8-bit Microcontroller with 256K Bytes In

Features
• High Performance, Low Power AVR® 8-Bit Microcontroller
• Advanced RISC Architecture
•
•
•
•
•
•
•
– 135 Powerful Instructions – Most Single Clock Cycle Execution
– 32 x 8 General Purpose Working Registers
– Fully Static Operation
– Up to 16 MIPS Throughput at 16 MHz
– On-Chip 2-cycle Multiplier
Non-volatile Program and Data Memories
– 64K/128K/256K Bytes of In-System Self-Programmable Flash
Endurance: 10,000 Write/Erase Cycles
– Optional Boot Code Section with Independent Lock Bits
In-System Programming by On-chip Boot Program
True Read-While-Write Operation
– 4K Bytes EEPROM
Endurance: 100,000 Write/Erase Cycles
– 8K Bytes Internal SRAM
– Up to 64K Bytes Optional External Memory Space
– 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
– Four 16-bit Timer/Counter with Separate Prescaler, Compare- and Capture Mode
– Real Time Counter with Separate Oscillator
– Four 8-bit PWM Channels
– Six/Twelve PWM Channels with Programmable Resolution from 2 to 16 Bits
(ATmega1281/2561, ATmega640/1280/2560)
– Output Compare Modulator
– 8/16-channel, 10-bit ADC
– Two/Four Programmable Serial USART (ATmega1281/2561,ATmega640/1280/2560)
– Master/Slave SPI Serial Interface
– Byte Oriented 2-wire Serial Interface
– 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
– External and Internal Interrupt Sources
– Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby,
and Extended Standby
I/O and Packages
– 51/86 Programmable I/O Lines (ATmega1281/2561, ATmega640/1280/2560)
– 64-lead (ATmega1281/2561)
– 100-lead (ATmega640/1280/2560)
– 100-lead TQFP (64-lead TQFP Option)
Temperature Range:
– -40°C to 85°C Industrial
Speed Grade:
– ATmega1281/2561V/ATmega640/1280/2560V:
0 - 4 MHz @ 1.8 - 5.5V, 0 - 8 MHz @ 2.7 - 5.5V
– ATmega640/1280/1281/2560/2561:
0 - 8 MHz @ 2.7 - 5.5V, 0 - 16 MHz @ 4.5 - 5.5V
8-bit
Microcontroller
with 256K Bytes
In-System
Programmable
Flash
ATmega1281/25
61/V
ATmega640/128
0/2560/V
Advance
Information
2549A–AVR–03/05
2
PA1 (AD1)
80
PA2 (AD2)
81
PJ7
82
PA0 (AD0)
83
GND
84
VCC
85
PK6 (ADC14/PCINT22)
86
PK7 (ADC15/PCINT23)
87
PK4 (ADC12/PCINT20)
88
PK5 (ADC13/PCINT21)
89
PK2 (ADC10/PCINT18)
90
PK3 (ADC11/PCINT19)
91
PK0 (ADC8/PCINT16)
92
PK1 (ADC9/PCINT17)
PF7 (ADC7/TDI)
93
PF5 (ADC5/TMS)
94
PF6 (ADC6/TDO)
95
PF3 (ADC3)
96
PF4 (ADC4/TCK)
97
PF1 (ADC1)
98
PF2 (ADC2)
AREF
GND
100 99
PF0 (ADC0)
Figure 1. Pinout ATmega640/1280/2560
AVCC
79
78
77
76
(OC0B) PG5
1
75
PA3 (AD3)
(RXD0/PCINT8) PE0
2
74
PA4 (AD4)
(TXD0) PE1
3
73
PA5 (AD5)
(XCK0/AIN0) PE2
4
72
PA6 (AD6)
(OC3A/AIN1) PE3
5
71
PA7 (AD7)
(OC3B/INT4) PE4
6
70
PG2 (ALE)
(OC3C/INT5) PE5
7
69
PJ6 (PCINT15)
(T3/INT6) PE6
8
68
PJ5 (PCINT14)
(CLKO/ICP3/INT7) PE7
9
67
PJ4 (PCINT13)
VCC
10
66
PJ3 (PCINT12)
GND
11
65
PJ2 (XCK3/PCINT11)
(RXD2) PH0
12
64
PJ1 (TXD3/PCINT10)
(TXD2) PH1
13
63
PJ0 (RXD3/PCINT9)
(XCK2) PH2
14
62
GND
(OC4A) PH3
15
61
VCC
(OC4B) PH4
16
60
PC7 (A15)
(OC4C) PH5
17
59
PC6 (A14)
(OC2B) PH6
18
58
PC5 (A13)
(SS/PCINT0) PB0
19
57
PC4 (A12)
(SCK/PCINT1) PB1
20
56
PC3 (A11)
(MOSI/PCINT2) PB2
21
55
PC2 (A10)
(MISO/PCINT3) PB3
22
54
PC1 (A9)
(OC2A/PCINT4) PB4
23
53
PC0 (A8)
(OC1A/PCINT5) PB5
24
52
PG1 (RD)
(OC1B/PCINT6) PB6
25
51
PG0 (WR)
INDEX CORNER
42
43
44
45
46
47
48
49
50
(T1) PD6
41
(T0) PD7
40
(ICP1) PD4
39
(XCK1) PD5
GND
XTAL2
38
(TXD1/INT3) PD3
VCC
37
(RXD1/INT2) PD2
RESET
36
(SCL/INT0) PD0
(TOSC2) PG3
(TOSC1) PG4
35
(SDA/INT1) PD1
(T4) PH7
34
PL6
33
PL7
32
(OC5B) PL4
31
(OC5C) PL5
30
(OC5A) PL3
29
(T5) PL2
28
(ICP5) PL1
27
XTAL1
26
(OC0A/OC1C/PCINT7) PB7
ATmega640/1280/2560
(ICP4) PL0
Pin Configurations
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
PF7 (ADC7/TDI)
GND
VCC
PA0 (AD0)
PA1 (AD1)
PA2 (AD2)
54
53
52
51
50
49
PF5 (ADC5/TMS)
57
PF6 (ADC6/TDO)
PF4 (ADC4/TCK)
58
55
PF3 (ADC3)
59
56
PF1 (ADC1)
PF2 (ADC2)
60
AREF
PF0 (ADC0)
62
63
61
AVCC
GND
(OC0B) PG5
1
48
PA3 (AD3)
(RXD0/PCINT8/PDI) PE0
2
47
PA4 (AD4)
(TXD0/PDO) PE1
3
46
PA5 (AD5)
(XCK0/AIN0) PE2
4
45
PA6 (AD6)
(OC3A/AIN1) PE3
5
44
PA7 (AD7)
(OC3B/INT4) PE4
6
43
PG2 (ALE)
(OC3C/INT5) PE5
7
42
PC7 (A15)
(T3/INT6) PE6
8
41
PC6 (A14)
(ICP3/CLKO/INT7) PE7
9
40
PC5 (A13)
(SS/PCINT0) PB0
10
39
PC4 (A12)
(SCK/ PCINT1) PB1
11
38
PC3 (A11)
(MOSI/ PCINT2) PB2
12
37
PC2 (A10)
(MISO/ PCINT3) PB3
13
36
PC1 (A9)
(OC2A/ PCINT4) PB4
14
35
PC0 (A8)
(OC1A/PCINT5) PB5
15
34
PG1 (RD)
(OC1B/PCINT6) PB6
16
33
PG0 (WR)
Note:
Disclaimer
64
Figure 2. Pinout ATmega1281/2561
INDEX CORNER
27
28
29
30
31
32
(RXD1/INT2) PD2
(TXD1/INT3) PD3
(ICP1) PD4
(XCK1) PD5
(T1) PD6
(T0) PD7
23
26
22
XTAL2
(SDA/INT1) PD1
21
VCC
GND
25
20
RESET
24
19
(TOSC1) PG4
XTAL1
18
(TOSC2) PG3
(SCL/INT0) PD0
17
(OC0A/OC1C/PCINT7) PB7
ATmega1281/2561
The large center pad underneath the QFN/MLF package is made of metal and internally
connected 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.
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.
3
2549A–AVR–03/05
Overview
The ATmega640/1280/1281/2560/2561 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 ATmega640/1280/1281/2560/2561 achieves
throughputs approaching 1 MIPS per MHz allowing the system designer to optimize power consumption versus processing
speed.
Block Diagram
Figure 3. Block Diagram
PF7..0
PK7..0
PORT F (8)
PORT K (8)
PJ7..0
PE7..0
VCC
RESET
GND
Power
Supervision
POR / BOD &
RESET
PORT J (8)
PORT E (8)
Watchdog
Timer
Watchdog
Oscillator
Analog
Comparator
JTAG
A/D
Converter
EEPROM
Internal
Bandgap reference
USART 0
XTAL1
Oscillator
Circuits /
Clock
Generation
16bit T/C 3
USART 3
16bit T/C 5
XTAL2
CPU
PA7..0
PORT A (8)
16bit T/C 4
USART 1
PG5..0
PORT G (6)
XRAM
PC7..0
PORT C (8)
TWI
FLASH
SPI
SRAM
16bit T/C 1
8bit T/C 0
USART 2
8bit T/C 2
NOTE:
Shaded parts only appear
in the 100 pin version.
The ADC, T/C4 and T/C5
only have full functionality
in the 100 pin version.
4
PORT D (8)
PORT B (8)
PORT H (8)
PORT L (8)
PD7..0
PB7..0
PH7..0
PL7..0
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
The AVR core combines a rich instruction set with 32 general purpose working registers.
All the 32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing
two independent registers to be accessed in one single instruction executed in one clock
cycle. The resulting architecture is more code efficient while achieving throughputs up to
ten times faster than conventional CISC microcontrollers.
The ATmega640/1280/1281/2560/2561 provides the following features: 64K/128K/256K
bytes of In-System Programmable Flash with Read-While-Write capabilities, 4K bytes
EEPROM, 8K bytes SRAM, 54/86 general purpose I/O lines, 32 general purpose working registers, Real Time Counter (RTC), six flexible Timer/Counters with compare
modes and PWM, 4 USARTs, a byte oriented 2-wire Serial Interface, a 16-channel, 10bit ADC with optional differential input stage with programmable gain, programmable
Watchdog Timer with Internal Oscillator, an SPI serial port, IEEE std. 1149.1 compliant
JTAG test interface, also used for accessing the On-chip Debug system and programming and six software selectable power saving modes. The Idle mode stops the CPU
while allowing the SRAM, Timer/Counters, SPI port, and interrupt system to continue
functioning. The Power-down mode saves the register contents but freezes the Oscillator, disabling all other chip functions until the next interrupt or Hardware Reset. In
Power-save mode, the asynchronous timer continues to run, allowing the user to maintain a timer base while the rest of the device is sleeping. The ADC Noise Reduction
mode stops the CPU and all I/O modules except Asynchronous Timer and ADC, to minimize switching noise during ADC conversions. In Standby mode, the Crystal/Resonator
Oscillator is running while the rest of the device is sleeping. This allows very fast start-up
combined with low power consumption. In Extended Standby mode, both the main
Oscillator and the Asynchronous Timer continue to run.
The device is manufactured using Atmel’s high-density nonvolatile memory technology.
The On-chip ISP Flash allows the program memory to be reprogrammed in-system
through an SPI serial interface, by a conventional nonvolatile memory programmer, or
by an On-chip Boot program running on the AVR core. The boot program can use any
interface to download the application program in the application Flash memory. Software in the Boot Flash section will continue to run while the Application Flash section is
updated, providing true Read-While-Write operation. By combining an 8-bit RISC CPU
with In-System Self-Program mable Flash on a monolithic ch ip, the Atmel
ATmega640/1280/1281/2560/2561 is a powerful microcontroller that provides a highly
flexible and cost effective solution to many embedded control applications.
The ATmega640/1280/1281/2560/2561 AVR is supported with a full suite of program
and system development tools including: C compilers, macro assemblers, program
debugger/simulators, in-circuit emulators, and evaluation kits.
5
2549A–AVR–03/05
Comparison Between ATmega1281/2561 and ATmega640/1280/2560
Each device in the ATmega640/1280/1281/2560/2561 family differs only in memory size
and number of pins. Table 1 summarizes the different configurations for the six devices.
Table 1. Configuration Summary
Device
Flash
EEPROM
RAM
General
Purpose I/O pins
16 bits resolution
PWM channels
Serial
USARTs
ADC
Channels
ATmega640
64KB
4KB
8KB
86
12
4
16
ATmega1280
128KB
4KB
8KB
86
12
4
16
ATmega1281
128KB
4KB
8KB
54
6
2
8
ATmega2560
256KB
4KB
8KB
86
12
4
16
ATmega2561
256KB
4KB
8KB
54
6
2
8
Pin Descriptions
VCC
Digital supply voltage.
GND
Ground.
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.
Port A also serves the functions of various special features of the
ATmega640/1280/1281/2560/2561 as listed on page 88.
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
ATmega640/1280/1281/2560/2561 as listed on page 89.
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 C also serves the functions of special
ATmega640/1280/1281/2560/2561 as listed on page 92.
Port D (PD7..PD0)
6
features
of
the
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
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
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
ATmega640/1280/1281/2560/2561 as listed on page 94.
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 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
ATmega640/1280/1281/2560/2561 as listed on page 96.
Port F (PF7..PF0)
Port F serves as 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 symmetrical 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.
Port G (PG5..PG0)
Port G is a 6-bit 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
ATmega640/1280/1281/2560/2561 as listed on page 102.
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
ATmega640/1280/1281/2560/2561 as listed on page 104.
Port J (PJ7..PJ0)
Port J is a 8-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 capability. 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
ATmega640/1280/1281/2560/2561 as listed on page 106.
Port K (PK7..PK0)
Port K serves as analog inputs to the A/D Converter.
7
2549A–AVR–03/05
Port K is a 8-bit bi-directional I/O port with internal pull-up resistors (selected for each
bit). The Port K output buffers have symmetrical drive characteristics with both high sink
and source capability. As inputs, Port K pins that are externally pulled low will source
current if the pull-up resistors are activated. The Port K pins are tri-stated when a reset
condition becomes active, even if the clock is not running.
Port K also serves the functions of various special features of the
ATmega640/1280/1281/2560/2561 as listed on page 108.
Port L (PL7..PL0)
Port L is a 8-bit bi-directional I/O port with internal pull-up resistors (selected for each
bit). The Port L output buffers have symmetrical drive characteristics with both high sink
and source capability. As inputs, Port L pins that are externally pulled low will source
current if the pull-up resistors are activated. The Port L pins are tri-stated when a reset
condition becomes active, even if the clock is not running.
Port L also serves the functions of various special features of the
ATmega640/1280/1281/2560/2561 as listed on page 110.
RESET
Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the clock is not running. The minimum pulse length is given in Table
23 on page 58. Shorter pulses are not guaranteed to generate a reset.
XTAL1
Input to the inverting Oscillator amplifier and input to the internal clock operating circuit.
XTAL2
Output from the inverting Oscillator amplifier.
AVCC
AVCC is the supply voltage pin for Port F and the A/D Converter. It should be externally
connected 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.
AREF
This is the analog reference pin for the A/D Converter.
About Code
Examples
This documentation contains simple code examples that briefly show how to use various
parts of the device. 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 documentation for more details.
These code examples assume that the part specific header file is included before compilation. 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".
8
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
AVR CPU Core
Introduction
This section discusses the AVR core architecture in general. The main function of the
CPU core is to ensure correct program execution. The CPU must therefore be able to
access memories, perform calculations, control peripherals, and handle interrupts.
Architectural Overview
Figure 4. Block Diagram of the AVR Architecture
Data Bus 8-bit
Flash
Program
Memory
Program
Counter
Status
and Control
32 x 8
General
Purpose
Registrers
Control Lines
Direct Addressing
Instruction
Decoder
Indirect Addressing
Instruction
Register
Interrupt
Unit
SPI
Unit
Watchdog
Timer
ALU
Analog
Comparator
I/O Module1
Data
SRAM
I/O Module 2
I/O Module n
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 instruction is pre-fetched from the program memory. This concept
enables instructions to be executed in every clock cycle. The program memory is InSystem Reprogrammable Flash memory.
The fast-access Register File contains 32 x 8-bit general purpose working registers with
a single clock cycle access time. This allows single-cycle Arithmetic Logic Unit (ALU)
operation. In a typical ALU operation, two operands are output from the Register File,
9
2549A–AVR–03/05
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 operation, 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 format. Every program memory address contains a 16- or 32-bit instruction.
Program Flash memory space is divided in two sections, the Boot Program section and
the Application Program section. Both sections have dedicated Lock bits for write and
read/write protection. The SPM instruction that writes into the Application Flash memory
section must reside in the Boot Program section.
During interrupts and subroutine calls, the return address Program Counter (PC) is
stored on the Stack. The Stack is effectively allocated in the general data SRAM, and
consequently the Stack size is only limited by the total SRAM size and the usage of the
SRAM. All user programs must initialize the SP in the Reset routine (before subroutines
or interrupts are executed). The Stack Pointer (SP) is read/write accessible in the I/O
space. The data SRAM can easily be accessed through the five different addressing
modes supported in the AVR architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional
Global Interrupt Enable bit in the Status Register. All interrupts have a separate Interrupt
Vector in the Interrupt Vector table. The interrupts have priority in accordance with their
Interrupt Vector position. The lower the Interrupt Vector address, the higher the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as Control
Registers, SPI, and other I/O functions. The I/O Memory can be accessed directly, or as
the Data Space locations following those of the Register File, 0x20 - 0x5F. In addition,
the ATmega640/1280/1281/2560/2561 has Extended I/O space from 0x60 - 0x1FF in
SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used.
ALU – Arithmetic Logic
Unit
The high-performance AVR ALU operates in direct connection with all the 32 general
purpose working registers. Within a single clock cycle, arithmetic operations between
general purpose registers or between a register and an immediate are executed. The
ALU operations are divided into three main categories – arithmetic, logical, and bit-functions. Some implementations of the architecture also provide a powerful multiplier
supporting both signed/unsigned multiplication and fractional format. See the “Instruction Set” section for a detailed description.
Status Register
The Status Register contains information about the result of the most recently executed
arithmetic instruction. This information can be used for altering program flow in order to
perform conditional operations. Note that the Status Register is updated after all ALU
operations, as specified in the Instruction Set Reference. This will in many cases
10
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
remove the need for using the dedicated compare instructions, resulting in faster and
more compact code.
The Status Register is not automatically stored when entering an interrupt routine and
restored when returning from an interrupt. This must be handled by software.
The AVR Status Register – SREG – is defined as:
Bit
7
6
5
4
3
2
1
0
I
T
H
S
V
N
Z
C
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
SREG
• Bit 7 – I: Global Interrupt Enable
The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual interrupt enable control is then performed in separate control registers. If the Global
Interrupt Enable Register is cleared, none of the interrupts are enabled independent of
the individual interrupt enable settings. The I-bit is cleared by hardware after an interrupt
has occurred, and is set by the RETI instruction to enable subsequent interrupts. The Ibit can also be set and cleared by the application with the SEI and CLI instructions, as
described in the instruction set reference.
• Bit 6 – T: Bit Copy Storage
The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or
destination for the operated bit. A bit from a register in the Register File can be copied
into T by the BST instruction, and a bit in T can be copied into a bit in a register in the
Register File by the BLD instruction.
• Bit 5 – H: Half Carry Flag
The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry Is
useful in BCD arithmetic. See the “Instruction Set Description” for detailed information.
• Bit 4 – S: Sign Bit, S = N
⊕V
The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement Overflow Flag V. See the “Instruction Set Description” for detailed information.
• Bit 3 – V: Two’s Complement Overflow Flag
The Two’s Complement Overflow Flag V supports two’s complement arithmetics. See
the “Instruction Set Description” for detailed information.
• Bit 2 – N: Negative Flag
The Negative Flag N indicates a negative result in an arithmetic or logic operation. See
the “Instruction Set Description” for detailed information.
• Bit 1 – Z: Zero Flag
The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the
“Instruction Set Description” for detailed information.
• Bit 0 – C: Carry Flag
The Carry Flag C indicates a carry in an arithmetic or logic operation. See the “Instruction Set Description” for detailed information.
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2549A–AVR–03/05
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 5 shows the structure of the 32 general purpose working registers in the CPU.
Figure 5. AVR CPU General Purpose Working Registers
7
0
Addr.
R0
0x00
R1
0x01
R2
0x02
…
R13
0x0D
General
R14
0x0E
Purpose
R15
0x0F
Working
R16
0x10
Registers
R17
0x11
…
R26
0x1A
X-register Low Byte
R27
0x1B
X-register High Byte
R28
0x1C
Y-register Low Byte
R29
0x1D
Y-register High Byte
R30
0x1E
Z-register Low Byte
R31
0x1F
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 5, 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 implemented as SRAM locations, this memory organization provides great
flexibility in access of the registers, as the X-, Y- and Z-pointer registers can be set to
index any register in the file.
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The X-register, Y-register, and
Z-register
The registers R26..R31 have some added functions to their general purpose usage.
These registers are 16-bit address pointers for indirect addressing of the data space.
The three indirect address registers X, Y, and Z are defined as described in Figure 6.
Figure 6. The X-, Y-, and Z-registers
15
X-register
XH
XL
7
0
R27 (0x1B)
YH
YL
7
0
R29 (0x1D)
Z-register
0
R26 (0x1A)
15
Y-register
0
7
0
7
0
R28 (0x1C)
15
ZH
7
0
ZL
7
R31 (0x1F)
0
0
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).
Stack Pointer
The Stack is mainly used for storing temporary data, for storing local variables and for
storing return addresses after interrupts and subroutine calls. The Stack Pointer Register always points to the top of the Stack. Note that the Stack is implemented as growing
from higher memory locations to lower memory locations. This implies that a Stack
PUSH command decreases the Stack Pointer.
The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt Stacks are located. This Stack space in the data SRAM must be defined by the
program before any subroutine calls are executed or interrupts are enabled. The Stack
Pointer must be set to point above 0x0200. The initial value of the stack pointer is the
last address of the internal SRAM. The Stack Pointer is decremented by one when data
is pushed onto the Stack with the PUSH instruction, and it is decremented by three
when the return address is pushed onto the Stack with subroutine call or interrupt. The
Stack Pointer is incremented by one when data is popped from the Stack with the POP
instruction, and it is incremented by three when data is popped from the Stack with
return from subroutine RET or return from interrupt RETI.
The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually used is implementation dependent. Note that the data space in some
implementations of the AVR architecture is so small that only SPL is needed. In this
case, the SPH Register will not be present.
Bit
Read/Write
Initial Value
15
14
13
12
11
10
9
8
SP15
SP14
SP13
SP12
SP11
SP10
SP9
SP8
SPH
SP7
SP6
SP5
SP4
SP3
SP2
SP1
SP0
SPL
7
6
5
4
3
2
1
0
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
0
0
1
0
0
0
0
1
1
1
1
1
1
1
1
1
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2549A–AVR–03/05
Extended Z-pointer Register
for ELPM/SPM - RAMPZ
Bit
7
6
5
4
3
2
1
0
RAMPZ7
RAMPZ6
RAMPZ5
RAMPZ4
RAMPZ3
RAMPZ2
RAMPZ1
RAMPZ0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
RAMPZ
For ELPM/SPM instructions, the Z-pointer is a concatenation of RAMPZ, ZH, and ZL, as
shown in Figure 7. Note that LPM is not affected by the RAMPZ setting.
Figure 7. The Z-pointer used by ELPM and SPM
Bit (
Individually)
7
Bit (Z-pointer)
23
0
7
16
15
RAMPZ
0
7
8
7
ZH
0
ZL
0
The actual number of bits is implementation dependent. Unused bits in an implementation will always read as zero. For compatibility with future devices, be sure to write these
bits to zero.
Extended Indirect Register EIND
Bit
7
6
5
4
3
2
1
0
EIND7
EIND6
EIND5
EIND4
EIND3
EIND2
EIND1
EIND0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
EIND
For EICALL/EIJMP instructions, the Indirect-pointer to the subroutine/routine is a concatenation of EIND, ZH, and ZL, as shown in Figure 8. Note that ICALL and IJMP are
not affected by the EIND setting.
Figure 8. The Indirect-pointer used by EICALL and EIJMP
Bit (Individually)
7
0
7
EIND
Bit (Indirectpointer)
23
0
7
ZH
16
15
0
ZL
8
7
0
The actual number of bits is implementation dependent. Unused bits in an implementation will always read as zero. For compatibility with future devices, be sure to write these
bits to zero.
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 9 shows the parallel instruction fetches and instruction executions enabled by the
Harvard 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.
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ATmega640/1280/1281/2560/2561
Figure 9. 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 10 shows the internal timing concept for the Register File. In a single clock cycle
an ALU operation using two register operands is executed, and the result is stored back
to the destination register.
Figure 10. Single Cycle ALU Operation
T1
T2
T3
T4
clkCPU
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
Reset and Interrupt
Handling
The AVR provides several different interrupt sources. These interrupts and the separate
Reset Vector each have a separate program vector in the program memory space. All
interrupts are assigned individual enable bits which must be written logic one together
with the Global Interrupt Enable bit in the Status Register in order to enable the interrupt.
Depending on the Program Counter value, interrupts may be automatically disabled
when Boot Lock bits BLB02 or BLB12 are programmed. This feature improves software
security. See the section “Memory Programming” on page 335 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 69.
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 69 for more information. The Reset Vector can also be
moved to the start of the Boot Flash section by programming the BOOTRST Fuse, see
“Memory Programming” on page 335.
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts
are disabled. The user software can write logic one to the I-bit to enable nested 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.
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There are basically two types of interrupts. The first type is triggered by an event that
sets the Interrupt Flag. For these interrupts, the Program Counter is vectored to the
actual Interrupt Vector in order to execute the interrupt handling routine, and hardware
clears the corresponding Interrupt Flag. Interrupt Flags can also be cleared by writing a
logic one to the flag bit position(s) to be cleared. If an interrupt condition occurs while the
corresponding interrupt enable bit is cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled, or the flag is cleared by software. Similarly, if one or
more interrupt conditions occur while the Global Interrupt Enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the Global Interrupt
Enable bit is set, and will then be executed by order of priority.
The second type of interrupts will trigger as long as the interrupt condition is present.
These interrupts do not necessarily have Interrupt Flags. If the interrupt condition disappears before the interrupt is enabled, the interrupt will not be triggered.
When the AVR exits from an interrupt, it will always return to the main program and execute one more instruction before any pending interrupt is served.
Note that the Status Register is not automatically stored when entering an interrupt routine, nor restored when returning from an interrupt routine. This must be handled by
software.
When using the CLI instruction to disable interrupts, the interrupts will be immediately
disabled. No interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the CLI instruction. The following example shows how this can be used to
avoid interrupts during the timed EEPROM write sequence..
Assembly Code Example
in
cli
r16, SREG
; store SREG value
; disable interrupts during timed sequence
sbi EECR, EEMPE
; start EEPROM write
sbi EECR, EEPE
out SREG, 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<<EEMPE); /* start EEPROM write */
EECR |= (1<<EEPE);
SREG = cSREG; /* restore SREG value (I-bit) */
When using the SEI instruction to enable interrupts, the instruction following SEI will be
executed before any pending interrupts, as shown in this example.
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ATmega640/1280/1281/2560/2561
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) */
Interrupt Response Time
The interrupt execution response for all the enabled AVR interrupts is five clock cycles
minimum. After five clock cycles the program vector address for the actual interrupt handling routine is executed. During these five 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 five 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 five clock cycles. During these five
clock cycles, the Program Counter (three bytes) is popped back from the Stack, the
Stack Pointer is incremented by three, and the I-bit in SREG is set.
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ATmega640/1280/1281/2560/2561
AVR ATmega640/1280/1281/2560/2561 Memories
This section describes the different memories in the ATmega640/1280/1281/2560/2561.
The AVR architecture has two main memory spaces, the Data Memory and the Program
Memory space. In addition, the ATmega640/1280/1281/2560/2561 features an
EEPROM Memory for data storage. All three memory spaces are linear and regular.
In-System
Reprogrammable Flash
Program Memory
The ATmega640/1280/1281/2560/2561 contains 64K/128K/256K bytes On-chip In-System Reprogrammable Flash memory for program storage. Since all AVR instructions
are 16 or 32 bits wide, the Flash is organized as 32K/64K/128K x 16. For software security, the Flash Program memory space is divided into two sections, Boot Program
section and Application Program section.
The Flash memory has an endurance of at least 10,000 write/erase cycles. The
ATmega640/1280/1281/2560/2561 Program Counter (PC) is 15/16/17 bits wide, thus
addressing the 32K/64K/128K program memory locations. The operation of Boot Program section and associated Boot Lock bits for software protection are described in
detail in “Boot Loader Support – Read-While-Write Self-Programming” on page 317.
“Memory Programming” on page 335 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 and ELPM - Extended Load
Program Memory instruction description).
Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Timing” on page 14.
Figure 11. Program Memory Map
Program Memory
0x0000
Application Flash Section
Boot Flash Section
0x7FFF/0xFFFF/0x1FFFF
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SRAM Data Memory
Figure 12 shows how the ATmega640/1280/1281/2560/2561 SRAM Memory is
organized.
The ATmega640/1280/1281/2560/2561 is a complex microcontroller with more peripheral units than can be supported within the 64 location reserved in the Opcode for the IN
and OUT instructions. For the Extended I/O space from $060 - $1FF in SRAM, only the
ST/STS/STD and LD/LDS/LDD instructions can be used.
The first 4,608/8,704 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 416 locations of Extended I/O memory and the next 8,192 locations address the internal data
SRAM.
An
optional
external
data
SRAM
can
be
used
with
the
ATmega640/1280/1281/2560/2561. This SRAM will occupy an area in the remaining
address locations in the 64K address space. This area starts at the address following
the internal SRAM. The Register file, I/O, Extended I/O and Internal SRAM occupies the
lowest 4,608/8,704 bytes, so when using 64KB (65,536 bytes) of External Memory,
60,478/56,832 Bytes of External Memory are available. See “External Memory Interface” on page 29 for details on how to take advantage of the external memory map.
When the addresses accessing the SRAM memory space exceeds the internal data
memory locations, the external data SRAM is accessed using the same instructions as
for the internal data memory access. When the internal data memories are accessed,
the read and write strobe pins (PG0 and PG1) are inactive during the whole access
cycle. External SRAM operation is enabled by setting the SRE bit in the XMCRA
Register.
Accessing external SRAM takes one additional clock cycle per byte compared to access
of the internal SRAM. This means that the commands LD, ST, LDS, STS, LDD, STD,
PUSH, and POP take one additional clock cycle. If the Stack is placed in external
SRAM, interrupts, subroutine calls and returns take three clock cycles extra because the
three-byte program counter is pushed and popped, and external memory access does
not take advantage of the internal pipe-line memory access. When external SRAM interface is used with wait-state, one-byte external access takes two, three, or four additional
clock cycles for one, two, and three wait-states respectively. Interrupts, subroutine calls
and returns will need five, seven, or nine clock cycles more than specified in the instruction set manual for one, two, and three wait-states.
The five different addressing modes for the data memory cover: Direct, Indirect with Displacement, Indirect, Indirect with Pre-decrement, and Indirect with Post-increment. In
the Register file, registers R26 to R31 feature the indirect addressing pointer registers.
The direct addressing reaches the entire data space.
The Indirect with Displacement mode reaches 63 address locations from the base
address given by the Y- or Z-register.
When using register indirect addressing modes with automatic pre-decrement and postincrement, the address registers X, Y, and Z are decremented or incremented.
The 32 general purpose working registers, 64 I/O registers, and the 4,196/8,192 bytes of
internal data SRAM in the ATmega640/1280/1281/2560/2561 are all accessible through
all these addressing modes. The Register File is described in “General Purpose Register File” on page 12.
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ATmega640/1280/1281/2560/2561
Figure 12. Data Memory Map
Data Memory
32 Registers
64 I/O Registers
416 Ext I/O Reg.
$0000 - $001F
$0020 - $005F
$0060 - $01FF
$0200
Internal SRAM
(8192 x 8)
$21FF
$2200
External SRAM
(0 - 64K x 8)
$FFFF
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Data Memory Access Times
This section describes the general access timing concepts for internal memory access.
The internal data SRAM access is performed in two clkCPU cycles as described in Figure
13.
Figure 13. On-chip Data SRAM Access Cycles
T1
T2
T3
clkCPU
Address
Compute Address
Address valid
Write
Data
WR
Read
Data
RD
Memory Access Instruction
EEPROM Data Memory
Next Instruction
The ATmega640/1280/1281/2560/2561 contains 4K bytes of data EEPROM memory. It
is organized as a separate data space, in which single bytes can be read and written.
The EEPROM has an endurance of at least 100,000 write/erase cycles. The access
between the EEPROM and the CPU is described in the following, specifying the
EEPROM Address Registers, the EEPROM Data Register, and the EEPROM Control
Register.
For a detailed description of SPI, JTAG and Parallel data downloading to the EEPROM,
see page 349, page 353, and page 338 respectively.
EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space.
The write access time for the EEPROM is given in Table 3. A self-timing function, however, lets the user software detect when the next byte can be written. If the user code
contains instructions that write the EEPROM, some precautions must be taken. In
heavily filtered power supplies, 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 “Preventing EEPROM Corruption” on page
27. for details on how to avoid problems in these situations.
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to the description of the EEPROM Control Register for details on this.
When the EEPROM is read, the CPU is halted for four clock cycles before the next
instruction is executed. When the EEPROM is written, the CPU is halted for two clock
cycles before the next instruction is executed.
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ATmega640/1280/1281/2560/2561
The EEPROM Address
Register – EEARH and EEARL
Bit
Read/Write
Initial Value
15
14
13
12
11
10
9
8
–
–
–
–
EEAR11
EEAR10
EEAR9
EEAR8
EEARH
EEAR7
EEAR6
EEAR5
EEAR4
EEAR3
EEAR2
EEAR1
EEAR0
EEARL
7
6
5
4
3
2
1
0
R
R
R
R
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
0
0
0
0
X
X
X
X
X
X
X
X
X
X
X
X
• Bits 15..12 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bits 11..0 – EEAR8..0: EEPROM Address
The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address
in the 4K bytes EEPROM space. The EEPROM data bytes are addressed linearly
between 0 and 4096. The initial value of EEAR is undefined. A proper value must be
written before the EEPROM may be accessed.
The EEPROM Data Register –
EEDR
Bit
7
6
5
4
3
2
1
0
MSB
LSB
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
EEDR
• Bits 7..0 – EEDR7.0: EEPROM Data
For the EEPROM write operation, the EEDR Register contains the data to be written to
the EEPROM in the address given by the EEAR Register. For the EEPROM read operation, the EEDR contains the data read out from the EEPROM at the address given by
EEAR.
The EEPROM Control Register
– EECR
Bit
7
6
5
4
3
2
1
0
–
–
EEPM1
EEPM0
EERIE
EEMPE
EEPE
EERE
Read/Write
R
R
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
X
X
0
0
X
0
EECR
• Bits 7..6 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bits 5, 4 – EEPM1 and EEPM0: EEPROM Programming Mode Bits
The EEPROM Programming mode bit setting defines which programming action that will
be triggered when writing EEPE. It is possible to program data in one atomic operation
(erase the old value and program the new value) or to split the Erase and Write operations in two different operations. The Programming times for the different modes are
shown in Table 2. While EEPE is set, any write to EEPMn will be ignored. During reset,
the EEPMn bits will be reset to 0b00 unless the EEPROM is busy programming.
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2549A–AVR–03/05
Table 2. EEPROM Mode Bits
EEPM1
EEPM0
Programming
Time
0
0
3.4 ms
Erase and Write in one operation (Atomic Operation)
0
1
1.8 ms
Erase Only
1
0
1.8 ms
Write Only
1
1
–
Operation
Reserved for future use
• Bit 3 – EERIE: EEPROM Ready Interrupt Enable
Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set.
Writing EERIE to zero disables the interrupt. The EEPROM Ready interrupt generates a
constant interrupt when EEPE is cleared.
• Bit 2 – EEMPE: EEPROM Master Programming Enable
The EEMPE bit determines whether setting EEPE to one causes the EEPROM to be
written. When EEMPE is set, setting EEPE within four clock cycles will write data to the
EEPROM at the selected address If EEMPE is zero, setting EEPE will have no effect.
When EEMPE has been written to one by software, hardware clears the bit to zero after
four clock cycles. See the description of the EEPE bit for an EEPROM write procedure.
• Bit 1 – EEPE: EEPROM Programming Enable
The EEPROM Write Enable Signal EEPE is the write strobe to the EEPROM. When
address and data are correctly set up, the EEPE bit must be written to one to write the
value into the EEPROM. The EEMPE bit must be written to one before a logical one is
written to EEPE, otherwise no EEPROM write takes place. The following procedure
should be followed when writing the EEPROM (the order of steps 3 and 4 is not
essential):
1. Wait until EEPE becomes zero.
2. Wait until SELFPRGEN 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 EEMPE bit while writing a zero to EEPE in EECR.
6. Within four clock cycles after setting EEMPE, write a logical one to EEPE.
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 “Memory Programming” on page 335 for details about Boot
programming.
Caution: An interrupt between step 5 and step 6 will make the write cycle fail, since the
EEPROM Master Write Enable will time-out. If an interrupt routine accessing the
EEPROM is interrupting another EEPROM access, the EEAR or EEDR Register will be
modified, causing the interrupted EEPROM access to fail. It is recommended to have
the Global Interrupt Flag cleared during all the steps to avoid these problems.
When the write access time has elapsed, the EEPE bit is cleared by hardware. The user
software can poll this bit and wait for a zero before writing the next byte. When EEPE
has been set, the CPU is halted for two cycles before the next instruction is executed.
24
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
• 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 EEPE 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 3 lists the typical
programming time for EEPROM access from the CPU.
Table 3. EEPROM Programming Time
Symbol
EEPROM write
(from CPU)
Number of Calibrated RC Oscillator Cycles
Typ Programming Time
26,368
3.3 ms
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2549A–AVR–03/05
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 globally) so that no interrupts will occur during execution of these functions. The
examples also assume that no Flash Boot Loader is present in the software. If such
code is present, the EEPROM write function must also wait for any ongoing SPM command to finish.
Assembly Code Example()
EEPROM_write:
; Wait for completion of previous write
sbic EECR,EEPE
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 EEMPE
sbi
EECR,EEMPE
; Start eeprom write by setting EEPE
sbi
EECR,EEPE
ret
C Code Example(1)
void EEPROM_write(unsigned int uiAddress, unsigned char ucData)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address and Data Registers */
EEAR = uiAddress;
EEDR = ucData;
/* Write logical one to EEMPE */
EECR |= (1<<EEMPE);
/* Start eeprom write by setting EEPE */
EECR |= (1<<EEPE);
}
Note:
26
1. See “About Code Examples” on page 8.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
The next code examples show assembly and C functions for reading the EEPROM. The
examples assume that interrupts are controlled so that no interrupts will occur during
execution of these functions.
Assembly Code Example(1)
EEPROM_read:
; Wait for completion of previous write
sbic EECR,EEPE
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(1)
unsigned char EEPROM_read(unsigned int uiAddress)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEPE))
;
/* Set up address register */
EEAR = uiAddress;
/* Start eeprom read by writing EERE */
EECR |= (1<<EERE);
/* Return data from Data Register */
return EEDR;
}
Note:
Preventing EEPROM
Corruption
1. See “About Code Examples” on page 8.
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. Secondly, 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 completed provided that the power supply voltage is
sufficient.
27
2549A–AVR–03/05
I/O Memory
The I/O space definition of the ATmega640/1280/1281/2560/2561 is shown in “Register
Summary” on page 385.
All ATmega640/1280/1281/2560/2561 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
ATmega640/1280/1281/2560/2561 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 - 0x1FF 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 registers 0x00 to 0x1F only.
The I/O and peripherals control registers are explained in later sections.
General Purpose I/O Registers The ATmega640/1280/1281/2560/2561 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.
General Purpose I/O Register
2 – GPIOR2
Bit
7
6
5
4
3
2
1
MSB
General Purpose I/O Register
1 – GPIOR1
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
7
6
5
4
3
2
1
Bit
MSB
General Purpose I/O Register
0 – GPIOR0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
7
6
5
4
3
2
1
Bit
GPIOR2
0
LSB
MSB
28
0
LSB
GPIOR1
0
LSB
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
GPIOR0
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
External Memory
Interface
With all the features the External Memory Interface provides, it is well suited to operate
as an interface to memory devices such as External SRAM and Flash, and peripherals
such as LCD-display, A/D, and D/A. The main features are:
• Four different wait-state settings (including no wait-state).
• Independent wait-state setting for different extErnal Memory sectors (configurable sector
size).
• The number of bits dedicated to address high byte is selectable.
• Bus keepers on data lines to minimize current consumption (optional).
Overview
When the eXternal MEMory (XMEM) is enabled, address space outside the internal
SRAM becomes available using the dedicated External Memory pins (see Figure 2 on
page 3, Table 36 on page 88, Table 42 on page 92, and Table 54 on page 102). The
memory configuration is shown in Figure 14.
Figure 14. External Memory with Sector Select
Memory Configuration A
0x0000
Internal memory
0x21FF
0x2200
Lower sector
SRW01
SRW00
SRL[2..0]
External Memory
(0-60K x 8)
Upper sector
SRW11
SRW10
0xFFFF
Using the External Memory
Interface
The interface consists of:
•
AD7:0: Multiplexed low-order address bus and data bus.
•
A15:8: High-order address bus (configurable number of bits).
•
ALE: Address latch enable.
•
RD: Read strobe.
•
WR: Write strobe.
The control bits for the External Memory Interface are located in two registers, the External Memory Control Register A – XMCRA, and the External Memory Control Register B
– XMCRB.
When the XMEM interface is enabled, the XMEM interface will override the setting in the
data direction registers that corresponds to the ports dedicated to the XMEM interface.
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2549A–AVR–03/05
For details about the port override, see the alternate functions in section “I/O-Ports” on
page 81. The XMEM interface will auto-detect whether an access is internal or external.
If the access is external, the XMEM interface will output address, data, and the control
signals on the ports according to Figure 16 (this figure shows the wave forms without
wait-states). When ALE goes from high-to-low, there is a valid address on AD7:0. ALE is
low during a data transfer. When the XMEM interface is enabled, also an internal access
will cause activity on address, data and ALE ports, but the RD and WR strobes will not
toggle during internal access. When the External Memory Interface is disabled, the normal pin and data direction settings are used. Note that when the XMEM interface is
disabled, the address space above the internal SRAM boundary is not mapped into the
internal SRAM. Figure 15 illustrates how to connect an external SRAM to the AVR using
an octal latch (typically “74 x 573” or equivalent) which is transparent when G is high.
Address Latch Requirements
Due to the high-speed operation of the XRAM interface, the address latch must be
selected with care for system frequencies above 8 MHz @ 4V and 4 MHz @ 2.7V.
When operating at conditions above these frequencies, the typical old style 74HC series
latch becomes inadequate. The External Memory Interface is designed in compliance to
the 74AHC series latch. However, most latches can be used as long they comply with
the main timing parameters. The main parameters for the address latch are:
•
D to Q propagation delay (tPD).
•
Data setup time before G low (tSU).
•
Data (address) hold time after G low (TH).
The External Memory Interface is designed to guaranty minimum address hold time
after G is asserted low of th = 5 ns. Refer to tLAXX_LD/tLLAXX_ST in “External Data Memory
Timing” Tables 169 through Tables 176 on pages 376 - 378. The D-to-Q propagation
delay (tPD) must be taken into consideration when calculating the access time requirement of the external component. The data setup time before G low (t SU ) must not
exceed address valid to ALE low (tAVLLC) minus PCB wiring delay (dependent on the
capacitive load).
Figure 15. External SRAM Connected to the AVR
D[7:0]
AD7:0
D
ALE
G
AVR
A15:8
RD
WR
Pull-up and Bus-keeper
Q
A[7:0]
SRAM
A[15:8]
RD
WR
The pull-ups on the AD7:0 ports may be activated if the corresponding Port register is
written to one. To reduce power consumption in sleep mode, it is recommended to disable the pull-ups by writing the Port register to zero before entering sleep.
The XMEM interface also provides a bus-keeper on the AD7:0 lines. The bus-keeper
can be disabled and enabled in software as described in “External Memory Control Register B – XMCRB” on page 35. When enabled, the bus-keeper will keep the previous
value on the AD7:0 bus while these lines are tri-stated by the XMEM interface.
30
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Timing
External Memory devices have different timing requirements. To meet these requirements, the XMEM interface provides four different wait-states as shown in Table 5. It is
important to consider the timing specification of the External Memory device before
selecting the wait-state. The most important parameters are the access time for the
external memory compared to the set-up requirement. The access time for the External
Memory is defined to be the time from receiving the chip select/address until the data of
this address actually is driven on the bus. The access time cannot exceed the time from
the ALE pulse must be asserted low until data is stable during a read sequence (See
tLLRL+ tRLRH - tDVRH in Tables 169 through Tables 176 on pages 376 - 378). The different
wait-states are set up in software. As an additional feature, it is possible to divide the
external memory space in two sectors with individual wait-state settings. This makes it
possible to connect two different memory devices with different timing requirements to
the same XMEM interface. For XMEM interface timing details, please refer to Table 169
to Table 176 and Figure 161 to Figure 164 in the “External Data Memory Timing” on
page 376.
Note that the XMEM interface is asynchronous and that the waveforms in the following
figures are related to the internal system clock. The skew between the internal and
external clock (XTAL1) is not guarantied (varies between devices temperature, and supply voltage). Consequently, the XMEM interface is not suited for synchronous operation.
Figure 16. External Data Memory Cycles without Wait-state (SRWn1=0 and SRWn0=0)
T1
T2
T3
T4
System Clock (CLKCPU )
ALE
A15:8
Prev. addr.
DA7:0
Prev. data
Address
DA7:0 (XMBK = 0)
Prev. data
Address
DA7:0 (XMBK = 1)
Prev. data
Address
XX
Write
Address
Data
WR
XXXXX
Data
XXXXXXXX
Read
Data
RD
Note:
1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper
sector) or SRW00 (lower sector). The ALE pulse in period T4 is only present if the
next instruction accesses the RAM (internal or external).
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2549A–AVR–03/05
Figure 17. External Data Memory Cycles with SRWn1 = 0 and SRWn0 = 1(1)
T1
T2
T3
T4
T5
System Clock (CLKCPU )
ALE
A15:8
Prev. addr.
DA7:0
Prev. data
Address
DA7:0 (XMBK = 0)
Prev. data
Address
DA7:0 (XMBK = 1)
Prev. data
XX
Write
Address
Data
WR
Address
Read
Data
Data
RD
Note:
1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper
sector) or SRW00 (lower sector).
The ALE pulse in period T5 is only present if the next instruction accesses the RAM
(internal or external).
Figure 18. External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 0(1)
T1
T2
T3
T4
T5
T6
System Clock (CLKCPU )
ALE
A15:8
Prev. addr.
DA7:0
Prev. data
Address
DA7:0 (XMBK = 0)
Prev. data
Address
DA7:0 (XMBK = 1)
Prev. data
XX
Data
Write
Address
WR
Data
Read
Address
Data
RD
Note:
32
1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper
sector) or SRW00 (lower sector).
The ALE pulse in period T6 is only present if the next instruction accesses the RAM
(internal or external).
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 19. External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 1(1)
T1
T2
T3
T4
T5
T6
T7
System Clock (CLKCPU )
ALE
A15:8
Prev. addr.
DA7:0
Prev. data
Address
DA7:0 (XMBK = 0)
Prev. data
Address
DA7:0 (XMBK = 1)
Prev. data
XX
Write
Address
Data
WR
Address
Read
Data
Data
RD
Note:
External Memory Control
Register A – XMCRA
1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper
sector) or SRW00 (lower sector).
The ALE pulse in period T7 is only present if the next instruction accesses the RAM
(internal or external).
Bit
7
6
5
4
3
2
1
0
SRE
SRL2
SRL1
SRL0
SRW11
SRW10
SRW01
SRW00
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
XMCRA
• Bit 7 – SRE: External SRAM/XMEM Enable
Writing SRE to one enables the External Memory Interface.The pin functions AD7:0,
A15:8, ALE, WR, and RD are activated as the alternate pin functions. The SRE bit overrides any pin direction settings in the respective data direction registers. Writing SRE to
zero, disables the External Memory Interface and the normal pin and data direction settings are used.
• Bit 6..4 – SRL2:0: Wait-state Sector Limit
It is possible to configure different wait-states for different External Memory addresses.
The external memory address space can be divided in two sectors that have separate
wait-state bits. The SRL2, SRL1, and SRL0 bits select the split of the sectors, see Table
4 and Figure 14. By default, the SRL2, SRL1, and SRL0 bits are set to zero and the
entire external memory address space is treated as one sector. When the entire SRAM
address space is configured as one sector, the wait-states are configured by the
SRW11 and SRW10 bits.
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2549A–AVR–03/05
Table 4. Sector limits with different settings of SRL2..0
SRL2
SRL1
SRL0
Sector Limits
0
0
x
Lower sector = N/A
Upper sector = 0x2200 - 0xFFFF
0
1
0
Lower sector = 0x2200 - 0x3FFF
Upper sector = 0x4000 - 0xFFFF
0
1
1
Lower sector = 0x2200 - 0x5FFF
Upper sector = 0x6000 - 0xFFFF
1
0
0
Lower sector = 0x2200 - 0x7FFF
Upper sector = 0x8000 - 0xFFFF
1
0
1
Lower sector = 0x2200 - 0x9FFF
Upper sector = 0xA000 - 0xFFFF
1
1
0
Lower sector = 0x2200 - 0xBFFF
Upper sector = 0xC000 - 0xFFFF
1
1
1
Lower sector = 0x2200 - 0xDFFF
Upper sector = 0xE000 - 0xFFFF
• Bit 3..2 – SRW11, SRW10: Wait-state Select Bits for Upper Sector
The SRW11 and SRW10 bits control the number of wait-states for the upper sector of
the external memory address space, see Table 5.
• Bit 1..0 – SRW01, SRW00: Wait-state Select Bits for Lower Sector
The SRW01 and SRW00 bits control the number of wait-states for the lower sector of
the external memory address space, see Table 5.
Table 5. Wait States(1)
SRWn1
SRWn0
0
0
No wait-states
0
1
Wait one cycle during read/write strobe
1
0
Wait two cycles during read/write strobe
1
1
Note:
34
Wait States
Wait two cycles during read/write and wait one cycle before driving out
new address
1. n = 0 or 1 (lower/upper sector).
For further details of the timing and wait-states of the External Memory Interface, see
Figures 16 through Figures 19 for how the setting of the SRW bits affects the timing.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
External Memory Control
Register B – XMCRB
Bit
7
6
5
4
3
2
1
0
XMBK
–
–
–
–
XMM2
XMM1
XMM0
Read/Write
R/W
R
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
XMCRB
• Bit 7– XMBK: External Memory Bus-keeper Enable
Writing XMBK to one enables the bus keeper on the AD7:0 lines. When the bus keeper
is enabled, AD7:0 will keep the last driven value on the lines even if the XMEM interface
has tri-stated the lines. Writing XMBK to zero disables the bus keeper. XMBK is not
qualified with SRE, so even if the XMEM interface is disabled, the bus keepers are still
activated as long as XMBK is one.
