B
The COM1A1:0 and COM1B1:0 control the Output Compare pins (OC1A and OC1B respec-
tively) behavior. If one or both of the COM1A1:0 bits are written to one, the OC1A output
overrides the normal port functionality of the I/O pin it is connected to. If one or both of the
COM1B1:0 bit are written to one, the OC1B output overrides the normal port functionality of the
I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit correspond-
ing to the OC1A or OC1B pin must be set in order to enable the output driver.
When the OC1A or OC1B is connected to the pin, the function of the COM1x1:0 bits is depen-
dent of the WGM13:0 bits setting. Table 15-2 shows the COM1x1:0 bit functionality when the
WGM13:0 bits are set to a Normal or a CTC mode (non-PWM).
Table 15-2. Compare Output Mode, non-PWM
COM1A1/COM1B1
COM1A0/COM1B0
Description
0
0
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).
1
1
0
1
Set OC1A/OC1B on Compare Match (Set output to
high level).
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Table 15-3 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the fast
PWM mode.
Table 15-3. Compare Output Mode, Fast PWM(1)
COM1A1/COM1B1
COM1A0/COM1B0
Description
0
0
Normal port operation, OC1A/OC1B disconnected.
WGM13:0 = 14 or 15: Toggle OC1A on Compare
Match, OC1B disconnected (normal port operation).
For all other WGM1 settings, normal port operation,
OC1A/OC1B disconnected.
0
1
Clear OC1A/OC1B on Compare Match, set
OC1A/OC1B at BOTTOM (non-inverting mode)
1
1
0
1
Set OC1A/OC1B on Compare Match, clear
OC1A/OC1B at BOTTOM (inverting mode)
Note:
1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. In
this case the compare match is ignored, but the set or clear is done at BOTTOM. See Section
“15.9.3” on page 120. for more details.
Table 15-4 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 15-4. Compare Output Mode, Phase Correct and Phase and Frequency Correct
PWM(1)
COM1A1/COM1B1
COM1A0/COM1B0
Description
0
0
Normal port operation, OC1A/OC1B disconnected.
WGM13:0 = 9 or 11: Toggle OC1A on Compare
Match, OC1B disconnected (normal port operation).
For all other WGM1 settings, normal port operation,
OC1A/OC1B disconnected.
0
1
Clear OC1A/OC1B on Compare Match when up-
counting. Set OC1A/OC1B on Compare Match when
counting down.
1
1
0
1
Set OC1A/OC1B on Compare Match when up-
counting. Clear OC1A/OC1B on Compare Match
when counting down.
Note:
1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. See
Section “15.9.4” on page 122. for more details.
• Bit 1:0 – WGM11:0: Waveform Generation Mode
Combined with the WGM13:2 bits found in the TCCR1B Register, these bits control the counting
sequence of the counter, the source for maximum (TOP) counter value, and what type of wave-
form generation to be used, see Table 15-14 on page 130. Modes of operation supported by the
Timer/Counter unit are: Normal mode (counter), Clear Timer on Compare match (CTC) mode,
and three types of Pulse Width Modulation (PWM) modes. (See ”Modes of Operation” on page
119).
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Figure 15-14. Waveform Generation Mode Bit Description(1)
WGM12
(CTC1)
WGM11
WGM10
Timer/Counter Mode of
Update of
OCR1x at
TOV1 Flag
Set on
Mode
WGM13
(PWM11) (PWM10) Operation
TOP
0
1
2
3
4
5
6
7
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
Normal
0xFFFF
0x00FF
0x01FF
0x03FF
OCR1A
0x00FF
0x01FF
0x03FF
Immediate
TOP
MAX
PWM, Phase Correct, 8-bit
PWM, Phase Correct, 9-bit
PWM, Phase Correct, 10-bit
CTC
BOTTOM
BOTTOM
BOTTOM
MAX
TOP
TOP
Immediate
BOTTOM
BOTTOM
BOTTOM
Fast PWM, 8-bit
TOP
Fast PWM, 9-bit
TOP
Fast PWM, 10-bit
TOP
PWM, Phase and Frequency
Correct
8
9
1
1
0
0
0
0
0
1
ICR1
BOTTOM
BOTTOM
BOTTOM
BOTTOM
PWM, Phase and Frequency
Correct
OCR1A
10
11
12
13
14
15
1
1
1
1
1
1
0
0
1
1
1
1
1
1
0
0
1
1
0
1
0
1
0
1
PWM, Phase Correct
PWM, Phase Correct
CTC
ICR1
OCR1A
ICR1
–
TOP
BOTTOM
BOTTOM
MAX
TOP
Immediate
–
(Reserved)
–
Fast PWM
ICR1
OCR1A
BOTTOM
BOTTOM
TOP
Fast PWM
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.
15.11.2 TCCR1B – Timer/Counter1 Control Register B
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
0
CS10
R/W
0
(0x81)
TCCR1B
Read/Write
Initial Value
R
0
• Bit 7 – ICNC1: Input Capture Noise Canceler
Setting this bit (to one) activates the Input Capture Noise Canceler. When the noise canceler is
activated, the input from the Input Capture pin (ICP1) is filtered. The filter function requires four
successive equal valued samples of the ICP1 pin for changing its output. The Input Capture is
therefore delayed by four Oscillator cycles when the noise canceler is enabled.
• Bit 6 – ICES1: Input Capture Edge Select
This bit selects which edge on the Input Capture pin (ICP1) that is used to trigger a capture
event. When the ICES1 bit is written to zero, a falling (negative) edge is used as trigger, and
when the ICES1 bit is written to one, a rising (positive) edge will trigger the capture.
When a capture is triggered according to the ICES1 setting, the counter value is copied into the
Input Capture Register (ICR1). The event will also set the Input Capture Flag (ICF1), and this
can be used to cause an Input Capture Interrupt, if this interrupt is enabled.
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When the ICR1 is used as TOP value (see description of the WGM13:0 bits located in the
TCCR1A and the TCCR1B Register), the ICP1 is disconnected and consequently the Input Cap-
ture function is disabled.
• Bit 5 – Reserved Bit
This bit is reserved for future use. For ensuring compatibility with future devices, this bit must be
written to zero when TCCR1B is written.
• Bit 4:3 – WGM13:2: Waveform Generation Mode
See TCCR1A Register description.
• Bit 2:0 – CS12:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter, see Figure
15-10 and Figure 15-11.
Figure 15-15. Clock Select Bit Description
CS12
CS11
CS10
Description
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
No clock source (Timer/Counter stopped).
clkI/O/1 (No prescaling)
clkI/O/8 (From prescaler)
clkI/O/64 (From prescaler)
clkI/O/256 (From prescaler)
clkI/O/1024 (From prescaler)
External clock source on T1 pin. Clock on falling edge.
External clock source on T1 pin. Clock on rising edge.
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.
15.11.3 TCCR1C – Timer/Counter1 Control Register C
Bit
(0x82)
7
6
5
–
4
–
3
–
2
–
1
–
0
–
FOC1A
FOC1B
TCCR1C
Read/Write
Initial Value
R/W
0
R/W
0
R
0
R
0
R
0
R
0
R
0
R
0
• Bit 7 – FOC1A: Force Output Compare for Unit A
• Bit 6 – FOC1B: Force Output Compare for Unit B
The FOC1A/FOC1B bits are only active when the WGM13:0 bits specifies a non-PWM mode.
However, for ensuring compatibility with future devices, these bits must be set to zero when
TCCR1A is written when operating in a PWM mode. When writing a logical one to the
FOC1A/FOC1B bit, an immediate compare match is forced on the Waveform Generation unit.
The OC1A/OC1B output is changed according to its COM1x1:0 bits setting. Note that the
FOC1A/FOC1B bits are implemented as strobes. Therefore it is the value present in the
COM1x1:0 bits that determine the effect of the forced compare.
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A FOC1A/FOC1B strobe will not generate any interrupt nor will it clear the timer in Clear Timer
on Compare match (CTC) mode using OCR1A as TOP. The FOC1A/FOC1B bits are always
read as zero.
15.11.4 TCNT1H and TCNT1L – Timer/Counter1
Bit
7
6
5
4
3
2
1
0
(0x85)
TCNT1[15:8]
TCNT1[7:0]
TCNT1H
TCNT1L
(0x84)
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
R/W
R/W
0
R/W
0
R/W
0
R/W
0
0
The two Timer/Counter I/O locations (TCNT1H and TCNT1L, combined TCNT1) give direct
access, both for read and for write operations, to the Timer/Counter unit 16-bit counter. To
ensure that both the high and low bytes are read and written simultaneously when the CPU
accesses these registers, the access is performed using an 8-bit temporary High Byte Register
(TEMP). This temporary register is shared by all the other 16-bit registers. See Section “15.3” on
page 110.
Modifying the counter (TCNT1) while the counter is running introduces a risk of missing a com-
pare match between TCNT1 and one of the OCR1x Registers.
Writing to the TCNT1 Register blocks (removes) the compare match on the following timer clock
for all compare units.
15.11.5 OCR1AH and OCR1AL – Output Compare Register 1 A
Bit
7
6
5
4
3
2
1
0
(0x89)
OCR1A[15:8]
OCR1A[7:0]
OCR1AH
OCR1AL
(0x88)
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
R/W
R/W
R/W
0
R/W
0
R/W
0
0
0
15.11.6 OCR1BH and OCR1BL – Output Compare Register 1 B
Bit
7
6
5
4
3
2
1
0
(0x8B)
OCR1B[15:8]
OCR1B[7:0]
OCR1BH
OCR1BL
(0x8A)
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
R/W
R/W
R/W
0
R/W
0
R/W
0
0
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 Section “15.3” on page 110.
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15.11.7 ICR1H and ICR1L – Input Capture Register 1
Bit
7
6
5
4
3
2
1
0
(0x87)
ICR1[15:8]
ICR1[7:0]
ICR1H
ICR1L
(0x86)
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 Input Capture is updated with the counter (TCNT1) value each time an event occurs on the
ICP1 pin (or optionally on the Analog Comparator output for Timer/Counter1). The Input Capture
can be used for defining the counter TOP value.
The Input Capture Register is 16-bit in size. To ensure that both the high and low bytes are read
simultaneously when the CPU accesses these registers, the access is performed using an 8-bit
temporary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit
registers. See Section “15.3” on page 110.
15.11.8 TIMSK1 – Timer/Counter1 Interrupt Mask Register
Bit
(0x6F)
7
6
5
4
–
3
–
2
OCIE1B
R/W
0
1
OCIE1A
R/W
0
0
TOIE1
R/W
0
–
–
ICIE1
TIMSK1
Read/Write
Initial Value
R
0
R
0
R/W
0
R
0
R
0
• Bit 5 – ICIE1: Timer/Counter1, Input Capture Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally
enabled), the Timer/Counter1 Input Capture interrupt is enabled. The corresponding Interrupt
Vector (See Section “11.” on page 53.) is executed when the ICF1 Flag, located in TIFR1, is set.
• Bit 2 – OCIE1B: Timer/Counter1, Output Compare B Match Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally
enabled), the Timer/Counter1 Output Compare B Match interrupt is enabled. The corresponding
Interrupt Vector (See Section “11.” on page 53.) is executed when the OCF1B Flag, located in
TIFR1, is set.
• Bit 1 – OCIE1A: Timer/Counter1, Output Compare A Match Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally
enabled), the Timer/Counter1 Output Compare A Match interrupt is enabled. The corresponding
Interrupt Vector (See Section “11.” on page 53.) is executed when the OCF1A Flag, located in
TIFR1, is set.
• Bit 0 – TOIE1: Timer/Counter1, Overflow Interrupt Enable
When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally
enabled), the Timer/Counter1 Overflow interrupt is enabled. The corresponding Interrupt Vector
(See Section “11.” on page 53.) is executed when the TOV1 Flag, located in TIFR1, is set.
15.11.9 TIFR1 – Timer/Counter1 Interrupt Flag Register
Bit
0x16 (0x36)
7
6
5
4
–
3
–
2
OCF1B
R/W
0
1
OCF1A
R/W
0
0
TOV1
R/W
0
–
–
ICF1
R/W
0
TIFR1
Read/Write
Initial Value
R
0
R
0
R
0
R
0
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• Bit 5 – ICF1: Timer/Counter1, Input Capture Flag
This flag is set when a capture event occurs on the ICP1 pin. When the Input Capture Register
(ICR1) is set by the WGM13:0 to be used as the TOP value, the ICF1 Flag is set when the coun-
ter reaches the TOP value.
ICF1 is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively,
ICF1 can be cleared by writing a logic one to its bit location.
• Bit 2 – OCF1B: Timer/Counter1, Output Compare B Match Flag
This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output
Compare Register B (OCR1B).
Note that a Forced Output Compare (FOC1B) strobe will not set the OCF1B Flag.
OCF1B is automatically cleared when the Output Compare Match B Interrupt Vector is exe-
cuted. Alternatively, OCF1B can be cleared by writing a logic one to its bit location.
• Bit 1 – OCF1A: Timer/Counter1, Output Compare A Match Flag
This flag is set in the timer clock cycle after the counter (TCNT1) value matches the Output
Compare Register A (OCR1A).
Note that a Forced Output Compare (FOC1A) strobe will not set the OCF1A Flag.
OCF1A is automatically cleared when the Output Compare Match A Interrupt Vector is exe-
cuted. Alternatively, OCF1A can be cleared by writing a logic one to its bit location.
• Bit 0 – 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 15-14 on page 130 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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16. 8-bit Timer/Counter2 with PWM and Asynchronous Operation
16.1 Features
• Single Compare Unit Counter
• Clear Timer on Compare Match (Auto Reload)
• Glitch-free, Phase Correct Pulse Width Modulator (PWM)
• Frequency Generator
• 10-bit Clock Prescaler
• Overflow and Compare Match Interrupt Sources (TOV2 and OCF2A)
• Allows Clocking from External 32 kHz Watch Crystal Independent of the I/O Clock
16.2 Overview
Timer/Counter2 is a general purpose, single compare unit, 8-bit Timer/Counter module. A simpli-
fied block diagram of the 8-bit Timer/Counter is shown in Figure 16-1. For the actual placement
of I/O pins, refer to ”Pinout ATmega3250P” 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 ”Register Description” on page 149.
Figure 16-1. 8-bit Timer/Counter Block Diagram
TCCRnx
count
TOVn
(Int.Req.)
clear
Control Logic
TOP
direction
clkTn
TOSC1
BOTTOM
T/C
Oscillator
Prescaler
TOSC2
Timer/Counter
TCNTn
= 0
= 0xFF
clkI/O
OCnx
(Int.Req.)
Waveform
Generation
OCnx
=
OCRnx
clkI/O
Synchronized Status flags
Synchronization Unit
clkASY
Status flags
ASSRn
asynchronous mode
select (ASn)
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16.2.1
Registers
The Timer/Counter (TCNT2) and Output Compare Register (OCR2A) are 8-bit registers. Inter-
rupt request (shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag Register
(TIFR2). All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK2).
TIFR2 and TIMSK2 are not shown in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or asynchronously clocked from
the TOSC1/2 pins, as detailed later in this section. The asynchronous operation is controlled by
the Asynchronous Status Register (ASSR). The Clock Select logic block controls which clock
source the Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inac-
tive when no clock source is selected. The output from the Clock Select logic is referred to as the
timer clock (clkT2).
The double buffered Output Compare Register (OCR2A) is compared with the Timer/Counter
value at all times. The result of the compare can be used by the Waveform Generator to gener-
ate a PWM or variable frequency output on the Output Compare pin (OC2A). See Section “16.5”
on page 137. for details. The compare match event will also set the Compare Flag (OCF2A)
which can be used to generate an Output Compare interrupt request.
16.2.2
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 16-1 are also used extensively throughout the section.
Table 16-1. Definitions of Timer/Counter values.
BOTTOM
MAX
The counter reaches the BOTTOM when it becomes zero (0x00).
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 OCR2A Register. The assignment is depen-
dent on the mode of operation.
TOP
16.3 Timer/Counter Clock Sources
The Timer/Counter can be clocked by an internal synchronous or an external asynchronous
clock source. The clock source clkT2 is by default equal to the MCU clock, clkI/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 ”ASSR
– Asynchronous Status Register” on page 151. For details on clock sources and prescaler, see
”Timer/Counter Prescaler” on page 148.
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16.4 Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
16-2 shows a block diagram of the counter and its surrounding environment.
Figure 16-2. Counter Unit Block Diagram
TOVn
(Int.Req.)
DATA BUS
TOSC1
count
T/C
Oscillator
clk Tn
clear
TCNTn
Control Logic
Prescaler
direction
TOSC2
clk
bottom
top
I/O
Signal description (internal signals):
count
direction
clear
Increment or decrement TCNT2 by 1.
Selects between increment and decrement.
Clear TCNT2 (set all bits to zero).
Timer/Counter clock.
clkT2
top
Signalizes that TCNT2 has reached maximum value.
Signalizes that TCNT2 has reached minimum value (zero).
bottom
Depending on the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clkT2). clkT2 can 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 clkT2 is 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 (TCCR2A). There are close connections between how the
counter behaves (counts) and how waveforms are generated on the Output Compare output
OC2A. For more details about advanced counting sequences and waveform generation, see
”Modes of Operation” on page 140.
The Timer/Counter Overflow Flag (TOV2) is set according to the mode of operation selected by
the WGM21:0 bits. TOV2 can be used for generating a CPU interrupt.
16.5 Output Compare Unit
The 8-bit comparator continuously compares TCNT2 with the Output Compare Register
(OCR2A). Whenever TCNT2 equals OCR2A, the comparator signals a match. A match will set
the Output Compare Flag (OCF2A) at the next timer clock cycle. If enabled (OCIE2A = 1), the
Output Compare Flag generates an Output Compare interrupt. The OCF2A Flag is automatically
cleared when the interrupt is executed. Alternatively, the OCF2A Flag can be cleared by soft-
ware 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 WGM21:0 bits and Com-
pare Output mode (COM2A1:0) bits. The max and bottom signals are used by the Waveform
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Generator for handling the special cases of the extreme values in some modes of operation
(”Modes of Operation” on page 140).
Figure 16-3 shows a block diagram of the Output Compare unit.
Figure 16-3. Output Compare Unit, Block Diagram
DATA BUS
OCRnx
TCNTn
=
(8-bit Comparator )
OCFnx (Int.Req.)
top
bottom
FOCn
Waveform Generator
OCnx
WGMn1:0
COMnX1:0
The OCR2A 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 OCR2A 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 OCR2A Register access may seem complex, but this is not case. When the double buffer-
ing is enabled, the CPU has access to the OCR2A Buffer Register, and if double buffering is
disabled the CPU will access the OCR2A directly.
16.5.1
16.5.2
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 (FOC2A) bit. Forcing compare match will not set the
OCF2A Flag or reload/clear the timer, but the OC2A pin will be updated as if a real compare
match had occurred (the COM2A1:0 bits settings define whether the OC2A 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 OCR2A to be initial-
ized to the same value as TCNT2 without triggering an interrupt when the Timer/Counter clock is
enabled.
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16.5.3
Using the Output Compare Unit
Since writing TCNT2 in any mode of operation will block all compare matches for one timer clock
cycle, there are risks involved when changing TCNT2 when using the Output Compare unit,
independently of whether the Timer/Counter is running or not. If the value written to TCNT2
equals the OCR2A 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
counting down.
The setup of the OC2A should be performed before setting the Data Direction Register for the
port pin to output. The easiest way of setting the OC2A value is to use the Force Output Com-
pare (FOC2A) strobe bit in Normal mode. The OC2A Register keeps its value even when
changing between Waveform Generation modes.
Be aware that the COM2A1:0 bits are not double buffered together with the compare value.
Changing the COM2A1:0 bits will take effect immediately.
16.6 Compare Match Output Unit
The Compare Output mode (COM2A1:0) bits have two functions. The Waveform Generator
uses the COM2A1:0 bits for defining the Output Compare (OC2A) state at the next compare
match. Also, the COM2A1:0 bits control the OC2A pin output source. Figure 16-4 shows a sim-
plified schematic of the logic affected by the COM2A1:0 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 Regis-
ters (DDR and PORT) that are affected by the COM2A1:0 bits are shown. When referring to the
OC2A state, the reference is for the internal OC2A Register, not the OC2A pin.
Figure 16-4. Compare Match Output Unit, Schematic
COMnx1
Waveform
Generator
COMnx0
FOCnx
D
Q
1
0
OCnx
Pin
OCnx
D
Q
PORT
D
Q
DDR
clkI/O
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The general I/O port function is overridden by the Output Compare (OC2A) from the Waveform
Generator if either of the COM2A1:0 bits are set. However, the OC2A pin direction (input or out-
put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction
Register bit for the OC2A pin (DDR_OC2A) must be set as output before the OC2A value is vis-
ible 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 OC2A state before the
output is enabled. Note that some COM2A1:0 bit settings are reserved for certain modes of
operation. See ”Register Description” on page 149.
16.6.1
Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM2A1:0 bits differently in normal, CTC, and PWM modes.
For all modes, setting the COM2A1:0 = 0 tells the Waveform Generator that no action on the
OC2A Register is to be performed on the next compare match. For compare output actions in
the non-PWM modes refer to Table 16-3 on page 150. For fast PWM mode, refer to Table 16-4
on page 150, and for phase correct PWM refer to Table 16-5 on page 150.
A change of the COM2A1:0 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
FOC2A strobe bits.
16.7 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 (COM2A1:0) bits. The Compare Output mode bits do not affect the counting sequence,
while the Waveform Generation mode bits do. The COM2A1:0 bits control whether the PWM
output generated should be inverted or not (inverted or non-inverted PWM). For non-PWM
modes the COM2A1:0 bits control whether the output should be set, cleared, or toggled at a
compare match (See Section “16.6” on page 139.).
For detailed timing information refer to ”Timer/Counter Timing Diagrams” on page 145.
16.7.1
Normal Mode
The simplest mode of operation is the Normal mode (WGM21:0 = 0). In this mode the counting
direction is always up (incrementing), and no counter clear is performed. The counter simply
overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bot-
tom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV2) will be set in the same
timer clock cycle as the TCNT2 becomes zero. The TOV2 Flag in this case behaves like a ninth
bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt
that automatically clears the TOV2 Flag, the timer resolution can be increased by software.
There are no special cases to consider in the Normal mode, a new counter value can be written
anytime.
The Output Compare unit can be used to generate interrupts at some given time. Using the Out-
put Compare to generate waveforms in Normal mode is not recommended, since this will
occupy too much of the CPU time.
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16.7.2
Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM21:0 = 2), the OCR2A Register is used to
manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter
value (TCNT2) matches the OCR2A. The OCR2A 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 16-5. The counter value (TCNT2)
increases until a compare match occurs between TCNT2 and OCR2A, and then counter
(TCNT2) is cleared.
Figure 16-5. CTC Mode, Timing Diagram
OCnx Interrupt Flag Set
TCNTn
OCnx
(Toggle)
(COMnx1:0 = 1)
1
2
3
4
Period
An interrupt can be generated each time the counter value reaches the TOP value by using the
OCF2A Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating
the TOP value. However, changing the TOP to a value close to BOTTOM when the counter is
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 OCR2A is lower than the cur-
rent 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 OC2A output can be set to toggle its logical
level on each compare match by setting the Compare Output mode bits to toggle mode
(COM2A1:0 = 1). The OC2A value will not be visible on the port pin unless the data direction for
the pin is set to output. The waveform generated will have a maximum frequency of fOC2A
clk_I/O/2 when OCR2A is set to zero (0x00). The waveform frequency is defined by the following
equation:
=
f
f
clk_I/O
f
= -------------------------------------------------
OCnx
2 ⋅ N ⋅ (1 + OCRnx)
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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16.7.3
Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM21:0 = 3) provides a high frequency
PWM waveform generation option. The fast PWM differs from the other PWM option by its sin-
gle-slope operation. The counter counts from BOTTOM to MAX then restarts from BOTTOM. In
non-inverting Compare Output mode, the Output Compare (OC2A) is cleared on the compare
match between TCNT2 and OCR2A, and set at BOTTOM. In inverting Compare Output mode,
the output is set on compare match and cleared at BOTTOM. Due to the single-slope operation,
the operating frequency of the fast PWM mode can be twice as high as the phase correct PWM
mode that uses dual-slope operation. This high frequency makes the fast PWM mode well suited
for power regulation, rectification, and DAC applications. High frequency allows physically small
sized external components (coils, capacitors), and therefore reduces total system cost.
In fast PWM mode, the counter is incremented until the counter value matches the MAX value.
The counter is then cleared at the following timer clock cycle. The timing diagram for the fast
PWM mode is shown in Figure 16-6. The TCNT2 value is in the timing diagram shown as a his-
togram 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 OCR2A and TCNT2.
Figure 16-6. Fast PWM Mode, Timing Diagram
OCRnx Interrupt Flag Set
OCRnx Update and
TOVn Interrupt Flag Set
TCNTn
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCnx
OCnx
1
2
3
4
5
6
7
Period
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches MAX. If the inter-
rupt is enabled, the interrupt handler routine can be used for updating the compare value.
In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC2A pin.
Setting the COM2A1:0 bits to two will produce a non-inverted PWM and an inverted PWM output
can be generated by setting the COM2A1:0 to three (See Table 16-4 on page 150). The actual
OC2A value will only be visible on the port pin if the data direction for the port pin is set as out-
put. The PWM waveform is generated by setting (or clearing) the OC2A Register at the compare
match between OCR2A and TCNT2, and clearing (or setting) the OC2A 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
= -----------------
OCnxPWM
N ⋅ 256
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2A Register represent special cases when generating a PWM
waveform output in the fast PWM mode. If the OCR2A is set equal to BOTTOM, the output will
be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR2A equal to MAX will result
in a constantly high or low output (depending on the polarity of the output set by the COM2A1:0
bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-
ting OC2A to toggle its logical level on each compare match (COM2A1:0 = 1). The waveform
generated will have a maximum frequency of foc2 = fclk_I/O/2 when OCR2A is set to zero. This fea-
ture is similar to the OC2A toggle in CTC mode, except the double buffer feature of the Output
Compare unit is enabled in the fast PWM mode.
16.7.4
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 (OC2A) is cleared on the compare match
between TCNT2 and OCR2A while counting up, and set on the compare match while counting
down. 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 sym-
metric 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 16-7.
The TCNT2 value is in the timing diagram shown as a histogram for illustrating the dual-slope
operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal
line marks on the TCNT2 slopes represent compare matches between OCR2A and TCNT2.
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Figure 16-7. Phase Correct PWM Mode, Timing Diagram
OCnx Interrupt Flag Set
OCRnx Update
TOVn Interrupt Flag Set
TCNTn
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCnx
OCnx
1
2
3
Period
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches BOTTOM. The
Interrupt Flag can be used to generate an interrupt each time the counter reaches the BOTTOM
value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the
OC2A pin. Setting the COM2A1:0 bits to two will produce a non-inverted PWM. An inverted
PWM output can be generated by setting the COM2A1:0 to three (See Table 16-5 on page 150).
The actual OC2A 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 OC2A Register at the
compare match between OCR2A and TCNT2 when the counter increments, and setting (or
clearing) the OC2A Register at compare match between OCR2A and TCNT2 when the counter
decrements. The PWM frequency for the output when using phase correct PWM can be calcu-
lated by the following equation:
f
clk_I/O
f
= -----------------
OCnxPCPWM
N ⋅ 510
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2A Register represent special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR2A 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 16-7 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 BOT-
TOM. There are two cases that give a transition without Compare Match.
• OCR2A changes its value from MAX, like in Figure 16-7. When the OCR2A value is MAX the
OCn pin value is the same as the result of a down-counting compare match. To ensure
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symmetry around BOTTOM the OCn value at MAX must correspond to the result of an up-
counting Compare Match.
• The timer starts counting from a value higher than the one in OCR2A, and for that reason
misses the Compare Match and hence the OCn change that would have happened on the way
up.
16.8 Timer/Counter Timing Diagrams
The following figures show the Timer/Counter in synchronous mode, and the timer clock (clkT2)
is therefore shown as a clock enable signal. In asynchronous mode, clkI/O should be replaced by
the Timer/Counter Oscillator clock. The figures include information on when Interrupt Flags are
set. Figure 16-8 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 16-8. Timer/Counter Timing Diagram, no Prescaling
clkI/O
clkTn
(clkI/O/1)
TCNTn
TOVn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
Figure 16-9 shows the same timing data, but with the prescaler enabled.
Figure 16-9. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O/8)
TCNTn
TOVn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
Figure 16-10 shows the setting of OCF2A in all modes except CTC mode.
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Figure 16-10. Timer/Counter Timing Diagram, Setting of OCF2A, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O/8)
TCNTn
OCRnx
OCFnx
OCRnx - 1
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
Figure 16-11 shows the setting of OCF2A and the clearing of TCNT2 in CTC mode.
Figure 16-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Pres-
caler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O/8)
TCNTn
(CTC)
TOP - 1
TOP
BOTTOM
BOTTOM + 1
OCRnx
TOP
OCFnx
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16.9 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, OCR2A, and TCCR2A might be corrupted. A
safe procedure for switching clock source is:
1. Disable the Timer/Counter2 interrupts by clearing OCIE2A and TOIE2.
2. Select clock source by setting AS2 as appropriate.
3. Write new values to TCNT2, OCR2A, and TCCR2A.
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 CPU main clock frequency must be more than four times the Oscillator frequency.
• When writing to one of the registers TCNT2, OCR2A, or TCCR2A, 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 OCR2A write in progress. To
detect that a transfer to the destination register has taken place, the Asynchronous Status
Register – ASSR has been implemented.
• When entering Power-save or ADC Noise Reduction mode after having written to TCNT2,
OCR2A, or TCCR2A, the user must wait until the written register has been updated if
Timer/Counter2 is used to wake up the device. Otherwise, the MCU will enter sleep mode
before the changes are effective. This is particularly important if the Output Compare2 interrupt
is used to wake up the device, since the Output Compare function is disabled during writing to
OCR2A or TCNT2. If the write cycle is not finished, and the MCU enters sleep mode before the
OCR2UB bit returns to zero, the device will never receive a compare match interrupt, and the
MCU will not wake up.
• If Timer/Counter2 is used to wake the device up from Power-save or ADC Noise Reduction
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 reentering
sleep mode is less than one TOSC1 cycle, correct interrupt handling is not guaranteed. If the
user is in doubt whether the time before re-entering Power-save or ADC Noise Reduction
mode is sufficient, the following algorithm can be used to ensure that one TOSC1 cycle has
elapsed:
1. Write a value to TCCR2A, TCNT2, or OCR2A.
2. Wait until the corresponding Update Busy Flag in ASSR returns to zero.
3. Enter Power-save or ADC Noise Reduction 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.
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• Description of wake up from Power-save or ADC Noise Reduction 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
(clkI/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 OCR2A or TCCR2A.
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 3 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.
16.9.1
Timer/Counter Prescaler
Figure 16-12. Prescaler for Timer/Counter2
clkI/O
clkT2S
10-BIT T/C PRESCALER
Clear
TOSC1
AS2
PSR2
0
CS20
CS21
CS22
TIMER/COUNTER2 CLOCK SOURCE
clkT2
The clock source for Timer/Counter2 is named clkT2S. clkT2S is by default connected to the main
system I/O clock clkIO. By setting the AS2 bit in ASSR, Timer/Counter2 is asynchronously
clocked from the TOSC1 pin. This enables use of Timer/Counter2 as a Real Time Counter
(RTC). When AS2 is set, pins TOSC1 and TOSC2 are disconnected from Port C. A crystal can
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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. If
applying an external clock on TOSC1, the EXCLK bit in ASSR must be set.
For Timer/Counter2, the possible prescaled selections are: clkT2S/8, clkT2S/32, clkT2S/64,
clkT2S/128, clkT2S/256, and clkT2S/1024. Additionally, clkT2S as well as 0 (stop) may be selected.
Setting the PSR2 bit in GTCCR resets the prescaler. This allows the user to operate with a pre-
dictable prescaler.
16.10 Register Description
16.10.1 TCCR2A – Timer/Counter Control Register A
Bit
7
FOC2A
W
6
WGM20
R/W
0
5
COM2A1
R/W
4
COM2A0
R/W
3
WGM21
R/W
0
2
CS22
R/W
0
1
CS21
R/W
0
0
CS20
R/W
0
(0xB0)
TCCR2A
Read/Write
Initial Value
0
0
0
• Bit 7 – FOC2A: Force Output Compare A
The FOC2A bit is only active when the WGM bits specify a non-PWM mode. However, for ensur-
ing compatibility with future devices, this bit must be set to zero when TCCR2A is written when
operating in PWM mode. When writing a logical one to the FOC2A bit, an immediate compare
match is forced on the Waveform Generation unit. The OC2A output is changed according to its
COM2A1:0 bits setting. Note that the FOC2A bit is implemented as a strobe. Therefore it is the
value present in the COM2A1:0 bits that determines the effect of the forced compare.
A FOC2A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using
OCR2A as TOP.
The FOC2A 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 operation supported
by the Timer/Counter unit are: Normal mode, Clear Timer on Compare match (CTC) mode, and
two types of Pulse Width Modulation (PWM) modes. See Table 16-2 and ”Modes of Operation”
on page 140.
Table 16-2. Waveform Generation Mode Bit Description(1)
WGM21
(CTC2)
WGM20
(PWM2)
Timer/Counter Mode of
Operation
Update of
OCR2A at
TOV2 Flag
Set on
Mode
TOP
0
1
2
3
0
0
1
1
0
1
0
1
Normal
0xFF
0xFF
OCR2A
0xFF
Immediate
TOP
MAX
PWM, Phase Correct
CTC
BOTTOM
MAX
Immediate
BOTTOM
Fast PWM
MAX
Note:
1. The CTC2 and PWM2 bit definition names are now obsolete. Use the WGM21:0 definitions.
However, the functionality and location of these bits are compatible with previous versions of
the timer.
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• Bit 5:4 – COM2A1:0: Compare Match Output Mode A
These bits control the Output Compare pin (OC2A) behavior. If one or both of the COM2A1:0
bits are set, the OC2A output overrides the normal port functionality of the I/O pin it is connected
to. However, note that the Data Direction Register (DDR) bit corresponding to OC2A pin must be
set in order to enable the output driver.
When OC2A is connected to the pin, the function of the COM2A1:0 bits depends on the
WGM21:0 bit setting. Table 16-3 shows the COM2A1:0 bit functionality when the WGM21:0 bits
are set to a normal or CTC mode (non-PWM).
Table 16-3. Compare Output Mode, non-PWM Mode
COM2A1
COM2A0
Description
0
0
1
1
0
1
0
1
Normal port operation, OC2A disconnected.
Toggle OC2A on compare match.
Clear OC2A on compare match.
Set OC2A on compare match.
Table 16-4 shows the COM2A1:0 bit functionality when the WGM21:0 bits are set to fast PWM
mode.
Table 16-4. Compare Output Mode, Fast PWM Mode(1)
COM2A1
COM2A0
Description
0
0
0
1
Normal port operation, OC2A disconnected.
Reserved
Clear OC2A on compare match, set OC2A at BOTTOM,
(non-inverting mode)
1
1
0
1
Set OC2A on compare match, clear OC2A at BOTTOM,
(inverting mode)
Note:
1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case, the com-
pare match is ignored, but the set or clear is done at TOP. See ”Fast PWM Mode” on page 142
for more details.
Table 16-5 shows the COM21:0 bit functionality when the WGM21:0 bits are set to phase cor-
rect PWM mode.
Table 16-5. Compare Output Mode, Phase Correct PWM Mode(1)
COM2A1
COM2A0
Description
0
0
0
1
Normal port operation, OC2A disconnected.
Reserved
Clear OC2A on compare match when up-counting. Set OC2A on
compare match when counting down.
1
1
0
1
Set OC2A on compare match when up-counting. Clear OC2A on
compare match when counting down.
Note:
1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case, the com-
pare match is ignored, but the set or clear is done at TOP. See ”Phase Correct PWM Mode” on
page 143 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
16-6.
Table 16-6. Clock Select Bit Description
CS22
CS21
CS20
Description
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
No clock source (Timer/Counter stopped).
clkT2S/(No prescaling)
clkT2S/8 (From prescaler)
clkT2S/32 (From prescaler)
clkT2S/64 (From prescaler)
clkT2S/128 (From prescaler)
clkT S/256 (From prescaler)
2
clkT S/1024 (From prescaler)
2
16.10.2 TCNT2 – Timer/Counter Register
Bit
7
6
5
4
3
2
1
0
(0xB2)
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 OCR2A Register.
16.10.3 OCR2A – Output Compare Register A
Bit
7
6
5
4
3
2
1
0
(0xB3)
OCR2A[7:0]
OCR2A
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 A 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 OC2A pin.
16.10.4 ASSR – Asynchronous Status Register
Bit
(0xB6)
7
6
5
–
4
EXCLK
R/W
0
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
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• Bit 4 – EXCLK: Enable External Clock Input
When EXCLK is written to one, and asynchronous clock is selected, the external clock input buf-
fer is enabled and an external clock can be input on Timer Oscillator 1 (TOSC1) pin instead of a
32 kHz crystal. Writing to EXCLK should be done before asynchronous operation is selected.
Note that the crystal Oscillator will only run when this bit is zero.
• Bit 3 – AS2: Asynchronous Timer/Counter2
When AS2 is written to zero, Timer/Counter2 is clocked from the I/O clock, clkI/O. When AS2 is
written to one, Timer/Counter2 is clocked from a crystal Oscillator connected to the Timer Oscil-
lator 1 (TOSC1) pin. When the value of AS2 is changed, the contents of TCNT2, OCR2A, and
TCCR2A might be corrupted.
• Bit 2 – TCN2UB: Timer/Counter2 Update Busy
When Timer/Counter2 operates asynchronously and TCNT2 is written, this bit becomes set.
When TCNT2 has been updated from the temporary storage register, this bit is cleared by hard-
ware. A logical zero in this bit indicates that TCNT2 is ready to be updated with a new value.
• Bit 1 – OCR2UB: Output Compare Register2 Update Busy
When Timer/Counter2 operates asynchronously and OCR2A is written, this bit becomes set.
When OCR2A has been updated from the temporary storage register, this bit is cleared by hard-
ware. A logical zero in this bit indicates that OCR2A is ready to be updated with a new value.
• Bit 0 – TCR2UB: Timer/Counter Control Register2 Update Busy
When Timer/Counter2 operates asynchronously and TCCR2A is written, this bit becomes set.
When TCCR2A has been updated from the temporary storage register, this bit is cleared by
hardware. A logical zero in this bit indicates that TCCR2A is ready to be updated with a new
value.
If a write is performed to any of the three Timer/Counter2 Registers while its update busy flag is
set, the updated value might get corrupted and cause an unintentional interrupt to occur.
The mechanisms for reading TCNT2, OCR2A, and TCCR2A are different. When reading
TCNT2, the actual timer value is read. When reading OCR2A or TCCR2A, the value in the tem-
porary storage register is read.
16.10.5 TIMSK2 – Timer/Counter2 Interrupt Mask Register
Bit
(0x70)
7
6
5
4
–
3
–
2
–
1
OCIE2A
R/W
0
0
TOIE2
R/W
0
–
–
–
TIMSK2
Read/Write
Initial Value
R
0
R
0
R
0
R
0
R
0
R
0
• Bit 1 – OCIE2A: Timer/Counter2 Output Compare Match A Interrupt Enable
When the OCIE2A bit is written to one and the I-bit in the Status Register is set (one), the
Timer/Counter2 Compare Match A interrupt is enabled. The corresponding interrupt is executed
if a compare match in Timer/Counter2 occurs, i.e., when the OCF2A bit is set in the Timer/Coun-
ter 2 Interrupt Flag Register – TIFR2.
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• Bit 0 – 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/Counter2 Interrupt
Flag Register – TIFR2.
16.10.6 TIFR2 – Timer/Counter2 Interrupt Flag Register
Bit
0x17 (0x37)
7
6
5
–
4
–
3
–
2
–
1
OCF2A
R/W
0
0
TOV2
R/W
0
–
–
TIFR2
Read/Write
Initial Value
R
0
R
0
R
0
R
0
R
0
R
0
• Bit 1 – OCF2A: Output Compare Flag 2 A
The OCF2A bit is set (one) when a compare match occurs between the Timer/Counter2 and the
data in OCR2A – Output Compare Register2. OCF2A is cleared by hardware when executing
the corresponding interrupt handling vector. Alternatively, OCF2A is cleared by writing a logic
one to the flag. When the I-bit in SREG, OCIE2A (Timer/Counter2 Compare match Interrupt
Enable), and OCF2A are set (one), the Timer/Counter2 Compare match Interrupt is executed.
• Bit 0 – TOV2: Timer/Counter2 Overflow Flag
The TOV2 bit is set (one) when an overflow occurs in Timer/Counter2. TOV2 is cleared by hard-
ware when executing the corresponding interrupt handling vector. Alternatively, TOV2 is cleared
by writing a logic one to the flag. When the SREG I-bit, TOIE2A (Timer/Counter2 Overflow Inter-
rupt Enable), and TOV2 are set (one), the Timer/Counter2 Overflow interrupt is executed. In
PWM mode, this bit is set when Timer/Counter2 changes counting direction at 0x00.
16.10.7 GTCCR – General Timer/Counter Control Register
Bit
0x23 (0x43)
7
6
5
4
–
3
–
2
–
1
PSR2
R/W
0
0
PSR10
R/W
0
TSM
–
–
GTCCR
Read/Write
Initial Value
R/W
0
R
0
R
0
R
0
R
0
R
0
• Bit 1 – PSR2: Prescaler Reset Timer/Counter2
When this bit is one, the Timer/Counter2 prescaler will be reset. This bit is normally cleared
immediately by hardware. If the bit is written when Timer/Counter2 is operating in asynchronous
mode, the bit will remain one until the prescaler has been reset. The bit will not be cleared by
hardware if the TSM bit is set. Refer to the description of the ”Bit 7 – TSM: Timer/Counter Syn-
chronization Mode” on page 105 for a description of the Timer/Counter Synchronization mode.
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17. SPI – Serial Peripheral Interface
17.1 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
17.2 Overview
The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the
ATmega325P/3250P and peripheral devices or between several AVR devices. A simplified block
diagram of the Serial Peripheral Interface is shown in Figure 17-1.
The PRSPI bit in ”PRR – Power Reduction Register” on page 44 must be written to zero to
enable the SPI module.
Figure 17-1. SPI Block Diagram(1)
DIVIDER
/2/4/8/16/32/64/128
Note:
1. Refer to Figure 1-1 on page 2, and Table 13-3 on page 71 for SPI pin placement.
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The interconnection between Master and Slave CPUs with SPI is shown in Figure 17-2 on page
155. The system consists of two shift Registers, and a Master clock generator. The SPI Master
initiates the communication cycle when pulling low the Slave Select SS pin of the desired Slave.
Master and Slave prepare the data to be sent in their respective shift Registers, and the Master
generates the required clock pulses on the SCK line to interchange data. Data is always shifted
from 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.
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 generator stops, setting the end of
Transmission Flag (SPIF). If the SPI Interrupt Enable bit (SPIE) in the SPCR Register is set, an
interrupt is requested. The Master may continue to shift the next byte by writing it into SPDR, or
signal the end of packet by pulling high the Slave Select, SS line. The last incoming byte will be
kept in the Buffer Register for later use.
When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated as long
as the SS pin is driven high. In this state, software may update the contents of the SPI Data
Register, SPDR, but the data will not be shifted out by incoming clock pulses on the SCK pin
until the SS pin is driven low. As one byte has been completely shifted, the end of Transmission
Flag, SPIF is set. If the SPI Interrupt Enable bit, SPIE, in the SPCR Register is set, an interrupt
is requested. The Slave may continue to place new data to be sent into SPDR before reading
the incoming data. The last incoming byte will be kept in the Buffer Register for later use.
Figure 17-2. SPI Master-slave Interconnection
SHIFT
ENABLE
The system is single buffered in the transmit direction and double buffered in the receive direc-
tion. This means that bytes to be transmitted cannot be written to the SPI Data Register before
the entire shift cycle is completed. When receiving data, however, a received character must be
read from the SPI Data Register before the next character has been completely shifted in. Oth-
erwise, the first byte is lost.
In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure
correct sampling of the clock signal, the minimum low and high period should be:
Low period: longer than 2 CPU clock cycles.
High period: longer than 2 CPU clock cycles.
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When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden
according to Table 17-1. For more details on automatic port overrides, refer to ”Alternate Port
Functions” on page 69.
Table 17-1. SPI Pin Overrides(1)
Pin
MOSI
MISO
SCK
SS
Direction, Master SPI
User Defined
Input
Direction, Slave SPI
Input
User Defined
Input
User Defined
User Defined
Input
Note:
1. See ”Alternate Functions of Port B” on page 71 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 perform a simple transmission. DDR_SPI in the examples must be replaced by the actual Data Direction Register controlling the SPI pins. DD_MOSI, DD_MISO and DD_SCK must be replaced by the actual data direction bits for these pins. 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. See Section “5.” on page 10.
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The following code examples show how to initialize the SPI as a Slave and how to perform a
simple reception.
Assembly Code Example(1)
SPI_SlaveInit:
; Set MISO output, all others input
ldi 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. See Section “5.” on page 10.
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17.3 SS Pin Functionality
17.3.1
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 synchronous
with the master clock generator. When the SS pin is driven high, the SPI slave will immediately
reset the send and receive logic, and drop any partially received data in the Shift Register.
17.3.2
Master Mode
When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the
direction of the SS pin.
If SS is configured as an output, the pin is a general output pin which does not affect the SPI
system. Typically, the pin will be driving the SS pin of the SPI Slave.
If SS is configured as an input, it must be held high to ensure Master SPI operation. If the SS pin
is driven low by peripheral circuitry when the SPI is configured as a Master with the SS pin
defined as an input, the SPI system interprets this as another master selecting the SPI as a
slave and starting to send data to it. To avoid bus contention, the SPI system takes the following
actions:
1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a result of
the SPI becoming a Slave, the MOSI and SCK pins become inputs.
2. The SPIF Flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in SREG is
set, the interrupt routine will be executed.
Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possi-
bility that SS is driven low, the interrupt should always check that the MSTR bit is still set. If the
MSTR bit has been cleared by a slave select, it must be set by the user to re-enable SPI Master
mode.
17.4 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
17-3 and Figure 17-4. Data bits are shifted out and latched in on opposite edges of the SCK sig-
nal, ensuring sufficient time for data signals to stabilize. This is clearly seen by summarizing
Table 17-2 and Table 17-3, as done below:
Table 3. CPOL 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
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Figure 17-3. SPI Transfer Format with CPHA = 0
SCK (CPOL = 0)
mode 0
SCK (CPOL = 1)
mode 2
SAMPLE I
MOSI/MISO
CHANGE 0
MOSI PIN
CHANGE 0
MISO PIN
SS
MSB first (DORD = 0) MSB
LSB first (DORD = 1) LSB
Bit 6
Bit 1
Bit 5
Bit 2
Bit 4
Bit 3
Bit 3
Bit 4
Bit 2
Bit 5
Bit 1
Bit 6
LSB
MSB
Figure 17-4. 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 3
Bit 4
Bit 2
Bit 5
Bit 1
Bit 6
LSB
MSB
17.5 Register Description
17.5.1
SPCR – SPI Control Register
Bit
7
SPIE
R/W
0
6
5
4
3
CPOL
R/W
0
2
1
0
0x2C (0x4C)
Read/Write
Initial Value
SPE
R/W
0
DORD
R/W
0
MSTR
R/W
0
CPHA
R/W
0
SPR1
R/W
0
SPR0
R/W
0
SPCR
• 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.
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• 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 be cleared,
and SPIF in SPSR will become set. The user will then have to set MSTR to re-enable SPI Mas-
ter mode.
• Bit 3 – CPOL: Clock Polarity
When this bit is written to one, SCK is high when idle. When CPOL is written to zero, SCK is low
when idle. Refer to Figure 17-3 on page 160 and Figure 17-4 on page 160 for an example. The
CPOL functionality is summarized below:
Table 17-2. 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 17-3 on page 160 and Figure 17-4 on page 160 for an
example. The CPOL functionality is summarized below:
Table 17-3. 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 fosc is
shown in the following table:
Table 17-4. Relationship Between SCK and the Oscillator Frequency
SPI2X
SPR1
SPR0
SCK Frequency
fosc/4
0
0
0
0
0
0
1
1
0
1
0
1
fosc/16
fosc/64
fosc/128
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Table 17-4. Relationship Between SCK and the Oscillator Frequency (Continued)
SPI2X
SPR1
SPR0
SCK Frequency
fosc/2
1
1
1
1
0
0
1
1
0
1
0
1
fosc/8
fosc/32
fosc/64
17.5.2
SPSR – SPI Status Register
Bit
7
SPIF
R
6
5
–
4
–
3
–
2
–
1
–
0
SPI2X
R/W
0
0x2D (0x4D)
Read/Write
Initial Value
WCOL
SPSR
R
0
R
0
R
0
R
0
R
0
R
0
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 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 17-4 on page 161). This means that the minimum SCK period will
be two CPU clock periods. When the SPI is configured as Slave, the SPI is only guaranteed to
work at fosc/4 or lower.
The SPI interface on the ATmega325P/3250P is also used for program memory and EEPROM
downloading or uploading. See page 285 for serial programming and verification.
17.5.3
SPDR – SPI Data Register
Bit
7
6
5
4
3
2
1
0
0x2E (0x4E)
Read/Write
Initial Value
MSB
R/W
X
LSB
R/W
X
SPDR
R/W
X
R/W
X
R/W
X
R/W
X
R/W
X
R/W
X
Undefined
The SPI Data Register is a read/write register used for data transfer between the Register File
and the SPI Shift Register. Writing to the register initiates data transmission. Reading the regis-
ter causes the Shift Register Receive buffer to be read.
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18. USART0
18.1 Features
• Full Duplex Operation (Independent Serial Receive and Transmit Registers)
• Asynchronous or Synchronous Operation
• Master or Slave Clocked Synchronous Operation
• High Resolution Baud Rate Generator
• Supports Serial Frames with 5, 6, 7, 8, or 9 Data Bits and 1 or 2 Stop Bits
• Odd or Even Parity Generation and Parity Check Supported by Hardware
• Data OverRun Detection
• Framing Error Detection
• Noise Filtering Includes False Start Bit Detection and Digital Low Pass Filter
• Three Separate Interrupts on TX Complete, TX Data Register Empty and RX Complete
• Multi-processor Communication Mode
• Double Speed Asynchronous Communication Mode
18.2 Overview
The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a
highly flexible serial communication device.
A simplified block diagram of the USART Transmitter is shown in Figure 18-1 on page 164. CPU
accessible I/O Registers and I/O pins are shown in bold.
The Power Reduction USART bit, PRUSART0, in ”PRR – Power Reduction Register” on page
44 must be written to zero to enable USART0 module.
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Figure 18-1. USART Block Diagram(1)
Clock Generator
UBRR[H:L]
OSC
BAUD RATE GENERATOR
SYNC LOGIC
PIN
CONTROL
XCK
TxD
RxD
Transmitter
TX
CONTROL
UDR (Transmit)
PARITY
GENERATOR
PIN
CONTROL
TRANSMIT SHIFT REGISTER
Receiver
CLOCK
RECOVERY
RX
CONTROL
DATA
RECOVERY
PIN
CONTROL
RECEIVE SHIFT REGISTER
PARITY
CHECKER
UDR (Receive)
UCSRA
UCSRB
UCSRC
Note:
1. Refer to Figure 1-1 on page 2, Figure 1-2 on page 3, ”Alternate Functions of Port E” on page
75 for USART pin placement.
The dashed boxes in the block diagram separate the three main parts of the USART (listed from
the top): Clock Generator, Transmitter and Receiver. Control Registers are shared by all units.
The Clock Generation logic consists of synchronization logic for external clock input used by
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 (UDRn). The Receiver supports the same frame formats as the Transmitter, and
can detect Frame Error, Data OverRun and Parity Errors.
18.2.1
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.
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• 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 UDRn must only be read once for each incoming data! More important is
the fact that the Error Flags (FEn and DORn) and the ninth data bit (RXB8n) are buffered with
the data in the receive buffer. Therefore the status bits must always be read before the UDRn
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 18-1) if the Buffer Registers are
full, until a new start bit is detected. The USART is therefore more resistant to Data OverRun
(DORn) error conditions.
The following control bits have changed name, but have same functionality and register location:
• CHR9 is changed to UCSZn2.
• OR is changed to DORn.
18.3 Clock Generation
The Clock Generation logic generates the base clock for the Transmitter and Receiver. The
USART supports four modes of clock operation: Normal asynchronous, Double Speed asyn-
chronous, Master synchronous and Slave synchronous mode. The UMSELn bit in USART
Control and Status Register C (UCSRnC) selects between asynchronous and synchronous
operation. Double Speed (asynchronous mode only) is controlled by the U2Xn found in the
UCSRnA Register. When using synchronous mode (UMSELn = 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 18-2 shows a block diagram of the clock generation logic.
Figure 18-2. 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
Pin
xcko
DDR_XCK
UCPOL
1
0
Signal description:
txclk
Transmitter clock (Internal Signal).
Receiver base clock (Internal Signal).
rxclk
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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).
18.3.1
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 18-2 on page 165.
The USART Baud Rate Register (UBRRn) and the down-counter connected to it function as a
programmable prescaler or baud rate generator. The down-counter, running at system clock
(fosc), is loaded with the UBRRn value each time the counter has counted down to zero or when
the UBRRnL Register is written. A clock is generated each time the counter reaches zero. This
clock is the baud rate generator clock output (= fosc/(UBRRn+1)). The Transmitter divides the
baud rate generator clock output by 2, 8 or 16 depending on mode. The baud rate generator out-
put is used directly by the Receiver’s clock and data recovery units. However, the recovery units
use a state machine that uses 2, 8 or 16 states depending on mode set by the state of the
UMSELn, U2Xn and DDR_XCK bits.
Table 18-3 contains equations for calculating the baud rate (in bits per second) and for calculat-
ing the UBRRn value for each mode of operation using an internally generated clock source.
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Figure 18-3. Equations for Calculating Baud Rate Register Setting
Equation for Calculating Baud
Rate(1)
Equation for Calculating UBRRn
Value
Operating Mode
f
f
Asynchronous Normal
mode (U2Xn = 0)
OSC
OSC
BAUD = -----------------------------------------
16(UBRRn + 1)
UBRRn = ----------------------- – 1
16BAUD
Asynchronous Double
Speed mode
(U2Xn = 1)
f
f
OSC
OSC
BAUD = --------------------------------------
8(UBRRn + 1)
UBRRn = -------------------- – 1
8BAUD
f
f
Synchronous Master
mode
OSC
OSC
BAUD = --------------------------------------
2(UBRRn + 1)
UBRRn = -------------------- – 1
2BAUD
Note:
1. The baud rate is defined to be the transfer rate in bit per second (bps)
BAUD
Baud rate (in bits per second, bps)
fOSC
System Oscillator clock frequency
UBRRn
Contents of the UBRRnH and UBRRnL Registers, (0-4095)
Some examples of UBRRn values for some system clock frequencies are found in Table 18-3
(see page 187).
18.3.2
Double Speed Operation (U2Xn)
The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. 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.
18.3.3
External Clock
External clocking is used by the synchronous slave modes of operation. The description in this
section refers to Figure 18-2 for details.
External clock input from the XCK pin is sampled by a synchronization register to minimize the
chance of meta-stability. The output from the synchronization register must then pass through
an edge detector before it can be used by the Transmitter and Receiver. This process intro-
duces a two CPU clock period delay and therefore the maximum external XCK clock frequency
is limited by the following equation:
f
OSC
f
< -----------
XCK
4
Note that fosc depends on the stability of the system clock source. It is therefore recommended to
add some margin to avoid possible loss of data due to frequency variations.
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18.3.4
Synchronous Clock Operation
When synchronous mode is used (UMSELn = 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 18-4. Synchronous Mode XCK Timing.
UCPOL = 1
XCK
RxD / TxD
Sample
Sample
UCPOL = 0
XCK
RxD / TxD
The UCPOLn bit UCRSC selects which XCK clock edge is used for data sampling and which is
used for data change. As Figure 18-4 shows, when UCPOLn is zero the data will be changed at
rising XCK edge and sampled at falling XCK edge. If UCPOLn is set, the data will be changed at
falling XCK edge and sampled at rising XCK edge.
18.4 Frame Formats
A serial frame is defined to be one character of data bits with synchronization bits (start and stop
bits), and optionally a parity bit for error checking. The USART accepts all 30 combinations of
the following as valid frame formats:
• 1 start bit
• 5, 6, 7, 8, or 9 data bits
• no, even or odd parity bit
• 1 or 2 stop bits
A frame starts with the start bit followed by the least significant data bit. Then the next data bits,
up to a total of nine, are succeeding, ending with the most significant bit. If enabled, the parity bit
is inserted after the data bits, before the stop bits. When a complete frame is transmitted, it can
be directly followed by a new frame, or the communication line can be set to an idle (high) state.
Figure 18-5 illustrates the possible combinations of the frame formats. Bits inside brackets are
optional.
Figure 18-5. Frame Formats
FRAME
(IDLE)
St
0
1
2
3
4
[5]
[6]
[7]
[8]
[P] Sp1 [Sp2] (St / IDLE)
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St
Start bit, always low.
Data bits (0 to 8).
(n)
P
Parity bit. Can be odd or even.
Stop bit, always high.
Sp
IDLE
No transfers on the communication line (RxD or TxD). An IDLE line must
high.
be
The frame format used by the USART is set by the UCSZn2:0, UPMn1:0 and USBSn bits in
UCSRnB and UCSRnC. 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 (UCSZn2:0) bits select the number of data bits in the frame. The
USART Parity mode (UPMn1:0) bits enable and set the type of parity bit. The selection between
one or two stop bits is done by the USART Stop Bit Select (USBSn) bit. The Receiver ignores
the second stop bit. An FEn (Frame Error) will therefore only be detected in the cases where the
first stop bit is zero.
18.4.1
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
P
= d
= d
⊕ … ⊕ d ⊕ d ⊕ d ⊕ d ⊕ 0
3 2 1 0
even
n – 1
n – 1
⊕ … ⊕ d ⊕ d ⊕ d ⊕ d ⊕ 1
odd
3 2 1 0
Peven
Podd
dn
Parity bit using even parity
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.
18.5 USART Initialization
The USART has to be initialized before any communication can take place. The initialization pro-
cess normally consists of setting the baud rate, setting frame format and enabling the
Transmitter or the Receiver depending on the usage. For interrupt driven USART operation, the
Global Interrupt Flag should be cleared (and interrupts globally disabled) when doing the
initialization.
Before doing a re-initialization with changed baud rate or frame format, be sure that there are no
ongoing transmissions during the period the registers are changed. The TXCn Flag can be used
to check that the Transmitter has completed all transfers, and the RXCn Flag can be used to
check that there are no unread data in the receive buffer. Note that the TXCn Flag must be
cleared before each transmission (before UDRn is written) if it is used for this purpose.
The following simple USART initialization code examples show one assembly and one C func-
tion that are equal in functionality. The examples assume asynchronous operation using polling
(no interrupts enabled) and a fixed frame format. The baud rate is given as a function parameter.
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For the assembly code, the baud rate parameter is assumed to be stored in the r17:r16
Registers.
Assembly Code Example(1)
USART_Init:
; Set baud rate
out UBRR0H, r17
out UBRR0L, r16
; Enable receiver and transmitter
ldi r16, (1<<RXEN0)|(1<<TXEN0)
out UCSR0B,r16
; Set frame format: 8data, 2stop bit
ldi r16, (1<<USBS0)|(3<<UCSZ00)
out UCSR0C,r16
ret
C Code Example(1)
#define FOSC 1843200 // Clock Speed
#define BAUD 9600
#define MYUBRR FOSC/16/BAUD-1
void main( void )
{
...
USART_Init(MYUBRR)
...
}
void USART_Init( unsigned int ubrr)
{
/* Set baud rate */
UBRR0H = (unsigned char)(ubrr>>8);
UBRR0L = (unsigned char)ubrr;
/* Enable receiver and transmitter */
UCSR0B = (1<<RXEN0)|(1<<TXEN0);
/* Set frame format: 8data, 2stop bit */
UCSR0C = (1<<USBS0)|(3<<UCSZ00);
}
Note:
1. See Section “5.” on page 10.
More advanced initialization routines can be made that include frame format as parameters, dis-
able interrupts and so on. However, many applications use a fixed setting of the baud and
control registers, and for these types of applications the initialization code can be placed directly
in the main routine, or be combined with initialization code for other I/O modules.
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18.6 Data Transmission – The USART Transmitter
The USART Transmitter is enabled by setting the Transmit Enable (TXENn) bit in the UCSRnB
Register. When the Transmitter is enabled, the normal port operation of the TxD pin is overrid-
den by the USART and given the function as the Transmitter’s serial output. The baud rate,
mode of operation and frame format must be set up once before doing any transmissions. If syn-
chronous operation is used, the clock on the XCK pin will be overridden and used as
transmission clock.
18.6.1
Sending Frames with 5 to 8 Data Bit
A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The
CPU can load the transmit buffer by writing to the UDRn 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 previous frame is transmitted. When the Shift Register is
loaded with new data, it will transfer one complete frame at the rate given by the Baud Register,
U2Xn bit or by XCK depending on mode of operation.
The following code examples show a simple USART transmit function based on polling of the
Data Register Empty (UDREn) Flag. When using frames with less than eight bits, the most sig-
nificant bits written to the UDRn 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 UCSR0A,UDRE0
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out UDR0,r16
ret
C Code Example(1)
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSR0A & (1<<UDRE0)) )
;
/* Put data into buffer, sends the data */
UDR0 = data;
}
Note:
1. See Section “5.” on page 10.
The function simply waits for the transmit buffer to be empty by checking the UDREn Flag,
before loading it with new data to be transmitted. If the Data Register Empty interrupt is utilized,
the interrupt routine writes the data into the buffer.
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18.6.2
Sending Frames with 9 Data Bit
If 9-bit characters are used (UCSZ = 7), the ninth bit must be written to the TXB8n bit in
UCSRnB before the low byte of the character is written to UDRn. 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)(2)
USART_Transmit:
; Wait for empty transmit buffer
sbis UCSR0A,UDRE0
rjmp USART_Transmit
; Copy 9th bit from r17 to TXB80
cbi UCSR0B,TXB80
sbrc r17,0
sbi UCSR0B,TXB80
; Put LSB data (r16) into buffer, sends the data
out UDR0,r16
ret
C Code Example(1)(2)
void USART_Transmit( unsigned int data )
{
/* Wait for empty transmit buffer */
while ( !( UCSR0A & (1<<UDRE0))) )
;
/* Copy 9th bit to TXB80 */
UCSR0B &= ~(1<<TXB80);
if ( data & 0x0100 )
UCSR0B |= (1<<TXB80);
/* Put data into buffer, sends the data */
UDR0 = data;
}
Notes: 1. These transmit functions are written to be general functions. They can be optimized if the con-
tents of the UCSRnB is static. For example, only the TXB80 bit of the UCSRnB Register is
used after initialization.
2. See Section “5.” on page 10.
The ninth bit can be used for indicating an address frame when using multi processor communi-
cation mode or for other protocol handling as for example synchronization.
18.6.3
Transmitter Flags and Interrupts
The USART Transmitter has two flags that indicate its state: USART Data Register Empty
(UDREn) and Transmit Complete (TXCn). Both flags can be used for generating interrupts.
The Data Register Empty (UDREn) 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
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contains data to be transmitted that has not yet been moved into the Shift Register. For compat-
ibility with future devices, always write this bit to zero when writing the UCSRnA Register.
When the Data Register Empty Interrupt Enable (UDRIEn) bit in UCSRnB is written to one, the
USART Data Register Empty Interrupt will be executed as long as UDREn is set (provided that
global interrupts are enabled). UDREn is cleared by writing UDRn. When interrupt-driven data
transmission is used, the Data Register Empty interrupt routine must either write new data to
UDRn in order to clear UDREn or disable the Data Register Empty interrupt, otherwise a new
interrupt will occur once the interrupt routine terminates.
The Transmit Complete (TXCn) 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 TXCn 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 TXCn Flag is useful in half-duplex commu-
nication interfaces (like the RS-485 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 (TXCIEn) bit in UCSRnB is set, the USART
Transmit Complete Interrupt will be executed when the TXCn Flag becomes set (provided that
global interrupts are enabled). When the transmit complete interrupt is used, the interrupt han-
dling routine does not have to clear the TXCn Flag, this is done automatically when the interrupt
is executed.
18.6.4
18.6.5
Parity Generator
The Parity Generator calculates the parity bit for the serial frame data. When parity bit is enabled
(UPMn1 = 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 TXENn to zero) will not become effective until ongo-
ing and pending transmissions 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 pin.
18.7 Data Reception – The USART Receiver
The USART Receiver is enabled by writing the Receive Enable (RXENn) bit in the UCSRnB
Register to one. When the Receiver is enabled, the normal pin operation of the RxD pin is over-
ridden 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.
18.7.1
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 UDRn I/O location.
The following code example shows a simple USART receive function based on polling of the
Receive Complete (RXCn) Flag. When using frames with less than eight bits the most significant
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bits of the data read from the UDRn 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 UCSR0A, RXC0
rjmp USART_Receive
; Get and return received data from buffer
in
r16, UDR0
ret
C Code Example(1)
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSR0A & (1<<RXC0)) )
;
/* Get and return received data from buffer */
return UDR0;
}
Note:
1. See Section “5.” on page 10.
The function simply waits for data to be present in the receive buffer by checking the RXCn Flag,
before reading the buffer and returning the value.
18.7.2
Receiving Frames with 9 Data Bits
If 9-bit characters are used (UCSZ=7), the ninth bit must be read from the RXB8n bit in UCSRnB
before reading the low bits from the UDRn. This rule applies to the FEn, DORn and UPEn Status
Flags as well. Read status from UCSRnA, then data from UDRn. Reading the UDRn I/O location
will change the state of the receive buffer FIFO and consequently the TXB8n, FEn, DORn and
UPEn 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 nine bit
characters and the status bits.
Assembly Code Example(1)
USART_Receive:
; Wait for data to be received
sbis UCSR0A, RXC0
rjmp USART_Receive
; Get status and 9th bit, then data from buffer
in
in
in
r18, UCSR0A
r17, UCSR0B
r16, UDRn
; If error, return -1
andi r18,(1<<FE0)|(1<<DOR0)|(1<<UPE0)
breq USART_ReceiveNoError
ldi r17, HIGH(-1)
ldi r16, LOW(-1)
USART_ReceiveNoError:
; Filter the 9th bit, then return
lsr r17
andi r17, 0x01
ret
C Code Example(1)
unsigned int USART_Receive( void )
{
unsigned char status, resh, resl;
/* Wait for data to be received */
while ( !(UCSR0A & (1<<RXC0)) )
;
/* Get status and 9th bit, then data */
/* from buffer */
status = UCSR0A;
resh = UCSR0B;
resl = UDRn;
/* If error, return -1 */
if ( status & (1<<FE0)|(1<<DOR0)|(1<<UPE0) )
return -1;
/* Filter the 9th bit, then return */
resh = (resh >> 1) & 0x01;
return ((resh << 8) | resl);
}
Note:
1. See Section “5.” on page 10.
The receive function example reads all the I/O Registers into the Register File before any com-
putation is done. This gives an optimal receive buffer utilization since the buffer location read will
be free to accept new data as early as possible.
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18.7.3
Receive Compete Flag and Interrupt
The USART Receiver has one flag that indicates the Receiver state.
The Receive Complete (RXCn) Flag indicates if there are unread data present in the receive buf-
fer. This flag is one when unread data exist in the receive buffer, and zero when the receive
buffer is empty (i.e., does not contain any unread data). If the Receiver is disabled (RXENn = 0),
the receive buffer will be flushed and consequently the RXCn bit will become zero.
When the Receive Complete Interrupt Enable (RXCIEn) in UCSRnB is set, the USART Receive
Complete interrupt will be executed as long as the RXCn Flag is set (provided that global inter-
rupts are enabled). When interrupt-driven data reception is used, the receive complete routine
must read the received data from UDRn in order to clear the RXCn Flag, otherwise a new inter-
rupt will occur once the interrupt routine terminates.
18.7.4
Receiver Error Flags
The USART Receiver has three Error Flags: Frame Error (FEn), Data OverRun (DORn) and
Parity Error (UPEn). All can be accessed by reading UCSRnA. 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 UCSRnA must be read before the
receive buffer (UDRn), since reading the UDRn 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 UCSRnA is written for upward
compatibility of future USART implementations. None of the Error Flags can generate interrupts.
The Frame Error (FEn) Flag indicates the state of the first stop bit of the next readable frame
stored in the receive buffer. The FEn Flag is zero when the stop bit was correctly read (as one),
and the FEn 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 FEn
Flag is not affected by the setting of the USBSn bit in UCSRnC 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 UCSRnA.
The Data OverRun (DORn) Flag indicates data loss due to a receiver buffer full condition. A
Data OverRun occurs when the receive buffer is full (two characters), it is a new character wait-
ing in the Receive Shift Register, and a new start bit is detected. If the DORn Flag is set there
was one or more serial frame lost between the frame last read from UDRn, and the next frame
read from UDRn. For compatibility with future devices, always write this bit to zero when writing
to UCSRnA. The DORn Flag is cleared when the frame received was successfully moved from
the Shift Register to the receive buffer.
The Parity Error (UPEn) Flag indicates that the next frame in the receive buffer had a Parity
Error when received. If Parity Check is not enabled the UPEn bit will always be read zero. For
compatibility with future devices, always set this bit to zero when writing to UCSRnA. For more
details see ”Parity Bit Calculation” on page 169 and ”Parity Checker” on page 176.
18.7.5
Parity Checker
The Parity Checker is active when the high USART Parity mode (UPMn1) bit is set. Type of Par-
ity Check to be performed (odd or even) is selected by the UPMn0 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
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with the received data and stop bits. The Parity Error (UPEn) Flag can then be read by software
to check if the frame had a Parity Error.
The UPEn bit is set if the next character that can be read from the receive buffer had a Parity
Error when received and the Parity Checking was enabled at that point (UPMn1 = 1). This bit is
valid until the receive buffer (UDRn) is read.
18.7.6
18.7.7
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 RXENn 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 UDRn I/O location until the RXCn Flag
is cleared. The following code example shows how to flush the receive buffer.
Assembly Code Example(1)
USART_Flush:
sbis UCSR0A, RXC0
ret
in
rjmp USART_Flush
C Code Example(1)
r16, UDR0
void USART_Flush( void )
{
unsigned char dummy;
while ( UCSR0A & (1<<RXC0) ) dummy = UDR0;
}
Note:
1. See Section “5.” on page 10.
18.8 Asynchronous Data Reception
The USART includes a clock recovery and a data recovery unit for handling asynchronous data
reception. The clock recovery logic is used for synchronizing the internally generated baud rate
clock to the incoming asynchronous serial frames at the RxD pin. The data recovery logic sam-
ples and low pass filters each incoming bit, thereby improving the noise immunity of the
Receiver. The asynchronous reception operational range depends on the accuracy of the inter-
nal baud rate clock, the rate of the incoming frames, and the frame size in number of bits.
18.8.1
Asynchronous Clock Recovery
The clock recovery logic synchronizes internal clock to the incoming serial frames. Figure 18-6
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 hor-
izontal arrows illustrate the synchronization variation due to the sampling process. Note the
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larger time variation when using the Double Speed mode (U2Xn = 1) of operation. Samples
denoted zero are samples done when the RxD line is idle (i.e., no communication activity).
Figure 18-6. Start Bit Sampling
RxD
IDLE
START
BIT 0
Sample
(U2X = 0)
0
0
1
1
2
3
2
4
5
3
6
7
4
8
9
5
10
11
6
12
13
7
14
15
8
16
1
1
2
3
Sample
(U2X = 1)
0
2
When the clock recovery logic detects a high (idle) to low (start) transition on the RxD line, the
start bit detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in
the figure. The clock recovery logic then uses samples 8, 9, and 10 for Normal mode, and sam-
ples 4, 5, and 6 for Double Speed mode (indicated with sample numbers inside boxes on the
figure), to decide if a valid start bit is received. If two or more of these three samples have logical
high levels (the majority wins), the start bit is rejected as a noise spike and the Receiver starts
looking for the next high to low-transition. If however, a valid start bit is detected, the clock recov-
ery logic is synchronized and the data recovery can begin. The synchronization process is
repeated for each start bit.
18.8.2
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 18-7 shows the sampling of the data bits and
the parity bit. Each of the samples is given a number that is equal to the state of the recovery
unit.
Figure 18-7. Sampling of Data and Parity Bit
RxD
BIT n
Sample
(U2X = 0)
1
1
2
3
2
4
5
3
6
7
4
8
9
5
10
11
6
12
13
7
14
15
8
16
1
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 18-8 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 18-8. Stop Bit Sampling and Next Start Bit Sampling
(A)
(B)
(C)
RxD
STOP 1
Sample
(U2X = 0)
1
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 (FEn) 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 18-8. For Double Speed mode the first low level must be delayed to
(B). (C) marks a stop bit of full length. The early start bit detection influences the operational
range of the Receiver.
18.8.3
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 18-1) base frequency, the Receiver will not be able to synchronize the frames to the start
bit.
The following equations can be used to calculate the ratio of the incoming data rate and internal
receiver baud rate.
(D + 1)S
S – 1 + D ⋅ S + S
(D + 2)S
(D + 1)S + S
R
= ------------------------------------------
R
= -----------------------------------
slow
fast
F
M
D
S
Sum of character size and parity size (D = 5 to 10 bit)
Samples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed
mode.
SF
First sample number used for majority voting. SF = 8 for normal speed and SF = 4
for Double Speed mode.
SM
Middle sample number used for majority voting. SM = 9 for normal speed and
SM = 5 for Double Speed mode.
Rslow
is the ratio of the slowest incoming data rate that can be accepted in relation to the
receiver baud rate. Rfast is the ratio of the fastest incoming data rate that can be
accepted in relation to the receiver baud rate.
Table 18-1 and Table 18-2 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 18-1. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode
(U2Xn = 0)
D
Recommended Max
Receiver Error (%)
# (Data+Parity Bit)
R
slow (%)
93.20
94.12
94.81
95.36
95.81
96.17
Rfast (%)
106.67
105.79
105.11
104.58
104.14
103.78
Max Total Error (%)
+6.67/-6.8
5
6
3.0
2.5
2.0
2.0
1.5
1.5
+5.79/-5.88
+5.11/-5.19
+4.58/-4.54
+4.14/-4.19
+3.78/-3.83
7
8
9
10
Table 18-2. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode
(U2Xn = 1)
D
Recommended Max
Receiver Error (%)
# (Data+Parity Bit)
Rslow (%)
94.12
94.92
95.52
96.00
96.39
96.70
Rfast (%)
105.66
104.92
104,35
103.90
103.53
103.23
Max Total Error (%)
+5.66/-5.88
5
6
2.5
2.0
1.5
1.5
1.5
1.0
+4.92/-5.08
7
+4.35/-4.48
8
+3.90/-4.00
9
+3.53/-3.61
10
+3.23/-3.30
The recommendations of the maximum receiver baud rate error was made under the assump-
tion that the Receiver and Transmitter equally divides the maximum total error.
There are two possible sources for the receivers baud rate error. The Receiver’s system clock
(XTAL) will always have some minor instability over the supply voltage range and the tempera-
ture range. When using a crystal to generate the system clock, this is rarely a problem, but for a
resonator the system clock may differ more than 2% depending of the resonators tolerance. The
second source for the error is more controllable. The baud rate generator can not always do an
exact division of the system frequency to get the baud rate wanted. In this case an UBRRn value
that gives an acceptable low error can be used if possible.
18.9 Multi-processor Communication Mode
Setting the Multi-processor Communication mode (MPCMn) bit in UCSRnA enables a filtering
function of incoming frames received by the USART Receiver. Frames that do not contain
address information will be ignored and not put into the receive buffer. This effectively reduces
the number of incoming frames that has to be handled by the CPU, in a system with multiple
MCUs that communicate via the same serial bus. The Transmitter is unaffected by the MPCMn
setting, but has to be used differently when it is a part of a system utilizing the Multi-processor
Communication mode.
If the Receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop bit indi-
cates if the frame contains data or address information. If the Receiver is set up for frames with
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nine data bits, then the ninth bit (RXB8n) 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.
18.9.1
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 (TXB8n) must be set when an address frame (TXB8n = 1) or cleared when a data frame
(TXB = 0) is being transmitted. The slave MCUs must in this case be set to use a 9-bit character
frame format.
The following procedure should be used to exchange data in Multi-processor Communication
mode:
1. All Slave MCUs are in Multi-processor Communication mode (MPCMn in UCSRnA is
set).
2. The Master MCU sends an address frame, and all slaves receive and read this frame. In
the Slave MCUs, the RXCn Flag in UCSRnA will be set as normal.
3. Each Slave MCU reads the UDRn Register and determines if it has been selected. If so,
it clears the MPCMn bit in UCSRnA, otherwise it waits for the next address byte and
keeps the MPCMn 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 MPCMn bit set, will ignore the data frames.
5. When the last data frame is received by the addressed MCU, the addressed MCU sets
the MPCMn 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 character size set-
ting. If 5- to 8-bit character frames are used, the Transmitter must be set to use two stop bit
(USBSn = 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 MPCMn bit. The
MPCMn bit shares the same I/O location as the TXCn Flag and this might accidentally be
cleared when using SBI or CBI instructions.
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18.10 Register Description
18.10.1 UDRn – USART I/O Data Register n
Bit
7
6
5
4
3
2
1
0
RXBn[7:0]
TXBn[7:0]
UDRn (Read)
UDRn (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 UDRn. The Transmit Data Buffer Reg-
ister (TXB) will be the destination for data written to the UDRn Register location. Reading the
UDRn 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.
The transmit buffer can only be written when the UDREn Flag in the UCSRnA Register is set.
Data written to UDRn when the UDREn Flag is not set, will be ignored by the USART Transmit-
ter. 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.
18.10.2 UCSRnA – USART Control and Status Register n A
Bit
7
6
5
4
FEn
R
3
DORn
R
2
UPEn
R
1
U2Xn
R/W
0
0
MPCMn
R/W
0
RXCn
TXCn
UDREn
UCSRnA
Read/Write
Initial Value
R
0
R/W
0
R
1
0
0
0
• Bit 7 – RXCn: 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 disabled, the receive
buffer will be flushed and consequently the RXCn bit will become zero. The RXCn Flag can be
used to generate a Receive Complete interrupt (see description of the RXCIEn bit).
• Bit 6 – TXCn: 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 (UDRn). The TXCn Flag bit is auto-
matically cleared when a transmit complete interrupt is executed, or it can be cleared by writing
a one to its bit location. The TXCn Flag can generate a Transmit Complete interrupt (see
description of the TXCIEn bit).
182
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• Bit 5 – UDREn: USART Data Register Empty
The UDREn Flag indicates if the transmit buffer (UDRn) is ready to receive new data. If UDREn
is one, the buffer is empty, and therefore ready to be written. The UDREn Flag can generate a
Data Register Empty interrupt (see description of the UDRIEn bit).
UDREn is set after a reset to indicate that the Transmitter is ready.
• Bit 4 – FEn: 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 (UDRn) is read. The FEn bit is zero when the stop bit of received data is one.
Always set this bit to zero when writing to UCSRnA.
• Bit 3 – DORn: 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 (UDRn) is read. Always set this
bit to zero when writing to UCSRnA.
• Bit 2 – UPEn: USART 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 (UPMn1 = 1). This bit is valid until the receive buffer
(UDRn) is read. Always set this bit to zero when writing to UCSRnA.
• Bit 1 – U2Xn: Double the USART Transmission Speed
This bit only has effect for the asynchronous operation. Write this bit to zero when using syn-
chronous operation.
Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively dou-
bling the transfer rate for asynchronous communication.
• Bit 0 – MPCMn: Multi-processor Communication Mode
This bit enables the Multi-processor Communication mode. When the MPCMn bit is written to
one, all the incoming frames received by the USART Receiver that do not contain address infor-
mation will be ignored. The Transmitter is unaffected by the MPCMn setting. For more detailed
information see ”Multi-processor Communication Mode” on page 180.
18.10.3 UCSRnB – USART Control and Status Register n B
Bit
7
6
5
4
RXENn
R/W
0
3
TXENn
R/W
0
2
UCSZn2
R/W
1
0
TXB8n
R/W
0
RXCIEn
TXCIEn
UDRIEn
RXB8n
UCSRnB
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
R
0
0
• Bit 7 – RXCIEn: RX Complete Interrupt Enable
Writing this bit to one enables interrupt on the RXCn Flag. A USART Receive Complete interrupt
will be generated only if the RXCIEn bit is written to one, the Global Interrupt Flag in SREG is
written to one and the RXCn bit in UCSRnA is set.
183
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• Bit 6 – TXCIEn: TX Complete Interrupt Enable
Writing this bit to one enables interrupt on the TXCn Flag. A USART Transmit Complete interrupt
will be generated only if the TXCIEn bit is written to one, the Global Interrupt Flag in SREG is
written to one and the TXCn bit in UCSRnA is set.
• Bit 5 – UDRIEn: USART Data Register Empty Interrupt Enable
Writing this bit to one enables interrupt on the UDREn Flag. A Data Register Empty interrupt will
be generated only if the UDRIEn bit is written to one, the Global Interrupt Flag in SREG is written
to one and the UDREn bit in UCSRnA is set.
• Bit 4 – RXEN0: Receiver Enable
Writing this bit to one enables the USART Receiver. The Receiver will override normal port oper-
ation for the RxD pin when enabled. Disabling the Receiver will flush the receive buffer
invalidating the FEn, DORn, and UPEn Flags.
• Bit 3 – TXENn: Transmitter Enable
Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port
operation for the TxD pin when enabled. The disabling of the Transmitter (writing TXENn 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 trans-
mitted. When disabled, the Transmitter will no longer override the TxD port.
• Bit 2 – UCSZn2: Character Size
The UCSZn2 bits combined with the UCSZn1:0 bit in UCSRnC sets the number of data bits
(Character SiZe) in a frame the Receiver and Transmitter use.
• Bit 1 – RXB8n: Receive Data Bit 8
RXB8n 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 UDRn.
• Bit 0 – TXB8n: Transmit Data Bit 8
TXB8n 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 UDRn.
18.10.4 UCSRnC – USART Control and Status Register n C
Bit
7
6
5
4
UPMn0
R/W
0
3
USBSn
R/W
0
2
UCSZn1
R/W
1
UCSZn0
R/W
0
UCPOLn
R/W
–
UMSELn
UPMn1
UCSRnC
Read/Write
Initial Value
R
0
R/W
0
R/W
0
1
1
0
• Bit 6 – UMSELn: USART Mode Select
This bit selects between asynchronous and synchronous mode of operation.
Figure 18-9. UMSELn Bit Settings
UMSELn
Mode
0
1
Asynchronous Operation
Synchronous Operation
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• Bit 5:4 – UPMn1:0: Parity Mode
These bits enable and set type of parity generation and check. If enabled, the Transmitter will
automatically generate and send the parity of the transmitted data bits within each frame. The
Receiver will generate a parity value for the incoming data and compare it to the UPMn0 setting.
If a mismatch is detected, the UPEn Flag in UCSRnA will be set.
Figure 18-10. UPMn Bits Settings
UPMn1
UPMn0
Parity Mode
0
0
1
1
0
1
0
1
Disabled
Reserved
Enabled, Even Parity
Enabled, Odd Parity
• Bit 3 – USBSn: Stop Bit Select
This bit selects the number of stop bits to be inserted by the Transmitter. The Receiver ignores
this setting.
Figure 18-11. USBSn Bit Settings
USBSn
Stop Bit(s)
1-bit
0
1
2-bit
• Bit 2:1 – UCSZn1:0: Character Size
The UCSZn1:0 bits combined with the UCSZn2 bit in UCSRnB sets the number of data bits
(Character SiZe) in a frame the Receiver and Transmitter use.
Figure 18-12. UCSZ Bits Settings
UCSZn2
UCSZn1
UCSZn0
Character Size
5-bit
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
6-bit
7-bit
8-bit
Reserved
Reserved
Reserved
9-bit
• Bit 0 – UCPOLn: Clock Polarity
This bit is used for synchronous mode only. Write this bit to zero when asynchronous mode is
used. The UCPOLn bit sets the relationship between data output change and data input sample,
and the synchronous clock (XCK).
185
8023F–AVR–07/09
Figure 18-13. UCPOLn Bit Settings
Transmitted Data Changed (Output
Received Data Sampled (Input
on RxD Pin)
UCPOLn
of TxD Pin)
0
1
Rising XCK Edge
Falling XCK Edge
Falling XCK Edge
Rising XCK Edge
18.10.5 UBRRnL and UBRRnH – USART Baud Rate Registers n
Bit
15
14
13
12
11
10
9
8
–
–
–
–
UBRRn[11:8]
UBRRnH
UBRRnL
UBRRn[7:0]
7
R
6
R
5
R
4
R
3
R/W
R/W
0
2
R/W
R/W
0
1
R/W
R/W
0
0
R/W
R/W
0
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
R/W
0
0
0
0
0
0
0
0
0
• Bit 15:12 – Reserved Bits
These bits are reserved for future use. For compatibility with future devices, these bit must be
written to zero when UBRRnH is written.