• Bit 6..3 – Res: Reserved Bits
These bits are reserved and will always read as zero. When writing to this address location, write these bits to zero for compatibility with future devices.
• Bit 2..0 – XMM2, XMM1, XMM0: External Memory High Mask
When the External Memory is enabled, all Port C pins are default used for the high
address byte. If the full 60KB address space is not required to access the External Memory, some, or all, Port C pins can be released for normal Port Pin function as described
in Table 6. As described in “Using all 64KB Locations of External Memory” on page 36,
it is possible to use the XMMn bits to access all 64KB locations of the External Memory.
Table 6. Port C Pins Released as Normal Port Pins when the External Memory is
Enabled
Using all Locations of
External Memory Smaller than
64 KB
XMM2
XMM1
XMM0
# Bits for External Memory Address
Released Port Pins
0
0
0
8 (Full 56KB space)
None
0
0
1
7
PC7
0
1
0
6
PC7 - PC6
0
1
1
5
PC7 - PC5
1
0
0
4
PC7 - PC4
1
0
1
3
PC7 - PC3
1
1
0
2
PC7 - PC2
1
1
1
No Address high bits
Full Port C
Since the external memory is mapped after the internal memory as shown in Figure 14,
the external memory is not addressed when addressing the first 8,704 bytes of data
space. It may appear that the first 8,704 bytes of the external memory are inaccessible
(external memory addresses 0x0000 to 0x21FF). However, when connecting an external memory smaller than 64 KB, for example 32 KB, these locations are easily accessed
simply by addressing from address 0x8000 to 0xA1FF. Since the External Memory
Address bit A15 is not connected to the external memory, addresses 0x8000 to 0xA1FF
will appear as addresses 0x0000 to 0x21FF for the external memory. Addressing above
address 0xA1FF is not recommended, since this will address an external memory location that is already accessed by another (lower) address. To the Application software,
the external 32 KB memory will appear as one linear 32 KB address space from 0x2200
to 0xA1FF. This is illustrated in Figure 20.
35
2549A–AVR–03/05
Figure 20. Address Map with 32 KB External Memory
Memory Configuration A
AVR Memory Map
0x0000
External 32K SRAM
0x0000
Internal Memory
0x21FF
0x2200
0x7FFF
0x8000
External
0x7FFF
Memory
0x90FF
0x9100
(Unused)
0xFFFF
Using all 64KB Locations of
External Memory
Since the External Memory is mapped after the Internal Memory as shown in Figure 14,
only 56KB of External Memory is available by default (address space 0x0000 to 0x21FF
is reserved for internal memory). However, it is possible to take advantage of the entire
External Memory by masking the higher address bits to zero. This can be done by using
the XMMn bits and control by software the most significant bits of the address. By setting Port C to output 0x00, and releasing the most significant bits for normal Port Pin
operation, the Memory Interface will address 0x0000 - 0x2FFF. See the following code
examples.
Care must be exercised using this option as most of the memory is masked away.
36
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Assembly Code Example(1)
;
;
;
;
;
OFFSET is defined to 0x4000 to ensure
external memory access
Configure Port C (address high byte) to
output 0x00 when the pins are released
for normal Port Pin operation
ldi
r16, 0xFF
out
DDRC, r16
ldi
r16, 0x00
out
PORTC, r16
; release PC7:6
ldi
r16, (1<<XMM1)
sts
XMCRB, r16
; write 0xAA to address 0x0001 of external
; memory
ldi
r16, 0xaa
sts
0x0001+OFFSET, r16
; re-enable PC7:6 for external memory
ldi
r16, (0<<XMM1)
sts
XMCRB, r16
; store 0x55 to address (OFFSET + 1) of
; external memory
ldi
r16, 0x55
sts
0x0001+OFFSET, r16
C Code Example(1)
#define OFFSET 0x4000
void XRAM_example(void)
{
unsigned char *p = (unsigned char *) (OFFSET + 1);
DDRC = 0xFF;
PORTC = 0x00;
XMCRB = (1<<XMM1);
*p = 0xaa;
XMCRB = 0x00;
*p = 0x55;
}
Note:
1. See “About Code Examples” on page 8.
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2549A–AVR–03/05
38
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
System Clock and
Clock Options
Clock Systems and their
Distribution
Figure 21 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 Management and Sleep Modes” on page 51. The clock systems
are detailed below.
Figure 21. Clock Distribution
Asynchronous
Timer/Counter
General I/O
Modules
ADC
CPU Core
RAM
Flash and
EEPROM
clkADC
clkI/O
AVR Clock
Control Unit
clkASY
clkCPU
clkFLASH
Reset Logic
Source clock
System Clock
Prescaler
Watchdog Timer
Watchdog clock
Watchdog
Oscillator
Clock
Multiplexer
Timer/Counter
Oscillator
External Clock
Crystal
Oscillator
Low-frequency
Crystal Oscillator
Calibrated RC
Oscillator
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 interrupts 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 asynchronously when clkI/O is halted, TWI address recognition in
all sleep modes.
Flash Clock – clkFLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually
active simultaneously with the CPU clock.
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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 7. Device Clocking Options Select(1)
Device Clocking Option
CKSEL3..0
Low Power Crystal Oscillator
1111 - 1000
Full Swing Crystal Oscillator
0111 - 0110
Low Frequency Crystal Oscillator
0101 - 0100
Internal 128 kHz RC Oscillator
0011
Calibrated Internal RC Oscillator
0010
External Clock
0000
Reserved
0001
Note:
1. For all fuses “1” means unprogrammed while “0” means programmed.
Default Clock Source
The device is shipped with internal RC oscillator at 8.0MHz and with the fuse CKDIV8
programmed, resulting in 1.0MHz system clock. The startup time is set to maximum and
time-out period enabled. (CKSEL = "0010", SUT = "10", CKDIV8 = "0"). The default setting ensures that all users can make their desired clock source setting using any
available programming interface.
Clock Startup Sequence
Any clock source needs a sufficient VCC to start oscillating and a minimum number of
oscillating cycles before it can be considered stable.
To ensure sufficient VCC, the device issues an internal reset with a time-out delay (tTOUT)
after the device reset is released by all other reset sources. “On-chip Debug System” on
page 56 describes the start conditions for the internal reset. The delay (tTOUT) is timed
from the Watchdog Oscillator and the number of cycles in the delay is set by the SUTx
and CKSELx fuse bits. The selectable delays are shown in Table 8. The frequency of
the Watchdog Oscillator is voltage dependent as shown in
“ATmega640/1280/1281/2560/2561 Typical Characteristics – Preliminary Data” on page
381.
Table 8. Number of Watchdog Oscillator Cycles
40
Typ Time-out (VCC = 5.0V)
Typ Time-out (VCC = 3.0V)
Number of Cycles
0 ms
0 ms
0
4.1 ms
4.3 ms
512
65 ms
69 ms
8K (8,192)
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ATmega640/1280/1281/2560/2561
Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum
Vcc. The delay will not monitor the actual voltage and it will be required to select a delay
longer than the Vcc rise time. If this is not possible, an internal or external Brown-Out
Detection circuit should be used. A BOD circuit will ensure sufficient Vcc before it
releases the reset, and the time-out delay can be disabled. Disabling the time-out delay
without utilizing a Brown-Out Detection circuit is not recommended.
The oscillator is required to oscillate for a minimum number of cycles before the clock is
considered stable. An internal ripple counter monitors the oscillator output clock, and
keeps the internal reset active for a given number of clock cycles. The reset is then
released and the device will start to execute. The recommended oscillator start-up time
is dependent on the clock type, and varies from 6 cycles for an externally applied clock
to 32K cycles for a low frequency crystal.
The start-up sequence for the clock includes both the time-out delay and the start-up
time when the device starts up from reset. When starting up from Power-save or Powerdown mode, Vcc is assumed to be at a sufficient level and only the start-up time is
included.
Low Power Crystal
Oscillator
Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier
which can be configured for use as an On-chip Oscillator, as shown in Figure 22. Either
a quartz crystal or a ceramic resonator may be used.
This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the
XTAL2 output. It gives the lowest power consumption, but is not capable of driving other
clock inputs, and may be more susceptible to noise in noisy environments. In these
cases, refer to the “Full Swing Crystal Oscillator” on page 43.
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 9. For ceramic resonators,
the capacitor values given by the manufacturer should be used.
Figure 22. Crystal Oscillator Connections
C2
C1
XTAL2
XTAL1
GND
The Low Power 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 9.
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2549A–AVR–03/05
Table 9. Low Power Crystal Oscillator Operating Modes(3)
Frequency Range(1) (MHz)
CKSEL3..1
Recommended Range for Capacitors
C1 and C2 (pF)
0.4 - 0.9
100(2)
–
0.9 - 3.0
101
12 - 22
3.0 - 8.0
110
12 - 22
8.0 - 16.0
111
12 - 22
Notes:
1. The frequency ranges are preliminary values. Actual values are TBD.
2. This option should not be used with crystals, only with ceramic resonators.
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. It
must be ensured that the resulting divided clock meets the frequency specification of
the device.
The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown
in Table 10.
Table 10. Start-up Times for the Low Power Crystal Oscillator Clock Selection
Oscillator Source /
Power Conditions
Additional Delay
from Reset
(VCC = 5.0V)
CKSEL0
SUT1..0
(1)
0
00
Ceramic resonator, fast
rising power
258 CK
14CK + 4.1 ms
Ceramic resonator,
slowly rising power
258 CK
14CK + 65 ms(1)
0
01
Ceramic resonator,
BOD enabled
1K CK
14CK(2)
0
10
Ceramic resonator, fast
rising power
1K CK
14CK + 4.1 ms(2)
0
11
Ceramic resonator,
slowly rising power
1K CK
14CK + 65 ms(2)
1
00
Crystal Oscillator, BOD
enabled
16K CK
14CK
Crystal Oscillator, fast
rising power
16K CK
14CK + 4.1 ms
Crystal Oscillator,
slowly rising power
16K CK
14CK + 65 ms
Notes:
42
Start-up Time from
Power-down and
Power-save
1
1
1
01
10
11
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 frequency of the device, and if frequency stability at start-up is
not important for the application.
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
Full Swing Crystal
Oscillator
Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier
which can be configured for use as an On-chip Oscillator, as shown in Figure 22. Either
a quartz crystal or a ceramic resonator may be used.
This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is useful for driving other clock inputs and in noisy environments. The current
consumption is higher than the “Low Power Crystal Oscillator” on page 41. Note that the
Full Swing Crystal Oscillator will only operate for Vcc = 2.7 - 5.5 volts.
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 12. For ceramic resonators,
the capacitor values given by the manufacturer should be used.
The operating mode is selected by the fuses CKSEL3..1 as shown in Table 11.
Table 11. Full Swing Crystal Oscillator operating modes(2)
Frequency Range(1) (MHz)
CKSEL3..1
0.4 - 16
Notes:
Recommended Range for Capacitors
C1 and C2 (pF)
011
12 - 22
1. The frequency ranges are preliminary values. Actual values are TBD.
2. 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. It
must be ensured that the resulting divided clock meets the frequency specification of
the device.
Figure 23. Crystal Oscillator Connections
C2
C1
XTAL2
XTAL1
GND
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2549A–AVR–03/05
Table 12. Start-up Times for the Full Swing Crystal Oscillator Clock Selection
Oscillator Source /
Power Conditions
Additional Delay
from Reset
(VCC = 5.0V)
CKSEL0
SUT1..0
(1)
0
00
Ceramic resonator, fast
rising power
258 CK
14CK + 4.1 ms
Ceramic resonator,
slowly rising power
258 CK
14CK + 65 ms(1)
0
01
Ceramic resonator,
BOD enabled
1K CK
14CK(2)
0
10
Ceramic resonator, fast
rising power
1K CK
14CK + 4.1 ms(2)
0
11
Ceramic resonator,
slowly rising power
1K CK
14CK + 65 ms(2)
1
00
Crystal Oscillator, BOD
enabled
16K CK
14CK
Crystal Oscillator, fast
rising power
16K CK
14CK + 4.1 ms
Crystal Oscillator,
slowly rising power
16K CK
14CK + 65 ms
Notes:
44
Start-up Time from
Power-down and
Power-save
1
1
1
01
10
11
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 frequency of the device, and if frequency stability at start-up is
not important for the application.
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
Low Frequency Crystal
Oscillator
The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low
Frequency Crystal Oscillator. The crystal should be connected as shown in Figure 22.
When this Oscillator is selected, start-up times are determined by the SUT Fuses and
CKSEL0 as shown in Table 13.
Table 13. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Power Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(VCC = 5.0V)
SUT1..0
0
00
1K CK
Fast rising power
1K CK
14CK + 4.1 ms(1)
0
01
1K CK
(1)
0
10
0
11
14CK + 65 ms
Reserved
BOD enabled
32K CK
14CK
1
00
Fast rising power
32K CK
14CK + 4.1 ms
1
01
Slowly rising power
32K CK
14CK + 65 ms
1
10
1
11
Reserved
Note:
Calibrated Internal RC
Oscillator
CKSEL0
BOD enabled
Slowly rising power
14CK
(1)
1. These options should only be used if frequency stability at start-up is not important
for the application.
The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. The frequency
is nominal value at 3V and 25°C. The device is shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 48 for more details. This clock may
be selected as the system clock by programming the CKSEL Fuses as shown in Table
14. If selected, it will operate with no external components. During reset, hardware loads
the calibration byte into the OSCCAL Register and thereby automatically calibrates the
RC Oscillator. At 3V and 25°C, this calibration gives a frequency of 8 MHz ± 1%. The
oscillator can be calibrated to any frequency in the range 7.3 - 8.1 MHz within ±1%
accuracy, by changing the OSCCAL register. When this Oscillator is used as the chip
clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the
Reset Time-out. For more information on the pre-programmed calibration value, see the
section “Calibration Byte” on page 338.
Table 14. Internal Calibrated RC Oscillator Operating Modes(1)(3)
Notes:
Frequency Range(2) (MHz)
CKSEL3..0
7.3 - 8.1
0010
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.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as
shown in Table 15 on page 46.
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2549A–AVR–03/05
Table 15. Start-up times for the internal calibrated RC Oscillator clock selection
Start-up Time from Powerdown and Power-save
Additional Delay from
Reset (VCC = 5.0V)
SUT1..0
BOD enabled
6 CK
14CK
00
Fast rising power
6 CK
14CK + 4.1 ms
01
Slowly rising power
6 CK
14CK + 65 ms(1)
10
Power Conditions
Reserved
Note:
Oscillator Calibration Register
– OSCCAL
11
1. The device is shipped with this option selected.
Bit
Read/Write
Initial Value
7
6
5
4
3
2
1
0
CAL7
CAL6
CAL5
CAL4
CAL3
CAL2
CAL1
CAL0
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
OSCCAL
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. The factory-calibrated value
is automatically written to this register during chip reset, giving an oscillator frequency of
8.0 MHz at 25°C. The application software can write this register to change the oscillator
frequency. The oscillator can be calibrated to any frequency in the range 7.3 - 8.1 MHz
within ±1% accuracy. 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 frequency 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. Incrementing CAL6..0 by 1 will give a frequency increment of
less than 2% in the frequency range 7.3 - 8.1 MHz.
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ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
128 kHz Internal
Oscillator
The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz.
The frequency is nominal at 3V and 25°C. This clock may be select as the system clock
by programming the CKSEL Fuses to “11” as shown in Table 16.
Table 16. 128 kHz Internal Oscillator Operating Modes
Note:
Nominal Frequency
CKSEL3..0
128 kHz
0011
1. The frequency is preliminary value. Actual value is TBD.
When this clock source is selected, start-up times are determined by the SUT Fuses as
shown in Table 17.
Table 17. Start-up Times for the 128 kHz Internal Oscillator
Start-up Time from Powerdown and Power-save
Additional Delay from
Reset
SUT1..0
BOD enabled
6 CK
14CK
00
Fast rising power
6 CK
14CK + 4 ms
01
Slowly rising power
6 CK
14CK + 64 ms
10
Power Conditions
Reserved
External Clock
11
To drive the device from an external clock source, XTAL1 should be driven as shown in
Figure 24. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Figure 24. External Clock Drive Configuration
NC
XTAL2
EXTERNAL
CLOCK
SIGNAL
XTAL1
GND
When this clock source is selected, start-up times are determined by the SUT Fuses as
shown in Table 20..
Table 18. Crystal Oscillator Clock Frequency
Nominal Frequency
CKSEL3..0
0 - 16 MHz
0000
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2549A–AVR–03/05
Table 19. Start-up Times for the External Clock Selection
Start-up Time from Powerdown and Power-save
Additional Delay from
Reset (VCC = 5.0V)
SUT1..0
BOD enabled
6 CK
14CK
00
Fast rising power
6 CK
14CK + 4.1 ms
01
Slowly rising power
6 CK
14CK + 65 ms
10
Power Conditions
Reserved
11
When applying an external clock, it is required to avoid sudden changes in the applied
clock frequency to ensure stable operation of the MCU. A variation in frequency of more
than 2% from one clock cycle to the next can lead to unpredictable behavior. If changes
of more than 2% is required, ensure that the MCU is kept in Reset during the changes.
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 48 for details.
Clock Output Buffer
The device can output the system clock on the CLKO pin. To enable the output, the
CKOUT Fuse has to be programmed. This mode is suitable when the chip clock is used
to drive other circuits on the system. The clock also will be output during reset, and the
normal operation of I/O pin will be overridden when the fuse is programmed. Any clock
source, including the internal RC Oscillator, can be selected when the clock is output on
CLKO. If the System Clock Prescaler is used, it is the divided system clock that is
output.
Timer/Counter Oscillator
The device can operate its Timer/Counter2 from an external 32.768 kHz watch crystal or
a external clock source. See Figure 22 on page 41 for crystal connection.
Applying an external clock source to TOSC1 requires EXCLK in the ASSR Register written to logic one. See “Asynchronous operation of the Timer/Counter” on page 189 for
further description on selecting external clock as input instead of a 32 kHz crystal.
System Clock Prescaler
The ATmega640/1280/1281/2560/2561 has a system clock prescaler, and the system
clock can be divided by setting the “Clock Prescale Register – CLKPR” on page 49. This
feature can be used to decrease the system clock frequency and the 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
20.
When switching between prescaler settings, the System Clock Prescaler ensures that
no glitches occurs in the clock system. It also ensures 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 the other 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
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ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
the previous clock period, and T2 is the period corresponding to the new prescaler
setting.
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.
Clock Prescale Register –
CLKPR
Bit
7
6
5
4
3
2
1
0
CLKPCE
–
–
–
CLKPS3
CLKPS2
CLKPS1
CLKPS0
Read/Write
R/W
R
R
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
CLKPR
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 synchronous peripherals is reduced when a division factor is used. The division factors are given in Table 20.
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 operating 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.
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Table 20. Clock Prescaler Select
50
CLKPS3
CLKPS2
CLKPS1
CLKPS0
Clock Division Factor
0
0
0
0
1
0
0
0
1
2
0
0
1
0
4
0
0
1
1
8
0
1
0
0
16
0
1
0
1
32
0
1
1
0
64
0
1
1
1
128
1
0
0
0
256
1
0
0
1
Reserved
1
0
1
0
Reserved
1
0
1
1
Reserved
1
1
0
0
Reserved
1
1
0
1
Reserved
1
1
1
0
Reserved
1
1
1
1
Reserved
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Power Management
and Sleep Modes
Sleep modes enable the application to shut down unused modules in the MCU, thereby
saving power. The AVR provides various sleep modes allowing the user to tailor the
power consumption to the application’s requirements.
To enter any of the five sleep modes, the SE bit in 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 (Idle, ADC Noise Reduction, Power-down, Powersave, or Standby) will be activated by the SLEEP instruction. See Table 21 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.
Figure 21 on page 39 presents the different clock systems in the
ATmega640/1280/1281/2560/2561, and their distribution. The figure is helpful in selecting an appropriate sleep mode.
Sleep Mode Control Register –
SMCR
The Sleep Mode Control Register contains control bits for power management.
Bit
7
6
5
4
3
2
1
0
–
–
–
–
SM2
SM1
SM0
SE
Read/Write
R
R
R
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
SMCR
• 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 21.
Table 21. Sleep Mode Select
Note:
SM2
SM1
SM0
Sleep Mode
0
0
0
Idle
0
0
1
ADC Noise Reduction
0
1
0
Power-down
0
1
1
Power-save
1
0
0
Reserved
1
0
1
Reserved
1
1
0
Standby(1)
1
1
1
Extended Standby(1)
1. Standby modes are 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.
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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,
2-wire Serial Interface, Timer/Counters, 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 automatically when this mode is entered.
ADC Noise Reduction
Mode
When the SM2..0 bits are written to 001, the SLEEP instruction makes the MCU enter
ADC Noise Reduction mode, stopping the CPU but allowing the ADC, the external interrupts, 2-wire Serial Interface address match, 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 System Reset, a Watchdog interrupt, a Brown-out Reset, a 2-wire serial interface interrupt, a Timer/Counter2 interrupt, an SPM/EEPROM ready interrupt, an
external level interrupt on INT7:4 or a pin change interrupt can wakeup the MCU from
ADC Noise Reduction mode.
Power-down Mode
When the SM2..0 bits are written to 010, the SLEEP instruction makes the MCU enter
Power-down mode. In this mode, the external Oscillator is stopped, while the external
interrupts, the 2-wire Serial Interface, and the Watchdog continue operating (if enabled).
Only an External Reset, a Watchdog Reset, a Brown-out Reset, 2-wire Serial Interface
address match, an external level interrupt on INT7:4, an external interrupt on INT3:0, 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 75 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
40.
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 not running, Power-down mode is recommended instead of Powersave mode.
52
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ATmega640/1280/1281/2560/2561
The Timer/Counter2 can be clocked both synchronously and asynchronously in Powersave mode. If the Timer/Counter2 is not using the asynchronous clock, the
Timer/Counter Oscillator is stopped during sleep. If the Timer/Counter2 is not 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 available for the
Timer/Counter2.
Standby Mode
When the SM2..0 bits are 110 and an external crystal/resonator clock option is selected,
the SLEEP instruction makes the MCU enter Standby mode. This mode is identical to
Power-down with the exception that the Oscillator is kept running. From Standby mode,
the device wakes up in six clock cycles.
Extended Standby Mode
When the SM2..0 bits are 111 and an external crystal/resonator clock option is selected,
the SLEEP instruction makes the MCU enter Extended Standby mode. This mode is
identical to Power-save mode with the exception that the Oscillator is kept running.
From Extended Standby mode, the device wakes up in six clock cycles.
Table 22. Active Clock Domains and Wake-up Sources in the Different Sleep Modes.
Standby
(1)
X
X
X
X
X
X
X(2)
X(3)
X
X(2)
X
X
X
X(3)
X
(3)
X
(3)
X
(2)
clkASY
X
X
X
X
X
Extended
X
X(2)
X(3)
X
X(2)
Standby
Notes: 1. Only recommended with external crystal or resonator selected as clock source.
2. If Timer/Counter2 is running in asynchronous mode.
3. For INT7:4, only level interrupt.
Other I/O
WDT Interrupt
X
ADC
X
Power-down
Power-save
SPM/
EEPROM Ready
X
Timer2
X(2)
Timer Osc
Enabled
X
Main Clock
Source
Enabled
X
clkADC
TWI Address
Match
ADCNRM
Wake-up Sources
X
clkIO
X
Oscillators
INT7:0 and
Pin Change
Idle
clkFLASH
Sleep Mode
clkCPU
Active Clock Domains
X
X
X
X
X
X
X
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2549A–AVR–03/05
Power Reduction
Register
The Power Reduction Register, PRR, provides 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 can not be read or written. Resources used by the peripheral when
stopping the clock will remain occupied, hence the peripheral should in most cases 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 significantly reduce the
overall power consumption. See “Supply Current of IO modules” on page 381 for examples. In all other sleep modes, the clock is already stopped.
Power Reduction Register 0 PRR0
Bit
7
6
5
4
3
2
1
0
PRTWI
PRTIM2
PRTIM0
–
PRTIM1
PRSPI
PRUSART0
PRADC
Read/Write
R/W
R/W
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
PRR0
• Bit 7 - PRTWI: Power Reduction TWI
Writing a logic one to this bit shuts down the TWI by stopping the clock to the module.
When waking up the TWI again, the TWI should be re initialized to ensure proper
operation.
• Bit 6 - PRTIM2: Power Reduction Timer/Counter2
Writing a logic one to this bit shuts down the Timer/Counter2 module in synchronous
mode (AS2 is 0). When the Timer/Counter2 is enabled, operation will continue like
before the shutdown.
• Bit 5 - PRTIM0: Power Reduction Timer/Counter0
Writing a logic one to this bit shuts down the Timer/Counter0 module. When the
Timer/Counter0 is enabled, operation will continue like before the shutdown.
• Bit 4 - Res: Reserved bit
This bit is reserved bit and will always read as zero.
• Bit 3 - PRTIM1: Power Reduction Timer/Counter1
Writing a logic one to this bit shuts down the Timer/Counter1 module. When the
Timer/Counter1 is enabled, operation will continue like before the shutdown.
• Bit 2 - PRSPI: Power Reduction Serial Peripheral Interface
Writing a 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 - PRUSART0: Power Reduction USART0
Writing a logic one to this bit shuts down the USART0 by stopping the clock to the module. When waking up the USART0 again, the USART0 should be re initialized to ensure
proper operation.
• Bit 0 - PRADC: Power Reduction ADC
Writing a 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.
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ATmega640/1280/1281/2560/2561
Power Reduction Register 1 PRR1
Bit
7
6
5
4
3
2
1
0
–
–
PRTIM5
PRTIM4
PRTIM3
PRUSART3
PRUSART2
PRUSART1
Read/Write
R
R
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
PRR1
• Bit 7..6 - Res: Reserved bits
These bits are reserved and will always read as zero.
• Bit 5 - PRTIM5: Power Reduction Timer/Counter5
Writing a logic one to this bit shuts down the Timer/Counter5 module. When the
Timer/Counter5 is enabled, operation will continue like before the shutdown.
• Bit 4 - PRTIM4: Power Reduction Timer/Counter4
Writing a logic one to this bit shuts down the Timer/Counter4 module. When the
Timer/Counter4 is enabled, operation will continue like before the shutdown.
• Bit 3 - PRTIM3: Power Reduction Timer/Counter3
Writing a logic one to this bit shuts down the Timer/Counter3 module. When the
Timer/Counter3 is enabled, operation will continue like before the shutdown.
• Bit 2 - PRUSART3: Power Reduction USART3
Writing a logic one to this bit shuts down the USART3 by stopping the clock to the module. When waking up the USART3 again, the USART3 should be re initialized to ensure
proper operation.
• Bit 1 - PRUSART2: Power Reduction USART2
Writing a logic one to this bit shuts down the USART2 by stopping the clock to the module. When waking up the USART2 again, the USART2 should be re initialized to ensure
proper operation.
• Bit 0 - PRUSART1: Power Reduction USART1
Writing a logic one to this bit shuts down the USART1 by stopping the clock to the module. When waking up the USART1 again, the USART1 should be re initialized to ensure
proper operation.
Minimizing Power
Consumption
There are several issues to consider when trying to minimize the power consumption in
an AVR controlled system. In general, sleep modes should be used as much as possible, and the sleep mode should be selected so that as few as possible of the device’s
functions are operating. All functions not needed should be disabled. In particular, the
following modules may need special consideration when trying to achieve the lowest
possible power consumption.
Analog to Digital Converter
If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should
be disabled before entering any sleep mode. When the ADC is turned off and on again,
the next conversion will be an extended conversion. Refer to “Analog to Digital Converter” on page 274 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 271 for details on how to configure the Analog Comparator.
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2549A–AVR–03/05
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 significantly to the total current consumption. Refer to “Brown-out Detection” on page 59 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 Voltage Reference” on page 62 for details on the
start-up time.
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 consumption. Refer to “Interrupts” on page 69 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 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 85 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 (DIDR2, DIDR1 and DIDR0). Refer to “Digital Input Disable Register 2 – DIDR2” on
page 293, “Digital Input Disable Register 1 – DIDR1” on page 273 and “Digital Input Disable Register 0 – DIDR0” on page 293 for details.
On-chip Debug System
If the On-chip debug system is enabled by the OCDEN Fuse and the chip enters sleep
mode, the main clock source is enabled, and hence, always consumes power. In the
deeper sleep modes, this will contribute significantly to the total current consumption.
There are three alternative ways to disable the OCD system:
56
•
Disable the OCDEN Fuse.
•
Disable the JTAGEN Fuse.
•
Write one to the JTD bit in MCUCR.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
System Control
and Reset
Resetting the AVR
During reset, all I/O Registers are set to their initial values, and the program starts execution from the Reset Vector. The instruction placed at the Reset Vector must be a JMP
– Absolute Jump – instruction to the reset handling routine. If the program never
enables an interrupt source, the Interrupt Vectors are not used, and regular program
code can be placed at these locations. This is also the case if the Reset Vector is in the
Application section while the Interrupt Vectors are in the Boot section or vice versa. The
circuit diagram in Figure 25 shows the reset logic. Table 23 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 different selections for the delay period are presented in “Clock
Sources” on page 40.
Reset Sources
The ATmega640/1280/1281/2560/2561 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 301 for details.
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2549A–AVR–03/05
Figure 25. Reset Logic
DATA BUS
PORF
BORF
EXTRF
WDRF
JTRF
MCU Status
Register (MCUSR)
Power-on Reset
Circuit
Brown-out
Reset Circuit
BODLEVEL [2..0]
Pull-up Resistor
SPIKE
FILTER
JTAG Reset
Register
Watchdog
Oscillator
Clock
Generator
CK
Delay Counters
TIMEOUT
CKSEL[3:0]
SUT[1:0]
Table 23. Reset Characteristics(1)
Symbol
Min
Typ
Max
Units
Power-on Reset Threshold
Voltage (rising)
TBD
TBD
TBD
V
Power-on Reset Threshold
Voltage (falling)(2)
TBD
TBD
TBD
V
VRST
RESET Pin Threshold Voltage
TBD
TBD
TBD
V
tRST
Minimum pulse width on RESET
Pin
TBD
TBD
TBD
ns
VPOT
Notes:
Power-on Reset
Parameter
Condition
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)
A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level is defined in Table 23. The POR is activated whenever 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.
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ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 26. MCU Start-up, RESET Tied to VCC
VCC
RESET
VPOT
VRST
tTOUT
TIME-OUT
INTERNAL
RESET
Figure 27. MCU Start-up, RESET Extended Externally
VCC
RESET
VPOT
VRST
TIME-OUT
tTOUT
INTERNAL
RESET
External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer
than the minimum pulse width (see Table 23) 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 28. External Reset During Operation
CC
Brown-out Detection
ATmega640/1280/1281/2560/2561 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
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2549A–AVR–03/05
has a hysteresis to ensure spike free Brown-out Detection. The hysteresis on the detection level should be interpreted as VBOT+ = VBOT + VHYST/2 and VBOT- = VBOT - VHYST/2.
Table 24. BODLEVEL Fuse Coding(1)
BODLEVEL 2..0 Fuses
Min VBOT
Typ VBOT
Max VBOT
111
BOD Disabled
110
1.8
101
2.7
100
4.3
Units
V
011
010
Reserved
001
000
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 guarantees 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 = 110
for
ATmega640/1280/1281/2560/2561
and
BODLEVEL = 101
for
ATmega640/1280/2560/1L.
Table 25. Brown-out Characteristics
Symbol
Parameter
Min
VHYST
Brown-out Detector Hysteresis
tBOD
Min Pulse Width on Brown-out Reset
Typ
50
Max
Units
mV
ns
When the BOD is enabled, and VCC decreases to a value below the trigger level (VBOTin Figure 29), the Brown-out Reset is immediately activated. When VCC increases above
the trigger level (VBOT+ in Figure 29), the delay counter starts the MCU after the Timeout period tTOUT has expired.
The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level
for longer than tBOD given in Table 23.
Figure 29. Brown-out Reset During Operation
VCC
VBOT-
VBOT+
RESET
TIME-OUT
tTOUT
INTERNAL
RESET
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ATmega640/1280/1281/2560/2561
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. See “Watchdog Timer” on page 56. for details on operation of the Watchdog
Timer.
Figure 30. Watchdog Reset During Operation
CC
CK
MCU Status Register –
MCUSR
The MCU Status Register provides information on which reset source caused an MCU
reset.
Bit
7
6
5
4
3
2
1
0
–
–
–
JTRF
WDRF
BORF
EXTRF
PORF
Read/Write
R
R
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
MCUSR
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
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2549A–AVR–03/05
before another reset occurs, the source of the reset can be found by examining the
Reset Flags.
Internal Voltage
Reference
ATmega640/1280/1281/2560/2561 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.
Voltage Reference Enable
Signals and Start-up Time
The voltage reference has a start-up time that may influence the way it should be used.
The start-up time is given in Table 26. 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 [2..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 ADC is used. To reduce power consumption in Power-down mode, the user
can avoid the three conditions above to ensure that the reference is turned off before
entering Power-down mode.
Table 26. Internal Voltage Reference Characteristics(1)
Symbol
Condition
Min
Typ
Max
Units
TBD
1.1
TBD
V
VBG
Bandgap reference voltage
TBD
tBG
Bandgap reference start-up time
TBD
40
70
µs
IBG
Bandgap reference current
consumption
TBD
10
TBD
µA
Note:
62
Parameter
1. Values are guidelines only. Actual values are TBD.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Watchdog Timer
ATmega640/1280/1281/2560/2561 has an Enhanced Watchdog Timer (WDT). The
main features are:
• Clocked from separate On-chip Oscillator
• 3 Operating modes
– Interrupt
– System Reset
– Interrupt and System Reset
• Selectable Time-out period from 16ms to 8s
• Possible Hardware fuse Watchdog always on (WDTON) for fail-safe mode
128kHz
OSCILLATOR
WATCHDOG
RESET
WDE
OSC/2K
OSC/4K
OSC/8K
OSC/16K
OSC/32K
OSC/64K
OSC/128K
OSC/256K
OSC/512K
OSC/1024K
Figure 31. Watchdog Timer
WDP0
WDP1
WDP2
WDP3
MCU RESET
WDIF
WDIE
INTERRUPT
The Watchdog Timer (WDT) is a timer counting cycles of a separate on-chip 128 kHz
oscillator. The WDT gives an interrupt or a system reset when the counter reaches a
given time-out value. In normal operation mode, it is required that the system uses the
WDR - Watchdog Timer Reset - instruction to restart the counter before the time-out
value is reached. If the system doesn't restart the counter, an interrupt or system reset
will be issued.
In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can
be used to wake the device from sleep-modes, and also as a general system timer. One
example is to limit the maximum time allowed for certain operations, giving an interrupt
when the operation has run longer than expected. In System Reset mode, the WDT
gives a reset when the timer expires. This is typically used to prevent system hang-up in
case of runaway code. The third mode, Interrupt and System Reset mode, combines the
other two modes by first giving an interrupt and then switch to System Reset mode. This
mode will for instance allow a safe shutdown by saving critical parameters before a system reset.
The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer
to System Reset mode. With the fuse programmed the System Reset mode bit (WDE)
and Interrupt mode bit (WDIE) are locked to 1 and 0 respectively. To further ensure program security, alterations to the Watchdog set-up must follow timed sequences. The
sequence for clearing WDE and changing time-out configuration is as follows:
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2549A–AVR–03/05
1. In the same operation, write a logic one to the Watchdog change enable bit
(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, write the WDE and Watchdog prescaler bits
(WDP) as desired, but with the WDCE bit cleared. This must be done in one
operation.
The following code example shows one assembly and one C function for turning off the
Watchdog Timer. The example assumes that interrupts are controlled (e.g. by disabling
interrupts globally) so that no interrupts will occur during the execution of these
functions.
64
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Assembly Code Example(1)
WDT_off:
; Turn off global interrupt
cli
; Reset Watchdog Timer
wdr
; Clear WDRF in MCUSR
in
r16, MCUSR
andi
r16, (0xff & (0<<WDRF))
out
MCUSR, r16
; Write logical one to WDCE and WDE
; Keep old prescaler setting to prevent unintentional time-out
in
r16, WDTCSR
ori
r16, (1<<WDCE) | (1<<WDE)
out
WDTCSR, r16
; Turn off WDT
ldi
r16, (0<<WDE)
out
WDTCSR, r16
; Turn on global interrupt
sei
ret
C Code Example(1)
void WDT_off(void)
{
__disable_interrupt();
__watchdog_reset();
/* Clear WDRF in MCUSR */
MCUSR &= ~(1<<WDRF);
/* Write logical one to WDCE and WDE */
/* Keep old prescaler setting to prevent unintentional time-out
*/
WDTCSR |= (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCSR = 0x00;
__enable_interrupt();
}
Note:
1. The example code assumes that the part specific header file is included.
Note: If the Watchdog is accidentally enabled, for example by a runaway pointer or
brown-out condition, the device will be reset and the Watchdog Timer will stay enabled.
If the code is not set up to handle the Watchdog, this might lead to an eternal loop of
time-out resets. To avoid this situation, the application software should always clear the
Watchdog System Reset Flag (WDRF) and the WDE control bit in the initialisation routine, even if the Watchdog is not in use.
65
2549A–AVR–03/05
The following code example shows one assembly and one C function for changing the
time-out value of the Watchdog Timer.
Assembly Code Example(1)
WDT_Prescaler_Change:
; Turn off global interrupt
cli
; Reset Watchdog Timer
wdr
; Start timed sequence
in
r16, WDTCSR
ori
r16, (1<<WDCE) | (1<<WDE)
out
WDTCSR, r16
; --
Got four cycles to set the new values from here -
; Set new prescaler(time-out) value = 64K cycles (~0.5 s)
ldi
r16, (1<<WDE) | (1<<WDP2) | (1<<WDP0)
out
WDTCSR, r16
; --
Finished setting new values, used 2 cycles -
; Turn on global interrupt
sei
ret
C Code Example(1)
void WDT_Prescaler_Change(void)
{
__disable_interrupt();
__watchdog_reset();
/* Start timed
equence */
WDTCSR |= (1<<WDCE) | (1<<WDE);
/* Set new prescaler(time-out) value = 64K cycles (~0.5 s) */
WDTCSR
= (1<<WDE) | (1<<WDP2) | (1<<WDP0);
__enable_interrupt();
}
Note:
1. The example code assumes that the part specific header file is included.
Note: The Watchdog Timer should be reset before any change of the WDP bits, since a
change in the WDP bits can result in a time-out when switching to a shorter time-out
period.
66
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Watchdog Timer Control
Register - WDTCSR
Bit
7
6
5
4
3
2
1
0
WDIF
WDIE
WDP3
WDCE
WDE
WDP2
WDP1
WDP0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
X
0
0
0
WDTCSR
• Bit 7 - WDIF: Watchdog Interrupt Flag
This bit is set when a time-out occurs in the Watchdog Timer and the Watchdog Timer is
configured for interrupt. WDIF is cleared by hardware when executing the corresponding
interrupt handling vector. Alternatively, WDIF is cleared by writing a logic one to the flag.
When the I-bit in SREG and WDIE are set, the Watchdog Time-out Interrupt is
executed.
• Bit 6 - WDIE: Watchdog Interrupt Enable
When this bit is written to one and the I-bit in the Status Register is set, the Watchdog
Interrupt is enabled. If WDE is cleared in combination with this setting, the Watchdog
Timer is in Interrupt Mode, and the corresponding interrupt is executed if time-out in the
Watchdog Timer occurs.
If WDE is set, the Watchdog Timer is in Interrupt and System Reset Mode. The first
time-out in the Watchdog Timer will set WDIF. Executing the corresponding interrupt
vector will clear WDIE and WDIF automatically by hardware (the Watchdog goes to System Reset Mode). This is useful for keeping the Watchdog Timer security while using
the interrupt. To stay in Interrupt and System Reset Mode, WDIE must be set after each
interrupt. This should however not be done within the interrupt service routine itself, as
this might compromise the safety-function of the Watchdog System Reset mode. If the
interrupt is not executed before the next time-out, a System Reset will be applied.
Table 27. Watchdog Timer Configuration
WDTON
WDE
WDIE
Mode
Action on Time-out
0
0
0
Stopped
None
0
0
1
Interrupt Mode
Interrupt
0
1
0
System Reset Mode
Reset
0
1
1
Interrupt and System
Reset Mode
Interrupt, then go to
System Reset Mode
1
x
x
System Reset Mode
Reset
• Bit 4 - WDCE: Watchdog Change Enable
This bit is used in timed sequences for changing WDE and prescaler bits. To clear the
WDE bit, and/or change the prescaler bits, WDCE must be set.
Once written to one, hardware will clear WDCE after four clock cycles.
• Bit 3 - WDE: Watchdog System Reset Enable
WDE is overridden by WDRF in MCUSR. This means that WDE is always set when
WDRF is set. To clear WDE, WDRF must be cleared first. This feature ensures multiple
resets during conditions causing failure, and a safe start-up after the failure.
• Bit 5, 2..0 - WDP3..0: Watchdog Timer Prescaler 3, 2, 1 and 0
The WDP3..0 bits determine the Watchdog Timer prescaling when the Watchdog Timer
is running. The different prescaling values and their corresponding time-out periods are
shown in Table 28 on page 68.
67
2549A–AVR–03/05
.
Table 28. Watchdog Timer Prescale Select
WDP3
WDP2
WDP1
WDP0
Number of WDT Oscillator
Cycles
Typical Time-out at
VCC = 5.0V
0
0
0
0
2K (2048) cycles
16 ms
0
0
0
1
4K (4096) cycles
32 ms
0
0
1
0
8K (8192) cycles
64 ms
0
0
1
1
16K (16384) cycles
0.125 s
0
1
0
0
32K (32768) cycles
0.25 s
0
1
0
1
64K (65536) cycles
0.5 s
0
1
1
0
128K (131072) cycles
1.0 s
0
1
1
1
256K (262144) cycles
2.0 s
1
0
0
0
512K (524288) cycles
4.0 s
1
0
0
1
1024K (1048576) cycles
8.0 s
1
0
1
0
1
0
1
1
1
1
0
0
1
1
0
1
1
1
1
0
1
1
1
1
Reserved
68
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Interrupts
This section describes the specifics of the interrupt handling as performed in
ATmega640/1280/1281/2560/2561. For a general explanation of the AVR interrupt handling, refer to “Reset and Interrupt Handling” on page 15.
Interrupt Vectors in
Table 29. Reset and Interrupt Vectors
ATmega640/1280/1281/25
Vector
Program
60/2561
(2)
No.
Address
Source
Interrupt Definition
1
$0000(1)
RESET
External Pin, Power-on Reset, Brown-out Reset,
Watchdog Reset, and JTAG AVR Reset
2
$0002
INT0
External Interrupt Request 0
3
$0004
INT1
External Interrupt Request 1
4
$0006
INT2
External Interrupt Request 2
5
$0008
INT3
External Interrupt Request 3
6
$000A
INT4
External Interrupt Request 4
7
$000C
INT5
External Interrupt Request 5
8
$000E
INT6
External Interrupt Request 6
9
$0010
INT7
External Interrupt Request 7
10
$0012
PCINT0
Pin Change Interrupt Request 0
11
$0014
PCINT1
Pin Change Interrupt Request 1
(3)
12
$0016
PCINT2
Pin Change Interrupt Request 2
13
$0018
WDT
Watchdog Time-out Interrupt
14
$001A
TIMER2 COMPA
Timer/Counter2 Compare Match A
15
$001C
TIMER2 COMPB
Timer/Counter2 Compare Match B
16
$001E
TIMER2 OVF
Timer/Counter2 Overflow
17
$0020
TIMER1 CAPT
Timer/Counter1 Capture Event
18
$0022
TIMER1 COMPA
Timer/Counter1 Compare Match A
19
$0024
TIMER1 COMPB
Timer/Counter1 Compare Match B
20
$0026
TIMER1 COMPC
Timer/Counter1 Compare Match C
21
$0028
TIMER1 OVF
Timer/Counter1 Overflow
22
$002A
TIMER0 COMPA
Timer/Counter0 Compare Match A
23
$002C
TIMER0 COMPB
Timer/Counter0 Compare match B
24
$002E
TIMER0 OVF
Timer/Counter0 Overflow
25
$0030
SPI, STC
SPI Serial Transfer Complete
26
$0032
USART0 RX
USART0 Rx Complete
27
$0034
USART0 UDRE
USART0 Data Register Empty
28
$0036
USART0 TX
USART0 Tx Complete
29
$0038
ANALOG COMP
Analog Comparator
30
$003A
ADC
ADC Conversion Complete
31
$003C
EE READY
EEPROM Ready
69
2549A–AVR–03/05
Table 29. Reset and Interrupt Vectors (Continued)
Vector
No.
Program
Address(2)
Source
Interrupt Definition
32
$003E
TIMER3 CAPT
Timer/Counter3 Capture Event
33
$0040
TIMER3 COMPA
Timer/Counter3 Compare Match A
34
$0042
TIMER3 COMPB
Timer/Counter3 Compare Match B
35
$0044
TIMER3 COMPC
Timer/Counter3 Compare Match C
36
$0046
TIMER3 OVF
Timer/Counter3 Overflow
37
$0048
USART1 RX
USART1 Rx Complete
38
$004A
USART1 UDRE
USART1 Data Register Empty
39
$004C
USART1 TX
USART1 Tx Complete
40
$004E
TWI
2-wire Serial Interface
41
$0050
SPM READY
Store Program Memory Ready
(3)
42
$0052
TIMER4 CAPT
Timer/Counter4 Capture Event
43
$0054
TIMER4 COMPA
Timer/Counter4 Compare Match A
44
$0056
TIMER4 COMPB
Timer/Counter4 Compare Match B
45
$0058
TIMER4 COMPC
Timer/Counter4 Compare Match C
46
$005A
TIMER4 OVF
Timer/Counter4 Overflow
(3)
47
$005C
TIMER5 CAPT
Timer/Counter5 Capture Event
48
$005E
TIMER5 COMPA
Timer/Counter5 Compare Match A
49
$0060
TIMER5 COMPB
Timer/Counter5 Compare Match B
50
$0062
TIMER5 COMPC
Timer/Counter5 Compare Match C
51
$0064
TIMER5 OVF
Timer/Counter5 Overflow
52
$0066(3)
USART2 RX
USART2 Rx Complete
53
$0068(3)
USART2 UDRE
USART2 Data Register Empty
54
(3)
USART2 TX
USART2 Tx Complete
(3)
$006A
55
$006C
USART3 RX
USART3 Rx Complete
56
$006E(3))
USART3 UDRE
USART3 Data Register Empty
USART3 TX
USART3 Tx Complete
57
Notes:
(3)
$0070
1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader
address at reset, see “Memory Programming” on page 335.
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. Only available in ATmega640/1280/2560
Table 30 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.
70
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 30. Reset and Interrupt Vectors Placement(1)
BOOTRST
IVSEL
1
Note:
Reset Address
Interrupt Vectors Start Address
0
0x0000
0x0002
1
1
0x0000
Boot Reset Address + 0x0002
0
0
Boot Reset Address
0x0002
0
1
Boot Reset Address
Boot Reset Address + 0x0002
1. The Boot Reset Address is shown in Table 139 on page 330 through Table 147 on
page 334. 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 ATmega640/1280/1281/2560/2561 is:
Address
Labels
Code
Comments
0x0000
jmp
RESET
; Reset Handler
0x0002
jmp
INT0
; IRQ0 Handler
0x0004
jmp
INT1
; IRQ1 Handler
0x0006
jmp
INT2
; IRQ2 Handler
0x0008
jmp
INT3
; IRQ3 Handler
0x000A
jmp
INT4
; IRQ4 Handler
0x000C
jmp
INT5
; IRQ5 Handler
0x000E
jmp
INT6
; IRQ6 Handler
0x0010
jmp
INT7
; IRQ7 Handler
0x0012
jmp
PCINT0
; PCINT0 Handler
0x0014
jmp
PCINT1
; PCINT1 Handler
0x0016
jmp
PCINT2
; PCINT2 Handler
0X0018
jmp
WDT
; Watchdog Timeout Handler
0x001A
jmp
TIM2_COMPA
; Timer2 CompareA Handler
0x001C
jmp
TIM2_COMPB
; Timer2 CompareB Handler
0x001E
jmp
TIM2_OVF
; Timer2 Overflow Handler
0x0020
jmp
TIM1_CAPT
; Timer1 Capture Handler
0x0022
jmp
TIM1_COMPA
; Timer1 CompareA Handler
0x0024
jmp
TIM1_COMPB
; Timer1 CompareB Handler
0x0026
jmp
TIM1_COMPC
; Timer1 CompareC Handler
0x0028
jmp
TIM1_OVF
; Timer1 Overflow Handler
0x002A
jmp
TIM0_COMPA
; Timer0 CompareA Handler
0x002C
jmp
TIM0_COMPB
; Timer0 CompareB Handler
0x002E
jmp
TIM0_OVF
; Timer0 Overflow Handler
0x0030
jmp
SPI_STC
; SPI Transfer Complete Handler
0x0032
jmp
USART0_RXC
; USART0 RX Complete Handler
0x0034
jmp
USART0_UDRE
; USART0,UDR Empty Handler
0x0036
jmp
USART0_TXC
; USART0 TX Complete Handler
0x0038
jmp
ANA_COMP
; Analog Comparator Handler
0x003A
jmp
ADC
; ADC Conversion Complete Handler
0x003C
jmp
EE_RDY
; EEPROM Ready Handler
0x003E
jmp
TIM3_CAPT
; Timer3 Capture Handler
0x0040
jmp
TIM3_COMPA
; Timer3 CompareA Handler
0x0042
jmp
TIM3_COMPB
; Timer3 CompareB Handler
0x0044
jmp
TIM3_COMPC
; Timer3 CompareC Handler
0x0046
jmp
TIM3_OVF
; Timer3 Overflow Handler
0x0048
jmp
USART1_RXC
; USART1 RX Complete Handler
71
2549A–AVR–03/05
0x004A
jmp
USART1_UDRE
; USART1,UDR Empty Handler
0x004C
jmp
USART1_TXC
; USART1 TX Complete Handler
0x004E
jmp
TWI
; 2-wire Serial Handler
0x0050
jmp
SPM_RDY
; SPM Ready Handler
0x0052
jmp
TIM4_CAPT
; Timer4 Capture Handler
0x0054
jmp
TIM4_COMPA
; Timer4 CompareA Handler
0x0056
jmp
TIM4_COMPB
; Timer4 CompareB Handler
0x0058
jmp
TIM4_COMPC
; Timer4 CompareC Handler
0x005A
jmp
TIM4_OVF
; Timer4 Overflow Handler
0x005C
jmp
TIM5_CAPT
; Timer5 Capture Handler
0x005E
jmp
TIM5_COMPA
; Timer5 CompareA Handler
0x0060
jmp
TIM5_COMPB
; Timer5 CompareB Handler
0x0062
jmp
TIM5_COMPC
; Timer5 CompareC Handler
0x0064
jmp
TIM5_OVF
; Timer5 Overflow Handler
0x0066
jmp
USART2_RXC
; USART2 RX Complete Handler
0x0068
jmp
USART2_UDRE
; USART2,UDR Empty Handler
0x006A
jmp
USART2_TXC
; USART2 TX Complete Handler
0x006C
jmp
USART3_RXC
; USART3 RX Complete Handler
0x006E
jmp
USART3_UDRE
; USART3,UDR Empty Handler
0x0070
jmp
USART3_TXC
; USART3 TX Complete Handler
;
0x0072
ldi
r16, high(RAMEND)
; Main program start
0x0073
out
SPH,r16
; Set Stack Pointer to top of RAM
0x0074
ldi
r16, low(RAMEND)
0x0075
out
SPL,r16
0x0076
sei
0x0077
<instr> xxx
...