• Bit 11:0 – UBRR11:0: USART Baud Rate Register
This is a 12-bit register which contains the USART baud rate. The UBRRnH contains the four
most significant bits, and the UBRRnL contains the eight least significant bits of the USART
baud rate. Ongoing transmissions by the Transmitter and Receiver will be corrupted if the baud
rate is changed. Writing UBRRnL will trigger an immediate update of the baud rate prescaler.
18.11 Examples of Baud Rate Setting
For standard crystal and resonator frequencies, the most commonly used baud rates for asyn-
chronous operation can be generated by using the UBRRn settings in Table 18-3. UBRRn
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 resis-
tance when the error ratings are high, especially for large serial frames (see ”Asynchronous
Operational Range” on page 179). The error values are calculated using the following equation:
BaudRateClosest Match
⎛
⎞
Error[%] = ------------------------------------------------------- – 1 • 100%
⎝
⎠
BaudRate
186
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ATmega325P/3250P
Table 18-3. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
fosc = 1.0000 MHz fosc = 1.8432 MHz
U2Xn = 0 U2Xn = 1
UBRRn Error UBRRn Error UBRRn Error
fosc = 2.0000 MHz
Baud
Rate
(bps)
U2Xn = 0
UBRRn Error
U2Xn = 1
U2Xn = 0
U2Xn = 1
UBRRn Error
UBRRn Error
2400
4800
25
12
6
0.2%
0.2%
-7.0%
8.5%
8.5%
8.5%
-18.6%
8.5%
–
51
25
12
8
0.2%
0.2%
0.2%
-3.5%
-7.0%
8.5%
8.5%
8.5%
-18.6%
8.5%
–
47
23
11
7
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
-25.0%
0.0%
–
95
47
23
15
11
7
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
–
51
25
12
8
0.2%
0.2%
0.2%
-3.5%
-7.0%
8.5%
8.5%
8.5%
-18.6%
8.5%
–
103
51
25
16
12
8
0.2%
0.2%
0.2%
2.1%
0.2%
-3.5%
-7.0%
8.5%
8.5%
8.5%
–
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
3
3
1
2
2
5
2
6
0
1
1
3
1
3
–
1
1
2
1
2
–
–
0
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.
187
8023F–AVR–07/09
Table 18-4. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)
fosc = 3.6864 MHz fosc = 4.0000 MHz fosc = 7.3728 MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn UBRRn UBRRn UBRRn UBRRn UBRRn
Baud
Rate
(bps)
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
-7.8%
–
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
-7.8%
-7.8%
–
Error
0.2%
0.2%
0.2%
2.1%
0.2%
-3.5%
-7.0%
8.5%
8.5%
8.5%
8.5%
0.0%
–
Error
0.2%
0.2%
0.2%
-0.8%
0.2%
2.1%
0.2%
-3.5%
-7.0%
8.5%
8.5%
0.0%
0.0%
–
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
-7.8%
-7.8%
–
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
-7.8%
-7.8%
-7.8%
2400
4800
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
191
95
47
31
23
15
11
7
383
191
95
63
47
31
23
15
11
7
9600
14.4k
19.2k
28.8k
38.4k
57.6k
76.8k
115.2k
230.4k
250k
5
6
3
3
2
5
2
6
5
1
3
1
3
3
0
1
0
1
1
3
0
1
0
1
1
3
0.5M
–
0
–
0
0
1
1M
–
–
–
–
–
–
–
0
Max. (1)
230.4 kbps
460.8 kbps
250 kbps
0.5 Mbps
460.8 kbps
921.6 kbps
1.
UBRR = 0, Error = 0.0%
188
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ATmega325P/3250P
Table 4. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)
fosc = 8.0000 MHz fosc = 11.0592 MHz fosc = 14.7456 MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn UBRRn UBRRn UBRRn UBRRn UBRRn
Baud
Rate
(bps)
Error
0.2%
0.2%
0.2%
-0.8%
0.2%
2.1%
0.2%
-3.5%
-7.0%
8.5%
8.5%
0.0%
0.0%
–
Error
-0.1%
0.2%
0.2%
0.6%
0.2%
-0.8%
0.2%
2.1%
0.2%
-3.5%
8.5%
0.0%
0.0%
0.0%
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
-7.8%
–
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
-7.8%
-7.8%
–
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
-7.8%
-7.8%
-7.8%
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
5.3%
-7.8%
-7.8%
2400
4800
207
103
51
34
25
16
12
8
416
207
103
68
51
34
25
16
12
8
287
143
71
47
35
23
17
11
8
575
287
143
95
71
47
35
23
17
11
5
383
191
95
63
47
31
23
15
11
7
767
383
191
127
95
63
47
31
23
15
7
9600
14.4k
19.2k
28.8k
38.4k
57.6k
76.8k
115.2k
230.4k
250k
6
3
5
1
3
2
3
1
3
2
5
3
6
0.5M
0
1
–
2
1
3
1M
–
0
–
–
–
0
1
Max. (1)
0.5 Mbps
UBRR = 0, Error = 0.0%
1 Mbps
691.2 kbps
1.3824 Mbps
921.6 kbps
1.8432 Mbps
1.
189
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Table 5. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)
fosc = 16.0000 MHz fosc = 18.4320 MHz fosc = 20.0000 MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRRn UBRRn UBRRn UBRRn UBRRn UBRRn
Baud
Rate
(bps)
Error
-0.1%
0.2%
0.2%
0.6%
0.2%
-0.8%
0.2%
2.1%
0.2%
-3.5%
8.5%
0.0%
0.0%
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%
0.0%
0.0%
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
-7.8%
–
Error
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
2.4%
-7.8%
–
Error
0.0%
0.2%
0.2%
-0.2%
0.2%
0.9%
-1.4%
-1.4%
1.7%
-1.4%
8.5%
0.0%
–
Error
0.0%
0.0%
0.2%
-0.2%
0.2%
-0.2%
0.2%
0.9%
-1.4%
-1.4%
-1.4%
0.0%
0.0%
–
2400
4800
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
520
259
129
86
64
42
32
21
15
10
4
1041
520
259
173
129
86
9600
14.4k
19.2k
28.8k
38.4k
57.6k
76.8k
115.2k
230.4k
250k
64
42
32
21
3
4
10
3
7
4
8
4
9
0.5M
1
3
–
4
–
4
1M
0
1
–
–
–
–
–
–
Max. (1)
1 Mbps
UBRR = 0, Error = 0.0%
2 Mbps
1.152 Mbps
2.304 Mbps
1.25 Mbps
2.5 Mbps
1.
190
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ATmega325P/3250P
19. USI – Universal Serial Interface
19.1 Features
• Two-wire Synchronous Data Transfer (Master or Slave)
• Three-wire Synchronous Data Transfer (Master or Slave)
• Data Received Interrupt
• Wake up from Idle Mode
• In Two-wire Mode: Wake-up from All Sleep Modes, Including Power-down Mode
• Two-wire Start Condition Detector with Interrupt Capability
19.2 Overview
The Universal Serial Interface, USI, provides the basic hardware resources needed for serial
communication. Combined with a minimum of control software, the USI allows significantly
higher transfer rates and uses less code space than solutions based on software only. Interrupts
are included to minimize the processor load.
A simplified block diagram of the USI is shown on Figure 19-1. For the actual placement of I/O
pins, refer to ”Pinout ATmega3250P” on page 2 and ”Pinout ATmega325P” on page 3. 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 ”Register Descriptions” on page 199.
Figure 19-1. Universal Serial Interface, Block Diagram
(Output only)
DO
D
Q
LE
(Input/Open Drain)
DI/SDA
3
2
USIDR
1
0
TIM0 COMP
3
2
0
1
(Input/Open Drain)
USCK/SCL
4-bit Counter
1
0
CLOCK
HOLD
[1]
Two-wire Clock
Control Unit
USISR
2
USICR
The 8-bit Shift Register is directly accessible via the data bus and contains the incoming and
outgoing data. The register has no buffering so the data must be read as quickly as possible to
ensure that no data is lost. The most significant bit is connected to one of two output pins
depending of the wire mode configuration. A transparent latch is inserted between the Serial
Register Output and output pin, which delays the change of data output to the opposite clock
edge of the data input sampling. The serial input is always sampled from the Data Input (DI) pin
independent of the configuration.
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The 4-bit counter can be both read and written via the data bus, and can generate an overflow
interrupt. Both the Serial Register and the counter are clocked simultaneously by the same clock
source. This allows the counter to count the number of bits received or transmitted and generate
an interrupt when the transfer is complete. Note that when an external clock source is selected
the counter counts both clock edges. In this case the counter counts the number of edges, and
not the number of bits. The clock can be selected from three different sources: The USCK pin,
Timer/Counter0 Compare Match or from software.
The Two-wire clock control unit can generate an interrupt when a start condition is detected on
the Two-wire bus. It can also generate wait states by holding the clock pin low after a start con-
dition is detected, or after the counter overflows.
19.3 Functional Descriptions
19.3.1
Three-wire Mode
The USI Three-wire mode is compliant to the Serial Peripheral Interface (SPI) mode 0 and 1, but
does not have the slave select (SS) pin functionality. However, this feature can be implemented
in software if necessary. Pin names used by this mode are: DI, DO, and USCK.
Figure 19-2. Three-wire Mode Operation, Simplified Diagram
DO
DI
Bit7
Bit6
Bit5
Bit4
Bit3
Bit2
Bit1
Bit0
USCK
SLAVE
DO
DI
Bit7
Bit6
Bit5
Bit4
Bit3
Bit2
Bit1
Bit0
USCK
PORTxn
MASTER
Figure 19-2 shows two USI units operating in Three-wire mode, one as Master and one as
Slave. The two Shift Registers are interconnected in such way that after eight USCK clocks, the
data in each register are interchanged. The same clock also increments the USI’s 4-bit counter.
The Counter Overflow (interrupt) Flag, or USIOIF, can therefore be used to determine when a
transfer is completed. The clock is generated by the Master device software by toggling the
USCK pin via the PORT Register or by writing a one to the USITC bit in USICR.
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Figure 19-3. Three-wire Mode, Timing Diagram
( Reference )
1
2
3
4
5
6
7
8
CYCLE
USCK
USCK
DO
MSB
MSB
6
5
4
3
2
1
LSB
LSB
6
5
4
3
2
1
DI
A
B
C
D
E
The Three-wire mode timing is shown in Figure 19-3. At the top of the figure is a USCK cycle ref-
erence. One bit is shifted into the USI Shift Register (USIDR) for each of these cycles. The
USCK timing is shown for both external clock modes. In External Clock mode 0 (USICS0 = 0), DI
is sampled at positive edges, and DO is changed (Data Register is shifted by one) at negative
edges. External Clock mode 1 (USICS0 = 1) uses the opposite edges versus mode 0, i.e., sam-
ples data at negative and changes the output at positive edges. The USI clock modes
corresponds to the SPI data mode 0 and 1.
Referring to the timing diagram (Figure 19-3), a bus transfer involves the following steps:
1. The Slave device and Master device sets up its data output and, depending on the proto-
col used, enables its output driver (mark A and B). The output is set up by writing the
data to be transmitted to the Serial Data Register. Enabling of the output is done by set-
ting the corresponding bit in the port Data Direction Register. Note that point A and B
does not have any specific order, but both must be at least one half USCK cycle before
point C where the data is sampled. This must be done to ensure that the data setup
requirement is satisfied. The 4-bit counter is reset to zero.
2. The Master generates a clock pulse by software toggling the USCK line twice (C and D).
The bit value on the slave and master’s data input (DI) pin is sampled by the USI on the
first edge (C), and the data output is changed on the opposite edge (D). The 4-bit counter
will count both edges.
3. Step 2. is repeated eight times for a complete register (byte) transfer.
4. After eight clock pulses (i.e., 16 clock edges) the counter will overflow and indicate that
the transfer is completed. The data bytes transferred must now be processed before a
new transfer can be initiated. The overflow interrupt will wake up the processor if it is set
to Idle mode. Depending of the protocol used the slave device can now set its output to
high impedance.
19.3.2
SPI Master Operation Example
The following code demonstrates how to use the USI module as a SPI Master:
SPITransfer:
sts
ldi
sts
ldi
USIDR,r16
r16,(1<<USIOIF)
USISR,r16
r16,(1<<USIWM0)|(1<<USICS1)|(1<<USICLK)|(1<<USITC)
SPITransfer_loop:
sts
USICR,r16
lds
r16, USISR
r16, USIOIF
sbrs
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rjmp
lds
SPITransfer_loop
r16,USIDR
ret
The code is size optimized using only eight instructions (+ ret). The code example assumes that
the DO and USCK pins are enabled as output in the DDRE Register. The value stored in register
r16 prior to the function is called is transferred to the Slave device, and when the transfer is com-
pleted the data received from the Slave is stored back into the r16 Register.
The second and third instructions clears the USI Counter Overflow Flag and the USI counter
value. The fourth and fifth instruction set Three-wire mode, positive edge Shift Register clock,
count at USITC strobe, and toggle USCK. The loop is repeated 16 times.
The following code demonstrates how to use the USI module as a SPI Master with maximum
speed (fsck = fck/4):
SPITransfer_Fast:
sts
ldi
ldi
USIDR,r16
r16,(1<<USIWM0)|(0<<USICS0)|(1<<USITC)
r17,(1<<USIWM0)|(0<<USICS0)|(1<<USITC)|(1<<USICLK)
sts
sts
sts
sts
sts
sts
sts
sts
sts
sts
sts
sts
sts
sts
sts
sts
USICR,r16 ; MSB
USICR,r17
USICR,r16
USICR,r17
USICR,r16
USICR,r17
USICR,r16
USICR,r17
USICR,r16
USICR,r17
USICR,r16
USICR,r17
USICR,r16
USICR,r17
USICR,r16 ; LSB
USICR,r17
lds
r16,USIDR
ret
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19.3.3
SPI Slave Operation Example
The following code demonstrates how to use the USI module as a SPI Slave:
init:
ldi
sts
r16,(1<<USIWM0)|(1<<USICS1)
USICR,r16
...
SlaveSPITransfer:
sts
ldi
sts
USIDR,r16
r16,(1<<USIOIF)
USISR,r16
SlaveSPITransfer_loop:
lds
r16, USISR
sbrs
rjmp
lds
r16, USIOIF
SlaveSPITransfer_loop
r16,USIDR
ret
The code is size optimized using only eight instructions (+ ret). The code example assumes that
the DO is configured as output and USCK pin is configured as input in the DDR Register. The
value stored in register r16 prior to the function is called is transferred to the master device, and
when the transfer is completed the data received from the Master is stored back into the r16
Register.
Note that the first two instructions is for initialization only and needs only to be executed
once.These instructions sets Three-wire mode and positive edge Shift Register clock. The loop
is repeated until the USI Counter Overflow Flag is set.
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19.3.4
Two-wire Mode
The USI Two-wire mode is compliant to the Inter IC (TWI) bus protocol, but without slew rate lim-
iting on outputs and input noise filtering. Pin names used by this mode are SCL and SDA.
Figure 19-4. Two-wire Mode Operation, Simplified Diagram
VCC
SDA
Bit7
Bit6
Bit5
Bit4
Bit3
Bit2
Bit1
Bit0
SCL
HOLD
SCL
Two-wire Clock
Control Unit
SLAVE
SDA
SCL
Bit7
Bit6
Bit5
Bit4
Bit3
Bit2
Bit1
Bit0
PORTxn
MASTER
Figure 19-4 shows two USI units operating in Two-wire mode, one as Master and one as Slave.
It is only the physical layer that is shown since the system operation is highly dependent of the
communication scheme used. The main differences between the Master and Slave operation at
this level, is the serial clock generation which is always done by the Master, and only the Slave
uses the clock control unit. Clock generation must be implemented in software, but the shift
operation is done automatically by both devices. Note that only clocking on negative edge for
shifting data is of practical use in this mode. The slave can insert wait states at start or end of
transfer by forcing the SCL clock low. This means that the Master must always check if the SCL
line was actually released after it has generated a positive edge.
Since the clock also increments the counter, a counter overflow can be used to indicate that the
transfer is completed. The clock is generated by the master by toggling the USCK pin via the
PORT Register.
The data direction is not given by the physical layer. A protocol, like the one used by the TWI-
bus, must be implemented to control the data flow.
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Figure 19-5. Two-wire Mode, Typical Timing Diagram
SDA
1 - 7
8
9
1 - 8
9
1 - 8
9
SCL
S
P
ADDRESS
R/W
ACK
DATA
ACK
DATA
ACK
A
B
C
D
E
F
Referring to the timing diagram (Figure 19-5.), a bus transfer involves the following steps:
1. The a start condition is generated by the Master by forcing the SDA low line while the
SCL line is high (A). SDA can be forced low either by writing a zero to bit 7 of the Shift
Register, or by setting the corresponding bit in the PORT Register to zero. Note that the
Data Direction Register bit must be set to one for the output to be enabled. The slave
device’s start detector logic (Figure 19-6) detects the start condition and sets the USISIF
Flag. The flag can generate an interrupt if necessary.
2. In addition, the start detector will hold the SCL line low after the Master has forced an
negative edge on this line (B). This allows the Slave to wake up from sleep or complete
its other tasks before setting up the Shift Register to receive the address. This is done by
clearing the start condition flag and reset the counter.
3. The Master set the first bit to be transferred and releases the SCL line (C). The Slave
samples the data and shift it into the Serial Register at the positive edge of the SCL
clock.
4. After eight bits are transferred containing slave address and data direction (read or
write), the Slave counter overflows and the SCL line is forced low (D). If the slave is not
the one the Master has addressed, it releases the SCL line and waits for a new start
condition.
5. If the Slave is addressed it holds the SDA line low during the acknowledgment cycle
before holding the SCL line low again (i.e., the Counter Register must be set to 14 before
releasing SCL at (D)). Depending of the R/W bit the Master or Slave enables its output. If
the bit is set, a master read operation is in progress (i.e., the slave drives the SDA line)
The slave can hold the SCL line low after the acknowledge (E).
6. Multiple bytes can now be transmitted, all in same direction, until a stop condition is given
by the Master (F). Or a new start condition is given.
If the Slave is not able to receive more data it does not acknowledge the data byte it has last
received. When the Master does a read operation it must terminate the operation by force the
acknowledge bit low after the last byte transmitted.
Figure 19-6. Start Condition Detector, Logic Diagram
USISIF
CLOCK
HOLD
D Q
D Q
SDA
CLR
CLR
SCL
Write( USISIF)
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19.3.5
Start Condition Detector
The start condition detector is shown in Figure 19-6 The SDA line is delayed (in the range of 50
to 300 ns) to ensure valid sampling of the SCL line. The start condition detector is only enabled
in Two-wire mode.
The start condition detector is working asynchronously and can therefore wake up the processor
from the Power-down sleep mode. However, the protocol used might have restrictions on the
SCL hold time. Therefore, when using this feature in this case the Oscillator start-up time set by
the CKSEL Fuses (see ”Clock Systems and their Distribution” on page 29) must also be taken
into the consideration. Refer to the USISIF bit description on page 199 for further details.
19.3.6
Clock speed considerations.
Maximum frequency for SCL and SCK is fCK /4. This is also the maximum data transmit and
receieve rate in both two- and three-wire mode. In two-wire slave mode the Two-wire Clock Con-
trol Unit will hold the SCL low until the slave is ready to receive more data. This may reduce the
actual data rate in two-wire mode.
19.4 Alternative USI Usage
When the USI unit is not used for serial communication, it can be set up to do alternative tasks
due to its flexible design.
19.4.1
19.4.2
19.4.3
19.4.4
Half-duplex Asynchronous Data Transfer
By utilizing the Shift Register in Three-wire mode, it is possible to implement a more compact
and higher performance UART than by software only.
4-bit Counter
The 4-bit counter can be used as a stand-alone counter with overflow interrupt. Note that if the
counter is clocked externally, both clock edges will generate an increment.
12-bit Timer/Counter
Combining the USI 4-bit counter and Timer/Counter0 allows them to be used as a 12-bit
counter.
Edge Triggered External Interrupt
By setting the counter to maximum value (F) it can function as an additional external interrupt.
The Overflow Flag and Interrupt Enable bit are then used for the external interrupt. This feature
is selected by the USICS1 bit.
19.4.5
Software Interrupt
The counter overflow interrupt can be used as a software interrupt triggered by a clock strobe.
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19.5 Register Descriptions
19.5.1
USIDR – USI Data Register
Bit
7
MSB
R/W
0
6
5
4
3
2
1
0
(0xBA)
LSB
R/W
0
USIDR
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
The USI uses no buffering of the Serial Register, i.e., when accessing the Data Register
(USIDR) the Serial Register is accessed directly. If a serial clock occurs at the same cycle the
register is written, the register will contain the value written and no shift is performed. A (left) shift
operation is performed depending of the USICS1:0 bits setting. The shift operation can be con-
trolled by an external clock edge, by a Timer/Counter0 Compare Match, or directly by software
using the USICLK strobe bit. Note that even when no wire mode is selected (USIWM1:0 = 0)
both the external data input (DI/SDA) and the external clock input (USCK/SCL) can still be used
by the Shift Register.
The output pin in use, DO or SDA depending on the wire mode, is connected via the output latch
to the most significant bit (bit 7) of the Data Register. The output latch is open (transparent) dur-
ing the first half of a serial clock cycle when an external clock source is selected (USICS1 = 1),
and constantly open when an internal clock source is used (USICS1 = 0). The output will be
changed immediately when a new MSB written as long as the latch is open. The latch ensures
that data input is sampled and data output is changed on opposite clock edges.
Note that the corresponding Data Direction Register to the pin must be set to one for enabling
data output from the Shift Register.
19.5.2
USISR – USI Status Register
Bit
7
6
USIOIF
R/W
0
5
USIPF
R/W
0
4
USIDC
R
3
USICNT3
R/W
2
USICNT2
R/W
1
USICNT1
R/W
0
USICNT0
R/W
USISIF
R/W
0
USISR
(0xB9)
Read/Write
Initial Value
0
0
0
0
0
The Status Register contains Interrupt Flags, line Status Flags and the counter value.
• Bit 7 – USISIF: Start Condition Interrupt Flag
When Two-wire mode is selected, the USISIF Flag is set (to one) when a start condition is
detected. When output disable mode or Three-wire mode is selected, the flag is set when the 4-
bit counter is incremented.
An interrupt will be generated when the flag is set while the USISIE bit in USICR and the Global
Interrupt Enable Flag are set. The flag will only be cleared by writing a logical one to the USISIF
bit. Clearing this bit will release the start detection hold of USCL in Two-wire mode. A start con-
dition interrupt will wake up the processor from all sleep modes.
• Bit 6 – USIOIF: Counter Overflow Interrupt Flag
This flag is set (one) when the 4-bit counter overflows (i.e., at the transition from 15 to 0). An
interrupt will be generated when the flag is set while the USIOIE bit in USICR and the Global
Interrupt Enable Flag are set. The flag will only be cleared if a one is written to the USIOIF bit.
Clearing this bit will release the counter overflow hold of SCL in Two-wire mode. A counter over-
flow interrupt will wake up the processor from Idle sleep mode.
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• Bit 5 – USIPF: Stop Condition Flag
When Two-wire mode is selected, the USIPF Flag is set (one) when a stop condition is detected.
The flag is cleared by writing a one to this bit. Note that this is not an Interrupt Flag. This signal is
useful when implementing Two-wire bus master arbitration.
• Bit 4 – USIDC: Data Output Collision
This bit is logical one when bit 7 in the Shift Register differs from the physical pin value. The flag
is only valid when Two-wire mode is used. This signal is useful when implementing Two-wire
bus master arbitration.
• Bits 3:0 – USICNT3:0: Counter Value
These bits reflect the current 4-bit counter value. The 4-bit counter value can directly be read or
written by the CPU.
The 4-bit counter increments by one for each clock generated either by the external clock edge
detector, by a Timer/Counter0 Compare Match, or by software using USICLK or USITC strobe
bits. The clock source depends of the setting of the USICS1:0 bits. For external clock operation
a special feature is added that allows the clock to be generated by writing to the USITC strobe
bit. This feature is enabled by write a one to the USICLK bit while setting an external clock
source (USICS1 = 1).
Note that even when no wire mode is selected (USIWM1:0 = 0) the external clock input
(USCK/SCL) are can still be used by the counter.
19.5.3
USICR – USI Control Register
Bit
7
6
USIOIE
R/W
0
5
USIWM1
R/W
4
USIWM0
R/W
3
USICS1
R/W
0
2
USICS0
R/W
0
1
0
USITC
W
(0xB8)
USISIE
R/W
0
USICLK
USICR
Read/Write
Initial Value
W
0
0
0
0
The Control Register includes interrupt enable control, wire mode setting, Clock Select setting,
and clock strobe.
• Bit 7 – USISIE: Start Condition Interrupt Enable
Setting this bit to one enables the Start Condition detector interrupt. If there is a pending inter-
rupt when the USISIE and the Global Interrupt Enable Flag is set to one, this will immediately be
executed. Refer to the USISIF bit description on page 199 for further details.
• Bit 6 – USIOIE: Counter Overflow Interrupt Enable
Setting this bit to one enables the Counter Overflow interrupt. If there is a pending interrupt when
the USIOIE and the Global Interrupt Enable Flag is set to one, this will immediately be executed.
Refer to the USIOIF bit description on page 199 for further details.
• Bit 5:4 – USIWM1:0: Wire Mode
These bits set the type of wire mode to be used. Basically only the function of the outputs are
affected by these bits. Data and clock inputs are not affected by the mode selected and will
always have the same function. The counter and Shift Register can therefore be clocked exter-
nally, and data input sampled, even when outputs are disabled. The relations between
USIWM1:0 and the USI operation is summarized in Table 19-1.
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Table 19-1. Relations between USIWM1:0 and the USI Operation
USIWM1
USIWM0
Description
Outputs, clock hold, and start detector disabled. Port pins operates as
normal.
0
0
Three-wire mode. Uses DO, DI, and USCK pins.
The Data Output (DO) pin overrides the corresponding bit in the PORT
Register in this mode. However, the corresponding DDR bit still controls the
data direction. When the port pin is set as input the pins pull-up is controlled
by the PORT bit.
0
1
The Data Input (DI) and Serial Clock (USCK) pins do not affect the normal
port operation. When operating as master, clock pulses are software
generated by toggling the PORT Register, while the data direction is set to
output. The USITC bit in the USICR Register can be used for this purpose.
Two-wire mode. Uses SDA (DI) and SCL (USCK) pins(1)
.
The Serial Data (SDA) and the Serial Clock (SCL) pins are bi-directional and
uses open-collector output drives. The output drivers are enabled by setting
the corresponding bit for SDA and SCL in the DDR Register.
When the output driver is enabled for the SDA pin, the output driver will force
the line SDA low if the output of the Shift Register or the corresponding bit in
the PORT Register is zero. Otherwise the SDA line will not be driven (i.e., it is
released). When the SCL pin output driver is enabled the SCL line will be
forced low if the corresponding bit in the PORT Register is zero, or by the start
detector. Otherwise the SCL line will not be driven.
1
0
The SCL line is held low when a start detector detects a start condition and
the output is enabled. Clearing the Start Condition Flag (USISIF) releases the
line. The SDA and SCL pin inputs is not affected by enabling this mode. Pull-
ups on the SDA and SCL port pin are disabled in Two-wire mode.
Two-wire mode. Uses SDA and SCL pins.
Same operation as for the Two-wire mode described above, except that the
SCL line is also held low when a counter overflow occurs, and is held low until
the Counter Overflow Flag (USIOIF) is cleared.
1
1
Note:
1. The I and USCK pins are renamed to Serial Data (SDA) and Serial Clock (SCL) respectively to
avoid confusion between the modes of operation.
• Bit 3:2 – USICS1:0: Clock Source Select
These bits set the clock source for the Shift Register and counter. The data output latch ensures
that the output is changed at the opposite edge of the sampling of the data input (DI/SDA) when
using external clock source (USCK/SCL). When software strobe or Timer/Counter0 Compare
Match clock option is selected, the output latch is transparent and therefore the output is
changed immediately. Clearing the USICS1:0 bits enables software strobe option. When using
this option, writing a one to the USICLK bit clocks both the Shift Register and the counter. For
external clock source (USICS1 = 1), the USICLK bit is no longer used as a strobe, but selects
between external clocking and software clocking by the USITC strobe bit.
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Table 19-2 shows the relationship between the USICS1:0 and USICLK setting and clock source
used for the Shift Register and the 4-bit counter.
Table 19-2. Relations between the USICS1:0 and USICLK Setting
USICS1
USICS0
USICLK
Shift Register Clock Source
4-bit Counter Clock Source
0
0
0
No Clock
No Clock
Software clock strobe
(USICLK)
Software clock strobe
(USICLK)
0
0
0
1
1
Timer/Counter0 Compare
Match
Timer/Counter0 Compare
Match
X
1
1
1
1
0
1
0
1
0
0
1
1
External, positive edge
External, negative edge
External, positive edge
External, negative edge
External, both edges
External, both edges
Software clock strobe (USITC)
Software clock strobe (USITC)
• Bit 1 – USICLK: Clock Strobe
Writing a one to this bit location strobes the Shift Register to shift one step and the counter to
increment by one, provided that the USICS1:0 bits are set to zero and by doing so the software
clock strobe option is selected. The output will change immediately when the clock strobe is exe-
cuted, i.e., in the same instruction cycle. The value shifted into the Shift Register is sampled the
previous instruction cycle. The bit will be read as zero.
When an external clock source is selected (USICS1 = 1), the USICLK function is changed from
a clock strobe to a Clock Select Register. Setting the USICLK bit in this case will select the
USITC strobe bit as clock source for the 4-bit counter (see Table 19-2).
• Bit 0 – USITC: Toggle Clock Port Pin
Writing a one to this bit location toggles the USCK/SCL value either from 0 to 1, or from 1 to 0.
The toggling is independent of the setting in the Data Direction Register, but if the PORT value is
to be shown on the pin the DDRE4 must be set as output (to one). This feature allows easy clock
generation when implementing master devices. The bit will be read as zero.
When an external clock source is selected (USICS1 = 1) and the USICLK bit is set to one, writ-
ing to the USITC strobe bit will directly clock the 4-bit counter. This allows an early detection of
when the transfer is done when operating as a master device.
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20. Analog Comparator
20.1 Overview
The Analog Comparator compares the input values on the positive pin AIN0 and negative pin
AIN1. When the voltage on the positive pin AIN0 is higher than the voltage on the negative pin
AIN1, the Analog Comparator output, ACO, is set. The comparator’s output can be set to trigger
the Timer/Counter1 Input Capture function. In addition, the comparator can trigger a separate
interrupt, exclusive to the Analog Comparator. The user can select Interrupt triggering on com-
parator output rise, fall or toggle. A block diagram of the comparator and its surrounding logic is
shown in Figure 20-1.
The PRADC, in ”PRR – Power Reduction Register” on page 44 must be written to zero to use
the ADC input MUX.
Figure 20-1. Analog Comparator Block Diagram(2)
BANDGAP
REFERENCE
ACBG
ACME
ADEN
ADC MULTIPLEXER
OUTPUT(1)
Note:
1. See Table 20-1 on page 203.
2. Refer to Figure 1-1 on page 2, Figure 1-2 on page 3and ”Alternate Functions of Port E” on
page 75 for Analog Comparator pin placement.
20.2 Analog Comparator Multiplexed Input
It is possible to select any of the ADC7:0 pins to replace the negative input to the Analog Com-
parator. The ADC multiplexer is used to select this input, and consequently, the ADC must be
switched off to utilize this feature. If the Analog Comparator Multiplexer Enable bit (ACME in
ADCSRB) 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
20-1. If ACME is cleared or ADEN is set, AIN1 is applied to the negative input to the Analog
Comparator.
Table 20-1. Analog Comparator Multiplexed Input
ACME
ADEN
MUX2:0
xxx
Analog Comparator Negative Input
0
1
1
x
1
0
AIN1
AIN1
ADC0
xxx
000
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Table 20-1. Analog Comparator Multiplexed Input (Continued)
ACME
ADEN
MUX2:0
001
Analog Comparator Negative Input
1
1
1
1
1
1
1
0
0
0
0
0
0
0
ADC1
ADC2
ADC3
ADC4
ADC5
ADC6
ADC7
010
011
100
101
110
111
20.3 Register Description
20.3.1
ADCSRB – ADC Control and Status Register B
Bit
7
–
6
ACME
R/W
0
5
–
4
–
3
–
2
ADTS2
R/W
0
1
ADTS1
R/W
0
0
ADTS0
R/W
0
(0x7B)
ADCSRB
Read/Write
Initial Value
R
0
R
0
R
0
R
0
• Bit 6 – ACME: Analog Comparator Multiplexer Enable
When this bit is written logic one and the ADC is switched off (ADEN in ADCSRA is zero), the
ADC multiplexer selects the negative input to the Analog Comparator. When this bit is written
logic zero, AIN1 is applied to the negative input of the Analog Comparator. For a detailed
description of this bit, see ”Analog Comparator Multiplexed Input” on page 203.
20.3.2
ACSR – Analog Comparator Control and Status Register
Bit
0x30 (0x50)
7
6
5
4
3
ACIE
R/W
0
2
ACIC
R/W
0
1
ACIS1
R/W
0
0
ACIS0
R/W
0
ACD
ACBG
ACO
ACI
R/W
0
ACSR
Read/Write
Initial Value
R/W
0
R/W
0
R
N/A
• Bit 7 – ACD: Analog Comparator Disable
When this bit is written logic one, the power to the Analog Comparator is switched off. This bit
can be set at any time to turn off the Analog Comparator. This will reduce power consumption in
Active and Idle mode. When changing the ACD bit, the Analog Comparator Interrupt must be
disabled by clearing the ACIE bit in ACSR. Otherwise an interrupt can occur when the bit is
changed.
• Bit 6 – ACBG: Analog Comparator Bandgap Select
When this bit is set, a fixed bandgap reference voltage replaces the positive input to the Analog
Comparator. When this bit is cleared, AIN0 is applied to the positive input of the Analog Compar-
ator. When the bandgap reference is used as input to the analog comparator, it will take a
certain time for the voltage to stabilize. If not stabilized, the the first converison may give a wrong
value. See Section “10.3” on page 48.
• 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.
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• Bit 4 – ACI: Analog Comparator Interrupt Flag
This bit is set by hardware when a comparator output event triggers the interrupt mode defined
by ACIS1 and ACIS0. The Analog Comparator interrupt routine is executed if the ACIE bit is set
and the I-bit in SREG is set. ACI is cleared by hardware when executing the corresponding inter-
rupt handling vector. Alternatively, ACI is cleared by writing a logic one to the flag.
• Bit 3 – ACIE: Analog Comparator Interrupt Enable
When the ACIE bit is written logic one and the I-bit in the Status Register is set, the Analog Com-
parator interrupt is activated. When written logic zero, the interrupt is disabled.
• Bit 2 – ACIC: Analog Comparator Input Capture Enable
When written logic one, this bit enables the Input Capture function in Timer/Counter1 to be trig-
gered by the Analog Comparator. The comparator output is in this case directly connected to the
Input Capture front-end logic, making the comparator utilize the noise canceler and edge select
features of the Timer/Counter1 Input Capture interrupt. When written logic zero, no connection
between the Analog Comparator and the Input Capture function exists. To make the comparator
trigger the Timer/Counter1 Input Capture interrupt, the ICIE1 bit in the Timer Interrupt Mask
Register (TIMSK1) must be set.
• Bits 1, 0 – ACIS1, ACIS0: Analog Comparator Interrupt Mode Select
These bits determine which comparator events that trigger the Analog Comparator interrupt. The
different settings are shown in Table 20-2.
Table 20-2. ACIS1/ACIS0 Settings
ACIS1
ACIS0
Interrupt Mode
0
0
1
1
0
1
0
1
Comparator Interrupt on Output Toggle.
Reserved
Comparator Interrupt on Falling Output Edge.
Comparator Interrupt on Rising Output Edge.
When changing the ACIS1/ACIS0 bits, the Analog Comparator Interrupt must be disabled by
clearing its Interrupt Enable bit in the ACSR Register. Otherwise an interrupt can occur when the
bits are changed.
20.3.3
DIDR1 – Digital Input Disable Register 1
Bit
(0x7F)
7
6
5
–
4
–
3
–
2
–
1
AIN1D
R/W
0
0
AIN0D
R/W
0
–
–
R
0
DIDR1
Read/Write
Initial Value
R
0
R
0
R
0
R
0
R
0
• Bit 1, 0 – AIN1D, AIN0D: AIN1, AIN0 Digital Input Disable
When this bit is written logic one, the digital input buffer on the AIN1/0 pin is disabled. The corre-
sponding PIN Register bit will always read as zero when this bit is set. When an analog signal is
applied to the AIN1/0 pin and the digital input from this pin is not needed, this bit should be writ-
ten logic one to reduce power consumption in the digital input buffer.
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21. Analog to Digital Converter
21.1 Features
• 10-bit Resolution
• 0.5 LSB Integral Non-linearity
2 LSB Absolute Accuracy
•
• 13 µs - 260 µs Conversion Time (50 kHz to 1 MHz ADC clock)
• Up to 76.9 kSPS at Maximum Resolution (200 kHz ADC clock)
• Eight Multiplexed Single Ended Input Channels
• Optional Left Adjustment for ADC Result Readout
• 0 - VCC ADC Input Voltage Range
• Selectable 1.1V ADC Reference Voltage
• Free Running or Single Conversion Mode
• ADC Start Conversion by Auto Triggering on Interrupt Sources
• Interrupt on ADC Conversion Complete
• Sleep Mode Noise Canceler
21.2 Overview
The ATmega325P/3250P features a 10-bit successive approximation ADC. The ADC is con-
nected to an 8-channel Analog Multiplexer which allows eight single-ended voltage inputs
constructed from the pins of Port F. The single-ended voltage inputs refer to 0V (GND).
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 21-1.
The ADC has a separate analog supply voltage pin, AVCC. AVCC must not differ more than
0.3V from VCC. See the paragraph ”ADC Noise Canceler” on page 212 on how to connect this
pin.
Internal reference voltages of nominally 1.1V or AVCC are provided On-chip. The voltage refer-
ence may be externally decoupled at the AREF pin by a capacitor for better noise performance.
The PRADC, in ”PRR – Power Reduction Register” on page 44 must be written to zero to enable
the ADC module.
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Figure 21-1. Analog to Digital Converter Block Schematic
ADC CONVERSION
COMPLETE IRQ
INTERRUPT
FLAGS
ADTS[2:0]
8-BIT DATA BUS
15
0
ADC MULTIPLEXER
SELECT (ADMUX)
ADC CTRL. & STATUS
REGISTER (ADCSRA)
ADC DATA REGISTER
(ADCH/ADCL)
TRIGGER
SELECT
MUX DECODER
PRESCALER
START
CONVERSION LOGIC
AVCC
INTERNAL
REFERENCE
SAMPLE & HOLD
COMPARATOR
AREF
GND
10-BIT DAC
-
+
BANDGAP
REFERENCE
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADC1
ADC0
SINGLE ENDED / DIFFERENTIAL SELECTION
POS.
INPUT
MUX
ADC MULTIPLEXER
OUTPUT
DIFFERENTIAL
AMPLIFIER
+
-
NEG.
INPUT
MUX
21.3 Operation
The ADC converts an analog input voltage to a 10-bit digital value through successive approxi-
mation. The minimum value represents GND and the maximum value represents the voltage on
the AREF pin minus 1 LSB. Optionally, AVCC or an internal 1.1V reference voltage may be con-
nected 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. Volt-
age 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 conversion is lost. When ADCH is read,
ADC access to the ADCH and ADCL Registers is re-enabled.
The ADC has its own interrupt which can be triggered when a conversion completes. When ADC
access to the Data Registers is prohibited between reading of ADCH and ADCL, the interrupt
will trigger even if the result is lost.
21.4 Starting a Conversion
A single conversion is started by writing a logical one to the ADC Start Conversion bit, ADSC.
This bit stays high as long as the conversion is in progress and will be cleared by hardware
when the conversion is completed. If a different data channel is selected while a conversion is in
progress, the ADC will finish the current conversion before performing the channel change.
Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is
enabled by setting the ADC Auto Trigger Enable bit, ADATE in ADCSRA. The trigger source is
selected by setting the ADC Trigger Select bits, ADTS in ADCSRB (See description of the ADTS
bits for a list of the trigger sources). When a positive edge occurs on the selected trigger signal,
the ADC prescaler is reset and a conversion is started. This provides a method of starting con-
versions at fixed intervals. If the trigger signal still is set when the conversion completes, a new
conversion will not be started. If another positive edge occurs on the trigger signal during con-
version, the edge will be ignored. Note that an Interrupt Flag will be set even if the specific
interrupt is disabled or the Global Interrupt Enable bit in SREG is cleared. A conversion can thus
be triggered without causing an interrupt. However, the Interrupt Flag must be cleared in order to
trigger a new conversion at the next interrupt event.
Figure 21-2. ADC Auto Trigger Logic
ADTS[2:0]
PRESCALER
CLKADC
START
ADIF
ADATE
SOURCE 1
.
.
.
.
CONVERSION
LOGIC
EDGE
DETECTOR
SOURCE n
ADSC
Using the ADC Interrupt Flag as a trigger source makes the ADC start a new conversion as soon
as the ongoing conversion has finished. The ADC then operates in Free Running mode, con-
stantly sampling and updating the ADC Data Register. The first conversion must be started by
writing a logical one to the ADSC bit in ADCSRA. In this mode the ADC will perform successive
conversions independently of whether the ADC Interrupt Flag, ADIF is cleared or not.
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If Auto Triggering is enabled, single conversions can be started by writing ADSC in ADCSRA to
one. ADSC can also be used to determine if a conversion is in progress. The ADSC bit will be
read as one during a conversion, independently of how the conversion was started.
21.5 Prescaling and Conversion Timing
Figure 21-3. ADC Prescaler
ADEN
START
Reset
7-BIT ADC PRESCALER
CK
ADPS0
ADPS1
ADPS2
ADC CLOCK SOURCE
By default, the successive approximation circuitry requires an input clock frequency between 50
kHz and 200 kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the
input clock frequency to the ADC can be higher than 200 kHz to get a higher sample rate.
The ADC module contains a prescaler, which generates an acceptable ADC clock frequency
from any CPU frequency above 100 kHz. The prescaling is set by the ADPS bits in ADCSRA.
The prescaler starts counting from the moment the ADC is switched on by setting the ADEN bit
in ADCSRA. The prescaler keeps running for as long as the ADEN bit is set, and is continuously
reset when ADEN is low.
When initiating a single ended conversion by setting the ADSC bit in ADCSRA, the conversion
starts at the following rising edge of the ADC clock cycle.
A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched
on (ADEN in ADCSRA is set) takes 25 ADC clock cycles in order to initialize the analog circuitry.
When the bandgap reference voltage is used as input to the ADC, it will take a certain time for
the voltage to stabilize. If not stabilized the first value read after the first conversion may be
wrong.
The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conver-
sion and 13.5 ADC clock cycles after the start of an first conversion. When a conversion is
complete, the result is written to the ADC Data Registers, and ADIF is set. In Single Conversion
mode, ADSC is cleared simultaneously. The software may then set ADSC again, and a new
conversion will be initiated on the first rising ADC clock edge.
When Auto Triggering is used, the prescaler is reset when the trigger event occurs. This assures
a fixed delay from the trigger event to the start of conversion. In this mode, the sample-and-hold
takes place two ADC clock cycles after the rising edge on the trigger source signal. Three addi-
tional CPU clock cycles are used for synchronization logic. When using Differential mode, along
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with Auto triggering from a source other than the ADC Conversion Complete, each conversion
will require 25 ADC clocks. This is because the ADC must be disabled and re-enabled after
every conversion.
In Free Running mode, a new conversion will be started immediately after the conversion com-
pletes, while ADSC remains high. For a summary of conversion times, see Table 21-1.
Figure 21-4. ADC Timing Diagram, First Conversion (Single Conversion Mode)
Next
First Conversion
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
Sign and MSB of Result
LSB of Result
ADCH
ADCL
MUX and REFS
Update
Conversion
Complete
MUX and REFS
Update
Sample & Hold
Figure 21-5. 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
Sign and MSB of Result
LSB of Result
ADCL
Sample & Hold
Conversion
Complete
MUX and REFS
Update
MUX and REFS
Update
Figure 21-6. ADC Timing Diagram, Auto Triggered Conversion
One Conversion
Next Conversion
1
2
3
4
5
6
7
8
9
10
11
12
13
1
2
Cycle Number
ADC Clock
Trigger
Source
ADATE
ADIF
ADCH
ADCL
Sign and MSB of Result
LSB of Result
Sample &
Hold
Prescaler
Reset
Conversion
Complete
Prescaler
Reset
MUX and REFS
Update
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Figure 21-7. 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
Sign and MSB of Result
LSB of Result
Sample & Hold
Conversion
Complete
MUX and REFS
Update
Table 21-1. ADC Conversion Time
Sample & Hold (Cycles from
Start of Conversion)
Condition
Conversion Time (Cycles)
First conversion
13.5
1.5
2
25
13
Normal conversions, single ended
Auto Triggered conversions
13.5
21.6 Changing Channel or Reference Selection
The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a temporary
register to which the CPU has random access. This ensures that the channels and reference
selection only takes place at a safe point during the conversion. The channel and reference
selection is continuously updated until a conversion is started. Once the conversion starts, the
channel and reference selection is locked to ensure a sufficient sampling time for the ADC. Con-
tinuous updating resumes in the last ADC clock cycle before the conversion completes (ADIF in
ADCSRA is set). Note that the conversion starts on the following rising ADC clock edge after
ADSC is written. The user is thus advised not to write new channel or reference selection values
to ADMUX until one ADC clock cycle after ADSC is written.
If Auto Triggering is used, the exact time of the triggering event can be indeterministic. Special
care must be taken when updating the ADMUX Register, in order to control which conversion
will be affected by the new settings.
If both ADATE and ADEN is written to one, an interrupt event can occur at any time. If the
ADMUX Register is changed in this period, the user cannot tell if the next conversion is based
on the old or the new settings. ADMUX can be safely updated in the following ways:
1. When ADATE or ADEN is cleared.
2. During conversion, minimum one ADC clock cycle after the trigger event.
3. After a conversion, before the Interrupt Flag used as trigger source is cleared.
When updating ADMUX in one of these conditions, the new settings will affect the next ADC
conversion.
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21.6.1
ADC Input Channels
When changing channel selections, the user should observe the following guidelines to ensure
that the correct channel is selected:
In Single Conversion mode, always select the channel before starting the conversion. The chan-
nel selection may be changed one ADC clock cycle after writing one to ADSC. However, the
simplest method is to wait for the conversion to complete before changing the channel selection.
In Free Running mode, always select the channel before starting the first conversion. The chan-
nel selection may be changed one ADC clock cycle after writing one to ADSC. However, the
simplest method is to wait for the first conversion to complete, and then change the channel
selection. Since the next conversion has already started automatically, the next result will reflect
the previous channel selection. Subsequent conversions will reflect the new channel selection.
21.6.2
ADC Voltage Reference
The reference voltage for the ADC (VREF) indicates the conversion range for the ADC. Single
ended channels that exceed VREF will result in codes close to 0x3FF. VREF can be selected as
either AVCC, internal 1.1V reference, or external AREF pin.
AVCC is connected to the ADC through a passive switch. The internal 1.1V reference is gener-
ated from the internal bandgap reference (VBG) through an internal buffer. 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. VREF can
also be measured at the AREF pin with a high impedant voltmeter. Note that VREF is a high
impedant source, and only a capacitive load should be connected in a system.
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage. If no
external voltage is applied to the AREF pin, the user may switch between AVCC and 1.1V as
reference selection. The first ADC conversion result after switching reference voltage source
may be inaccurate, and the user is advised to discard this result.
21.7 ADC Noise Canceler
The ADC features a noise canceler that enables conversion during sleep mode to reduce noise
induced from the CPU core and other I/O peripherals. The noise canceler can be used with ADC
Noise Reduction and Idle mode. To make use of this feature, the following procedure should be
used:
1. Make sure that the ADC is enabled and is not busy converting. Single Conversion mode
must be selected and the ADC conversion complete interrupt must be enabled.
2. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once
the CPU has been halted.
3. If no other interrupts occur before the ADC conversion completes, the ADC interrupt will
wake up the CPU and execute the ADC Conversion Complete interrupt routine. If another
interrupt wakes up the CPU before the ADC conversion is complete, that interrupt will be
executed, and an ADC Conversion Complete interrupt request will be generated when
the ADC conversion completes. The CPU will remain in active mode until a new sleep
command is executed.
Note that the ADC will not be automatically turned off when entering other sleep modes than Idle
mode and ADC Noise Reduction mode. The user is advised to write zero to ADEN before enter-
ing such sleep modes to avoid excessive power consumption.
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21.7.1
Analog Input Circuitry
The analog input circuitry for single ended channels is illustrated in Figure 21-8. An analog
source applied to ADCn is subjected to the pin capacitance and input leakage of that pin, regard-
less of whether that channel is selected as input for the ADC. When the channel is selected, the
source must drive the S/H capacitor through the series resistance (combined resistance in the
input path).
The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or
less. If such a source is used, the sampling time will be negligible. If a source with higher imped-
ance is used, the sampling time will depend on how long time the source needs to charge the
S/H capacitor, with can vary widely. The user is recommended to only use low impedant sources
with slowly varying signals, since this minimizes the required charge transfer to the S/H
capacitor.
Signal components higher than the Nyquist frequency (fADC/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 21-8. Analog Input Circuitry
IIH
ADCn
1..100 kΩ
CS/H= 14 pF
IIL
VCC/2
21.7.2
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 VCC supply voltage via an
LC network as shown in Figure 21-9.
3. Use the ADC noise canceler function to reduce induced noise from the CPU.
4. If any ADC port pins are used as digital outputs, it is essential that these do not switch
while a conversion is in progress.
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Figure 21-9. ADC Power Connections
PA0 51
VCC
52
GND
53
(ADC7) PF7 54
(ADC6) PF6 55
(ADC5) PF5 56
(ADC4) PF4 57
(ADC3) PF3 58
(ADC2) PF2 59
(ADC1) PF1
(ADC0) PF0
60
61
10μΗ
62
63
64
AREF
GND
AVCC
100nF
1
Analog Ground Plane
21.7.3
ADC Accuracy Definitions
An n-bit single-ended ADC converts a voltage linearly between GND and VREF in 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 21-10. Offset Error
Output Code
Ideal ADC
Actual ADC
Offset
Error
VREF
Input Voltage
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• 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 21-11. Gain Error
Gain
Error
Output Code
Ideal ADC
Actual ADC
VREF
Input Voltage
• Integral Non-linearity (INL): After adjusting for offset and gain error, the INL is the maximum
deviation of an actual transition compared to an ideal transition for any code. Ideal value: 0
LSB.
Figure 21-12. Integral Non-linearity (INL)
Output Code
Ideal ADC
Actual ADC
V
Input Voltage
REF
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• Differential Non-linearity (DNL): The maximum deviation of the actual code width (the interval
between two adjacent transitions) from the ideal code width (1 LSB). Ideal value: 0 LSB.
Figure 21-13. Differential Non-linearity (DNL)
Output Code
0x3FF
1 LSB
DNL
0x000
0
VREF Input 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.
21.8 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 VIN is the voltage on the selected input pin and VREF the selected voltage reference (see
Table 6 on page 218 and Table 21-3 on page 219). 0x000 represents analog ground, and 0x3FF
represents the selected reference voltage minus one LSB.
(V
– V
) ⋅ 512
NEG
POS
ADC = ----------------------------------------------------
V
REF
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Figure 21-14. Differential Measurement Range
Output Code
0x1FF
0x000
0
Differential Input
VREF
- VREF
0x3FF
Voltage (Volts)
0x200
Table 21-2. Correlation Between Input Voltage and Output Codes
VADCn
Read Code
0x1FF
0x1FF
0x1FE
...
Corresponding Decimal Value
VADCm + VREF
511
511
510
...
511
VADCm
VADCm
...
+
+
/
512 VREF
510
/512 VREF
VADCm + 1/512 VREF
0x001
0x000
0x3FF
...
1
VADCm
0
VADCm - 1/512 VREF
-1
...
...
511
VADCm
-
/
512 VREF
0x201
0x200
-511
-512
VADCm - VREF
ADMUX = 0xFB (ADC3 - ADC2, 1.1V reference, left adjusted result)
Voltage on ADC3 is 300 mV, voltage on ADC2 is 500 mV.
ADCR = 512 * (300 - 500) / 1100 = -93 = 0x3A3.
ADCL will thus read 0xC0, and ADCH will read 0xD8. Writing zero to ADLAR right adjusts the
result: ADCL = 0xA3, ADCH = 0x03.
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21.9 Register Description
21.9.1
ADMUX – ADC Multiplexer Selection Register
Bit
7
REFS1
R/W
0
6
REFS0
R/W
0
5
ADLAR
R/W
0
4
MUX4
R/W
0
3
MUX3
R/W
0
2
MUX2
R/W
0
1
MUX1
R/W
0
0
MUX0
R/W
0
(0x7C)
ADMUX
Read/Write
Initial Value
• Bit 7:6 – REFS1:0: Reference Selection Bits
These bits select the voltage reference for the ADC, as shown in Table 6. If these bits are
changed during a conversion, the change will not go in effect until this conversion is complete
(ADIF in ADCSRA is set). The internal voltage reference options may not be used if an external
reference voltage is being applied to the AREF pin.
Table 6. Voltage Reference Selections for ADC
REFS1
REFS0
Voltage Reference Selection
0
0
1
1
0
1
0
1
AREF, Internal Vref turned off
AVCC with external capacitor at AREF pin
Reserved
Internal 1.1V Voltage Reference with external capacitor at AREF pin
•
Bit 5 – ADLAR: ADC Left Adjust Result
The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register.
Write one to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the
ADLAR bit will affect the ADC Data Register immediately, regardless of any ongoing conver-
sions. For a complete description of this bit, see ”ADCL and ADCH – The ADC Data Register” on
page 221.
• Bits 4:0 – MUX4:0: Analog Channel Selection Bits
The value of these bits selects which combination of analog inputs are connected to the ADC.
See Table 21-3 on page 219 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).
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Table 21-3. Input Channel Selections
MUX4:0
00000
00001
00010
00011
00100
00101
00110
00111
01000
01001
01010
01011
01100
01101
01110
01111
10000
10001
10010
10011
10100
10101
10110
10111
11000
11001
11010
11011
11100
11101
11110
11111
Single Ended Input
ADC0
Positive Differential Input
Negative Differential Input
ADC1
ADC2
ADC3
ADC4
ADC5
ADC6
ADC7
N/A
ADC0
ADC1
ADC2
ADC3
ADC4
ADC5
ADC6
ADC7
ADC0
ADC1
ADC2
ADC3
ADC4
ADC5
ADC1
ADC1
ADC1
ADC1
ADC1
ADC1
ADC1
ADC1
ADC2
ADC2
ADC2
ADC2
ADC2
ADC2
N/A
1.1V (VBG
)
N/A
0V (GND)
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21.9.2
ADCSRA – ADC Control and Status Register A
Bit
7
ADEN
R/W
0
6
ADSC
R/W
0
5
ADATE
R/W
0
4
ADIF
R/W
0
3
ADIE
R/W
0
2
ADPS2
R/W
0
1
ADPS1
R/W
0
0
ADPS0
R/W
0
(0x7A)
ADCSRA
Read/Write
Initial Value
• Bit 7 – ADEN: ADC Enable
Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the
ADC off while a conversion is in progress, will terminate this conversion.
• Bit 6 – ADSC: ADC Start Conversion
In Single Conversion mode, write this bit to one to start each conversion. In Free Running mode,
write this bit to one to start the first conversion. The first conversion after ADSC has been written
after the ADC has been enabled, or if ADSC is written at the same time as the ADC is enabled,
will take 25 ADC clock cycles instead of the normal 13. This first conversion performs initializa-
tion of the ADC.
ADSC will read as one as long as a conversion is in progress. When the conversion is complete,
it returns to zero. Writing zero to this bit has no effect.
• Bit 5 – ADATE: ADC Auto Trigger Enable
When this bit is written to one, Auto Triggering of the ADC is enabled. The ADC will start a con-
version on a positive edge of the selected trigger signal. The trigger source is selected by setting
the ADC Trigger Select bits, ADTS in ADCSRB.
• Bit 4 – ADIF: ADC Interrupt Flag
This bit is set when an ADC conversion completes and the Data Registers are updated. The
ADC Conversion Complete Interrupt is executed if the ADIE bit and the I-bit in SREG are set.
ADIF is cleared by hardware when executing the corresponding interrupt handling vector. Alter-
natively, ADIF is cleared by writing a logical one to the flag. Beware that if doing a Read-Modify-
Write on ADCSRA, a pending interrupt can be disabled. This also applies if the SBI and CBI
instructions are used.
• Bit 3 – ADIE: ADC Interrupt Enable
When this bit is written to one and the I-bit in SREG is set, the ADC Conversion Complete Inter-
rupt is activated.
• Bits 2:0 – ADPS2:0: ADC Prescaler Select Bits
These bits determine the division factor between the XTAL frequency and the input clock to the
ADC.
Table 21-4. ADC Prescaler Selections
ADPS2
ADPS1
ADPS0
Division Factor
0
0
0
0
0
0
1
1
0
1
0
1
2
2
4
8
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Table 21-4. ADC Prescaler Selections (Continued)
ADPS2
ADPS1
ADPS0
Division Factor
1
1
1
1
0
0
1
1
0
1
0
1
16
32
64
128
21.9.3
ADCL and ADCH – The ADC Data Register
21.9.3.1
ADLAR = 0
Bit
15
14
13
12
11
10
9
8
(0x79)
(0x78)
–
–
–
–
–
–
ADC9
ADC8
ADCH
ADCL
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADC1
ADC0
7
R
R
0
6
R
R
0
5
R
R
0
4
R
R
0
3
R
R
0
2
R
R
0
1
R
R
0
0
R
R
0
Read/Write
Initial Value
0
0
0
0
0
0
0
0
21.9.3.2
ADLAR = 1
Bit
15
14
13
12
11
10
9
8
(0x79)
(0x78)
ADC9
ADC8
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADCH
ADCL
ADC1
ADC0
–
5
–
4
–
3
–
2
–
1
–
0
7
R
R
0
6
R
R
0
Read/Write
Initial Value
R
R
0
R
R
0
R
R
0
R
R
0
R
R
0
R
R
0
0
0
0
0
0
0
0
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. Consequently, if the result is left
adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH. Otherwise,
ADCL must be read first, then ADCH.
The ADLAR bit in ADMUX, and the MUXn bits in ADMUX affect the way the result is read from
the registers. If ADLAR is set, the result is left adjusted. If ADLAR is cleared (default), the result
is right adjusted.
• ADC9:0: ADC Conversion Result
These bits represent the result from the conversion, as detailed in ”ADC Conversion Result” on
page 216.
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21.9.4
ADCSRB – ADC Control and Status Register B
Bit
7
–
6
ACME
R/W
0
5
–
4
–
3
–
2
ADTS2
R/W
0
1
ADTS1
R/W
0
0
ADTS0
R/W
0
(0x7B)
ADCSRB
Read/Write
Initial Value
R
0
R
0
R
0
R
0
• Bit 7 – Res: Reserved Bit
This bit is reserved for future use. To ensure compatibility with future devices, this bit must be
written to zero when ADCSRB is written.
• Bit 2:0 – ADTS2:0: ADC Auto Trigger Source
If ADATE in ADCSRA is written to one, the value of these bits selects which source will trigger
an ADC conversion. If ADATE is cleared, the ADTS2:0 settings will have no effect. A conversion
will be triggered by the rising edge of the selected Interrupt Flag. Note that switching from a trig-
ger source that is cleared to a trigger source that is set, will generate a positive edge on the
trigger signal. If ADEN in ADCSRA is set, this will start a conversion. Switching to Free Running
mode (ADTS[2:0]=0) will not cause a trigger event, even if the ADC Interrupt Flag is set.
Table 21-5. ADC Auto Trigger Source Selections
ADTS2
ADTS1
ADTS0
Trigger Source
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
Free Running mode
Analog Comparator
External Interrupt Request 0
Timer/Counter0 Compare MatchA
Timer/Counter0 Overflow
Timer/Counter1 Compare Match B
Timer/Counter1 Overflow
Timer/Counter1 Capture Event
21.9.5
DIDR0 – Digital Input Disable Register 0
Bit
7
ADC7D
R/W
0
6
ADC6D
R/W
0
5
ADC5D
R/W
0
4
3
ADC3D
R/W
0
2
ADC2D
R/W
0
1
ADC1D
R/W
0
0
ADC0D
R/W
0
(0x7E)
ADC4D
DIDR0
Read/Write
Initial Value
R/W
0
• Bit 7:0 – ADC7D:ADC0D: ADC7:0 Digital Input Disable
When this bit is written logic one, the digital input buffer on the corresponding ADC pin is dis-
abled. The corresponding PIN Register bit will always read as zero when this bit is set. When an
analog signal is applied to the ADC7:0 pin and the digital input from this pin is not needed, this
bit should be written logic one to reduce power consumption in the digital input buffer.
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22. JTAG Interface and On-chip Debug System
22.1 Features
• JTAG (IEEE std. 1149.1 Compliant) Interface
• Boundary-scan Capabilities According to the IEEE std. 1149.1 (JTAG) Standard
• Debugger Access to:
– All Internal Peripheral Units
– Internal and External RAM
– The Internal Register File
– Program Counter
– EEPROM and Flash Memories
• Extensive On-chip Debug Support for Break Conditions, Including
– AVR Break Instruction
– Break on Change of Program Memory Flow
– Single Step Break
– Program Memory Break Points on Single Address or Address Range
– Data Memory Break Points on Single Address or Address Range
• Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface
• On-chip Debugging Supported by AVR Studio®
22.2 Overview
The AVR IEEE std. 1149.1 compliant JTAG interface can be used for
• Testing PCBs by using the JTAG Boundary-scan capability
• Programming the non-volatile memories, Fuses and Lock bits
• On-chip debugging
A brief description is given in the following sections. Detailed descriptions for Programming via
the JTAG interface, and using the Boundary-scan Chain can be found in the sections ”Program-
ming via the JTAG Interface” on page 291 and ”IEEE 1149.1 (JTAG) Boundary-scan” on page
229, respectively. The On-chip Debug support is considered being private JTAG instructions,
and distributed within ATMEL and to selected third party vendors only.
Figure 22-1 shows a block diagram of the JTAG interface and the On-chip Debug system. The
TAP Controller is a state machine controlled by the TCK and TMS signals. The TAP Controller
selects either the JTAG Instruction Register or one of several Data Registers as the scan chain
(Shift Register) between the TDI – input and TDO – output. The Instruction Register holds JTAG
instructions controlling the behavior of a Data Register.
The ID-Register, Bypass Register, and the Boundary-scan Chain are the Data Registers used
for board-level testing. The JTAG Programming Interface (actually consisting of several physical
and virtual Data Registers) is used for serial programming via the JTAG interface. The Internal
Scan Chain and Break Point Scan Chain are used for On-chip debugging only.
22.3 TAP – Test Access Port
The JTAG interface is accessed through four of the AVR’s pins. In JTAG terminology, these pins
constitute the Test Access Port – TAP. These pins are:
• TMS: Test mode select. This pin is used for navigating through the TAP-controller state
machine.
• TCK: Test Clock. JTAG operation is synchronous to TCK.
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• TDI: Test Data In. Serial input data to be shifted in to the Instruction Register or Data Register
(Scan Chains).
• TDO: Test Data Out. Serial output data from Instruction Register or Data Register.
The IEEE std. 1149.1 also specifies an optional TAP signal; TRST – Test ReSeT – which is not
provided.
When the JTAGEN fuse is unprogrammed, these four TAP pins are normal port pins and the
TAP controller is in reset. When programmed and the JTD bit in MCUCR is cleared, the TAP
pins are internally pulled high and the JTAG is enabled for Boundary-scan and programming.
The device is shipped with this fuse programmed.
For the On-chip Debug system, in addition to the JTAG interface pins, the RESET pin is moni-
tored by the debugger to be able to detect external reset sources. The debugger can also pull
the RESET pin low to reset the whole system, assuming only open collectors on the reset line
are used in the application.
Figure 22-1. Block Diagram
I/O PORT 0
DEVICE BOUNDARY
BOUNDARY SCAN CHAIN
TDI
JTAG PROGRAMMING
INTERFACE
TDO
TCK
TMS
TAP
CONTROLLER
AVR CPU
INTERNAL
SCAN
CHAIN
FLASH
MEMORY
Address
Data
PC
Instruction
INSTRUCTION
REGISTER
ID
REGISTER
BREAKPOINT
UNIT
M
U
X
FLOW CONTROL
UNIT
BYPASS
REGISTER
DIGITAL
PERIPHERAL
UNITS
ANALOG
PERIPHERIAL
UNITS
Analog inputs
BREAKPOINT
SCAN CHAIN
JTAG / AVR CORE
COMMUNICATION
INTERFACE
ADDRESS
DECODER
OCD STATUS
AND CONTROL
Control & Clock lines
I/O PORT n
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Figure 22-2. TAP Controller State Diagram
1
Test-Logic-Reset
0
1
1
1
0
Run-Test/Idle
Select-DR Scan
Select-IR Scan
0
0
1
1
Capture-DR
Capture-IR
0
0
Shift-DR
0
Shift-IR
0
1
Exit1-DR
0
1
1
1
Exit1-IR
0
Pause-IR
1
Pause-DR
1
0
0
0
0
Exit2-DR
1
Exit2-IR
1
Update-DR
Update-IR
1
1
0
0
22.4 TAP Controller
The TAP controller is a 16-state finite state machine that controls the operation of the Boundary-
scan circuitry, JTAG programming circuitry, or On-chip Debug system. The state transitions
depicted in Figure 22-2 depend on the signal present on TMS (shown adjacent to each state
transition) at the time of the rising edge at TCK. The initial state after a Power-on Reset is Test-
Logic-Reset.
As a definition in this document, the LSB is shifted in and out first for all Shift Registers.
Assuming Run-Test/Idle is the present state, a typical scenario for using the JTAG interface is:
• At the TMS input, apply the sequence 1, 1, 0, 0 at the rising edges of TCK to enter the Shift
Instruction Register – Shift-IR state. While in this state, shift the four bits of the JTAG
instructions into the JTAG Instruction Register from the TDI input at the rising edge of TCK.
The TMS input must be held low during input of the 3 LSBs in order to remain in the Shift-IR
state. The MSB of the instruction is shifted in when this state is left by setting TMS high. While
the instruction is shifted in from the TDI pin, the captured IR-state 0x01 is shifted out on the
TDO pin. The JTAG Instruction selects a particular Data Register as path between TDI and
TDO and controls the circuitry surrounding the selected Data Register.
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• Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. The instruction is latched
onto the parallel output from the Shift Register path in the Update-IR state. The Exit-IR, Pause-
IR, and Exit2-IR states are only used for navigating the state machine.
• At the TMS input, apply the sequence 1, 0, 0 at the rising edges of TCK to enter the Shift Data
Register – Shift-DR state. While in this state, upload the selected Data Register (selected by
the present JTAG instruction in the JTAG Instruction Register) from the TDI input at the rising
edge of TCK. In order to remain in the Shift-DR state, the TMS input must be held low during
input of all bits except the MSB. The MSB of the data is shifted in when this state is left by
setting TMS high. While the Data Register is shifted in from the TDI pin, the parallel inputs to
the Data Register captured in the Capture-DR state is shifted out on the TDO pin.
• Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. If the selected Data
Register has a latched parallel-output, the latching takes place in the Update-DR state. The
Exit-DR, Pause-DR, and Exit2-DR states are only used for navigating the state machine.
As shown in the state diagram, the Run-Test/Idle state need not be entered between selecting
JTAG instruction and using Data Registers, and some JTAG instructions may select certain
functions to be performed in the Run-Test/Idle, making it unsuitable as an Idle state.
Note:
1. Independent of the initial state of the TAP Controller, the Test-Logic-Reset state can always be
entered by holding TMS high for five TCK clock periods.
For detailed information on the JTAG specification, refer to the literature listed in ”Bibliography”
on page 228.
22.5 Using the Boundary-scan Chain
A complete description of the Boundary-scan capabilities are given in the section ”IEEE 1149.1
(JTAG) Boundary-scan” on page 229.
22.6 Using the On-chip Debug System
As shown in Figure 22-1, the hardware support for On-chip Debugging consists mainly of
• A scan chain on the interface between the internal AVR CPU and the internal peripheral units.
• Break Point unit.
• Communication interface between the CPU and JTAG system.
All read or modify/write operations needed for implementing the Debugger are done by applying
AVR instructions via the internal AVR CPU Scan Chain. The CPU sends the result to an I/O
memory mapped location which is part of the communication interface between the CPU and the
JTAG system.
The Break Point Unit implements Break on Change of Program Flow, Single Step Break, two
Program Memory Break Points, and two combined Break Points. Together, the four Break
Points can be configured as either:
• 4 single Program Memory Break Points.
• 3 Single Program Memory Break Point + 1 single Data Memory Break Point.
• 2 single Program Memory Break Points + 2 single Data Memory Break Points.
• 2 single Program Memory Break Points + 1 Program Memory Break Point with mask (“range
Break Point”).
• 2 single Program Memory Break Points + 1 Data Memory Break Point with mask (“range Break
Point”).
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A debugger, like the AVR Studio, may however use one or more of these resources for its inter-
nal purpose, leaving less flexibility to the end-user.
A list of the On-chip Debug specific JTAG instructions is given in ”On-chip Debug Specific JTAG
Instructions” on page 227.
The JTAGEN Fuse must be programmed to enable the JTAG Test Access Port. In addition, the
OCDEN Fuse must be programmed and no Lock bits must be set for the On-chip debug system
to work. As a security feature, the On-chip debug system is disabled when either of the LB1 or
LB2 Lock bits are set. Otherwise, the On-chip debug system would have provided a back-door
into a secured device.
The AVR Studio enables the user to fully control execution of programs on an AVR device with
On-chip Debug capability, AVR In-Circuit Emulator, or the built-in AVR Instruction Set Simulator.
AVR Studio® supports source level execution of Assembly programs assembled with Atmel Cor-
poration’s AVR Assembler and C programs compiled with third party vendors’ compilers.
AVR Studio runs under Microsoft® Windows® 95/98/2000, Windows NT® and Windows XP®.
For a full description of the AVR Studio, please refer to the AVR Studio User Guide. Only high-
lights are presented in this document.
All necessary execution commands are available in AVR Studio, both on source level and on
disassembly level. The user can execute the program, single step through the code either by
tracing into or stepping over functions, step out of functions, place the cursor on a statement and
execute until the statement is reached, stop the execution, and reset the execution target. In
addition, the user can have an unlimited number of code Break Points (using the BREAK
instruction) and up to two data memory Break Points, alternatively combined as a mask (range)
Break Point.
22.7 On-chip Debug Specific JTAG Instructions
The On-chip debug support is considered being private JTAG instructions, and distributed within
ATMEL and to selected third party vendors only. Instruction opcodes are listed for reference.
22.7.1
22.7.2
22.7.3
22.7.4
PRIVATE0; 0x8
PRIVATE1; 0x9
PRIVATE2; 0xA
PRIVATE3; 0xB
Private JTAG instruction for accessing On-chip debug system.
Private JTAG instruction for accessing On-chip debug system.
Private JTAG instruction for accessing On-chip debug system.
Private JTAG instruction for accessing On-chip debug system.
22.8 Using the JTAG Programming Capabilities
Programming of AVR parts via JTAG is performed via the 4-pin JTAG port, TCK, TMS, TDI, and
TDO. These are the only pins that need to be controlled/observed to perform JTAG program-
ming (in addition to power pins). It is not required to apply 12V externally. The JTAGEN Fuse
must be programmed and the JTD bit in the MCUCR Register must be cleared to enable the
JTAG Test Access Port.
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The JTAG programming capability supports:
• Flash programming and verifying.
• EEPROM programming and verifying.
• Fuse programming and verifying.
• Lock bit programming and verifying.
The Lock bit security is exactly as in parallel programming mode. If the Lock bits LB1 or LB2 are
programmed, the OCDEN Fuse cannot be programmed unless first doing a chip erase. This is a
security feature that ensures no back-door exists for reading out the content of a secured
device.
The details on programming through the JTAG interface and programming specific JTAG
instructions are given in the section ”Programming via the JTAG Interface” on page 291.
22.9 Bibliography
For more information about general Boundary-scan, the following literature can be consulted:
• IEEE: IEEE Std. 1149.1-1990. IEEE Standard Test Access Port and Boundary-scan
Architecture, IEEE, 1993.
Colin Maunder: The Board Designers Guide to Testable Logic Circuits, Addison-Wesley, 1992.
22.10 Register Description
22.10.1 OCDR – On-chip Debug Register
Bit
7
6
5
4
3
2
1
0
0x31 (0x51)
Read/Write
Initial Value
MSB/IDRD
LSB
R/W
0
OCDR
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
The OCDR Register provides a communication channel from the running program in the micro-
controller to the debugger. The CPU can transfer a byte to the debugger by writing to this
location. At the same time, an internal flag; I/O Debug Register Dirty – IDRD – is set to indicate
to the debugger that the register has been written. When the CPU reads the OCDR Register the
7 LSB will be from the OCDR Register, while the MSB is the IDRD bit. The debugger clears the
IDRD bit when it has read the information.
In some AVR devices, this register is shared with a standard I/O location. In this case, the OCDR
Register can only be accessed if the OCDEN Fuse is programmed, and the debugger enables
access to the OCDR Register. In all other cases, the standard I/O location is accessed.
Refer to the debugger documentation for further information on how to use this register.
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23. IEEE 1149.1 (JTAG) Boundary-scan
23.1 Features
• JTAG (IEEE std. 1149.1 compliant) Interface
• Boundary-scan Capabilities According to the JTAG Standard
• Full Scan of all Port Functions as well as Analog Circuitry having Off-chip Connections
• Supports the Optional IDCODE Instruction
• Additional Public AVR_RESET Instruction to Reset the AVR
23.2 Overview
The Boundary-scan chain has the capability of driving and observing the logic levels on the digi-
tal I/O pins, as well as the boundary between digital and analog logic for analog circuitry having
off-chip connections. At system level, all ICs having JTAG capabilities are connected serially by
the TDI/TDO signals to form a long Shift Register. An external controller sets up the devices to
drive values at their output pins, and observe the input values received from other devices. The
controller compares the received data with the expected result. In this way, Boundary-scan pro-
vides a mechanism for testing interconnections and integrity of components on Printed Circuits
Boards by using the four TAP signals only.
The four IEEE 1149.1 defined mandatory JTAG instructions IDCODE, BYPASS, SAMPLE/PRE-
LOAD, and EXTEST, as well as the AVR specific public JTAG instruction AVR_RESET can be
used for testing the Printed Circuit Board. Initial scanning of the Data Register path will show the
ID-Code of the device, since IDCODE is the default JTAG instruction. It may be desirable to
have the AVR device in reset during test mode. If not reset, inputs to the device may be deter-
mined by the scan operations, and the internal software may be in an undetermined state when
exiting the test mode. Entering reset, the outputs of any port pin will instantly enter the high
impedance state, making the HIGHZ instruction redundant. If needed, the BYPASS instruction
can be issued to make the shortest possible scan chain through the device. The device can be
set in the reset state either by pulling the external RESET pin low, or issuing the AVR_RESET
instruction with appropriate setting of the Reset Data Register.
The EXTEST instruction is used for sampling external pins and loading output pins with data.
The data from the output latch will be driven out on the pins as soon as the EXTEST instruction
is loaded into the JTAG IR-Register. Therefore, the SAMPLE/PRELOAD should also be used for
setting initial values to the scan ring, to avoid damaging the board when issuing the EXTEST
instruction for the first time. SAMPLE/PRELOAD can also be used for taking a snapshot of the
external pins during normal operation of the part.
The JTAGEN Fuse must be programmed and the JTD bit in the I/O Register MCUCR must be
cleared to enable the JTAG Test Access Port.
When using the JTAG interface for Boundary-scan, using a JTAG TCK clock frequency higher
than the internal chip frequency is possible. The chip clock is not required to run.
23.3 Data Registers
The Data Registers relevant for Boundary-scan operations are:
• Bypass Register
• Device Identification Register
• Reset Register
• Boundary-scan Chain
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23.3.1
23.3.2
Bypass Register
The Bypass Register consists of a single Shift Register stage. When the Bypass Register is
selected as path between TDI and TDO, the register is reset to 0 when leaving the Capture-DR
controller state. The Bypass Register can be used to shorten the scan chain on a system when
the other devices are to be tested.
Device Identification Register
Figure 23-1 shows the structure of the Device Identification Register.
Table 23-1. The Format of the Device Identification Register
LSB
MSB
Bit
31
28
27
12
11
1
0
Device ID
Version
Part Number
Manufacturer ID
1
4 bits
16 bits
11 bits
1-bit
23.3.2.1
23.3.2.2
23.3.2.3
23.3.3
Version
Version is a 4-bit number identifying the revision of the component. The JTAG version number
follows the revision of the device. Revision A is 0x0, revision B is 0x1 and so on.
Part Number
The part number is a 16-bit code identifying the component. The JTAG Part Number for
ATmega325P/3250P is listed in Table 25-6 on page 274.
Manufacturer ID
The Manufacturer ID is a 11-bit code identifying the manufacturer. The JTAG manufacturer ID
for ATMEL is listed in Table 25-6 on page 274.
Reset Register
The Reset Register is a test Data Register used to reset the part. Since the AVR tri-states Port
Pins when reset, the Reset Register can also replace the function of the unimplemented optional
JTAG instruction HIGHZ.
A high value in the Reset Register corresponds to pulling the external Reset low. The part is
reset as long as there is a high value present in the Reset Register. Depending on the fuse set-
tings for the clock options, the part will remain reset for a reset time-out period (refer to ”Clock
Sources” on page 30) after releasing the Reset Register. The output from this Data Register is
not latched, so the reset will take place immediately, as shown in Figure 23-1.
230
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Figure 23-1. Reset Register
To
TDO
From Other Internal and
External Reset Sources
From
TDI
Internal reset
D
Q
ClockDR · AVR_RESET
23.3.4
Boundary-scan Chain
The Boundary-scan Chain has the capability of driving and observing the logic levels on the dig-
ital I/O pins, as well as the boundary between digital and analog logic for analog circuitry having
off-chip connections.
See ”Boundary-scan Chain” on page 232 for a complete description.
23.4 Boundary-scan Specific JTAG Instructions
The Instruction Register is 4-bit wide, supporting up to 16 instructions. Listed below are the
JTAG instructions useful for Boundary-scan operation. Note that the optional HIGHZ instruction
is not implemented, but all outputs with tri-state capability can be set in high-impedant state by
using the AVR_RESET instruction, since the initial state for all port pins is tri-state.
As a definition in this data sheet, the LSB is shifted in and out first for all Shift Registers.
The OPCODE for each instruction is shown behind the instruction name in hex format. The text
describes which Data Register is selected as path between TDI and TDO for each instruction.
23.4.1
EXTEST; 0x0
Mandatory JTAG instruction for selecting the Boundary-scan Chain as Data Register for testing
circuitry external to the AVR package. For port-pins, Pull-up Disable, Output Control, Output
Data, and Input Data are all accessible in the scan chain. For Analog circuits having off-chip
connections, the interface between the analog and the digital logic is in the scan chain. The con-
tents of the latched outputs of the Boundary-scan chain is driven out as soon as the JTAG IR-
Register is loaded with the EXTEST instruction.
The active states are:
• Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain.
• Shift-DR: The Internal Scan Chain is shifted by the TCK input.
• Update-DR: Data from the scan chain is applied to output pins.
23.4.2
IDCODE; 0x1
Optional JTAG instruction selecting the 32 bit ID-Register as Data Register. The ID-Register
consists of a version number, a device number and the manufacturer code chosen by JEDEC.
This is the default instruction after power-up.
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The active states are:
• Capture-DR: Data in the IDCODE Register is sampled into the Boundary-scan Chain.
• Shift-DR: The IDCODE scan chain is shifted by the TCK input.
23.4.3
SAMPLE_PRELOAD; 0x2
Mandatory JTAG instruction for pre-loading the output latches and taking a snap-shot of the
input/output pins without affecting the system operation. However, the output latches are not
connected to the pins. The Boundary-scan Chain is selected as Data Register.
The active states are:
• Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain.
• Shift-DR: The Boundary-scan Chain is shifted by the TCK input.
• Update-DR: Data from the Boundary-scan chain is applied to the output latches. However, the
output latches are not connected to the pins.
23.4.4
AVR_RESET; 0xC
The AVR specific public JTAG instruction for forcing the AVR device into the Reset mode or
releasing the JTAG reset source. The TAP controller is not reset by this instruction. The one bit
Reset Register is selected as Data Register. Note that the reset will be active as long as there is
a logic “one” in the Reset Chain. The output from this chain is not latched.
The active states are:
• Shift-DR: The Reset Register is shifted by the TCK input.
23.4.5
BYPASS; 0xF
Mandatory JTAG instruction selecting the Bypass Register for Data Register.
The active states are:
• Capture-DR: Loads a logic “0” into the Bypass Register.
• Shift-DR: The Bypass Register cell between TDI and TDO is shifted.
23.5 Boundary-scan Chain
The Boundary-scan chain has the capability of driving and observing the logic levels on the digi-
tal I/O pins, as well as the boundary between digital and analog logic for analog circuitry having
off-chip connection.
23.5.1
Scanning the Digital Port Pins
Figure 23-2 shows the Boundary-scan Cell for a bi-directional port pin with pull-up function. The
cell consists of a standard Boundary-scan cell for the Pull-up Enable – PUExn – function, and a
bi-directional pin cell that combines the three signals Output Control – OCxn, Output Data –
ODxn, and Input Data – IDxn, into only a two-stage Shift Register. The port and pin indexes are
not used in the following description
The Boundary-scan logic is not included in the figures in the Data Sheet. Figure 23-3 shows a
simple digital port pin as described in the section ”I/O-Ports” on page 63. The Boundary-scan
details from Figure 23-2 replaces the dashed box in Figure 23-3.
When no alternate port function is present, the Input Data – ID – corresponds to the PINxn Reg-
ister value (but ID has no synchronizer), Output Data corresponds to the PORT Register, Output
232
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ATmega325P/3250P
Control corresponds to the Data Direction – DD Register, and the Pull-up Enable – PUExn – cor-
responds to logic expression PUD · DDxn · PORTxn.
Digital alternate port functions are connected outside the dotted box in Figure 23-3 to make the
scan chain read the actual pin value. For Analog function, there is a direct connection from the
external pin to the analog circuit, and a scan chain is inserted on the interface between the digi-
tal logic and the analog circuitry.
Figure 23-2. Boundary-scan Cell for Bi-directional Port Pin with Pull-up Function.
ShiftDR
To Next Cell
EXTEST
Vcc
Pullup Enable (PUE)
0
1
FF2
Q
LD2
0
1
D
D
Q
G
Output Control (OC)
FF1
D Q
LD1
0
1
0
1
D
G
Q
Output Data (OD)
0
1
FF0
D
LD0
0
1
0
1
Q
D
G
Q
Input Data (ID)
From Last Cell
ClockDR
UpdateDR
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Figure 23-3. General Port Pin Schematic Diagram
See Boundary-scan
Description for Details!
PUExn
PUD
Q
D
DDxn
Q CLR
WDx
RDx
RESET
OCxn
Pxn
1
0
Q
D
PORTxn
ODxn
IDxn
Q CLR
RESET
WPx
WRx
SLEEP
RRx
SYNCHRONIZER
RPx
D
Q
D
L
Q
Q
PINxn
Q
CLK I/O
PUD:
PULLUP DISABLE
WDx:
RDx:
WRx:
RRx:
RPx:
WRITE DDRx
READ DDRx
WRITE PORTx
PUExn:
OCxn:
ODxn:
IDxn:
PULLUP ENABLE for pin Pxn
OUTPUT CONTROL for pin Pxn
OUTPUT DATA to pin Pxn
INPUT DATA from pin Pxn
SLEEP CONTROL
READ PORTx REGISTER
READ PORTx PIN
WRITE PINx REGISTER
I/O CLOCK
WPx:
CLK I/O
SLEEP:
:
23.5.2
Scanning the RESET Pin
The RESET pin accepts 5V active low logic for standard reset operation, and 12V active high
logic for High Voltage Parallel programming. An observe-only cell as shown in Figure 23-4 is
inserted both for the 5V reset signal; RSTT, and the 12V reset signal; RSTHV.
Figure 23-4. Observe-only Cell
To
Next
ShiftDR
Cell
From System Pin
To System Logic
FF1
0
1
D
Q
From
ClockDR
Previous
Cell
234
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ATmega325P/3250P
23.5.3
Scanning the Clock Pins
The AVR devices have many clock options selectable by fuses. These are: Internal RC Oscilla-
tor, External Clock, (High Frequency) Crystal Oscillator, Low-frequency Crystal Oscillator, and
Ceramic Resonator.
Figure 23-5 shows how each Oscillator with external connection is supported in the scan chain.