RESET:
...
...
; Enable interrupts
...
When the BOOTRST Fuse is unprogrammed, the Boot section size set to 8K 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
0x00000
RESET: ldi
Comments
0x00001
out
SPH,r16
0x00002
ldi
r16,low(RAMEND)
0x00003
0x00004
out
sei
SPL,r16
0x00005
<instr>
r16,high(RAMEND) ; Main program start
; Set Stack Pointer to top of RAM
; Enable interrupts
xxx
;
.org 0x1F002
0x1F002
jmp
EXT_INT0
; IRQ0 Handler
0x1F004
jmp
EXT_INT1
; IRQ1 Handler
...
...
...
;
0x1FO70
jmp
USART3_TXC
; USART3 TX Complete Handler
When the BOOTRST Fuse is programmed and the Boot section size set to 8K bytes, the
most typical and general program setup for the Reset and Interrupt Vector Addresses is:
72
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Address
Labels Code
Comments
.org 0x0002
0x00002
jmp
EXT_INT0
; IRQ0 Handler
0x00004
jmp
EXT_INT1
; IRQ1 Handler
...
...
...
;
0x00070
jmp
USART3_TXC
; USART3 TX Complete Handler
;
.org 0x1F000
0x1F000 RESET: ldi
r16,high(RAMEND) ; Main program start
0x1F001
out
SPH,r16
0x1F002
ldi
r16,low(RAMEND)
0x1F003
0x1F004
out
sei
SPL,r16
0x1F005
<instr>
; Set Stack Pointer to top of RAM
; Enable interrupts
xxx
When the BOOTRST Fuse is programmed, the Boot section size set to 8K 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
Comments
;
.org 0x1F000
0x1F000
0x1F002
jmp
jmp
RESET
EXT_INT0
; Reset handler
; IRQ0 Handler
0x1F004
jmp
EXT_INT1
; IRQ1 Handler
...
...
...
;
0x1F070
jmp
USART3_TXC
; USART3 TX Complete Handler
;
Moving Interrupts Between
Application and Boot Space
MCU Control Register –
MCUCR
0x1F072
RESET: ldi
0x1F073
out
SPH,r16
r16,high(RAMEND) ; Main program start
0x1F074
ldi
r16,low(RAMEND)
0x1F075
0x1F076
out
sei
SPL,r16
0x1FO77
<instr>
; Set Stack Pointer to top of RAM
; Enable interrupts
xxx
The General Interrupt Control Register controls the placement of the Interrupt Vector
table.
Bit
7
6
5
4
3
2
1
0
JTD
–
–
PUD
–
–
IVSEL
IVCE
Read/Write
R/W
R
R
R/W
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
MCUCR
• 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 determined by the BOOTSZ Fuses. Refer to the section “Memory Programming” on page 335 for details. To avoid unintentional changes of Interrupt Vector
tables, a special write procedure must be followed to change the IVSEL bit:
73
2549A–AVR–03/05
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 “Memory Programming” on page 335 for details on Boot Lock bits.
• Bit 0 – IVCE: Interrupt Vector Change Enable
The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is
cleared by hardware four cycles after it is written or when IVSEL is written. Setting the
IVCE bit will disable interrupts, as explained in the IVSEL description above. See Code
Example below.
Assembly Code Example
Move_interrupts:
; Enable change of Interrupt Vectors
ldi
r16, (1<<IVCE)
out
MCUCR, r16
; Move interrupts to Boot Flash section
ldi
r16, (1<<IVSEL)
out
MCUCR, r16
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);
}
74
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
External Interrupts
The External Interrupts are triggered by the INT7:0 pin or any of the PCINT23..0 pins.
Observe that, if enabled, the interrupts will trigger even if the INT7:0 or PCINT23..0 pins
are configured as outputs. This feature provides a way of generating a software
interrupt.
The Pin change interrupt PCI2 will trigger if any enabled PCINT23:16 pin toggles, Pin
change interrupt PCI1 if any enabled PCINT15:8 toggles and Pin change interrupts
PCI0 will trigger if any enabled PCINT7..0 pin toggles. PCMSK2, PCMSK1 and
PCMSK0 Registers control which pins contribute to the pin change interrupts. Pin
change interrupts on PCINT23 ..0 are detected asynchronously. This implies that these
interrupts can be used for waking the part also from sleep modes other than Idle mode.
The External Interrupts can be triggered by a falling or rising edge or a low level. This is
set up as indicated in the specification for the External Interrupt Control Registers –
EICRA (INT3:0) and EICRB (INT7:4). When the external interrupt is enabled and is configured as level triggered, the interrupt will trigger as long as the pin is held low. Note
that recognition of falling or rising edge interrupts on INT7:4 requires the presence of an
I/O clock, described in “Clock Systems and their Distribution” on page 39. Low level
interrupts and the edge interrupt on INT3:0 are detected asynchronously. This implies
that these interrupts can be used for waking the part also from sleep modes other than
Idle mode. The I/O clock is halted in all sleep modes except Idle mode.
Note that if a level triggered interrupt is used for wake-up from Power-down, 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 interrupt 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 39.
Pin Change Interrupt
Timing
An example of timing of a pin change interrupt is shown in Figure 32.
Figure 32.
pin_lat
PCINT(0)
LE
clk
D
pcint_in_(0)
Q
0
pcint_syn
pcint_setflag
PCIF
pin_sync
x
PCINT(0) in PCMSK(x)
clk
clk
PCINT(n)
pin_lat
pin_sync
pcint_in_(n)
pcint_syn
pcint_setflag
PCIF
75
2549A–AVR–03/05
External Interrupt Control
Register A – EICRA
The External Interrupt Control Register A contains control bits for interrupt sense
control.
Bit
7
6
5
4
3
2
1
0
ISC31
ISC30
ISC21
ISC20
ISC11
ISC10
ISC01
ISC00
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
EICRA
• Bits 7..0 – ISC31, ISC30 – ISC00, ISC00: External Interrupt 3 - 0 Sense Control
Bits
The External Interrupts 3 - 0 are activated by the external pins INT3:0 if the SREG I-flag
and the corresponding interrupt mask in the EIMSK is set. The level and edges on the
external pins that activate the interrupts are defined in Table 31. Edges on INT3..INT0
are registered asynchronously. Pulses on INT3:0 pins wider than the minimum pulse
width given in Table 32 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. If enabled,
a level triggered interrupt will generate an interrupt request as long as the pin is held
low. When changing the ISCn bit, an interrupt can occur. Therefore, it is recommended
to first disable INTn by clearing its Interrupt Enable bit in the EIMSK Register. Then, the
ISCn bit can be changed. Finally, the INTn interrupt flag should be cleared by writing a
logical one to its Interrupt Flag bit (INTFn) in the EIFR Register before the interrupt is reenabled.
Table 31. Interrupt Sense Control(1)
ISCn1
ISCn0
0
0
The low level of INTn generates an interrupt request.
0
1
Any edge of INTn generates asynchronously an interrupt request.
1
0
The falling edge of INTn generates asynchronously an interrupt request.
1
Note:
Description
1
The rising edge of INTn generates asynchronously an interrupt request.
1. n = 3, 2, 1or 0.
When changing the ISCn1/ISCn0 bits, the interrupt must be disabled by clearing its
Interrupt Enable bit in the EIMSK Register. Otherwise an interrupt can occur when
the bits are changed.
Table 32. Asynchronous External Interrupt Characteristics
Symbol
tINT
External Interrupt Control
Register B – EICRB
Bit
Parameter
Condition
Min
Typ
Minimum pulse width for
asynchronous external interrupt
Max
50
Units
ns
7
6
5
4
3
2
1
0
ISC71
ISC70
ISC61
ISC60
ISC51
ISC50
ISC41
ISC40
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
EICRB
• Bits 7..0 – ISC71, ISC70 - ISC41, ISC40: External Interrupt 7 - 4 Sense Control
Bits
The External Interrupts 7 - 4 are activated by the external pins INT7:4 if the SREG I-flag
and the corresponding interrupt mask in the EIMSK is set. The level and edges on the
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external pins that activate the interrupts are defined in Table 33. The value on the
INT7:4 pins are 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. Observe that CPU clock frequency can be
lower than the XTAL frequency if the XTAL divider is enabled. If low level interrupt is
selected, the low level must be held until the completion of the currently executing
instruction to generate an interrupt. If enabled, a level triggered interrupt will generate an
interrupt request as long as the pin is held low.
Table 33. Interrupt Sense Control(1)
ISCn1
ISCn0
0
0
The low level of INTn generates an interrupt request.
0
1
Any logical change on INTn generates an interrupt request
1
0
The falling edge between two samples of INTn generates an interrupt
request.
1
Note:
External Interrupt Mask
Register – EIMSK
Description
The rising edge between two samples of INTn generates an interrupt
request.
1. n = 7, 6, 5 or 4.
When changing the ISCn1/ISCn0 bits, the interrupt must be disabled by clearing its
Interrupt Enable bit in the EIMSK Register. Otherwise an interrupt can occur when
the bits are changed.
Bit
1
7
6
5
4
3
2
1
0
INT7
INT6
INT5
INT4
INT3
INT2
INT1
IINT0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
EIMSK
• Bits 7..0 – INT7 – INT0: External Interrupt Request 7 - 0 Enable
When an INT7 – INT0 bit is written to one and the I-bit in the Status Register (SREG) is
set (one), the corresponding external pin interrupt is enabled. The Interrupt Sense Control bits in the External Interrupt Control Registers – EICRA and EICRB – defines
whether the external interrupt is activated on rising or falling edge or level sensed. Activity on any of these pins will trigger an interrupt request even if the pin is enabled as an
output. This provides a way of generating a software interrupt.
External Interrupt Flag
Register – EIFR
Bit
7
6
5
4
3
2
1
0
INTF7
INTF6
INTF5
INTF4
INTF3
INTF2
INTF1
IINTF0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
EIFR
• Bits 7..0 – INTF7 - INTF0: External Interrupt Flags 7 - 0
When an edge or logic change on the INT7:0 pin triggers an interrupt request, INTF7:0
becomes set (one). If the I-bit in SREG and the corresponding interrupt enable bit,
INT7:0 in EIMSK, are set (one), the MCU will jump to the 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. These flags are always cleared when INT7:0 are configured as
level interrupt. Note that when entering sleep mode with the INT3:0 interrupts disabled,
the input buffers on these pins will be disabled. This may cause a logic change in inter-
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nal signals which will set the INTF3:0 flags. See “Digital Input Enable and Sleep Modes”
on page 85 for more information.
Pin Change Interrupt Control
Register - PCICR
Bit
7
6
5
4
3
2
1
0
–
–
–
PCIE2
PCIE1
PCIE0
Read/Write
R
R
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
PCICR
• Bit 2 – PCIE2: Pin Change Interrupt Enable 1
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 interrupt. The corresponding interrupt of Pin Change Interrupt Request is executed from the PCI2 Interrupt Vector. PCINT23..16 pins are enabled individually by the
PCMSK2 Register.
• Bit 1 – 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 interrupt. The corresponding interrupt of Pin Change Interrupt Request is executed from the PCI1 Interrupt Vector. PCINT15..8 pins are enabled individually by the
PCMSK1 Register.
• Bit 0 – 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 PCI0 Interrupt Vector. PCINT7..0 pins are enabled individually by the PCMSK0
Register.
Pin Change Interrupt Flag
Register – PCIFR
Bit
7
6
5
4
3
2
1
0
–
–
–
PCIF2
PCIF1
PCIF0
Read/Write
R
R
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
PCIFR
• Bit 2 – PCIF2: Pin Change Interrupt Flag 1
When a logic change on any PCINT23..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. Alternatively, the flag can be cleared by writing a logical one to
it.
• Bit 1 – 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. Alternatively, the flag can be cleared by writing a logical one to
it.
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• Bit 0 – 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. Alternatively, the flag can be cleared by writing a logical one to
it.
Pin Change Mask Register 2 –
PCMSK2
Bit
7
6
5
4
3
2
1
0
PCINT23
PCINT22
PCINT21
PCINT20
PCINT19
PCINT18
PCINT17
PCINT16
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
PCMSK2
• 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 PCICR 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.
Pin Change Mask Register 1 –
PCMSK1
Bit
7
6
5
4
3
2
1
0
PCINT15
PCINT14
PCINT13
PCINT12
PCINT11
PCINT10
PCINT9
PCINT8
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
PCMSK1
• 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.
Pin Change Mask Register 0 –
PCMSK0
Bit
7
6
5
4
3
2
1
0
PCINT7
PCINT6
PCINT5
PCINT4
PCINT3
PCINT2
PCINT1
PCINT0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
PCMSK0
• 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 PCICR is set, pin change interrupt is
enabled on the corresponding I/O pin. If PCINT7..0 is cleared, pin change interrupt on
the corresponding I/O pin is disabled.
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I/O-Ports
Introduction
All AVR ports have true Read-Modify-Write functionality when used as general digital
I/O ports. This means that the direction of one port pin can be changed without unintentionally changing the direction of any other pin with the SBI and CBI instructions. The
same applies when changing drive value (if configured as output) or enabling/disabling
of pull-up resistors (if configured as input). Each output buffer has symmetrical drive
characteristics with both high sink and source capability. The pin driver is strong enough
to drive LED displays directly. All port pins have individually selectable pull-up resistors
with a supply-voltage invariant resistance. All I/O pins have protection diodes to both
VCC and Ground as indicated in Figure 33. Refer to “Electrical Characteristics” on page
367 for a complete list of parameters.
Figure 33. I/O Pin Equivalent Schematic
Rpu
Logic
Pxn
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” represents the numbering letter for the port, and a lower case “n” represents the bit
number. However, when using the register or bit defines in a program, the precise form
must be used. For example, PORTB3 for bit no. 3 in Port B, here documented generally
as PORTxn. The physical I/O Registers and bit locations are listed in “Register Description for I/O-Ports” on page 112.
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 corresponding 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 82. Most port pins are multiplexed with alternate functions for the peripheral features on the device. How each alternate function interferes with the port pin is described
in “Alternate Port Functions” on page 86. Refer to the individual module sections for a
full description of the alternate functions.
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Note that enabling the alternate function of some of the port pins does not affect the use
of the other pins in the port as general digital I/O.
Ports as General Digital
I/O
The ports are bi-directional I/O ports with optional internal pull-ups. Figure 34 shows a
functional description of one I/O-port pin, here generically called Pxn.
Figure 34. General Digital I/O(1)
PUD
Q
D
DDxn
Q CLR
WDx
RESET
DATA BUS
RDx
1
Q
Pxn
D
0
PORTxn
Q CLR
RESET
WRx
SLEEP
WPx
RRx
SYNCHRONIZER
D
Q
L
Q
D
RPx
Q
PINxn
Q
clk I/O
PUD:
SLEEP:
clkI/O:
Note:
Configuring the Pin
PULLUP DISABLE
SLEEP CONTROL
I/O CLOCK
WDx:
RDx:
WRx:
RRx:
RPx:
WPx:
WRITE DDRx
READ DDRx
WRITE PORTx
READ PORTx REGISTER
READ PORTx PIN
WRITE PINx REGISTER
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.
Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in
“Register Description for I/O-Ports” on page 112, the DDxn bits are accessed at the
DDRx I/O address, the PORTxn bits at the PORTx I/O address, and the PINxn bits at
the PINx I/O address.
The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written
logic one, Pxn is configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input pin.
If PORTxn is written logic one when the pin is configured as an input pin, the pull-up
resistor is activated. To switch the pull-up resistor off, PORTxn has to be written logic
zero or the pin has to be configured as an output pin. The port pins are tri-stated when
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).
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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 acceptable, 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 34 summarizes the control signals for the pin value.
Table 34. Port Pin Configurations
Reading the Pin Value
DDxn
PORTxn
PUD
(in MCUCR)
I/O
Pull-up
0
0
X
Input
No
Tri-state (Hi-Z)
0
1
0
Input
Yes
Pxn will source current if ext. pulled
low.
0
1
1
Input
No
Tri-state (Hi-Z)
1
0
X
Output
No
Output Low (Sink)
1
1
X
Output
No
Output High (Source)
Comment
Independent of the setting of Data Direction bit DDxn, the port pin can be read through
the PINxn Register bit. As shown in Figure 34, the PINxn Register bit and the preceding
latch constitute a synchronizer. This is needed to avoid metastability if the physical pin
changes value near the edge of the internal clock, but it also introduces a delay. Figure
35 shows a timing diagram of the synchronization when reading an externally applied
pin value. The maximum and minimum propagation delays are denoted tpd,max and tpd,min
respectively.
Figure 35. Synchronization when Reading an Externally Applied Pin value
SYSTEM CLK
INSTRUCTIONS
XXX
XXX
in r17, PINx
SYNC LATCH
PINxn
r17
0x00
0xFF
t pd, max
t pd, min
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Consider the clock period starting shortly after the first falling edge of the system clock.
The latch is closed when the clock is low, and goes transparent when the clock is high,
as indicated by the shaded region of the “SYNC LATCH” signal. The signal value is
latched when the system clock goes low. It is clocked into the PINxn Register at the succeeding positive clock edge. As indicated by the two arrows 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
indicated in Figure 36. 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 36. Synchronization when Reading a Software Assigned Pin Value
SYSTEM CLK
r16
INSTRUCTIONS
0xFF
out PORTx, r16
nop
in r17, PINx
SYNC LATCH
PINxn
r17
0x00
0xFF
t pd
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:
Digital Input Enable and Sleep
Modes
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.
As shown in Figure 34, 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 86.
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.
Unconnected Pins
If some pins are unused, it is recommended to ensure that these pins have a defined
level. Even though most of the digital inputs are disabled in the deep sleep modes as
described above, floating inputs should be avoided to reduce current consumption in all
other modes where the digital inputs are enabled (Reset, Active mode and Idle mode).
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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.
Alternate Port Functions
Most port pins have alternate functions in addition to being general digital I/Os. Figure
37 shows how the port pin control signals from the simplified Figure 34 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 37. Alternate Port Functions(1)
PUOExn
PUOVxn
1
PUD
0
DDOExn
DDOVxn
1
Q
D
DDxn
0
Q CLR
WDx
PVOExn
RESET
RDx
1
DATA BUS
PVOVxn
1
Pxn
Q
0
D
0
PORTxn
PTOExn
Q CLR
DIEOExn
WPx
DIEOVxn
RESET
WRx
1
0
RRx
SLEEP
SYNCHRONIZER
D
SET
Q
RPx
Q
D
PINxn
L
CLR
Q
CLR
Q
clk I/O
DIxn
AIOxn
PUOExn:
PUOVxn:
DDOExn:
DDOVxn:
PVOExn:
PVOVxn:
DIEOExn:
DIEOVxn:
SLEEP:
PTOExn:
Note:
86
Pxn PULL-UP OVERRIDE ENABLE
Pxn PULL-UP OVERRIDE VALUE
Pxn DATA DIRECTION OVERRIDE ENABLE
Pxn DATA DIRECTION OVERRIDE VALUE
Pxn PORT VALUE OVERRIDE ENABLE
Pxn PORT VALUE OVERRIDE VALUE
Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE
Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE
SLEEP CONTROL
Pxn, PORT TOGGLE OVERRIDE ENABLE
PUD:
WDx:
RDx:
RRx:
WRx:
RPx:
WPx:
clkI/O:
DIxn:
AIOxn:
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
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.
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ATmega640/1280/1281/2560/2561
Table 35 summarizes the function of the overriding signals. The pin and port indexes
from Figure 37 are not shown in the succeeding tables. The overriding signals are generated internally in the modules having the alternate function.
Table 35. Generic Description of Overriding Signals for Alternate Functions
Signal Name
Full Name
Description
PUOE
Pull-up Override
Enable
If this signal is set, the pull-up enable is controlled by the
PUOV signal. If this signal is cleared, the pull-up is
enabled when {DDxn, PORTxn, PUD} = 0b010.
PUOV
Pull-up Override
Value
If PUOE is set, the pull-up is enabled/disabled when
PUOV is set/cleared, regardless of the setting of the
DDxn, PORTxn, and PUD Register bits.
DDOE
Data Direction
Override Enable
If this signal is set, the Output Driver Enable is controlled
by the DDOV signal. If this signal is cleared, the Output
driver is enabled by the DDxn Register bit.
DDOV
Data Direction
Override Value
If DDOE is set, the Output Driver is enabled/disabled
when DDOV is set/cleared, regardless of the setting of
the DDxn Register bit.
PVOE
Port Value
Override Enable
If this signal is set and the Output Driver is enabled, the
port value is controlled by the PVOV signal. If PVOE is
cleared, and the Output Driver is enabled, the port Value
is controlled by the PORTxn Register bit.
PVOV
Port Value
Override Value
If PVOE is set, the port value is set to PVOV, regardless
of the setting of the PORTxn Register bit.
PTOE
Port Toggle
Override Enable
If PTOE is set, the PORTxn Register bit is inverted.
DIEOE
Digital Input
Enable Override
Enable
If this bit is set, the Digital Input Enable is controlled by
the DIEOV signal. If this signal is cleared, the Digital Input
Enable is determined by MCU state (Normal mode, sleep
mode).
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).
DI
Digital Input
This is the Digital Input to alternate functions. In the
figure, the signal is connected to the output of the schmitt
trigger but before the synchronizer. Unless the Digital
Input is used as a clock source, the module with the
alternate function will use its own synchronizer.
AIO
Analog
Input/Output
This is the Analog Input/output to/from alternate
functions. The signal is connected directly to the pad, and
can be used bi-directionally.
The following subsections shortly describe the alternate functions for each port, and
relate the overriding signals to the alternate function. Refer to the alternate function
description for further details.
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MCU Control Register –
MCUCR
Bit
7
6
5
4
3
2
1
0
JTD
–
–
PUD
–
–
IVSEL
IVCE
Read/Write
R/W
R
R
R/W
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
MCUCR
• 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 “Configuring the Pin” on page 82 for more details about this feature.
Alternate Functions of Port A
The Port A has an alternate function as the address low byte and data lines for the
External Memory Interface.
Table 36. Port A Pins Alternate Functions
Port Pin
Alternate Function
PA7
AD7 (External memory interface address and data bit 7)
PA6
AD6 (External memory interface address and data bit 6)
PA5
AD5 (External memory interface address and data bit 5)
PA4
AD4 (External memory interface address and data bit 4)
PA3
AD3 (External memory interface address and data bit 3)
PA2
AD2 (External memory interface address and data bit 2)
PA1
AD1 (External memory interface address and data bit 1)
PA0
AD0 (External memory interface address and data bit 0)
Table 37 and Table 38 relates the alternate functions of Port A to the overriding signals
shown in Figure 37 on page 86.
Table 37. Overriding Signals for Alternate Functions in PA7..PA4
Signal
Name
PA7/AD7
PUOE
SRE
PA5/AD5
PA4/AD4
SRE
SRE
SRE
PUOV
~(WR | ADA ) •
PORTA7 • PUD
~(WR | ADA) •
PORTA6 • PUD
~(WR | ADA) •
PORTA5 • PUD
~(WR | ADA) •
PORTA4 • PUD
DDOE
SRE
SRE
SRE
SRE
DDOV
WR | ADA
WR | ADA
WR | ADA
WR | ADA
PVOE
SRE
SRE
SRE
SRE
PVOV
A7 • ADA | D7
OUTPUT • WR
A6 • ADA | D6
OUTPUT • WR
A5 • ADA | D5
OUTPUT • WR
A4 • ADA | D4
OUTPUT • WR
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
D7 INPUT
D6 INPUT
D5 INPUT
D4 INPUT
AIO
–
–
–
–
Note:
88
(1)
PA6/AD6
1. ADA is short for ADdress Active and represents the time when address is output. See
“External Memory Interface” on page 29 for details.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 38. Overriding Signals for Alternate Functions in PA3..PA0
Alternate Functions of Port B
Signal
Name
PA3/AD3
PA2/AD2
PA1/AD1
PA0/AD0
PUOE
SRE
SRE
SRE
SRE
PUOV
~(WR | ADA) •
PORTA3 • PUD
~(WR | ADA) •
PORTA2 • PUD
~(WR | ADA) •
PORTA1 • PUD
~(WR | ADA) •
PORTA0 • PUD
DDOE
SRE
SRE
SRE
SRE
DDOV
WR | ADA
WR | ADA
WR | ADA
WR | ADA
PVOE
SRE
SRE
SRE
SRE
PVOV
A3 • ADA | D3
OUTPUT • WR
A2• ADA | D2
OUTPUT • WR
A1 • ADA | D1
OUTPUT • WR
A0 • ADA | D0
OUTPUT • WR
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
D3 INPUT
D2 INPUT
D1 INPUT
D0 INPUT
AIO
–
–
–
–
The Port B pins with alternate functions are shown in Table 39.
Table 39. Port B Pins Alternate Functions
Port Pin
Alternate Functions
PB7
OC0A/OC1C/PCINT7 (Output Compare and PWM Output A for Timer/Counter0,
Output Compare and PWM Output C for Timer/Counter1 or Pin Change Interrupt 7)
PB6
OC1B/PCINT6 (Output Compare and PWM Output B for Timer/Counter1 or Pin
Change Interrupt 6)
PB5
OC1A/PCINT5 (Output Compare and PWM Output A for Timer/Counter1 or Pin
Change Interrupt 5)
PB4
OC2A/PCINT4 (Output Compare and PWM Output A for Timer/Counter2 or Pin
Change Interrupt 4)
PB3
MISO/PCINT3 (SPI Bus Master Input/Slave Output or Pin Change Interrupt 3)
PB2
MOSI/PCINT2 (SPI Bus Master Output/Slave Input or Pin Change Interrupt 2)
PB1
SCK/PCINT1 (SPI Bus Serial Clock or Pin Change Interrupt 1)
PB0
SS/PCINT0 (SPI Slave Select input or Pin Change Interrupt 0)
The alternate pin configuration is as follows:
• OC0A/OC1C/PCINT7, Bit 7
OC0A, Output Compare Match A output: The PB7 pin can serve as an external output
for the Timer/Counter0 Output Compare. The pin has to be configured as an output
(DDB7 set “one”) to serve this function. The OC0A pin is also the output pin for the PWM
mode timer function.
OC1C, Output Compare Match C output: The PB7 pin can serve as an external output
for the Timer/Counter1 Output Compare C. The pin has to be configured as an output
(DDB7 set (one)) to serve this function. The OC1C pin is also the output pin for the
PWM mode timer function.
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PCINT7, Pin Change Interrupt source 7: The PB7 pin can serve as an external interrupt
source.
• OC1B/PCINT6, 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.
PCINT6, Pin Change Interrupt source 6: The PB7 pin can serve as an external interrupt
source.
• OC1A/PCINT5, 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.
PCINT5, Pin Change Interrupt source 5: The PB7 pin can serve as an external interrupt
source.
• OC2A/PCINT4, Bit 4
OC2A, Output Compare Match output: The PB4 pin can serve as an external output for
the Timer/Counter2 Output Compare. The pin has to be configured as an output (DDB4
set (one)) to serve this function. The OC2A pin is also the output pin for the PWM mode
timer function.
PCINT4, Pin Change Interrupt source 4: The PB7 pin can serve as an external interrupt
source.
• MISO/PCINT3 – Port B, Bit 3
MISO: Master Data input, Slave Data output pin for SPI channel. When the SPI is
enabled as a master, this pin is configured as an input regardless of the setting of
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.
PCINT3, Pin Change Interrupt source 3: The PB7 pin can serve as an external interrupt
source.
• MOSI/PCINT2 – Port B, Bit 2
MOSI: SPI Master Data output, Slave Data input for SPI channel. When the SPI is
enabled as a slave, this pin is configured as an input regardless of the setting of 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.
PCINT2, Pin Change Interrupt source 2: The PB7 pin can serve as an external interrupt
source.
• SCK/PCINT1 – Port B, Bit 1
SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is
enabled as a slave, this pin is configured as an input regardless of the setting of DDB1.
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ATmega640/1280/1281/2560/2561
When the SPI0 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.
PCINT1, Pin Change Interrupt source 1: The PB7 pin can serve as an external interrupt
source.
• SS/PCINT0 – 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.
Table 40 and Table 41 relate the alternate functions of Port B to the overriding signals
shown in Figure 37 on page 86. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute
the MISO signal, while MOSI is divided into SPI MSTR OUTPUT and SPI SLAVE
INPUT.
PCINT0, Pin Change Interrupt source 0: The PB7 pin can serve as an external interrupt
source..
Table 40. Overriding Signals for Alternate Functions in PB7..PB4
Signal
Name
PB7/OC0A/OC1C
PB6/OC1B
PB5/OC1A
PB4/OC2
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
0
0
DDOV
0
0
0
0
PVOE
OC0/OC1C ENABLE
OC1B ENABLE
OC1A ENABLE
OC2 ENABLE
PVOV
OC0/OC1C
OC1B
OC1A
OC2B
DIEOE
PCINT7 • PCIE0
PCINT6 • PCIE0
PCINT5 • PCIE0
PCINT4 • PCIE0
DIEOV
1
1
1
1
DI
PCINT7 INPUT
PCINT6 INPUT
PCINT5 INPUT
PCINT4 INPUT
AIO
–
–
–
–
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2549A–AVR–03/05
Table 41. Overriding Signals for Alternate Functions in PB3..PB0
Alternate Functions of Port C
Signal
Name
PB3/MISO
PB2/MOSI
PB1/SCK
PB0/SS
PUOE
SPE • MSTR
SPE • MSTR
SPE • MSTR
SPE • MSTR
PUOV
PORTB3 • PUD
PORTB2 • PUD
PORTB1 • PUD
PORTB0 • PUD
DDOE
SPE • MSTR
SPE • MSTR
SPE • MSTR
SPE • MSTR
DDOV
0
0
0
0
PVOE
SPE • MSTR
SPE • MSTR
SPE • MSTR
0
PVOV
SPI SLAVE OUTPUT
SPI MSTR OUTPUT
SCK OUTPUT
0
DIEOE
PCINT3 • PCIE0
PCINT2 • PCIE0
PCINT1 •
PCIE0
PCINT0 •
PCIE0
DIEOV
1
1
1
1
DI
SPI MSTR INPUT
PCINT3 INPUT
SPI SLAVE INPUT
PCINT2 INPUT
SCK INPUT
PCINT1 INPUT
SPI SS
PCINT0 INPUT
AIO
–
–
–
–
The Port C alternate function is as follows:
Table 42. Port C Pins Alternate Functions
Port Pin
Alternate Function
PC7
A15(External Memory interface address bit 15)
PC6
A14(External Memory interface address bit 14)
PC5
A13(External Memory interface address bit 13)
PC4
A12(External Memory interface address bit 12)
PC3
A11(External Memory interface address bit 11)
PC2
A10(External Memory interface address bit 10)
PC1
A9(External Memory interface address bit 9)
PC0
A8(External Memory interface address bit 8)
Table 43 and Table 44 relate the alternate functions of Port C to the overriding signals
shown in Figure 37 on page 86.
92
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
Table 43. Overriding Signals for Alternate Functions in PC7..PC4
Signal
Name
PC7/A15
PC6/A14
PC5/A13
PC4/A12
PUOE
SRE • (XMM<1)
SRE • (XMM<2)
SRE • (XMM<3)
SRE • (XMM<4)
PUOV
0
0
0
0
DDOE
SRE • (XMM<1)
SRE • (XMM<2)
SRE • (XMM<3)
SRE • (XMM<4)
DDOV
1
1
1
1
PVOE
SRE • (XMM<1)
SRE • (XMM<2)
SRE • (XMM<3)
SRE • (XMM<4)
PVOV
A15
A14
A13
A12
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
–
–
–
–
AIO
–
–
–
–
Table 44. Overriding Signals for Alternate Functions in PC3..PC0
Signal
Name
PC3/A11
PC2/A10
PC1/A9
PC0/A8
PUOE
SRE • (XMM<5)
SRE • (XMM<6)
SRE • (XMM<7)
SRE • (XMM<7)
PUOV
0
0
0
0
DDOE
SRE • (XMM<5)
SRE • (XMM<6)
SRE • (XMM<7)
SRE • (XMM<7)
DDOV
1
1
1
1
PVOE
SRE • (XMM<5)
SRE • (XMM<6)
SRE • (XMM<7)
SRE • (XMM<7)
PVOV
A11
A10
A9
A8
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
–
–
–
–
AIO
–
–
–
–
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2549A–AVR–03/05
Alternate Functions of Port D
The Port D pins with alternate functions are shown in Table 45.
Table 45. Port D Pins Alternate Functions
Port Pin
Alternate Function
PD7
T0 (Timer/Counter0 Clock Input)
PD6
T1 (Timer/Counter1 Clock Input)
PD5
XCK1 (USART1 External Clock Input/Output)
PD4
ICP1 (Timer/Counter1 Input Capture Trigger)
PD3
INT3/TXD1 (External Interrupt3 Input or USART1 Transmit Pin)
PD2
INT2/RXD1 (External Interrupt2 Input or USART1 Receive Pin)
PD1
INT1/SDA (External Interrupt1 Input or TWI Serial DAta)
PD0
INT0/SCL (External Interrupt0 Input or TWI Serial CLock)
The alternate pin configuration is as follows:
• T0 – Port D, Bit 7
T0, Timer/Counter0 counter source.
• T1 – Port D, Bit 6
T1, Timer/Counter1 counter source.
• XCK1 – Port D, Bit 5
XCK1, USART1 External clock. The Data Direction Register (DDD5) controls whether
the clock is output (DDD5 set) or input (DDD5 cleared). The XCK1 pin is active only
when the USART1 operates in Synchronous mode.
• ICP1 – Port D, Bit 4
ICP1 – Input Capture Pin 1: The PD4 pin can act as an input capture pin for
Timer/Counter1.
• INT3/TXD1 – Port D, Bit 3
INT3, External Interrupt source 3: The PD3 pin can serve as an external interrupt source
to the MCU.
TXD1, Transmit Data (Data output pin for the USART1). When the USART1 Transmitter
is enabled, this pin is configured as an output regardless of the value of DDD3.
• INT2/RXD1 – Port D, Bit 2
INT2, External Interrupt source 2. The PD2 pin can serve as an External Interrupt
source to the MCU.
RXD1, Receive Data (Data input pin for the USART1). When the USART1 receiver is
enabled this pin is configured as an input regardless of the value of DDD2. When the
USART forces this pin to be an input, the pull-up can still be controlled by the PORTD2
bit.
94
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ATmega640/1280/1281/2560/2561
• INT1/SDA – Port D, Bit 1
INT1, External Interrupt source 1. The PD1 pin can serve as an external interrupt source
to the MCU.
SDA, 2-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to enable
the 2-wire Serial Interface, pin PD1 is disconnected from the port and becomes the
Serial Data I/O pin for the 2-wire Serial Interface. In this mode, there is a spike filter on
the pin to suppress spikes shorter than 50 ns on the input signal, and the pin is driven by
an open drain driver with slew-rate limitation.
• INT0/SCL – Port D, Bit 0
INT0, External Interrupt source 0. The PD0 pin can serve as an external interrupt source
to the MCU.
SCL, 2-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to enable
the 2-wire Serial Interface, pin PD0 is disconnected from the port and becomes the
Serial Clock I/O pin for the 2-wire Serial Interface. In this mode, there is a spike filter on
the pin to suppress spikes shorter than 50 ns on the input signal, and the pin is driven by
an open drain driver with slew-rate limitation.
Table 46 and Table 47 relates the alternate functions of Port D to the overriding signals
shown in Figure 37 on page 86.
Table 46. Overriding Signals for Alternate Functions PD7..PD4
Signal Name
PD7/T0
PD6/T1
PD5/XCK1
PD4/ICP1
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
XCK1 OUTPUT ENABLE
0
DDOV
0
0
1
0
PVOE
0
0
XCK1 OUTPUT ENABLE
0
PVOV
0
0
XCK1 OUTPUT
0
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
T0 INPUT
T1 INPUT
XCK1 INPUT
ICP1 INPUT
AIO
–
–
–
–
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2549A–AVR–03/05
Table 47. Overriding Signals for Alternate Functions in PD3..PD0(1)
Signal Name
PD3/INT3/TXD1
PD2/INT2/RXD1
PD1/INT1/SDA
PD0/INT0/SCL
PUOE
TXEN1
RXEN1
TWEN
TWEN
PUOV
0
PORTD2 • PUD
PORTD1 • PUD
PORTD0 • PUD
DDOE
TXEN1
RXEN1
TWEN
TWEN
DDOV
1
0
SDA_OUT
SCL_OUT
PVOE
TXEN1
0
TWEN
TWEN
PVOV
TXD1
0
0
0
DIEOE
INT3 ENABLE
INT2 ENABLE
INT1 ENABLE
INT0 ENABLE
DIEOV
1
1
1
1
DI
INT3 INPUT
INT2 INPUT/RXD1
INT1 INPUT
INT0 INPUT
AIO
–
–
SDA INPUT
SCL INPUT
Note:
Alternate Functions of Port E
1. When enabled, the 2-wire Serial Interface enables Slew-Rate controls on the output
pins PD0 and PD1. This is not shown in this table. In addition, spike filters are connected between the AIO outputs shown in the port figure and the digital logic of the
TWI module.
The Port E pins with alternate functions are shown in Table 48.
Table 48. Port E Pins Alternate Functions
Port Pin
Alternate Function
PE7
INT7/ICP3/CLK0 (External Interrupt 7 Input, Timer/Counter3 Input Capture Trigger
or Divided System Clock)
PE6
INT6/ T3 (External Interrupt 6 Input or Timer/Counter3 Clock Input)
PE5
INT5/OC3C (External Interrupt 5 Input or Output Compare and PWM Output C for
Timer/Counter3)
PE4
INT4/OC3B (External Interrupt4 Input or Output Compare and PWM Output B for
Timer/Counter3)
PE3
AIN1/OC3A (Analog Comparator Negative Input or Output Compare and PWM
Output A for Timer/Counter3)
PE2
AIN0/XCK0 (Analog Comparator Positive Input or USART0 external clock
input/output)
PE1
PDO(1)/TXD0 (Programming Data Output or USART0 Transmit Pin)
PE0
PDI(1)/RXD0/PCINT8 (Programming Data Input, USART0 Receive Pin or Pin
Change Interrupt 8)
Note:
1. Only for ATmega1281/2561. For ATmega1281/2561 these functions are placed
on MISO/MOSI pins.
• INT7/ICP3 – Port E, Bit 7
INT7, External Interrupt source 7: The PE7 pin can serve as an external interrupt
source.
ICP3 - Input Capture Pin 3: The PE7 pin can act as an input capture pin for
Timer/Counter3.
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ATmega640/1280/1281/2560/2561
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.
• INT6/T3 – Port E, Bit 6
INT6, External Interrupt source 6: The PE6 pin can serve as an external interrupt
source.
T3, Timer/Counter3 counter source.
• INT5/OC3C – Port E, Bit 5
INT5, External Interrupt source 5: The PE5 pin can serve as an External Interrupt
source.
OC3C, Output Compare Match C output: The PE5 pin can serve as an External output
for the Timer/Counter3 Output Compare C. The pin has to be configured as an output
(DDE5 set “one”) to serve this function. The OC3C pin is also the output pin for the
PWM mode timer function.
• INT4/OC3B – Port E, Bit 4
INT4, External Interrupt source 4: The PE4 pin can serve as an External Interrupt
source.
OC3B, Output Compare Match B output: The PE4 pin can serve as an External output
for the Timer/Counter3 Output Compare B. The pin has to be configured as an output
(DDE4 set (one)) to serve this function. The OC3B pin is also the output pin for the PWM
mode timer function.
• AIN1/OC3A – Port E, Bit 3
AIN1 – Analog Comparator Negative input. This pin is directly connected to the negative
input of the Analog Comparator.
OC3A, Output Compare Match A output: The PE3 pin can serve as an External output
for the Timer/Counter3 Output Compare A. The pin has to be configured as an output
(DDE3 set “one”) to serve this function. The OC3A pin is also the output pin for the PWM
mode timer function.
• AIN0/XCK0 – Port E, Bit 2
AIN0 – Analog Comparator Positive input. This pin is directly connected to the positive
input of the Analog Comparator.
XCK0, USART0 External clock. The Data Direction Register (DDE2) controls whether
the clock is output (DDE2 set) or input (DDE2 cleared). The XCK0 pin is active only
when the USART0 operates in Synchronous mode.
• PDO/TXD0 – Port E, Bit 1
PDO, SPI Serial Programming Data Output. During Serial Program Downloading, this
pin is used as data output line for the ATmega1281/2561. For ATmega640/1280/2560
this function is placed on MISO.
TXD0, USART0 Transmit pin.
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2549A–AVR–03/05
• PDI/RXD0/PCINT8 – Port E, Bit 0
PDI, SPI Serial Programming Data Input. During Serial Program Downloading, this pin
is used as data input line for the ATmega1281/2561. For ATmega640/1280/2560 this
function is placed on MOSI.
RXD0, 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
DDRE0. When the USART0 forces this pin to be an input, a logical one in PORTE0 will
turn on the internal pull-up.
PCINT8, Pin Change Interrupt source 8: The PE0 pin can serve as an external interrupt source.
Table 49 and Table 50 relates the alternate functions of Port E to the overriding signals
shown in Figure 37 on page 86.
Table 49. Overriding Signals for Alternate Functions PE7..PE4
98
Signal
Name
PE7/INT7/ICP3
PE6/INT6/T3
PE5/INT5/OC3C
PE4/INT4/OC3B
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
0
0
DDOV
0
0
0
0
PVOE
0
0
OC3C ENABLE
OC3B ENABLE
PVOV
0
0
OC3C
OC3B
DIEOE
INT7 ENABLE
INT6 ENABLE
INT5 ENABLE
INT4 ENABLE
DIEOV
1
1
1
1
DI
INT7 INPUT/ICP3
INPUT
INT7 INPUT/T3
INPUT
INT5 INPUT
INT4 INPUT
AIO
–
–
–
–
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 50. Overriding Signals for Alternate Functions in PE3..PE0
PE2/AIN0/XCK0
PE1/PDO(1)/
TXD0
PE0/PDI(1)/
RXD0/PCINT8
0
0
TXEN0
RXEN0
PUOV
0
0
0
PORTE0 • PUD
DDOE
0
XCK0 OUTPUT
ENABLE
TXEN0
RXEN0
DDOV
0
1
1
0
PVOE
OC3B ENABLE
XCK0 OUTPUT
ENABLE
TXEN0
0
PVOV
OC3B
XCK0 OUTPUT
TXD0
0
DIEOE
0
0
0
PCINT8 • PCIE1
DIEOV
0
0
0
1
DI
0
XCK0 INPUT
–
RXD0
PE0
0
0
0
PCINT8 INPUT
AIO
AIN1 INPUT
AIN0 INPUT
–
–
Signal
Name
PE3/AIN1/OC3A
PUOE
Note:
1. PDO/PDI only available at PE1/PE0 for ATmega1281/2561.
99
2549A–AVR–03/05
Alternate Functions of Port F
The Port F has an alternate function as analog input for the ADC as shown in Table 51.
If some Port F pins are configured as outputs, it is essential that these do not switch
when a conversion is in progress. This might corrupt the result of the conversion. 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 51. Port F Pins Alternate Functions
Port Pin
Alternate Function
PF7
ADC7/TDI (ADC input channel 7 or JTAG Test Data Input)
PF6
ADC6/TDO (ADC input channel 6 or JTAG Test Data Output)
PF5
ADC5/TMS (ADC input channel 5 or JTAG Test Mode Select)
PF4
ADC4/TCK (ADC input channel 4 or JTAG Test ClocK)
PF3
ADC3 (ADC input channel 3)
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 Register (scan chains). When the JTAG interface is enabled, this pin can not be
used as an I/O pin.
• 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.
The TDO pin is tri-stated unless TAP states that shift out data are entered.
• 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.
100
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
• ADC3 – ADC0 – Port F, Bit 3..0
Analog to Digital Converter, Channel 3..0.
Table 52. Overriding Signals for Alternate Functions in PF7..PF4
Signal
Name
PF7/ADC7/TDI
PF6/ADC6/TDO
PF5/ADC5/TMS
PF4/ADC4/TCK
PUOE
JTAGEN
JTAGEN
JTAGEN
JTAGEN
PUOV
1
0
1
1
DDOE
JTAGEN
JTAGEN
JTAGEN
JTAGEN
DDOV
0
SHIFT_IR +
SHIFT_DR
0
0
PVOE
0
JTAGEN
0
0
PVOV
0
TDO
0
0
DIEOE
JTAGEN
JTAGEN
JTAGEN
JTAGEN
DIEOV
0
0
0
0
DI
–
–
–
–
AIO
TDI/ADC7 INPUT
ADC6 INPUT
TMS/ADC5
INPUT
TCK/ADC4
INPUT
Table 53. 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
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
–
–
–
–
AIO
ADC3 INPUT
ADC2 INPUT
ADC1 INPUT
ADC0 INPUT
101
2549A–AVR–03/05
Alternate Functions of Port G
The Port G alternate pin configuration is as follows:
Table 54. Port G Pins Alternate Functions
Port Pin
Alternate Function
PG5
OC0B (Output Compare and PWM Output B for Timer/Counter0)
PG4
TOSC1 (RTC Oscillator Timer/Counter2)
PG3
TOSC2 (RTC Oscillator Timer/Counter2)
PG2
ALE (Address Latch Enable to external memory)
PG1
RD (Read strobe to external memory)
PG0
WR (Write strobe to external memory)
• OC0B – Port G, Bit 5
OC0B, Output Compare match B output: The PG5 pin can serve as an external output
for the TImer/Counter0 Output Compare. The pin has to be configured as an output
(DDG5 set) to serve this function. The OC0B pin is also the output pin for the PWM
mode timer function.
• TOSC1 – Port G, Bit 4
TOSC2, Timer Oscillator pin 1: When the AS2 bit in ASSR is set (one) to enable asynchronous clocking of Timer/Counter2, pin PG4 is disconnected from the port, and
becomes the input of the inverting Oscillator amplifier. In this mode, a Crystal Oscillator
is connected to this pin, and the pin can not be used as an I/O pin.
• TOSC2 – Port G, Bit 3
TOSC2, Timer Oscillator pin 2: When the AS2 bit in ASSR is set (one) to enable asynchronous clocking of Timer/Counter2, pin PG3 is disconnected from the port, and
becomes the inverting output of the Oscillator amplifier. In this mode, a Crystal Oscillator
is connected to this pin, and the pin can not be used as an I/O pin.
• ALE – Port G, Bit 2
ALE is the external data memory Address Latch Enable signal.
• RD – Port G, Bit 1
RD is the external data memory read control strobe.
• WR – Port G, Bit 0
WR is the external data memory write control strobe.
Table 55 and Table 56 relates the alternate functions of Port G to the overriding signals
shown in Figure 37 on page 86.
102
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 55. Overriding Signals for Alternate Functions in PG5..PG4
Signal Name
—
—
PG5/OC0B
PG4/TOSC1
PUOE
–
–
–
AS2
PUOV
–
–
–
0
DDOE
–
–
–
AS2
DDOV
–
–
–
0
PVOE
–
–
OC0B Enable
0
PVOV
–
–
OC0B
0
PTOE
–
–
–
–
DIEOE
–
–
–
AS2
DIEOV
–
–
–
EXCLK
DI
–
–
–
–
AIO
–
–
–
T/C2 OSC INPUT
Table 56. Overriding Signals for Alternate Functions in PG3..PG0
Signal Name
PG3/TOSC2
PG2/ALE/A7
PG1/RD
PG0/WR
PUOE
AS2 • EXCLK
SRE
SRE
SRE
PUOV
0
0
0
0
DDOE
AS2 • EXCLK
SRE
SRE
SRE
DDOV
0
1
1
1
PVOE
0
SRE
SRE
SRE
PVOV
0
ALE
RD
WR
PTOE
–
–
–
–
DIEOE
AS2 • EXCLK
0
0
0
DIEOV
0
0
0
0
DI
–
–
–
–
AIO
T/C2 OSC OUTPUT
–
–
–
103
2549A–AVR–03/05
Alternate Functions of Port H
The Port H alternate pin configuration is as follows:
Table 57. Port H Pins Alternate Functions
Port Pin
Alternate Function
PH7
T4 (Timer/Counter4 Clock Input)
PH6
OC2B(Output Compare and PWM Output B for Timer/Counter2)
PH5
OC4C(Output Compare and PWM Output C for Timer/Counter4)
PH4
OC4B(Output Compare and PWM Output B for Timer/Counter4)
PH3
OC4A(Output Compare and PWM Output A for Timer/Counter4)
PH2
XCK2 (USART2 External Clock)
PH1
TXD2 (USART2 Transmit Pin)
PH0
RXD2 (USART2 Receive Pin)
• T4 – Port H, Bit 7
T4, Timer/Counter4 counter source.
• OC2B – Port H, Bit 6
OC2B, Output Compare Match B output: The PH6 pin can serve as an external output for the
Timer/Counter2 Output Compare B. The pin has to be configured as an output (DDH6 set) to
serve this function. The OC2B pin is also the output pin for the PWM mode timer function.
• OC4C – Port H, Bit 5
OC4C, Output Compare Match C output: The PH5 pin can serve as an external output for the
Timer/Counter4 Output Compare C. The pin has to be configured as an output (DDH5 set) to
serve this function. The OC4C pin is also the output pin for the PWM mode timer function.
• OC4B – Port H, Bit 4
OC4B, Output Compare Match B output: The PH4 pin can serve as an external output for the
Timer/Counter2 Output Compare B. The pin has to be configured as an output (DDH4 set) to
serve this function. The OC4B pin is also the output pin for the PWM mode timer function.
• OC4A – Port H, Bit 3
OC4C, Output Compare Match A output: The PH3 pin can serve as an external output
for the Timer/Counter4 Output Compare A. The pin has to be configured as an output
(DDH3 set) to serve this function. The OC4A pin is also the output pin for the PWM
mode timer function.
• XCK2 – Port H, Bit 2
XCK2, USART2 External Clock: The Data Direction Register (DDH2) controls whether the clock is
output (DDH2 set) or input (DDH2 cleared). The XC2K pin is active only when the USART2 operates in synchronous mode.
• TXD2 – Port H, Bit 1
TXD2, USART2 Transmit Pin.
104
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
• RXD2 – Port H, Bit 0
RXD2, USART2 Receive pin: Receive Data (Data input pin for the USART2). When the
USART2 Receiver is enabled, this pin is configured as an input regardless of the value
of DDH0. When the USART2 forces this pin to be an input, a logical on in PORTH0 will
turn on the internal pull-up.