The Enable signal is supported with a general Boundary-scan cell, while the Oscillator/clock out-
put is attached to an observe-only cell. In addition to the main clock, the timer Oscillator is
scanned in the same way. The output from the internal RC Oscillator is not scanned, as this
Oscillator does not have external connections.
Figure 23-5. Boundary-scan Cells for Oscillators and Clock Options
XTAL1/TOSC1
XTAL2/TOSC2
To
Next
Cell
To
ShiftDR
EXTEST
Next
Cell
Oscillator
ShiftDR
From Digital Logic
0
1
To System Logic
ENABLE
OUTPUT
0
1
FF1
D
Q
D
G
Q
0
1
D
Q
From
ClockDR
UpdateDR
Previous
Cell
From
ClockDR
Previous
Cell
Table 23-2 summaries the scan registers for the external clock pin XTAL1, oscillators with
XTAL1/XTAL2 connections as well as 32kHz Timer Oscillator.
Table 23-2. Scan Signals for the Oscillator(1)(2)(3)
Scanned Clock Line
Enable Signal
Scanned Clock Line
Clock Option
when not Used
EXTCLKEN
EXTCLK (XTAL1)
External Clock
0
External Crystal
OSCON
OSCCK
1
1
External Ceramic Resonator
OSC32EN
OSC32CK
Low Freq. External Crystal
Notes: 1. Do not enable more than one clock source as main clock at a time.
2. Scanning an Oscillator output gives unpredictable results as there is a frequency drift between
the internal Oscillator and the JTAG TCK clock. If possible, scanning an external clock is
preferred.
3. The clock configuration is programmed by fuses. As a fuse is not changed run-time, the clock
configuration is considered fixed for a given application. The user is advised to scan the same
clock option as to be used in the final system. The enable signals are supported in the scan
chain because the system logic can disable clock options in sleep modes, thereby disconnect-
ing the Oscillator pins from the scan path if not provided.
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23.5.4
Scanning the Analog Comparator
The relevant Comparator signals regarding Boundary-scan are shown in Figure 23-6. The
Boundary-scan cell from Figure 23-7 is attached to each of these signals. The signals are
described in Table 23-3.
The Comparator need not be used for pure connectivity testing, since all analog inputs are
shared with a digital port pin as well.
Figure 23-6. Analog Comparator
BANDGAP
REFERENCE
ACBG
ACD
ACO
AC_IDLE
ACME
ADCEN
ADC MULTIPLEXER
OUTPUT
Figure 23-7. General Boundary-scan cell Used for Signals for Comparator and ADC
To
Next
ShiftDR
Cell
EXTEST
From Digital Logic/
From Analog Ciruitry
0
1
To Analog Circuitry/
To Digital Logic
0
1
D
Q
D
G
Q
From
ClockDR
UpdateDR
Previous
Cell
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Table 23-3. Boundary-scan Signals for the Analog Comparator
Direction as
Seen from the
Comparator
Recommended
Input when Not
in Use
Output Values when
Recommended Inputs
are Used
Signal
Name
Description
Turns off Analog
Comparator when
true
Depends upon µC code
being executed
AC_IDLE
ACO
input
1
Will become input
to µC code being
executed
Analog Comparator
Output
output
0
Uses output signal
from ADC mux when
true
Depends upon µC code
being executed
ACME
ACBG
input
input
0
0
Bandgap Reference
enable
Depends upon µC code
being executed
23.5.5
Scanning the ADC
Figure 23-8 shows a block diagram of the ADC with all relevant control and observe signals. The
Boundary-scan cell from Figure 23-4 is attached to each of these signals. The ADC need not be
used for pure connectivity testing, since all analog inputs are shared with a digital port pin as
well.
Figure 23-8. Analog to Digital Converter
VCCREN
AREF
IREFEN
1.11V
ref
To Comparator
PASSEN
MUXEN_7
ADC_7
MUXEN_6
ADC_6
MUXEN_5
ADC_5
MUXEN_4
ADC_4
ADCBGEN
SCTEST
1.22V
ref
EXTCH
MUXEN_3
ADC_3
PRECH
PRECH
AREF
AREF
DACOUT
COMP
MUXEN_2
ADC_2
MUXEN_1
ADC_1
MUXEN_0
ADC_0
DAC_9..0
ADCEN
10-bit DAC
+
-
COMP
ACTEN
+
NEGSEL_2
NEGSEL_1
NEGSEL_0
1x
-
HOLD
ADC_2
ADC_1
GNDEN
ST
ACLK
AMPEN
ADC_0
The signals are described briefly in Table 23-4.
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8023F–AVR–07/09
Table 23-4. Boundary-scan Signals for the ADC(1)
Output Values when
Direction as
Recommende recommended inputs are
Signal
Name
seen
from the ADC
d Input when
not in use
used, and CPU is not using the
ADC
Description
COMP
ACLK
Output
Input
Comparator Output
0
0
0
0
Clock signal to differential amplifier
implemented as Switch-cap filters
Enable path from differential amplifier to
the comparator
ACTEN
Input
Input
0
0
0
0
Enable Band-gap reference as negative
input to comparator
ADCBGEN
ADCEN
AMPEN
DAC_9
DAC_8
DAC_7
DAC_6
DAC_5
DAC_4
DAC_3
DAC_2
DAC_1
DAC_0
Input
Input
Input
Input
Input
Input
Input
Input
Input
Input
Input
Input
Power-on signal to the ADC
Power-on signal to the differential amplifier
Bit 9 of digital value to DAC
Bit 8 of digital value to DAC
Bit 7 of digital value to DAC
Bit 6 of digital value to DAC
Bit 5 of digital value to DAC
Bit 4 of digital value to DAC
Bit 3 of digital value to DAC
Bit 2 of digital value to DAC
Bit 1 of digital value to DAC
Bit 0 of digital value to DAC
0
0
1
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
Connect ADC channels 0 - 3 to by-pass
path around differential amplifier
EXTCH
GNDEN
Input
Input
1
0
1
0
Ground the negative input to comparator
when true
Sample & Hold signal. Sample analog
signal when low. Hold signal when high. If
differential amplifier are used, this signal
must go active when ACLK is high.
HOLD
Input
1
0
1
0
Enables Band-gap reference as AREF
signal to DAC
IREFEN
Input
MUXEN_7
MUXEN_6
MUXEN_5
MUXEN_4
MUXEN_3
MUXEN_2
MUXEN_1
MUXEN_0
Input
Input
Input
Input
Input
Input
Input
Input
Input Mux bit 7
Input Mux bit 6
Input Mux bit 5
Input Mux bit 4
Input Mux bit 3
Input Mux bit 2
Input Mux bit 1
Input Mux bit 0
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
1
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Table 23-4. Boundary-scan Signals for the ADC(1) (Continued)
Output Values when
Direction as
Recommende recommended inputs are
Signal
Name
seen
from the ADC
d Input when
not in use
used, and CPU is not using the
ADC
Description
Input Mux for negative input for differential
signal, bit 2
NEGSEL_2
NEGSEL_1
Input
Input
0
0
0
0
Input Mux for negative input for differential
signal, bit 1
Input Mux for negative input for differential
signal, bit 0
NEGSEL_0
PASSEN
PRECH
Input
Input
Input
0
1
1
0
1
1
Enable pass-gate of differential amplifier.
Precharge output latch of comparator.
(Active low)
Switch-cap TEST enable. Output from
differential amplifier send out to Port Pin
having ADC_4
SCTEST
Input
0
0
Output of differential amplifier will settle
faster if this signal is high first two ACLK
periods after AMPEN goes high.
ST
Input
Input
0
0
0
0
VCCREN
Selects Vcc as the ACC reference voltage.
Note:
1. Incorrect setting of the switches in Figure 23-8 will make signal contention and may damage the part. There are several input
choices to the S&H circuitry on the negative input of the output comparator in Figure 23-8. Make sure only one path is
selected from either one ADC pin, Bandgap reference source, or Ground.
If the ADC is not to be used during scan, the recommended input values from Table 23-4 should
be used. The user is recommended not to use the differential amplifier during scan. Switch-Cap
based differential amplifier require fast operation and accurate timing which is difficult to obtain
when used in a scan chain. Details concerning operations of the differential amplifier is therefore
not provided.
The AVR ADC is based on the analog circuitry shown in Figure 23-8 with a successive approxi-
mation algorithm implemented in the digital logic. When used in Boundary-scan, the problem is
usually to ensure that an applied analog voltage is measured within some limits. This can easily
be done without running a successive approximation algorithm: apply the lower limit on the digi-
tal DAC[9:0] lines, make sure the output from the comparator is low, then apply the upper limit
on the digital DAC[9:0] lines, and verify the output from the comparator to be high.
The ADC need not be used for pure connectivity testing, since all analog inputs are shared with
a digital port pin as well.
When using the ADC, remember the following
• The port pin for the ADC channel in use must be configured to be an input with pull-up disabled
to avoid signal contention.
• In Normal mode, a dummy conversion (consisting of 10 comparisons) is performed when
enabling the ADC. The user is advised to wait at least 200ns after enabling the ADC before
controlling/observing any ADC signal, or perform a dummy conversion before using the first
result.
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8023F–AVR–07/09
• The DAC values must be stable at the midpoint value 0x200 when having the HOLD signal low
(Sample mode).
As an example, consider the task of verifying a 1.5V 5% input signal at ADC channel 3 when
the power supply is 5.0V and AREF is externally connected to VCC
.
The lower limit is:
The upper limit is:
1024 ⋅ 1,5V ⋅ 0,95 ⁄ 5V = 291 = 0x123
1024 ⋅ 1,5V ⋅ 1,05 ⁄ 5V = 323 = 0x143
The recommended values from Table 23-4 are used unless other values are given in the algo-
rithm in Table 23-5. Only the DAC and port pin values of the Scan Chain are shown. The column
“Actions” describes what JTAG instruction to be used before filling the Boundary-scan Register
with the succeeding columns. The verification should be done on the data scanned out when
scanning in the data on the same row in the table.
Table 23-5. Algorithm for Using the ADC
PA3.
PA3.
Data
PA3.
Control
Pull-up_
Enable
Step
Actions
ADCEN
DAC
0x200
MUXEN
HOLD
PRECH
SAMPLE_PR
ELOAD
1
1
0x08
1
1
0
0
0
2
3
4
5
EXTEST
1
1
1
1
0x200
0x200
0x123
0x123
0x08
0x08
0x08
0x08
0
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
Verify the
COMP bit
scanned out to
be 0
6
1
0x200
0x08
1
1
0
0
0
7
1
1
1
1
0x200
0x200
0x143
0x143
0x08
0x08
0x08
0x08
0
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
8
9
10
Verify the
COMP bit
scanned out to
be 1
11
1
0x200
0x08
1
1
0
0
0
Using this algorithm, the timing constraint on the HOLD signal constrains the TCK clock fre-
quency. As the algorithm keeps HOLD high for five steps, the TCK clock frequency has to be at
least five times the number of scan bits divided by the maximum hold time, thold,max
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23.6 ATmega325P/3250P Boundary-scan Order
Table 23-6 and Table 23-7 shows the Scan order between TDI and TDO when the Boundary-
scan chain is selected as data path. Bit 0 is the LSB; the first bit scanned in, and the first bit
scanned out. The scan order follows the pin-out order as far as possible. Therefore, the bits of
Port A is scanned in the opposite bit order of the other ports. Exceptions from the rules are the
Scan chains for the analog circuits, which constitute the most significant bits of the scan chain
regardless of which physical pin they are connected to. In Figure 23-2, PXn. Data corresponds
to FF0, PXn. Control corresponds to FF1, and PXn. Pull-up_enable corresponds to FF2. Bit 4, 5,
6 and 7 of Port F is not in the scan chain, since these pins constitute the TAP pins when the
JTAG is enabled.
Table 23-6. ATmega325P Boundary-scan Order, 64-pin
Bit Number
197
Signal Name
AC_IDLE
ACO
Module
196
Comparator
195
ACME
194
AINBG
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Table 23-6. ATmega325P Boundary-scan Order, 64-pin (Continued)
Bit Number
193
192
191
190
189
188
187
186
185
184
183
182
181
180
179
178
177
176
175
174
173
172
171
170
169
168
167
166
165
164
163
162
161
160
159
158
Signal Name
COMP
Module
ACLK
ACTEN
PRIVATE_SIGNAL1(1)
ADCBGEN
ADCEN
AMPEN
DAC_9
DAC_8
DAC_7
DAC_6
DAC_5
ADC
DAC_4
DAC_3
DAC_2
DAC_1
DAC_0
EXTCH
GNDEN
HOLD
IREFEN
MUXEN_7
MUXEN_6
MUXEN_5
MUXEN_4
MUXEN_3
MUXEN_2
MUXEN_1
MUXEN_0
NEGSEL_2
NEGSEL_1
NEGSEL_0
PASSEN
PRECH
ST
VCCREN
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Table 23-6. ATmega325P Boundary-scan Order, 64-pin (Continued)
Bit Number
157
156
155
154
153
152
151
150
149
148
147
146
145
144
143
142
141
140
139
138
137
136
135
134
Signal Name
PE0.Data
Module
PE0.Control
PE0.Pull-up_Enable
PE1.Data
PE1.Control
PE1.Pull-up_Enable
PE2.Data
PE2.Control
PE2.Pull-up_Enable
PE3.Data
PE3.Control
PE3.Pull-up_Enable
PE4.Data
Port E
PE4.Control
PE4.Pull-up_Enable
PE5.Data
PE5.Control
PE5.Pull-up_Enable
PE6.Data
PE6.Control
PE6.Pull-up_Enable
PE7.Data
PE7.Control
PE7.Pull-up_Enable
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Table 23-6. ATmega325P Boundary-scan Order, 64-pin (Continued)
Bit Number
133
132
131
130
129
128
127
126
125
124
123
122
121
120
119
118
117
116
115
114
113
112
111
110
109
108
107
106
105
104
103
102
101
Signal Name
PB0.Data
Module
PB0.Control
PB0.Pull-up_Enable
PB1.Data
PB1.Control
PB1.Pull-up_Enable
PB2.Data
PB2.Control
PB2.Pull-up_Enable
PB3.Data
PB3.Control
PB3.Pull-up_Enable
PB4.Data
Port B
PB4.Control
PB4.Pull-up_Enable
PB5.Data
PB5.Control
PB5.Pull-up_Enable
PB6.Data
PB6.Control
PB6.Pull-up_Enable
PB7.Data
PB7.Control
PB7.Pull-up_Enable
PG3.Data
PG3.Control
PG3.Pull-up_Enable
PG4.Data
Port G
PG4.Control
PG4.Pull-up_Enable
PG5
(Observe Only)
RSTT
Reset Logic
(Observe-only)
RSTHV
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Table 23-6. ATmega325P Boundary-scan Order, 64-pin (Continued)
Bit Number
100
99
Signal Name
EXTCLKEN
Module
OSCON
Enable signals for main Clock/Oscillators
98
RCOSCEN
97
OSC32EN
96
EXTCLK (XTAL1)
OSCCK
95
Clock input and Oscillators for the main clock
(Observe-only)
94
RCCK
93
OSC32CK
92
PD0.Data
91
PD0.Control
PD0.Pull-up_Enable
PD1.Data
90
89
88
PD1.Control
PD1.Pull-up_Enable
PD2.Data
87
86
85
PD2.Control
PD2.Pull-up_Enable
PD3.Data
84
83
82
PD3.Control
PD3.Pull-up_Enable
PD4.Data
81
Port D
80
79
PD4.Control
PD4.Pull-up_Enable
PD5.Data
78
77
76
PD5.Control
PD5.Pull-up_Enable
PD6.Data
75
74
73
PD6.Control
PD6.Pull-up_Enable
PD7.Data
72
71
70
PD7.Control
PD7.Pull-up_Enable
PG0.Data
69
68
67
PG0.Control
PG0.Pull-up_Enable
PG1.Data
Port G
66
65
245
8023F–AVR–07/09
Table 23-6. ATmega325P Boundary-scan Order, 64-pin (Continued)
Bit Number
64
Signal Name
PG1.Control
Module
63
PG1.Pull-up_Enable
PC0.Data
62
61
PC0.Control
60
PC0.Pull-up_Enable
PC1.Data
59
58
PC1.Control
57
PC1.Pull-up_Enable
PC2.Data
56
55
PC2.Control
54
PC2.Pull-up_Enable
PC3.Data
53
52
PC3.Control
51
PC3.Pull-up_Enable
PC4.Data
Port C
50
49
PC4.Control
48
PC4.Pull-up_Enable
PC5.Data
47
46
PC5.Control
45
PC5.Pull-up_Enable
PC6.Data
44
43
PC6.Control
42
PC6.Pull-up_Enable
PC7.Data
41
40
PC7.Control
39
PC7.Pull-up_Enable
PG2.Data
38
37
PG2.Control
Port G
36
PG2.Pull-up_Enable
PA7.Data
35
34
PA7.Control
33
PA7.Pull-up_Enable
PA6.Data
32
Port A
31
PA6.Control
30
PA6.Pull-up_Enable
PA5.Data
29
246
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Table 23-6. ATmega325P Boundary-scan Order, 64-pin (Continued)
Bit Number
Signal Name
PA5.Control
Module
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
PA5.Pull-up_Enable
PA4.Data
PA4.Control
PA4.Pull-up_Enable
PA3.Data
PA3.Control
PA3.Pull-up_Enable
PA2.Data
PA2.Control
PA2.Pull-up_Enable
PA1.Data
PA1.Control
PA1.Pull-up_Enable
PA0.Data
PA0.Control
PA0.Pull-up_Enable
PF3.Data
PF3.Control
PF3.Pull-up_Enable
PF2.Data
8
7
PF2.Control
6
PF2.Pull-up_Enable
PF1.Data
Port F
5
4
PF1.Control
3
PF1.Pull-up_Enable
PF0.Data
2
1
PF0.Control
0
PF0.Pull-up_Enable
Note:
1. PRIVATE_SIGNAL1 should always be scanned in as zero.
247
8023F–AVR–07/09
Table 23-7. ATmega3250P Boundary-scan Order, 100-pin
Bit Number
242
241
240
239
238
237
236
235
234
233
232
231
230
229
228
227
226
225
224
223
222
221
220
219
218
217
216
215
214
213
212
211
210
209
208
Signal Name
AC_IDLE
ACO
Module
Comparator
ACME
AINBG
COMP
ACLK
ACTEN
PRIVATE_SIGNAL1(1)
ADCBGEN
ADCEN
AMPEN
DAC_9
DAC_8
DAC_7
DAC_6
DAC_5
DAC_4
DAC_3
DAC_2
DAC_1
ADC
DAC_0
EXTCH
GNDEN
HOLD
IREFEN
MUXEN_7
MUXEN_6
MUXEN_5
MUXEN_4
MUXEN_3
MUXEN_2
MUXEN_1
MUXEN_0
NEGSEL_2
NEGSEL_1
248
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Table 23-7. ATmega3250P Boundary-scan Order, 100-pin (Continued)
Bit Number
207
206
205
204
203
202
201
200
199
198
197
196
195
194
193
192
191
190
189
188
187
186
185
184
183
182
181
180
179
178
177
176
175
174
173
172
Signal Name
NEGSEL_0
Module
PASSEN
PRECH
ST
VCCREN
PE0.Data
PE0.Control
PE0.Pull-up_Enable
PE1.Data
PE1.Control
PE1.Pull-up_Enable
PE2.Data
PE2.Control
PE2.Pull-up_Enable
PE3.Data
PE3.Control
PE3.Pull-up_Enable
PE4.Data
Port E
PE4.Control
PE4.Pull-up_Enable
PE5.Data
PE5.Control
PE5.Pull-up_Enable
PE6.Data
PE6.Control
PE6.Pull-up_Enable
PE7.Data
PE7.Control
PE7.Pull-up_Enable
PJ0.Data
PJ0.Control
PJ0.Pull-up_Enable
PJ1.Data
Port J
Port B
PJ1.Control
PJ1.Pull-up_Enable
PB0.Data
249
8023F–AVR–07/09
Table 23-7. ATmega3250P Boundary-scan Order, 100-pin (Continued)
Bit Number
171
170
169
168
167
166
165
164
163
162
161
160
159
158
157
156
155
154
153
152
151
150
149
148
147
146
145
144
143
142
141
140
139
138
137
136
Signal Name
PB0.Control
PB0.Pull-up_Enable
PB1.Data
Module
PB1.Control
PB1.Pull-up_Enable
PB2.Data
PB2.Control
PB2.Pull-up_Enable
PB3.Data
PB3.Control
PB3.Pull-up_Enable
PB4.Data
PB4.Control
PB4.Pull-up_Enable
PB5.Data
PB5.Control
PB5.Pull-up_Enable
PB6.Data
PB6.Control
PB6.Pull-up_Enable
PB7.Data
PB7.Control
PB7.Pull-up_Enable
PG3.Data
PG3.Control
PG3.Pull-up_Enable
PG4.Data
Port G
PG4.Control
PG4.Pull-up_Enable
PG5
(Observe Only)
RSTT
Reset Logic
(Observe-only)
RSTHV
EXTCLKEN
OSCON
Enable signals for main Clock/Oscillators
RCOSCEN
OSC32EN
250
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Table 23-7. ATmega3250P Boundary-scan Order, 100-pin (Continued)
Bit Number
135
134
133
132
131
130
129
128
127
126
125
124
123
122
121
120
119
118
117
116
115
114
113
112
111
110
109
108
107
106
105
104
103
102
101
100
Signal Name
EXTCLK (XTAL1)
OSCCK
Module
Clock input and Oscillators for the main clock
(Observe-only)
RCCK
OSC32CK
PJ2.Data
PJ2.Control
PJ2.Pull-up_Enable
PJ3.Data
PJ3.Control
PJ3.Pull-up_Enable
PJ4.Data
PJ4.Control
Port J
PJ4.Pull-up_Enable
PJ5.Data
PJ5.Control
PJ5.Pull-up_Enable
PJ6.Data
PJ6.Control
PJ6.Pull-up_Enable
PD0.Data
PD0.Control
PD0.Pull-up_Enable
PD1.Data
PD1.Control
PD1.Pull-up_Enable
PD2.Data
PD2.Control
PD2.Pull-up_Enable
PD3.Data
Port D
PD3.Control
PD3.Pull-up_Enable
PD4.Data
PD4.Control
PD4.Pull-up_Enable
PD5.Data
PD5.Control
251
8023F–AVR–07/09
Table 23-7. ATmega3250P Boundary-scan Order, 100-pin (Continued)
Bit Number
99
Signal Name
PD5.Pull-up_Enable
PD6.Data
Module
98
97
PD6.Control
96
PD6.Pull-up_Enable
PD7.Data
95
94
PD7.Control
93
PD7.Pull-up_Enable
PG0.Data
92
91
PG0.Control
90
PG0.Pull-up_Enable
PG1.Data
Port G
89
88
PG1.Control
87
PG1.Pull-up_Enable
PC0.Data
86
85
PC0.Control
84
PC0.Pull-up_Enable
PC1.Data
83
82
PC1.Control
81
PC1.Pull-up_Enable
PC2.Data
80
79
PC2.Control
78
PC2.Pull-up_Enable
PC3.Data
Port C
77
76
PC3.Control
75
PC3.Pull-up_Enable
PC4.Data
74
73
PC4.Control
72
PC4.Pull-up_Enable
PC5.Data
71
70
PC5.Control
69
PC5.Pull-up_Enable
PH0.Data
68
67
PH0.Control
66
PH0.Pull-up_Enable
PH1.Data
Port H
65
64
PH1.Control
252
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Table 23-7. ATmega3250P Boundary-scan Order, 100-pin (Continued)
Bit Number
63
Signal Name
PH1.Pull-up_Enable
PH2.Data
Module
62
61
PH2.Control
60
PH2.Pull-up_Enable
PH3.Data
59
58
PH3.Control
57
PH3.Pull-up_Enable
PC6.Data
56
55
PC6.Control
54
PC6.Pull-up_Enable
PC7.Data
Port C
Port G
53
52
PC7.Control
51
PC7.Pull-up_Enable
PG2.Data
50
49
PG2.Control
48
PG2.Pull-up_Enable
PA7.Data
47
46
PA7.Control
45
PA7.Pull-up_Enable
PA6.Data
44
43
PA6.Control
42
PA6.Pull-up_Enable
PA5.Data
41
40
PA5.Control
39
PA5.Pull-up_Enable
PA4.Data
38
Port A
37
PA4.Control
36
PA4.Pull-up_Enable
PA3.Data
35
34
PA3.Control
33
PA3.Pull-up_Enable
PA2.Data
32
31
PA2.Control
30
PA2.Pull-up_Enable
PA1.Data
29
28
PA1.Control
253
8023F–AVR–07/09
Table 23-7. ATmega3250P Boundary-scan Order, 100-pin (Continued)
Bit Number
Signal Name
PA1.Pull-up_Enable
PA0.Data
Module
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
PA0.Control
PA0.Pull-up_Enable
PH4.Data
PH4.Control
PH4.Pull-up_Enable
PH5.Data
PH5.Control
PH5.Pull-up_Enable
PH6.Data
Port H
PH6.Control
PH6.Pull-up_Enable
PH7.Data
PH7.Control
PH7.Pull-up_Enable
PF3.Data
PF3.Control
PF3.Pull-up_Enable
PF2.Data
8
7
PF2.Control
6
PF2.Pull-up_Enable
PF1.Data
Port F
5
4
PF1.Control
3
PF1.Pull-up_Enable
PF0.Data
2
1
PF0.Control
0
PF0.Pull-up_Enable
Note:
1. PRIVATE_SIGNAL1 should always be scanned in as zero.
23.7 Boundary-scan Description Language Files
Boundary-scan Description Language (BSDL) files describe Boundary-scan capable devices in
a standard format used by automated test-generation software. The order and function of bits in
the Boundary-scan Data Register are included in this description. A BSDL file for
ATmega325P/3250P is available.
254
ATmega325P/3250P
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ATmega325P/3250P
23.8 Register Description
23.8.1
MCUCR – MCU Control Register
The MCU Control Register contains control bits for general MCU functions.
Bit
7
6
BODS
R
5
4
3
–
2
–
1
IVSEL
R/W
0
0
IVCE
R/W
0
0x35 (0x55)
Read/Write
Initial Value
JTD
R/W
0
BODSE
PUD
R/W
0
MCUCR
R
0
R
0
R
0
0
• Bit 7 – JTD: JTAG Interface Disable
When this bit is zero, the JTAG interface is enabled if the JTAGEN Fuse is programmed. If this
bit is one, the JTAG interface is disabled. In order to avoid unintentional disabling or enabling of
the JTAG interface, a timed sequence must be followed when changing this bit: The application
software must write this bit to the desired value twice within four cycles to change its value. Note
that this bit must not be altered when using the On-chip Debug system.
If the JTAG interface is left unconnected to other JTAG circuitry, the JTD bit should be set to
one. The reason for this is to avoid static current at the TDO pin in the JTAG interface.
23.8.2
MCUSR – MCU Status Register
The MCU Status Register provides information on which reset source caused an MCU reset.
Bit
7
–
6
–
5
–
4
3
2
BORF
1
0
0x34 (0x54)
Read/Write
Initial Value
JTRF
R/W
WDRF
R/W
EXTRF
R/W
PORF
R/W
MCUSR
R
0
R
0
R
0
R/W
See Bit Description
• Bit 4 – JTRF: JTAG Reset Flag
This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by
the JTAG instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic
zero to the flag.
255
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ATmega325P/3250P
24. Boot Loader Support – Read-While-Write Self-Programming
24.1 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 25-11 on page 276)
used during programming. The page organization does not affect normal operation.
24.2 Overview
The Boot Loader Support provides a real Read-While-Write Self-Programming mechanism for
downloading and uploading program code by the MCU itself. This feature allows flexible applica-
tion software updates controlled by the MCU using a Flash-resident Boot Loader program. The
Boot Loader program can use any available data interface and associated protocol to read code
and write (program) that code into the Flash memory, or read the code from the program mem-
ory. The program code within the Boot Loader section has the capability to write into the entire
Flash, including the Boot Loader memory. The Boot Loader can thus even modify itself, and it
can also erase itself from the code if the feature is not needed anymore. The size of the Boot
Loader memory is configurable with fuses and the Boot Loader has two separate sets of Boot
Lock bits which can be set independently. This gives the user a unique flexibility to select differ-
ent levels of protection.
24.3 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 24-2). The size of the different sections is configured by the
BOOTSZ Fuses as shown in Table 24-6 on page 268 and Figure 24-2. These two sections can
have different level of protection since they have different sets of Lock bits.
24.3.1
24.3.2
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 24-2 on page 260. The Application section can never store any
Boot Loader code since the SPM instruction is disabled when executed from the Application
section.
BLS – Boot Loader Section
While the Application section is used for storing the application code, the The Boot Loader soft-
ware must be located in the BLS since the SPM instruction can initiate a programming when
executing from the BLS only. The SPM instruction can access the entire Flash, including the
BLS itself. The protection level for the Boot Loader section can be selected by the Boot Loader
Lock bits (Boot Lock bits 1), see Table 24-3 on page 260.
256
8023F–AVR–07/09
24.4 Read-While-Write and No Read-While-Write Flash Sections
Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader soft-
ware update is dependent on which address that is being programmed. In addition to the two
sections that are configurable by the BOOTSZ Fuses as described above, the Flash is also
divided into two fixed sections, the Read-While-Write (RWW) section and the No Read-While-
Write (NRWW) section. The limit between the RWW- and NRWW sections is given in Table 24-
7 on page 268 and Figure 24-2 on page 259. The main difference between the two sections is:
• When erasing or writing a page located inside the RWW section, the NRWW section can be
read during the operation.
• When erasing or writing a page located inside the NRWW section, the CPU is halted during the
entire operation.
Note that the user software can never read any code that is located inside the RWW section dur-
ing a Boot Loader software operation. The syntax “Read-While-Write section” refers to which
section that is being programmed (erased or written), not which section that actually is being
read during a Boot Loader software update.
24.4.1
RWW – Read-While-Write Section
If a Boot Loader software update is programming a page inside the RWW section, it is possible
to read code from the Flash, but only code that is located in the NRWW section. During an on-
going programming, the software must ensure that the RWW section never is being read. If the
user software is trying to read code that is located inside the RWW section (i.e., by a
call/jmp/lpm or an interrupt) during programming, the software might end up in an unknown
state. To avoid this, the interrupts should either be disabled or moved to the Boot Loader sec-
tion. The Boot Loader section is always located in the NRWW section. The RWW Section Busy
bit (RWWSB) in the Store Program Memory Control and Status Register (SPMCSR) will be read
as logical one as long as the RWW section is blocked for reading. After a programming is com-
pleted, the RWWSB must be cleared by software before reading code located in the RWW
section. See Section “24.9.1” on page 269. for details on how to clear RWWSB.
24.4.2
NRWW – No Read-While-Write Section
The code located in the NRWW section can be read when the Boot Loader software is updating
a page in the RWW section. When the Boot Loader code updates the NRWW section, the CPU
is halted during the entire Page Erase or Page Write operation.
Table 24-1. Read-While-Write Features
Which Section Can
Which Section does the Z-pointer
Address During the Programming?
be Read During
Programming?
Is the CPU
Halted?
Read-While-Write
Supported?
RWW Section
NRWW Section
None
No
Yes
No
NRWW Section
Yes
257
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 24-1. 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
258
8023F–AVR–07/09
Figure 24-2. Memory Sections
Program Memory
BOOTSZ = '10'
Program Memory
BOOTSZ = '11'
0x0000
0x0000
Application Flash Section
Application Flash Section
End RWW
End RWW
Start NRWW
Start NRWW
Application Flash Section
Boot Loader Flash Section
Application Flash Section
Boot Loader Flash Section
End Application
End Application
Start Boot Loader
Flashend
Start Boot Loader
Flashend
0x0000
Program Memory
BOOTSZ = '01'
Program Memory
BOOTSZ = '00'
0x0000
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
Flashend
Note:
1. The parameters in the figure above are given in Table 24-6 on page 268.
24.5 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 independently. 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 24-2 and Table 24-3 for further details. The Boot Lock bits and general 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 pro-
gramming of the Flash memory by SPM instruction. Similarly, the general Read/Write Lock
(Lock Bit mode 1) does not control reading nor writing by LPM/SPM, if it is attempted.
259
ATmega325P/3250P
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ATmega325P/3250P
Table 24-2. 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
1
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
0
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 24-3. 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
1
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
0
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
260
8023F–AVR–07/09
24.6 Entering the Boot Loader Program
Entering the Boot Loader takes place by a jump or call from the application program. This may
be initiated by a trigger such as a command received via USART, or SPI interface. Alternatively,
the Boot Reset Fuse can be programmed so that the Reset Vector is pointing to the Boot Flash
start address after a reset. In this case, the Boot Loader is started after a reset. After the applica-
tion code is loaded, the program can start executing the application code. Note that the fuses
cannot be changed by the MCU itself. This means that once the Boot Reset Fuse is pro-
grammed, the Reset Vector will always point to the Boot Loader Reset and the fuse can only be
changed through the serial or parallel programming interface.
Table 24-4. Boot Reset Fuse(1)
BOOTRST
Reset Address
1
0
Reset Vector = Application Reset (address 0x0000)
Reset Vector = Boot Loader Reset (see Table 24-6 on page 268)
Note:
1. “1” means unprogrammed, “0” means programmed
24.7 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 25-11 on page 276), the Program Counter can
be treated as having two different sections. One section, consisting of the least significant bits, is
addressing the words within a page, while the most significant bits are addressing the pages.
This is shown in Figure 24-3. 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 program-
ming 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 24-3. Addressing the Flash During SPM(1)
BIT 15
ZPCMSB
ZPAGEMSB
1
0
0
Z - REGISTER
PCMSB
PAGEMSB
PCWORD
PROGRAM
COUNTER
PCPAGE
PAGE ADDRESS
WITHIN THE FLASH
WORD ADDRESS
WITHIN A PAGE
PROGRAM MEMORY
PAGE
PAGE
INSTRUCTION WORD
PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
Notes: 1. The different variables used in Figure 24-3 on page 262 are listed in Table 24-8 on page 268.
2. PCPAGE and PCWORD are listed in Table 25-11 on page 276.
24.8 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 buf-
fer is filled one word at a time using SPM and the buffer can be filled either before the Page
Erase command or between a Page Erase and a Page Write operation:
Alternative 1, fill the buffer before a Page Erase
• Fill temporary page buffer
• Perform a Page Erase
• Perform a Page Write
Alternative 2, fill the buffer after Page Erase
• Perform a Page Erase
• Fill temporary page buffer
• Perform a Page Write
If only a part of the page needs to be changed, the rest of the page must be stored (for example
in the temporary page buffer) before the erase, and then be rewritten. When using alternative 1,
the Boot Loader provides an effective Read-Modify-Write feature which allows the user software
to first read the page, do the necessary changes, and then write back the modified data. If alter-
native 2 is used, it is not possible to read the old data while loading since the page is already
erased. The temporary page buffer can be accessed in a random sequence. It is essential that
the page address used in both the Page Erase and Page Write operation is addressing the same
page. See ”Simple Assembly Code Example for a Boot Loader” on page 266 for an assembly
code example.
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24.8.1
Performing Page Erase by SPM
To execute Page Erase, set up the address in the Z-pointer, write “X0000011” to SPMCSR and
execute SPM within four clock cycles after writing SPMCSR. 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.
24.8.2
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 SPMCSR and execute SPM within four clock cycles after writing SPMCSR. 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
SPMCSR. It is also erased after a system reset. Note that it is not possible to write more than
one time to each address without erasing the temporary buffer.
If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be
lost.
24.8.3
Performing a Page Write
To execute Page Write, set up the address in the Z-pointer, write “X0000101” to SPMCSR and
execute SPM within four clock cycles after writing SPMCSR. 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.
24.8.4
24.8.5
24.8.6
Using the SPM Interrupt
If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the
SPMEN bit in SPMCSR is cleared. This means that the interrupt can be used instead of polling
the SPMCSR 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 53.
Consideration While Updating BLS
Special care must be taken if the user allows the Boot Loader section to be updated by leaving
Boot Lock bit11 unprogrammed. An accidental write to the Boot Loader itself can corrupt the
entire Boot Loader, and further software updates might be impossible. If it is not necessary to
change the Boot Loader software itself, it is recommended to program the Boot Lock bit11 to
protect the Boot Loader software from any internal software changes.
Prevent Reading the RWW Section During Self-Programming
During Self-Programming (either Page Erase or Page Write), the RWW section is always
blocked for reading. The user software itself must prevent that this section is addressed during
the self programming operation. The RWWSB in the SPMCSR 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 53, or the interrupts must be disabled. Before addressing
the RWW section after the programming is completed, the user software must clear the
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RWWSB by writing the RWWSRE. See ”Simple Assembly Code Example for a Boot Loader” on
page 266 for an example.
24.8.7
Setting the Boot Loader Lock Bits by SPM
To set the Boot Loader Lock bits and general Lock bits, write the desired data to R0, write
“X0001001” to SPMCSR and execute SPM within four clock cycles after writing SPMCSR.
Bit
7
6
5
4
3
2
1
0
R0
1
1
BLB12
BLB11
BLB02
BLB01
LB2
LB1
See Table 24-2 and Table 24-3 for how the different settings of the Boot Loader bits affect the
Flash access.
If bits 5:0 in R0 are cleared (zero), the corresponding Lock bit will be programmed if an SPM
instruction is executed within four cycles after BLBSET and SPMEN are set in SPMCSR. 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, and 6 in R0 to “1” when writing the Lock bits. When program-
ming the Lock bits the entire Flash can be read during the operation.
24.8.8
24.8.9
EEPROM Write Prevents Writing to SPMCSR
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 SPMCSR 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 SPMCSR. When an LPM instruc-
tion is executed within three CPU cycles after the BLBSET and SPMEN bits are set in SPMCSR,
the value of the Lock bits will be loaded in the destination register. The BLBSET and SPMEN
bits will auto-clear upon completion of reading the Lock bits or if no LPM instruction is executed
within three CPU cycles or no SPM instruction is executed within four CPU cycles. When BLB-
SET and SPMEN are cleared, LPM will work as described in the Instruction set Manual.
Bit
Rd
7
6
5
4
3
2
1
0
–
–
BLB12
BLB11
BLB02
BLB01
LB2
LB1
The algorithm for reading the Fuse Low byte is similar to the one described above for reading
the Lock bits. To read the Fuse Low byte, load the Z-pointer with 0x0000 and set the BLBSET
and SPMEN bits in SPMCSR. When an LPM instruction is executed within three cycles after the
BLBSET and SPMEN bits are set in the SPMCSR, the value of the Fuse Low byte (FLB) will be
loaded in the destination register as shown below. Refer to Table 25-5 on page 273 for a
detailed description and mapping of the Fuse Low byte.
Bit
Rd
7
6
5
4
3
2
1
0
FLB7
FLB6
FLB5
FLB4
FLB3
FLB2
FLB1
FLB0
Similarly, when reading the Fuse High byte, load 0x0003 in the Z-pointer. When an LPM instruc-
tion is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR,
the value of the Fuse High byte (FHB) will be loaded in the destination register as shown below.
Refer to Table 25-4 on page 273 for detailed description and mapping of the Fuse High byte.
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Bit
Rd
7
6
5
4
3
2
1
0
FHB7
FHB6
FHB5
FHB4
FHB3
FHB2
FHB1
FHB0
When reading the Extended Fuse byte, load 0x0002 in the Z-pointer. When an LPM instruction
is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the
value of the Extended Fuse byte (EFB) will be loaded in the destination register as shown below.
Refer to Table 25-3 on page 272 for detailed description and mapping of the Extended Fuse
byte.
Bit
Rd
7
6
5
4
3
2
1
0
–
–
–
–
–
EFB2
EFB1
EFB0
Fuse and Lock bits that are programmed, will be read as zero. Fuse and Lock bits that are
unprogrammed, will be read as one.
24.8.10 Preventing Flash Corruption
During periods of low VCC, the Flash program can be corrupted because the supply voltage is
too low for the CPU and the Flash to operate properly. These issues are the same as for board
level systems using the Flash, and the same design solutions should be applied.
A Flash program corruption can be caused by two situations when the voltage is too low. First, a
regular write sequence to the Flash requires a minimum voltage to operate correctly. Secondly,
the CPU itself can execute instructions incorrectly, if the supply voltage for executing instructions
is too low.
Flash corruption can easily be avoided by following these design recommendations (one is
sufficient):
1. If there is no need for a Boot Loader update in the system, program the Boot Loader Lock
bits to prevent any Boot Loader software updates.
2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage.
This can be done by enabling the internal Brown-out Detector (BOD) if the operating volt-
age matches the detection level. If not, an external low VCC 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 VCC. This will pre-
vent the CPU from attempting to decode and execute instructions, effectively protecting
the SPMCSR Register and thus the Flash from unintentional writes.
24.8.11 Programming Time for Flash when Using SPM
The calibrated RC Oscillator is used to time Flash accesses. Table 24-5 shows the typical pro-
gramming time for Flash accesses from the CPU.
Table 24-5. SPM Programming Time(1)
Symbol
Min Programming Time
Max Programming Time
Flash write (Page Erase, Page Write, and
write Lock bits by SPM)
3.7 ms
4.5 ms
Note:
1. Minimum and maximum programming time is per individual operation.
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24.8.12 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
.org SMALLBOOTSTART
Write_page:
;PAGESIZEB is page size in BYTES, not words
; Page Erase
ldi spmcrval, (1<<PGERS) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
; transfer data from RAM to Flash page buffer
ldi looplo, low(PAGESIZEB)
;init loop variable
ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256
Wrloop:
ld
ld
r0, Y+
r1, Y+
ldi spmcrval, (1<<SPMEN)
call Do_spm
adiw ZH:ZL, 2
sbiw loophi:looplo, 2
brne Wrloop
;use subi for PAGESIZEB<=256
; execute Page Write
subi ZL, low(PAGESIZEB)
sbci ZH, high(PAGESIZEB)
;restore pointer
;not required for PAGESIZEB<=256
ldi spmcrval, (1<<PGWRT) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
; read back and check, optional
ldi looplo, low(PAGESIZEB)
;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
jmp Error
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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, SPMCSR
sbrs temp1, RWWSB
ret
; If RWWSB is set, the RWW section is not ready yet
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
rjmp Return
Do_spm:
; check for previous SPM complete
Wait_spm:
in
temp1, SPMCSR
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 SPMCSR, spmcrval
spm
; restore SREG (to enable interrupts if originally enabled)
out SREG, temp2
ret
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24.8.13 ATmega325P/3250P Boot Loader Parameters
In Table 24-6 through Table 24-8, the parameters used in the description of the Self-Programming are given.
Table 24-6. Boot Size Configuration(1)
Boot Reset
Address
(Start
End
Application
Pages Flash Section
Boot Loader
Flash Section
Application
Section
Boot Loader
Section)
BOOTSZ1 BOOTSZ0
Boot Size
256 words
512 words
1024/ words
2048 words
1
1
0
0
1
0
1
0
4
8
0x0000-0x3EFF
0x0000-0x3DFF
0x0000-0x3BFF
0x0000-0x37FF
0x3F00-0x3FFF
0x3E00-0x3FFF
0x3C00-0x3FFF
0x3800-0x3FFF
0x3EFF
0x3DFF
0x3BFF
0x37FF
0x3F00
0x3E00
0x3C00
0x3800
16
32
Note:
1. The different BOOTSZ Fuse configurations are shown in Figure 24-2
Table 24-7. Read-While-Write Limit(1)
Section
Pages
224
Address
Read-While-Write section (RWW)
No Read-While-Write section (NRWW)
0x0000 - 0x37FF
0x3800 - 0x3FFF
32
Note:
1. For details about these two section, see ”NRWW – No Read-While-Write Section” on page 257 and ”RWW – Read-While-
Write Section” on page 257.
Table 24-8. Explanation of different variables used in Figure 24-3 and the mapping to the Z-pointer(1)
Corresponding
Variable
Z-value
Description
Most significant bit in the Program Counter. (Program Counter is 14 bits
PC[13:0])
PCMSB
13
5
Most significant bit which is used to address the words within one page
(64 words in a page requires six/seven bits PC [5:0]).
PAGEMSB
ZPCMSB
ZPAGEMSB
PCPAGE
Bit in Z-register that is mapped to PCMSB. Because Z0 is not used, the
ZPCMSB equals PCMSB + 1.
Z14
Z6
Bit in Z-register that is mapped to PCMSB. Because Z0 is not used, the
ZPAGEMSB equals PAGEMSB + 1.
Program Counter page address: Page select, for Page Erase and Page
Write
PC[13:6]
PC[5:0]
Z14:Z7
Z6:Z1
Program Counter word address: Word select, for filling temporary buffer
(must be zero during Page Write operation)
PCWORD
Note:
1. Z0: should be zero for all SPM commands, byte select for the LPM instruction. See ”Addressing the Flash During Self-Pro-
gramming” on page 261 for details about the use of Z-pointer during Self-Programming.
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24.9 Register Description
24.9.1
SPMCSR – Store Program Memory Control and Status Register
The Store Program Memory Control and Status Register contains the control bits needed to con-
trol 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
0
SPMEN
R/W
0
RWWSB
SPMCSR
0x37 (0x57)
Read/Write
Initial Value
R
0
R
0
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 SPMCSR 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 initi-
ated, 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 ATmega325P/3250P and always read as zero.
• Bit 4 – RWWSRE: Read-While-Write Section Read Enable
When programming (Page Erase or Page Write) to the RWW section, the RWW section is
blocked for reading (the RWWSB will be set by hardware). To re-enable the RWW section, the
user software must wait until the programming is completed (SPMEN will be cleared). Then, if
the RWWSRE bit is written to one at the same time as SPMEN, the next SPM instruction within
four clock cycles re-enables the RWW section. The RWW section cannot be re-enabled while
the Flash is busy with a Page Erase or a Page Write (SPMEN is set). If the RWWSRE bit is writ-
ten while the Flash is being loaded, the Flash load operation will abort and the data loaded will
be lost.
• Bit 3 – BLBSET: Boot Lock Bit Set
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock
cycles sets Boot Lock bits and general 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 SPMCSR Reg-
ister, will read either the Lock bits or the Fuse bits (depending on Z0 in the Z-pointer) into the
destination register. See ”Reading the Fuse and Lock Bits from Software” on page 264 for
details.
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• Bit 2 – PGWRT: Page Write
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock
cycles executes Page Write, with the data stored in the temporary buffer. The page address is
taken from the high part of the Z-pointer. The data in R1 and R0 are ignored. The PGWRT bit
will auto-clear upon completion of a Page Write, or if no SPM instruction is executed within four
clock cycles. The CPU is halted during the entire Page Write operation if the NRWW section is
addressed.
• Bit 1 – PGERS: Page Erase
If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock
cycles executes Page Erase. The page address is taken from the high part of the Z-pointer. The
data in R1 and R0 are ignored. The PGERS bit will auto-clear upon completion of a Page Erase,
or if no SPM instruction is executed within four clock cycles. The CPU is halted during the entire
Page Write operation if the NRWW section is addressed.
• Bit 0 – SPMEN: Store Program Memory Enable
This bit enables the SPM instruction for the next four clock cycles. If written to one together with
either RWWSRE, BLBSET, PGWRT’ or PGERS, the following SPM instruction will have a spe-
cial meaning, see description above. If only SPMEN is written, the following SPM instruction will
store the value in R1:R0 in the temporary page buffer addressed by the Z-pointer. The LSB of
the Z-pointer is ignored. The SPMEN bit will auto-clear upon completion of an SPM instruction,
or if no SPM instruction is executed within four clock cycles. During Page Erase and Page Write,
the SPMEN bit remains high until the operation is completed.
Writing any other combination than “10001”, “01001”, “00101”, “00011” or “00001” in the lower
five bits will have no effect.
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ATmega325P/3250P
25. Memory Programming
25.1 Program And Data Memory Lock Bits
The ATmega325P/3250P provides six Lock bits which can be left unprogrammed (“1”) or can be
programmed (“0”) to obtain the additional features listed in Table 25-2. The Lock bits can only be
erased to “1” with the Chip Erase command.
Table 25-1. Lock Bit Byte(1)
Lock Bit Byte
Bit No
Description
–
Default Value
7
6
5
4
3
2
1
0
1 (unprogrammed)
1 (unprogrammed)
1 (unprogrammed)
1 (unprogrammed)
1 (unprogrammed)
1 (unprogrammed)
1 (unprogrammed)
1 (unprogrammed)
–
BLB12
Boot Lock bit
Boot Lock bit
Boot Lock bit
Boot Lock bit
Lock bit
BLB11
BLB02
BLB01
LB2
LB1
Lock bit
Note:
Table 25-2. Lock Bit Protection Modes(1)(2)
Memory Lock Bits Protection Type
1. “1” means unprogrammed, “0” means programmed
LB Mode
LB2
LB1
1
1
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
0
0
Further programming and verification of the Flash and
EEPROM is disabled in Parallel and Serial Programming mode.
The Boot Lock bits and Fuse bits are locked in both Serial and
Parallel Programming mode.(1)
3
BLB0 Mode
BLB02
BLB01
No restrictions for SPM or LPM accessing the Application
section.
1
2
1
1
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
0
0
1
LPM executing from the Boot Loader section is not allowed to
read from the Application section. If Interrupt Vectors are placed
in the Boot Loader section, interrupts are disabled while
executing from the Application section.
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Table 25-2. Lock Bit Protection Modes(1)(2) (Continued)
Memory Lock Bits
Protection Type
BLB1 Mode
BLB12
BLB11
No restrictions for SPM or LPM accessing the Boot Loader
section.
1
2
1
1
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
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 and Boot Lock bits before programming the LB1 and LB2.
2. “1” means unprogrammed, “0” means programmed
25.2 Fuse Bits
The ATmega325P/3250P has three Fuse bytes. Table 25-3 - Table 25-5 describe briefly the
functionality 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 25-3. Extended Fuse Byte
Extended Fuse Byte
Bit No
Description
Default Value
–
–
–
–
–
7
–
1
6
5
4
5
2
1
0
–
1
–
1
–
1
–
1
BODLEVEL1(1)
BODLEVEL0(1)
RSTDISBL(2)
Brown-out Detector trigger level
Brown-out Detector trigger level
External Reset Disable
1 (unprogrammed)
1 (unprogrammed)
1 (unprogrammed)
Notes: 1. See ”System and Reset Characterizations” on page 308 for BODLEVEL Fuse decoding.
2. Port G, PG5 is input only. Pull-up is always on. See ”Alternate Functions of Port G” on page 79
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Table 25-4. Fuse High Byte
Fuse High Byte
Bit No
Description
Default Value
1 (unprogrammed, OCD
disabled)
OCDEN(4)
7
6
Enable OCD
0 (programmed, JTAG
enabled)
JTAGEN(5)
Enable JTAG
Enable Serial Program and Data
Downloading
0 (programmed, SPI
prog. enabled)
SPIEN(1)
WDTON(3)
EESAVE
5
4
3
Watchdog Timer always on
1 (unprogrammed)
EEPROM memory is preserved
through the Chip Erase
1 (unprogrammed,
EEPROM not preserved)
Select Boot Size (see Table 25-7 on
page 275 for details)
BOOTSZ1
2
0 (programmed)(2)
Select Boot Size (see Table 25-7 on
page 275 for details)
BOOTSZ0
BOOTRST
1
0
0 (programmed)(2)
1 (unprogrammed)
Select Reset Vector
Notes: 1. The SPIEN Fuse is not accessible in serial programming mode.
2. The default value of BOOTSZ1..0 results in maximum Boot Size. See Table 24-6 on page 268
for details.
3. See ”WDTCR – Watchdog Timer Control Register” on page 51 for details.
4. Never ship a product with the OCDEN Fuse programmed regardless of the setting of Lock bits
and JTAGEN Fuse. A programmed OCDEN Fuse enables some parts of the clock system to
be running in all sleep modes. This may increase the power consumption.
5. If the JTAG interface is left unconnected, the JTAGEN fuse should if possible be disabled. This
to avoid static current at the TDO pin in the JTAG interface.
Table 25-5. Fuse Low Byte
Fuse Low Byte
CKDIV8(4)
CKOUT(3)
SUT1
Bit No
Description
Default Value
7
6
5
4
3
2
1
0
Divide clock by 8
Clock output
0 (programmed)
1 (unprogrammed)
1 (unprogrammed)(1)
0 (programmed)(1)
0 (programmed)(2)
0 (programmed)(2)
1 (unprogrammed)(2)
0 (programmed)(2)
Select start-up time
Select start-up time
Select Clock source
Select Clock source
Select Clock source
Select Clock source
SUT0
CKSEL3
CKSEL2
CKSEL1
CKSEL0
Notes: 1. The default value of SUT1..0 results in maximum start-up time for the default clock source.
See ”System and Reset Characterizations” on page 308 for details.
2. The default setting of CKSEL3..0 results in internal RC Oscillator @ 8 MHz. See Table 8-6 on
page 33 for details.
3. The CKOUT Fuse allow the system clock to be output on PORTE7. See ”Clock Output Buffer”
on page 35 for details.
4. See ”System Clock Prescaler” on page 35 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.
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25.2.1
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.
25.3 Signature Bytes
All Atmel microcontrollers have a three-byte signature code which identifies the device. This
code can be read in both serial and parallel mode, also when the device is locked. The three
bytes reside in a separate address space. For the ATmega325P/3250P the signature bytes are
given in Table 25-6
Table 25-6. Device and JTAG ID
Signature Bytes Address
JTAG
Manufacture ID
Parts
0x000
0x001
0x95
0x95
0x002
Part Number
950D
ATmega325P
ATmega3250P
0x1E
0x1E
0x0D
0x0E
0x1F
0x1F
950E
25.4 Calibration Byte
The ATmega325P/3250P has a byte calibration value for the internal RC Oscillator. This byte
resides in the high byte of address 0x000 in the signature address space. During reset, this byte
is automatically written into the OSCCAL Register to ensure correct frequency of the calibrated
RC Oscillator.
25.5 Parallel Programming Parameters, Pin Mapping, and Commands
This section describes how to parallel program and verify Flash Program memory, EEPROM
Data memory, Memory Lock bits, and Fuse bits in the ATmega325P/3250P. Pulses are
assumed to be at least 250ns unless otherwise noted.
25.5.1
Signal Names
In this section, some pins of the ATmega325P/3250P are referenced by signal names describ-
ing their functionality during parallel programming, see Figure 25-1 on page 275 and Table 25-7
on page 275. Pins not described in the following table are referenced by pin names.
The XA1/XA0 pins determine the action executed when the XTAL1 pin is given a positive pulse.
The bit coding is shown in Table 25-9 on page 276.
When pulsing WR or OE, the command loaded determines the action executed. The different
Commands are shown in Table 25-10 on page 276.
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Figure 25-1. Parallel Programming
+5V
RDY/BSY
OE
PD1
PD2
PD3
PD4
PD5
PD6
PD7
VCC
+5V
WR
AVCC
BS1
PB7 - PB0
DATA
XA0
XA1
PAGEL
+12 V
BS2
RESET
PA0
XTAL1
GND
Table 25-7. 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
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
PD7
I
I
I
XTAL Action Bit 0
XTAL Action Bit 1
PAGEL
Program Memory and EEPROM data Page Load.
Byte Select 2 (“0” selects low byte, “1” selects 2’nd high
byte).
BS2
PA0
I
DATA
PB7-0
I/O Bi-directional Data bus (Output when OE is low).
Table 25-8. Pin Values Used to Enter Programming Mode
Pin
PAGEL
XA1
Symbol
Value
Prog_enable[3]
Prog_enable[2]
Prog_enable[1]
Prog_enable[0]
0
0
0
0
XA0
BS1
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Table 25-9. XA1 and XA0 Coding
XA1
XA0
Action when XTAL1 is Pulsed
0
0
Load Flash or EEPROM Address (High or low address byte determined by
BS1).
0
1
1
1
0
1
Load Data (High or Low data byte for Flash determined by BS1).
Load Command
No Action, Idle
Table 25-10. 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
Table 25-11. No. of Words in a Page and No. of Pages in the Flash
Flash Size
Page Size
PCWORD
No. of Pages
PCPAGE
PCMSB
16K words
(32K bytes)
64 words
PC[5:0]
256
PC[13:6]
13
Table 25-12. No. of Words in a Page and No. of Pages in the EEPROM
EEPROM Size
Page Size
PCWORD
No. of Pages
PCPAGE
EEAMSB
1K bytes
4 bytes
EEA[1:0]
256
EEA[13:2]
13
25.6 Parallel Programming
25.6.1
Enter Programming Mode
The following algorithm puts the device in Parallel (High-voltage) Programming mode:
1. Set Prog_enable pins listed in Table 25-8 on page 275 to “0000”, RESET pin and VCC to
0V.
2. Apply 4.5 - 5.5V between VCC and GND.
3. Ensure that VCC reaches at least 1.8V within the next 20 µs.
4. Wait 20 - 60 µs, and apply 11.5 - 12.5V to RESET.
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5. Keep the Prog_enable pins unchanged for at least 10µs after the High-voltage has been
applied to ensure the Prog_enable Signature has been latched.
6. Wait at least 300 µs before giving any parallel programming commands.
7. Exit Programming mode by power the device down or by bringing RESET pin to 0V.
If the rise time of the VCC is unable to fulfill the requirements listed above, the following alterna-
tive algorithm can be used.
1. Set Prog_enable pins listed in Table 25-8 on page 275 to “0000”, RESET pin to 0V and
V
CC to 0V.
2. Apply 4.5 - 5.5V between VCC and GND.
3. Monitor VCC, and as soon as VCC reaches 0.9 - 1.1V, apply 11.5 - 12.5V to RESET.
4. Keep the Prog_enable pins unchanged for at least 10µs after the High-voltage has been
applied to ensure the Prog_enable Signature has been latched.
5. Wait until VCC actually reaches 4.5 -5.5V before giving any parallel programming
commands.
6. Exit Programming mode by power the device down or by bringing RESET pin to 0V.
25.6.2
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.
25.6.3
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 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.
25.6.4
Programming the Flash
The Flash is organized in pages, see Table 25-11 on page 276. When programming the Flash,
the program data is latched into a page buffer. This allows one page of program data to be pro-
grammed simultaneously. The following procedure describes how to program the entire Flash
memory:
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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 25-3 for signal
waveforms)
F. Repeat B through E until the entire buffer is filled or until all data within the page is loaded.
While the lower bits in the address are mapped to words within the page, the higher bits address
the pages within the FLASH. This is illustrated in Figure 25-2 on page 279. 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. Give WR a negative pulse. This starts programming of the entire page of data. RDY/BSY
goes low.
2. Wait until RDY/BSY goes high (See Figure 25-3 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
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1. 1. Set XA1, XA0 to “10”. This enables command loading.
2. Set DATA to “0000 0000”. This is the command for No Operation.
3. Give XTAL1 a positive pulse. This loads the command, and the internal write signals are
reset.
Figure 25-2. Addressing the Flash Which is Organized in Pages(1)
PCMSB
PAGEMSB
PROGRAM
COUNTER
PCPAGE
PCWORD
PAGE ADDRESS
WITHIN THE FLASH
WORD ADDRESS
WITHIN A PAGE
PROGRAM MEMORY
PAGE
PAGE
INSTRUCTION WORD
PCWORD[PAGEMSB:0]:
00
01
02
PAGEEND
Note:
1. PCPAGE and PCWORD are listed in Table 25-11 on page 276.
Figure 25-3. 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
ADDR. HIGH
XX
XX
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.
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25.6.5
Programming the EEPROM
The EEPROM is organized in pages, see Table 25-12 on page 276. 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 277 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 25-4 for
signal waveforms).
Figure 25-4. Programming the EEPROM Waveforms
K
A
G
B
C
E
B
C
E
L
0x11
ADDR. HIGH
ADDR. LOW
DATA
ADDR. LOW
DATA
XX
XX
DATA
XA1
XA0
BS1
XTAL1
WR
RDY/BSY
RESET +12V
OE
PAGEL
BS2
25.6.6
Reading the Flash
The algorithm for reading the Flash memory is as follows (refer to ”Programming the Flash” on
page 277 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”.
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25.6.7
Reading the EEPROM
The algorithm for reading the EEPROM memory is as follows (refer to ”Programming the Flash”
on page 277 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”.
25.6.8
25.6.9
Programming the Fuse Low Bits
The algorithm for programming the Fuse Low bits is as follows (refer to ”Programming the Flash”
on page 277 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. Give WR a negative pulse and wait for RDY/BSY to go high.
Programming the Fuse High Bits
The algorithm for programming the Fuse High bits is as follows (refer to ”Programming the
Flash” on page 277 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.
25.6.10 Programming the Extended Fuse Bits
The algorithm for programming the Extended Fuse bits is as follows (refer to ”Programming the
Flash” on page 277 for details on Command and Data loading):
1. 1. A: Load Command “0100 0000”.
2. 2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit.
3. 3. Set BS1 to “0” and BS2 to “1”. This selects extended data byte.
4. 4. Give WR a negative pulse and wait for RDY/BSY to go high.
5. 5. Set BS2 to “0”. This selects low data byte.
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Figure 25-5. Programming the FUSES Waveforms
Write Fuse Low byte
Write Fuse high byte
Write Extended Fuse byte
A
C
A
C
A
C
0x40
DATA
XX
0x40
DATA
XX
0x40
DATA
XX
DATA
XA1
XA0
BS1
BS2
XTAL1
WR
RDY/BSY
RESET +12V
OE
PAGEL
25.6.11 Programming the Lock Bits
The algorithm for programming the Lock bits is as follows (refer to ”Programming the Flash” on
page 277 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. If LB mode 3 is programmed
(LB1 and LB2 is programmed), it is not possible to program the Boot Lock bits by any
External Programming mode.
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.
25.6.12 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 277 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 “1”, and BS1 to “0”. The status of the Extended Fuse bits can now
be read at DATA (“0” means programmed).
5. 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).
6. Set OE to “1”.
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Figure 25-6. Mapping Between BS1, BS2 and the Fuse and Lock Bits During Read
0
Fuse Low Byte
Extended Fuse Byte
Lock Bits
0
1
1
0
DATA
BS2
BS1
Fuse High Byte
1
BS2
25.6.13 Reading the Signature Bytes
The algorithm for reading the Signature bytes is as follows (refer to ”Programming the Flash” on
page 277 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”.
25.6.14 Reading the Calibration Byte
The algorithm for reading the Calibration byte is as follows (refer to ”Programming the Flash” on
page 277 for details on Command and Address loading):
1. A: Load Command “0000 1000”.
2. B: Load Address Low Byte, 0x00.
3. Set OE to “0”, and BS1 to “1”. The Calibration byte can now be read at DATA.
4. Set OE to “1”.
25.6.15 Parallel Programming Characteristics
Figure 25-7. Parallel Programming Timing, Including some General Timing Requirements
tXLWL
tXHXL
XTAL1
tDVXH
tXLDX
Data & Contol
(DATA, XA0/1, BS1, BS2)
tBVPH
tPLBX tBVWL
tWLBX
PAGEL
tPHPL
tWLWH
WR
tPLWL
WLRL
RDY/BSY
tWLRH
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Figure 25-8. 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)
tXLPH
tXLXH
tPLXH
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 25-7 (i.e., tDVXH, tXHXL, and tXLDX) also apply to load-
ing operation.
Figure 25-9. 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)
tXLOL
XTAL1
BS1
tBVDV
tOLDV
OE
tOHDZ
ADDR1 (Low Byte)
DATA (High Byte)
DATA
ADDR0 (Low Byte)
DATA (Low Byte)
XA0
XA1
Note:
1. The timing requirements shown in Figure 25-7 (i.e., tDVXH, tXHXL, and tXLDX) also apply to read-
ing operation.
Table 25-13. Parallel Programming Characteristics, VCC = 5V 10%
Symbol
VPP
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
IPP
μA
ns
tDVXH
tXLXH
tXHXL
tXLDX
tXLWL
67
200
150
67
ns
ns
Data and Control Hold after XTAL1 Low
XTAL1 Low to WR Low
ns
0
ns
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Table 25-13. Parallel Programming Characteristics, VCC = 5V 10% (Continued)
Symbol
tXLPH
Parameter
Min
0
Typ
Max
Units
ns
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
BS1 Valid to WR Low
tPLXH
150
67
150
67
67
67
67
150
0
ns
tBVPH
tPHPL
ns
ns
tPLBX
ns
tWLBX
tPLWL
tBVWL
tWLWH
tWLRL
tWLRH
tWLRH_CE
tXLOL
ns
ns
ns
WR Pulse Width Low
ns
WR Low to RDY/BSY Low
WR Low to RDY/BSY High(1)
WR Low to RDY/BSY High for Chip Erase(2)
XTAL1 Low to OE Low
1
4.5
9
μs
ms
ms
ns
3.7
7.5
0
tBVDV
tOLDV
tOHDZ
BS1 Valid to DATA valid
OE Low to DATA Valid
0
250
250
250
ns
ns
OE High to DATA Tri-stated
ns
Notes: 1. tWLRH is valid for the Write Flash, Write EEPROM, Write Fuse bits and Write Lock bits
commands.
2. tWLRH_CE is valid for the Chip Erase command.
25.7 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 (out-
put). After RESET is set low, the Programming Enable instruction needs to be executed first
before program/erase operations can be executed. NOTE, in Table 25-14 on page 285, the pin
mapping for SPI programming is listed. Not all parts use the SPI pins dedicated for the internal
SPI interface.
25.7.1
Serial Programming Pin Mapping
Table 25-14. Pin Mapping Serial Programming
Symbol
MOSI
MISO
SCK
Pins
PB2
PB3
PB1
I/O
Description
Serial Data in
Serial Data out
Serial Clock
I
O
I
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Figure 25-10. Serial Programming and Verify(1)
+1.8 - 5.5V
+1.8 - 5.5V
VCC
(2)
MOSI
MISO
AVCC
SCK
XTAL1
RESET
GND
Notes: 1. If the device is clocked by the internal Oscillator, it is no need to connect a clock source to the
XTAL1 pin.
2. VCC - 0.3V < AVCC < VCC + 0.3V, however, AVCC should always be within 1.8 - 5.5V
When programming the EEPROM, an auto-erase cycle is built into the self-timed programming
operation (in the Serial mode ONLY) and there is no need to first execute the Chip Erase
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:
Low: > 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles for fck >= 12 MHz
High: > 2 CPU clock cycles for fck < 12 MHz, 3 CPU clock cycles for fck >= 12 MHz
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ATmega325P/3250P
Serial Programming
Algorithm
When writing serial data to the ATmega325P/3250P, data is clocked on the rising edge of SCK.
When reading data from the ATmega325P/3250P, data is clocked on the falling edge of SCK.
See Figure 25-11 for timing details.
To program and verify the ATmega325P/3250P in the serial programming mode, the following
sequence is recommended (See four byte instruction formats in Table 25-16):
1. Power-up sequence:
Apply power between VCC and GND while RESET and SCK are set to “0”. In some sys-
tems, the programmer can not guarantee that SCK is held low during power-up. In this
case, RESET must be given a positive pulse of at least two CPU clock cycles duration
after SCK has been set to “0”.
2. Wait for at least 20 ms and enable serial programming by sending the Programming
Enable serial instruction to pin MOSI.
3. The serial programming instructions will not work if the communication is out of synchro-
nization. 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 25-11 on
page 276. The memory page is loaded one byte at a time by supplying the 6 LSB of the
address and data together with the Load Program Memory Page instruction. To ensure
correct loading of the page, the data low byte must be loaded before data high byte is
applied for a given address. The Program Memory Page is stored by loading the Write
Program Memory Page instruction with the 8 MSB of the address. If polling is not used,
the user must wait at least tWD_FLASH before issuing the next page. (See Table 25-15.)
Accessing the serial programming interface before the Flash write operation completes
can result in incorrect programming.
5. A: 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 (RDY/BSY) is not used, the
user must wait at least tWD_EEPROM before issuing the next byte (See Table 25-15.) In a
chip erased device, no 0xFFs in the data file(s) need to be programmed.
B: The EEPROM array is programmed one page at a time. The Memory page is loaded
one byte at a time by supplying the 2 LSB of the address and data together with the Load
EEPROM Memory Page instruction. The EEPROM Memory Page is stored by loading
the Write EEPROM Memory Page Instruction with the 4 MSB of the address. When using
EEPROM page access only byte locations loaded with the Load EEPROM Memory Page
instruction is altered. The remaining locations remain unchanged. If polling (RDY/BSY) is
not used, the used must wait at least tWD_EEPROM before issuing the next page (See Table
25-12). In a chip erased device, no 0xFF in the data file(s) need to be programmed.
6. Any memory location can be verified by using the Read instruction which returns the con-
tent at the selected address at serial output MISO.
7. At the end of the programming session, RESET can be set high to commence normal
operation.
8. Power-off sequence (if needed):
Set RESET to “1”.
Turn VCC power off.
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Table 25-15. Minimum Wait Delay Before Writing the Next Flash or EEPROM Location
Symbol
Minimum Wait Delay
4.5 ms
tWD_FUSE
tWD_FLASH
tWD_EEPROM
tWD_ERASE
4.5 ms
9.0 ms
9.0 ms
Figure 25-11. Serial Programming Waveforms
SERIAL DATA INPUT
(MOSI)
MSB
LSB
LSB
SERIAL DATA OUTPUT
(MISO)
MSB
SERIAL CLOCK INPUT
(SCK)
SAMPLE
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ATmega325P/3250P
Serial Programming
Instruction set
Table 25-16 and Figure 25-12 on page 290 describes the Instruction set.
Table 25-16. Serial Programming Instruction Set
Instruction Format
Instruction/Operation
Byte 1
$AC
Byte 2
Byte 3
$00
Byte4
$00
Programming Enable
$53
$80
$00
Chip Erase (Program Memory/EEPROM)
Poll RDY/BSY
$AC
$00
$00
$F0
$00
data byte out
Load Instructions
Load Extended Address byte(1)
Load Program Memory Page, High byte
Load Program Memory Page, Low byte
$4D
$48
$40
$00
$00
$00
Extended adr
adr LSB
$00
high data byte in
low data byte in
adr LSB
0000 00aa
/
Load EEPROM Memory Page (page access)
$C1
$00
data byte in
0000 0aaa
Read Instructions
Read Program Memory, High byte
Read Program Memory, Low byte
$28
$20
adr MSB
adr MSB
adr LSB
adr LSB
high data byte out
low data byte out
0000 00aa
/
Read EEPROM Memory
$A0
aaaa aaaa
data byte out
0000 0aaa
Read Lock bits
$58
$30
$50
$58
$50
$38
$00
$00
$00
$08
$08
$00
$00
0000 000aa
$00
data byte out
data byte out
data byte out
data byte out
data byte out
data byte out
Read Signature Byte
Read Fuse bits
Read Fuse High bits
Read Extended Fuse Bits
Read Calibration Byte
Write Instructions(6)
Write Program Memory Page
$00
$00
$00
$4C
$C0
adr MSB
adr LSB
$00
0000 00aa
/
Write EEPROM Memory
aaaa aaaa
data byte in
0000 0aaa
0000 00aa
/
aaaa aa00
/
Write EEPROM Memory Page (page access)
$C2
$00
0000 0aaa
aaaa a000
Write Lock bits
$AC
$AC
$AC
$AC
$E0
$A0
$A8
$A4
$00
$00
$00
$00
data byte in
data byte in
data byte in
data byte in
Write Fuse bits
Write Fuse High bits
Write Extended Fuse Bits
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Notes: 1. Not all instructions are applicable for all parts
2. a = address
3. Bits are programmed ‘0’, unprogrammed ‘1’.
4. To ensure future compatibility, unused Fuses and Lock bits should be unprogrammed (‘1’) .
5. Refer to the correspondig section for Fuse and Lock bits, Calibration and Signature bytes and
Page size.
6. Instructions accessing program memory use word address. This address may be random
within the page range.
7. See htt://www.atmel.com/avr for Application Notes regarding programming and programmers.
If the LSB in RDY/BSY data byte out is ‘1’, a programming operation is still pending. Wait until
this bit returns ‘0’ before the next instruction is carried out.
Within the same page, the low data byte must be loaded prior to the high data byte.
After data is loaded to the page buffer, program the EEPROM page, see Figure 25-12.
Figure 25-12. Serial Programming Instruction example
Serial Programming Instruction
Load Program Memory Page (High/Low Byte)/
Load EEPROM Memory Page (page access)
Write Program Memory Page/
Write EEPROM Memory Page
Byte 1
Byte 2
Byte 3
Byte 4
Byte 1
Byte 2
Byte 3
Byte 4
Adr MSB
Adr LSB
Adr MSB
Adr LSB
Bit 15 B
0
Bit 15 B
0
Page Buffer
Page Offset
Page 0
Page 1
Page 2
Page Number
Page N-1
Program Memory/
EEPROM Memory
25.7.2
SPI Serial Programming Characteristics
For characteristics of the SPI module, see ”SPI Timing Characteristics” on page 310.
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ATmega325P/3250P
25.8 Programming via the JTAG Interface
Programming through the JTAG interface requires control of the four JTAG specific pins: TCK,
TMS, TDI, and TDO. Control of the reset and clock pins is not required.
To be able to use the JTAG interface, the JTAGEN Fuse must be programmed. The device is
default shipped with the fuse programmed. In addition, the JTD bit in MCUCR must be cleared.
Alternatively, if the JTD bit is set, the external reset can be forced low. Then, the JTD bit will be
cleared after two chip clocks, and the JTAG pins are available for programming. This provides a
means of using the JTAG pins as normal port pins in Running mode while still allowing In-Sys-
tem Programming via the JTAG interface. Note that this technique can not be used when using
the JTAG pins for Boundary-scan or On-chip Debug. In these cases the JTAG pins must be ded-
icated for this purpose.
During programming the clock frequency of the TCK Input must be less than the maximum fre-
quency of the chip. The System Clock Prescaler can not be used to divide the TCK Clock Input
into a sufficiently low frequency.
As a definition in this data sheet, the LSB is shifted in and out first of all Shift Registers.
25.8.1
Programming Specific JTAG Instructions
The Instruction Register is 4-bit wide, supporting up to 16 instructions. The JTAG instructions
useful for programming are listed below.
The OPCODE for each instruction is shown behind the instruction name in hex format. The text
describes which Data Register is selected as path between TDI and TDO for each instruction.
The Run-Test/Idle state of the TAP controller is used to generate internal clocks. It can also be
used as an idle state between JTAG sequences. The state machine sequence for changing the
instruction word is shown in Figure 25-13.
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Figure 25-13. State Machine Sequence for Changing the Instruction Word
1
Test-Logic-Reset
0
1
1
1
0
Run-Test/Idle
Select-DR Scan
Select-IR Scan
0
0
1
1
Capture-DR
Capture-IR
0
0
Shift-DR
0
Shift-IR
0
1
Exit1-DR
0
1
Exit1-IR
0
1
1
Pause-DR
1
0
Pause-IR
1
0
0
0
Exit2-DR
1
Exit2-IR
1
Update-DR
Update-IR
1
1
0
0
25.8.2
AVR_RESET (0xC)
The AVR specific public JTAG instruction for setting the AVR device in the Reset mode or taking
the device out from the Reset mode. The TAP controller is not reset by this instruction. The one
bit Reset Register is selected as Data Register. Note that the reset will be active as long as there
is a logic “one” in the Reset Chain. The output from this chain is not latched.
The active states are:
• Shift-DR: The Reset Register is shifted by the TCK input.
25.8.3
PROG_ENABLE (0x4)
The AVR specific public JTAG instruction for enabling programming via the JTAG port. The 16-
bit Programming Enable Register is selected as Data Register. The active states are the
following:
• Shift-DR: The programming enable signature is shifted into the Data Register.
• Update-DR: The programming enable signature is compared to the correct value, and
Programming mode is entered if the signature is valid.
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ATmega325P/3250P
25.8.4
PROG_COMMANDS (0x5)
The AVR specific public JTAG instruction for entering programming commands via the JTAG
port. The 15-bit Programming Command Register is selected as Data Register. The active
states are the following:
• Capture-DR: The result of the previous command is loaded into the Data Register.
• Shift-DR: The Data Register is shifted by the TCK input, shifting out the result of the previous
command and shifting in the new command.
• Update-DR: The programming command is applied to the Flash inputs
• Run-Test/Idle: One clock cycle is generated, executing the applied command (not always
required, see Table 25-17 below).
25.8.5
PROG_PAGELOAD (0x6)
The AVR specific public JTAG instruction to directly load the Flash data page via the JTAG port.
An 8-bit Flash Data Byte Register is selected as the Data Register. This is physically the 8 LSBs
of the Programming Command Register. The active states are the following:
• Shift-DR: The Flash Data Byte Register is shifted by the TCK input.
• Update-DR: The content of the Flash Data Byte Register is copied into a temporary register. A
write sequence is initiated that within 11 TCK cycles loads the content of the temporary
register into the Flash page buffer. The AVR automatically alternates between writing the low
and the high byte for each new Update-DR state, starting with the low byte for the first Update-
DR encountered after entering the PROG_PAGELOAD command. The Program Counter is
pre-incremented before writing the low byte, except for the first written byte. This ensures that
the first data is written to the address set up by PROG_COMMANDS, and loading the last
location in the page buffer does not make the program counter increment into the next page.
25.8.6
PROG_PAGEREAD (0x7)
The AVR specific public JTAG instruction to directly capture the Flash content via the JTAG port.
An 8-bit Flash Data Byte Register is selected as the Data Register. This is physically the 8 LSBs
of the Programming Command Register. The active states are the following:
• Capture-DR: The content of the selected Flash byte is captured into the Flash Data Byte
Register. The AVR automatically alternates between reading the low and the high byte for each
new Capture-DR state, starting with the low byte for the first Capture-DR encountered after
entering the PROG_PAGEREAD command. The Program Counter is post-incremented after
reading each high byte, including the first read byte. This ensures that the first data is captured
from the first address set up by PROG_COMMANDS, and reading the last location in the page
makes the program counter increment into the next page.
• Shift-DR: The Flash Data Byte Register is shifted by the TCK input.
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25.8.7
Data Registers
The Data Registers are selected by the JTAG instruction registers described in section ”Pro-
gramming Specific JTAG Instructions” on page 291. The Data Registers relevant for
programming operations are:
• Reset Register
• Programming Enable Register
• Programming Command Register
• Flash Data Byte Register
25.8.8
Reset Register
The Reset Register is a Test Data Register used to reset the part during programming. It is
required to reset the part before entering Programming mode.
A high value in the Reset Register corresponds to pulling the external reset low. The part is reset
as long as there is a high value present in the Reset Register. Depending on the Fuse settings
for the clock options, the part will remain reset for a Reset Time-out period (refer to ”Clock
Sources” on page 30) after releasing the Reset Register. The output from this Data Register is
not latched, so the reset will take place immediately, as shown in Figure 23-1 on page 231.
25.8.9
Programming Enable Register
The Programming Enable Register is a 16-bit register. The contents of this register is compared
to the programming enable signature, binary code 0b1010_0011_0111_0000. When the con-
tents of the register is equal to the programming enable signature, programming via the JTAG
port is enabled. The register is reset to 0 on Power-on Reset, and should always be reset when
leaving Programming mode.
Figure 25-14. Programming Enable Register
TDI
0xA370
D
D
Q
A
T
A
Programming Enable
=
ClockDR & PROG_ENABLE
TDO
25.8.10 Programming Command Register
The Programming Command Register is a 15-bit register. This register is used to serially shift in
programming commands, and to serially shift out the result of the previous command, if any. The
JTAG Programming Instruction Set is shown in Table 25-17. The state sequence when shifting
in the programming commands is illustrated in Figure 25-16.
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ATmega325P/3250P
Figure 25-15. Programming Command Register
TDI
S
T
R
O
B
E
S
Flash
EEPROM
Fuses
A
D
D
R
E
S
S
/
Lock Bits
D
A
T
A
TDO
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Table 25-17. JTAG Programming Instruction Set
a = address high bits, b = address low bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in, x = don’t care
Instruction
TDI Sequence
TDO Sequence
Notes
0100011_10000000
0110001_10000000
0110011_10000000
0110011_10000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
1a. Chip Erase
1b. Poll for Chip Erase Complete
2a. Enter Flash Write
0110011_10000000
0100011_00010000
0000111_aaaaaaaa
0000011_bbbbbbbb
0010011_iiiiiiii
xxxxxox_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(2)
(9)
2b. Load Address High Byte
2c. Load Address Low Byte
2d. Load Data Low Byte
2e. Load Data High Byte
0010111_iiiiiiii
0110111_00000000
1110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
2f. Latch Data
(1)
(1)
0110111_00000000
0110101_00000000
0110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
2g. Write Flash Page
2h. Poll for Page Write Complete
3a. Enter Flash Read
0110111_00000000
0100011_00000010
0000111_aaaaaaaa
0000011_bbbbbbbb
xxxxxox_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(2)
(9)
3b. Load Address High Byte
3c. Load Address Low Byte
0110010_00000000
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
xxxxxxx_oooooooo
3d. Read Data Low and High Byte
Low byte
High byte
4a. Enter EEPROM Write
4b. Load Address High Byte
4c. Load Address Low Byte
4d. Load Data Byte
0100011_00010001
0000111_aaaaaaaa
0000011_bbbbbbbb
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(9)
0110111_00000000
1110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
4e. Latch Data
(1)
(1)
0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
4f. Write EEPROM Page
4g. Poll for Page Write Complete
5a. Enter EEPROM Read
0110011_00000000
0100011_00000011
0000111_aaaaaaaa
xxxxxox_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(2)
(9)
5b. Load Address High Byte
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ATmega325P/3250P
Table 25-17. JTAG Programming Instruction Set (Continued)
a = address high bits, b = address low bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in, x = don’t care
Instruction
TDI Sequence
TDO Sequence
Notes
5c. Load Address Low Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
0110011_bbbbbbbb
0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
5d. Read Data Byte
6a. Enter Fuse Write
0100011_01000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
6b. Load Data Low Byte(6)
0010011_iiiiiiii
(3)
(1)
0111011_00000000
0111001_00000000
0111011_00000000
0111011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
6c. Write Fuse Extended Byte
6d. Poll for Fuse Write Complete
6e. Load Data Low Byte(7)
0110111_00000000
xxxxxox_xxxxxxxx
(2)
(3)
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
0110111_00000000
0110101_00000000
0110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
6f. Write Fuse High Byte
(1)
6g. Poll for Fuse Write Complete
6h. Load Data Low Byte(7)
0110111_00000000
xxxxxox_xxxxxxxx
(2)
(3)
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
6i. Write Fuse Low Byte
(1)
6j. Poll for Fuse Write Complete
7a. Enter Lock Bit Write
7b. Load Data Byte(9)
0110011_00000000
0100011_00100000
0010011_11iiiiii
xxxxxox_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(2)
(4)
0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
7c. Write Lock Bits
(1)
(2)
7d. Poll for Lock Bit Write complete
8a. Enter Fuse/Lock Bit Read
0110011_00000000
0100011_00000100
xxxxxox_xxxxxxxx
xxxxxxx_xxxxxxxx
0111010_00000000
0111011_00000000
xxxxxxx_xxxxxxxx
8b. Read Extended Fuse Byte(6)
8c. Read Fuse High Byte(7)
8d. Read Fuse Low Byte(8)
8e. Read Lock Bits(9)
xxxxxxx_oooooooo
0111110_00000000
0111111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
(5)
xxxxxxx_xxoooooo
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Table 25-17. JTAG Programming Instruction Set (Continued)
a = address high bits, b = address low bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in, x = don’t care
Instruction
TDI Sequence
TDO Sequence
Notes
0111010_00000000
0111110_00000000
0110010_00000000
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
xxxxxxx_oooooooo
xxxxxxx_oooooooo
xxxxxxx_oooooooo
(5)
Fuse Ext. byte
Fuse High byte
Fuse Low byte
Lock bits
8f. Read Fuses and Lock Bits
9a. Enter Signature Byte Read
9b. Load Address Byte
0100011_00001000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
0000011_bbbbbbbb
0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
9c. Read Signature Byte
xxxxxxx_oooooooo
10a. Enter Calibration Byte Read
10b. Load Address Byte
0100011_00001000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
0000011_bbbbbbbb
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
10c. Read Calibration Byte
xxxxxxx_oooooooo
0100011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
11a. Load No Operation Command
Notes: 1. This command sequence is not required if the seven MSB are correctly set by the previous command sequence (which is
normally the case).
2. Repeat until o = “1”.
3. Set bits to “0” to program the corresponding Fuse, “1” to unprogram the Fuse.
4. Set bits to “0” to program the corresponding Lock bit, “1” to leave the Lock bit unchanged.
5. “0” = programmed, “1” = unprogrammed.