Table 58. Overriding Signals for Alternate Functions in PH7..PH4
Signal Name
PH7/T4
PH6/OC2B
PH5/OC4C
PH4/OC4B
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
0
0
DDOV
0
0
0
0
PVOE
0
OC2B ENABLE
OC4C ENABLE
OC4B ENABLE
PVOV
0
OC2B
OC4C
OC4B
PTOE
–
–
–
–
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
T4 INPUT
0
0
0
AIO
–
–
–
–
Table 59. Overriding Signals for Alternate Functions in PH3..PH0
Signal Name
PH3/OC4A
PH2/XCK2
PH1/TXD2
PH0/RXD2
PUOE
0
0
TXEN2
RXEN2
PUOV
0
0
0
PORTH0 • PUD
DDOE
0
XCK2 OUTPUT
ENABLE
TXEN2
RXEN2
DDOV
0
1
1
0
PVOE
OC4A ENABLE
XCK2 OUTPUT
ENABLE
TXEN2
0
PVOV
OC4A
XCK2
TXD2
0
PTOE
–
–
–
–
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
0
XC2K INPUT
0
RXD2
AIO
–
–
–
–
105
2549A–AVR–03/05
Alternate Functions of Port J
The Port J alternate pin configuration is as follows:
Table 60. Port J Pins Alternate Functions
Port Pin
Alternate Function
PJ7
–
PJ6
PCINT15 (Pin Change Interrupt 15)
PJ5
PCINT14 (Pin Change Interrupt 14)
PJ4
PCINT13 (Pin Change Interrupt 13)
PJ3
PCINT12 (Pin Change Interrupt 12)
PJ2
XCK3/PCINT11 (USART3 External Clock or Pin Change Interrupt 11)
PJ1
TXD3/PCINT10 (USART3 Transmit Pin or Pin Change Interrupt 10)
PJ0
RXD3/PCINT9 (USART3 Receive Pin or Pin Change Interrupt 9)
• PCINT15:12 - Port J, Bit 6:3
PCINT15:12, Pin Change Interrupt Source 15:12. The PJ6:3 pins can serve as External
Interrupt Sources
• XCK2/PCINT11 - Port J, Bit 2
XCK2, USART 2 External Clock. The Data Direction Register (DDJ2) controls whether
the clock is output (DDJ2 set) or input (DDJ2 cleared). The XCK2 pin is active only when
the USART2 operates in synchronous mode.
PCINT11, Pin Change Interrupt Source 11. The PJ2 pins can serve as External Interrupt
Sources
• TXD3/PCINT10 - Port J, Bit 1
TXD3, USART3 Transmit pin
PCINT10, Pin Change Interrupt Source 10. The PJ2 pins can serve as External Interrupt
Sources
• RXD3/PCINT9 - Port J, Bit 0
RXD3, USART3 Receive pin. Receive Data (Data input pin for the USART3). When the
USART3 Receiver is enabled, this pin is configured as an input regardless of the value
of DDJ0. When the USART3 forces this pin to be an input, a logical one in PORTJ0 will
turn on the internal pull-up.
PCINT9, Pin Change Interrupt Source 9. The PJ2 pins can serve as External Interrupt
Sources
Table 61 and Table 62 relates the alternate functions of Port J to the overriding signals
shown in Figure 37 on page 86
106
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 61. Overriding Signals for Alternate Functions in PJ7..PJ4
Signal Name
PJ7
PJ6/ PCINT15
PJ5/ PCINT14
PJ4/ PCINT13
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
PCINT15·PCIE1
PCINT14·PCIE1
PCINT13·PCIE1
DIEOV
0
1
1
1
DI
0
PCINT15 INPUT
PCINT14 INPUT
PCINT13 INPUT
AIO
-
-
-
-
Table 62. Overriding Signals for Alternate Functions in PJ3..PJ0
Signal Name
PJ3/PCINT12
PJ2/XCK3/PCIN
T11
PJ1/TXD3/PCIN
T10
PJ0/RXD3/PCIN
T9
PUOE
0
0
TXEN3
RXEN3
PUOV
0
0
0
PORTJ0·PUD
DDOE
0
XCK3 OUTPUT
ENABLE
TXEN3
RXEN3
DDOV
0
1
1
0
PVOE
0
XCK3 OUTPUT
ENABLE
TXEN3
0
PVOV
0
XCK3
TXD3
0
PTOE
-
-
-
-
DIEOE
PCINT12·PCIE1
PCINT11·PCIE1
PCINT10·PCIE1
PCINT9·PCIE1
DIEOV
1
1
1
1
DI
PCINT12 INPUT
PCINT11 INPUT
XCK3 INPUT
PCINT10 INPUT
PCINT9 INPUT
RXD3
AIO
-
-
-
-
107
2549A–AVR–03/05
Alternate Functions of Port K
The Port K alternate pin configuration is as follows:
Table 63. Port K Pins Alternate Functions
Port Pin
Alternate Function
PK7
ADC15/PCINT23 (ADC Input Channel 15 or Pin Change Interrupt 23)
PK6
ADC14/PCINT22 (ADC Input Channel 14 or Pin Change Interrupt 22)
PK5
ADC13/PCINT21 (ADC Input Channel 13 or Pin Change Interrupt 21)
PK4
ADC12/PCINT20 (ADC Input Channel 12 or Pin Change Interrupt 20)
PK3
ADC11/PCINT19 (ADC Input Channel 11 or Pin Change Interrupt 19)
PK2
ADC10/PCINT18 (ADC Input Channel 10 or Pin Change Interrupt 18)
PK1
ADC9/PCINT17 (ADC Input Channel 9 or Pin Change Interrupt 17)
PK0
ADC8 /PCINT16 (ADC Input Channel 8 or Pin Change Interrupt 16)
• ADC15:8/PCINT23:16 – Port K, Bit 7:0
ADC15:8, Analog to Digital Converter, Channel 15 - 8.
PCINT23:16, Pin Change Interrupt Source 23:16. The PK7:0 pins can serve as External
Interrupt Sources.
Table 64. Overriding Signals for Alternate Functions in PK7..PK4
108
Signal Name
PK7/ADC15/
PCINT23
PK6/ADC14/
PCINT22
PK5/ADC13/
PCINT21
PK4/ADC12/
PCINT20
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
PCINT23 •
PCIE2
PCINT22 •
PCIE2
PCINT21 •
PCIE2
PCINT20 •
PCIE2
DIEOV
1
1
1
1
DI
PCINT23
INPUT
PCINT22
INPUT
PCINT21
INPUT
PCINT20
INPUT
AIO
ADC15 INPUT
ADC14 INPUT
ADC13 INPUT
ADC12 INPUT
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 65. Overriding Signals for Alternate Functions in PK3..PK0
Signal Name
PK3/ADC11/
PCINT19
PK2/ADC10/
PCINT18
PK1/ADC9/
PCINT17
PK0/ADC8/
PCINT16
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
PCINT19 •
PCIE2
PCINT18 •
PCIE2
PCINT17 •
PCIE2
PCINT16 •
PCIE2
DIEOV
1
1
1
1
DI
PCINT19 INPUT
PCINT18 INPUT
PCINT17
INPUT
PCINT16
INPUT
AIO
ADC11 INPUT
ADC10INPUT
ADC9 INPUT
ADC8 INPUT
109
2549A–AVR–03/05
Alternate Functions of Port L
The Port L alternate pin configuration is as follows:
Table 66. Port L Pins Alternate Functions
Port Pin
Alternate Function
PL7
–
PL6
–
PL5
OC5C (Output Compare and PWM Output C for Timer/Counter5)
PL4
OC5B (Output Compare and PWM Output B for Timer/Counter5)
PL3
OC5A (Output Compare and PWM Output A for Timer/Counter5)
PL2
T5 (Timer/Counter5 Clock Input)
PL1
ICP5 (Timer/Counter5 Input Capture Trigger)
PL0
ICP4 (Timer/Counter4 Input Capture Trigger)
• OC5C – Port L, Bit 5
OC5C, Output Compare Match C output: The PL5 pin can serve as an external output
for the Timer/Counter5 Output Compare C. The pin has to be configured as an output
(DDL5 set) to serve this function. The OC5C pin is also the output pin for the PWM
mode timer function.
• OC5B – Port L, Bit 4
OC5B, Output Compare Match B output: The PL4 pin can serve as an external output
for the Timer/Counter 5 Output Compare B. The pin has to be configured as an output
(DDL4 set) to serve this function. The OC5B pin is also the output pin for the PWM
mode timer function.
• OC5A – Port L, Bit 3
OC5A, Output Compare Match A output: The PL3 pin can serve as an external output
for the Timer/Counter 5 Output Compare A. The pin has to be configured as an output
(DDL3 set) to serve this function. The OC5A pin is also the output pin for the PWM
mode timer function.
• T5 – Port L, Bit 2
T5, Timer/Counter5 counter source.
• ICP5 – Port L, Bit 1
ICP5, Input Capture Pin 5: The PL1 pin can serve as an Input Capture pin for
Timer/Counter5.
• ICP4 – Port L, Bit 0
ICP4, Input Capture Pin 4: The PL0 pin can serve as an Input Capture pin for
Timer/Counter4.
110
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 67 and Table 68 relates the alternate functions of Port L to the overriding signals
shown in Figure 37 on page 86.
Table 67. Overriding Signals for Alternate Functions in PL7..PL4
Signal Name
PL7
PL6
PL5/OC5C
PL4/OC5B
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
–
–
0
0
DDOV
–
–
0
0
PVOE
–
–
OC5C ENABLE
OC5B ENABLE
PVOV
–
–
OC5C
OC5B
PTOE
–
–
–
–
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
0
0
0
0
AIO
–
–
–
–
Table 68. Overriding Signals for Alternate Functions in PL3..PL0
Signal Name
PL3/OC5A
PL2/T5
PL1/ICP5
PL0/ICP4
PUOE
0
0
0
0
PUOV
0
0
0
0
DDOE
0
0
0
0
DDOV
0
0
0
0
PVOE
OC5A ENABLE
0
0
0
PVOV
OC5A
0
0
0
PTOE
–
–
–
–
DIEOE
0
0
0
0
DIEOV
0
0
0
0
DI
0
T5 INPUT
ICP5 INPUT
ICP4 INPUT
AIO
–
–
–
–
111
2549A–AVR–03/05
Register Description for I/O-Ports
Port A Data Register – PORTA
Bit
Port A Data Direction Register
– DDRA
Port A Input Pins Address –
PINA
7
6
5
4
3
2
1
0
PORTA7
PORTA6
PORTA5
PORTA4
PORTA3
PORTA2
PORTA1
PORTA0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
DDA7
DDA6
DDA5
DDA4
DDA3
DDA2
DDA1
DDA0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
PINA7
PINA6
PINA5
PINA4
PINA3
PINA2
PINA1
PINA0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PORTA
DDRA
PINA
Port B Data Register – PORTB
Bit
Port B Data Direction Register
– DDRB
Port B Input Pins Address –
PINB
7
6
5
4
3
2
1
0
PORTB7
PORTB6
PORTB5
PORTB4
PORTB3
PORTB2
PORTB1
PORTB0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
DDB7
DDB6
DDB5
DDB4
DDB3
DDB2
DDB1
DDB0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
PINB7
PINB6
PINB5
PINB4
PINB3
PINB2
PINB1
PINB0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PORTB
DDRB
PINB
Port C Data Register – PORTC
Bit
Port C Data Direction Register
– DDRC
Port C Input Pins Address –
PINC
112
7
6
5
4
3
2
1
0
PORTC7
PORTC6
PORTC5
PORTC4
PORTC3
PORTC2
PORTC1
PORTC0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
DDC7
DDC6
DDC5
DDC4
DDC3
DDC2
DDC1
DDC0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
PINC7
PINC6
PINC5
PINC4
PINC3
PINC2
PINC1
PINC0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PORTC
DDRC
PINC
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Port D Data Register – PORTD
Bit
Port D Data Direction Register
– DDRD
Port D Input Pins Address –
PIND
7
6
5
4
3
2
1
0
PORTD7
PORTD6
PORTD5
PORTD4
PORTD3
PORTD2
PORTD1
PORTD0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
DDD7
DDD6
DDD5
DDD4
DDD3
DDD2
DDD1
DDD0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
PIND7
PIND6
PIND5
PIND4
PIND3
PIND2
PIND1
PIND0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PORTD
DDRD
PIND
Port E Data Register – PORTE
Bit
Port E Data Direction Register
– DDRE
Port E Input Pins Address –
PINE
7
6
5
4
3
2
1
0
PORTE7
PORTE6
PORTE5
PORTE4
PORTE3
PORTE2
PORTE1
PORTE0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
DDE7
DDE6
DDE5
DDE4
DDE3
DDE2
DDE1
DDE0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
PINE7
PINE6
PINE5
PINE4
PINE3
PINE2
PINE1
PINE0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PORTE
DDRE
PINE
Port F Data Register – PORTF
Bit
Port F Data Direction Register
– DDRF
Port F Input Pins Address –
PINF
7
6
5
4
3
2
1
0
PORTF7
PORTF6
PORTF5
PORTF4
PORTF3
PORTF2
PORTF1
PORTF0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
DDF7
DDF6
DDF5
DDF4
DDF3
DDF2
DDF1
DDF0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
PINF7
PINF6
PINF5
PINF4
PINF3
PINF2
PINF1
PINF0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PORTF
DDRF
PINF
113
2549A–AVR–03/05
Port G Data Register – PORTG
Bit
Port G Data Direction Register
– DDRG
Port G Input Pins Address –
PING
7
6
5
4
3
2
1
0
–
–
PORTG5
PORTG4
PORTG3
PORTG2
PORTG1
PORTG0
Read/Write
R
R
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
7
6
5
4
3
2
1
0
–
–
DDG5
DDG4
DDG3
DDG2
DDG1
DDG0
Read/Write
R
R
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
–
–
PING5
PING4
PING3
PING2
PING1
PING0
Read/Write
R
R
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
N/A
N/A
N/A
N/A
N/A
N/A
Bit
PORTG
DDRG
PING
Port H Data Register – PORTH
Bit
Port H Data Direction Register
– DDRH
Port H Input Pins Address –
PINH
7
6
5
4
3
2
1
0
PORTH7
PORTH6
PORTH5
PORTH4
PORTH3
PORTH2
PORTH1
PORTH0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
DDH7
DDH6
DDH5
DDH4
DDH3
DDH2
DDH1
DDH0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
PINH5
PINH5
PINH5
PINH4
PINH3
PINGH
PINH1
PINH0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PORTH
DDRH
PINH
Port J Data Register – PORTJ
Bit
Port J Data Direction Register
– DDRJ
Port J Input Pins Address –
PINJ
114
7
6
5
4
3
2
1
0
PORTJ7
PORTJ6
PORTJ5
PORTJ4
PORTJ3
PORTJ2
PORTJ1
PORTJ0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
DDJ7
DDJ6
DDJ5
DDJ4
DDJ3
DDJ2
DDJ1
DDJ0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
PINJ5
PINJ5
PINJ5
PINJ4
PINJ3
PINGJ
PINJ1
PINJ0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PORTJ
DDRJ
PINJ
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Port K Data Register – PORTK
Bit
Port K Data Direction Register
– DDRK
Port K Input Pins Address –
PINK
7
6
5
4
3
2
1
0
PORTK7
PORTK6
PORTK5
PORTK4
PORTK3
PORTK2
PORTK1
PORTK0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
DDK7
DDK6
DDK5
DDK4
DDK3
DDK2
DDK1
DDK0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
PINK5
PINK5
PINK5
PINK4
PINK3
PINGK
PINK1
PINK0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PORTK
DDRK
PINK
Port L Data Register – PORTL
Bit
Port L Data Direction Register
– DDRL
Port L Input Pins Address –
PINL
7
6
5
4
3
2
1
0
PORTL7
PORTL6
PORTL5
PORTL4
PORTL3
PORTL2
PORTL1
PORTL0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
DDL7
DDL6
DDL5
DDL4
DDL3
DDL2
DDL1
DDL0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
PINL5
PINL5
PINL5
PINL4
PINL3
PINGL
PINL1
PINL0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PORTL
DDRL
PINL
115
2549A–AVR–03/05
116
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
8-bit Timer/Counter0
with PWM
Timer/Counter0 is a general purpose 8-bit Timer/Counter module, with two independent
Output Compare Units, and with PWM support. It allows accurate program execution
timing (event management) and wave generation. The main features are:
• Two Independent Output Compare Units
• Double Buffered Output Compare Registers
• Clear Timer on Compare Match (Auto Reload)
• Glitch Free, Phase Correct Pulse Width Modulator (PWM)
• Variable PWM Period
• Frequency Generator
• Three Independent Interrupt Sources (TOV0, OCF0A, and OCF0B)
Overview
A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 38. For the
actual placement of I/O pins, refer to “Pinout ATmega640/1280/2560” on page 2. CPU
accessible I/O Registers, including I/O bits and I/O pins, are shown in bold. The devicespecific I/O Register and bit locations are listed in the “8-bit Timer/Counter Register
Description” on page 128.
Figure 38. 8-bit Timer/Counter Block Diagram
Count
Clear
Direction
TOVn
(Int.Req.)
Control Logic
clkTn
Clock Select
Edge
Detector
TOP
Tn
BOTTOM
( From Prescaler )
Timer/Counter
TCNTn
=
=0
OCnA
(Int.Req.)
Waveform
Generation
=
OCnA
DATA BUS
OCRnA
Fixed
TOP
Value
Waveform
Generation
=
OCnB
OCRnB
TCCRnA
Registers
OCnB
(Int.Req.)
TCCRnB
The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are
8-bit registers. Interrupt 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 Register (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).
The double buffered Output Compare Registers (OCR0A and OCR0B) are compared
with the Timer/Counter value at all times. The result of the compare can be used by the
Waveform Generator to generate a PWM or variable frequency output on the Output
117
2549A–AVR–03/05
Compare pins (OC0A and OC0B). See “Output Compare Unit” on page 119. for details.
The Compare Match event will also set the Compare Flag (OCF0A or OCF0B) which
can be used to generate an Output Compare interrupt request.
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 Compare Unit, in this case Compare Unit A or Compare Unit B. 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 69 are also used extensively throughout the document.
Table 69. Definitions
BOTTOM
The counter reaches the BOTTOM when it becomes 0x00.
MAX
The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
TOP
The counter reaches the TOP when it becomes equal to the highest
value in the count sequence. The TOP value can be assigned to be the
fixed value 0xFF (MAX) or the value stored in the OCR0A Register. The
assignment is dependent on the mode of operation.
Timer/Counter Clock
Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock
source is selected by the Clock Select logic which is controlled by the Clock Select
(CS02:0) bits located in the Timer/Counter Control Register (TCCR0B). For details on
clock sources and prescaler, see “Timer/Counter0, Timer/Counter1, Timer/Counter3,
Timer/Counter4, and Timer/Counter5 Prescalers” on page 169.
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure 39 shows a block diagram of the counter and its surroundings.
Figure 39. Counter Unit Block Diagram
TOVn
(Int.Req.)
DATA BUS
Clock Select
count
TCNTn
clear
Control Logic
clkTn
Edge
Detector
Tn
direction
( From Prescaler )
bottom
top
Signal description (internal signals):
118
count
Increment or decrement TCNT0 by 1.
direction
Select between increment and decrement.
clear
Clear TCNT0 (set all bits to zero).
clkTn
Timer/Counter clock, referred to as clkT0 in the following.
top
Signalize that TCNT0 has reached maximum value.
bottom
Signalize that TCNT0 has reached minimum value (zero).
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
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) and the WGM02 bit located in
the Timer/Counter Control Register B (TCCR0B). There are close connections between
how the counter behaves (counts) and how waveforms are generated on the Output
Compare outputs OC0A and OC0B. For more details about advanced counting
sequences and waveform generation, see “Modes of Operation” on page 121.
The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation
selected by the WGM02:0 bits. TOV0 can be used for generating a CPU interrupt.
Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers (OCR0A and OCR0B). Whenever TCNT0 equals OCR0A or OCR0B, the
comparator signals a match. A match will set the Output Compare Flag (OCF0A or
OCF0B) at the next timer clock cycle. If the corresponding interrupt is enabled, the Output Compare Flag generates an Output Compare interrupt. The Output Compare Flag is
automatically cleared when the interrupt is executed. Alternatively, the 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
WGM02:0 bits and Compare Output mode (COM0x1:0) bits. The max and bottom signals are used by the Waveform Generator for handling the special cases of the extreme
values in some modes of operation (“Modes of Operation” on page 121).
Figure 40 shows a block diagram of the Output Compare unit.
Figure 40. Output Compare Unit, Block Diagram
DATA BUS
OCRnx
TCNTn
= (8-bit Comparator )
OCFnx (Int.Req.)
top
bottom
Waveform Generator
OCnx
FOCn
WGMn1:0
COMnX1:0
119
2549A–AVR–03/05
The OCR0x Registers are 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 OCR0x Compare Registers 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 OCR0x Register access may seem complex, but this is not case. When the double
buffering is enabled, the CPU has access to the OCR0x Buffer Register, and if double
buffering is disabled the CPU will access the OCR0x directly.
Force Output Compare
In non-PWM waveform generation modes, the match output of the comparator can be
forced by writing a one to the Force Output Compare (FOC0x) bit. Forcing Compare
Match will not set the OCF0x Flag or reload/clear the timer, but the OC0x pin will be
updated as if a real Compare Match had occurred (the COM0x1:0 bits settings define
whether the OC0x pin is set, cleared or toggled).
Compare Match Blocking by
TCNT0 Write
All CPU write operations to the TCNT0 Register will block any Compare Match that
occur in the next timer clock cycle, even when the timer is stopped. This feature allows
OCR0x to be initialized to the same value as TCNT0 without triggering an interrupt when
the Timer/Counter clock is enabled.
Using the Output Compare
Unit
Since writing TCNT0 in any mode of operation will block all Compare Matches for one
timer clock cycle, there are risks involved when changing TCNT0 when using the Output
Compare Unit, independently of whether the Timer/Counter is running or not. If the
value written to TCNT0 equals the OCR0x 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 down-counting.
The setup of the OC0x should be performed before setting the Data Direction Register
for the port pin to output. The easiest way of setting the OC0x value is to use the Force
Output Compare (FOC0x) strobe bits in Normal mode. The OC0x Registers keep their
values even when changing between Waveform Generation modes.
Be aware that the COM0x1:0 bits are not double buffered together with the compare
value. Changing the COM0x1:0 bits will take effect immediately.
Compare Match Output
Unit
120
The Compare Output mode (COM0x1:0) bits have two functions. The Waveform Generator uses the COM0x1:0 bits for defining the Output Compare (OC0x) state at the next
Compare Match. Also, the COM0x1:0 bits control the OC0x pin output source. Figure 41
shows a simplified schematic of the logic affected by the COM0x1:0 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 COM0x1:0
bits are shown. When referring to the OC0x state, the reference is for the internal OC0x
Register, not the OC0x pin. If a system reset occur, the OC0x Register is reset to “0”.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 41. Compare Match Output Unit, Schematic
COMnx1
COMnx0
FOCn
Waveform
Generator
D
Q
1
OCnx
DATA BUS
D
0
OCnx
Pin
Q
PORT
D
Q
DDR
clk I/O
The general I/O port function is overridden by the Output Compare (OC0x) from the
Waveform Generator if either of the COM0x1:0 bits are set. However, the OC0x pin
direction (input or output) is still controlled by the Data Direction Register (DDR) for the
port pin. The Data Direction Register bit for the OC0x pin (DDR_OC0x) must be set as
output before the OC0x value is visible on the pin. The port override function is independent of the Waveform Generation mode.
The design of the Output Compare pin logic allows initialization of the OC0x state before
the output is enabled. Note that some COM0x1:0 bit settings are reserved for certain
modes of operation. See “8-bit Timer/Counter Register Description” on page 128.
Compare Output Mode and
Waveform Generation
The Waveform Generator uses the COM0x1:0 bits differently in Normal, CTC, and PWM
modes. For all modes, setting the COM0x1:0 = 0 tells the Waveform Generator that no
action on the OC0x Register is to be performed on the next Compare Match. For compare output actions in the non-PWM modes refer to Table 70 on page 128. For fast
PWM mode, refer to Table 71 on page 128, and for phase correct PWM refer to Table
72 on page 129.
A change of the COM0x1:0 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 FOC0x strobe bits.
Modes of Operation
The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare
pins, is defined by the combination of the Waveform Generation mode (WGM02:0) and
Compare Output mode (COM0x1:0) bits. The Compare Output mode bits do not affect
the counting sequence, while the Waveform Generation mode bits do. The COM0x1:0
bits control whether the PWM output generated should be inverted or not (inverted or
non-inverted PWM). For non-PWM modes the COM0x1:0 bits control whether the output should be set, cleared, or toggled at a Compare Match (See “Compare Match
Output Unit” on page 146.).
For detailed timing information see “Timer/Counter Timing Diagrams” on page 126.
121
2549A–AVR–03/05
Normal Mode
The simplest mode of operation is the Normal mode (WGM02:0 = 0). In this mode the
counting direction is always up (incrementing), and no counter clear is performed. The
counter simply overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then
restarts from the bottom (0x00). In normal operation the Timer/Counter Overflow Flag
(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 Output Compare to generate waveforms in Normal mode is not recommended,
since this will occupy too much of the CPU time.
Clear Timer on Compare
Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM02:0 = 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 42. The counter value
(TCNT0) increases until a Compare Match occurs between TCNT0 and OCR0A, and
then counter (TCNT0) is cleared.
Figure 42. CTC Mode, Timing Diagram
OCnx Interrupt Flag Set
TCNTn
OCn
(Toggle)
Period
(COMnx1:0 = 1)
1
2
3
4
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 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 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 the pin is set to output. The waveform generated will have a maxi-
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ATmega640/1280/1281/2560/2561
mum frequency of fOC0 = fclk_I/O/2 when OCR0A is set to zero (0x00). The waveform
frequency is defined by the following equation:
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.
Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM02:0 = 3 or 7) provides a high
frequency PWM waveform generation option. The fast PWM differs from the other PWM
option by its single-slope operation. The counter counts from BOTTOM to TOP then
restarts from BOTTOM. TOP is defined as 0xFF when WGM2:0 = 3, and OCR0A when
WGM2:0 = 7. In non-inverting Compare Output mode, the Output Compare (OC0x) is
cleared on the Compare Match between TCNT0 and OCR0x, 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 TOP
value. The counter is then cleared at the following timer clock cycle. The timing diagram
for the fast PWM mode is shown in Figure 43. The TCNT0 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 TCNT0
slopes represent Compare Matches between OCR0x and TCNT0.
Figure 43. Fast PWM Mode, Timing Diagram
OCRnx Interrupt Flag Set
OCRnx Update and
TOVn Interrupt Flag Set
TCNTn
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
4
5
6
7
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. If
the interrupt 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
OC0x pins. Setting the COM0x1:0 bits to two will produce a non-inverted PWM and an
123
2549A–AVR–03/05
inverted PWM output can be generated by setting the COM0x1:0 to three: Setting the
COM0A1:0 bits to one allows the OC0A pin to toggle on Compare Matches if the
WGM02 bit is set. This option is not available for the OC0B pin (See Table 71 on page
128). The actual OC0x value will only be visible on the port pin if the data direction for
the port pin is set as output. The PWM waveform is generated by setting (or clearing)
the OC0x Register at the Compare Match between OCR0x and TCNT0, and clearing (or
setting) the OC0x 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 ⋅ 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 setting OC0x to toggle its logical level on each Compare Match (COM0x1:0 = 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 Output Compare unit is enabled in the fast PWM mode.
Phase Correct PWM Mode
The phase correct PWM mode (WGM02:0 = 1 or 5) 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 TOP and then
from TOP to BOTTOM. TOP is defined as 0xFF when WGM2:0 = 1, and OCR0A when
WGM2:0 = 5. In non-inverting Compare Output mode, the Output Compare (OC0x) is
cleared on the Compare Match between TCNT0 and OCR0x while upcounting, and set
on the Compare Match while down-counting. 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.
In phase correct PWM mode the counter is incremented until the counter value matches
TOP. When the counter reaches TOP, it changes the count direction. The TCNT0 value
will be equal to TOP for one timer clock cycle. The timing diagram for the phase correct
PWM mode is shown on Figure 44. 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 OCR0x and TCNT0.
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ATmega640/1280/1281/2560/2561
Figure 44. Phase Correct PWM Mode, Timing Diagram
OCnx Interrupt Flag Set
OCRnx Update
TOVn Interrupt Flag Set
TCNTn
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
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 OC0x pins. Setting the COM0x1:0 bits to two will produce a non-inverted PWM. An
inverted PWM output can be generated by setting the COM0x1:0 to three: Setting the
COM0A0 bits to one allows the OC0A pin to toggle on Compare Matches if the WGM02
bit is set. This option is not available for the OC0B pin (See Table 72 on page 129). The
actual OC0x 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 OC0x
Register at the Compare Match between OCR0x and TCNT0 when the counter increments, and setting (or clearing) the OC0x Register at Compare Match between OCR0x
and TCNT0 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 = ----------------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 44 OCnx has a transition from high to low even
though there is no Compare Match. The point of this transition is to guarantee symmetry
around BOTTOM. There are two cases that give a transition without Compare Match.
•
OCR0A changes its value from MAX, like in Figure 44. When the OCR0A value is
MAX the OCn pin value is the same as the result of a down-counting Compare
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2549A–AVR–03/05
Match. To ensure symmetry around BOTTOM the OCn value at MAX must
correspond to the result of an up-counting Compare Match.
•
Timer/Counter Timing
Diagrams
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.
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 45 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 45. Timer/Counter Timing Diagram, no Prescaling
clkI/O
clkTn
(clkI/O /1)
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
TOVn
Figure 46 shows the same timing data, but with the prescaler enabled.
Figure 46. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
TOVn
Figure 47 shows the setting of OCF0B in all modes and OCF0A in all modes except
CTC mode and PWM mode, where OCR0A is TOP.
Figure 47. Timer/Counter Timing Diagram, Setting of OCF0x, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
OCRnx - 1
OCRnx
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
OCFnx
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ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 48 shows the setting of OCF0A and the clearing of TCNT0 in CTC mode and fast
PWM mode where OCR0A is TOP.
Figure 48. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with
Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
(CTC)
OCRnx
TOP - 1
TOP
BOTTOM
BOTTOM + 1
TOP
OCFnx
127
2549A–AVR–03/05
8-bit Timer/Counter
Register Description
Timer/Counter Control
Register A – TCCR0A
Bit
7
6
5
4
3
2
1
0
COM0A1
COM0A0
COM0B1
COM0B0
–
–
WGM01
WGM00
Read/Write
R/W
R/W
R/W
R/W
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TCCR0A
• Bits 7:6 – COM01A:0: Compare Match Output A 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.
When OC0A is connected to the pin, the function of the COM0A1:0 bits depends on the
WGM02:0 bit setting. Table 70 shows the COM0A1:0 bit functionality when the
WGM02:0 bits are set to a normal or CTC mode (non-PWM).
Table 70. Compare Output Mode, non-PWM Mode
COM0A1
COM0A0
Description
0
0
Normal port operation, OC0A disconnected.
0
1
Toggle OC0A on Compare Match
1
0
Clear OC0A on Compare Match
1
1
Set OC0A on Compare Match
Table 71 shows the COM0A1:0 bit functionality when the WGM01:0 bits are set to fast
PWM mode.
Table 71. Compare Output Mode, Fast PWM Mode(1)
COM0A1
COM0A0
0
0
Normal port operation, OC0A disconnected.
0
1
WGM02 = 0: Normal Port Operation, OC0A Disconnected.
WGM02 = 1: Toggle OC0A on Compare Match.
1
0
Clear OC0A on Compare Match, set OC0A at TOP
1
1
Set OC0A on Compare Match, clear OC0A at TOP
Note:
128
Description
1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case,
the Compare Match is ignored, but the set or clear is done at TOP. See “Fast PWM
Mode” on page 123 for more details.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 72 shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to
phase correct PWM mode.
Table 72. Compare Output Mode, Phase Correct PWM Mode(1)
COM0A1
COM0A0
0
0
Normal port operation, OC0A disconnected.
0
1
WGM02 = 0: Normal Port Operation, OC0A Disconnected.
WGM02 = 1: Toggle OC0A on Compare Match.
1
0
Clear OC0A on Compare Match when up-counting. Set OC0A on
Compare Match when down-counting.
1
1
Set OC0A on Compare Match when up-counting. Clear OC0A on
Compare Match when down-counting.
Note:
Description
1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case,
the Compare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page 124 for more details.
• Bits 5:4 – COM0B1:0: Compare Match Output B Mode
These bits control the Output Compare pin (OC0B) behavior. If one or both of the
COM0B1:0 bits are set, the OC0B 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 OC0B pin must be set in order to enable the output driver.
When OC0B is connected to the pin, the function of the COM0B1:0 bits depends on the
WGM02:0 bit setting. Table 70 shows the COM0A1:0 bit functionality when the
WGM02:0 bits are set to a normal or CTC mode (non-PWM).
Table 73. Compare Output Mode, non-PWM Mode
COM01
COM00
Description
0
0
Normal port operation, OC0B disconnected.
0
1
Toggle OC0B on Compare Match
1
0
Clear OC0B on Compare Match
1
1
Set OC0B on Compare Match
Table 71 shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to fast
PWM mode.
Table 74. Compare Output Mode, Fast PWM Mode(1)
COM01
COM00
0
0
Normal port operation, OC0B disconnected.
0
1
Reserved
1
0
Clear OC0B on Compare Match, set OC0B at TOP
1
1
Set OC0B on Compare Match, clear OC0B at TOP
Note:
Description
1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case,
the Compare Match is ignored, but the set or clear is done at TOP. See “Fast PWM
Mode” on page 123 for more details.
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Table 72 shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to
phase correct PWM mode.
Table 75. Compare Output Mode, Phase Correct PWM Mode(1)
COM0A1
COM0A0
0
0
Normal port operation, OC0B disconnected.
0
1
Reserved
1
0
Clear OC0B on Compare Match when up-counting. Set OC0B on
Compare Match when down-counting.
1
1
Set OC0B on Compare Match when up-counting. Clear OC0B on
Compare Match when down-counting.
Note:
Description
1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case,
the Compare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page 124 for more details.
• Bits 3, 2 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bits 1:0 – WGM01:0: Waveform Generation Mode
Combined with the WGM02 bit found in the TCCR0B Register, these bits control the
counting sequence of the counter, the source for maximum (TOP) counter value, and
what type of waveform generation to be used, see Table 76. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear Timer on Compare
Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes (see
“Modes of Operation” on page 147).
Table 76. Waveform Generation Mode Bit Description
TOP
Update of
OCRx at
TOV Flag
Set on(1)(2)
Mode
WGM2
WGM1
WGM0
0
0
0
0
Normal
0xFF
Immediate
MAX
1
0
0
1
PWM, Phase
Correct
0xFF
TOP
BOTTOM
2
0
1
0
CTC
OCRA
Immediate
MAX
3
0
1
1
Fast PWM
0xFF
TOP
MAX
4
1
0
0
Reserved
–
–
–
5
1
0
1
PWM, Phase
Correct
OCRA
TOP
BOTTOM
6
1
1
0
Reserved
–
–
–
7
1
1
1
Fast PWM
OCRA
TOP
TOP
Notes:
130
Timer/Counter
Mode of
Operation
1. MAX
= 0xFF
2. BOTTOM = 0x00
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Timer/Counter Control
Register B – TCCR0B
Bit
7
6
5
4
3
2
1
0
FOC0A
FOC0B
–
–
WGM02
CS02
CS01
CS00
Read/Write
W
W
R
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TCCR0B
• Bit 7 – FOC0A: Force Output Compare A
The FOC0A bit is only active when the WGM bits specify a non-PWM mode.
However, for ensuring compatibility with future devices, this bit must be set to zero when
TCCR0B is written when operating in PWM mode. When writing a logical one to the
FOC0A bit, an immediate Compare 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 – FOC0B: Force Output Compare B
The FOC0B bit is only active when the WGM bits specify a non-PWM mode.
However, for ensuring compatibility with future devices, this bit must be set to zero when
TCCR0B is written when operating in PWM mode. When writing a logical one to the
FOC0B bit, an immediate Compare Match is forced on the Waveform Generation unit.
The OC0B output is changed according to its COM0B1:0 bits setting. Note that the
FOC0B bit is implemented as a strobe. Therefore it is the value present in the
COM0B1:0 bits that determines the effect of the forced compare.
A FOC0B strobe will not generate any interrupt, nor will it clear the timer in CTC mode
using OCR0B as TOP.
The FOC0B bit is always read as zero.
• Bits 5:4 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bit 3 – WGM02: Waveform Generation Mode
See the description in the “Timer/Counter Control Register A – TCCR0A” on page 128.
• Bits 2:0 – CS02:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter.
Table 77. Clock Select Bit Description
CS02
CS01
CS00
Description
0
0
0
No clock source (Timer/Counter stopped)
0
0
1
clkI/O/(No prescaling)
0
1
0
clkI/O/8 (From prescaler)
0
1
1
clkI/O/64 (From prescaler)
1
0
0
clkI/O/256 (From prescaler)
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2549A–AVR–03/05
Table 77. Clock Select Bit Description (Continued)
CS02
CS01
CS00
Description
1
0
1
clkI/O/1024 (From prescaler)
1
1
0
External clock source on T0 pin. Clock on falling edge.
1
1
1
External clock source on T0 pin. Clock on rising edge.
If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will
clock the counter even if the pin is configured as an output. This feature allows software
control of the counting.
Timer/Counter Register –
TCNT0
Bit
7
6
5
4
3
2
1
0
TCNT0[7:0]
TCNT0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
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 OCR0x Registers.
Output Compare Register A –
OCR0A
Bit
7
6
5
4
3
2
1
0
OCR0A[7:0]
OCR0A
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
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.
Output Compare Register B –
OCR0B
Bit
7
6
5
4
3
2
1
0
OCR0B[7:0]
OCR0B
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
The Output Compare Register B 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 OC0B pin.
Timer/Counter Interrupt Mask
Register – TIMSK0
Bit
7
6
5
4
3
2
1
0
–
–
–
–
–
OCIE0B
OCIE0A
TOIE0
Read/Write
R
R
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TIMSK0
• Bits 7..3, 0 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bit 2 – OCIE0B: Timer/Counter Output Compare Match B Interrupt Enable
When the OCIE0B bit is written to one, and the I-bit in the Status Register is set, the
Timer/Counter Compare Match B interrupt is enabled. The corresponding interrupt is
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ATmega640/1280/1281/2560/2561
executed if a Compare Match in Timer/Counter occurs, i.e., when the OCF0B bit is set in
the Timer/Counter Interrupt Flag Register – TIFR0.
• 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, 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 Timer/Counter 0 Interrupt Flag Register – TIFR0.
• 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, the
Timer/Counter0 Overflow interrupt is enabled. The corresponding interrupt is executed if
an overflow in Timer/Counter0 occurs, i.e., when the TOV0 bit is set in the
Timer/Counter 0 Interrupt Flag Register – TIFR0.
Timer/Counter 0 Interrupt Flag
Register – TIFR0
Bit
7
6
5
4
3
2
1
0
–
–
–
–
–
OCF0B
OCF0A
TOV0
Read/Write
R
R
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TIFR0
• Bits 7..3, 0 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bit 2 – OCF0B: Timer/Counter 0 Output Compare B Match Flag
The OCF0B bit is set when a Compare Match occurs between the Timer/Counter and
the data in OCR0B – Output Compare Register0 B. OCF0B is cleared by hardware
when executing the corresponding interrupt handling vector. Alternatively, OCF0B is
cleared by writing a logic one to the flag. When the I-bit in SREG, OCIE0B
(Timer/Counter Compare B Match Interrupt Enable), and OCF0B are set, the
Timer/Counter Compare Match Interrupt is executed.
• Bit 1 – OCF0A: Timer/Counter 0 Output Compare A Match Flag
The OCF0A bit is set 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, the Timer/Counter0 Compare
Match Interrupt is executed.
• Bit 0 – TOV0: Timer/Counter0 Overflow Flag
The bit TOV0 is set when an overflow occurs in Timer/Counter0. TOV0 is cleared by
hardware when executing the corresponding interrupt handling vector. Alternatively,
TOV0 is cleared by writing a logic one to the flag. When the SREG I-bit, TOIE0
(Timer/Counter0 Overflow Interrupt Enable), and TOV0 are set, the Timer/Counter0
Overflow interrupt is executed.
The setting of this flag is dependent of the WGM02:0 bit setting. Refer to Table 76,
“Waveform Generation Mode Bit Description” on page 130.
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16-bit Timer/Counter
(Timer/Counter1,
Timer/Counter3,
Timer/Counter4 and
Timer/Counter5)
The 16-bit Timer/Counter unit allows accurate program execution timing (event management), wave generation, and signal timing measurement. The main features are:
• True 16-bit Design (i.e., Allows 16-bit PWM)
• Three 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
• Twenty independent interrupt sources (TOV1, OCF1A, OCF1B, OCF1C, ICF1, TOV3,
OCF3A, OCF3B, OCF3C, ICF3, TOV4, OCF4A, OCF4B, OCF4C, ICF4, TOV5, OCF5A,
OCF5B, OCF5C and ICF5)
Overview
Most register and bit references in this section are written in general form. A lower case
“n” replaces the Timer/Counter number, and a lower case “x” replaces the Output Compare unit channel. However, when using the register or bit defines in a program, the
precise form must be used, i.e., TCNT1 for accessing Timer/Counter1 counter value
and so on.
A simplified block diagram of the 16-bit Timer/Counter is shown in Figure 49. For the
actual placement of I/O pins, see “Pinout ATmega640/1280/2560” on page 2 and
“Pinout ATmega1281/2561” 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 “16-bit Timer/Counter Register Description” on page 157.
The Power Reduction Timer/Counter1 bit, PRTIM1, in “Power Reduction Register 0 PRR0” on page 54 must be written to zero to enable Timer/Counter1 module.
The Power Reduction Timer/Counter3 bit, PRTIM3, in “Power Reduction Register 1 PRR1” on page 55 must be written to zero to enable Timer/Counter3 module.
The Power Reduction Timer/Counter4 bit, PRTIM4, in “Power Reduction Register 1 PRR1” on page 55 must be written to zero to enable Timer/Counter4 module.
The Power Reduction Timer/Counter5 bit, PRTIM5, in “Power Reduction Register 1 PRR1” on page 55 must be written to zero to enable Timer/Counter5 module.
Timer/Counter4 and Timer/Counter5 only have full functionality in the
ATmega640/1280/2560.
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Figure 49. 16-bit Timer/Counter Block Diagram(1)
Count
Clear
Direction
TOVn
(Int.Req.)
Control Logic
TCLK
Clock Select
Edge
Detector
TOP
Tn
BOTTOM
( From Prescaler )
Timer/Counter
TCNTn
=
=0
OCFnA
(Int.Req.)
Waveform
Generation
=
OCnA
OCRnA
OCFnB
(Int.Req.)
Fixed
TOP
Values
Waveform
Generation
DATABUS
=
OCnB
OCRnB
OCFnC
(Int.Req.)
Waveform
Generation
=
OCnC
OCRnC
( From Analog
Comparator Ouput )
ICFn (Int.Req.)
Edge
Detector
ICRn
Noise
Canceler
ICPn
TCCRnA
Note:
Registers
TCCRnB
TCCRnC
1. Refer to Figure 1 on page 2, Table 38 on page 89, and Table 44 on page 93 for
Timer/Counter1 and 3 and 3 pin placement and description.
The Timer/Counter (TCNTn), Output Compare Registers (OCRnA/B/C), and Input Capture Register (ICRn) 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 137. The Timer/Counter Control Registers (TCCRnA/B/C)
are 8-bit registers and have no CPU access restrictions. Interrupt requests (shorten as
Int.Req.) signals are all visible in the Timer Interrupt Flag Register (TIFRn). All interrupts
are individually masked with the Timer Interrupt Mask Register (TIMSKn). TIFRn and
TIMSKn are not shown in the figure since these registers are shared by other timer
units.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock
source on the Tn 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 (clkTn).
The double buffered Output Compare Registers (OCRnA/B/C) are compared with the
Timer/Counter value at all time. The result of the compare can be used by the Waveform
Generator to generate a PWM or variable frequency output on the Output Compare pin
(OCnA/B/C). See “Output Compare Units” on page 144.. The compare match event will
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also set the Compare Match Flag (OCFnA/B/C) which can be used to generate an Output Compare interrupt request.
The Input Capture Register can capture the Timer/Counter value at a given external
(edge triggered) event on either the Input Capture pin (ICPn) or on the Analog Comparator pins (See “Analog Comparator” on page 271.) The Input Capture unit includes a
digital filtering unit (Noise Canceler) for reducing the chance of capturing noise spikes.
The TOP value, or maximum Timer/Counter value, can in some modes of operation be
defined by either the OCRnA Register, the ICRn Register, or by a set of fixed values.
When using OCRnA as TOP value in a PWM mode, the OCRnA 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 ICRn Register can be used as an alternative, freeing the OCRnA to be
used as PWM output.
Definitions
The following definitions are used extensively throughout the document:
Table 78. Definitions
Accessing 16-bit
Registers
BOTTOM
The counter reaches the BOTTOM when it becomes 0x0000.
MAX
The counter reaches its MAXimum when it becomes 0xFFFF (decimal 65535).
TOP
The counter reaches the TOP when it becomes equal to the highest value in the
count sequence. The TOP value can be assigned to be one of the fixed values:
0x00FF, 0x01FF, or 0x03FF, or to the value stored in the OCRnA or ICRn
Register. The assignment is dependent of the mode of operation.
The TCNTn, OCRnA/B/C, and ICRn 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 Temporary
Register in the same clock cycle as the low byte is read.
Not all 16-bit accesses uses the Temporary Register for the high byte. Reading the
OCRnA/B/C 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 OCRnA/B/C and ICRn Registers. Note that when using “C”,
the compiler handles the 16-bit access.
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Assembly Code Examples(1)
...
; Set TCNTn to 0x01FF
ldi r17,0x01
ldi r16,0xFF
out TCNTnH,r17
out TCNTnL,r16
; Read TCNTn into r17:r16
in
r16,TCNTnL
in
r17,TCNTnH
...
C Code Examples(1)
unsigned int i;
...
/* Set TCNTn to 0x01FF */
TCNTn = 0x1FF;
/* Read TCNTn into i */
i = TCNTn;
...
Note:
1. See “About Code Examples” on page 8.
The assembly code example returns the TCNTn 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 Registers, then the result of the access outside the interrupt will be corrupted.
Therefore, when both the main code and the interrupt code update the temporary register, the main code must disable the interrupts during the 16-bit access.
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The following code examples show how to do an atomic read of the TCNTn Register
contents. Reading any of the OCRnA/B/C or ICRn Registers can be done by using the
same principle.
Assembly Code Example(1)
TIM16_ReadTCNTn:
; Save global interrupt flag
in
r18,SREG
; Disable interrupts
cli
; Read TCNTn into r17:r16
in
r16,TCNTnL
in
r17,TCNTnH
; Restore global interrupt flag
out SREG,r18
ret
C Code Example(1)
unsigned int TIM16_ReadTCNTn( void )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
__disable_interrupt();
/* Read TCNTn into i */
i = TCNTn;
/* Restore global interrupt flag */
SREG = sreg;
return i;
}
Note:
1. See “About Code Examples” on page 8.
The assembly code example returns the TCNTn value in the r17:r16 register pair.
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The following code examples show how to do an atomic write of the TCNTn Register
contents. Writing any of the OCRnA/B/C or ICRn Registers can be done by using the
same principle.
Assembly Code Example(1)
TIM16_WriteTCNTn:
; Save global interrupt flag
in
r18,SREG
; Disable interrupts
cli
; Set TCNTn to r17:r16
out TCNTnH,r17
out TCNTnL,r16
; Restore global interrupt flag
out SREG,r18
ret
C Code Example(1)
void TIM16_WriteTCNTn( unsigned int i )
{
unsigned char sreg;
unsigned int i;
/* Save global interrupt flag */
sreg = SREG;
/* Disable interrupts */
__disable_interrupt();
/* Set TCNTn to i */
TCNTn = i;
/* Restore global interrupt flag */
SREG = sreg;
}
Note:
1. See “About Code Examples” on page 8.
The assembly code example requires that the r17:r16 register pair contains the value to
be written to TCNTn.
Reusing the Temporary High
Byte Register
140
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.
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Timer/Counter Clock
Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock
source is selected by the Clock Select logic which is controlled by the Clock Select
(CSn2:0) bits located in the Timer/Counter control Register B (TCCRnB). For details on
clock sources and prescaler, see “Timer/Counter0, Timer/Counter1, Timer/Counter3,
Timer/Counter4, and Timer/Counter5 Prescalers” on page 169.
Counter Unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional
counter unit. Figure 50 shows a block diagram of the counter and its surroundings.
Figure 50. Counter Unit Block Diagram
DATA BUS
(8-bit)
TOVn
(Int.Req.)
TEMP (8-bit)
Clock Select
Count
TCNTnH (8-bit)
TCNTnL (8-bit)
TCNTn (16-bit Counter)
Clear
Direction
Control Logic
clkTn
Edge
Detector
Tn
( From Prescaler )
TOP
BOTTOM
Signal description (internal signals):
Count
Increment or decrement TCNTn by 1.
Direction
Select between increment and decrement.
Clear
Clear TCNTn (set all bits to zero).
clkTn
Timer/Counter clock.
TOP
Signalize that TCNTn has reached maximum value.
BOTTOM
Signalize that TCNTn has reached minimum value (zero).
The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High
(TCNTnH) containing the upper eight bits of the counter, and Counter Low (TCNTnL)
containing the lower eight bits. The TCNTnH Register can only be indirectly accessed
by the CPU. When the CPU does an access to the TCNTnH I/O location, the CPU
accesses the high byte temporary register (TEMP). The temporary register is updated
with the TCNTnH value when the TCNTnL is read, and TCNTnH is updated with the
temporary register value when TCNTnL 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 TCNTn 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 (clkTn). The clkTn can be generated from an external or
internal clock source, selected by the Clock Select bits (CSn2:0). When no clock source
is selected (CSn2:0 = 0) the timer is stopped. However, the TCNTn value can be
accessed by the CPU, independent of whether clkTn 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 Waveform Generation mode
bits (WGMn3:0) located in the Timer/Counter Control Registers A and B (TCCRnA and
TCCRnB). There are close connections between how the counter behaves (counts) and
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how waveforms are generated on the Output Compare outputs OCnx. For more details
about advanced counting sequences and waveform generation, see “Modes of Operation” on page 147.
The Timer/Counter Overflow Flag (TOVn) is set according to the mode of operation
selected by the WGMn3:0 bits. TOVn can be used for generating a CPU interrupt.
Input Capture Unit
The Timer/Counter incorporates an input capture unit that can capture external events
and give them a time-stamp indicating time of occurrence. The external signal indicating
an event, or multiple events, can be applied via the ICPn pin or alternatively, for the
Timer/Counter1 only, via the Analog Comparator unit. The time-stamps can then be
used to calculate frequency, duty-cycle, and other features of the signal applied. Alternatively the time-stamps can be used for creating a log of the events.
The Input Capture unit is illustrated by the block diagram shown in Figure 51. 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 51. Input Capture Unit Block Diagram
DATA BUS
(8-bit)
TEMP (8-bit)
ICRnH (8-bit)
WRITE
ICRnL (8-bit)
TCNTnH (8-bit)
ICRn (16-bit Register)
ACO*
ACIC*
Analog
Comparator
TCNTnL (8-bit)
TCNTn (16-bit Counter)
ICNC
ICES
Noise
Canceler
Edge
Detector
ICFn (Int.Req.)
ICPn
Note:
The Analog Comparator Output (ACO) can only trigger the Timer/Counter1 ICP – not
Timer/Counter3, 4 or 5.
When a change of the logic level (an event) occurs on the Input Capture Pin (ICPn),
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 (TCNTn) is written to the Input Capture Register (ICRn). The
Input Capture Flag (ICFn) is set at the same system clock as the TCNTn value is copied
into ICRn Register. If enabled (TICIEn = 1), the input capture flag generates an input
capture interrupt. The ICFn flag is automatically cleared when the interrupt is executed.
Alternatively the ICFn flag can be cleared by software by writing a logical one to its I/O
bit location.
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Reading the 16-bit value in the Input Capture Register (ICRn) is done by first reading the
low byte (ICRnL) and then the high byte (ICRnH). When the low byte is read the high
byte is copied into the high byte Temporary Register (TEMP). When the CPU reads the
ICRnH I/O location it will access the TEMP Register.
The ICRn Register can only be written when using a Waveform Generation mode that
utilizes the ICRn Register for defining the counter’s TOP value. In these cases the
Waveform Generation mode (WGMn3:0) bits must be set before the TOP value can be
written to the ICRn Register. When writing the ICRn Register the high byte must be written to the ICRnH I/O location before the low byte is written to ICRnL.
For more information on how to access the 16-bit registers refer to “Accessing 16-bit
Registers” on page 137.
Input Capture Trigger Source
The main trigger source for the input capture unit is the Input Capture Pin (ICPn).
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 (ICPn) and the Analog Comparator output (ACO) inputs are
sampled using the same technique as for the Tn pin (Figure 62 on page 169). The edge
detector is also identical. However, when the noise canceler is enabled, additional logic
is inserted before the edge detector, which increases the delay by four system clock
cycles. Note that the input of the noise canceler and edge detector is always enabled
unless the Timer/Counter is set in a Waveform Generation mode that uses ICRn to
define TOP.
An input capture can be triggered by software by controlling the port of the ICPn pin.
Noise Canceler
The noise canceler improves noise immunity by using a simple digital filtering scheme.
The noise canceler input is monitored over four samples, and all four must be equal for
changing the output that in turn is used by the edge detector.
The noise canceler is enabled by setting the Input Capture Noise Canceler (ICNCn) bit
in Timer/Counter Control Register B (TCCRnB). When enabled the noise canceler introduces additional four system clock cycles of delay from a change applied to the input, to
the update of the ICRn Register. The noise canceler uses the system clock and is therefore not affected by the prescaler.
Using the Input Capture Unit
The main challenge when using the Input Capture unit is to assign enough processor
capacity for handling the incoming events. The time between two events is critical. If the
processor has not read the captured value in the ICRn Register before the next event
occurs, the ICRn 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 ICRn Register should be read as early in the
interrupt 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 ICRn Register has been read. After a change of the edge, the Input Capture Flag
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(ICFn) must be cleared by software (writing a logical one to the I/O bit location). For
measuring frequency only, the clearing of the ICFn Flag is not required (if an interrupt
handler is used).
Output Compare Units
The 16-bit comparator continuously compares TCNTn with the Output Compare Register (OCRnx). If TCNT equals OCRnx the comparator signals a match. A match will set
the Output Compare Flag (OCFnx) at the next timer clock cycle. If enabled (OCIEnx =
1), the Output Compare Flag generates an Output Compare interrupt. The OCFnx Flag
is automatically cleared when the interrupt is executed. Alternatively the OCFnx Flag
can be cleared by software by writing a logical one to its I/O bit location. The Waveform
Generator uses the match signal to generate an output according to operating mode set
by the Waveform Generation mode (WGMn3:0) bits and Compare Output mode
(COMnx1:0) bits. The TOP and BOTTOM signals are used by the Waveform Generator
for handling the special cases of the extreme values in some modes of operation (See
“Modes of Operation” on page 147.)
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 52 shows a block diagram of the Output Compare unit. The small “n” in the register and bit names indicates the device number (n = n for Timer/Counter n), and the “x”
indicates Output Compare unit (A/B/C). The elements of the block diagram that are not
directly a part of the Output Compare unit are gray shaded.
Figure 52. Output Compare Unit, Block Diagram
DATA BUS
(8-bit)
TEMP (8-bit)
OCRnxH Buf. (8-bit)
OCRnxL Buf. (8-bit)
TCNTnH (8-bit)
OCRnx Buffer (16-bit Register)
OCRnxH (8-bit)
TCNTnL (8-bit)
TCNTn (16-bit Counter)
OCRnxL (8-bit)
OCRnx (16-bit Register)
= (16-bit Comparator )
OCFnx (Int.Req.)
TOP
BOTTOM
Waveform Generator
WGMn3:0
OCnx
COMnx1:0
The OCRnx 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 OCRnx Compare Register to either TOP or BOTTOM of the counting
sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical
PWM pulses, thereby making the output glitch-free.
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The OCRnx Register access may seem complex, but this is not case. When the double
buffering is enabled, the CPU has access to the OCRnx Buffer Register, and if double
buffering is disabled the CPU will access the OCRnx 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
OCRnx Registers must be done via the TEMP Register since the compare of all 16 bits
is done continuously. The high byte (OCRnxH) 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 (OCRnxL) is written to the lower eight bits, the high byte
will be copied into the upper 8-bits of either the OCRnx buffer or OCRnx 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 137.
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 (FOCnx) bit. Forcing compare
match will not set the OCFnx Flag or reload/clear the timer, but the OCnx pin will be
updated as if a real compare match had occurred (the COMn1:0 bits settings define
whether the OCnx pin is set, cleared or toggled).
Compare Match Blocking by
TCNTn Write
All CPU writes to the TCNTn Register will block any compare match that occurs in the
next timer clock cycle, even when the timer is stopped. This feature allows OCRnx to be
initialized to the same value as TCNTn without triggering an interrupt when the
Timer/Counter clock is enabled.
Using the Output Compare
Unit
Since writing TCNTn in any mode of operation will block all compare matches for one
timer clock cycle, there are risks involved when changing TCNTn when using any of the
Output Compare channels, independent of whether the Timer/Counter is running or not.
If the value written to TCNTn equals the OCRnx value, the compare match will be
missed, resulting in incorrect waveform generation. Do not write the TCNTn 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 TCNTn value
equal to BOTTOM when the counter is downcounting.
The setup of the OCnx should be performed before setting the Data Direction Register
for the port pin to output. The easiest way of setting the OCnx value is to use the Force
Output Compare (FOCnx) strobe bits in Normal mode. The OCnx Register keeps its
value even when changing between Waveform Generation modes.
Be aware that the COMnx1:0 bits are not double buffered together with the compare
value. Changing the COMnx1:0 bits will take effect immediately.
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Compare Match Output
Unit
The Compare Output mode (COMnx1:0) bits have two functions. The Waveform Generator uses the COMnx1:0 bits for defining the Output Compare (OCnx) state at the next
compare match. Secondly the COMnx1:0 bits control the OCnx pin output source. Figure 53 shows a simplified schematic of the logic affected by the COMnx1:0 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
COMnx1:0 bits are shown. When referring to the OCnx state, the reference is for the
internal OCnx Register, not the OCnx pin. If a system reset occur, the OCnx Register is
reset to “0”.
Figure 53. Compare Match Output Unit, Schematic
COMnx1
COMnx0
FOCnx
Waveform
Generator
D
Q
1
OCnx
DATA BUS
D
0
OCnx
Pin
Q
PORT
D
Q
DDR
clk I/O
The general I/O port function is overridden by the Output Compare (OCnx) from the
Waveform Generator if either of the COMnx1:0 bits are set. However, the OCnx pin
direction (input or output) is still controlled by the Data Direction Register (DDR) for the
port pin. The Data Direction Register bit for the OCnx pin (DDR_OCnx) must be set as
output before the OCnx value is visible on the pin. The port override function is generally
independent of the Waveform Generation mode, but there are some exceptions. Refer
to Table 79, Table 80 and Table 81 for details.
The design of the Output Compare pin logic allows initialization of the OCnx state before
the output is enabled. Note that some COMnx1:0 bit settings are reserved for certain
modes of operation. See “16-bit Timer/Counter Register Description” on page 157.
The COMnx1:0 bits have no effect on the Input Capture unit.
Compare Output Mode and
Waveform Generation
146
The Waveform Generator uses the COMnx1:0 bits differently in normal, CTC, and PWM
modes. For all modes, setting the COMnx1:0 = 0 tells the Waveform Generator that no
action on the OCnx Register is to be performed on the next compare match. For compare output actions in the non-PWM modes refer to Table 79 on page 158. For fast
PWM mode refer to Table 80 on page 158, and for phase correct and phase and frequency correct PWM refer to Table 81 on page 159.
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A change of the COMnx1:0 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 FOCnx strobe bits.
Modes of Operation
The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare
pins, is defined by the combination of the Waveform Generation mode (WGMn3:0) and
Compare Output mode (COMnx1:0) bits. The Compare Output mode bits do not affect
the counting sequence, while the Waveform Generation mode bits do. The COMnx1:0
bits control whether the PWM output generated should be inverted or not (inverted or
non-inverted PWM). For non-PWM modes the COMnx1:0 bits control whether the output should be set, cleared or toggle at a compare match (See “Compare Match Output
Unit” on page 146.)
For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 154.
Normal Mode
The simplest mode of operation is the Normal mode (WGMn3:0 = 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 (TOVn) will be set in the same timer clock cycle as the TCNTn becomes zero.
The TOVn 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 TOVn
Flag, the timer resolution can be increased by software. There are no special cases to
consider in the Normal mode, a new counter value can be written anytime.
The Input Capture unit is easy to use in Normal mode. However, observe that the 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.
Clear Timer on Compare
Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGMn3:0 = 4 or 12), the OCRnA or ICRn
Register are used to manipulate the counter resolution. In CTC mode the counter is
cleared to zero when the counter value (TCNTn) matches either the OCRnA (WGMn3:0
= 4) or the ICRn (WGMn3:0 = 12). The OCRnA or ICRn define the top value for the
counter, hence also its resolution. This mode allows greater control of the compare
match output frequency. It also simplifies the operation of counting external events.
The timing diagram for the CTC mode is shown in Figure 54. The counter value
(TCNTn) increases until a compare match occurs with either OCRnA or ICRn, and then
counter (TCNTn) is cleared.
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Figure 54. CTC Mode, Timing Diagram
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
TCNTn
OCnA
(Toggle)
Period
(COMnA1:0 = 1)
1
2
3
4
An interrupt can be generated at each time the counter value reaches the TOP value by
either using the OCFnA or ICFn Flag according to the register used to define the TOP
value. If the interrupt is enabled, the interrupt handler routine can be used for updating
the TOP value. However, changing the TOP to a value close to BOTTOM when the
counter is running with none or a low prescaler value must be done with care since the
CTC mode does not have the double buffering feature. If the new value written to
OCRnA or ICRn is lower than the current value of TCNTn, the counter will miss the compare match. The counter will then have to count to its maximum value (0xFFFF) and
wrap around starting at 0x0000 before the compare match can occur. In many cases
this feature is not desirable. An alternative will then be to use the fast PWM mode using
OCRnA for defining TOP (WGMn3:0 = 15) since the OCRnA then will be double
buffered.
For generating a waveform output in CTC mode, the OCnA output can be set to toggle
its logical level on each compare match by setting the Compare Output mode bits to toggle mode (COMnA1:0 = 1). The OCnA value will not be visible on the port pin unless the
data direction for the pin is set to output (DDR_OCnA = 1). The waveform generated will
have a maximum frequency of fOCnA = fclk_I/O/2 when OCRnA is set to zero (0x0000). The
waveform frequency is defined by the following equation:
f clk_I/O
f OCnA = -------------------------------------------------2 ⋅ N ⋅ ( 1 + OCRnA )
The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOVn Flag is set in the same timer clock cycle
that the counter counts from MAX to 0x0000.
Fast PWM Mode
148
The fast Pulse Width Modulation or fast PWM mode (WGMn3:0 = 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 (OCnx) is set on the compare match between TCNTn and OCRnx, and
cleared at TOP. In inverting Compare Output mode output is cleared on compare match
and set at TOP. Due to the single-slope operation, the operating frequency of the fast
PWM mode can be twice as high as the phase correct and phase and frequency correct
PWM modes that use dual-slope operation. This high frequency makes the fast PWM
mode well suited for power regulation, rectification, and DAC applications. High frequency allows physically small sized external components (coils, capacitors), 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
ICRn or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to
0x0003), and the maximum resolution is 16-bit (ICRn or OCRnA 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 (WGMn3:0 = 5, 6, or 7), the value in
ICRn (WGMn3:0 = 14), or the value in OCRnA (WGMn3:0 = 15). The counter is then
cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is
shown in Figure 55. The figure shows fast PWM mode when OCRnA or ICRn is used to
define TOP. The TCNTn 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 TCNTn slopes represent compare
matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a compare match occurs.
Figure 55. 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
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
4
5
6
7
8
The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches TOP. In
addition the OCnA or ICFn Flag is set at the same timer clock cycle as TOVn is set
when either OCRnA or ICRn is used for defining the TOP value. If one of the interrupts
are enabled, the interrupt handler routine can be used for updating the TOP and 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
TCNTn and the OCRnx. Note that when using fixed TOP values the unused bits are
masked to zero when any of the OCRnx Registers are written.
The procedure for updating ICRn differs from updating OCRnA when used for defining
the TOP value. The ICRn Register is not double buffered. This means that if ICRn 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 ICRn value written is lower than the current value of TCNTn.
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 OCRnA Register however, is
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double buffered. This feature allows the OCRnA I/O location to be written anytime.
When the OCRnA I/O location is written the value written will be put into the OCRnA
Buffer Register. The OCRnA Compare Register will then be updated with the value in
the Buffer Register at the next timer clock cycle the TCNTn matches TOP. The update is
done at the same timer clock cycle as the TCNTn is cleared and the TOVn Flag is set.
Using the ICRn Register for defining TOP works well when using fixed TOP values. By
using ICRn, the OCRnA Register is free to be used for generating a PWM output on
OCnA. However, if the base PWM frequency is actively changed (by changing the TOP
value), using the OCRnA 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
OCnx pins. Setting the COMnx1:0 bits to two will produce a non-inverted PWM and an
inverted PWM output can be generated by setting the COMnx1:0 to three (see Table on
page 158). The actual OCnx value will only be visible on the port pin if the data direction
for the port pin is set as output (DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare match between OCRnx and TCNTn,
and clearing (or setting) the OCnx 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 OCRnx Register represents special cases when generating
a PWM waveform output in the fast PWM mode. If the OCRnx is set equal to BOTTOM
(0x0000) the output will be a narrow spike for each TOP+1 timer clock cycle. Setting the
OCRnx equal to TOP will result in a constant high or low output (depending on the polarity of the output set by the COMnx1:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved
by setting OCnA to toggle its logical level on each compare match (COMnA1:0 = 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 fOCnA = fclk_I/O/2 when OCRnA is set to
zero (0x0000). This feature is similar to the OCnA toggle in CTC mode, except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode.
Phase Correct PWM Mode
The phase correct Pulse Width Modulation or phase correct PWM mode (WGMn3:0 = 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 (OCnx) is cleared on the compare match between TCNTn
and OCRnx while upcounting, and set on the compare match while downcounting. In
inverting Output Compare mode, the operation is inverted. The dual-slope operation has
lower maximum operation frequency than single slope operation. However, due to the
symmetric feature of the dual-slope PWM modes, these modes are preferred for motor
control applications.
The PWM resolution for the phase correct PWM mode can be fixed to 8-, 9-, or 10-bit, or
defined by either ICRn or OCRnA. The minimum resolution allowed is 2-bit (ICRn or
OCRnA set to 0x0003), and the maximum resolution is 16-bit (ICRn or OCRnA set to
MAX). The PWM resolution in bits can be calculated by using the following equation:
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( TOP + 1 )
R PCPWM = log
----------------------------------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 (WGMn3:0 = 1, 2, or 3), the
value in ICRn (WGMn3:0 = 10), or the value in OCRnA (WGMn3:0 = 11). The counter
has then reached the TOP and changes the count direction. The TCNTn value will be
equal to TOP for one timer clock cycle. The timing diagram for the phase correct PWM
mode is shown on Figure 56. The figure shows phase correct PWM mode when OCRnA
or ICRn is used to define TOP. The TCNTn 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 TCNTn slopes represent compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be
set when a compare match occurs.
Figure 56. 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
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
4
The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches BOTTOM. When either OCRnA or ICRn is used for defining the TOP value, the OCnA or
ICFn Flag is set accordingly at the same timer clock cycle as the OCRnx 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
TCNTn and the OCRnx. Note that when using fixed TOP values, the unused bits are
masked to zero when any of the OCRnx Registers are written. As the third period shown
in Figure 56 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 OCRnx Register. Since the OCRnx update occurs
at TOP, the PWM period starts and ends at TOP. This implies that the length of the falling slope is determined by the previous TOP value, while the length of the rising slope is
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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 OCnx pins. Setting the COMnx1:0 bits to two will produce a non-inverted PWM and
an inverted PWM output can be generated by setting the COMnx1:0 to three (See Table
81 on page 159). The actual OCnx value will only be visible on the port pin if the data
direction for the port pin is set as output (DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare match between OCRnx
and TCNTn when the counter increments, and clearing (or setting) the OCnx Register at
compare match between OCRnx and TCNTn 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 OCRnx Register represent special cases when generating a
PWM waveform output in the phase correct PWM mode. If the OCRnx is set equal to
BOTTOM the output will be continuously low and if set equal to TOP the output will be
continuously high for non-inverted PWM mode. For inverted PWM the output will have
the opposite logic values. If OCR1A is used to define the TOP value (WGM13:0 = 11)
and COM1A1:0 = 1, the OC1A output will toggle with a 50% duty cycle.
Phase and Frequency Correct
PWM Mode
The phase and frequency correct Pulse Width Modulation, or phase and frequency correct PWM mode (WGMn3:0 = 8 or 9) provides a high resolution phase and frequency
correct PWM waveform generation option. The phase and frequency correct PWM
mode is, like the phase correct PWM mode, based on a dual-slope operation. The
counter counts repeatedly from BOTTOM (0x0000) to TOP and then from TOP to BOTTOM. In non-inverting Compare Output mode, the Output Compare (OCnx) is cleared
on the compare match between TCNTn and OCRnx while upcounting, and set on the
compare match while downcounting. In inverting Compare Output mode, the operation
is inverted. The dual-slope operation gives a lower maximum operation frequency compared to the single-slope operation. However, due to the symmetric feature of the dualslope 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 OCRnx Register is updated by the OCRnx Buffer Register,
(see Figure 56 and Figure 57).
The PWM resolution for the phase and frequency correct PWM mode can be defined by
either ICRn or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to
0x0003), and the maximum resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM
resolution in bits can be calculated using the following equation:
log ( TOP + 1 )
R PFCPWM = ----------------------------------log ( 2 )
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In phase and frequency correct PWM mode the counter is incremented until the counter
value matches either the value in ICRn (WGMn3:0 = 8), or the value in OCRnA
(WGMn3:0 = 9). The counter has then reached the TOP and changes the count direction. The TCNTn 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 57.
The figure shows phase and frequency correct PWM mode when OCRnA or ICRn is
used to define TOP. The TCNTn 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 TCNTn slopes represent compare
matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a compare match occurs.
Figure 57. 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
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
4
The Timer/Counter Overflow Flag (TOVn) is set at the same timer clock cycle as the
OCRnx Registers are updated with the double buffer value (at BOTTOM). When either
OCRnA or ICRn is used for defining the TOP value, the OCnA or ICFn Flag set when
TCNTn 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
TCNTn and the OCRnx.
As Figure 57 shows the output generated is, in contrast to the phase correct mode, symmetrical in all periods. Since the OCRnx Registers are updated at BOTTOM, the length
of the rising and the falling slopes will always be equal. This gives symmetrical output
pulses and is therefore frequency correct.
Using the ICRn Register for defining TOP works well when using fixed TOP values. By
using ICRn, the OCRnA Register is free to be used for generating a PWM output on
OCnA. However, if the base PWM frequency is actively changed by changing the TOP
value, using the OCRnA as TOP is clearly a better choice due to its double buffer
feature.
In phase and frequency correct PWM mode, the compare units allow generation of
PWM waveforms on the OCnx pins. Setting the COMnx1:0 bits to two will produce a
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non-inverted PWM and an inverted PWM output can be generated by setting the
COMnx1:0 to three (See Table 81 on page 159). The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output (DDR_OCnx). The
PWM waveform is generated by setting (or clearing) the OCnx Register at the compare
match between OCRnx and TCNTn when the counter increments, and clearing (or setting) the OCnx Register at compare match between OCRnx and TCNTn 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 OCRnx Register represents special cases when generating
a PWM waveform output in the phase correct PWM mode. If the OCRnx is set equal to
BOTTOM the output will be continuously low and if set equal to TOP the output will be
set to high for non-inverted PWM mode. For inverted PWM the output will have the
opposite logic values. If OCR1A is used to define the TOP value (WGM13:0 = 9) and
COM1A1:0 = 1, the OC1A output will toggle with a 50% duty cycle.
Timer/Counter Timing
Diagrams
The Timer/Counter is a synchronous design and the timer clock (clkTn) 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 OCRnx Register is updated with the
OCRnx buffer value (only for modes utilizing double buffering). Figure 58 shows a timing
diagram for the setting of OCFnx.
Figure 58. Timer/Counter Timing Diagram, Setting of OCFnx, no Prescaling
clkI/O
clkTn
(clkI/O /1)
TCNTn
OCRnx - 1
OCRnx
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
OCFnx
Figure 59 shows the same timing data, but with the prescaler enabled.
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Figure 59. Timer/Counter Timing Diagram, Setting of OCFnx, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
OCRnx - 1
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
OCRnx
OCFnx
Figure 60 shows the count sequence close to TOP in various modes. When using phase
and frequency correct PWM mode the OCRnx 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 TOVn Flag
at BOTTOM.
Figure 60. Timer/Counter Timing Diagram, no Prescaling
clkI/O
clkTn
(clkI/O /1)
TCNTn
(CTC and FPWM)
TCNTn
(PC and PFC PWM)
TOP - 1
TOP
BOTTOM
BOTTOM + 1
TOP - 1
TOP
TOP - 1
TOP - 2
TOVn (FPWM)
and ICFn (if used
as TOP)
OCRnx
(Update at TOP)
Old OCRnx Value
New OCRnx Value
Figure 61 shows the same timing data, but with the prescaler enabled.
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Figure 61. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O/8)
TCNTn
(CTC and FPWM)
TCNTn
(PC and PFC PWM)
TOP - 1
TOP
BOTTOM
BOTTOM + 1
TOP - 1
TOP
TOP - 1
TOP - 2
TOVn (FPWM)
and ICF n (if used
as TOP)
OCRnx
(Update at TOP)
156
Old OCRnx Value
New OCRnx Value
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ATmega640/1280/1281/2560/2561
16-bit Timer/Counter
Register Description
Timer/Counter1 Control
Register A – TCCR1A
Timer/Counter3 Control
Register A – TCCR3A
Timer/Counter4 Control
Register A – TCCR4A
Timer/Counter5 Control
Register A – TCCR5A
Bit
7
6
5
4
3
2
1
0
COM1A1
COM1A0
COM1B1
COM1B0
COM1C1
COM1C0
WGM11
WGM10
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
COM3A1
COM3A0
COM3B1
COM3B0
COM3C1
COM3C0
WGM31
WGM30
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
COM4A1
COM4A0
COM4B1
COM4B0
COM4C1
COM4C0
WGM41
WGM40
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
COM5A1
COM5A0
COM5B1
COM5B0
COM5C1
COM5C0
WGM51
WGM50
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TCCR1A
TCCR3A
TCCR4A
TCCR5A
• Bit 7:6 – COMnA1:0: Compare Output Mode for Channel A
• Bit 5:4 – COMnB1:0: Compare Output Mode for Channel B
• Bit 3:2 – COMnC1:0: Compare Output Mode for Channel C
The COMnA1:0, COMnB1:0, and COMnC1:0 control the output compare pins (OCnA,
OCnB, and OCnC respectively) behavior. If one or both of the COMnA1:0 bits are written to one, the OCnA output overrides the normal port functionality of the I/O pin it is
connected to. If one or both of the COMnB1:0 bits are written to one, the OCnB output
overrides the normal port functionality of the I/O pin it is connected to. If one or both of
the COMnC1:0 bits are written to one, the OCnC 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 OCnA, OCnB or OCnC pin must be set in order to
enable the output driver.
When the OCnA, OCnB or OCnC is connected to the pin, the function of the COMnx1:0
bits is dependent of the WGMn3:0 bits setting. Table 79 shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to a normal or a CTC mode (non-PWM).
157
2549A–AVR–03/05
.
Table 79. Compare Output Mode, non-PWM
COMnA1/COMnB1/
COMnC1
COMnA0/COMnB0/
COMnC0
0
0
Normal port operation, OCnA/OCnB/OCnC
disconnected.
0
1
Toggle OCnA/OCnB/OCnC on compare
match.
1
0
Clear OCnA/OCnB/OCnC on compare
match (set output to low level).
1
1
Set OCnA/OCnB/OCnC on compare match
(set output to high level).
Description
Table 80 shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the
fast PWM mode.
Table 80. Compare Output Mode, Fast PWM
COMnA1/COMnB1/
COMnC0
COMnA0/COMnB0/
COMnC0
0
0
Normal port operation, OCnA/OCnB/OCnC
disconnected.
0
1
WGM13:0 = 14 or 15: Toggle OC1A on
Compare Match, OC1B and OC1C
disconnected (normal port operation). For all
other WGM1 settings, normal port operation,
OC1A/OC1B/OC1C disconnected.
1
0
Clear OCnA/OCnB/OCnC on compare
match, set OCnA/OCnB/OCnC at TOP
1
1
Set OCnA/OCnB/OCnC on compare match,
clear OCnA/OCnB/OCnC at TOP
Note:
Description
A special case occurs when OCRnA/OCRnB/OCRnC equals TOP and
COMnA1/COMnB1/COMnC1 is set. In this case the compare match is ignored, but the
set or clear is done at TOP. See “Fast PWM Mode” on page 148. for more details.
Table 81 shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the
phase correct and frequency correct PWM mode.
158
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 81. Compare Output Mode, Phase Correct and Phase and Frequency Correct
PWM
COMnA1/COMnB/
COMnC1
COMnA0/COMnB0/
COMnC0
0
0
Normal port operation, OCnA/OCnB/OCnC
disconnected.
0
1
WGM13:0 = 8, 9 10 or 11: Toggle OC1A on
Compare Match, OC1B and OC1C
disconnected (normal port operation). For all
other WGM1 settings, normal port operation,
OC1A/OC1B/OC1C disconnected.
1
0
Clear OCnA/OCnB/OCnC on compare
match when up-counting. Set
OCnA/OCnB/OCnC on compare match
when downcounting.
1
1
Set OCnA/OCnB/OCnC on compare match
when up-counting. Clear
OCnA/OCnB/OCnC on compare match
when downcounting.
Note:
Description
A special case occurs when OCRnA/OCRnB/OCRnC equals TOP and
COMnA1/COMnB1//COMnC1 is set. See “Phase Correct PWM Mode” on page 150. for
more details.
• Bit 1:0 – WGMn1:0: Waveform Generation Mode
Combined with the WGMn3:2 bits found in the TCCRnB Register, these bits control the
counting sequence of the counter, the source for maximum (TOP) counter value, and
what type of waveform generation to be used, see Table 82. 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 147.).
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2549A–AVR–03/05
Table 82. Waveform Generation Mode Bit Description(1)
Mode
WGMn3
WGMn2
(CTCn)
WGMn1
(PWMn1)
WGMn0
(PWMn0)
Timer/Counter Mode of
Operation
TOP
Update of
OCRnx at
TOVn Flag
Set on
0
0
0
0
0
Normal
0xFFFF
Immediate
MAX
1
0
0
0
1
PWM, Phase Correct, 8-bit
0x00FF
TOP
BOTTOM
2
0
0
1
0
PWM, Phase Correct, 9-bit
0x01FF
TOP
BOTTOM
3
0
0
1
1
PWM, Phase Correct, 10-bit
0x03FF
TOP
BOTTOM
4
0
1
0
0
CTC
OCRnA
Immediate
MAX
5
0
1
0
1
Fast PWM, 8-bit
0x00FF
TOP
TOP
6
0
1
1
0
Fast PWM, 9-bit
0x01FF
TOP
TOP
7
0
1
1
1
Fast PWM, 10-bit
0x03FF
TOP
TOP
8
1
0
0
0
PWM, Phase and Frequency
Correct
ICRn
BOTTOM
BOTTOM
9
1
0
0
1
PWM, Phase and Frequency
Correct
OCRnA
BOTTOM
BOTTOM
10
1
0
1
0
PWM, Phase Correct
ICRn
TOP
BOTTOM
11
1
0
1
1
PWM, Phase Correct
OCRnA
TOP
BOTTOM
12
1
1
0
0
CTC
ICRn
Immediate
MAX
13
1
1
0
1
(Reserved)
–
–
–
14
1
1
1
0
Fast PWM
ICRn
TOP
TOP
15
1
1
1
1
Fast PWM
OCRnA
TOP
TOP
Note:
1. The CTCn and PWMn1:0 bit definition names are obsolete. Use the WGMn2:0 definitions. However, the functionality and
location of these bits are compatible with previous versions of the timer.
Timer/Counter1 Control
Register B – TCCR1B
Timer/Counter3 Control
Register B – TCCR3B
Timer/Counter4 Control
Register B – TCCR4B
160
Bit
7
6
5
4
3
2
1
0
ICNC1
ICES1
–
WGM13
WGM12
CS12
CS11
CS10
Read/Write
R/W
R/W
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
ICNC3
ICES3
–
WGM33
WGM32
CS32
CS31
CS30
Read/Write
R/W
R/W
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
ICNC4
ICES4
–
WGM43
WGM42
CS42
CS41
CS40
Read/Write
R/W
R/W
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TCCR1B
TCCR3B
TCCR4B
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Timer/Counter5 Control
Register B – TCCR5B
Bit
7
6
5
4
3
2
1
0
ICNC5
ICES5
–
WGM53
WGM52
CS52
CS51
CS50
Read/Write
R/W
R/W
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TCCR5B
• Bit 7 – ICNCn: 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 (ICPn) is filtered. The filter
function requires four successive equal valued samples of the ICPn pin for changing its
output. The input capture is therefore delayed by four Oscillator cycles when the noise
canceler is enabled.
• Bit 6 – ICESn: Input Capture Edge Select
This bit selects which edge on the Input Capture Pin (ICPn) that is used to trigger a capture event. When the ICESn bit is written to zero, a falling (negative) edge is used as
trigger, and when the ICESn bit is written to one, a rising (positive) edge will trigger the
capture.
When a capture is triggered according to the ICESn setting, the counter value is copied
into the Input Capture Register (ICRn). The event will also set the Input Capture Flag
(ICFn), and this can be used to cause an Input Capture Interrupt, if this interrupt is
enabled.
When the ICRn is used as TOP value (see description of the WGMn3:0 bits located in
the TCCRnA and the TCCRnB Register), the ICPn is disconnected and consequently
the input capture function is disabled.
• Bit 5 – Reserved Bit
This bit is reserved for future use. For ensuring compatibility with future devices, this bit
must be written to zero when TCCRnB is written.
• Bit 4:3 – WGMn3:2: Waveform Generation Mode
See TCCRnA Register description.
• Bit 2:0 – CSn2:0: Clock Select
The three clock select bits select the clock source to be used by the Timer/Counter, see
Figure 58 and Figure 59.
161
2549A–AVR–03/05
Table 83. Clock Select Bit Description
CSn2
CSn1
CSn0
Description
0
0
0
No clock source. (Timer/Counter stopped)
0
0
1
clkI/O/1 (No prescaling
0
1
0
clkI/O/8 (From prescaler)
0
1
1
clkI/O/64 (From prescaler)
1
0
0
clkI/O/256 (From prescaler)
1
0
1
clkI/O/1024 (From prescaler)
1
1
0
External clock source on Tn pin. Clock on falling edge
1
1
1
External clock source on Tn pin. Clock on rising edge
If external pin modes are used for the Timer/Countern, transitions on the Tn pin will
clock the counter even if the pin is configured as an output. This feature allows software
control of the counting.
Timer/Counter1 Control
Register C – TCCR1C
Timer/Counter3 Control
Register C – TCCR3C
Timer/Counter4 Control
Register C – TCCR4C
Timer/Counter5 Control
Register C – TCCR5C
Bit
7
6
5
4
3
2
1
0
FOC1A
FOC1B
FOC1C
–
–
–
–
–
Read/Write
W
W
W
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
FOC3A
FOC3B
FOC3C
–
–
–
–
–
Read/Write
W
W
W
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
FOC4A
FOC4B
FOC4C
–
–
–
–
–
Read/Write
W
W
W
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
FOC5A
FOC5B
FOC3C
–
–
–
–
–
Read/Write
W
W
W
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
TCCR1C
TCCR3C
TCCR4C
TCCR5C
• Bit 7 – FOCnA: Force Output Compare for Channel A
• Bit 6 – FOCnB: Force Output Compare for Channel B
• Bit 5 – FOCnC: Force Output Compare for Channel C
The FOCnA/FOCnB/FOCnC bits are only active when the WGMn3:0 bits specifies a
non-PWM mode. When writing a logical one to the FOCnA/FOCnB/FOCnC bit, an
immediate compare match is forced on the waveform generation unit. The
OCnA/OCnB/OCnC output is changed according to its COMnx1:0 bits setting. Note that
the FOCnA/FOCnB/FOCnC bits are implemented as strobes. Therefore it is the value
present in the COMnx1:0 bits that determine the effect of the forced compare.
A FOCnA/FOCnB/FOCnC strobe will not generate any interrupt nor will it clear the timer
in Clear Timer on Compare Match (CTC) mode using OCRnA as TOP.
162
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
The FOCnA/FOCnB/FOCnB bits are always read as zero.
• Bit 4:0 – Reserved Bits
These bits are reserved for future use. For ensuring compatibility with future devices,
these bits must be written to zero when TCCRnC is written.
Timer/Counter1 – TCNT1H
and TCNT1L
Bit
7
6
5
4
3
2
1
0
TCNT1[15:8]
TCNT1H
TCNT1[7:0]
Timer/Counter3 – TCNT3H
and TCNT3L
TCNT1L
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
TCNT3[15:8]
TCNT3H
TCNT3[7:0]
Timer/Counter4 – TCNT4H
and TCNT4L
TCNT3L
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
TCNT4[15:8]
TCNT4H
TCNT4[7:0]
Timer/Counter5 – TCNT5H
and TCNT5L
TCNT4L
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
TCNT5[15:8]
TCNT5H
TCNT5[7:0]
TCNT5L
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
The two Timer/Counter I/O locations (TCNTnH and TCNTnL, combined TCNTn) give
direct access, both for read and for write operations, to the Timer/Counter unit 16-bit
counter. To ensure that both the high and low bytes are read and written simultaneously
when the CPU accesses these registers, the access is performed using an 8-bit temporary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit
registers. See “Accessing 16-bit Registers” on page 137.
Modifying the counter (TCNTn) while the counter is running introduces a risk of missing
a compare match between TCNTn and one of the OCRnx Registers.
Writing to the TCNTn Register blocks (removes) the compare match on the following
timer clock for all compare units.
163
2549A–AVR–03/05
Output Compare Register 1 A
– OCR1AH and OCR1AL
Bit
7
6
5
4
3
2
1
0
OCR1A[15:8]
OCR1AH
OCR1A[7:0]
Output Compare Register 1 B
– OCR1BH and OCR1BL
OCR1AL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
OCR1B[15:8]
OCR1BH
OCR1B[7:0]
Output Compare Register 1 C
– OCR1CH and OCR1CL
OCR1BL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
OCR1C[15:8]
OCR1CH
OCR1C[7:0]
Output Compare Register 3 A
– OCR3AH and OCR3AL
OCR1CL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
OCR3A[15:8]
OCR3AH
OCR3A[7:0]
Output Compare Register 3 B
– OCR3BH and OCR3BL
OCR3AL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
OCR3B[15:8]
OCR3BH
OCR3B[7:0]
Output Compare Register 3 C
– OCR3CH and OCR3CL
OCR3BL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
OCR3C[15:8]
OCR3CH
OCR3C[7:0]
Output Compare Register 4 A
– OCR4AH and OCR4AL
OCR3CL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
OCR4A[15:8]
OCR4AH
OCR4A[7:0]
Output Compare Register 4 B
– OCR4BH and OCR4BL
OCR4AL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
OCR4B[15:8]
OCR4BH
OCR4B[7:0]
164
OCR4BL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Output Compare Register 4 C
– OCR4CH and OCR4CL
Bit
7
6
5
4
3
2
1
0
OCR4C[15:8]
OCR4CH
OCR4C[7:0]
Output Compare Register 5 A
– OCR5AH and OCR5AL
OCR4CL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
OCR5A[15:8]
OCR5AH
OCR5A[7:0]
Output Compare Register 5 B
– OCR5BH and OCR5BL
OCR5AL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
OCR5B[15:8]
OCR5BH
OCR5B[7:0]
Output Compare Register 5 C
– OCR5CH and OCR5CL
OCR5BL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
OCR5C[15:8]
OCR5CH
OCR5C[7:0]
OCR5CL
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
The Output Compare Registers contain a 16-bit value that is continuously compared
with the counter value (TCNTn). A match can be used to generate an Output Compare
interrupt, or to generate a waveform output on the OCnx pin.
The Output Compare Registers are 16-bit in size. To ensure that both the high and low
bytes are written simultaneously when the CPU writes to these registers, the access is
performed using an 8-bit temporary High Byte Register (TEMP). This temporary register
is shared by all the other 16-bit registers. See “Accessing 16-bit Registers” on page 137.
Input Capture Register 1 –
ICR1H and ICR1L
Bit
7
6
5
4
3
2
1
0
ICR1[15:8]
ICR1H
ICR1[7:0]
Input Capture Register 3 –
ICR3H and ICR3L
ICR1L
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
ICR3[15:8]
ICR3H
ICR3[7:0]
ICR3L
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
165
2549A–AVR–03/05
Input Capture Register 4 –
ICR4H and ICR4L
Bit
7
6
5
4
3
2
1
0
ICR4[15:8]
ICR4H
ICR4[7:0]
Input Capture Register 5 –
ICR5H and ICR5L
ICR4L
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
ICR5[15:8]
ICR5H
ICR5[7:0]
ICR5L
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
The Input Capture is updated with the counter (TCNTn) value each time an event occurs
on the ICPn pin (or optionally on the Analog Comparator output for Timer/Counter1).
The Input Capture can be used for defining the counter TOP value.
The Input Capture Register is 16-bit in size. To ensure that both the high and low bytes
are read simultaneously when the CPU accesses these registers, the access is performed using an 8-bit temporary High Byte Register (TEMP). This temporary register is
shared by all the other 16-bit registers. See “Accessing 16-bit Registers” on page 137.
Timer/Counter1 Interrupt
Mask Register – TIMSK1
Timer/Counter3 Interrupt
Mask Register – TIMSK3
Timer/Counter4 Interrupt
Mask Register – TIMSK4
Timer/Counter5 Interrupt
Mask Register – TIMSK5
Bit
7
6
5
4
3
2
1
0
–
–
ICIE1
–
OCIE1C
OCIE1B
OCIE1A
TOIE1
Read/Write
R
R
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
–
–
ICIE3
–
OCIE3C
OCIE3B
OCIE3A
TOIE3
Read/Write
R
R
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
–
–
ICIE4
–
OCIE4C
OCIE4B
OCIE4A
TOIE4
Read/Write
R
R
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
–
–
ICIE5
–
OCIE5C
OCIE5B
OCIE5A
TOIE5
Read/Write
R
R
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TIMSK1
TIMSK3
TIMSK4
TIMSK5
• Bit 5 – ICIEn: Timer/Countern, 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/Countern Input Capture interrupt is enabled. The
corresponding Interrupt Vector (See “Interrupts” on page 69.) is executed when the
ICFn Flag, located in TIFRn, is set.
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• Bit 3 – OCIEnC: Timer/Countern, Output Compare C 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/Countern Output Compare C Match interrupt is enabled. The
corresponding Interrupt Vector (See “Interrupts” on page 69.) is executed when the
OCFnC Flag, located in TIFRn, is set.
• Bit 2 – OCIEnB: Timer/Countern, 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/Countern Output Compare B Match interrupt is enabled. The
corresponding Interrupt Vector (See “Interrupts” on page 69.) is executed when the
OCFnB Flag, located in TIFRn, is set.
• Bit 1 – OCIEnA: Timer/Countern, 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/Countern Output Compare A Match interrupt is enabled. The
corresponding Interrupt Vector (See “Interrupts” on page 69.) is executed when the
OCFnA Flag, located in TIFRn, is set.
• Bit 0 – TOIEn: Timer/Countern, 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/Countern Overflow interrupt is enabled. The corresponding
Interrupt Vector (See “Interrupts” on page 69.) is executed when the TOVn Flag, located
in TIFRn, is set.
Timer/Counter1 Interrupt Flag
Register – TIFR1
Timer/Counter3 Interrupt Flag
Register – TIFR3
Timer/Counter4 Interrupt Flag
Register – TIFR4
Timer/Counter5 Interrupt Flag
Register – TIFR5
Bit
7
6
5
4
3
2
1
0
–
–
ICF1
–
OCF1C
OCF1B
OCF1A
TOV1
Read/Write
R
R
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
–
–
ICF3
–
OCF3C
OCF3B
OCF3A
TOV3
Read/Write
R
R
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
–
–
ICF4
–
OCF4C
OCF4B
OCF4A
TOV4
Read/Write
R
R
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
–
–
ICF5
–
OCF5C
OCF5B
OCF5A
TOV5
Read/Write
R
R
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TIFR1
TIFR3
TIFR4
TIFR5
• Bit 5 – ICFn: Timer/Countern, Input Capture Flag
This flag is set when a capture event occurs on the ICPn pin. When the Input Capture
Register (ICRn) is set by the WGMn3:0 to be used as the TOP value, the ICFn Flag is
set when the counter reaches the TOP value.
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ICFn is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively, ICFn can be cleared by writing a logic one to its bit location.
• Bit 3– OCFnC: Timer/Countern, Output Compare C Match Flag
This flag is set in the timer clock cycle after the counter (TCNTn) value matches the Output Compare Register C (OCRnC).
Note that a Forced Output Compare (FOCnC) strobe will not set the OCFnC Flag.
OCFnC is automatically cleared when the Output Compare Match C Interrupt Vector is
executed. Alternatively, OCFnC can be cleared by writing a logic one to its bit location.
• Bit 2 – OCFnB: Timer/Counter1, Output Compare B Match Flag
This flag is set in the timer clock cycle after the counter (TCNTn) value matches the Output Compare Register B (OCRnB).
Note that a Forced Output Compare (FOCnB) strobe will not set the OCFnB Flag.
OCFnB is automatically cleared when the Output Compare Match B Interrupt Vector is
executed. Alternatively, OCFnB 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 (TCNTn value matches the Output Compare Register A (OCRnA).
Note that a Forced Output Compare (FOCnA) strobe will not set the OCFnA Flag.
OCFnA is automatically cleared when the Output Compare Match A Interrupt Vector is
executed. Alternatively, OCFnA can be cleared by writing a logic one to its bit location.
• Bit 0 – TOVn: Timer/Countern, Overflow Flag
The setting of this flag is dependent of the WGMn3:0 bits setting. In Normal and CTC
modes, the TOVn Flag is set when the timer overflows. Refer to Table 82 on page 160
for the TOVn Flag behavior when using another WGMn3:0 bit setting.
TOVn is automatically cleared when the Timer/Countern Overflow Interrupt Vector is
executed. Alternatively, TOVn can be cleared by writing a logic one to its bit location.
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Timer/Counter0,
Timer/Counter1,
Timer/Counter3,
Timer/Counter4, and
Timer/Counter5
Prescalers
Timer/Counter0, 1, 3, 4, and 5 share the same prescaler module, but the
Timer/Counters can have different prescaler settings. The description below applies to
all Timer/Counters. Tn is used as a general name, n = 0, 1, 3, 4, or 5.
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 the Timer/Counter Tn. 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
execution. However, care must be taken if the other Timer/Counter that shares the
same prescaler also uses prescaling. A prescaler reset will affect the prescaler period
for all Timer/Counters it is connected to.
External Clock Source
An external clock source applied to the Tn pin can be used as Timer/Counter clock
(clkTn). The Tn 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 62 shows a functional equivalent block diagram of the Tn 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.
The edge detector generates one clkTn pulse for each positive (CSn2:0 = 7) or negative
(CSn2:0 = 6) edge it detects.
Figure 62. Tn/T0 Pin Sampling
Tn
D
Q
D
Q
D
Tn_sync
(To Clock
Select Logic)
Q
LE
clk I/O
Synchronization
Edge Detector
The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system
clock cycles from an edge has been applied to the Tn pin to the counter is updated.
Enabling and disabling of the clock input must be done when Tn has been stable for at
least one system clock cycle, otherwise it is a risk that a false Timer/Counter clock pulse
is generated.
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Each half period of the external clock applied must be longer than one system clock
cycle to ensure correct sampling. The external clock must be guaranteed to have less
than half the system clock frequency (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 frequency (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 63. Prescaler for synchronous Timer/Counters
clk I/O
Clear
PSR10
Tn
Synchronization
Tn
Synchronization
CSn0
CSn0
CSn1
CSn1
CSn2
CSn2
TIMER/COUNTERn CLOCK SOURCE
clkTn
General Timer/Counter
Control Register – GTCCR
Bit
TIMER/COUNTERn CLOCK SOURCE
clkTn
7
6
5
4
3
2
TSM
–
–
–
–
–
1
0
Read/Write
R/W
R
R
R
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
PSRASY PSRSYNC
GTCCR
• 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 PSRASY and PSRSYNC bits is kept, hence keeping the corresponding prescaler 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 configuration. When the TSM bit is written
to zero, the PSRASY and PSRSYNC bits are cleared by hardware, and the
Timer/Counters start counting simultaneously.
• Bit 0 – PSRSYNC: Prescaler Reset for Synchronous Timer/Counters
When this bit is one, Timer/Counter0 and Timer/Counter1, Timer/Counter3,
Timer/Counter4 and Timer/Counter5 prescaler will be Reset. This bit is normally cleared
immediately by hardware, except if the TSM bit is set. Note that Timer/Counter0,
Timer/Counter1, Timer/Counter3, Timer/Counter4 and Timer/Counter5 share the same
prescaler and a reset of this prescaler will affect all timers.
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Output Compare
Modulator
(OCM1C0A)
Overview
The Output Compare Modulator (OCM) allows generation of waveforms modulated with
a carrier frequency. The modulator uses the outputs from the Output Compare Unit C of
the 16-bit Timer/Counter1 and the Output Compare Unit of the 8-bit Timer/Counter0. For
more details about these Timer/Counters see “Timer/Counter0, Timer/Counter1,
Timer/Counter3, Timer/Counter4, and Timer/Counter5 Prescalers” on page 169 and “8bit Timer/Counter2 with PWM and Asynchronous Operation” on page 173.
Figure 64. Output Compare Modulator, Block Diagram
Timer/Counter 1
OC1C
Pin
Timer/Counter 0
OC1C /
OC0A / PB7
OC0A
When the modulator is enabled, the two output compare channels are modulated
together as shown in the block diagram (Figure 64).
Description
The Output Compare unit 1C and Output Compare unit 2 shares the PB7 port pin for
output. The outputs of the Output Compare units (OC1C and OC0A) overrides the normal PORTB7 Register when one of them is enabled (i.e., when COMnx1:0 is not equal
to zero). When both OC1C and OC0A are enabled at the same time, the modulator is
automatically enabled.
The functional equivalent schematic of the modulator is shown on Figure 65. The schematic includes part of the Timer/Counter units and the port B pin 7 output driver circuit.
Figure 65. Output Compare Modulator, Schematic
COMA01
COMA00
Vcc
COM1C1
COM1C0
( From Waveform Generator )
Modulator
0
D
1
Q
1
OC1C
Pin
0
( From Waveform Generator )
D
Q
OC1C /
OC0A/ PB7
OC0A
D
Q
D
PORTB7
Q
DDRB7
DATABUS
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When the modulator is enabled the type of modulation (logical AND or OR) can be
selected by the PORTB7 Register. Note that the DDRB7 controls the direction of the
port independent of the COMnx1:0 bit setting.
Timing Example
Figure 66 illustrates the modulator in action. In this example the Timer/Counter1 is set to
operate in fast PWM mode (non-inverted) and Timer/Counter0 uses CTC waveform
mode with toggle Compare Output mode (COMnx1:0 = 1).
Figure 66. Output Compare Modulator, Timing Diagram
clk I/O
OC1C
(FPWM Mode)
OC0A
(CTC Mode)
PB7
(PORTB7 = 0)
PB7
(PORTB7 = 1)
(Period)
1
2
3
In this example, Timer/Counter2 provides the carrier, while the modulating signal is generated by the Output Compare unit C of the Timer/Counter1.
The resolution of the PWM signal (OC1C) is reduced by the modulation. The reduction
factor is equal to the number of system clock cycles of one period of the carrier (OC0A).
In this example the resolution is reduced by a factor of two. The reason for the reduction
is illustrated in Figure 66 at the second and third period of the PB7 output when
PORTB7 equals zero. The period 2 high time is one cycle longer than the period 3 high
time, but the result on the PB7 output is equal in both periods.
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8-bit Timer/Counter2
with PWM and
Asynchronous
Operation
Timer/Counter2 is a general purpose, single channel, 8-bit Timer/Counter module. The
main features are:
• Single Channel 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, OCF2A and OCF2B)
• Allows Clocking from External 32 kHz Watch Crystal Independent of the I/O Clock
Overview
A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 60.. For the
actual placement of I/O pins, see “Pin Configurations” on page 2. CPU accessible I/O
Registers, including I/O bits and I/O pins, are shown in bold. The device-specific I/O
Register and bit locations are listed in the “8-bit Timer/Counter Register Description” on
page 184.
The Power Reduction Timer/Counter2 bit, PRTIM2, in “Power Reduction Register 0 PRR0” on page 54 must be written to zero to enable Timer/Counter2 module.
Figure 67. 8-bit Timer/Counter Block Diagram
Count
TOVn
(Int.Req.)