6. The bit mapping for Fuses Extended byte is listed in Table 25-3 on page 272
7. The bit mapping for Fuses High byte is listed in Table 25-4 on page 273
8. The bit mapping for Fuses Low byte is listed in Table 25-5 on page 273
9. The bit mapping for Lock bits byte is listed in Table 25-1 on page 271
10. Address bits exceeding PCMSB and EEAMSB (Table 25-11 and Table 25-12) are don’t care
11. All TDI and TDO sequences are represented by binary digits (0b...).
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ATmega325P/3250P
Figure 25-16. State Machine Sequence for Changing/Reading the Data Word
1
Test-Logic-Reset
0
1
1
1
0
Run-Test/Idle
Select-DR Scan
Select-IR Scan
0
0
1
1
Capture-DR
Capture-IR
0
0
Shift-DR
0
Shift-IR
0
1
Exit1-DR
0
1
1
1
Exit1-IR
0
Pause-IR
1
Pause-DR
1
0
0
0
0
Exit2-DR
1
Exit2-IR
1
Update-DR
Update-IR
1
1
0
0
25.8.11 Flash Data Byte Register
The Flash Data Byte Register provides an efficient way to load the entire Flash page buffer
before executing Page Write, or to read out/verify the content of the Flash. A state machine sets
up the control signals to the Flash and senses the strobe signals from the Flash, thus only the
data words need to be shifted in/out.
The Flash Data Byte Register actually consists of the 8-bit scan chain and a 8-bit temporary reg-
ister. During page load, the Update-DR state copies the content of the scan chain over to the
temporary register and initiates a write sequence that within 11 TCK cycles loads the content of
the temporary register into the Flash page buffer. The AVR automatically alternates between
writing the low and the high byte for each new Update-DR state, starting with the low byte for the
first Update-DR encountered after entering the PROG_PAGELOAD command. The Program
Counter is pre-incremented before writing the low byte, except for the first written byte. This
ensures that the first data is written to the address set up by PROG_COMMANDS, and loading
the last location in the page buffer does not make the Program Counter increment into the next
page.
During Page Read, the content of the selected Flash byte is captured into the Flash Data Byte
Register during the Capture-DR state. The AVR automatically alternates between reading the
low and the high byte for each new Capture-DR state, starting with the low byte for the first Cap-
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ture-DR encountered after entering the PROG_PAGEREAD command. The Program Counter is
post-incremented after reading each high byte, including the first read byte. This ensures that
the first data is captured from the first address set up by PROG_COMMANDS, and reading the
last location in the page makes the program counter increment into the next page.
Figure 25-17. Flash Data Byte Register
STROBES
State
Machine
TDI
ADDRESS
Flash
EEPROM
Fuses
Lock Bits
D
A
T
A
TDO
The state machine controlling the Flash Data Byte Register is clocked by TCK. During normal
operation in which eight bits are shifted for each Flash byte, the clock cycles needed to navigate
through the TAP controller automatically feeds the state machine for the Flash Data Byte Regis-
ter with sufficient number of clock pulses to complete its operation transparently for the user.
However, if too few bits are shifted between each Update-DR state during page load, the TAP
controller should stay in the Run-Test/Idle state for some TCK cycles to ensure that there are at
least 11 TCK cycles between each Update-DR state.
25.8.12 Programming Algorithm
All references below of type “1a”, “1b”, and so on, refer to Table 25-17.
25.8.13 Entering Programming Mode
1. Enter JTAG instruction AVR_RESET and shift 1 in the Reset Register.
2. Enter instruction PROG_ENABLE and shift 0b1010_0011_0111_0000 in the Program-
ming Enable Register.
25.8.14 Leaving Programming Mode
1. Enter JTAG instruction PROG_COMMANDS.
2. Disable all programming instructions by using no operation instruction 11a.
3. Enter instruction PROG_ENABLE and shift 0b0000_0000_0000_0000 in the program-
ming Enable Register.
4. Enter JTAG instruction AVR_RESET and shift 0 in the Reset Register.
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25.8.15 Performing Chip Erase
1. Enter JTAG instruction PROG_COMMANDS.
2. Start Chip Erase using programming instruction 1a.
3. Poll for Chip Erase complete using programming instruction 1b, or wait for tWLRH_CE (refer
to Table 25-13 on page 284).
25.8.16 Programming the Flash
Before programming the Flash a Chip Erase must be performed, see “Performing Chip Erase”
on page 301.
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash write using programming instruction 2a.
3. Load address High byte using programming instruction 2b.
4. Load address Low byte using programming instruction 2c.
5. Load data using programming instructions 2d, 2e and 2f.
6. Repeat steps 4 and 5 for all instruction words in the page.
7. Write the page using programming instruction 2g.
8. Poll for Flash write complete using programming instruction 2h, or wait for tWLRH (refer to
Table 25-13 on page 284).
9. Repeat steps 3 to 7 until all data have been programmed.
A more efficient data transfer can be achieved using the PROG_PAGELOAD instruction:
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash write using programming instruction 2a.
3. Load the page address using programming instructions 2b and 2c. PCWORD (refer to
Table 25-11 on page 276) is used to address within one page and must be written as 0.
4. Enter JTAG instruction PROG_PAGELOAD.
5. Load the entire page by shifting in all instruction words in the page byte-by-byte, starting
with the LSB of the first instruction in the page and ending with the MSB of the last
instruction in the page. Use Update-DR to copy the contents of the Flash Data Byte Reg-
ister into the Flash page location and to auto-increment the Program Counter before
each new word.
6. Enter JTAG instruction PROG_COMMANDS.
7. Write the page using programming instruction 2g.
8. Poll for Flash write complete using programming instruction 2h, or wait for tWLRH (refer to
Table 25-13 on page 284).
9. Repeat steps 3 to 8 until all data have been programmed.
25.8.17 Reading the Flash
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash read using programming instruction 3a.
3. Load address using programming instructions 3b and 3c.
4. Read data using programming instruction 3d.
5. Repeat steps 3 and 4 until all data have been read.
A more efficient data transfer can be achieved using the PROG_PAGEREAD instruction:
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash read using programming instruction 3a.
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3. Load the page address using programming instructions 3b and 3c. PCWORD (refer to
Table 25-11 on page 276) is used to address within one page and must be written as 0.
4. Enter JTAG instruction PROG_PAGEREAD.
5. Read the entire page (or Flash) by shifting out all instruction words in the page (or Flash),
starting with the LSB of the first instruction in the page (Flash) and ending with the MSB
of the last instruction in the page (Flash). The Capture-DR state both captures the data
from the Flash, and also auto-increments the program counter after each word is read.
Note that Capture-DR comes before the shift-DR state. Hence, the first byte which is
shifted out contains valid data.
6. Enter JTAG instruction PROG_COMMANDS.
7. Repeat steps 3 to 6 until all data have been read.
25.8.18 Programming the EEPROM
Before programming the EEPROM a Chip Erase must be performed, see “Performing Chip
Erase” on page 301.
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable EEPROM write using programming instruction 4a.
3. Load address High byte using programming instruction 4b.
4. Load address Low byte using programming instruction 4c.
5. Load data using programming instructions 4d and 4e.
6. Repeat steps 4 and 5 for all data bytes in the page.
7. Write the data using programming instruction 4f.
8. Poll for EEPROM write complete using programming instruction 4g, or wait for tWLRH
(refer to Table 25-13 on page 284).
9. Repeat steps 3 to 8 until all data have been programmed.
Note that the PROG_PAGELOAD instruction can not be used when programming the EEPROM.
25.8.19 Reading the EEPROM
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable EEPROM read using programming instruction 5a.
3. Load address using programming instructions 5b and 5c.
4. Read data using programming instruction 5d.
5. Repeat steps 3 and 4 until all data have been read.
Note that the PROG_PAGEREAD instruction can not be used when reading the EEPROM.
25.8.20 Programming the Fuses
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Fuse write using programming instruction 6a.
3. Load data high byte using programming instructions 6b. A bit value of “0” will program the
corresponding fuse, a “1” will unprogram the fuse.
4. Write Fuse High byte using programming instruction 6c.
5. Poll for Fuse write complete using programming instruction 6d, or wait for tWLRH (refer to
Table 25-13 on page 284).
6. Load data low byte using programming instructions 6e. A “0” will program the fuse, a “1”
will unprogram the fuse.
7. Write Fuse low byte using programming instruction 6f.
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8. Poll for Fuse write complete using programming instruction 6g, or wait for tWLRH (refer to
Table 25-13 on page 284).
25.8.21 Programming the Lock Bits
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Lock bit write using programming instruction 7a.
3. Load data using programming instructions 7b. A bit value of “0” will program the corre-
sponding lock bit, a “1” will leave the lock bit unchanged.
4. Write Lock bits using programming instruction 7c.
5. Poll for Lock bit write complete using programming instruction 7d, or wait for tWLRH (refer
to Table 25-13 on page 284).
25.8.22 Reading the Fuses and Lock Bits
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Fuse/Lock bit read using programming instruction 8a.
3. To read all Fuses and Lock bits, use programming instruction 8e.
To only read Fuse High byte, use programming instruction 8b.
To only read Fuse Low byte, use programming instruction 8c.
To only read Lock bits, use programming instruction 8d.
25.8.23 Reading the Signature Bytes
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Signature byte read using programming instruction 9a.
3. Load address 0x00 using programming instruction 9b.
4. Read first signature byte using programming instruction 9c.
5. Repeat steps 3 and 4 with address 0x01 and address 0x02 to read the second and third
signature bytes, respectively.
25.8.24 Reading the Calibration Byte
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Calibration byte read using programming instruction 10a.
3. Load address 0x00 using programming instruction 10b.
Read the calibration byte using programming instruction 10c.
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26. Electrical Characteristics
26.1 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 VCC+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 VCC and GND Pins................................ 200.0 mA
26.2 DC Characteristics
TA = -40⋅C to 85⋅C, VCC = 1.8V to 5.5V (unless otherwise noted)
Symbol
Parameter
Condition
Min.
Typ.
Max.
Units
(1)
(1)
Input Low Voltage, Except
XTAL1 pin
VCC = 1.8V - 2.4V
VCC = 2.4V - 5.5V
-0.5
-0.5
0.2VCC
0.3VCC
VIL
V
Input Low Voltage, XTAL1
pin
(1)
VIL1
V
CC = 1.8V - 5.5V
-0.5
0.1VCC
V
V
Input High Voltage,
Except XTAL1 and
RESET pins
(2)
VCC = 1.8V - 2.4V
VCC = 2.4V - 5.5V
0.7VCC
0.6VCC
VCC + 0.5
VCC + 0.5
VIH
(2)
(2)
(2)
Input High Voltage,
XTAL1 pin
VCC = 1.8V - 2.4V
VCC = 2.4V - 5.5V
0.8VCC
0.7VCC
VCC + 0.5
VCC + 0.5
VIH1
VIH2
VOL
VOL1
VOH
VOH1
IIL
V
V
Input High Voltage,
RESET pin
(2)
VCC = 1.8V - 5.5V
0.85VCC
VCC + 0.5
Output Low Voltage(3)
Port A, C, D, E, F, G, H, J
,
IOL = 10mA, VCC = 5V
IOL = 5mA, VCC = 3V
0.9
0.6
V
Output Low Voltage(3)
Port B
,
IOL = 20mA, VCC = 5V
0.9
0.6
V
I
OL = 10mA, VCC = 3V
Output High Voltage(4)
Port A, C, D, E, F, G, H, J
,
IOH = -10mA, VCC = 5V
IOH = -5mA, VCC = 3V
4.2
2.3
V
Output High Voltage(4)
Port B
,
IOH = -20mA, VCC = 5V
IOH = -10mA, VCC = 3V
4.2
2.3
V
Input Leakage
Current I/O Pin
VCC = 5.5V, pin low
(absolute value)
1
1
µA
µA
Input Leakage
Current I/O Pin
VCC = 5.5V, pin high
(absolute value)
IIH
RRST
RPU
Reset Pull-up Resistor
I/O Pin Pull-up Resistor
20
20
100
100
kΩ
kΩ
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TA = -40⋅C to 85⋅C, VCC = 1.8V to 5.5V (unless otherwise noted) (Continued)
Symbol
Parameter
Condition
Min.
Typ.
0.5
2.6
9
Max.
0.75
3.5
12
Units
mA
mA
mA
mA
mA
mA
Active 1 MHz, VCC = 2V
Active 4 MHz, VCC = 3V
Active 8 MHz, VCC = 5V
Idle 1 MHz, VCC = 2V
Idle 4 MHz, VCC = 3V
Idle 8 MHz, VCC = 5V
32 kHz TOSC enabled,
Power Supply Current
0.14
0.75
2.9
0.25
1.5
5
ICC
0.75
1
µA
µA
VCC = 1.8V
Power-save mode
32 kHz TOSC enabled,
VCC = 3V
WDT enabled, VCC = 3V
WDT disabled, VCC = 3V
6.7
0.2
15
2
µA
µA
Power-down mode(5)
VCC = 5V
Analog Comparator
Input Offset Voltage
VACIO
IACLK
tACID
<10
40
50
mV
nA
ns
Vin = VCC/2
Analog Comparator
Input Leakage Current
VCC = 5V
Vin = VCC/2
-50
Analog Comparator
Propagation Delay
VCC = 2.7V
VCC = 4.0V
750
500
Notes: 1. “Max” means the highest value where the pin is guaranteed to be read as low
2. “Min” means the lowest value where the pin is guaranteed to be read as high
3. Although each I/O port can sink more than the test conditions (20 mA at VCC = 5V, 10 mA at VCC = 3V for Port B and 10 mA
at VCC = 5V, 5 mA at VCC = 3V for all other ports) under steady state conditions (non-transient), the following must be
observed:
TQFP and QFN/MLF Package:
1] The sum of all IOL, for all ports, should not exceed 400 mA.
2] The sum of all IOL, for ports A0 - A7, C4 - C7, G2 should not exceed 100 mA.
3] The sum of all IOL, for ports B0 - B7, E0 - E7, G3 - G5 should not exceed 100 mA.
4] The sum of all IOL, for ports D0 - D7, C0 - C3, G0 - G1 should not exceed 100 mA.
5] The sum of all IOL, for ports F0 - F7, should not exceed 100 mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater
than the listed test condition.
4. Although each I/O port can source more than the test conditions (20 mA at VCC = 5V, 10 mA at VCC = 3V for Port B and 10mA
at VCC = 5V, 5 mA at VCC = 3V for all other ports) under steady state conditions (non-transient), the following must be
observed:
TQFP and QFN/MLF Package:
1] The sum of all IOL, for all ports, should not exceed 400 mA.
2] The sum of all IOL, for ports A0 - A7, C4 - C7, G2 should not exceed 100 mA.
3] The sum of all IOL, for ports B0 - B7, E0 - E7, G3 - G5 should not exceed 100 mA.
4] The sum of all IOL, for ports D0 - D7, C0 - C3, G0 - G1 should not exceed 100 mA.
5] The sum of all IOL, for ports F0 - F7, should not exceed 100 mA.
If IOH exceeds the test condition, VOH may exceed the related specification. Pins are not guaranteed to source current
greater than the listed test condition.
5. Typical values at 25⋅C.
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26.3 Speed Grades
Figure 26-1. Maximum Frequency vs. VCC (4 - 10 MHz).
10 MHz
4 MHz
Safe Operating Area
1.8V
2.7V
5.5V
Figure 26-2. Maximum Frequency vs. VCC (10 -20 MHz).
20 MHz
10 MHz
Safe Operating Area
2.7V
4.5V
5.5V
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26.4 Clock Characterizations
26.4.1
Calibrated Internal RC Oscillator Accuracy
Table 26-1. Calibration Accuracy of Internal RC Oscillator
Frequency
VCC
Temperature
Calibration Accuracy
Factory
Calibration
8.0 MHz
3V
25⋅C
10%
User
Calibration
1.8V - 5.5V(1)
2.7V - 5.5V(2)
7.3 - 8.1 MHz
-40⋅C - 85⋅C
1%
Notes: 1. Voltage range for ATmega325PV/3250PV/645PV/6450PV.
2. Voltage range for ATmega325P/3250P.
26.4.2
External Clock Drive Waveforms
Figure 26-3. External Clock Drive Waveforms
V
IH1
V
IL1
26.4.3
External Clock Drive
Table 26-2. External Clock Drive
VCC=1.8-5.5V
VCC=2.7-5.5V
VCC=4.5-5.5V
Symbol
Parameter
Min.
Max.
Min.
Max.
Min.
Max.
Units
Oscillator
Frequency
1/tCLCL
0
4
0
10
0
20
MHz
tCLCL
tCHCX
tCLCX
tCLCH
tCHCL
Clock Period
High Time
Low Time
Rise Time
Fall Time
1000
400
100
40
50
20
20
ns
ns
ns
μs
μs
400
40
2.0
2.0
1.6
1.6
0.5
0.5
Change in period
from one clock
cycle to the next
ΔtCLCL
2
2
2
%
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26.5 System and Reset Characterizations
Table 26-3. Reset, Brown-out, and Internal Voltage Reference Characteristics
Symbol
VRST
tRST
Parameter
Condition
VCC = 3V
VCC = 3V
Min
Typ
Max
Units
V
RESET Pin Threshold Voltage
Minimum pulse width on RESET Pin
Brown-out Detector Hysteresis
Min Pulse Width on Brown-out Reset
0.2 VCC
0.85 VCC
800
50
2
ns
VHYST
tBOD
mV
µs
VCC = 2.7V,
TA = 25°C
VBG
tBG
IBG
Bandgap reference voltage
1.0
1.1
40
15
1.2
70
V
VCC = 2.7V,
TA = 25°C
Bandgap reference start-up time
Bandgap reference current consumption
µs
µA
VCC = 2.7V,
TA = 25°C
26.6 Standard Power-on Reset
This implementation of power-on reset existed in early versions of ATmega325P/3250P. The
table below describes the characteristics of this power-on reset and it is valid for the following
devices, only:
• ATmega325P revision B and older
• ATmega3250P revision B and older
Table 26-4. Reset, Brown-out, and Internal Voltage Reference Characteristics,
TA = -40°C to 85°C
Symbol
VPOT
Parameter
Min
0.7
Typ
1.0
0.9
Max
1.4
1.3
4.5
Units
V
Power-on Reset Threshold Voltage (rising)
Power-on Reset Threshold Voltage (falling)(1)
Power-on Slope Rate
0.05
0.01
V
SRON
V/ms
Note:
1. The Power-on Reset will not work unless the supply voltage has been below VPOT (falling)
26.7 Enhanced Power-on Reset
This implementation of power-on reset exists in newer versions of ATmega325P/3250P. The
table below describes the characteristics of this power-on reset and it is valid for the following
devices only:
• ATmega325P revision C and newer
• ATmega3250P revision C and newer
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Table 26-5. Reset, Brown-out and Internal Voltage Reference Characteristics(1),
TA = -40°C to 85°C
Symbol
VPOT
Parameter
Min
1.1
Typ
1.4
1.3
Max
1.6
1.6
10
Units
V
Power-on Reset Threshold Voltage (rising)
Power-on Reset Threshold Voltage (falling)(2)
Power-on Slope Rate
0.6
V
SRON
0.01
V/ms
Notes: 1. Values are guidelines only. Actual values are TBD.
2. The Power-on Reset will not work unless the supply voltage has been below VPOT (falling)
26.8 Brown-out Detection
Table 26-6. BODLEVEL Fuse Coding(1)
BODLEVEL 1:0 Fuses
Min VBOT
Typ VBOT
Max VBOT
Units
11
10
01
00
BOD Disabled
1.7
2.5
4.1
1.8
2.7
4.3
2
2.9
4.5
V
Note:
1. VBOT may be below nominal minimum operating voltage for some devices. For devices where
this is the case, the device is tested down to VCC = VBOT during the production test. This guar-
antees that a Brown-Out Reset will occur before VCC drops to a voltage where correct
operation of the microcontroller is no longer guaranteed. The test is performed using
BODLEVEL = 10 for ATmega325P/3250PV and BODLEVEL = 01 for ATmega325P/3250PL.
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26.9 SPI Timing Characteristics
See Figure 26-4 and Figure 26-5 for details.
Table 26-7. SPI Timing Parameters
Description
Mode
Master
Master
Master
Master
Master
Master
Master
Master
Slave
Min
Typ
Max
1
2
SCK period
SCK high/low
Rise/Fall time
Setup
See Table 17-4 on page 161
50% duty cycle
3
3.6
10
4
5
Hold
10
ns
6
Out to SCK
SCK to out
SCK to out high
SS low to out
SCK period
SCK high/low(1)
Rise/Fall time
Setup
0.5 • tsck
10
7
8
10
9
15
10
11
12
13
14
15
16
17
18
Slave
4 • tck
2 • tck
Slave
Slave
1.6
µs
ns
Slave
10
tck
Hold
Slave
SCK to out
SCK to SS high
SS high to tri-state
SS low to SCK
Slave
15
10
Slave
20
Slave
Slave
20 • tck
Notes: 1. In SPI Programming mode the minimum SCK high/low period is:
- 2 tCLCL for fCK < 12 MHz
- 3 tCLCL for fCK > 12 MHz
Figure 26-4. SPI Interface Timing Requirements (Master Mode)
SS
6
1
SCK
(CPOL = 0)
2
2
SCK
(CPOL = 1)
4
5
3
MISO
(Data Input)
MSB
...
LSB
7
8
MOSI
(Data Output)
MSB
...
LSB
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Figure 26-5. SPI Interface Timing Requirements (Slave Mode)
SS
10
16
9
SCK
(CPOL = 0)
11
11
SCK
(CPOL = 1)
13
14
12
MOSI
(Data Input)
MSB
...
LSB
15
17
MISO
(Data Output)
MSB
...
LSB
X
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26.10 ADC Characteristics – Preliminary Data
Table 26-8. ADC Characteristics
Symbol
Parameter
Condition
Min
Typ
10
8
Max
Units
Bits
Single Ended Conversion
Differential Conversion
Single Ended Conversion
Resolution
Bits
V
REF = 4V, VCC = 4V,
2
2.5
LSB
LSB
ADC clock = 200 kHz
Single Ended Conversion
VREF = 4V, VCC = 4V,
2.5
ADC clock = 1 MHz
Absolute accuracy (Including
INL, DNL, quantization error,
gain and offset error)
Single Ended Conversion
VREF = 4V, VCC = 4V,
2
LSB
LSB
ADC clock = 200 kHz
Noise Reduction Mode
Single Ended Conversion
VREF = 4V, VCC = 4V,
2.5
ADC clock = 1 MHz
Noise Reduction Mode
Single Ended Conversion
VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
Integral Non-Linearity (INL)
1.25
0.25
2
LSB
LSB
LSB
LSB
Single Ended Conversion
VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
Differential Non-Linearity
(DNL)
Single Ended Conversion
VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
Gain Error
Single Ended Conversion
Offset Error
VREF = 4V, VCC = 4V,
1.25
ADC clock = 200 kHz
Conversion Time
Free Running Conversion
Single Ended Conversion
13
50
260
1000
µs
kHz
V
Clock Frequency
AVCC
VREF
Analog Supply Voltage
V
CC - 0.3
1.0
VCC + 0.3
AVCC
AVCC - 0.5
VREF
Single Ended Conversion
Differential Conversion
Single Ended Channels
Differential Channels
Single Ended Channels
Differential Channels(1)
Single Ended Channels
Differential Channels
V
Reference Voltage
Pin Input Voltage
Input Range
1.0
V
GND
GND
GND
V
AVCC
VREF
V
VIN
V
-0.85VREF
VREF
V
38.5
4
kHz
kHz
Input Bandwidth
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Table 26-8. ADC Characteristics
Symbol
VINT
Parameter
Condition
Min
Typ
1.1
32
Max
Units
V
Internal Voltage Reference
Reference Input Resistance
Analog Input Resistance
1.0
1.2
RREF
RAIN
kΩ
100
MΩ
Notes: 1. Voltage difference between channels.
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27. Typical Characteristics
The following charts show typical behavior. These figures are not tested during manufacturing.
All current consumption measurements are performed with all I/O pins configured as inputs and
with internal pull-ups enabled. A square wave generator with rail-to-rail output is used as clock
source.
All Active- and Idle current consumption measurements are done with all bits in the PRR register
set and thus, the corresponding I/O modules are turned off. Also the Analog Comparator is dis-
abled during these measurements.The power consumption in Power-down mode is independent
of clock selection.
The current consumption is a function of several factors such as: operating voltage, operating
frequency, loading of I/O pins, switching rate of I/O pins, code executed and ambient tempera-
ture. The dominating factors are operating voltage and frequency.
The current drawn from capacitive loaded pins may be estimated (for one pin) as CL*VCC*f where
CL = load capacitance, VCC = operating voltage and f = average switching frequency of I/O pin.
The parts are characterized at frequencies higher than test limits. Parts are not guaranteed to
function properly at frequencies higher than the ordering code indicates.
The difference between current consumption in Power-down mode with Watchdog Timer
enabled and Power-down mode with Watchdog Timer disabled represents the differential cur-
rent drawn by the Watchdog Timer.
27.1 Active Supply Current
Figure 27-1. Active Supply Current vs. Low Frequenzy (0.1 - 1.0 MHz)
ACTIVE SUPPLY CURRENT vs. LOW FREQUENCY
0.1 - 1.0 MHz
1.8
5.5 V
5.0 V
1.6
1.4
1.2
1
4.5 V
4.0 V
3.3 V
2.7 V
0.8
0.6
0.4
0.2
0
1.8 V
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Frequency (MHz)
314
8023F–AVR–07/09
Figure 27-2. Active Supply Current vs. Frequenzy (1 - 20 MHz)
ACTIVE SUPPLY CURRENT vs. FREQUENCY
1 - 20 MHz
25
5.5 V
5.0 V
4.5 V
20
15
10
4.0 V
3.3 V
5
2.7 V
1.8 V
0
0
2
4
6
8
10
12
14
16
18
20
Frequency (MHz)
Figure 27-3. Active Supply Current vs. VCC (Internal RC Oscillator, 8 MHz)
ACTIVE SUPPLY CURRENT vs. V
CC
INTERNAL RC OSCILLATOR, 8 MHz
12
85 °C
25 °C
-40 °C
10
8
6
4
2
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
315
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-4. Active Supply Current vs. VCC (Internal RC Oscillator, 1 MHz)
ACTIVE SUPPLY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 1 MHz
2.5
2
85 °C
25 °C
-40 °C
1.5
1
0.5
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
27.2 Idle Supply Current
Figure 27-5. Idle Supply Current vs. Low Frequenzy (0.1 - 1.0 MHz)
IDLE SUPPLY CURRENT vs. LOW FREQUENCY
0.1 - 1.0 MHz
0.6
0.5
0.4
0.3
0.2
0.1
0
5.5 V
5.0 V
4.5 V
4.0 V
3.3 V
2.7 V
1.8 V
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Frequency (MHz)
316
8023F–AVR–07/09
Figure 27-6. Idle Supply Current vs. Frequenzy (1 - 20 MHz)
IDLE SUPPLY CURRENT vs. FREQUENCY
1 - 20 MHz
8
5.5V
5.0V
4.5V
7
6
5
4
3
4.0V
3.3V
2
2.7V
1
1.8V
0
0
2
4
6
8
10
12
14
16
18
20
Frequency (MHz)
Figure 27-7. Idle Supply Current vs. VCC (Internal RC Oscillator, 8 MHz)
IDLE SUPPLY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 8 MHz
3.5
85 °C
25 °C
-40 °C
3
2.5
2
1.5
1
0.5
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
317
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-8. Idle Supply Current vs. VCC (Internal RC Oscillator, 1 MHz)
IDLE SUPPLY CURRENT vs. VCC
INTERNAL RC OSCILLATOR, 1 MHz
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
85 °C
25 °C
-40 °C
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
27.3 Supply Current of I/O modules
The tables and formulas below can be used to calculate the additional current consumption for
the different I/O modules in Active and Idle mode. The enabling or disabling of the I/O modules
are controlled by the Power Reduction Register. See ”PRR – Power Reduction Register” on
page 44 for details.
Table 27-1. Additional Current Consumption for the different I/O modules (absolute values)
PRR bit
Typical numbers
V
CC = 2V, F = 1MHz
VCC = 3V, F = 4MHz
VCC = 5V, F = 8MHz
395 µA
PRADC
19 µA
9 µA
102 µA
PRUSART0
PRSPI
57 µA
68 µA
30 µA
240 µA
276 µA
116 µA
11 µA
5 µA
PRTIM1
Table 27-2. Additional Current Consumption (percentage) in Active and Idle mode
Additional Current consumption
compared to Active with external
clock
(see Figure 27-1 and Figure 27-2)
Additional Current consumption
compared to Idle with external clock
(see Figure 27-5 and Figure 27-6)
PRR bit
PRADC
PRUSART0
PRSPI
4.2%
2.3%
2.7%
1.2%
9.0%
4.9%
5.9%
2.6%
PRTIM1
318
8023F–AVR–07/09
It is possible to calculate the typical current consumption based on the numbers from Table 27-2
for other VCC and frequency settings than listed in Table 27-1.
27.3.1
Example
Calculate the expected current consumption in idle mode with USART0, TIMER1, and SPI
enabled at VCC = 3.0V and F = 1MHz. Table 27-2 shows that we need to add 4.9% for the
USART0, 5.9% for the SPI, and 2.6% for the TIMER1 module. From Figure 27-5, we find that
the idle current consumption is ~0.21mA at VCC = 3.0V and F = 1MHz. The total current con-
sumption in idle mode with USART0, TIMER1, and SPI enabled, gives:
ICCtotal ≈ 0,21mA • (1 + 0,049 + 0,059 + 0,026) ≈ 0,24mA
27.4 Power-down Supply Current
Figure 27-9. Power-down Supply Current vs. VCC (Watchdog Timer Disabled)
POWER-DOWN SUPPLY CURRENT vs. VCC
WATCHDOG TIMER DISABLED
3
2.5
2
85 °C
1.5
1
-40 °C
25 °C
0.5
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
319
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-10. Power-down Supply Current vs. VCC (Watchdog Timer Enabled)
POWER-DOWN SUPPLY CURRENT vs. VCC
WATCHDOG TIMER ENABLED
20
18
16
14
12
10
8
85 °C
-40 °C
25 °C
6
4
2
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
V
CC (V)
27.5 Power-save Supply Current
Figure 27-11. Power-save Supply Current vs. VCC (Watchdog Timer Disabled)
POWER-SAVE SUPPLY CURRENT vs. VCC
WATCHDOG TIMER DISABLED
3.5
3
85 °C
2.5
2
1.5
1
25 °C
-40 °C
0.5
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
320
8023F–AVR–07/09
27.6 Standby Supply Current
Figure 27-12. Standby Supply Current vs. VCC (Watchdog Timer Disabled)
STANDBY SUPPLY CURRENT vs. VCC
WATCHDOG TIMER DISABLED
0.2
0.18
0.16
0.14
0.12
0.1
6MHz_xtal
6MHz_res
4MHz_res
4MHz_xtal
0.08
0.06
0.04
0.02
0
2MHz_res
2MHz_xtal
1MHz_res
450kHz_res
32kHz_xtal
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (MHz)
27.7 Pin Pull-up
Figure 27-13. I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 1.8 V)
I/O PIN PULL-UP RESISTOR CURRENT vs. INPUT VOLTAGE
VCC = 1.8V
60
50
40
30
20
10
0
25 °C
-40 °C
85 °C
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
VOP (V)
321
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-14. I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 2.7 V)
I/O PIN PULL-UP RESISTOR CURRENT vs. INPUT VOLTAGE
VCC = 2.7V
80
70
60
50
40
30
20
10
0
25 °C
85 °C
-40 °C
0
0.5
1
1.5
2
2.5
3
VOP (V)
Figure 27-15. I/O Pin Pull-up Resistor Current vs. Input Voltage (VCC = 5 V)
I/O PIN PULL-UP RESISTOR CURRENT vs. INPUT VOLTAGE
VCC = 5V
160
140
120
100
80
60
40
25 °C
20
85 °C
0
-40 °C
0
1
2
3
4
5
6
VOP (V)
322
8023F–AVR–07/09
Figure 27-16. Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 1.8 V)
RESET PULL-UP RESISTOR CURRENT vs. RESET PIN VOLTAGE
VCC = 1.8V
40
35
30
25
20
15
10
5
25 °C
-40 °C
85 °C
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
VRESET(V)
Figure 27-17. Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 2.7 V)
RESET PULL-UP RESISTOR CURRENT vs. RESET PIN VOLTAGE
VCC = 2.7V
70
60
50
40
30
20
10
0
25 °C
-40 °C
85 °C
0
0.5
1
1.5
2
2.5
3
VRESET(V)
323
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-18. Reset Pull-up Resistor Current vs. Reset Pin Voltage (VCC = 5 V)
RESET PULL-UP RESISTOR CURRENT vs. RESET PIN VOLTAGE
VCC = 5V
120
100
80
60
40
20
0
25 °C
-40 °C
85 °C
0
1
2
3
4
5
6
VRESET(V)
27.8 Pin Driver Strength
Figure 27-19. I/O Pin Sink Current vs. Output Voltage, Port B (VCC = 1.8 V)
I/O PIN SINK CURRENT vs. OUTPUT VOLTAGE, PORT B
VCC = 1.8V
14
12
-40 °C
10
8
25 °C
85 °C
6
4
2
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
VOL (V)
324
8023F–AVR–07/09
Figure 27-20. I/O Pin Sink Current vs. Output Voltage, Port B (VCC = 2.7 V)
I/O PIN SINK CURRENT vs. OUTPUT VOLTAGE, PORT B
VCC = 2.7V
40
35
30
25 °C
25
20
15
10
5
0
0
0.5
1
1.5
2
2.5
VOL (V)
Figure 27-21. I/O Pin Sink Current vs. Output Voltage, Port B (VCC = 5 V)
I/O PIN SINK CURRENT vs. OUTPUT VOLTAGE, PORT B
VCC = 5V
80
70
60
25 °C
50
40
30
20
10
0
0
0.5
1
1.5
2
2.5
VOL (V)
325
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-22. I/O Pin Sink Current vs. Output Voltage, Port A,C,D,E,F,G,H,J (VCC = 1.8 V)
I/O PIN SINK CURRENT vs. OUTPUT VOLTAGE, PORT A, C, D, E, F, G, H, J
VCC1.8V
7
6
-40 °C
25 °C
85 °C
5
4
3
2
1
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
VOL (V)
Figure 27-23. I/O Pin Sink Current vs. Output Voltage, Port A,C,D,E,F,G,H,J (VCC = 2.7 V)
I/O PIN SINK CURRENT vs. OUTPUT VOLTAGE, PORT A, C, D, E, F, G, H, J
VCC = 2.7V
16
14
25 °C
12
10
8
6
4
2
0
0
0
.
5
1
1.
5
2
2.5
V
(V)
OL
326
8023F–AVR–07/09
Figure 27-24. I/O Pin Sink Current vs. Output Voltage, Port A,C,D,E,F,G,H,J (VCC = 5 V)
I/O PIN SINK CURRENT vs. OUTPUT VOLTAGE, PORT A,C, D, E, F, G, H, J
VCC = 5V
40
35
30
25 °C
25
20
15
10
5
0
0
0.5
1
1.5
2
2.5
VOL (V)
Figure 27-25. I/O Pin Source Current vs. Output Voltage, Port B (VCC = 1.8 V)
I/O PIN SOURCE CURRENT vs. OUTPUT VOLTAGE, PORT B
VCC = 1.8V
8
7
6
5
4
3
2
1
0
-40 °C
25 °C
85 °C
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
VOH(V)
327
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-26. I/O Pin Source Current vs. Output Voltage, Port B (VCC = 2.7 V)
I/O PIN SOURCE CURRENT vs. OUTPUT VOLTAGE, PORT B
VCC = 2.7V
25
20
15
10
5
25 °C
0
0
0.5
1
1.5
2
2.5
3
VOH(V)
Figure 27-27. I/O Pin Source Current vs. Output Voltage, Port B (VCC = 5 V)
I/O PIN SOURCE CURRENT vs. OUTPUT VOLTAGE, PORT B
VCC = 5V
70
60
50
25 °C
40
30
20
10
0
0
1
2
3
4
5
6
VOH (V)
328
8023F–AVR–07/09
Figure 27-28. I/O Pin Source Current vs. Output Voltage, Port A,C,D,E,F,G,H,J (VCC = 1.8 V)
I/O PIN SOURCE CURRENT vs. OUTPUT VOLTAGE, PORT A, C, D, E, F, G, H, J
VCC = 1.8V
6
-40 °C
5
25 °C
85 °C
4
3
2
1
0
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
VOH(V)
Figure 27-29. I/O Pin Source Current vs. Output Voltage, Port A,C,D,E,F,G,H,J (VCC = 2.7 V)
I/O PIN SOURCE CURRENT vs. OUTPUT VOLTAGE, PORT A, C, D, E, F, G, H, J
VCC = 2.7
20
18
16
14
25 °C
12
10
8
6
4
2
0
0
0.5
1
1.5
2
2.5
3
VOH(V)
329
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-30. I/O Pin Source Current vs. Output Voltage, Port A,C,D,E,F,G,H,J (VCC = 5 V)
I/O PIN SOURCE CURRENT vs. OUTPUT VOLTAGE, PORT A, C, D, E, F, G, H, J
VCC = 5V
60
50
25 °C
40
30
20
10
0
0
1
2
3
4
5
6
VOH (V)
27.9 Pin Threshold and Hysteresis
Figure 27-31. I/O Pin Input Threshold Voltage vs. VCC, Port B (VIH, IO Pin Read as ¨1¨)
I/O PIN INPUT THRESHOLD VOLTAGE vs. VCC, PORT B
VIH, IO PIN READ AS '1'
3
-40 °C
25 °C
2.5
85 °C
2
1.5
1
0.5
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
330
8023F–AVR–07/09
Figure 27-32. I/O Pin Input Threshold Voltage vs. VCC, Port B (VIL, IO Pin Read as ¨0¨)
I/O PIN INPUT THRESHOLD VOLTAGE vs. VCC, PORT B
VIL, IO PIN READ AS '0'
2.5
85 °C
25 °C
2
-40 °C
1.5
1
0.5
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 27-33. I/O Pin Input Hysteresis vs. VCC, Port B
I/O PIN INPUT HYSTERESIS vs. VCC, PORT B
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-40 °C
25 °C
85 °C
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
331
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-34. I/O Pin Input Threshold Voltage vs. VCC, Port A,C,D,E,F,G,H,J (VIH, IO Pin Read
as ¨1¨)
I/O PIN INPUT THRESHOLD VOLTAGE vs. VCC, PORT A, C, D, E, F, G, H, J
VIH, IO PIN READ AS '1'
-40 °C
3
25 °C
85 °C
2.5
2
1.5
1
0.5
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 27-35. I/O Pin Input Threshold Voltage vs. VCC, Port A,C,D,E,F,G,H,J (VIL, IO Pin Read
as ¨0¨)
I/O PIN INPUT THRESHOLD VOLTAGE vs. VCC, PORT A, C, D, E, F, G, H, J
VIL, IO PIN READ AS '0'
85 °C
25 °C
-40 °C
2.5
2
1.5
1
0.5
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
332
8023F–AVR–07/09
Figure 27-36. I/O Pin Hysteresis vs. VCC, Port A,C,D,E,F,G,H,J
I/O PIN INPUT HYSTERESIS vs. VCC, PORT A, C, D, E, F, G, H, J
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-40 °C
25 °C
85 °C
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 27-37. Reset Input Threshold Voltage vs. VCC (VIH, IO Pin Read as ¨1¨)
RESET INPUT THRESHOLD VOLTAGE vs. VCC
VIH, IO PIN READ AS '1'
2.5
2
-40 °C
25 °C
85 °C
1.5
1
0.5
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
333
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-38. Reset Input Threshold Voltage vs. VCC (VIL, IO Pin Read as ¨0¨)
RESET INPUT THRESHOLD VOLTAGE vs. VCC
VIL, IO PIN READ AS '0'
2.5
2
85 °C
25 °C
-40 °C
1.5
1
0.5
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 27-39. Reset Pin Input Hysteresis vs. VCC
RESET PIN INPUT HYSTERESIS vs. VCC
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-40 °C
25 °C
85 °C
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
334
8023F–AVR–07/09
27.10 Internal Oscillator Speed
Figure 27-40. Watchdog Oscillator Frequenzy vs. VCC
WATCHDOG OSCILLATOR FREQUENCY vs. VCC
170
160
150
140
130
-40 °C
25 °C
85 °C
1.5
2
2.5
3
3.5
4
4.5
5
5.5
V
CC (V)
Figure 27-41. Watchdog Oscillator Frequenzy vs. Temperature
WATCHDOG OSCILLATOR FREQUENCY vs. TEMPERATURE
170
165
160
155
150
145
140
5.5 V
4.0 V
3.3 V
2.7 V
2.1 V
-60
-40
-20
0
20
40
60
80
100
Temperature
335
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-42. Calibrated 8 MHz RC Oscillator Frequenzy vs. VCC
CALIBRATED 8 MHz RC OSCILLATOR FREQUENCY vs. V
CC
8.4
8.2
8
85 °C
25 °C
-40 °C
7.8
7.6
7.4
7.2
1.5
2
2.5
3
3.5
4
4.5
5
5,5
VCC (V)
Figure 27-43. Calibrated 8 MHz RC Oscillator Frequenzy vs. Temperature
CALIBRATED 8 MHz RC OSCILLATOR FREQUENCY vs. TEMPERATURE
8.4
8.2
8
5.5 V
4.0 V
3.3 V
2.7 V
1.8 V
7.8
7.6
7.4
-60
-40
-20
0
20
40
60
80
100
Temperature (C)
336
8023F–AVR–07/09
Figure 27-44. Calibrated 8 MHz RC Oscillator Frequenzy vs. Osccal Value
CALIBRATED 8 MHz RC OSCILLATOR FREQUENCY vs. OSCCAL VALUE
16
14
12
10
8
85 °C
25 °C
-40 °C
6
4
2
0
0
16
32
48
64
80
96 112 128 144 160 176 192 208 224 240 256
OSCCAL (X1)
27.11 Current Consumption of Peripheral Units
Figure 27-45. Brownout Detector Current vs. VCC
BROWNOUT DETECTOR CURRENT vs. VCC
30
25
20
15
10
5
85 °C
25 °C
-40 °C
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
337
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-46. ADC Current vs. VCC (AREF = AVCC
)
ADC CURRENT vs. VCC
AREF = AV
CC
500
450
400
350
300
250
200
150
100
50
-40 °C
25 °C
85 °C
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
Figure 27-47. AREF External Reference Current vs. VCC
AREF EXTERNAL REFERENCE CURRENT vs. VCC
180
160
140
120
100
80
-40 °C
25 °C
85 °C
60
40
20
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
338
8023F–AVR–07/09
Figure 27-48. Watchdog Timer Current vs. VCC
WATCHDOG TIMER CURRENT vs. VCC
18
16
14
12
10
8
-40 °C
85 °C
25 °C
6
4
2
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
Figure 27-49. Analog Comparator Current vs. VCC
ANALOG COMPARATOR CURRENT vs. VCC
100
90
80
70
60
50
40
30
20
10
0
-40 °C
25 °C
85 °C
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
339
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Figure 27-50. Programming Current vs. VCC
PROGRAMMING CURRENT vs. Vcc
12
10
8
-40 °C
25 °C
85 °C
6
4
2
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
VCC (V)
27.12 Current Consumption in Reset and Reset Pulswidth
Figure 27-51. Reset Supply Current vs. Low Frequenzy (0.1 - 1.0 MHz)
RESET SUPPLY CURRENT vs. LOW FREQUENCY
0.1 - 1.0 MHz
0.25
0.2
0.15
0.1
0.05
0
5.5 V
5.0 V
4.5 V
4.0 V
3.3 V
2.7 V
1.8 V
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Frequency (MHz)
340
8023F–AVR–07/09
Figure 27-52. Reset Supply Current vs. Frequenzy (1 - 20 MHz)
RESET SUPPLY CURRENT vs. FREQUENCY
1 - 20 MHz
4
3.5
3
5.5 V
5.0 V
4.5 V
2.5
2
4.0 V
1.5
3.3 V
1
2.7 V
0.5
1.8 V
0
0
2
4
6
8
10
12
14
16
18
20
Frequency (MHz)
Figure 27-53. Minimum Reset Pulse Width vs. VCC
MINIMUM RESET PULSE WIDTH vs. VCC
2500
2000
1500
1000
500
85 °C
25 °C
-40 °C
0
0
1
2
3
4
5
6
VCC (V)
341
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
28. Register Summary
Note:
Registers with bold type only available in ATmega3250P.