Clear
Direction
Control Logic
clkTn
TOSC1
T/C
Oscillator
TOP
BOTTOM
TOSC2
Prescaler
clkI/O
Timer/Counter
TCNTn
=
=0
OCnA
(Int.Req.)
Waveform
Generation
=
OCnA
OCRnA
DATA BUS
Fixed
TOP
Value
Waveform
Generation
=
OCRnB
OCnB
(Int.Req.)
Synchronized Status flags
Synchronization Unit
OCnB
clkI/O
clkASY
Status flags
ASSRn
TCCRnA
Registers
asynchronous mode
select (ASn)
TCCRnB
The Timer/Counter (TCNT2) and Output Compare Register (OCR2A and OCR2B) are
8-bit registers. Interrupt request (abbreviated to 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.
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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 inactive 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 and OCR2B) are compared
with the Timer/Counter value at all times. The result of the compare can be used by the
Waveform Generator to generate a PWM or variable frequency output on the Output
Compare pins (OC2A and OC2B). See “Output Compare Unit” on page 175. for details.
The compare match event will also set the Compare Flag (OCF2A or OCF2B) which can
be used to generate an Output Compare interrupt request.
Definitions
Many register and bit references in this document are written in general form. A lower
case “n” replaces the Timer/Counter number, in this case 2. However, when using the
register or bit defines in a program, the precise form must be used, i.e., TCNT2 for
accessing Timer/Counter2 counter value and so on.
The definitions in Table 84 are also used extensively throughout the section.
Table 84. Definitions
BOTTOM
The counter reaches the BOTTOM when it becomes zero (0x00).
MAX
The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
TOP
The counter reaches the TOP when it becomes equal to the highest
value in the count sequence. The TOP value can be assigned to be the
fixed value 0xFF (MAX) or the value stored in the OCR2A Register. The
assignment is dependent on the mode of operation.
Timer/Counter Clock
Sources
The Timer/Counter can be clocked by an internal synchronous or an external asynchronous clock source. The clock source 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 “Asynchronous Status Register – ASSR” on page 189. For
details on clock sources and prescaler, see “Timer/Counter Prescaler” on page 193.
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure 68 shows a block diagram of the counter and its surrounding environment.
Figure 68. Counter Unit Block Diagram
TOVn
(Int.Req.)
DATA BUS
TOSC1
count
TCNTn
clear
clk Tn
Control Logic
Prescaler
T/C
Oscillator
direction
bottom
174
TOSC2
top
clkI/O
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Signal description (internal signals):
count
Increment or decrement TCNT2 by 1.
direction
Selects between increment and decrement.
clear
Clear TCNT2 (set all bits to zero).
clkTn
Timer/Counter clock, referred to as clkT2 in the following.
top
Signalizes that TCNT2 has reached maximum value.
bottom
Signalizes that TCNT2 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or 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) and the WGM22 located in the
Timer/Counter Control Register B (TCCR2B). There are close connections between
how the counter behaves (counts) and how waveforms are generated on the Output
Compare outputs OC2A and OC2B. For more details about advanced counting
sequences and waveform generation, see “Modes of Operation” on page 178.
The Timer/Counter Overflow Flag (TOV2) is set according to the mode of operation
selected by the WGM22:0 bits. TOV2 can be used for generating a CPU interrupt.
Output Compare Unit
The 8-bit comparator continuously compares TCNT2 with the Output Compare Register
(OCR2A and OCR2B). Whenever TCNT2 equals OCR2A or OCR2B, the comparator
signals a match. A match will set the Output Compare Flag (OCF2A or OCF2B) at the
next timer clock cycle. If the corresponding interrupt is enabled, the Output Compare
Flag generates an Output Compare interrupt. The Output Compare Flag is automatically
cleared when the interrupt is executed. Alternatively, the Output Compare 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
WGM22:0 bits and Compare Output mode (COM2x1:0) bits. The max and bottom signals are used by the Waveform Generator for handling the special cases of the extreme
values in some modes of operation (“Modes of Operation” on page 178).
Figure 58 on page 154 shows a block diagram of the Output Compare unit.
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Figure 69. Output Compare Unit, Block Diagram
DATA BUS
OCRnx
TCNTn
= (8-bit Comparator )
OCFnx (Int.Req.)
top
bottom
Waveform Generator
OCnx
FOCn
WGMn1:0
COMnX1:0
The OCR2x 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
OCR2x 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 OCR2x Register access may seem complex, but this is not case. When the double
buffering is enabled, the CPU has access to the OCR2x Buffer Register, and if double
buffering is disabled the CPU will access the OCR2x directly.
Force Output Compare
In non-PWM waveform generation modes, the match output of the comparator can be
forced by writing a one to the Force Output Compare (FOC2x) bit. Forcing compare
match will not set the OCF2x Flag or reload/clear the timer, but the OC2x pin will be
updated as if a real compare match had occurred (the COM2x1:0 bits settings define
whether the OC2x 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
OCR2x to be initialized to the same value as TCNT2 without triggering an interrupt when
the Timer/Counter clock is enabled.
Using the Output Compare
Unit
Since writing TCNT2 in any mode of operation will block all compare matches for one
timer clock cycle, there are risks involved when changing TCNT2 when using the Output
Compare channel, independently of whether the Timer/Counter is running or not. If the
value written to TCNT2 equals the OCR2x value, the compare match will be missed,
resulting in incorrect waveform generation. Similarly, do not write the TCNT2 value
equal to BOTTOM when the counter is downcounting.
The setup of the OC2x should be performed before setting the Data Direction Register
for the port pin to output. The easiest way of setting the OC2x value is to use the Force
Output Compare (FOC2x) strobe bit in Normal mode. The OC2x Register keeps its
value even when changing between Waveform Generation modes.
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Be aware that the COM2x1:0 bits are not double buffered together with the compare
value. Changing the COM2x1:0 bits will take effect immediately.
Compare Match Output
Unit
The Compare Output mode (COM2x1:0) bits have two functions. The Waveform Generator uses the COM2x1:0 bits for defining the Output Compare (OC2x) state at the next
compare match. Also, the COM2x1:0 bits control the OC2x pin output source. Figure 70
shows a simplified schematic of the logic affected by the COM2x1:0 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 COM2x1:0
bits are shown. When referring to the OC2x state, the reference is for the internal OC2x
Register, not the OC2x pin.
Figure 70. Compare Match Output Unit, Schematic
COMnx1
COMnx0
FOCnx
Waveform
Generator
D
Q
1
OCnx
DATA BUS
D
0
OCnx
Pin
Q
PORT
D
Q
DDR
clk I/O
The general I/O port function is overridden by the Output Compare (OC2x) from the
Waveform Generator if either of the COM2x1:0 bits are set. However, the OC2x pin
direction (input or output) is still controlled by the Data Direction Register (DDR) for the
port pin. The Data Direction Register bit for the OC2x pin (DDR_OC2x) must be set as
output before the OC2x value is visible on the pin. The port override function is independent of the Waveform Generation mode.
The design of the Output Compare pin logic allows initialization of the OC2x state before
the output is enabled. Note that some COM2x1:0 bit settings are reserved for certain
modes of operation. See “8-bit Timer/Counter Register Description” on page 184.
Compare Output Mode and
Waveform Generation
The Waveform Generator uses the COM2x1:0 bits differently in normal, CTC, and PWM
modes. For all modes, setting the COM2x1:0 = 0 tells the Waveform Generator that no
action on the OC2x Register is to be performed on the next compare match. For compare output actions in the non-PWM modes refer to Table 88 on page 185. For fast
PWM mode, refer to Table 89 on page 185, and for phase correct PWM refer to Table
90 on page 186.
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A change of the COM2x1:0 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 FOC2x strobe bits.
Modes of Operation
The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare
pins, is defined by the combination of the Waveform Generation mode (WGM22:0) and
Compare Output mode (COM2x1:0) bits. The Compare Output mode bits do not affect
the counting sequence, while the Waveform Generation mode bits do. The COM2x1:0
bits control whether the PWM output generated should be inverted or not (inverted or
non-inverted PWM). For non-PWM modes the COM2x1:0 bits control whether the output should be set, cleared, or toggled at a compare match (See “Compare Match Output
Unit” on page 177.).
For detailed timing information refer to “Timer/Counter Timing Diagrams” on page 182.
Normal Mode
The simplest mode of operation is the Normal mode (WGM22:0 = 0). In this mode the
counting direction is always up (incrementing), and no counter clear is performed. The
counter simply overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then
restarts from the bottom (0x00). In normal operation the Timer/Counter Overflow Flag
(TOV2) will be set in the same timer clock cycle as the TCNT2 becomes zero. The
TOV2 Flag in this case behaves like a ninth bit, except that it is only set, not cleared.
However, combined with the timer overflow interrupt that automatically clears the TOV2
Flag, the timer resolution can be increased by software. There are no special cases to
consider in the Normal mode, a new counter value can be written anytime.
The Output Compare unit can be used to generate interrupts at some given time. Using
the Output Compare to generate waveforms in Normal mode is not recommended,
since this will occupy too much of the CPU time.
Clear Timer on Compare
Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGM22:0 = 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 Table 71. The counter value (TCNT2)
increases until a compare match occurs between TCNT2 and OCR2A, and then counter
(TCNT2) is cleared.
Figure 71. CTC Mode, Timing Diagram
OCnx Interrupt Flag Set
TCNTn
OCnx
(Toggle)
Period
(COMnx1:0 = 1)
1
2
3
4
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
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used for updating the TOP value. However, changing TOP to a value close to BOTTOM
when the counter is running with none or a low prescaler value must be done with care
since the CTC mode does not have the double buffering feature. If the new value written
to OCR2A is lower than the current value of TCNT2, the counter will miss the compare
match. The counter will then have to count to its maximum value (0xFF) and wrap
around starting at 0x00 before the compare match can occur.
For generating a waveform output in CTC mode, the 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 = fclk_I/O/2 when OCR2A is set to zero (0x00). The waveform frequency is defined by the following equation:
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.
Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM22:0 = 3 or 7) provides a high
frequency PWM waveform generation option. The fast PWM differs from the other PWM
option by its single-slope operation. The counter counts from BOTTOM to TOP then
restarts from BOTTOM. TOP is defined as 0xFF when WGM22:0 = 3, and OCR2A when
MGM22:0 = 7. In non-inverting Compare Output mode, the Output Compare (OC2x) is
cleared on the compare match between TCNT2 and OCR2x, 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 TOP
value. The counter is then cleared at the following timer clock cycle. The timing diagram
for the fast PWM mode is shown in Figure 61. The TCNT2 value is in the timing diagram
shown as a histogram for illustrating the single-slope operation. The diagram includes
non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT2
slopes represent compare matches between OCR2x and TCNT2.
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Figure 72. Fast PWM Mode, Timing Diagram
OCRnx Interrupt Flag Set
OCRnx Update and
TOVn Interrupt Flag Set
TCNTn
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
4
5
6
7
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches TOP. If
the interrupt 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
OC2x pin. Setting the COM2x1:0 bits to two will produce a non-inverted PWM and an
inverted PWM output can be generated by setting the COM2x1:0 to three. TOP is
defined as 0xFF when WGM2:0 = 3, and OCR2A when WGM2:0 = 7 (See Table 86 on
page 184). The actual OC2x value will only be visible on the port pin if the data direction
for the port pin is set as output. The PWM waveform is generated by setting (or clearing)
the OC2x Register at the compare match between OCR2x and TCNT2, and clearing (or
setting) the OC2x 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 ⋅ 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 setting OC2x to toggle its logical level on each compare match (COM2x1:0 = 1). The
waveform generated will have a maximum frequency of foc2 = fclk_I/O/2 when OCR2A is
set to zero. This feature 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.
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Phase Correct PWM Mode
The phase correct PWM mode (WGM22:0 = 1 or 5) 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 TOP and then
from TOP to BOTTOM. TOP is defined as 0xFF when WGM22:0 = 1, and OCR2A when
MGM22:0 = 5. In non-inverting Compare Output mode, the Output Compare (OC2x) is
cleared on the compare match between TCNT2 and OCR2x while upcounting, and set
on the compare match while downcounting. In inverting Output Compare mode, the
operation is inverted. The dual-slope operation has lower maximum operation frequency
than single slope operation. However, due to the symmetric feature of the dual-slope
PWM modes, these modes are preferred for motor control applications.
In phase correct PWM mode the counter is incremented until the counter value matches
TOP. When the counter reaches TOP, it changes the count direction. The TCNT2 value
will be equal to TOP for one timer clock cycle. The timing diagram for the phase correct
PWM mode is shown on Figure 73. 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 OCR2x and TCNT2.
Figure 73. Phase Correct PWM Mode, Timing Diagram
OCnx Interrupt Flag Set
OCRnx Update
TOVn Interrupt Flag Set
TCNTn
OCnx
(COMnx1:0 = 2)
OCnx
(COMnx1:0 = 3)
Period
1
2
3
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 OC2x pin. Setting the COM2x1:0 bits to two will produce a non-inverted PWM. An
inverted PWM output can be generated by setting the COM2x1:0 to three. TOP is
defined as 0xFF when WGM2:0 = 3, and OCR2A when MGM2:0 = 7 (See Table 87 on
page 185). The actual OC2x 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 OC2x Register at the compare match between OCR2x and TCNT2 when the
counter increments, and setting (or clearing) the OC2x Register at compare match
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between OCR2x and TCNT2 when the counter decrements. The PWM frequency for the
output when using phase correct PWM can be calculated by the following equation:
f clk_I/O
f 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 73 OCnx has a transition from high to low even
though there is no Compare Match. The point of this transition is to guarantee symmetry
around BOTTOM. There are two cases that give a transition without Compare Match.
Timer/Counter Timing
Diagrams
•
OCR2A changes its value from MAX, like in Figure 73. When the OCR2A value is
MAX the OCn pin value is the same as the result of a down-counting compare
match. To ensure symmetry around BOTTOM the OCn value at MAX must
correspond to the result of an up-counting Compare Match.
•
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.
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 74 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 74. Timer/Counter Timing Diagram, no Prescaling
clkI/O
clkTn
(clkI/O /1)
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
TOVn
Figure 75 shows the same timing data, but with the prescaler enabled.
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Figure 75. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
MAX - 1
MAX
BOTTOM
BOTTOM + 1
TOVn
Figure 76 shows the setting of OCF2A in all modes except CTC mode.
Figure 76. Timer/Counter Timing Diagram, Setting of OCF2A, with Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
OCRnx - 1
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx Value
OCRnx
OCFnx
Figure 77 shows the setting of OCF2A and the clearing of TCNT2 in CTC mode.
Figure 77. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with
Prescaler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O /8)
TCNTn
(CTC)
OCRnx
TOP - 1
TOP
BOTTOM
BOTTOM + 1
TOP
OCFnx
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8-bit Timer/Counter
Register Description
Timer/Counter Control
Register A – TCCR2A
Bit
7
6
5
4
3
2
1
0
COM2A1
COM2A0
COM2B1
COM2B0
–
–
WGM21
WGM20
Read/Write
R/W
R/W
R/W
R/W
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TCCR2A
• Bits 7:6 – COM2A1:0: Compare Match Output A Mode
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 the 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
WGM22:0 bit setting. Table 85 shows the COM2A1:0 bit functionality when the
WGM22:0 bits are set to a normal or CTC mode (non-PWM).
Table 85. Compare Output Mode, non-PWM Mode
COM2A1
COM2A0
Description
0
0
Normal port operation, OC0A disconnected.
0
1
Toggle OC2A on Compare Match
1
0
Clear OC2A on Compare Match
1
1
Set OC2A on Compare Match
Table 86 shows the COM2A1:0 bit functionality when the WGM21:0 bits are set to fast
PWM mode.
Table 86. Compare Output Mode, Fast PWM Mode(1)
COM2A1
COM2A0
0
0
Normal port operation, OC2A disconnected.
0
1
WGM22 = 0: Normal Port Operation, OC0A Disconnected.
WGM22 = 1: Toggle OC2A on Compare Match.
1
0
Clear OC2A on Compare Match, set OC2A at TOP
1
1
Set OC2A on Compare Match, clear OC2A at TOP
Note:
184
Description
1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case,
the Compare Match is ignored, but the set or clear is done at TOP. See “Fast PWM
Mode” on page 179 for more details.
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Table 87 shows the COM2A1:0 bit functionality when the WGM22:0 bits are set to
phase correct PWM mode.
Table 87. Compare Output Mode, Phase Correct PWM Mode(1)
COM2A1
COM2A0
0
0
Normal port operation, OC2A disconnected.
0
1
WGM22 = 0: Normal Port Operation, OC2A Disconnected.
WGM22 = 1: Toggle OC2A on Compare Match.
1
0
Clear OC2A on Compare Match when up-counting. Set OC2A on
Compare Match when down-counting.
1
1
Set OC2A on Compare Match when up-counting. Clear OC2A on
Compare Match when down-counting.
Note:
Description
1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case,
the Compare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page 181 for more details.
• Bits 5:4 – COM2B1:0: Compare Match Output B Mode
These bits control the Output Compare pin (OC2B) behavior. If one or both of the
COM2B1:0 bits are set, the OC2B 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 OC2B pin must be set in order to enable the output driver.
When OC2B is connected to the pin, the function of the COM2B1:0 bits depends on the
WGM22:0 bit setting. Table 88 shows the COM2B1:0 bit functionality when the
WGM22:0 bits are set to a normal or CTC mode (non-PWM).
Table 88. Compare Output Mode, non-PWM Mode
COM2B1
COM2B0
Description
0
0
Normal port operation, OC2B disconnected.
0
1
Toggle OC2B on Compare Match
1
0
Clear OC2B on Compare Match
1
1
Set OC2B on Compare Match
Table 89 shows the COM2B1:0 bit functionality when the WGM22:0 bits are set to fast
PWM mode.
Table 89. Compare Output Mode, Fast PWM Mode(1)
COM2B1
COM2B0
0
0
Normal port operation, OC2B disconnected.
0
1
Reserved
1
0
Clear OC2B on Compare Match, set OC2B at TOP
1
1
Set OC2B on Compare Match, clear OC2B at TOP
Note:
Description
1. A special case occurs when OCR2B equals TOP and COM2B1 is set. In this case,
the Compare Match is ignored, but the set or clear is done at TOP. See “Fast PWM
Mode” on page 179 for more details.
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Table 90 shows the COM2B1:0 bit functionality when the WGM22:0 bits are set to
phase correct PWM mode.
Table 90. Compare Output Mode, Phase Correct PWM Mode(1)
COM2B1
COM2B0
0
0
Normal port operation, OC2B disconnected.
0
1
Reserved
1
0
Clear OC2B on Compare Match when up-counting. Set OC2B on
Compare Match when down-counting.
1
1
Set OC2B on Compare Match when up-counting. Clear OC2B on
Compare Match when down-counting.
Note:
Description
1. A special case occurs when OCR2B equals TOP and COM2B1 is set. In this case,
the Compare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page 181 for more details.
• Bits 3, 2 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bits 1:0 – WGM21:0: Waveform Generation Mode
Combined with the WGM22 bit found in the TCCR2B Register, these bits control the
counting sequence of the counter, the source for maximum (TOP) counter value, and
what type of waveform generation to be used, see Table 91. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear Timer on Compare
Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes (see
“Modes of Operation” on page 178).
Table 91. Waveform Generation Mode Bit Description
TOP
Update of
OCRx at
TOV Flag
Set on(1)(2)
Mode
WGM2
WGM1
WGM0
0
0
0
0
Normal
0xFF
Immediate
MAX
1
0
0
1
PWM, Phase
Correct
0xFF
TOP
BOTTOM
2
0
1
0
CTC
OCRA
Immediate
MAX
3
0
1
1
Fast PWM
0xFF
TOP
MAX
4
1
0
0
Reserved
–
–
–
5
1
0
1
PWM, Phase
Correct
OCRA
TOP
BOTTOM
6
1
1
0
Reserved
–
–
–
7
1
1
1
Fast PWM
OCRA
TOP
TOP
Notes:
186
Timer/Counter
Mode of
Operation
1. MAX= 0xFF
2. BOTTOM= 0x00
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ATmega640/1280/1281/2560/2561
Timer/Counter Control
Register B – TCCR2B
Bit
7
6
5
4
3
2
1
0
FOC2A
FOC2B
–
–
WGM22
CS22
CS21
CS20
Read/Write
W
W
R
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TCCR2B
• Bit 7 – FOC2A: Force Output Compare A
The FOC2A bit is only active when the WGM bits specify a non-PWM mode.
However, for ensuring compatibility with future devices, this bit must be set to zero when
TCCR2B 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 – FOC2B: Force Output Compare B
The FOC2B bit is only active when the WGM bits specify a non-PWM mode.
However, for ensuring compatibility with future devices, this bit must be set to zero when
TCCR2B is written when operating in PWM mode. When writing a logical one to the
FOC2B bit, an immediate Compare Match is forced on the Waveform Generation unit.
The OC2B output is changed according to its COM2B1:0 bits setting. Note that the
FOC2B bit is implemented as a strobe. Therefore it is the value present in the
COM2B1:0 bits that determines the effect of the forced compare.
A FOC2B strobe will not generate any interrupt, nor will it clear the timer in CTC mode
using OCR2B as TOP.
The FOC2B bit is always read as zero.
• Bits 5:4 – Res: Reserved Bits
These bits are reserved bits and will always read as zero.
• Bit 3 – WGM22: Waveform Generation Mode
See the description in the “Timer/Counter Control Register A – TCCR2A” on page 184.
• 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 92.
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Table 92. Clock Select Bit Description
CS22
CS21
CS20
Description
0
0
0
No clock source (Timer/Counter stopped).
0
0
1
clkT2S/(No prescaling)
0
1
0
clkT2S/8 (From prescaler)
0
1
1
clkT2S/32 (From prescaler)
1
0
0
clkT2S/64 (From prescaler)
1
0
1
clkT2S/128 (From prescaler)
1
1
0
clkT2S/256 (From prescaler)
1
1
1
clkT2S/1024 (From prescaler)
If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will
clock the counter even if the pin is configured as an output. This feature allows software
control of the counting.
Timer/Counter Register –
TCNT2
Bit
7
6
5
4
3
2
1
0
TCNT2[7:0]
TCNT2
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
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 OCR2x Registers.
Output Compare Register A –
OCR2A
Bit
7
6
5
4
3
2
1
0
OCR2A[7:0]
OCR2A
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
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.
Output Compare Register B –
OCR2B
Bit
7
6
5
4
3
2
1
0
OCR2B[7:0]
OCR2B
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
The Output Compare Register B 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 OC2B pin.
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Asynchronous operation
of the Timer/Counter
Asynchronous Status
Register – ASSR
Bit
7
6
5
4
3
2
1
0
–
EXCLK
AS2
TCN2UB
OCR2AUB
OCR2BUB
TCR2AUB
TCR2BUB
Read/Write
R
R/W
R/W
R
R
R
R
R
Initial Value
0
0
0
0
0
0
0
0
ASSR
• Bit 6 – EXCLK: Enable External Clock Input
When EXCLK is written to one, and asynchronous clock is selected, the external clock
input buffer 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 5 – 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 Oscillator 1 (TOSC1) pin. When the value of AS2 is changed, the contents of
TCNT2, OCR2A, OCR2B, TCCR2A and TCCR2B might be corrupted.
• Bit 4 – TCN2UB: Timer/Counter2 Update Busy
When Timer/Counter2 operates asynchronously and TCNT2 is written, this bit becomes
set. When TCNT2 has been updated from the temporary storage register, this bit is
cleared by hardware. A logical zero in this bit indicates that TCNT2 is ready to be
updated with a new value.
• Bit 3 – OCR2AUB: 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 hardware. A logical zero in this bit indicates that OCR2A is ready to be
updated with a new value.
• Bit 2 – OCR2BUB: Output Compare Register2 Update Busy
When Timer/Counter2 operates asynchronously and OCR2B is written, this bit becomes
set. When OCR2B has been updated from the temporary storage register, this bit is
cleared by hardware. A logical zero in this bit indicates that OCR2B is ready to be
updated with a new value.
• Bit 1 – TCR2AUB: 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.
• Bit 0 – TCR2BUB: Timer/Counter Control Register2 Update Busy
When Timer/Counter2 operates asynchronously and TCCR2B is written, this bit
becomes set. When TCCR2B has been updated from the temporary storage register,
this bit is cleared by hardware. A logical zero in this bit indicates that TCCR2B is ready
to be updated with a new value.
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If a write is performed to any of the five 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, OCR2B, TCCR2A and TCCR2B are different. When reading TCNT2, the actual timer value is read. When reading OCR2A,
OCR2B, TCCR2A and TCCR2B the value in the temporary storage register is read.
Asynchronous Operation of
Timer/Counter2
When Timer/Counter2 operates asynchronously, some considerations must be taken.
•
Warning: When switching between asynchronous and synchronous clocking of
Timer/Counter2, the Timer Registers TCNT2, OCR2x, and TCCR2x might be
corrupted. A safe procedure for switching clock source is:
1. Disable the Timer/Counter2 interrupts by clearing OCIE2x and TOIE2.
2. Select clock source by setting AS2 as appropriate.
3. Write new values to TCNT2, OCR2x, and TCCR2x.
4. To switch to asynchronous operation: Wait for TCN2UB, OCR2xUB, and
TCR2xUB.
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, OCR2x, or TCCR2x, 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 five mentioned registers have
their individual temporary register, which means that e.g. writing to TCNT2 does not
disturb an OCR2x 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, OCR2x, or TCCR2x, 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
any of the Output Compare2 interrupt is used to wake up the device, since the
Output Compare function is disabled during writing to OCR2x or TCNT2. If the write
cycle is not finished, and the MCU enters sleep mode before the corresponding
OCR2xUB 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 re-entering sleep mode is less than one TOSC1 cycle, the
interrupt will not occur, and the device will fail to wake up. If the user is in doubt
whether the time before re-entering Power-save or 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 TCCR2x, TCNT2, or OCR2x.
2. Wait until the corresponding Update Busy Flag in ASSR returns to zero.
3. Enter Power-save or ADC Noise Reduction mode.
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•
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 Powerdown or Standby mode due to unstable clock signal upon start-up, no matter
whether the Oscillator is in use or a clock signal is applied to the TOSC1 pin.
•
Description of wake up from Power-save or 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 OCR2x or TCCR2x.
2. Wait for the corresponding Update Busy Flag to be cleared.
3. Read TCNT2.
•
Timer/Counter2 Interrupt
Mask Register – TIMSK2
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.
Bit
7
6
5
4
3
2
1
0
–
–
–
–
–
OCIE2B
OCIE2A
TOIE2
Read/Write
R
R
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TIMSK2
• Bit 2 – OCIE2B: Timer/Counter2 Output Compare Match B Interrupt Enable
When the OCIE2B bit is written to one and the I-bit in the Status Register is set (one),
the Timer/Counter2 Compare Match B interrupt is enabled. The corresponding interrupt
is executed if a compare match in Timer/Counter2 occurs, i.e., when the OCF2B bit is
set in the Timer/Counter 2 Interrupt Flag Register – TIFR2.
• 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/Counter 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.
Timer/Counter2 Interrupt Flag
Register – TIFR2
Bit
7
6
5
4
3
2
1
0
–
–
–
–
–
OCF2B
OCF2A
TOV2
Read/Write
R
R
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TIFR2
• Bit 2 – OCF2B: Output Compare Flag 2 B
The OCF2B bit is set (one) when a compare match occurs between the Timer/Counter2
and the data in OCR2B – Output Compare Register2. OCF2B is cleared by hardware
when executing the corresponding interrupt handling vector. Alternatively, OCF2B is
cleared by writing a logic one to the flag. When the I-bit in SREG, OCIE2B
(Timer/Counter2 Compare match Interrupt Enable), and OCF2B are set (one), the
Timer/Counter2 Compare match Interrupt is executed.
• 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 hardware when executing the corresponding interrupt handling vector. Alternatively,
TOV2 is cleared by writing a logic one to the flag. When the SREG I-bit, TOIE2A
(Timer/Counter2 Overflow Interrupt Enable), and TOV2 are set (one), the
Timer/Counter2 Overflow interrupt is executed. In PWM mode, this bit is set when
Timer/Counter2 changes counting direction at 0x00.
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Figure 78. Prescaler for Timer/Counter2
clkT2S
PSRASY
clkT2S/1024
clkT2S/256
clkT2S/8
AS2
clkT2S/128
10-BIT T/C PRESCALER
Clear
TOSC1
clkT2S/64
clkI/O
clkT2S/32
Timer/Counter Prescaler
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 then be connected between the TOSC1 and TOSC2 pins to serve
as an independent clock source for Timer/Counter2. The Oscillator is optimized for use
with a 32.768 kHz crystal. Applying an external clock source to TOSC1 is not
recommended.
For Timer/Counter2, the possible prescaled selections are: 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 PSRASY bit in GTCCR resets the prescaler. This allows the user
to operate with a predictable prescaler.
General Timer/Counter
Control Register – GTCCR
Bit
7
6
5
4
3
2
TSM
–
–
–
–
–
1
0
Read/Write
R/W
R
R
R
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
PSRASY PSRSYNC
GTCCR
• Bit 1 – PSRASY: 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 Synchronization Mode” on page 170 for a description of the
Timer/Counter Synchronization mode.
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Serial Peripheral
Interface – SPI
The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer
between the ATmega640/1280/1281/2560/2561 and peripheral devices or between several AVR devices. The ATmega640/1280/1281/2560/2561 SPI includes the following
features:
• Full-duplex, Three-wire Synchronous Data Transfer
• Master or Slave Operation
• LSB First or MSB First Data Transfer
• Seven Programmable Bit Rates
• End of Transmission Interrupt Flag
• Write Collision Flag Protection
• Wake-up from Idle Mode
• Double Speed (CK/2) Master SPI Mode
USART can also be used in Master SPI mode, see “USART in SPI Mode” on page 231.
The Power Reduction SPI bit, PRSPI, in “Power Reduction Register 0 - PRR0” on page
54 on page 50 must be written to zero to enable SPI module.
Figure 79. SPI Block Diagram(1)
SPI2X
SPI2X
DIVIDER
/2/4/8/16/32/64/128
Note:
1. Refer to Figure 1 on page 2, and Table 39 on page 89 for SPI pin placement.
The interconnection between Master and Slave CPUs with SPI is shown in Figure 80.
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 inter-
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change data. Data is always shifted from Master to Slave on the Master Out – Slave In,
MOSI, line, and from Slave to Master on the Master In – Slave Out, MISO, line. After
each data packet, the Master will synchronize the Slave by pulling high the Slave Select,
SS, line.
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 80. SPI Master-slave Interconnection
SHIFT
ENABLE
The system is single buffered in the transmit direction and double buffered in the receive
direction. This means that bytes to be transmitted cannot be written to the SPI Data
Register before the entire shift cycle is completed. When receiving data, however, a
received character must be read from the SPI Data Register before the next character
has been completely shifted in. Otherwise, the first byte is lost.
In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To
ensure correct sampling of the clock signal, the frequency of the SPI clock should never
exceed fosc/4.
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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 93. For more details on automatic port overrides, refer to
“Alternate Port Functions” on page 86.
Table 93. SPI Pin Overrides(1)
Pin
Direction, Master SPI
Direction, Slave SPI
MOSI
User Defined
Input
MISO
Input
User Defined
SCK
User Defined
Input
SS
User Defined
Input
Note:
1. See “Alternate Functions of Port B” on page 89 for a detailed description of how to
define the direction of the user defined SPI pins.
The following code examples show how to initialize the SPI as a Master and how to perform a simple transmission. DDR_SPI in the examples must be replaced by the actual
Data Direction Register controlling the SPI pins. DD_MOSI, DD_MISO and DD_SCK
must be replaced by the actual data direction bits for these pins. E.g. if MOSI is placed
on pin PB5, replace DD_MOSI with DDB5 and DDR_SPI with DDRB.
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Assembly Code Example(1)
SPI_MasterInit:
; Set MOSI and SCK output, all others input
ldi
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:
198
1. See “About Code Examples” on page 8.
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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 “About Code Examples” on page 8.
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SS Pin Functionality
Slave Mode
When the SPI is configured as a Slave, the Slave Select (SS) pin is always input. When
SS is held low, the SPI is activated, and MISO becomes an output if configured so by
the user. All other pins are inputs. When SS is driven high, all pins are inputs, and the
SPI is passive, which means that it will not receive incoming data. Note that the SPI
logic will be reset once the SS pin is driven high.
The SS pin is useful for packet/byte synchronization to keep the slave bit counter synchronous with the master clock generator. When the SS pin is driven high, the SPI slave
will immediately reset the send and receive logic, and drop any partially received data in
the Shift Register.
Master Mode
When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine
the direction of the SS pin.
If SS is configured as an output, the pin is a general output pin which does not affect the
SPI system. Typically, the pin will be driving the SS pin of the SPI Slave.
If SS is configured as an input, it must be held high to ensure Master SPI operation. If
the SS pin is driven low by peripheral circuitry when the SPI is configured as a Master
with the SS pin defined as an input, the SPI system interprets this as another master
selecting the SPI as a slave and starting to send data to it. To avoid bus contention, the
SPI system takes the following actions:
1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a
result of the SPI becoming a Slave, the MOSI and SCK pins become inputs.
2. The SPIF Flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in
SREG is set, the interrupt routine will be executed.
Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a
possibility that SS is driven low, the interrupt should always check that the MSTR bit is
still set. If the MSTR bit has been cleared by a slave select, it must be set by the user to
re-enable SPI Master mode.
SPI Control Register – SPCR
Bit
7
6
5
4
3
2
1
0
SPIE
SPE
DORD
MSTR
CPOL
CPHA
SPR1
SPR0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
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.
• Bit 6 – SPE: SPI Enable
When the SPE bit is written to one, the SPI is enabled. This bit must be set to enable
any SPI operations.
• Bit 5 – DORD: Data Order
When the DORD bit is written to one, the LSB of the data word is transmitted first.
When the DORD bit is written to zero, the MSB of the data word is transmitted first.
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• Bit 4 – MSTR: Master/Slave Select
This bit selects Master SPI mode when written to one, and Slave SPI mode when written
logic zero. If SS is configured as an input and is driven low while MSTR is set, MSTR will
be cleared, and SPIF in SPSR will become set. The user will then have to set MSTR to
re-enable SPI Master mode.
• Bit 3 – CPOL: Clock Polarity
When this bit is written to one, SCK is high when idle. When CPOL is written to zero,
SCK is low when idle. Refer to Figure 81 and Figure 82 for an example. The CPOL functionality is summarized below:
Table 94. CPOL Functionality
CPOL
Leading Edge
Trailing Edge
0
Rising
Falling
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 81 and Figure 82 for an example.
The CPOL functionality is summarized below:
Table 95. CPHA Functionality
CPHA
Leading Edge
Trailing Edge
0
Sample
Setup
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 96. Relationship Between SCK and the Oscillator Frequency
SPI2X
SPR1
SPR0
0
0
0
0
0
1
0
1
0
0
1
1
1
0
0
1
0
1
1
1
0
1
1
1
SCK Frequency
fosc/4
fosc/16
fosc/64
fosc/128
fosc/2
fosc/8
fosc/32
fosc/64
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SPI Status Register – SPSR
Bit
7
6
5
4
3
2
1
0
SPIF
WCOL
–
–
–
–
–
SPI2X
Read/Write
R
R
R
R
R
R
R
R/W
Initial Value
0
0
0
0
0
0
0
0
SPSR
• Bit 7 – SPIF: SPI Interrupt Flag
When a serial transfer is complete, the SPIF Flag is set. An interrupt is generated if
SPIE in SPCR is set and global interrupts are enabled. If SS is an input and is driven low
when the SPI is in Master mode, this will also set the SPIF Flag. SPIF is cleared by
hardware when executing the corresponding interrupt handling vector. Alternatively, the
SPIF bit is cleared by first reading the SPI Status Register with SPIF set, then accessing
the SPI Data Register (SPDR).
• Bit 6 – WCOL: Write COLlision Flag
The WCOL bit is set if the SPI Data Register (SPDR) is written during a data transfer.
The WCOL bit (and the SPIF bit) are cleared by first reading the SPI Status Register
with WCOL set, and then accessing the SPI Data Register.
• Bit 5..1 – Res: Reserved Bits
These bits are reserved bits 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 96). 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 ATmega640/1280/1281/2560/2561 is also used for program
memory and EEPROM downloading or uploading. See “Serial Downloading” on page
349 for serial programming and verification.
SPI Data Register – SPDR
Bit
7
6
5
4
3
2
1
MSB
0
LSB
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
X
X
X
X
X
X
X
X
SPDR
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 register causes the Shift Register Receive buffer to be read.
Data Modes
202
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 81 and Figure 82. Data bits are shifted out and latched in on opposite edges of the SCK signal, ensuring sufficient time for data signals to stabilize. This is
clearly seen by summarizing Table 94 and Table 95, as done below:
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Table 97. CPOL Functionality
Leading Edge
Trailing eDge
SPI Mode
CPOL=0, CPHA=0
Sample (Rising)
Setup (Falling)
0
CPOL=0, CPHA=1
Setup (Rising)
Sample (Falling)
1
CPOL=1, CPHA=0
Sample (Falling)
Setup (Rising)
2
CPOL=1, CPHA=1
Setup (Falling)
Sample (Rising)
3
Figure 81. 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 82. 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
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USART
The Universal Synchronous and Asynchronous serial Receiver and Transmitter
(USART) is a highly flexible serial communication device. The main features are:
• Full Duplex Operation (Independent Serial Receive and Transmit Registers)
• Asynchronous or Synchronous Operation
• Master or Slave Clocked Synchronous Operation
• High Resolution Baud Rate Generator
• Supports Serial Frames with 5, 6, 7, 8, or 9 Data Bits and 1 or 2 Stop Bits
• Odd or Even Parity Generation and Parity Check Supported by Hardware
• Data OverRun Detection
• Framing Error Detection
• Noise Filtering Includes False Start Bit Detection and Digital Low Pass Filter
• Three Separate Interrupts on TX Complete, TX Data Register Empty and RX Complete
• Multi-processor Communication Mode
• Double Speed Asynchronous Communication Mode
Quad USART
The ATmega640/1280/1281/2560/2561 has four USART’s, USART0, USART1,
USART2, and USART3. The functionality for all four USART’s is described below.
USART0, USART1, USART2, and USART3 have different I/O registers as shown in
“Register Summary” on page 385.
Overview
A simplified block diagram of the USART Transmitter is shown in Figure 83 on page
206. CPU accessible I/O Registers and I/O pins are shown in bold.
The Power Reducion USART0 bit, PRUSART0, in “Power Reduction Register 0 PRR0” on page 54 must be disabled by writing a logical zero to it.
The Power Reducion USART1 bit, PRUSART1, in “Power Reduction Register 1 PRR1” on page 55 must be disabled by writing a logical zero to it.
The Power Reducion USART2 bit, PRUSART2, in “Power Reduction Register 1 PRR1” on page 55 must be disabled by writing a logical zero to it.
The Power Reducion USART3 bit, PRUSART3, in “Power Reduction Register 1 PRR1” on page 55 must be disabled by writing a logical zero to it.
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Figure 83. USART Block Diagram(1)
Clock Generator
UBRR[H:L]
OSC
BAUD RATE GENERATOR
SYNC LOGIC
PIN
CONTROL
XCK
Transmitter
TX
CONTROL
UDR (Transmit)
DATA BUS
PARITY
GENERATOR
TxD
Receiver
CLOCK
RECOVERY
RX
CONTROL
RECEIVE SHIFT REGISTER
DATA
RECOVERY
PIN
CONTROL
UDR (Receive)
PARITY
CHECKER
UCSRA
Note:
PIN
CONTROL
TRANSMIT SHIFT REGISTER
UCSRB
RxD
UCSRC
1. See Figure 1 on page 2, Figure 2 on page 3, Table 45 on page 94, Table 48 on page
96, Table 57 on page 104 and Table 60 on page 106 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 XCKn (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.
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Clock Generation
The Clock Generation logic generates the base clock for the Transmitter and Receiver.
The USARTn supports four modes of clock operation: Normal asynchronous, Double
Speed asynchronous, 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 XCKn pin (DDR_XCKn) controls whether the
clock source is internal (Master mode) or external (Slave mode). The XCKn pin is only
active when using synchronous mode.
Figure 84 shows a block diagram of the clock generation logic.
Figure 84. Clock Generation Logic, Block Diagram
UBRR
U2X
fosc
Prescaling
Down-Counter
UBRR+1
/2
/4
/2
0
1
0
OSC
DDR_XCK
xcki
XCK
Pin
Sync
Register
Edge
Detector
0
UCPOL
txclk
UMSEL
1
xcko
DDR_XCK
1
1
0
rxclk
Signal description:
txclk
Transmitter clock (Internal Signal).
rxclk
Receiver base clock (Internal Signal).
xcki
Input from XCK pin (internal Signal). Used for synchronous slave operation.
xcko
Clock output to XCK pin (Internal Signal). Used for synchronous master
operation.
fOSC
XTAL pin frequency (System Clock).
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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 84.
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 UBRRLn 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 output is used directly by the
Receiver’s clock and data recovery units. However, the recovery units use a state
machine that uses 2, 8 or 16 states depending on mode set by the state of the UMSELn,
U2Xn and DDR_XCKn bits.
Table 98 contains equations for calculating the baud rate (in bits per second) and for
calculating the UBRRn value for each mode of operation using an internally generated
clock source.
Table 98. Equations for Calculating Baud Rate Register Setting
Operating Mode
Equation for Calculating
Baud Rate(1)
Equation for Calculating
UBRR Value
f OSC
-–1
UBRRn = ----------------------16BAUD
Asynchronous Normal
mode (U2Xn = 0)
f OSC
BAUD = ----------------------------------------16 ( UBRRn + 1 )
Asynchronous Double
Speed mode (U2Xn =
1)
f OSC
UBRRn = -------------------–1
8BAUD
f OSC
BAUD = -------------------------------------8 ( UBRRn + 1 )
f OSC
UBRRn = -------------------–1
2BAUD
Synchronous Master
mode
f OSC
BAUD = -------------------------------------2 ( UBRRn + 1 )
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 UBRRHn and UBRRLn Registers, (0-4095)
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Some examples of UBRRn values for some system clock frequencies are found in Table
106 on page 227.
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.
External Clock
External clocking is used by the synchronous slave modes of operation. The description
in this section refers to Figure 84 for details.
External clock input from the XCKn pin is sampled by a synchronization register to minimize the chance of meta-stability. The output from the synchronization register must
then pass through an edge detector before it can be used by the Transmitter and
Receiver. This process introduces a two CPU clock period delay and therefore the maximum external XCKn 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.
Synchronous Clock Operation When synchronous mode is used (UMSELn = 1), the XCKn 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
RxDn) is sampled at the opposite XCKn clock edge of the edge the data output (TxDn)
is changed.
Figure 85. Synchronous Mode XCKn Timing.
UCPOL = 1
XCK
RxD / TxD
Sample
UCPOL = 0
XCK
RxD / TxD
Sample
The UCPOLn bit UCRSC selects which XCKn clock edge is used for data sampling and
which is used for data change. As Figure 85 shows, when UCPOLn is zero the data will
be changed at rising XCKn edge and sampled at falling XCKn edge. If UCPOLn is set,
the data will be changed at falling XCKn edge and sampled at rising XCKn edge.
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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 86 illustrates the possible
combinations of the frame formats. Bits inside brackets are optional.
Figure 86. Frame Formats
FRAME
(IDLE)
St
0
1
2
3
4
[5]
[6]
[7]
[8]
[P]
Sp1 [Sp2]
(St / IDLE)
St
Start bit, always low.
(n)
Data bits (0 to 8).
P
Parity bit. Can be odd or even.
Sp
Stop bit, always high.
IDLE
No transfers on the communication line (RxDn or TxDn). An IDLE line must be
high.
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 FE (Frame Error) will therefore only be
detected in the cases where the first stop bit is zero.
Parity Bit Calculation
The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is
used, the result of the exclusive or is inverted. The relation between the parity bit and
data bits is as follows::
P even = d n – 1 ⊕ … ⊕ d 3 ⊕ d 2 ⊕ d 1 ⊕ d 0 ⊕ 0
P odd = d n – 1 ⊕ … ⊕ d 3 ⊕ d 2 ⊕ d 1 ⊕ d 0 ⊕ 1
210
Peven
Parity bit using even parity
Podd
Parity bit using odd parity
dn
Data bit n of the character
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If used, the parity bit is located between the last data bit and first stop bit of a serial
frame.
USART Initialization
The USART has to be initialized before any communication can take place. The initialization process normally consists of setting the baud rate, setting frame format and
enabling the Transmitter or the Receiver depending on the usage. For interrupt driven
USART operation, the Global Interrupt Flag should be cleared (and interrupts globally
disabled) when doing the initialization.
Before doing a re-initialization with changed baud rate or frame format, be sure that
there are no ongoing transmissions during the period the registers are changed. The
TXCn Flag can be used to check that the Transmitter has completed all transfers, and
the RXC Flag can be used to check that there are no unread data in the receive buffer.
Note that the 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 function that are equal in functionality. The examples assume asynchronous operation using polling (no interrupts enabled) and a fixed frame format. The baud rate is
given as a function parameter. For the assembly code, the baud rate parameter is
assumed to be stored in the r17:r16 Registers.
Assembly Code Example(1)
USART_Init:
; Set baud rate
out
UBRRHn, r17
out
UBRRLn, r16
; Enable receiver and transmitter
ldi
r16, (1<<RXENn)|(1<<TXENn)
out
UCSRnB,r16
; Set frame format: 8data, 2stop bit
ldi
r16, (1<<USBSn)|(3<<UCSZn0)
out
UCSRnC,r16
ret
C Code Example(1)
void USART_Init( unsigned int baud )
{
/* Set baud rate */
UBRRHn = (unsigned char)(baud>>8);
UBRRLn = (unsigned char)baud;
/* Enable receiver and transmitter */
UCSRnB = (1<<RXENn)|(1<<TXENn);
/* Set frame format: 8data, 2stop bit */
UCSRnC = (1<<USBSn)|(3<<UCSZn0);
}
Note:
1. See “About Code Examples” on page 8.
More advanced initialization routines can be made that include frame format as parameters, disable interrupts and so on. However, many applications use a fixed setting of the
baud and control registers, and for these types of applications the initialization code can
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be placed directly in the main routine, or be combined with initialization code for other
I/O modules.
Data Transmission – The
USART Transmitter
The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the
UCSRnB Register. When the Transmitter is enabled, the normal port operation of the
TxDn pin is overridden by the USART and given the function as the Transmitter’s serial
output. The baud rate, mode of operation and frame format must be set up once before
doing any transmissions. If synchronous operation is used, the clock on the XCKn pin
will be overridden and used as transmission clock.
Sending Frames with 5 to 8
Data Bit
A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The CPU can load the transmit buffer by writing to the 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 XCKn
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 significant 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 UCSRnA,UDREn
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out
UDRn,r16
ret
C Code Example(1)
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) )
;
/* Put data into buffer, sends the data */
UDRn = data;
}
Note:
1. See “About Code Examples” on page 8.
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.
Sending Frames with 9 Data
Bit
212
If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8 bit in
UCSRnB before the low byte of the character is written to UDRn. The following code
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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 UCSRnA,UDREn
rjmp USART_Transmit
; Copy 9th bit from r17 to TXB8
cbi
UCSRnB,TXB8
sbrc r17,0
sbi
UCSRnB,TXB8
; Put LSB data (r16) into buffer, sends the data
out
UDRn,r16
ret
C Code Example(1)(2)
void USART_Transmit( unsigned int data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn))) )
;
/* Copy 9th bit to TXB8 */
UCSRnB &= ~(1<<TXB8);
if ( data & 0x0100 )
UCSRnB |= (1<<TXB8);
/* Put data into buffer, sends the data */
UDRn = data;
}
Notes:
1. These transmit functions are written to be general functions. They can be optimized if
the contents of the UCSRnB is static. For example, only the TXB8 bit of the UCSRnB
Register is used after initialization.
2. See “About Code Examples” on page 8.
The ninth bit can be used for indicating an address frame when using multi processor
communication mode or for other protocol handling as for example synchronization.
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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 contains data to be transmitted that has not yet been moved into the Shift
Register. For compatibility with future devices, always write this bit to zero when writing
the 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 communication 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 handling routine does not have to clear the TXCn Flag, this is done
automatically when the interrupt is executed.
Parity Generator
The Parity Generator calculates the parity bit for the serial frame data. When parity bit is
enabled (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 TXEN to zero) will not become effective
until ongoing and pending transmissions are completed, i.e., when the Transmit Shift
Register and Transmit Buffer Register do not contain data to be transmitted. When disabled, the Transmitter will no longer override the TxDn pin.
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 RxDn pin is overridden by the USART and given the function as the Receiver’s serial
input. The baud rate, mode of operation and frame format must be set up once before
any serial reception can be done. If synchronous operation is used, the clock on the
XCKn pin will be used as transfer clock.
Receiving Frames with 5 to 8
Data Bits
The Receiver starts data reception when it detects a valid start bit. Each bit that follows
the start bit will be sampled at the baud rate or XCKn 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.
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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 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 UCSRnA, RXCn
rjmp USART_Receive
; Get and return received data from buffer
in
r16, UDRn
ret
C Code Example(1)
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) )
;
/* Get and return received data from buffer */
return UDRn;
}
Note:
1. See “About Code Examples” on page 8.
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.
Receiving Frames with 9 Data
Bits
If 9-bit characters are used (UCSZn=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.
The following code example shows a simple USART receive function that handles both
nine bit characters and the status bits.
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Assembly Code Example(1)
USART_Receive:
; Wait for data to be received
sbis UCSRnA, RXCn
rjmp USART_Receive
; Get status and 9th bit, then data from buffer
in
r18, UCSRnA
in
r17, UCSRnB
in
r16, UDRn
; If error, return -1
andi r18,(1<<FEn)|(1<<DORn)|(1<<UPEn)
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 ( !(UCSRnA & (1<<RXCn)) )
;
/* Get status and 9th bit, then data */
/* from buffer */
status = UCSRnA;
resh = UCSRnB;
resl = UDRn;
/* If error, return -1 */
if ( status & (1<<FEn)|(1<<DORn)|(1<<UPEn) )
return -1;
/* Filter the 9th bit, then return */
resh = (resh >> 1) & 0x01;
return ((resh << 8) | resl);
}
Note:
1. See “About Code Examples” on page 8.
The receive function example reads all the I/O Registers into the Register File before
any computation is done. This gives an optimal receive buffer utilization since the buffer
location read will be free to accept new data as early as possible.
Receive Compete Flag and
Interrupt
216
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 buffer. This flag is one when unread data exist in the receive buffer, and zero
when the receive buffer is empty (i.e., does not contain any unread data). If the Receiver
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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 interrupts 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 interrupt will occur once the interrupt routine terminates.
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 waiting 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 210 and “Parity
Checker” on page 217.
Parity Checker
The Parity Checker is active when the high USART Parity mode (UPMn1) bit is set.
Type of Parity 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 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.
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 RxDn port pin. The
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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 UCSRnA, RXCn
ret
r16, UDRn
in
rjmp USART_Flush
C Code Example(1)
void USART_Flush( void )
{
unsigned char dummy;
while ( UCSRnA & (1<<RXCn) ) dummy = UDRn;
}
Note:
1. See “About Code Examples” on page 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 RxDn pin.
The data recovery logic samples and low pass filters each incoming bit, thereby improving the noise immunity of the Receiver. The asynchronous reception operational range
depends on the accuracy of the internal baud rate clock, the rate of the incoming
frames, and the frame size in number of bits.
Asynchronous Clock
Recovery
The clock recovery logic synchronizes internal clock to the incoming serial frames. Figure 87 illustrates the sampling process of the start bit of an incoming frame. The sample
rate is 16 times the baud rate for Normal mode, and eight times the baud rate for Double
Speed mode. The horizontal arrows illustrate the synchronization variation due to the
sampling process. Note the larger time variation when using the Double Speed mode
(U2Xn = 1) of operation. Samples denoted zero are samples done when the RxDn line
is idle (i.e., no communication activity).
Figure 87. Start Bit Sampling
RxD
IDLE
START
BIT 0
Sample
(U2X = 0)
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
2
3
Sample
(U2X = 1)
0
1
2
3
4
5
6
7
8
1
2
When the clock recovery logic detects a high (idle) to low (start) transition on the RxDn
line, the start bit detection sequence is initiated. Let sample 1 denote the first zero-sam-
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ple as shown in the figure. The clock recovery logic then uses samples 8, 9, and 10 for
Normal mode, and samples 4, 5, and 6 for Double Speed mode (indicated with sample
numbers inside boxes on the figure), to decide if a valid start bit is received. If two or
more of these three samples have logical high levels (the majority wins), the start bit is
rejected as a noise spike and the Receiver starts looking for the next high to low-transition. If however, a valid start bit is detected, the clock recovery logic is synchronized and
the data recovery can begin. The synchronization process is repeated for each start bit.
Asynchronous Data Recovery
When the receiver clock is synchronized to the start bit, the data recovery can begin.
The data recovery unit uses a state machine that has 16 states for each bit in Normal
mode and eight states for each bit in Double Speed mode. Figure 88 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 88. Sampling of Data and Parity Bit
RxD
BIT n
Sample
(U2X = 0)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
Sample
(U2X = 1)
1
2
3
4
5
6
7
8
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 RxDn 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 89 shows the sampling of the stop bit and the earliest possible beginning of the
start bit of the next frame.
Figure 89. Stop Bit Sampling and Next Start Bit Sampling
RxD
STOP 1
(A)
(B)
(C)
Sample
(U2X = 0)
1
2
3
4
5
6
7
8
9
10
0/1
0/1
0/1
Sample
(U2X = 1)
1
2
3
4
5
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 89. For Double Speed mode the first low
level must be delayed to (B). (C) marks a stop bit of full length. The early start bit detection influences the operational range of the Receiver.
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Asynchronous Operational
Range
The operational range of the Receiver is dependent on the mismatch between the
received bit rate and the internally generated baud rate. If the Transmitter is sending
frames at too fast or too slow bit rates, or the internally generated baud rate of the
Receiver does not have a similar (see Table 99) 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
R slow = ------------------------------------------S – 1 + D ⋅ S + SF
( D + 2 )S
R fast = ----------------------------------( D + 1 )S + S M
D
Sum of character size and parity size (D = 5 to 10 bit)
S
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 99 and Table 100 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 99. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode
(U2Xn = 0)
D
# (Data+Parity Bit)
Rslow (%)
Rfast (%)
Max Total Error (%)
Recommended Max
Receiver Error (%)
5
93.20
106.67
+6.67/-6.8
± 3.0
6
94.12
105.79
+5.79/-5.88
± 2.5
7
94.81
105.11
+5.11/-5.19
± 2.0
8
95.36
104.58
+4.58/-4.54
± 2.0
9
95.81
104.14
+4.14/-4.19
± 1.5
10
96.17
103.78
+3.78/-3.83
± 1.5
Table 100. Recommended Maximum Receiver Baud Rate Error for Double Speed
Mode (U2Xn = 1)
D
# (Data+Parity Bit)
Rslow (%)
Rfast (%)
Max Total Error (%)
Recommended Max
Receiver Error (%)
5
94.12
105.66
+5.66/-5.88
± 2.5
6
94.92
104.92
+4.92/-5.08
± 2.0
7
95.52
104,35
+4.35/-4.48
± 1.5
8
96.00
103.90
+3.90/-4.00
± 1.5
9
96.39
103.53
+3.53/-3.61
± 1.5
10
96.70
103.23
+3.23/-3.30
± 1.0
The recommendations of the maximum receiver baud rate error was made under the
assumption that the Receiver and Transmitter equally divides the maximum total error.
There are two possible sources for the receivers baud rate error. The Receiver’s system
clock (XTAL) will always have some minor instability over the supply voltage range and
the temperature range. When using a crystal to generate the system clock, this is rarely
a problem, but for a resonator the system clock may differ more than 2% depending of
the resonators tolerance. The second source for the error is more controllable. The baud
rate generator can not always do an exact division of the system frequency to get the
baud rate wanted. In this case an UBRR value that gives an acceptable low error can be
used if possible.
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 indicates if the frame contains data or address information. If the Receiver is set up
for frames with nine data bits, then the ninth bit (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.
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The Multi-processor Communication mode enables several slave MCUs to receive data
from a master MCU. This is done by first decoding an address frame to find out which
MCU has been addressed. If a particular slave MCU has been addressed, it will receive
the following data frames as normal, while the other slave MCUs will ignore the received
frames until another address frame is received.
Using MPCMn
For an MCU to act as a master MCU, it can use a 9-bit character frame format (UCSZn
= 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 setting. 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.
USART Register
Description
USART I/O Data Register n–
UDRn
Bit
7
6
5
4
3
2
1
0
RXB[7:0]
UDRn (Read)
TXB[7:0]
UDRn (Write)
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
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 Register (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.
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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 Transmitter. When data is written to the transmit buffer, and the Transmitter is
enabled, the Transmitter will load the data into the Transmit Shift Register when the
Shift Register is empty. Then the data will be serially transmitted on the TxDn pin.
The receive buffer consists of a two level FIFO. The FIFO will change its state whenever
the receive buffer is accessed. Due to this behavior of the receive buffer, do not use
Read-Modify-Write instructions (SBI and CBI) on this location. Be careful when using bit
test instructions (SBIC and SBIS), since these also will change the state of the FIFO.
USART Control and Status
Register A – UCSRnA
Bit
7
6
5
4
3
2
1
0
RXCn
TXCn
UDREn
FEn
DORn
UPEn
U2Xn
MPCMn
Read/Write
R
R/W
R
R
R
R
R/W
R/W
Initial Value
0
0
1
0
0
0
0
0
UCSRnA
• 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 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 can generate a
Transmit Complete interrupt (see description of the TXCIEn bit).
• 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.
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• 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
synchronous operation.
Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively doubling 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 information will be ignored. The Transmitter is unaffected by the MPCMn setting. For more detailed information see “Multi-processor Communication Mode” on page
221.
USART Control and Status
Register n B – UCSRnB
Bit
7
6
5
4
3
2
1
0
RXCIEn
TXCIEn
UDRIEn
RXENn
TXENn
UCSZn2
RXB8n
TXB8n
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R
R/W
Initial Value
0
0
0
0
0
0
0
0
UCSRnB
• Bit 7 – RXCIEn: RX Complete Interrupt Enable n
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.
• Bit 6 – TXCIEn: TX Complete Interrupt Enable n
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 n
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 – RXENn: Receiver Enable n
Writing this bit to one enables the USART Receiver. The Receiver will override normal
port operation for the RxDn pin when enabled. Disabling the Receiver will flush the
receive buffer invalidating the FEn, DORn, and UPEn Flags.
• Bit 3 – TXENn: Transmitter Enable n
Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port operation for the TxDn 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
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do not contain data to be transmitted. When disabled, the Transmitter will no longer
override the TxDn port.
• Bit 2 – UCSZn2: Character Size n
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 n
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 n
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.
USART Control and Status
Register n C – UCSRnC
Bit
7
6
5
4
3
2
1
0
UMSELn1
UMSELn0
UPMn1
UPMn0
USBSn
UCSZn1
UCSZn0
UCPOLn
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
1
1
0
UCSRnC
• Bits 7:6 – UMSELn1:0 USART Mode Select
These bits select the mode of operation of the USARTn as shown in Table 101..
Table 101. UMSELn Bits Settings
Note:
UMSELn1
UMSELn0
Mode
0
0
Asynchronous USART
0
1
Synchronous USART
1
0
(Reserved)
1
1
Master SPI (MSPIM)(1)
1. See “USART in SPI Mode” on page 231 for full description of the Master SPI Mode
(MSPIM) operation
• Bits 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 UPMn setting. If a mismatch is detected, the UPEn Flag in UCSRnA will be
set.
Table 102. UPMn Bits Settings
UPMn1
UPMn0
Parity Mode
0
0
Disabled
0
1
Reserved
1
0
Enabled, Even Parity
1
1
Enabled, Odd Parity
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• 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.
Table 103. USBS Bit Settings
USBSn
Stop Bit(s)
0
1-bit
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.
Table 104. UCSZn Bits Settings
UCSZn2
UCSZn1
UCSZn0
Character Size
0
0
0
5-bit
0
0
1
6-bit
0
1
0
7-bit
0
1
1
8-bit
1
0
0
Reserved
1
0
1
Reserved
1
1
0
Reserved
1
1
1
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 (XCKn).
Table 105. UCPOLn Bit Settings
Transmitted Data Changed (Output
of TxDn Pin)
Received Data Sampled (Input on
RxDn Pin)
0
Rising XCKn Edge
Falling XCKn Edge
1
Falling XCKn Edge
Rising XCKn Edge
UCPOLn
USART Baud Rate Registers –
UBRRLn and UBRRHn
Bit
15
14
13
12
–
–
–
–
11
10
9
8
UBRR[11:8]
UBRRHn
UBRR[7:0]
7
Read/Write
Initial Value
226
6
5
UBRRLn
4
3
2
1
0
R
R
R
R
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
• 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 UBRRH is written.
• Bit 11:0 – UBRR11:0: USART Baud Rate Register
This is a 12-bit register which contains the USART baud rate. The UBRRH contains the
four most significant bits, and the UBRRL contains the eight least significant bits of the
USART baud rate. Ongoing transmissions by the Transmitter and Receiver will be corrupted if the baud rate is changed. Writing UBRRL will trigger an immediate update of
the baud rate prescaler.
Examples of Baud Rate
Setting
For standard crystal and resonator frequencies, the most commonly used baud rates for
asynchronous operation can be generated by using the UBRR settings in Table 106 to
Table 109. UBRR values which yield an actual baud rate differing less than 0.5% from
the target baud rate, are bold in the table. Higher error ratings are acceptable, but the
Receiver will have less noise resistance when the error ratings are high, especially for
large serial frames (see “Asynchronous Operational Range” on page 220). The error
values are calculated using the following equation:
BaudRate Closest Match
- – 1⎞ • 100%
Error[%] = ⎛ ------------------------------------------------------⎝
⎠
BaudRate
Table 106. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
fosc = 1.0000 MHz
fosc = 1.8432 MHz
fosc = 2.0000 MHz
Baud
Rate
(bps)
UBRR
2400
25
0.2%
51
0.2%
47
0.0%
95
0.0%
51
0.2%
103
0.2%
4800
12
0.2%
25
0.2%
23
0.0%
47
0.0%
25
0.2%
51
0.2%
9600
6
-7.0%
12
0.2%
11
0.0%
23
0.0%
12
0.2%
25
0.2%
14.4k
3
8.5%
8
-3.5%
7
0.0%
15
0.0%
8
-3.5%
16
2.1%
19.2k
2
8.5%
6
-7.0%
5
0.0%
11
0.0%
6
-7.0%
12
0.2%
28.8k
1
8.5%
3
8.5%
3
0.0%
7
0.0%
3
8.5%
8
-3.5%
38.4k
1
-18.6%
2
8.5%
2
0.0%
5
0.0%
2
8.5%
6
-7.0%
57.6k
0
8.5%
1
8.5%
1
0.0%
3
0.0%
1
8.5%
3
8.5%
76.8k
–
–
1
-18.6%
1
-25.0%
2
0.0%
1
-18.6%
2
8.5%
115.2k
–
–
0
8.5%
0
0.0%
1
0.0%
0
8.5%
1
8.5%
230.4k
–
–
–
–
–
–
0
0.0%
–
–
–
–
250k
–
–
–
–
–
–
–
–
–
–
0
0.0%
U2Xn = 0
Max. (1)
1.
Error
U2Xn = 1
UBRR
62.5 kbps
Error
125 kbps
U2Xn = 0
UBRR
Error
115.2 kbps
U2Xn = 1
UBRR
Error
230.4 kbps
U2Xn = 0
UBRR
Error
125 kbps
U2Xn = 1
UBRR
Error
250 kbps
UBRR = 0, Error = 0.0%
227
2549A–AVR–03/05
Table 107. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)
fosc = 3.6864 MHz
fosc = 4.0000 MHz
fosc = 7.3728 MHz
Baud
Rate
(bps)
UBRR
2400
95
0.0%
191
0.0%
103
0.2%
207
0.2%
191
0.0%
383
0.0%
4800
47
0.0%
95
0.0%
51
0.2%
103
0.2%
95
0.0%
191
0.0%
9600
23
0.0%
47
0.0%
25
0.2%
51
0.2%
47
0.0%
95
0.0%
14.4k
15
0.0%
31
0.0%
16
2.1%
34
-0.8%
31
0.0%
63
0.0%
19.2k
11
0.0%
23
0.0%
12
0.2%
25
0.2%
23
0.0%
47
0.0%
28.8k
7
0.0%
15
0.0%
8
-3.5%
16
2.1%
15
0.0%
31
0.0%
38.4k
5
0.0%
11
0.0%
6
-7.0%
12
0.2%
11
0.0%
23
0.0%
57.6k
3
0.0%
7
0.0%
3
8.5%
8
-3.5%
7
0.0%
15
0.0%
76.8k
2
0.0%
5
0.0%
2
8.5%
6
-7.0%
5
0.0%
11
0.0%
115.2k
1
0.0%
3
0.0%
1
8.5%
3
8.5%
3
0.0%
7
0.0%
230.4k
0
0.0%
1
0.0%
0
8.5%
1
8.5%
1
0.0%
3
0.0%
250k
0
-7.8%
1
-7.8%
0
0.0%
1
0.0%
1
-7.8%
3
-7.8%
0.5M
–
–
0
-7.8%
–
–
0
0.0%
0
-7.8%
1
-7.8%
–
–
–
–
–
–
–
–
–
–
0
-7.8%
1M
Max.
1.
228
(1)
U2Xn = 0
U2Xn = 1
Error
UBRR
230.4 kbps
U2Xn = 0
Error
460.8 kbps
UBRR
U2Xn = 1
Error
250 kbps
UBRR
U2Xn = 0
Error
0.5 Mbps
UBRR
U2Xn = 1
Error
460.8 kbps
UBRR
Error
921.6 kbps
UBRR = 0, Error = 0.0%
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 108. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)
fosc = 11.0592 MHz
fosc = 8.0000 MHz
fosc = 14.7456 MHz
Baud
Rate
(bps)
UBRR
2400
207
0.2%
416
-0.1%
287
0.0%
575
0.0%
383
0.0%
767
0.0%
4800
103
0.2%
207
0.2%
143
0.0%
287
0.0%
191
0.0%
383
0.0%
9600
51
0.2%
103
0.2%
71
0.0%
143
0.0%
95
0.0%
191
0.0%
14.4k
34
-0.8%
68
0.6%
47
0.0%
95
0.0%
63
0.0%
127
0.0%
19.2k
25
0.2%
51
0.2%
35
0.0%
71
0.0%
47
0.0%
95
0.0%
28.8k
16
2.1%
34
-0.8%
23
0.0%
47
0.0%
31
0.0%
63
0.0%
38.4k
12
0.2%
25
0.2%
17
0.0%
35
0.0%
23
0.0%
47
0.0%
57.6k
8
-3.5%
16
2.1%
11
0.0%
23
0.0%
15
0.0%
31
0.0%
76.8k
6
-7.0%
12
0.2%
8
0.0%
17
0.0%
11
0.0%
23
0.0%
115.2k
3
8.5%
8
-3.5%
5
0.0%
11
0.0%
7
0.0%
15
0.0%
230.4k
1
8.5%
3
8.5%
2
0.0%
5
0.0%
3
0.0%
7
0.0%
250k
1
0.0%
3
0.0%
2
-7.8%
5
-7.8%
3
-7.8%
6
5.3%
0.5M
0
0.0%
1
0.0%
–
–
2
-7.8%
1
-7.8%
3
-7.8%
–
–
0
0.0%
–
–
–
–
0
-7.8%
1
-7.8%
U2Xn = 0
1M
Max.
1.
(1)
Error
U2Xn = 1
UBRR
0.5 Mbps
Error
1 Mbps
U2Xn = 0
UBRR
Error
691.2 kbps
U2Xn = 1
UBRR
Error
1.3824 Mbps
U2Xn = 0
UBRR
Error
921.6 kbps
U2Xn = 1
UBRR
Error
1.8432 Mbps
UBRR = 0, Error = 0.0%
229
2549A–AVR–03/05
Table 109. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies (Continued)
fosc = 16.0000 MHz
fosc = 18.4320 MHz
fosc = 20.0000 MHz
Baud
Rate
(bps)
UBRR
2400
416
-0.1%
832
0.0%
479
0.0%
959
0.0%
520
0.0%
1041
0.0%
4800
207
0.2%
416
-0.1%
239
0.0%
479
0.0%
259
0.2%
520
0.0%
9600
103
0.2%
207
0.2%
119
0.0%
239
0.0%
129
0.2%
259
0.2%
14.4k
68
0.6%
138
-0.1%
79
0.0%
159
0.0%
86
-0.2%
173
-0.2%
19.2k
51
0.2%
103
0.2%
59
0.0%
119
0.0%
64
0.2%
129
0.2%
28.8k
34
-0.8%
68
0.6%
39
0.0%
79
0.0%
42
0.9%
86
-0.2%
38.4k
25
0.2%
51
0.2%
29
0.0%
59
0.0%
32
-1.4%
64
0.2%
57.6k
16
2.1%
34
-0.8%
19
0.0%
39
0.0%
21
-1.4%
42
0.9%
76.8k
12
0.2%
25
0.2%
14
0.0%
29
0.0%
15
1.7%
32
-1.4%
115.2k
8
-3.5%
16
2.1%
9
0.0%
19
0.0%
10
-1.4%
21
-1.4%
230.4k
3
8.5%
8
-3.5%
4
0.0%
9
0.0%
4
8.5%
10
-1.4%
250k
3
0.0%
7
0.0%
4
-7.8%
8
2.4%
4
0.0%
9
0.0%
0.5M
1
0.0%
3
0.0%
–
–
4
-7.8%
–
–
4
0.0%
0
0.0%
1
0.0%
–
–
–
–
–
–
–
–
1M
Max.
1.
230
(1)
U2Xn = 0
Error
U2Xn = 1
UBRR
1 Mbps
Error
2 Mbps
U2Xn = 0
UBRR
Error
1.152 Mbps
U2Xn = 1
UBRR
Error
2.304 Mbps
U2Xn = 0
UBRR
Error
1.25 Mbps
U2Xn = 1
UBRR
Error
2.5 Mbps
UBRR = 0, Error = 0.0%
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
USART in SPI Mode
The Universal Synchronous and Asynchronous serial Receiver and Transmitter
(USART) can be set to a master SPI compliant mode of operation. The Master SPI
Mode (MSPIM) has the following features:
• Full Duplex, Three-wire Synchronous Data Transfer
• Master Operation
• Supports all four SPI Modes of Operation (Mode 0, 1, 2, and 3)
• LSB First or MSB First Data Transfer (Configurable Data Order)
• Queued Operation (Double Buffered)
• High Resolution Baud Rate Generator
• High Speed Operation (fXCKmax = fCK/2)
• Flexible Interrupt Generation
Overview
Setting both UMSELn1:0 bits to one enables the USART in MSPIM logic. In this mode of
operation the SPI master control logic takes direct control over the USART resources.
These resources include the transmitter and receiver shift register and buffers, and the
baud rate generator. The parity generator and checker, the data and clock recovery
logic, and the RX and TX control logic is disabled. The USART RX and TX control logic
is replaced by a common SPI transfer control logic. However, the pin control logic and
interrupt generation logic is identical in both modes of operation.
The I/O register locations are the same in both modes. However, some of the functionality of the control registers changes when using MSPIM.
Clock Generation
The Clock Generation logic generates the base clock for the Transmitter and Receiver.
For USART MSPIM mode of operation only internal clock generation (i.e. master operation) is supported. The Data Direction Register for the XCKn pin (DDR_XCKn) must
therefore be set to one (i.e. as output) for the USART in MSPIM to operate correctly.
Preferably the DDR_XCKn should be set up before the USART in MSPIM is enabled
(i.e. TXENn and RXENn bit set to one).
The internal clock generation used in MSPIM mode is identical to the USART synchronous master mode. The baud rate or UBRRn setting can therefore be calculated using
the same equations, see Table 110:
Table 110. Equations for Calculating Baud Rate Register Setting
Operating Mode
Equation for Calculating Baud
Rate(1)
Equation for Calculating
UBRRn Value
f OSC
BAUD = -------------------------------------2 ( UBRRn + 1 )
f OSC
UBRRn = -------------------–1
2BAUD
Synchronous Master
mode
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)
SPI Data Modes and
Timing
There are four combinations of XCKn (SCK) phase and polarity with respect to serial
data, which are determined by control bits UCPHAn and UCPOLn. The data transfer
231
2549A–AVR–03/05
timing diagrams are shown in Figure 90. Data bits are shifted out and latched in on
opposite edges of the XCKn signal, ensuring sufficient time for data signals to stabilize.
The UCPOLn and UCPHAn functionality is summarized in Table 111. Note that changing the setting of any of these bits will corrupt all ongoing communication for both the
Receiver and Transmitter.
Table 111. UCPOLn and UCPHAn FunctionalityUCPOLn
UCPHAn
SPI Mode
Leading Edge
Trailing Edge
0
0
0
Sample (Rising)
Setup (Falling)
0
1
1
Setup (Rising)
Sample (Falling)
1
0
2
Sample (Falling)
Setup (Rising)
1
1
3
Setup (Falling)
Sample (Rising)
Figure 90. UCPHAn and UCPOLn data transfer timing diagrams.
UCPHA=0
UCPHA=1
UCPOL=0
Frame Formats
UCPOL=1
XCK
XCK
Data setup (TXD)
Data setup (TXD)
Data sample (RXD)
Data sample (RXD)
XCK
XCK
Data setup (TXD)
Data setup (TXD)
Data sample (RXD)
Data sample (RXD)
A serial frame for the MSPIM is defined to be one character of 8 data bits. The USART
in MSPIM mode has two valid frame formats:
•
8-bit data with MSB first
•
8-bit data with LSB first
A frame starts with the least or most significant data bit. Then the next data bits, up to a
total of eight, are succeeding, ending with the most or least significant bit accordingly.
When a complete frame is transmitted, a new frame can directly follow it, or the communication line can be set to an idle (high) state.
The UDORDn bit in UCSRnC sets the frame format used by the USART in MSPIM
mode. 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.
16-bit data transfer can be achieved by writing two data bytes to UDRn. A UART transmit complete interrupt will then signal that the 16-bit value has been shifted out.
232
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
USART MSPIM Initialization
The USART in MSPIM mode has to be initialized before any communication can take
place. The initialization process normally consists of setting the baud rate, setting master mode of operation (by setting DDR_XCKn to one), setting frame format and enabling
the Transmitter and the Receiver. Only the transmitter can operate independently. For
interrupt driven USART operation, the Global Interrupt Flag should be cleared (and thus
interrupts globally disabled) when doing the initialization.
Note:
To ensure immediate initialization of the XCKn output the baud-rate register (UBRRn)
must be zero at the time the transmitter is enabled. Contrary to the normal mode USART
operation the UBRRn must then be written to the desired value after the transmitter is
enabled, but before the first transmission is started. Setting UBRRn to zero before
enabling the transmitter is not necessary if the initialization is done immediately after a
reset since UBRRn is reset to zero.
Before doing a re-initialization with changed baud rate, data mode, or frame format, be
sure that there is 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 function that are equal in functionality. The examples assume polling (no interrupts
enabled). The baud rate is given as a function parameter. For the assembly code, the
baud rate parameter is assumed to be stored in the r17:r16 registers.
233
2549A–AVR–03/05
Assembly Code Example(1)
USART_Init:
clr r18
out UBRRnH,r18
out UBRRnL,r18
; Setting the XCKn port pin as output, enables master mode.
sbi XCKn_DDR, XCKn
; Set MSPI mode of operation and SPI data mode 0.
ldi r18, (1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn)
out UCSRnC,r18
; Enable receiver and transmitter.
ldi r18, (1<<RXENn)|(1<<TXENn)
out UCSRnB,r18
; Set baud rate.
; IMPORTANT: The Baud Rate must be set after the transmitter is enabled!
out UBRRnH, r17
out UBRRnL, r18
ret
C Code Example(1)
void USART_Init( unsigned int baud )
{
UBRRn = 0;
/* Setting the XCKn port pin as output, enables master mode. */
XCKn_DDR |= (1<<XCKn);
/* Set MSPI mode of operation and SPI data mode 0. */
UCSRnC = (1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn);
/* Enable receiver and transmitter. */
UCSRnB = (1<<RXENn)|(1<<TXENn);
/* Set baud rate. */
/* IMPORTANT: The Baud Rate must be set after the transmitter is enabled
*/
UBRRn = baud;
}
Note:
Data Transfer
1. See “About Code Examples” on page 8.
Using the USART in MSPI mode requires the Transmitter to be enabled, i.e. the TXENn
bit in the UCSRnB register is set to one. When the Transmitter is enabled, the normal
port operation of the TxDn pin is overridden and given the function as the Transmitter's
serial output. Enabling the receiver is optional and is done by setting the RXENn bit in
the UCSRnB register to one. When the receiver is enabled, the normal pin operation of
the RxDn pin is overridden and given the function as the Receiver's serial input. The
XCKn will in both cases be used as the transfer clock.
After initialization the USART is ready for doing data transfers. A data transfer is initiated
by writing to the UDRn I/O location. This is the case for both sending and receiving data
since the transmitter controls the transfer clock. The data written to UDRn is moved from
234
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
the transmit buffer to the shift register when the shift register is ready to send a new
frame.
Note:
To keep the input buffer in sync with the number of data bytes transmitted, the UDRn register must be read once for each byte transmitted. The input buffer operation is identical
to normal USART mode, i.e. if an overflow occurs the character last received will be lost,
not the first data in the buffer. This means that if four bytes are transferred, byte 1 first,
then byte 2, 3, and 4, and the UDRn is not read before all transfers are completed, then
byte 3 to be received will be lost, and not byte 1.
The following code examples show a simple USART in MSPIM mode transfer function
based on polling of the Data Register Empty (UDREn) Flag and the Receive Complete
(RXCn) Flag. 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 and the
data received will be available in the same register (R16) after the function returns.
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. The function then waits for data
to be present in the receive buffer by checking the RXCn Flag, before reading the buffer
and returning the value..
Assembly Code Example(1)
USART_MSPIM_Transfer:
; Wait for empty transmit buffer
sbis UCSRnA, UDREn
rjmp USART_MSPIM_Transfer
; Put data (r16) into buffer, sends the data
out UDRn,r16
; Wait for data to be received
USART_MSPIM_Wait_RXCn:
sbis UCSRnA, RXCn
rjmp USART_MSPIM_Wait_RXCn
; Get and return received data from buffer
in r16, UDRn
ret
C Code Example(1)
unsigned char USART_Receive( void )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) );
/* Put data into buffer, sends the data */
UDRn = data;
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) );
/* Get and return received data from buffer */
return UDRn;
}
Note:
Transmitter and Receiver
Flags and Interrupts
1. See “About Code Examples” on page 8.
The RXCn, TXCn, and UDREn flags and corresponding interrupts in USART in MSPIM
mode are identical in function to the normal USART operation. However, the receiver
error status flags (FE, DOR, and PE) are not in use and is always read as zero.
235
2549A–AVR–03/05
Disabling the Transmitter or
Receiver
The disabling of the transmitter or receiver in USART in MSPIM mode is identical in
function to the normal USART operation.
USART MSPIM Register
Description
The following section describes the registers used for SPI operation using the USART.
USART MSPIM I/O Data
Register - UDRn
The function and bit description of the USART data register (UDRn) in MSPI mode is
identical to normal USART operation. See “USART I/O Data Register n– UDRn” on
page 222.
USART MSPIM Control and
Status Register n A - UCSRnA
•
Bit
7
6
5
4
3
2
1
RXCn
TXCn
UDREn
-
-
-
-
0
-
Read/Write
R/W
R/W
R/W
R
R
R
R
R
Initial Value
0
0
0
0
0
1
1
0
UCSRnA
• 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 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 can generate a
Transmit Complete interrupt (see description of the TXCIEn bit).
• 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 UDRIE bit).
UDREn is set after a reset to indicate that the Transmitter is ready.
• Bit 4:0 - Reserved Bits in MSPI mode
When in MSPI mode, these bits are reserved for future use. For compatibility with future
devices, these bits must be written to zero when UCSRnA is written.
USART MSPIM Control and
Status Register n B - UCSRnB
Bit
7
6
5
4
3
2
1
RXCIEn
TXCIEn
UDRIE
RXENn
TXENn
-
-
0
-
Read/Write
R/W
R/W
R/W
R/W
R/W
R
R
R
Initial Value
0
0
0
0
0
1
1
0
UCSRnB
• 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.
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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 - UDRIE: 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 UDRIE 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 - RXENn: Receiver Enable
Writing this bit to one enables the USART Receiver in MSPIM mode. The Receiver will
override normal port operation for the RxDn pin when enabled. Disabling the Receiver
will flush the receive buffer. Only enabling the receiver in MSPI mode (i.e. setting
RXENn=1 and TXENn=0) has no meaning since it is the transmitter that controls the
transfer clock and since only master mode is supported.
• Bit 3 - TXENn: Transmitter Enable
Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port operation for the TxDn 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 transmitted. When disabled, the Transmitter will no longer
override the TxDn port.
• Bit 2:0 - Reserved Bits in MSPI mode
When in MSPI mode, these bits are reserved for future use. For compatibility with future
devices, these bits must be written to zero when UCSRnB is written.
USART MSPIM Control and
Status Register n C - UCSRnC
Bit
7
6
5
4
3
2
1
0
UMSELn1
UMSELn0
-
-
-
UDORDn
UCPHAn
UCPOLn
Read/Write
R/W
R/W
R
R
R
R/W
R/W
R/W
Initial Value
0
0
0
0
0
1
1
0
UCSRnC
• Bit 7:6 - UMSELn1:0: USART Mode Select
These bits select the mode of operation of the USART as shown in Table 112. See
“USART Control and Status Register n C – UCSRnC” on page 225 for full description of
the normal USART operation. The MSPIM is enabled when both UMSELn bits are set to
one. The UDORDn, UCPHAn, and UCPOLn can be set in the same write operation
where the MSPIM is enabled.
Table 112. UMSELn Bits Settings
UMSELn1
UMSELn0
Mode
0
0
Asynchronous USART
0
1
Synchronous USART
1
0
(Reserved)
1
1
Master SPI (MSPIM)
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• Bit 5:3 - Reserved Bits in MSPI mode
When in MSPI mode, these bits are reserved for future use. For compatibility with future
devices, these bits must be written to zero when UCSRnC is written.
• Bit 2 - UDORDn: Data Order
When set to one the LSB of the data word is transmitted first. When set to zero the MSB
of the data word is transmitted first. Refer to the Frame Formats section page 4 for
details.
• Bit 1 - UCPHAn: Clock Phase
The UCPHAn bit setting determine if data is sampled on the leasing edge (first) or tailing
(last) edge of XCKn. Refer to the SPI Data Modes and Timing section page 4 for details.
• Bit 0 - UCPOLn: Clock Polarity
The UCPOLn bit sets the polarity of the XCKn clock. The combination of the UCPOLn
and UCPHAn bit settings determine the timing of the data transfer. Refer to the SPI Data
Modes and Timing section page 4 for details.
USART MSPIM Baud Rate
Registers - UBRRnL and
UBRRnH
The function and bit description of the baud rate registers in MSPI mode is identical to
normal USART operation. See “USART Baud Rate Registers – UBRRLn and UBRRHn”
on page 226.
AVR USART MSPIM vs.
AVR SPI
The USART in MSPIM mode is fully compatible with the AVR SPI regarding:
•
Master mode timing diagram.
•
The UCPOLn bit functionality is identical to the SPI CPOL bit.
•
The UCPHAn bit functionality is identical to the SPI CPHA bit.
•
The UDORDn bit functionality is identical to the SPI DORD bit.
However, since the USART in MSPIM mode reuses the USART resources, the use of
the USART in MSPIM mode is somewhat different compared to the SPI. In addition to
differences of the control register bits, and that only master operation is supported by
the USART in MSPIM mode, the following features differ between the two modules:
•
The USART in MSPIM mode includes (double) buffering of the transmitter. The SPI
has no buffer.
•
The USART in MSPIM mode receiver includes an additional buffer level.
•
The SPI WCOL (Write Collision) bit is not included in USART in MSPIM mode.
•
The SPI double speed mode (SPI2X) bit is not included. However, the same effect is
achieved by setting UBRRn accordingly.
•
Interrupt timing is not compatible.
•
Pin control differs due to the master only operation of the USART in MSPIM mode.
A comparison of the USART in MSPIM mode and the SPI pins is shown in Table 113 on
page 239.
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Table 113. Comparison of USART in MSPIM mode and SPI pins.
USART_MSPIM
SPI
Comment
TxDn
MOSI
Master Out only
RxDn
MISO
Master In only
XCKn
SCK
(Functionally identical)
(N/A)
SS
Not supported by USART in MSPIM
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2-wire Serial Interface
Features
•
•
•
•
•
•
•
•
•
•
2-wire Serial Interface
Bus Definition
The 2-wire Serial Interface (TWI) is ideally suited for typical microcontroller applications.
The TWI protocol allows the systems designer to interconnect up to 128 different
devices using only two bi-directional bus lines, one for clock (SCL) and one for data
(SDA). The only external hardware needed to implement the bus is a single pull-up
resistor for each of the TWI bus lines. All devices connected to the bus have individual
addresses, and mechanisms for resolving bus contention are inherent in the TWI
protocol.
Simple yet Powerful and Flexible Communication Interface, only two Bus Lines needed
Both Master and Slave Operation Supported
Device can Operate as Transmitter or Receiver
7-bit Address Space Allows up to 128 Different Slave Addresses
Multi-master Arbitration Support
Up to 400 kHz Data Transfer Speed
Slew-rate Limited Output Drivers
Noise Suppression Circuitry Rejects Spikes on Bus Lines
Fully Programmable Slave Address with General Call Support
Address Recognition Causes Wake-up When AVR is in Sleep Mode
Figure 91. TWI Bus Interconnection
VCC
Device 1
Device 2
Device 3
........
Device n
R1
R2
SDA
SCL
TWI Terminology
The following definitions are frequently encountered in this section.
Table 114. TWI Terminology
Term
Description
Master
The device that initiates and terminates a transmission. The Master also
generates the SCL clock.
Slave
The device addressed by a Master.
Transmitter
The device placing data on the bus.
Receiver
The device reading data from the bus.
The Power Reduction TWI bit, PRTWI bit in “Power Reduction Register 0 - PRR0” on
page 54 must be written to zero to enable the 2-wire Serial Interface.
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Electrical Interconnection
As depicted in Figure 91, both bus lines are connected to the positive supply voltage
through pull-up resistors. The bus drivers of all TWI-compliant devices are open-drain or
open-collector. This implements a wired-AND function which is essential to the operation of the interface. A low level on a TWI bus line is generated when one or more TWI
devices output a zero. A high level is output when all TWI devices trim-state their outputs, allowing the pull-up resistors to pull the line high. Note that all AVR devices
connected to the TWI bus must be powered in order to allow any bus operation.
The number of devices that can be connected to the bus is only limited by the bus
capacitance limit of 400 pF and the 7-bit slave address space. A detailed specification of
the electrical characteristics of the TWI is given in “SPI Timing Characteristics” on page
372. Two different sets of specifications are presented there, one relevant for bus
speeds below 100 kHz, and one valid for bus speeds up to 400 kHz.
Data Transfer and Frame
Format
Transferring Bits
Each data bit transferred on the TWI bus is accompanied by a pulse on the clock line.
The level of the data line must be stable when the clock line is high. The only exception
to this rule is for generating start and stop conditions.
Figure 92. Data Validity
SDA
SCL
Data Stable
Data Stable
Data Change
START and STOP Conditions
242
The Master initiates and terminates a data transmission. The transmission is initiated
when the Master issues a START condition on the bus, and it is terminated when the
Master issues a STOP condition. Between a START and a STOP condition, the bus is
considered busy, and no other master should try to seize control of the bus. A special
case occurs when a new START condition is issued between a START and STOP condition. This is referred to as a REPEATED START condition, and is used when the
Master wishes to initiate a new transfer without relinquishing control of the bus. After a
REPEATED START, the bus is considered busy until the next STOP. This is identical to
the START behavior, and therefore START is used to describe both START and
REPEATED START for the remainder of this datasheet, unless otherwise noted. As
depicted below, START and STOP conditions are signalled by changing the level of the
SDA line when the SCL line is high.
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Figure 93. START, REPEATED START and STOP conditions
SDA
SCL
START
Address Packet Format
STOP
REPEATED START
START
STOP
All address packets transmitted on the TWI bus are 9 bits long, consisting of 7 address
bits, one READ/WRITE control bit and an acknowledge bit. If the READ/WRITE bit is
set, a read operation is to be performed, otherwise a write operation should be performed. When a Slave recognizes that it is being addressed, it should acknowledge by
pulling SDA low in the ninth SCL (ACK) cycle. If the addressed Slave is busy, or for
some other reason can not service the Master’s request, the SDA line should be left
high in the ACK clock cycle. The Master can then transmit a STOP condition, or a
REPEATED START condition to initiate a new transmission. An address packet consisting of a slave address and a READ or a WRITE bit is called SLA+R or SLA+W,
respectively.
The MSB of the address byte is transmitted first. Slave addresses can freely be allocated by the designer, but the address 0000 000 is reserved for a general call.
When a general call is issued, all slaves should respond by pulling the SDA line low in
the ACK cycle. A general call is used when a Master wishes to transmit the same message to several slaves in the system. When the general call address followed by a Write
bit is transmitted on the bus, all slaves set up to acknowledge the general call will pull
the SDA line low in the ack cycle. The following data packets will then be received by all
the slaves that acknowledged the general call. Note that transmitting the general call
address followed by a Read bit is meaningless, as this would cause contention if several
slaves started transmitting different data.
All addresses of the format 1111 xxx should be reserved for future purposes.
Figure 94. Address Packet Format
Addr MSB
Addr LSB
R/W
ACK
7
8
9
SDA
SCL
1
2
START
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Data Packet Format
All data packets transmitted on the TWI bus are nine bits long, consisting of one data
byte and an acknowledge bit. During a data transfer, the Master generates the clock and
the START and STOP conditions, while the Receiver is responsible for acknowledging
the reception. An Acknowledge (ACK) is signalled by the Receiver pulling the SDA line
low during the ninth SCL cycle. If the Receiver leaves the SDA line high, a NACK is signalled. When the Receiver has received the last byte, or for some reason cannot receive
any more bytes, it should inform the Transmitter by sending a NACK after the final byte.
The MSB of the data byte is transmitted first.
Figure 95. Data Packet Format
Data MSB
Data LSB
ACK
8
9
Aggregate
SDA
SDA from
Transmitter
SDA from
Receiver
SCL from
Master
1
2
7
SLA+R/W
Combining Address and Data
Packets into a Transmission
STOP, REPEATED
START or Next
Data Byte
Data Byte
A transmission basically consists of a START condition, a SLA+R/W, one or more data
packets and a STOP condition. An empty message, consisting of a START followed by
a STOP condition, is illegal. Note that the Wired-ANDing of the SCL line can be used to
implement handshaking between the Master and the Slave. The Slave can extend the
SCL low period by pulling the SCL line low. This is useful if the clock speed set up by the
Master is too fast for the Slave, or the Slave needs extra time for processing between
the data transmissions. The Slave extending the SCL low period will not affect the SCL
high period, which is determined by the Master. As a consequence, the Slave can
reduce the TWI data transfer speed by prolonging the SCL duty cycle.
Figure 96 shows a typical data transmission. Note that several data bytes can be transmitted between the SLA+R/W and the STOP condition, depending on the software
protocol implemented by the application software.
Figure 96. Typical Data Transmission
Addr MSB
Addr LSB
R/W
ACK
Data MSB
7
8
9
1
Data LSB
ACK
8
9
SDA
SCL
1
START
244
2
SLA+R/W
2
7
Data Byte
STOP
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Multi-master Bus
The TWI protocol allows bus systems with several masters. Special concerns have
Systems, Arbitration and been taken in order to ensure that transmissions will proceed as normal, even if two or
more masters initiate a transmission at the same time. Two problems arise in multi-masSynchronization
ter systems:
•
An algorithm must be implemented allowing only one of the masters to complete the
transmission. All other masters should cease transmission when they discover that
they have lost the selection process. This selection process is called arbitration.
When a contending master discovers that it has lost the arbitration process, it
should immediately switch to Slave mode to check whether it is being addressed by
the winning master. The fact that multiple masters have started transmission at the
same time should not be detectable to the slaves, i.e. the data being transferred on
the bus must not be corrupted.
•
Different masters may use different SCL frequencies. A scheme must be devised to
synchronize the serial clocks from all masters, in order to let the transmission
proceed in a lockstep fashion. This will facilitate the arbitration process.
The wired-ANDing of the bus lines is used to solve both these problems. The serial
clocks from all masters will be wired-ANDed, yielding a combined clock with a high
period equal to the one from the Master with the shortest high period. The low period of
the combined clock is equal to the low period of the Master with the longest low period.
Note that all masters listen to the SCL line, effectively starting to count their SCL high
and low time-out periods when the combined SCL line goes high or low, respectively.
Figure 97. SCL Synchronization Between Multiple Masters
TA low
TA high
SCL from
Master A
SCL from
Master B
SCL Bus
Line
TBlow
Masters Start
Counting Low Period
TBhigh
Masters Start
Counting High Period
Arbitration is carried out by all masters continuously monitoring the SDA line after outputting data. If the value read from the SDA line does not match the value the Master
had output, it has lost the arbitration. Note that a Master can only lose arbitration when it
outputs a high SDA value while another Master outputs a low value. The losing Master
should immediately go to Slave mode, checking if it is being addressed by the winning
Master. The SDA line should be left high, but losing masters are allowed to generate a
clock signal until the end of the current data or address packet. Arbitration will continue
until only one Master remains, and this may take many bits. If several masters are trying
to address the same Slave, arbitration will continue into the data packet.
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Figure 98. Arbitration Between Two Masters
START
SDA from
Master A
Master A Loses
Arbitration, SDAA SDA
SDA from
Master B
SDA Line
Synchronized
SCL Line
Note that arbitration is not allowed between:
•
A REPEATED START condition and a data bit.
•
A STOP condition and a data bit.
•
A REPEATED START and a STOP condition.
It is the user software’s responsibility to ensure that these illegal arbitration conditions
never occur. This implies that in multi-master systems, all data transfers must use the
same composition of SLA+R/W and data packets. In other words: All transmissions
must contain the same number of data packets, otherwise the result of the arbitration is
undefined.
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Overview of the TWI
Module
The TWI module is comprised of several submodules, as shown in Figure 99. All registers drawn in a thick line are accessible through the AVR data bus.
Figure 99. Overview of the TWI Module
Slew-rate
Control
SDA
Spike
Filter
Slew-rate
Control
Spike
Filter
Bus Interface Unit
START / STOP
Control
Spike Suppression
Arbitration detection
Address/Data Shift
Register (TWDR)
Address Match Unit
Address Register
(TWAR)
Address Comparator
Bit Rate Generator
Prescaler
Bit Rate Register
(TWBR)
Ack
Control Unit
Status Register
(TWSR)
Control Register
(TWCR)
State Machine and
Status control
TWI Unit
SCL
SCL and SDA Pins
These pins interface the AVR TWI with the rest of the MCU system. The output drivers
contain a slew-rate limiter in order to conform to the TWI specification. The input stages
contain a spike suppression unit removing spikes shorter than 50 ns. Note that the internal pull-ups in the AVR pads can be enabled by setting the PORT bits corresponding to
the SCL and SDA pins, as explained in the I/O Port section. The internal pull-ups can in
some systems eliminate the need for external ones.
Bit Rate Generator Unit
This unit controls the period of SCL when operating in a Master mode. The SCL period
is controlled by settings in the TWI Bit Rate Register (TWBR) and the Prescaler bits in
the TWI Status Register (TWSR). Slave operation does not depend on Bit Rate or Prescaler settings, but the CPU clock frequency in the Slave must be at least 16 times higher
than the SCL frequency. Note that slaves may prolong the SCL low period, thereby
reducing the average TWI bus clock period. The SCL frequency is generated according
to the following equation:
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CPU Clock frequency
SCL frequency = ----------------------------------------------------------TWPS
16 + 2(TWBR) ⋅ 4
•
TWBR = Value of the TWI Bit Rate Register.
•
TWPS = Value of the prescaler bits in the TWI Status Register.
Note:
Bus Interface Unit
TWBR should be 10 or higher if the TWI operates in Master mode. If TWBR is lower than
10, the Master may produce an incorrect output on SDA and SCL for the reminder of the
byte. The problem occurs when operating the TWI in Master mode, sending Start + SLA
+ R/W to a Slave (a Slave does not need to be connected to the bus for the condition to
happen).
This unit contains the Data and Address Shift Register (TWDR), a START/STOP Controller and Arbitration detection hardware. The TWDR contains the address or data
bytes to be transmitted, or the address or data bytes received. In addition to the 8-bit
TWDR, the Bus Interface Unit also contains a register containing the (N)ACK bit to be
transmitted or received. This (N)ACK Register is not directly accessible by the application software. However, when receiving, it can be set or cleared by manipulating the
TWI Control Register (TWCR). When in Transmitter mode, the value of the received
(N)ACK bit can be determined by the value in the TWSR.
The START/STOP Controller is responsible for generation and detection of START,
REPEATED START, and STOP conditions. The START/STOP controller is able to
detect START and STOP conditions even when the AVR MCU is in one of the sleep
modes, enabling the MCU to wake up if addressed by a Master.
If the TWI has initiated a transmission as Master, the Arbitration Detection hardware
continuously monitors the transmission trying to determine if arbitration is in process. If
the TWI has lost an arbitration, the Control Unit is informed. Correct action can then be
taken and appropriate status codes generated.
Address Match Unit
The Address Match unit checks if received address bytes match the seven-bit address
in the TWI Address Register (TWAR). If the TWI General Call Recognition Enable
(TWGCE) bit in the TWAR is written to one, all incoming address bits will also be compared against the General Call address. Upon an address match, the Control Unit is
informed, allowing correct action to be taken. The TWI may or may not acknowledge its
address, depending on settings in the TWCR. The Address Match unit is able to compare addresses even when the AVR MCU is in sleep mode, enabling the MCU to wake
up if addressed by a Master. If another interrupt (e.g., INT0) occurs during TWI Powerdown address match and wakes up the CPU, the TWI aborts operation and return to it’s
idle state. If this cause any problems, ensure that TWI Address Match is the only
enabled interrupt when entering Power-down.
Control Unit
The Control unit monitors the TWI bus and generates responses corresponding to settings in the TWI Control Register (TWCR). When an event requiring the attention of the
application occurs on the TWI bus, the TWI Interrupt Flag (TWINT) is asserted. In the
next clock cycle, the TWI Status Register (TWSR) is updated with a status code identifying the event. The TWSR only contains relevant status information when the TWI
Interrupt Flag is asserted. At all other times, the TWSR contains a special status code
indicating that no relevant status information is available. As long as the TWINT Flag is
set, the SCL line is held low. This allows the application software to complete its tasks
before allowing the TWI transmission to continue.
The TWINT Flag is set in the following situations:
•
248
After the TWI has transmitted a START/REPEATED START condition.
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•
After the TWI has transmitted SLA+R/W.
•
After the TWI has transmitted an address byte.
•
After the TWI has lost arbitration.
•
After the TWI has been addressed by own slave address or general call.
•
After the TWI has received a data byte.
•
After a STOP or REPEATED START has been received while still addressed as a
Slave.
•
When a bus error has occurred due to an illegal START or STOP condition.
TWI Register Description
TWI Bit Rate Register – TWBR
Bit
7
6
5
4
3
2
1
0
TWBR7
TWBR6
TWBR5
TWBR4
TWBR3
TWBR2
TWBR1
TWBR0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
TWBR
• Bits 7..0 – TWI Bit Rate Register
TWBR selects the division factor for the bit rate generator. The bit rate generator is a
frequency divider which generates the SCL clock frequency in the Master modes. See
“Bit Rate Generator Unit” on page 247 for calculating bit rates.
TWI Control Register – TWCR
Bit
7
6
5
4
3
2
1
0
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Read/Write
R/W
R/W
R/W
R/W
R
R/W
R
R/W
Initial Value
0
0
0
0
0
0
0
0
TWCR
The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to
initiate a Master access by applying a START condition to the bus, to generate a
Receiver acknowledge, to generate a stop condition, and to control halting of the bus
while the data to be written to the bus are written to the TWDR. It also indicates a write
collision if data is attempted written to TWDR while the register is inaccessible.
• Bit 7 – TWINT: TWI Interrupt Flag
This bit is set by hardware when the TWI has finished its current job and expects application software response. If the I-bit in SREG and TWIE in TWCR are set, the MCU will
jump to the TWI Interrupt Vector. While the TWINT Flag is set, the SCL low period is
stretched. The TWINT Flag must be cleared by software by writing a logic one to it. Note
that this flag is not automatically cleared by hardware when executing the interrupt routine. Also note that clearing this flag starts the operation of the TWI, so all accesses to
the TWI Address Register (TWAR), TWI Status Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing this flag.
• Bit 6 – TWEA: TWI Enable Acknowledge Bit
The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to one, the ACK pulse is generated on the TWI bus if the following conditions are
met:
1. The device’s own slave address has been received.
2. A general call has been received, while the TWGCE bit in the TWAR is set.
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3. A data byte has been received in Master Receiver or Slave Receiver mode.
By writing the TWEA bit to zero, the device can be virtually disconnected from the 2-wire
Serial Bus temporarily. Address recognition can then be resumed by writing the TWEA
bit to one again.
• Bit 5 – TWSTA: TWI START Condition Bit
The application writes the TWSTA bit to one when it desires to become a Master on the
2-wire Serial Bus. The TWI hardware checks if the bus is available, and generates a
START condition on the bus if it is free. However, if the bus is not free, the TWI waits
until a STOP condition is detected, and then generates a new START condition to claim
the bus Master status. TWSTA must be cleared by software when the START condition
has been transmitted.