Address
Name
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
PORTJ
Bit 7
-
Bit 6
-
Bit 5
-
Bit 4
-
Bit 3
-
Bit 2
-
Bit 1
-
Bit 0
-
Page
(0xFF)
(0xFE)
(0xFD)
(0xFC)
(0xFB)
(0xFA)
(0xF9)
(0xF8)
(0xF7)
(0xF6)
(0xF5)
(0xF4)
(0xF3)
(0xF2)
(0xF1)
(0xF0)
(0xEF)
(0xEE)
(0xED)
(0xEC)
(0xEB)
(0xEA)
(0xE9)
(0xE8)
(0xE7)
(0xE6)
(0xE5)
(0xE4)
(0xE3)
(0xE2)
(0xE1)
(0xE0)
(0xDF)
(0xDE)
(0xDD)
(0xDC)
(0xDB)
(0xDA)
(0xD9)
(0xD8)
(0xD7)
(0xD6)
(0xD5)
(0xD4)
(0xD3)
(0xD2)
(0xD1)
(0xD0)
(0xCF)
(0xCE)
(0xCD)
(0xCC)
(0xCB)
(0xCA)
(0xC9)
(0xC8)
(0xC7)
(0xC6)
(0xC5)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
PORTJ6
PORTJ5
PORTJ4
PORTJ3
PORTJ2
PORTJ1
PORTJ0
88
88
88
87
88
88
DDRJ
-
DDJ6
DDJ5
DDJ4
DDJ3
DDJ2
DDJ1
DDJ0
PINJ
-
PINJ6
PINJ5
PINJ4
PINJ3
PINJ2
PINJ1
PINJ0
PORTH
PORTH7
PORTH6
PORTH5
PORTH4
PORTH3
PORTH2
PORTH1
PORTH0
DDRH
DDH7
DDH6
DDH5
DDH4
DDH3
DDH2
DDH1
DDH0
PINH
PINH7
PINH6
PINH5
PINH4
PINH3
PINH2
PINH1
PINH0
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
UDR0
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
USART0 Data Register
182
186
UBRR0H
USART0 Baud Rate Register High
342
8023F–AVR–07/09
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Page
UBRR0L
USART0 Baud Rate Register Low
186
(0xC4)
(0xC3)
(0xC2)
(0xC1)
(0xC0)
(0xBF)
(0xBE)
(0xBD)
(0xBC)
(0xBB)
(0xBA)
(0xB9)
(0xB8)
(0xB7)
(0xB6)
(0xB5)
(0xB4)
(0xB3)
(0xB2)
(0xB1)
(0xB0)
(0xAF)
(0xAE)
(0xAD)
(0xAC)
(0xAB)
(0xAA)
(0xA9)
(0xA8)
(0xA7)
(0xA6)
(0xA5)
(0xA4)
(0xA3)
(0xA2)
(0xA1)
(0xA0)
(0x9F)
(0x9E)
(0x9D)
(0x9C)
(0x9B)
(0x9A)
(0x99)
(0x98)
(0x97)
(0x96)
(0x95)
(0x94)
(0x93)
(0x92)
(0x91)
(0x90)
(0x8F)
(0x8E)
(0x8D)
(0x8C)
(0x8B)
(0x8A)
(0x89)
(0x88)
(0x87)
(0x86)
Reserved
UCSR0C
UCSR0B
UCSR0A
Reserved
Reserved
Reserved
Reserved
Reserved
USIDR
-
-
-
-
-
-
-
-
-
UMSEL0
UPM01
UPM00
USBS0
UCSZ01
UCSZ00
UCPOL0
184
183
182
RXCIE0
TXCIE0
UDRIE0
RXEN0
TXEN0
UCSZ02
RXB80
TXB80
RXC0
TXC0
UDRE0
FE0
DOR0
UPE0
U2X0
MPCM0
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
USI Data Register
199
199
200
USISR
USISIF
USIOIF
USIPF
USIDC
USICNT3
USICNT2
USICNT1
USICNT0
USICR
USISIE
USIOIE
USIWM1
USIWM0
USICS1
USICS0
USICLK
USITC
Reserved
ASSR
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
EXCLK
AS2
TCN2UB
OCR2UB
TCR2UB
151
Reserved
Reserved
OCR2A
-
-
-
-
-
-
-
-
-
-
Timer/Counter 2 Output Compare Register A
Timer/Counter2
151
151
TCNT2
Reserved
TCCR2A
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
OCR1BH
OCR1BL
OCR1AH
OCR1AL
ICR1H
-
-
-
-
-
-
-
-
FOC2A
WGM20
COM2A1
COM2A0
WGM21
CS22
CS21
CS20
149
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Timer/Counter1 Output Compare Register B High
Timer/Counter1 Output Compare Register B Low
Timer/Counter1 Output Compare Register A High
Timer/Counter1 Output Compare Register A Low
Timer/Counter1 Input Capture Register High
Timer/Counter1 Input Capture Register Low
132
132
132
132
133
133
ICR1L
343
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Address
Name
TCNT1H
Bit 7
Bit 6
Bit 5
Bit 4
Timer/Counter1 High
Bit 3
Bit 2
Bit 1
Bit 0
Page
132
(0x85)
(0x84)
TCNT1L
Reserved
TCCR1C
TCCR1B
TCCR1A
DIDR1
Timer/Counter1 Low
132
-
FOC1A
ICNC1
COM1A1
-
-
-
-
-
-
-
-
(0x83)
FOC1B
ICES1
COM1A0
-
-
-
-
-
CS12
-
-
-
131
130
128
205
222
(0x82)
-
WGM13
WGM12
CS11
WGM11
AIN1D
ADC1D
-
CS10
WGM10
AIN0D
ADC0D
-
(0x81)
COM1B1
COM1B0
-
(0x80)
-
-
ADC4D
-
-
ADC3D
-
-
(0x7F)
DIDR0
ADC7D
-
ADC6D
-
ADC5D
-
ADC2D
-
(0x7E)
Reserved
ADMUX
ADCSRB
ADCSRA
ADCH
(0x7D)
REFS1
-
REFS0
ACME
ADSC
ADLAR
-
MUX4
-
MUX3
-
MUX2
ADTS2
ADPS2
MUX1
ADTS1
ADPS1
MUX0
ADTS0
ADPS0
218
204/222
220
(0x7C)
(0x7B)
ADEN
ADATE
ADIF
ADIE
(0x7A)
ADC Data Register High
ADC Data Register Low
221
(0x79)
ADCL
221
(0x78)
Reserved
Reserved
Reserved
Reserved
PCMSK3
Reserved
Reserved
TIMSK2
TIMSK1
TIMSK0
PCMSK2
PCMSK1
PCMSK0
Reserved
EICRA
-
-
-
-
-
-
-
-
(0x77)
-
-
-
-
-
-
-
-
(0x76)
-
-
-
-
-
-
-
-
(0x75)
-
-
-
-
-
-
-
-
(0x74)
-
PCINT30
PCINT29
PCINT28
PCINT27
PCINT26
PCINT25
PCINT24
62
(0x73)
-
-
-
-
-
-
-
-
-
-
(0x72)
-
-
-
-
-
-
(0x71)
-
-
-
-
-
-
OCIE2A
OCIE1A
OCIE0A
PCINT17
PCINT9
PCINT1
-
TOIE2
TOIE1
TOIE0
PCINT16
PCINT8
PCINT0
-
152
133
104
62
(0x70)
-
-
ICIE1
-
-
OCIE1B
(0x6F)
-
-
-
-
-
-
(0x6E)
PCINT23
PCINT22
PCINT21
PCINT20
PCINT19
PCINT18
(0x6D)
PCINT15
PCINT14
PCINT13
PCINT12
PCINT11
PCINT10
62
(0x6C)
PCINT7
PCINT6
PCINT5
PCINT4
PCINT3
PCINT2
62
(0x6B)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
(0x6A)
ISC01
-
ISC00
-
59
(0x69)
Reserved
Reserved
OSCCAL
Reserved
PRR
(0x68)
-
-
(0x67)
Oscillator Calibration Register [CAL7..0]
36
44
(0x66)
-
-
-
-
-
-
-
-
-
-
(0x65)
-
-
PRTIM1
PRSPI
PSUSART0
PRADC
(0x64)
Reserved
Reserved
CLKPR
-
-
-
-
-
-
-
-
(0x63)
-
-
-
-
-
CLKPS3
WDE
V
-
-
-
(0x62)
CLKPCE
-
-
-
WDCE
S
CLKPS2
WDP2
N
CLKPS1
WDP1
Z
CLKPS0
WDP0
C
36
51
13
15
15
(0x61)
WDTCR
SREG
-
I
-
-
(0x60)
T
H
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)
SPH
Stack Pointer High
Stack Pointer Low
SPL
Reserved
Reserved
Reserved
Reserved
Reserved
SPMCSR
Reserved
MCUCR
MCUSR
SMCR
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
SPMIE
RWWSB
RWWSRE
BLBSET
PGWRT
PGERS
SPMEN
269
JTD
BODS
BODSE
PUD
-
WDRF
SM2
-
-
BORF
SM1
-
IVSEL
EXTRF
SM0
-
IVCE
PORF
SE
56/85/255
-
-
-
JTRF
50
43
-
-
-
-
Reserved
OCDR
-
-
-
OCDR5
ACO
-
-
OCDR4
ACI
-
IDRD/OCDR7
OCDR6
ACBG
-
OCDR3
ACIE
-
OCDR2
ACIC
-
OCDR1
ACIS1
-
OCDR0
ACIS0
-
228
204
ACSR
ACD
-
Reserved
SPDR
-
SPI Data Register
162
162
160
27
SPSR
SPIF
SPIE
WCOL
SPE
-
-
-
-
-
SPI2X
SPR0
SPCR
DORD
MSTR
CPOL
CPHA
SPR1
GPIOR2
GPIOR1
Reserved
Reserved
OCR0A
General Purpose I/O Register
General Purpose I/O Register
27
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Timer/Counter0 Output Compare A
104
344
8023F–AVR–07/09
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Page
TCNT0
Timer/Counter0
104
0x26 (0x46)
0x25 (0x45)
0x24 (0x44)
0x23 (0x43)
0x22 (0x42)
0x21 (0x41)
0x20 (0x40)
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)
0x01 (0x21)
0x00 (0x20)
Reserved
TCCR0A
GTCCR
EEARH
EEARL
EEDR
-
FOC0A
TSM
-
-
-
-
-
-
CS02
-
-
-
WGM00
COM0A1
COM0A0
WGM01
CS01
PSR2
CS00
PSR10
102
105/153
23
-
-
-
-
-
-
-
-
EEPROM Address Register High
EEPROM Address Register Low
EEPROM Data Register
23
23
EECR
-
-
-
-
EERIE
EEMWE
EEWE
EERE
24
GPIOR0
EIMSK
EIFR
General Purpose I/O Register
28
PCIE3
PCIF3
-
PCIE2
PCIF2
-
PCIE1
PCIF1
-
PCIE0
PCIF0
-
-
-
-
-
INT0
INTF0
-
60
-
-
61
Reserved
Reserved
Reserved
Reserved
TIFR2
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
OCF2A
OCF1A
OCF0A
PORTG1
DDG1
PING1
PORTF1
DDF1
PINF1
PORTE1
DDE1
PINE1
PORTD1
DDD1
PIND1
PORTC1
DDC1
PINC1
PORTB1
DDB1
PINB1
PORTA1
DDA1
PINA1
TOV2
TOV1
TOV0
PORTG0
DDG0
PING0
PORTF0
DDF0
PINF0
PORTE0
DDE0
PINE0
PORTD0
DDD0
PIND0
PORTC0
DDC0
PINC0
PORTB0
DDB0
PINB0
PORTA0
DDA0
PINA0
153
133
105
87
87
87
87
87
87
86
86
87
86
86
86
86
86
86
85
85
85
85
85
85
TIFR1
-
-
ICF1
-
-
-
OCF1B
-
TIFR0
-
-
-
-
PORTG
DDRG
PING
-
-
-
PORTG4
DDG4
PING4
PORTF4
DDF4
PINF4
PORTE4
DDE4
PINE4
PORTD4
DDD4
PIND4
PORTC4
DDC4
PINC4
PORTB4
DDB4
PINB4
PORTA4
DDA4
PINA4
PORTG3
DDG3
PING3
PORTF3
DDF3
PINF3
PORTE3
DDE3
PINE3
PORTD3
DDD3
PIND3
PORTC3
DDC3
PINC3
PORTB3
DDB3
PINB3
PORTA3
DDA3
PINA3
PORTG2
DDG2
PING2
PORTF2
DDF2
PINF2
PORTE2
DDE2
PINE2
PORTD2
DDD2
PIND2
PORTC2
DDC2
PINC2
PORTB2
DDB2
PINB2
PORTA2
DDA2
PINA2
-
-
-
-
-
PING5
PORTF5
DDF5
PINF5
PORTE5
DDE5
PINE5
PORTD5
DDD5
PIND5
PORTC5
DDC5
PINC5
PORTB5
DDB5
PINB5
PORTA5
DDA5
PINA5
PORTF
DDRF
PORTF7
DDF7
PINF7
PORTE7
DDE7
PINE7
PORTD7
DDD7
PIND7
PORTC7
DDC7
PINC7
PORTB7
DDB7
PINB7
PORTA7
DDA7
PINA7
PORTF6
DDF6
PINF6
PORTE6
DDE6
PINE6
PORTD6
DDD6
PIND6
PORTC6
DDC6
PINC6
PORTB6
DDB6
PINB6
PORTA6
DDA6
PINA6
PINF
PORTE
DDRE
PINE
PORTD
DDRD
PIND
PORTC
DDRC
PINC
PORTB
DDRB
PINB
PORTA
DDRA
PINA
Notes: 1. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory addresses
should never be written.
2. 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.
3. Some of the Status Flags are cleared by writing a logical one to them. Note that, unlike most other AVRs, the CBI and SBI
instructions will only operate on the specified bit, and can therefore be used on registers containing such Status Flags. The
CBI and SBI instructions work with registers 0x00 to 0x1F only.
4. 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. The ATmega325P/3250P is
a complex microcontroller with more peripheral units than can be supported within the 64 location reserved in Opcode for
the IN and OUT instructions. For the Extended I/O space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and
LD/LDS/LDD instructions can be used.
345
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
29. Instruction Set Summary
Mnemonics
Operands
Description
Operation
Flags
#Clocks
ARITHMETIC AND LOGIC INSTRUCTIONS
ADD
Rd, Rr
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,H
Z,C,N,V,S
Z,C,N,V,H
Z,C,N,V,H
Z,C,N,V,H
Z,C,N,V,H
Z,C,N,V,S
Z,N,V
1
1
2
1
1
1
1
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
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,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
Clear Register
Rd ← Rd • Rd
Z,N,V
CLR
Rd
Rd ← Rd ⊕ Rd
Rd ← 0xFF
Z,N,V
SER
Rd
Set Register
None
MUL
Rd, Rr
Rd, Rr
Rd, Rr
Rd, Rr
Rd, Rr
Rd, Rr
Multiply Unsigned
R1:R0 ← Rd x Rr
R1:R0 ← Rd x Rr
R1:R0 ← Rd x Rr
R1:R0 ← (Rd x Rr) << 1
R1:R0 ← (Rd x Rr) << 1
R1:R0 ← (Rd x Rr) << 1
Z,C
MULS
MULSU
FMUL
FMULS
FMULSU
Multiply Signed
Z,C
Multiply Signed with Unsigned
Fractional Multiply Unsigned
Fractional Multiply Signed
Fractional Multiply Signed with Unsigned
Z,C
Z,C
Z,C
Z,C
BRANCH INSTRUCTIONS
RJMP
IJMP
k
Relative Jump
PC ← PC + k + 1
None
None
None
None
None
None
None
I
2
2
Indirect Jump to (Z)
PC ← Z
JMP
k
k
Direct Jump
PC ← k
3
RCALL
ICALL
CALL
RET
Relative Subroutine Call
Indirect Call to (Z)
PC ← PC + k + 1
3
PC ← Z
3
k
Direct Subroutine Call
Subroutine Return
PC ← k
4
PC ← STACK
4
RETI
Interrupt Return
PC ← STACK
4
CPSE
CP
Rd,Rr
Compare, Skip if Equal
Compare
if (Rd = Rr) PC ← PC + 2 or 3
Rd − Rr
None
Z, N,V,C,H
Z, N,V,C,H
Z, N,V,C,H
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
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 = 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
1/2/3
1/2/3
1/2/3
1/2/3
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
Rr, b
P, b
P, b
s, k
s, k
k
BRBS
BRBC
BREQ
BRNE
BRCS
BRCC
BRSH
BRLO
BRMI
BRPL
BRGE
BRLT
BRHS
BRHC
BRTS
BRTC
BRVS
k
Branch if Not Equal
k
Branch if Carry Set
k
Branch if Carry Cleared
Branch if Same or Higher
Branch if Lower
k
k
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
k
k
k
k
Branch if T Flag Cleared
Branch if Overflow Flag is Set
k
346
8023F–AVR–07/09
Mnemonics
Operands
Description
Operation
Flags
#Clocks
BRVC
BRIE
BRID
k
k
k
Branch if Overflow Flag is Cleared
if (V = 0) then PC ← PC + k + 1
if ( I = 1) then PC ← PC + k + 1
if ( I = 0) then PC ← PC + k + 1
None
1/2
1/2
1/2
Branch if Interrupt Enabled
Branch if Interrupt Disabled
None
None
BIT AND BIT-TEST INSTRUCTIONS
SBI
P,b
P,b
Rd
Rd
Rd
Rd
Rd
Rd
s
Set Bit in I/O Register
Clear Bit in I/O Register
Logical Shift Left
I/O(P,b) ← 1
None
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
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
C
N
N
Z
Clear Carry
C ← 0
Set Negative Flag
N ← 1
Clear Negative Flag
Set Zero Flag
N ← 0
Z ← 1
Clear Zero Flag
Z ← 0
Z
Global Interrupt Enable
Global Interrupt Disable
Set Signed Test Flag
Clear Signed Test Flag
Set Twos Complement Overflow.
Clear Twos Complement Overflow
Set T in SREG
I ← 1
I
CLI
I ← 0
I
SES
CLS
SEV
CLV
SET
CLT
SEH
CLH
S ← 1
S
S
V
V
T
S ← 0
V ← 1
V ← 0
T ← 1
Clear T in SREG
T ← 0
T
Set Half Carry Flag in SREG
Clear Half Carry Flag in SREG
H ← 1
H
H
H ← 0
DATA TRANSFER INSTRUCTIONS
MOV
MOVW
LDI
LD
Rd, Rr
Rd, Rr
Rd, K
Move Between Registers
Copy Register Word
Rd ← Rr
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
3
3
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
LD
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
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
ST
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
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
LPM
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
Rd ← P
1
1
OUT
Out Port
P ← Rr
347
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
Mnemonics
Operands
Description
Operation
Flags
#Clocks
PUSH
POP
Rr
Rd
Push Register on Stack
Pop Register from Stack
STACK ← Rr
Rd ← STACK
None
2
2
None
MCU CONTROL INSTRUCTIONS
NOP
No Operation
Sleep
None
None
None
None
1
1
SLEEP
WDR
(see specific descr. for Sleep function)
(see specific descr. for WDR/timer)
For On-chip Debug Only
Watchdog Reset
Break
1
BREAK
N/A
348
8023F–AVR–07/09
30. Ordering Information
30.1 ATmega325P
Speed (MHz)(3)
Power Supply
Ordering Code(2)
Package Type(1)
Operational Range
ATmega325PV-10AU
ATmega325PV-10MU
64A
64M1
Industrial
(-40⋅C to 85⋅C)
10
1.8 - 5.5V
ATmega325P-20AU
ATmega325P-20MU
64A
64M1
Industrial
(-40⋅C to 85⋅C)
20
2.7 - 5.5V
Notes: 1. This device can also be supplied in wafer form. Please contact your local Atmel sales office for detailed ordering information
and minimum quantities.
2. Pb-free packaging complies to the European Directive for Restriction of Hazardous Substances (RoHS directive). Also
Halide free and fully Green.
3. For Speed vs. VCC see Figure 26-1 on page 306 and Figure 26-2 on page 306.
Package Type
64A
64-lead, 14 x 14 x 1.0 mm, Thin Profile Plastic Quad Flat Package (TQFP)
64M1
64-pad, 9 x 9 x 1.0 mm, Quad Flat No-Lead/Micro Lead Frame Package (QFN/MLF)
349
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
30.2 ATmega3250P
Speed (MHz)(3)
Power Supply
Ordering Code(2)
Package Type(1)
Operational Range
Industrial
(-40⋅C to 85⋅C)
10
1.8 - 5.5V
2.7 - 5.5V
ATmega3250PV-10AU
100A
100A
Industrial
(-40⋅C to 85⋅C)
20
ATmega3250P-20AU
Notes: 1. This device can also be supplied in wafer form. Please contact your local Atmel sales office for detailed ordering information
and minimum quantities.
2. Pb-free packaging complies to the European Directive for Restriction of Hazardous Substances (RoHS directive). Also
Halide free and fully Green.
3. For Speed vs. VCC see Figure 26-1 on page 306 and Figure 26-2 on page 306.
Package Type
100A
100-lead, 14 x 14 x 1.0 mm, 0.5 mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP)
350
8023F–AVR–07/09
31. Packaging Information
31.1 64A
PIN 1
B
PIN 1 IDENTIFIER
E1
E
e
D1
D
C
0°~7°
A2
A
A1
L
COMMON DIMENSIONS
(Unit of Measure = mm)
MIN
–
MAX
1.20
NOM
–
NOTE
SYMBOL
A
A1
A2
D
0.05
0.95
15.75
13.90
15.75
13.90
0.30
0.09
0.45
–
0.15
1.00
16.00
14.00
16.00
14.00
–
1.05
16.25
D1
E
14.10 Note 2
16.25
Notes:
E1
B
14.10 Note 2
0.45
1.This package conforms to JEDEC reference MS-026, Variation AEB.
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.
C
–
0.20
3. Lead coplanarity is 0.10 mm maximum.
L
–
0.75
e
0.80 TYP
10/5/2001
TITLE
DRAWING NO. REV.
2325 Orchard Parkway
San Jose, CA 95131
64A, 64-lead, 14 x 14 mm Body Size, 1.0 mm Body Thickness,
0.8 mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP)
64A
B
R
351
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
31.2 64M1
D
Marked Pin# 1 ID
E
SEATING PLANE
C
A1
TOP VIEW
A
K
0.08
C
L
Pin #1 Corner
SIDE VIEW
D2
Pin #1
Triangle
Option A
1
2
3
COMMON DIMENSIONS
(Unit of Measure = mm)
MIN
0.80
–
MAX
1.00
0.05
0.30
9.10
NOM
0.90
0.02
0.25
9.00
NOTE
SYMBOL
E2
Option B
Option C
A
Pin #1
Chamfer
(C 0.30)
A1
b
0.18
8.90
D
D2
E
5.20
5.40
9.00
5.60
9.10
K
Pin #1
Notch
(0.20 R)
8.90
e
b
E2
e
5.20
5.40
0.50 BSC
0.40
5.60
BOTTOM VIEW
L
0.35
0.45
1.55
K
1.25
1.40
1. JEDEC Standard MO-220, (SAW Singulation) Fig. 1, VMMD.
2. Dimension and tolerance conform to ASMEY14.5M-1994.
Note:
5/25/06
DRAWING NO. REV.
64M1
TITLE
2325 Orchard Parkway
San Jose, CA 95131
64M1, 64-pad, 9 x 9 x 1.0 mm Body, Lead Pitch 0.50 mm,
5.40 mm Exposed Pad, Micro Lead Frame Package (MLF)
G
R
352
8023F–AVR–07/09
31.3 100A
PIN 1
B
PIN 1 IDENTIFIER
E1
E
e
D1
D
C
0˚~7˚
A2
A
A1
L
COMMON DIMENSIONS
(Unit of Measure = mm)
MIN
–
MAX
1.20
NOM
NOTE
SYMBOL
A
–
–
A1
A2
D
0.05
0.95
15.75
13.90
15.75
13.90
0.17
0.09
0.45
0.15
1.00
16.00
14.00
16.00
14.00
–
1.05
16.25
D1
E
14.10 Note 2
16.25
Notes:
1. This package conforms to JEDEC reference MS-026, Variation AED.
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
14.10 Note 2
0.27
C
–
0.20
3. Lead coplanarity is 0.08 mm maximum.
L
–
0.75
e
0.50 TYP
10/5/2001
TITLE
DRAWING NO. REV.
2325 Orchard Parkway
San Jose, CA 95131
100A, 100-lead, 14 x 14 mm Body Size, 1.0 mm Body Thickness,
0.5 mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP)
100A
C
R
353
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
32. Errata
32.1 ATmega325P rev. A
• Interrupts may be lost when writing the timer registers in the asynchronous timer.
• Using BOD disable will make the chip reset.
1. Interrupts may be lost when writing the timer registers in the asynchronous timer.
The interrupt will be lost if a timer register that is synchronous timer clock is written when the
asynchronous Timer/Counter register (TCNTx) is 0x00.
Problem Fix/Workaround
Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor
0x00 before writing to the asynchronous Timer Control Register (TCCRx), asynchronous
Timer Counter Register (TCNTx), or asynchronous Output Compare Register (OCRx).
2. Using BOD disable will make the chip reset.
If the part enters sleep with the BOD turned off with the BOD disable option
enabled, a BOD reset will be generated at wakeup and the chip will reset.
Problem Fix/Workaround
Do not use BOD disable
32.2 ATmega325P rev. B
• Interrupts may be lost when writing the timer registers in the asynchronous timer.
1. Interrupts may be lost when writing the timer registers in the asynchronous timer.
The interrupt will be lost if a timer register that is synchronous timer clock is written when the
asynchronous Timer/Counter register (TCNTx) is 0x00.
Problem Fix/Workaround
Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor
0x00 before writing to the asynchronous Timer Control Register (TCCRx), asynchronous
Timer Counter Register (TCNTx), or asynchronous Output Compare Register (OCRx).
32.3 ATmega325P rev. C
• Interrupts may be lost when writing the timer registers in the asynchronous timer.
1. Interrupts may be lost when writing the timer registers in the asynchronous timer.
The interrupt will be lost if a timer register that is synchronous timer clock is written when the
asynchronous Timer/Counter register (TCNTx) is 0x00.
Problem Fix/Workoround
Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor
0x00 before writing to the asynchronous Timer Control Register (TCCRx), asynchronous
Timer Counter Register (TCNTx), or asynchronous Output Compare Register (OCRx).
354
8023F–AVR–07/09
32.4 ATmega3250P rev. A
• Interrupts may be lost when writing the timer registers in the asynchronous timer.
• Using BOD disable will make the chip reset.
1. Interrupts may be lost when writing the timer registers in the asynchronous timer.
The interrupt will be lost if a timer register that is synchronous timer clock is written when the
asynchronous Timer/Counter register (TCNTx) is 0x00.
Problem Fix/Workaround
Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor
0x00 before writing to the asynchronous Timer Control Register (TCCRx), asynchronous
Timer Counter Register (TCNTx), or asynchronous Output Compare Register (OCRx).
2. Using BOD disable will make the chip reset.
If the part enters sleep with the BOD turned off with the BOD disable option
enabled, a BOD reset will be generated at wakeup and the chip will reset.
Problem Fix/Workaround
Do not use BOD disable
32.5 ATmega3250P rev. B
• Interrupts may be lost when writing the timer registers in the asynchronous timer.
1. Interrupts may be lost when writing the timer registers in the asynchronous timer.
The interrupt will be lost if a timer register that is synchronous timer clock is written when the
asynchronous Timer/Counter register (TCNTx) is 0x00.
Problem Fix/Workaround
Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor
0x00 before writing to the asynchronous Timer Control Register (TCCRx), asynchronous
Timer Counter Register (TCNTx), or asynchronous Output Compare Register (OCRx).
32.6 ATmega3250P rev. C
• Interrupts may be lost when writing the timer registers in the asynchronous timer.
1. Interrupts may be lost when writing the timer registers in the asynchronous timer.
The interrupt will be lost if a timer register that is synchronous timer clock is written when the
asynchronous Timer/Counter register (TCNTx) is 0x00.
Problem Fix/Workaround
Always check that the asynchronous Timer/Counter register neither have the value 0xFF nor
0x00 before writing to the asynchronous Timer Control Register (TCCRx), asynchronous
Timer Counter Register (TCNTx), or asynchronous Output Compare Register (OCRx).
355
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
33. Datasheet Revision History
Please note that the referring page numbers in this section are referring to this document.The
referring revision in this section are referring to the document revision.
33.1 Rev.8023F– 07/09
1.
Updated ”Errata” on page 354.
2.
Updated the last page with Atmel’s new addresses.
33.2 Rev.8023E– 06/08
1.
2.
3.
4.
5.
6.
7.
Updated description of ”Stack Pointer” on page 15.
Updated description of ”Low-frequency Crystal Oscillator” on page 32.
Added notes to the overview section of ”External Interrupts” on page 58.
Updated address of ”PORTA – Port A Data Register” on page 85.
Bullet five updated in ”Asynchronous Operation of Timer/Counter2” on page 147.
Updated ”System and Reset Characterizations” on page 308.
Added Errata for ”ATmega325P rev. C” on page 354 and ”ATmega3250P rev. C” on
page 355.
33.3 Rev.8023D – 08/07
1.
2.
Added ”Data Retention” on page 10.
Updated Device and JTAG ID in ”Signature Bytes” on page 274.
33.4 Rev.8023C – 08/07
1.
Updated ”Errata” on page 354.
33.5 Rev.8023B – 08/07
1.
2.
3.
4.
Updated ”Features” on page 1.
Updated DEVICE and JTAG ID in Table 25-6 on page 274
Updated ”System and Reset Characterizations” on page 308.
Updated ”Typical Characteristics” on page 314
33.6 Rev.8023A – 12/06
1.
Initial version.
356
8023F–AVR–07/09
ATmega325P/3250P
Table of Contents
Features..................................................................................................... 1
1
2
Pin Configurations ................................................................................... 2
1.1Disclaimer ..................................................................................................................3
Overview ................................................................................................... 3
2.1Block Diagram ...........................................................................................................4
2.2Comparison between ATmega325P and ATmega3250P ..........................................5
2.3Pin Descriptions ........................................................................................................6
3
4
5
6
Resources ................................................................................................. 9
Data Retention ........................................................................................ 10
About Code Examples ........................................................................... 10
AVR CPU Core ........................................................................................ 11
6.1Overview .................................................................................................................11
6.2Architectural Overview ............................................................................................11
6.3ALU – Arithmetic Logic Unit ....................................................................................12
6.4AVR Status Register ...............................................................................................13
6.5General Purpose Register File ................................................................................14
6.6Stack Pointer ...........................................................................................................15
6.7Instruction Execution Timing ...................................................................................16
6.8Reset and Interrupt Handling ..................................................................................17
7
AVR Memories ........................................................................................ 19
7.1Overview .................................................................................................................19
7.2In-System Reprogrammable Flash Program Memory .............................................19
7.3SRAM Data Memory ...............................................................................................20
7.4EEPROM Data Memory ..........................................................................................21
7.5I/O Memory ..............................................................................................................22
7.6Register Description ................................................................................................23
8
System Clock and Clock Options ......................................................... 29
8.1Clock Systems and their Distribution .......................................................................29
8.2Clock Sources .........................................................................................................30
8.3Default Clock Source ...............................................................................................30
8.4Crystal Oscillator .....................................................................................................31
8.5Low-frequency Crystal Oscillator .............................................................................32
i
8023F–AVR–07/09
8.6Calibrated Internal RC Oscillator .............................................................................33
8.7External Clock .........................................................................................................34
8.8Clock Output Buffer .................................................................................................35
8.9Timer/Counter Oscillator .........................................................................................35
8.10System Clock Prescaler ........................................................................................35
8.11Register Description ..............................................................................................36
9
Power Management and Sleep Modes ................................................. 38
9.1Overview .................................................................................................................38
9.2Sleep Modes ...........................................................................................................38
9.3BOD Disable ............................................................................................................39
9.4Idle Mode .................................................................................................................39
9.5ADC Noise Reduction Mode ...................................................................................39
9.6Power-down Mode ..................................................................................................40
9.7Power-save Mode ...................................................................................................40
9.8Standby Mode .........................................................................................................40
9.9Power Reduction Register .......................................................................................40
9.10Minimizing Power Consumption ............................................................................41
9.11Register Description ..............................................................................................43
10 System Control and Reset .................................................................... 45
10.1Resetting the AVR .................................................................................................45
10.2Reset Sources .......................................................................................................45
10.3Internal Voltage Reference ....................................................................................48
10.4Watchdog Timer ....................................................................................................49
10.5Register Description ..............................................................................................50
11 Interrupts ................................................................................................ 53
11.1Overview ...............................................................................................................53
11.2Interrupt Vectors ....................................................................................................53
11.3Register Description ..............................................................................................56
12 External Interrupts ................................................................................. 58
12.1Overview ...............................................................................................................58
12.2Pin Change Interrupt Timing .................................................................................59
12.3Register Description ..............................................................................................59
13 I/O-Ports .................................................................................................. 63
13.1Overview ...............................................................................................................63
ii
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ATmega325P/3250P
13.2Ports as General Digital I/O ...................................................................................64
13.3Alternate Port Functions ........................................................................................69
13.4Register Description ..............................................................................................85
14 8-bit Timer/Counter0 with PWM ............................................................ 89
14.1Features ................................................................................................................89
14.2Overview ...............................................................................................................89
14.3Timer/Counter Clock Sources ...............................................................................90
14.4Counter Unit ..........................................................................................................90
14.5Output Compare Unit ............................................................................................92
14.6Compare Match Output Unit ..................................................................................93
14.7Modes of Operation ...............................................................................................94
14.8Timer/Counter Timing Diagrams ...........................................................................99
14.9Timer/Counter0 and Timer/Counter1 Prescalers .................................................100
14.10Register Description ..........................................................................................102
15 16-bit Timer/Counter1 .......................................................................... 107
15.1Features ..............................................................................................................107
15.2Overview .............................................................................................................107
15.3Accessing 16-bit Registers ..................................................................................110
15.4Timer/Counter Clock Sources .............................................................................112
15.5Counter Unit ........................................................................................................113
15.6Input Capture Unit ...............................................................................................114
15.7Output Compare Units .........................................................................................116
15.8Compare Match Output Unit ................................................................................117
15.9Modes of Operation .............................................................................................119
15.10Timer/Counter Timing Diagrams .......................................................................126
15.11Register Description ..........................................................................................128
16 8-bit Timer/Counter2 with PWM and Asynchronous Operation ...... 135
16.1Features ..............................................................................................................135
16.2Overview .............................................................................................................135
16.3Timer/Counter Clock Sources .............................................................................136
16.4Counter Unit ........................................................................................................137
16.5Output Compare Unit ..........................................................................................137
16.6Compare Match Output Unit ................................................................................139
16.7Modes of Operation .............................................................................................140
16.8Timer/Counter Timing Diagrams .........................................................................145
iii
8023F–AVR–07/09
16.9Asynchronous Operation of Timer/Counter2 .......................................................147
16.10Register Description ..........................................................................................149
17 SPI – Serial Peripheral Interface ......................................................... 154
17.1Features ..............................................................................................................154
17.2Overview .............................................................................................................154
17.3SS Pin Functionality ............................................................................................159
17.4Data Modes .........................................................................................................159
17.5Register Description ............................................................................................160
18 USART0 ................................................................................................. 163
18.1Features ..............................................................................................................163
18.2Overview .............................................................................................................163
18.3Clock Generation .................................................................................................165
18.4Frame Formats ....................................................................................................168
18.5USART Initialization ............................................................................................169
18.6Data Transmission – The USART Transmitter ....................................................171
18.7Data Reception – The USART Receiver .............................................................173
18.8Asynchronous Data Reception ............................................................................177
18.9Multi-processor Communication Mode ................................................................180
18.10Register Description ..........................................................................................182
18.11Examples of Baud Rate Setting ........................................................................186
19 USI – Universal Serial Interface .......................................................... 191
19.1Features ..............................................................................................................191
19.2Overview .............................................................................................................191
19.3Functional Descriptions .......................................................................................192
19.4Alternative USI Usage .........................................................................................198
19.5Register Descriptions ..........................................................................................199
20 Analog Comparator ............................................................................. 203
20.1Overview .............................................................................................................203
20.2Analog Comparator Multiplexed Input .................................................................203
20.3Register Description ............................................................................................204
21 Analog to Digital Converter ................................................................ 206
21.1Features ..............................................................................................................206
21.2Overview .............................................................................................................206
21.3Operation .............................................................................................................207
iv
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
21.4Starting a Conversion ..........................................................................................208
21.5Prescaling and Conversion Timing ......................................................................209
21.6Changing Channel or Reference Selection .........................................................211
21.7ADC Noise Canceler ...........................................................................................212
21.8ADC Conversion Result ......................................................................................216
21.9Register Description ............................................................................................218
22 JTAG Interface and On-chip Debug System ..................................... 223
22.1Features ..............................................................................................................223
22.2Overview .............................................................................................................223
22.3TAP – Test Access Port ......................................................................................223
22.4TAP Controller .....................................................................................................225
22.5Using the Boundary-scan Chain ..........................................................................226
22.6Using the On-chip Debug System .......................................................................226
22.7On-chip Debug Specific JTAG Instructions .........................................................227
22.8Using the JTAG Programming Capabilities .........................................................227
22.9Bibliography .........................................................................................................228
22.10Register Description ..........................................................................................228
23 IEEE 1149.1 (JTAG) Boundary-scan ................................................... 229
23.1Features ..............................................................................................................229
23.2Overview .............................................................................................................229
23.3Data Registers .....................................................................................................229
23.4Boundary-scan Specific JTAG Instructions .........................................................231
23.5Boundary-scan Chain ..........................................................................................232
23.6ATmega325P/3250P Boundary-scan Order ........................................................241
23.7Boundary-scan Description Language Files ........................................................254
23.8Register Description ............................................................................................255
24 Boot Loader Support – Read-While-Write Self-Programming ......... 256
24.1Features ..............................................................................................................256
24.2Overview .............................................................................................................256
24.3Application and Boot Loader Flash Sections .......................................................256
24.4Read-While-Write and No Read-While-Write Flash Sections ..............................257
24.5Boot Loader Lock Bits .........................................................................................259
24.6Entering the Boot Loader Program ......................................................................261
24.7Addressing the Flash During Self-Programming .................................................261
24.8Self-Programming the Flash ................................................................................262
v
8023F–AVR–07/09
24.9Register Description ............................................................................................269
25 Memory Programming ......................................................................... 271
25.1Program And Data Memory Lock Bits .................................................................271
25.2Fuse Bits .............................................................................................................272
25.3Signature Bytes ...................................................................................................274
25.4Calibration Byte ...................................................................................................274
25.5Parallel Programming Parameters, Pin Mapping, and Commands .....................274
25.6Parallel Programming ..........................................................................................276
25.7Serial Downloading .............................................................................................285
25.8Programming via the JTAG Interface ..................................................................291
26 Electrical Characteristics .................................................................... 304
26.1Absolute Maximum Ratings* ...............................................................................304
26.2DC Characteristics ..............................................................................................304
26.3Speed Grades .....................................................................................................306
26.4Clock Characterizations ......................................................................................307
26.5System and Reset Characterizations ..................................................................308
26.6Standard Power-on Reset ...................................................................................308
26.7Enhanced Power-on Reset .................................................................................308
26.8Brown-out Detection ............................................................................................309
26.9SPI Timing Characteristics ..................................................................................310
26.10ADC Characteristics – Preliminary Data ............................................................312
27 Typical Characteristics ....................................................................... 314
27.1Active Supply Current ..........................................................................................314
27.2Idle Supply Current ..............................................................................................316
27.3Supply Current of I/O modules ............................................................................318
27.4Power-down Supply Current ...............................................................................319
27.5Power-save Supply Current ................................................................................320
27.6Standby Supply Current ......................................................................................321
27.7Pin Pull-up ...........................................................................................................321
27.8Pin Driver Strength ..............................................................................................324
27.9Pin Threshold and Hysteresis .............................................................................330
27.10Internal Oscillator Speed ...................................................................................335
27.11Current Consumption of Peripheral Units ..........................................................337
27.12Current Consumption in Reset and Reset Pulswidth ........................................340
28 Register Summary ............................................................................... 342
vi
ATmega325P/3250P
8023F–AVR–07/09
ATmega325P/3250P
29 Instruction Set Summary .................................................................... 346
30 Ordering Information ........................................................................... 349
30.1ATmega325P ......................................................................................................349
30.2ATmega3250P ....................................................................................................350
31 Packaging Information ........................................................................ 351
31.164A ......................................................................................................................351
31.264M1 ...................................................................................................................352
31.3100A ....................................................................................................................353
32 Errata ..................................................................................................... 354
32.1ATmega325P rev. A ............................................................................................354
32.2ATmega325P rev. B ............................................................................................354
32.3ATmega3250P rev. A ..........................................................................................355
32.4ATmega3250P rev. B ..........................................................................................355
33 Datasheet Revision History ................................................................ 356
33.1Rev.8023E– 06/08 ...............................................................................................356
33.2Rev.8023D – 08/07 .............................................................................................356
33.3Rev.8023C – 08/07 .............................................................................................356
33.4Rev.8023B – 08/07 ..............................................................................................356
33.5Rev.8023A – 12/06 ..............................................................................................356
Table of Contents....................................................................................... i
vii
8023F–AVR–07/09
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8023F–AVR–07/09
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