• Bit 4 – TWSTO: TWI STOP Condition Bit
Writing the TWSTO bit to one in Master mode will generate a STOP condition on the 2wire Serial Bus. When the STOP condition is executed on the bus, the TWSTO bit is
cleared automatically. In Slave mode, setting the TWSTO bit can be used to recover
from an error condition. This will not generate a STOP condition, but the TWI returns to
a well-defined unaddressed Slave mode and releases the SCL and SDA lines to a high
impedance state.
• Bit 3 – TWWC: TWI Write Collision Flag
The TWWC bit is set when attempting to write to the TWI Data Register – TWDR when
TWINT is low. This flag is cleared by writing the TWDR Register when TWINT is high.
• Bit 2 – TWEN: TWI Enable Bit
The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is
written to one, the TWI takes control over the I/O pins connected to the SCL and SDA
pins, enabling the slew-rate limiters and spike filters. If this bit is written to zero, the TWI
is switched off and all TWI transmissions are terminated, regardless of any ongoing
operation.
• Bit 1 – Res: Reserved Bit
This bit is a reserved bit and will always read as zero.
• Bit 0 – TWIE: TWI Interrupt Enable
When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will
be activated for as long as the TWINT Flag is high.
TWI Status Register – TWSR
Bit
7
6
5
4
3
2
1
0
TWS7
TWS6
TWS5
TWS4
TWS3
–
TWPS1
TWPS0
Read/Write
R
R
R
R
R
R
R/W
R/W
Initial Value
1
1
1
1
1
0
0
0
TWSR
• Bits 7..3 – TWS: TWI Status
These 5 bits reflect the status of the TWI logic and the 2-wire Serial Bus. The different
status codes are described later in this section. Note that the value read from TWSR
contains both the 5-bit status value and the 2-bit prescaler value. The application
250
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
designer should mask the prescaler bits to zero when checking the Status bits. This
makes status checking independent of prescaler setting. This approach is used in this
datasheet, unless otherwise noted.
• Bit 2 – Res: Reserved Bit
This bit is reserved and will always read as zero.
• Bits 1..0 – TWPS: TWI Prescaler Bits
These bits can be read and written, and control the bit rate prescaler.
Table 115. TWI Bit Rate Prescaler
TWPS1
TWPS0
Prescaler Value
0
0
1
0
1
4
1
0
16
1
1
64
To calculate bit rates, see “Bit Rate Generator Unit” on page 247. The value of
TWPS1..0 is used in the equation.
TWI Data Register – TWDR
Bit
7
6
5
4
3
2
1
0
TWD7
TWD6
TWD5
TWD4
TWD3
TWD2
TWD1
TWD0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
1
1
1
1
1
1
1
1
TWDR
In Transmit mode, TWDR contains the next byte to be transmitted. In Receive mode, the
TWDR contains the last byte received. It is writable while the TWI is not in the process of
shifting a byte. This occurs when the TWI Interrupt Flag (TWINT) is set by hardware.
Note that the Data Register cannot be initialized by the user before the first interrupt
occurs. The data in TWDR remains stable as long as TWINT is set. While data is shifted
out, data on the bus is simultaneously shifted in. TWDR always contains the last byte
present on the bus, except after a wake up from a sleep mode by the TWI interrupt. In
this case, the contents of TWDR is undefined. In the case of a lost bus arbitration, no
data is lost in the transition from Master to Slave. Handling of the ACK bit is controlled
automatically by the TWI logic, the CPU cannot access the ACK bit directly.
• Bits 7..0 – TWD: TWI Data Register
These eight bits constitute the next data byte to be transmitted, or the latest data byte
received on the 2-wire Serial Bus.
TWI (Slave) Address Register
– TWAR
Bit
7
6
5
4
3
2
1
0
TWA6
TWA5
TWA4
TWA3
TWA2
TWA1
TWA0
TWGCE
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
1
1
1
1
1
1
1
0
TWAR
The TWAR should be loaded with the 7-bit Slave address (in the seven most significant
bits of TWAR) to which the TWI will respond when programmed as a Slave Transmitter
or Receiver, and not needed in the Master modes. In multimaster systems, TWAR must
be set in masters which can be addressed as Slaves by other Masters.
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The LSB of TWAR is used to enable recognition of the general call address (0x00).
There is an associated address comparator that looks for the slave address (or general
call address if enabled) in the received serial address. If a match is found, an interrupt
request is generated.
• Bits 7..1 – TWA: TWI (Slave) Address Register
These seven bits constitute the slave address of the TWI unit.
• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit
If set, this bit enables the recognition of a General Call given over the 2-wire Serial Bus.
TWI (Slave) Address Mask
Register – TWAMR
Bit
7
6
5
4
3
2
1
0
TWAM[6:0]
–
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R
Initial Value
0
0
0
0
0
0
0
0
TWAMR
• Bits 7..1 – TWAM: TWI Address Mask
The TWAMR can be loaded with a 7-bit Slave Address mask. Each of the bits in
TWAMR can mask (disable) the corresponding address bit in the TWI Address Register
(TWAR). If the mask bit is set to one then the address match logic ignores the compare
between the incoming address bit and the corresponding bit in TWAR. Figure 100
shows the address match logic in detail.
Figure 100. TWI Address Match Logic, Block Diagram
TWAR0
Address
Match
Address
Bit 0
TWAMR0
Address Bit Comparator 0
Address Bit Comparator 6..1
• Bit 0 – Res: Reserved Bit
This bit is reserved and will always read as zero.
Using the TWI
The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus
events, like reception of a byte or transmission of a START condition. Because the TWI
is interrupt-based, the application software is free to carry on other operations during a
TWI byte transfer. Note that the TWI Interrupt Enable (TWIE) bit in TWCR together with
the Global Interrupt Enable bit in SREG allow the application to decide whether or not
assertion of the TWINT Flag should generate an interrupt request. If the TWIE bit is
cleared, the application must poll the TWINT Flag in order to detect actions on the TWI
bus.
When the TWINT Flag is asserted, the TWI has finished an operation and awaits application response. In this case, the TWI Status Register (TWSR) contains a value
252
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ATmega640/1280/1281/2560/2561
indicating the current state of the TWI bus. The application software can then decide
how the TWI should behave in the next TWI bus cycle by manipulating the TWCR and
TWDR Registers.
Figure 101 is a simple example of how the application can interface to the TWI hardware. In this example, a Master wishes to transmit a single data byte to a Slave. This
description is quite abstract, a more detailed explanation follows later in this section. A
simple code example implementing the desired behavior is also presented.
Application
Action
Figure 101. Interfacing the Application to the TWI in a Typical Transmission
1. Application
writes to TWCR to
initiate
transmission of
START
TWI
Hardware
Action
TWI bus
3. Check TWSR to see if START was
sent. Application loads SLA+W into
TWDR, and loads appropriate control
signals into TWCR, makin sure that
TWINT is written to one,
and TWSTA is written to zero.
START
2. TWINT set.
Status code indicates
START condition sent
SLA+W
5. Check TWSR to see if SLA+W was
sent and ACK received.
Application loads data into TWDR, and
loads appropriate control signals into
TWCR, making sure that TWINT is
written to one
A
4. TWINT set.
Status code indicates
SLA+W sent, ACK
received
Data
7. Check TWSR to see if data was sent
and ACK received.
Application loads appropriate control
signals to send STOP into TWCR,
making sure that TWINT is written to one
A
6. TWINT set.
Status code indicates
data sent, ACK received
STOP
Indicates
TWINT set
1. The first step in a TWI transmission is to transmit a START condition. This is
done by writing a specific value into TWCR, instructing the TWI hardware to
transmit a START condition. Which value to write is described later on. However,
it is important that the TWINT bit is set in the value written. Writing a one to
TWINT clears the flag. The TWI will not start any operation as long as the
TWINT bit in TWCR is set. Immediately after the application has cleared TWINT,
the TWI will initiate transmission of the START condition.
2. When the START condition has been transmitted, the TWINT Flag in TWCR is
set, and TWSR is updated with a status code indicating that the START condition
has successfully been sent.
3. The application software should now examine the value of TWSR, to make sure
that the START condition was successfully transmitted. If TWSR indicates otherwise, the application software might take some special action, like calling an
error routine. Assuming that the status code is as expected, the application must
load SLA+W into TWDR. Remember that TWDR is used both for address and
data. After TWDR has been loaded with the desired SLA+W, a specific value
must be written to TWCR, instructing the TWI hardware to transmit the SLA+W
present in TWDR. Which value to write is described later on. However, it is
important that the TWINT bit is set in the value written. Writing a one to TWINT
clears the flag. The TWI will not start any operation as long as the TWINT bit in
TWCR is set. Immediately after the application has cleared TWINT, the TWI will
initiate transmission of the address packet.
4. When the address packet has been transmitted, the TWINT Flag in TWCR is set,
and TWSR is updated with a status code indicating that the address packet has
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2549A–AVR–03/05
successfully been sent. The status code will also reflect whether a Slave
acknowledged the packet or not.
5. The application software should now examine the value of TWSR, to make sure
that the address packet was successfully transmitted, and that the value of the
ACK bit was as expected. If TWSR indicates otherwise, the application software
might take some special action, like calling an error routine. Assuming that the
status code is as expected, the application must load a data packet into TWDR.
Subsequently, a specific value must be written to TWCR, instructing the TWI
hardware to transmit the data packet present in TWDR. Which value to write is
described later on. However, it is important that the TWINT bit is set in the value
written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the application
has cleared TWINT, the TWI will initiate transmission of the data packet.
6. When the data packet has been transmitted, the TWINT Flag in TWCR is set,
and TWSR is updated with a status code indicating that the data packet has successfully been sent. The status code will also reflect whether a Slave
acknowledged the packet or not.
7. The application software should now examine the value of TWSR, to make sure
that the data packet was successfully transmitted, and that the value of the ACK
bit was as expected. If TWSR indicates otherwise, the application software might
take some special action, like calling an error routine. Assuming that the status
code is as expected, the application must write a specific value to TWCR,
instructing the TWI hardware to transmit a STOP condition. Which value to write
is described later on. However, it is important that the TWINT bit is set in the
value written. Writing a one to TWINT clears the flag. The TWI will not start any
operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate transmission of the STOP
condition. Note that TWINT is NOT set after a STOP condition has been sent.
Even though this example is simple, it shows the principles involved in all TWI transmissions. These can be summarized as follows:
•
When the TWI has finished an operation and expects application response, the
TWINT Flag is set. The SCL line is pulled low until TWINT is cleared.
•
When the TWINT Flag is set, the user must update all TWI Registers with the value
relevant for the next TWI bus cycle. As an example, TWDR must be loaded with the
value to be transmitted in the next bus cycle.
•
After all TWI Register updates and other pending application software tasks have
been completed, TWCR is written. When writing TWCR, the TWINT bit should be
set. Writing a one to TWINT clears the flag. The TWI will then commence executing
whatever operation was specified by the TWCR setting.
In the following an assembly and C implementation of the example is given. Note that
the code below assumes that several definitions have been made, for example by using
include-files.
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ATmega640/1280/1281/2560/2561
Assembly Code Example
1
ldi
r16, (1<<TWINT)|(1<<TWSTA)|
(1<<TWEN)
out
2
TWCR = (1<<TWINT)|(1<<TWSTA)|
Comments
Send START condition
(1<<TWEN)
TWCR, r16
wait1:
in
C Example
r16,TWCR
while (!(TWCR & (1<<TWINT)))
;
sbrs r16,TWINT
Wait for TWINT Flag set. This
indicates that the START condition
has been transmitted
rjmp wait1
3
in
r16,TWSR
andi r16, 0xF8
cpi
if ((TWSR & 0xF8) != START)
ERROR();
r16, START
brne ERROR
4
ldi
r16, SLA_W
TWDR = SLA_W;
out
TWDR, r16
TWCR = (1<<TWINT) | (1<<TWEN);
ldi
r16, (1<<TWINT) | (1<<TWEN)
out
TWCR, r16
wait2:
in
r16,TWCR
while (!(TWCR & (1<<TWINT)))
;
sbrs r16,TWINT
rjmp wait2
5
in
r16,TWSR
andi r16, 0xF8
cpi
if ((TWSR & 0xF8) != MT_SLA_ACK)
ERROR();
r16, MT_SLA_ACK
brne ERROR
6
ldi
r16, DATA
TWDR = DATA;
out
TWDR, r16
TWCR = (1<<TWINT) | (1<<TWEN);
ldi
r16, (1<<TWINT) | (1<<TWEN)
out
TWCR, r16
wait3:
in
r16,TWCR
while (!(TWCR & (1<<TWINT)))
;
sbrs r16,TWINT
rjmp wait3
7
in
r16,TWSR
andi r16, 0xF8
cpi
if ((TWSR & 0xF8) != MT_DATA_ACK)
ERROR();
r16, MT_DATA_ACK
brne ERROR
ldi
r16, (1<<TWINT)|(1<<TWEN)|
(1<<TWSTO)
out
TWCR = (1<<TWINT)|(1<<TWEN)|
Check value of TWI Status
Register. Mask prescaler bits. If
status different from START go to
ERROR
Load SLA_W into TWDR Register.
Clear TWINT bit in TWCR to start
transmission of address
Wait for TWINT Flag set. This
indicates that the SLA+W has been
transmitted, and ACK/NACK has
been received.
Check value of TWI Status
Register. Mask prescaler bits. If
status different from MT_SLA_ACK
go to ERROR
Load DATA into TWDR Register.
Clear TWINT bit in TWCR to start
transmission of data
Wait for TWINT Flag set. This
indicates that the DATA has been
transmitted, and ACK/NACK has
been received.
Check value of TWI Status
Register. Mask prescaler bits. If
status different from
MT_DATA_ACK go to ERROR
Transmit STOP condition
(1<<TWSTO);
TWCR, r16
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2549A–AVR–03/05
Transmission Modes
The TWI can operate in one of four major modes. These are named Master Transmitter
(MT), Master Receiver (MR), Slave Transmitter (ST) and Slave Receiver (SR). Several
of these modes can be used in the same application. As an example, the TWI can use
MT mode to write data into a TWI EEPROM, MR mode to read the data back from the
EEPROM. If other masters are present in the system, some of these might transmit data
to the TWI, and then SR mode would be used. It is the application software that decides
which modes are legal.
The following sections describe each of these modes. Possible status codes are
described along with figures detailing data transmission in each of the modes. These figures contain the following abbreviations:
S: START condition
Rs: REPEATED START condition
R: Read bit (high level at SDA)
W: Write bit (low level at SDA)
A: Acknowledge bit (low level at SDA)
A: Not acknowledge bit (high level at SDA)
Data: 8-bit data byte
P: STOP condition
SLA: Slave Address
In Figure 103 to Figure 109, circles are used to indicate that the TWINT Flag is set. The
numbers in the circles show the status code held in TWSR, with the prescaler bits
masked to zero. At these points, actions must be taken by the application to continue or
complete the TWI transfer. The TWI transfer is suspended until the TWINT Flag is
cleared by software.
When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate software action. For each status code, the required software action and details of
the following serial transfer are given in Table 116 to Table 119. Note that the prescaler
bits are masked to zero in these tables.
Master Transmitter Mode
256
In the Master Transmitter mode, a number of data bytes are transmitted to a Slave
Receiver (see Figure 102). In order to enter a Master mode, a START condition must be
transmitted. The format of the following address packet determines whether Master
Transmitter or Master Receiver mode is to be entered. If SLA+W is transmitted, MT
mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes
mentioned in this section assume that the prescaler bits are zero or are masked to zero.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 102. Data Transfer in Master Transmitter Mode
VCC
Device 1
Device 2
MASTER
TRANSMITTER
SLAVE
RECEIVER
Device 3
........
R1
Device n
R2
SDA
SCL
A START condition is sent by writing the following value to TWCR:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
1
0
X
1
0
X
TWEN must be set to enable the 2-wire Serial Interface, TWSTA must be written to one
to transmit a START condition and TWINT must be written to one to clear the TWINT
Flag. The TWI will then test the 2-wire Serial Bus and generate a START condition as
soon as the bus becomes free. After a START condition has been transmitted, the
TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (see Table
116). In order to enter MT mode, SLA+W must be transmitted. This is done by writing
SLA+W to TWDR. Thereafter the TWINT bit should be cleared (by writing it to one) to
continue the transfer. This is accomplished by writing the following value to TWCR:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
0
0
X
1
0
X
When SLA+W have been transmitted and an acknowledgement bit has been received,
TWINT is set again and a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x18, 0x20, or 0x38. The appropriate action to be taken
for each of these status codes is detailed in Table 116.
When SLA+W has been successfully transmitted, a data packet should be transmitted.
This is done by writing the data byte to TWDR. TWDR must only be written when
TWINT is high. If not, the access will be discarded, and the Write Collision bit (TWWC)
will be set in the TWCR Register. After updating TWDR, the TWINT bit should be
cleared (by writing it to one) to continue the transfer. This is accomplished by writing the
following value to TWCR:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
0
0
X
1
0
X
This scheme is repeated until the last byte has been sent and the transfer is ended by
generating a STOP condition or a repeated START condition. A STOP condition is generated by writing the following value to TWCR:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
0
1
X
1
0
X
A REPEATED START condition is generated by writing the following value to TWCR:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
1
0
X
1
0
X
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2549A–AVR–03/05
After a repeated START condition (state 0x10) the 2-wire Serial Interface can access
the same Slave again, or a new Slave without transmitting a STOP condition. Repeated
START enables the Master to switch between Slaves, Master Transmitter mode and
Master Receiver mode without losing control of the bus.
Table 116. Status codes for Master Transmitter Mode
Status Code
(TWSR)
Prescaler Bits
are 0
Application Software Response
Status of the 2-wire Serial Bus
and 2-wire Serial Interface Hardware
To/from TWDR
0x08
A START condition has been
transmitted
0x10
A repeated START condition
has been transmitted
0x18
0x20
0x28
0x30
0x38
258
SLA+W has been transmitted;
ACK has been received
SLA+W has been transmitted;
NOT ACK has been received
Data byte has been transmitted;
ACK has been received
Data byte has been transmitted;
NOT ACK has been received
Arbitration lost in SLA+W or data
bytes
To TWCR
STA
STO
TWINT
TWEA
Load SLA+W
0
0
1
X
SLA+W will be transmitted;
ACK or NOT ACK will be received
Load SLA+W or
0
0
1
X
Load SLA+R
0
0
1
X
SLA+W will be transmitted;
ACK or NOT ACK will be received
SLA+R will be transmitted;
Logic will switch to Master Receiver mode
Load data byte or
0
0
1
X
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
1
1
X
Load data byte or
0
0
1
X
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
1
1
X
Load data byte or
0
0
1
X
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
1
1
X
Load data byte or
0
0
1
X
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
1
1
X
No TWDR action or
0
0
1
X
No TWDR action
1
0
1
X
Next Action Taken by TWI Hardware
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
2-wire Serial Bus will be released and not addressed
Slave mode entered
A START condition will be transmitted when the bus becomes free
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 103. Formats and States in the Master Transmitter Mode
MT
Successfull
transmission
to a slave
receiver
S
SLA
$08
W
A
DATA
$18
A
P
$28
Next transfer
started with a
repeated start
condition
RS
SLA
W
$10
Not acknowledge
received after the
slave address
A
R
P
$20
MR
Not acknowledge
received after a data
byte
A
P
$30
Arbitration lost in slave
address or data byte
A or A
Other master
continues
$38
Arbitration lost and
addressed as slave
A
$68
From master to slave
From slave to master
Master Receiver Mode
A or A
Other master
continues
$38
Other master
continues
$78
DATA
To corresponding
states in slave mode
$B0
A
n
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero
In the Master Receiver mode, a number of data bytes are received from a Slave Transmitter (Slave see Figure 104). In order to enter a Master mode, a START condition must
be transmitted. The format of the following address packet determines whether Master
Transmitter or Master Receiver mode is to be entered. If SLA+W is transmitted, MT
mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes
mentioned in this section assume that the prescaler bits are zero or are masked to zero.
259
2549A–AVR–03/05
Figure 104. Data Transfer in Master Receiver Mode
VCC
Device 1
Device 2
MASTER
RECEIVER
SLAVE
TRANSMITTER
Device 3
........
R1
Device n
R2
SDA
SCL
A START condition is sent by writing the following value to TWCR:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
1
0
X
1
0
X
TWEN must be written to one to enable the 2-wire Serial Interface, TWSTA must be
written to one to transmit a START condition and TWINT must be set to clear the TWINT
Flag. The TWI will then test the 2-wire Serial Bus and generate a START condition as
soon as the bus becomes free. After a START condition has been transmitted, the
TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (See Table
116). In order to enter MR mode, SLA+R must be transmitted. This is done by writing
SLA+R to TWDR. Thereafter the TWINT bit should be cleared (by writing it to one) to
continue the transfer. This is accomplished by writing the following value to TWCR:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
0
0
X
1
0
X
When SLA+R have been transmitted and an acknowledgement bit has been received,
TWINT is set again and a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x38, 0x40, or 0x48. The appropriate action to be taken
for each of these status codes is detailed in Table 117. Received data can be read from
the TWDR Register when the TWINT Flag is set high by hardware. This scheme is
repeated until the last byte has been received. After the last byte has been received, the
MR should inform the ST by sending a NACK after the last received data byte. The
transfer is ended by generating a STOP condition or a repeated START condition. A
STOP condition is generated by writing the following value to TWCR:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
0
1
X
1
0
X
A REPEATED START condition is generated by writing the following value to TWCR:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
1
0
X
1
0
X
After a repeated START condition (state 0x10) the 2-wire Serial Interface can access
the same Slave again, or a new Slave without transmitting a STOP condition. Repeated
260
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
START enables the Master to switch between Slaves, Master Transmitter mode and
Master Receiver mode without losing control over the bus.
Table 117. Status codes for Master Receiver Mode
Status Code
(TWSR)
Prescaler Bits
are 0
Application Software Response
Status of the 2-wire Serial Bus
and 2-wire Serial Interface Hardware
To TWCR
To/from TWDR
TWINT
TWEA
0x08
A START condition has been
transmitted
Load SLA+R
0
0
1
X
SLA+R will be transmitted
ACK or NOT ACK will be received
0x10
A repeated START condition
has been transmitted
Load SLA+R or
0
0
1
X
Load SLA+W
0
0
1
X
SLA+R will be transmitted
ACK or NOT ACK will be received
SLA+W will be transmitted
Logic will switch to Master Transmitter mode
No TWDR action or
0
0
1
X
No TWDR action
1
0
1
X
No TWDR action or
0
0
1
0
No TWDR action
0
0
1
1
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
1
1
X
0
0x38
0x40
0x48
Arbitration lost in SLA+R or NOT
ACK bit
SLA+R has been transmitted;
ACK has been received
SLA+R has been transmitted;
NOT ACK has been received
STA
STO
0x50
Data byte has been received;
ACK has been returned
Read data byte or
0
0
1
Read data byte
0
0
1
1
0x58
Data byte has been received;
NOT ACK has been returned
Read data byte or
Read data byte or
1
0
0
1
1
1
X
X
Read data byte
1
1
1
X
Next Action Taken by TWI Hardware
2-wire Serial Bus will be released and not addressed
Slave mode will be entered
A START condition will be transmitted when the bus
becomes free
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Repeated START will be transmitted
STOP condition will be transmitted and TWSTO Flag will
be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Repeated START will be transmitted
STOP condition will be transmitted and TWSTO Flag will
be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
261
2549A–AVR–03/05
Figure 105. Formats and States in the Master Receiver Mode
MR
Successfull
reception
from a slave
receiver
S
SLA
R
$08
A
DATA
A
$40
DATA
A
$50
P
$58
Next transfer
started with a
repeated start
condition
RS
SLA
R
$10
Not acknowledge
received after the
slave address
A
W
P
$48
MT
Arbitration lost in slave
address or data byte
A or A
Other master
continues
A
$38
Arbitration lost and
addressed as slave
A
$68
From master to slave
$38
Other master
continues
$78
To corresponding
states in slave mode
$B0
DATA
A
From slave to master
Slave Receiver Mode
Other master
continues
n
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero
In the Slave Receiver mode, a number of data bytes are received from a Master Transmitter (see Figure 106). All the status codes mentioned in this section assume that the
prescaler bits are zero or are masked to zero.
Figure 106. Data transfer in Slave Receiver mode
VCC
Device 1
Device 2
SLAVE
RECEIVER
MASTER
TRANSMITTER
Device 3
........
Device n
R1
R2
SDA
SCL
To initiate the Slave Receiver mode, TWAR and TWCR must be initialized as follows:
TWAR
value
262
TWA6
TWA5
TWA4
TWA3
TWA2
TWA1
TWA0
TWGCE
Device’s Own Slave Address
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
The upper 7 bits are the address to which the 2-wire Serial Interface will respond when
addressed by a Master. If the LSB is set, the TWI will respond to the general call
address (0x00), otherwise it will ignore the general call address.
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
0
1
0
0
0
1
0
X
TWEN must be written to one to enable the TWI. The TWEA bit must be written to one
to enable the acknowledgement of the device’s own slave address or the general call
address. TWSTA and TWSTO must be written to zero.
When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its
own slave address (or the general call address if enabled) followed by the data direction
bit. If the direction bit is “0” (write), the TWI will operate in SR mode, otherwise ST mode
is entered. After its own slave address and the write bit have been received, the TWINT
Flag is set and a valid status code can be read from TWSR. The status code is used to
determine the appropriate software action. The appropriate action to be taken for each
status code is detailed in Table 118. The Slave Receiver mode may also be entered if
arbitration is lost while the TWI is in the Master mode (see states 0x68 and 0x78).
If the TWEA bit is reset during a transfer, the TWI will return a “Not Acknowledge” (“1”)
to SDA after the next received data byte. This can be used to indicate that the Slave is
not able to receive any more bytes. While TWEA is zero, the TWI does not acknowledge
its own slave address. However, the 2-wire Serial Bus is still monitored and address recognition may resume at any time by setting TWEA. This implies that the TWEA bit may
be used to temporarily isolate the TWI from the 2-wire Serial Bus.
In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the
TWEA bit is set, the interface can still acknowledge its own slave address or the general
call address by using the 2-wire Serial Bus clock as a clock source. The part will then
wake up from sleep and the TWI will hold the SCL clock low during the wake up and
until the TWINT Flag is cleared (by writing it to one). Further data reception will be carried out as normal, with the AVR clocks running as normal. Observe that if the AVR is
set up with a long start-up time, the SCL line may be held low for a long time, blocking
other data transmissions.
Note that the 2-wire Serial Interface Data Register – TWDR does not reflect the last byte
present on the bus when waking up from these Sleep modes.
263
2549A–AVR–03/05
Table 118. Status Codes for Slave Receiver Mode
Status Code
(TWSR)
Prescaler Bits
are 0
Application Software Response
Status of the 2-wire Serial Bus and
2-wire Serial Interface Hardware
To TWCR
To/from TWDR
STA
STO
TWINT
TWEA
No TWDR action or
X
0
1
0
0x60
Own SLA+W has been received;
ACK has been returned
No TWDR action
X
0
1
1
0x68
Arbitration lost in SLA+R/W as
Master; own SLA+W has been
received; ACK has been returned
No TWDR action or
X
0
1
0
No TWDR action
X
0
1
1
0x70
General call address has been
received; ACK has been returned
No TWDR action or
X
0
1
0
No TWDR action
X
0
1
1
0x78
Arbitration lost in SLA+R/W as
Master; General call address has
been received; ACK has been
returned
No TWDR action or
X
0
1
0
No TWDR action
X
0
1
1
0x80
Previously addressed with own
SLA+W; data has been received;
ACK has been returned
Read data byte or
X
0
1
0
Read data byte
X
0
1
1
0x88
Previously addressed with own
SLA+W; data has been received;
NOT ACK has been returned
Read data byte or
0
0
1
0
Read data byte or
0
0
1
1
Read data byte or
1
0
1
0
Read data byte
1
0
1
1
0x90
Previously addressed with
general call; data has been received; ACK has been returned
Read data byte or
X
0
1
0
Read data byte
X
0
1
1
0x98
Previously addressed with
general call; data has been
received; NOT ACK has been
returned
Read data byte or
0
0
1
0
Read data byte or
0
0
1
1
Read data byte or
1
0
1
0
Read data byte
1
0
1
1
No action
0
0
1
0
0
0
1
1
1
0
1
0
1
0
1
1
0xA0
264
A STOP condition or repeated
START condition has been
received while still addressed as
Slave
Next Action Taken by TWI Hardware
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 107. Formats and States in the Slave Receiver Mode
Reception of the own
slave address and one or
more data bytes. All are
acknowledged
S
SLA
W
A
DATA
$60
A
DATA
$80
Last data byte received
is not acknowledged
A
P or S
$80
$A0
A
P or S
$88
Arbitration lost as master
and addressed as slave
A
$68
Reception of the general call
address and one or more data
bytes
General Call
A
DATA
$70
A
DATA
$90
Last data byte received is
not acknowledged
A
P or S
$90
$A0
A
P or S
$98
Arbitration lost as master and
addressed as slave by general call
A
$78
From master to slave
From slave to master
DATA
A
n
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero
265
2549A–AVR–03/05
Slave Transmitter Mode
In the Slave Transmitter mode, a number of data bytes are transmitted to a Master
Receiver (see Figure 108). All the status codes mentioned in this section assume that
the prescaler bits are zero or are masked to zero.
Figure 108. Data Transfer in Slave Transmitter Mode
VCC
Device 1
Device 2
SLAVE
TRANSMITTER
MASTER
RECEIVER
........
Device 3
R1
Device n
R2
SDA
SCL
To initiate the Slave Transmitter mode, TWAR and TWCR must be initialized as follows:
TWAR
TWA6
TWA5
value
TWA4
TWA3
TWA2
TWA1
TWA0
TWGCE
Device’s Own Slave Address
The upper seven bits are the address to which the 2-wire Serial Interface will respond
when addressed by a Master. If the LSB is set, the TWI will respond to the general call
address (0x00), otherwise it will ignore the general call address.
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
0
1
0
0
0
1
0
X
TWEN must be written to one to enable the TWI. The TWEA bit must be written to one
to enable the acknowledgement of the device’s own slave address or the general call
address. TWSTA and TWSTO must be written to zero.
When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its
own slave address (or the general call address if enabled) followed by the data direction
bit. If the direction bit is “1” (read), the TWI will operate in ST mode, otherwise SR mode
is entered. After its own slave address and the write bit have been received, the TWINT
Flag is set and a valid status code can be read from TWSR. The status code is used to
determine the appropriate software action. The appropriate action to be taken for each
status code is detailed in Table 119. The Slave Transmitter mode may also be entered if
arbitration is lost while the TWI is in the Master mode (see state 0xB0).
If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of
the transfer. State 0xC0 or state 0xC8 will be entered, depending on whether the Master
Receiver transmits a NACK or ACK after the final byte. The TWI is switched to the not
addressed Slave mode, and will ignore the Master if it continues the transfer. Thus the
Master Receiver receives all “1” as serial data. State 0xC8 is entered if the Master
demands additional data bytes (by transmitting ACK), even though the Slave has transmitted the last byte (TWEA zero and expecting NACK from the Master).
While TWEA is zero, the TWI does not respond to its own slave address. However, the
2-wire Serial Bus is still monitored and address recognition may resume at any time by
setting TWEA. This implies that the TWEA bit may be used to temporarily isolate the
TWI from the 2-wire Serial Bus.
266
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the
TWEA bit is set, the interface can still acknowledge its own slave address or the general
call address by using the 2-wire Serial Bus clock as a clock source. The part will then
wake up from sleep and the TWI will hold the SCL clock will low during the wake up and
until the TWINT Flag is cleared (by writing it to one). Further data transmission will be
carried out as normal, with the AVR clocks running as normal. Observe that if the AVR is
set up with a long start-up time, the SCL line may be held low for a long time, blocking
other data transmissions.
Note that the 2-wire Serial Interface Data Register – TWDR does not reflect the last byte
present on the bus when waking up from these sleep modes.
Table 119. Status Codes for Slave Transmitter Mode
Status Code
(TWSR)
Prescaler Bits
are 0
0xA8
0xB0
0xB8
0xC0
0xC8
Application Software Response
Status of the 2-wire Serial Bus and
2-wire Serial Interface Hardware
To TWCR
To/from TWDR
STA
STO
TWINT
TWEA
Load data byte or
X
0
1
0
Load data byte
X
0
1
1
Arbitration lost in SLA+R/W as
Master; own SLA+R has been
received; ACK has been returned
Load data byte or
X
0
1
0
Load data byte
X
0
1
1
Data byte in TWDR has been
transmitted; ACK has been
received
Load data byte or
X
0
1
0
Load data byte
X
0
1
1
Data byte in TWDR has been
transmitted; NOT ACK has been
received
No TWDR action or
0
0
1
0
No TWDR action or
0
0
1
1
No TWDR action or
1
0
1
0
No TWDR action
1
0
1
1
No TWDR action or
0
0
1
0
No TWDR action or
0
0
1
1
No TWDR action or
1
0
1
0
No TWDR action
1
0
1
1
Own SLA+R has been received;
ACK has been returned
Last data byte in TWDR has been
transmitted (TWEA = “0”); ACK
has been received
Next Action Taken by TWI Hardware
Last data byte will be transmitted and NOT ACK should
be received
Data byte will be transmitted and ACK should be received
Last data byte will be transmitted and NOT ACK should
be received
Data byte will be transmitted and ACK should be received
Last data byte will be transmitted and NOT ACK should
be received
Data byte will be transmitted and ACK should be received
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”
Switched to the not addressed Slave mode;
no recognition of own SLA or GCA;
a START condition will be transmitted when the bus
becomes free
Switched to the not addressed Slave mode;
own SLA will be recognized;
GCA will be recognized if TWGCE = “1”;
a START condition will be transmitted when the bus
becomes free
267
2549A–AVR–03/05
Figure 109. Formats and States in the Slave Transmitter Mode
Reception of the own
slave address and one or
more data bytes
S
SLA
R
A
DATA
$A8
Arbitration lost as master
and addressed as slave
A
DATA
$B8
A
P or S
$C0
A
$B0
Last data byte transmitted.
Switched to not addressed
slave (TWEA = '0')
A
All 1's
P or S
$C8
DATA
From master to slave
From slave to master
Miscellaneous States
A
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero
n
There are two status codes that do not correspond to a defined TWI state, see Table
120.
Status 0xF8 indicates that no relevant information is available because the TWINT Flag
is not set. This occurs between other states, and when the TWI is not involved in a serial
transfer.
Status 0x00 indicates that a bus error has occurred during a 2-wire Serial Bus transfer.
A bus error occurs when a START or STOP condition occurs at an illegal position in the
format frame. Examples of such illegal positions are during the serial transfer of an
address byte, a data byte, or an acknowledge bit. When a bus error occurs, TWINT is
set. To recover from a bus error, the TWSTO Flag must set and TWINT must be cleared
by writing a logic one to it. This causes the TWI to enter the not addressed Slave mode
and to clear the TWSTO Flag (no other bits in TWCR are affected). The SDA and SCL
lines are released, and no STOP condition is transmitted.
Table 120. Miscellaneous States
Status Code
(TWSR)
Prescaler Bits
are 0
Application Software Response
Status of the 2-wire Serial Bus
and 2-wire Serial Interface Hardware
To TWCR
To/from TWDR
0xF8
No relevant state information
available; TWINT = “0”
No TWDR action
0x00
Bus error due to an illegal
START or STOP condition
No TWDR action
268
STA
STO
TWINT
TWEA
No TWCR action
0
1
1
Next Action Taken by TWI Hardware
Wait or proceed current transfer
X
Only the internal hardware is affected, no STOP condition is sent on the bus. In all cases, the bus is released
and TWSTO is cleared.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Combining Several TWI
Modes
In some cases, several TWI modes must be combined in order to complete the desired
action. Consider for example reading data from a serial EEPROM. Typically, such a
transfer involves the following steps:
1. The transfer must be initiated.
2. The EEPROM must be instructed what location should be read.
3. The reading must be performed.
4. The transfer must be finished.
Note that data is transmitted both from Master to Slave and vice versa. The Master must
instruct the Slave what location it wants to read, requiring the use of the MT mode. Subsequently, data must be read from the Slave, implying the use of the MR mode. Thus,
the transfer direction must be changed. The Master must keep control of the bus during
all these steps, and the steps should be carried out as an atomical operation. If this principle is violated in a multimaster system, another Master can alter the data pointer in the
EEPROM between steps 2 and 3, and the Master will read the wrong data location.
Such a change in transfer direction is accomplished by transmitting a REPEATED
START between the transmission of the address byte and reception of the data. After a
REPEATED START, the Master keeps ownership of the bus. The following figure shows
the flow in this transfer.
Figure 110. Combining Several TWI Modes to Access a Serial EEPROM
Master Transmitter
S
SLA+W
A
ADDRESS
S = START
A
Rs
SLA+R
A
DATA
A
Rs = REPEATED START
Transmitted from master to slave
Multi-master Systems
and Arbitration
Master Receiver
P
P = STOP
Transmitted from slave to master
If multiple masters are connected to the same bus, transmissions may be initiated simultaneously by one or more of them. The TWI standard ensures that such situations are
handled in such a way that one of the masters will be allowed to proceed with the transfer, and that no data will be lost in the process. An example of an arbitration situation is
depicted below, where two masters are trying to transmit data to a Slave Receiver.
Figure 111. An Arbitration Example
VCC
Device 1
Device 2
Device 3
MASTER
TRANSMITTER
MASTER
TRANSMITTER
SLAVE
RECEIVER
........
Device n
R1
R2
SDA
SCL
Several different scenarios may arise during arbitration, as described below:
269
2549A–AVR–03/05
•
Two or more masters are performing identical communication with the same Slave.
In this case, neither the Slave nor any of the masters will know about the bus
contention.
•
Two or more masters are accessing the same Slave with different data or direction
bit. In this case, arbitration will occur, either in the READ/WRITE bit or in the data
bits. The masters trying to output a one on SDA while another Master outputs a zero
will lose the arbitration. Losing masters will switch to not addressed Slave mode or
wait until the bus is free and transmit a new START condition, depending on
application software action.
•
Two or more masters are accessing different slaves. In this case, arbitration will
occur in the SLA bits. Masters trying to output a one on SDA while another Master
outputs a zero will lose the arbitration. Masters losing arbitration in SLA will switch to
Slave mode to check if they are being addressed by the winning Master. If
addressed, they will switch to SR or ST mode, depending on the value of the
READ/WRITE bit. If they are not being addressed, they will switch to not addressed
Slave mode or wait until the bus is free and transmit a new START condition,
depending on application software action.
This is summarized in Figure 112. Possible status values are given in circles.
Figure 112. Possible Status Codes Caused by Arbitration
START
SLA
Data
Arbitration lost in SLA
Own
Address / General Call
received
No
STOP
Arbitration lost in Data
38
TWI bus will be released and not addressed slave mode will be entered
A START condition will be transmitted when the bus becomes free
Yes
Direction
Write
68/78
Read
B0
270
Data byte will be received and NOT ACK will be returned
Data byte will be received and ACK will be returned
Last data byte will be transmitted and NOT ACK should be received
Data byte will be transmitted and ACK should be received
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
Analog Comparator
The Analog Comparator compares the input values on the positive pin AIN0 and negative pin AIN1. When the voltage on the positive pin AIN0 is higher than the voltage on
the negative pin AIN1, the Analog Comparator output, ACO, is set. The comparator’s
output can be set to trigger the Timer/Counter1 Input Capture function. In addition, the
comparator can trigger a separate interrupt, exclusive to the Analog Comparator. The
user can select Interrupt triggering on comparator output rise, fall or toggle. A block diagram of the comparator and its surrounding logic is shown in Figure 113.
The Power Reduction ADC bit, PRADC, in “Power Reduction Register 0 - PRR0” on
page 54 must be disabled by writing a logical zero to be able to use the ADC input MUX.
Figure 113. Analog Comparator Block Diagram(2)
BANDGAP
REFERENCE
ACBG
ACME
ADEN
ADC MULTIPLEXER
OUTPUT (1)
Notes:
ADC Control and Status
Register B – ADCSRB
1. See Table 122 on page 273.
2. Refer to Figure 1 on page 2 and Table 38 on page 89 for Analog Comparator pin
placement.
Bit
7
6
5
4
3
2
1
0
–
ACME
–
–
MUX5
ADTS2
ADTS1
ADTS0
Read/Write
R
R/W
R
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
ADCSRB
• 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 273.
Analog Comparator Control
and Status Register – ACSR
Bit
7
6
5
4
3
2
1
0
ACD
ACBG
ACO
ACI
ACIE
ACIC
ACIS1
ACIS0
Read/Write
R/W
R/W
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
N/A
0
0
0
0
0
ACSR
• 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
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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 Comparator. See “Internal Voltage Reference” on page 62.
• Bit 5 – ACO: Analog Comparator Output
The output of the Analog Comparator is synchronized and then directly connected to
ACO. The synchronization introduces a delay of 1 - 2 clock cycles.
• Bit 4 – ACI: Analog Comparator Interrupt Flag
This bit is set by hardware when a comparator output event triggers the interrupt mode
defined by ACIS1 and ACIS0. The Analog Comparator interrupt routine is executed if
the ACIE bit is set and the I-bit in SREG is set. ACI is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, ACI is cleared by writing a
logic one to the flag.
• Bit 3 – ACIE: Analog Comparator Interrupt Enable
When the ACIE bit is written logic one and the I-bit in the Status Register is set, the Analog Comparator interrupt is activated. When written logic zero, the interrupt is disabled.
• Bit 2 – ACIC: Analog Comparator Input Capture Enable
When written logic one, this bit enables the input capture function in Timer/Counter1 to
be triggered by the Analog Comparator. The comparator output is in this case directly
connected to the input capture front-end logic, making the comparator utilize the noise
canceler and edge select features of the Timer/Counter1 Input Capture interrupt. When
written logic zero, no connection between the Analog Comparator and the input capture
function exists. To make the comparator trigger the Timer/Counter1 Input Capture 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 121.
Table 121. ACIS1/ACIS0 Settings
ACIS1
ACIS0
Interrupt Mode
0
0
Comparator Interrupt on Output Toggle.
0
1
Reserved
1
0
Comparator Interrupt on Falling Output Edge.
1
1
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.
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Analog Comparator
Multiplexed Input
It is possible to select any of the ADC15..0 pins to replace the negative input to the Analog Comparator. The ADC multiplexer is used to select this input, and consequently, the
ADC must be switched off to utilize this feature. If the Analog Comparator Multiplexer
Enable bit (ACME in ADCSRB) is set and the ADC is switched off (ADEN in ADCSRA is
zero), MUX5 and MUX2..0 in ADMUX select the input pin to replace the negative input
to the Analog Comparator, as shown in Table 122. If ACME is cleared or ADEN is set,
AIN1 is applied to the negative input to the Analog Comparator.
Table 122. Analog Comparator Mulitiplexed Input
Digital Input Disable Register
1 – DIDR1
ACME
ADEN
MUX5
MUX2..0
0
x
x
xxx
AIN1
1
1
x
xxx
AIN1
1
0
0
000
ADC0
1
0
0
001
ADC1
1
0
0
010
ADC2
1
0
0
011
ADC3
1
0
0
100
ADC4
1
0
0
101
ADC5
1
0
0
110
ADC6
1
0
0
111
ADC7
1
0
1
000
ADC8
1
0
1
001
ADC9
1
0
1
010
ADC10
1
0
1
011
ADC11
1
0
1
100
ADC12
1
0
1
101
ADC13
1
0
1
110
ADC14
1
0
1
111
ADC15
Bit
Analog Comparator Negative Input
7
6
5
4
3
2
1
0
–
–
–
–
–
–
AIN1D
AIN0D
Read/Write
R
R
R
R
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
DIDR1
• 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 corresponding 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 written logic one to reduce power consumption in the digital
input buffer.
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Analog to Digital
Converter
Features
•
•
•
•
•
•
•
•
•
•
•
•
•
•
10-bit Resolution
0.5 LSB Integral Non-linearity
± 2 LSB Absolute Accuracy
65 - 260 µs Conversion Time
Up to 15 kSPS at Maximum Resolution
16 Multiplexed Single Ended Input Channels
14 Differential input channels
4 Differential Input Channels with Optional Gain of 10x and 200x
Optional Left Adjustment for ADC Result Readout
0 - VCC ADC Input Voltage Range
Selectable 2.56V or 1.1V ADC Reference Voltage
Free Running or Single Conversion Mode
Interrupt on ADC Conversion Complete
Sleep Mode Noise Canceler
The ATmega640/1280/1281/2560/2561 features a 10-bit successive approximation
ADC. The ADC is connected to an 8-channel Analog Multiplexer which allows eight single-ended voltage inputs constructed from the pins of Port A. The single-ended voltage
inputs refer to 0V (GND).
The device also supports 16 differential voltage input combinations. Four of the differential inputs (ADC1 & ADC0, ADC & ADC2, ADC9 & ADC8 and ADC11 & ADC10) are
equipped with a programmable gain stage, providing amplification steps of 0dB (1x),
20dB (10x) or 46dB (200x) on the differential input voltage before the ADC conversion.
The 16 channels are split in two sections of 8 channels where in each section seven differential analog input channels share a common negative terminal (ADC1/ADC9), while
any other ADC input in that section can be selected as the positive input terminal. If 1x
or 10x gain is used, 8 bit resolution can be expected. If 200x gain is used, 7 bit resolution can be expected.
The ADC contains a Sample and Hold circuit which ensures that the input voltage to the
ADC is held at a constant level during conversion. A block diagram of the ADC is shown
in Figure 114.
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 281 on how to
connect this pin.
Internal reference voltages of nominally 1.1V, 2.56V or AVCC are provided On-chip.
The voltage reference may be externally decoupled at the AREF pin by a capacitor for
better noise performance.
The Power Reduction ADC bit, PRADC, in “Power Reduction Register 0 - PRR0” on
page 54 must be disabled by writing a logical zero to enable the ADC.
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Figure 114. Analog to Digital Converter Block Schematic
ADC CONVERSION
COMPLETE IRQ
INTERRUPT
FLAGS
ADTS[2:0]
ADPS1
ADPS0
ADC[9:0]
ADIF
ADIF
ADFR
ADPS2
ADEN
PRESCALER
GAIN SELECTION
CHANNEL SELECTION
INTERNAL
REFERENCE
(1.1V/2.56V)
0
ADC DATA REGISTER
(ADCH/ADCL)
TRIGGER
SELECT
MUX DECODER
AVCC
15
ADC CTRL & STATUS
REGISTER A (ADCSRA)
ADSC
MUX5
ADC CTRL & STATUS
REGISTER B (ADCSRB)
MUX0
MUX3
MUX1
MUX2
MUX4
ADLAR
REFS[1:0]
ADC MULTIPLEXER SELECT
(ADMUX)
ADIE
8-BIT DATABUS
START
CONVERSION LOGIC
SAMPLE & HOLD
COMPARATOR
10-bit DAC
AREF
+
GND
SINGLE ENDED / DIFFERENTIAL SELECTION
BANDGAP (1.1V)
REFERENCE
ADC
MULTIPLEXER
OUTPUT
ADC15
ADC14
ADC13
ADC12
+
ADC11
GAIN
AMPLIFIER
-
ADC10
ADC9
POS
INPUT
MUX
ADC8
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADC1
ADC0
NEG INPUT
MUX
Operation
The ADC converts an analog input voltage to a 10-bit digital value through successive
approximation. The minimum value represents GND and the maximum value represents
the voltage on the AREF pin minus 1 LSB. Optionally, AVCC or an internal 1.1V reference voltage may be connected to the AREF pin by writing to the REFSn bits in the
ADMUX Register. The internal voltage reference may thus be decoupled by an external
capacitor at the AREF pin to improve noise immunity.
The analog input channel 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. A selection of ADC input pins can be selected as
positive and negative inputs to the differential amplifier.
If differential channels are selected, the voltage difference between the selected input
channel pair then becomes the analog input to the ADC. If single ended channels are
used, the amplifier is bypassed altogether.
The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Voltage reference and input channel selections will not go into effect until ADEN is set. The ADC
does not consume power when ADEN is cleared, so it is recommended to switch off the
ADC before entering power saving sleep modes.
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The ADC generates a 10-bit result which is presented in the ADC Data Registers,
ADCH and ADCL. By default, the result is presented right adjusted, but can optionally
be presented left adjusted by setting the ADLAR bit in ADMUX.
If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to
read ADCH. Otherwise, ADCL must be read first, then ADCH, to ensure that the content
of the Data Registers belongs to the same conversion. Once ADCL is read, ADC access
to Data Registers is blocked. This means that if ADCL has been read, and a conversion
completes before ADCH is read, neither register is updated and the result from the conversion is lost. When ADCH is read, ADC access to the ADCH and ADCL Registers is
re-enabled.
The ADC has its own interrupt which can be triggered when a conversion completes.
When ADC access to the Data Registers is prohibited between reading of ADCH and
ADCL, the interrupt will trigger even if the result is lost.
Starting a Conversion
A single conversion is started by writing a logical one to the ADC Start Conversion bit,
ADSC. This bit stays high as long as the conversion is in progress and will be cleared by
hardware when the conversion is completed. If a different data channel is selected while
a conversion is in progress, the ADC will finish the current conversion before performing
the channel change.
Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is enabled by setting the ADC Auto Trigger Enable bit, ADATE in ADCSRA. The
trigger source is selected by setting the ADC Trigger Select bits, ADTS in 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 conversions 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 conversion, 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 115. ADC Auto Trigger Logic
ADTS[2:0]
PRESCALER
START
ADIF
CLKADC
ADATE
SOURCE 1
.
.
.
.
SOURCE n
CONVERSION
LOGIC
EDGE
DETECTOR
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, constantly sampling and updating the ADC Data Register. The first
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conversion must be started by writing a logical one to the ADSC bit in ADCSRA. In this
mode the ADC will perform successive conversions independently of whether the ADC
Interrupt Flag, ADIF is cleared or not.
If Auto Triggering is enabled, single conversions can be started by writing ADSC in
ADCSRA to one. ADSC can also be used to determine if a conversion is in progress.
The ADSC bit will be read as one during a conversion, independently of how the conversion was started.
Prescaling and
Conversion Timing
Figure 116. ADC Prescaler
ADEN
START
Reset
7-BIT ADC PRESCALER
CK/128
CK/64
CK/32
CK/16
CK/8
CK/4
CK/2
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.
The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal
conversion and 13.5 ADC clock cycles after the start of an first conversion. When a 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 additional CPU clock cycles are used for synchronization logic.
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In Free Running mode, a new conversion will be started immediately after the conversion completes, while ADSC remains high. For a summary of conversion times, see
Table 123.
Figure 117. ADC Timing Diagram, First Conversion (Single Conversion Mode)
Next
Conversion
First Conversion
Cycle Number
1
2
12
13
14
16
15
17
18
19
20
21
22
23
24
25
1
2
3
ADC Clock
ADEN
ADSC
ADIF
Sign and MSB of Result
ADCH
LSB of Result
ADCL
MUX and REFS
Update
Conversion
Complete
Sample & Hold
MUX and REFS
Update
Figure 118. ADC Timing Diagram, Single Conversion
One Conversion
Cycle Number
1
2
3
4
5
6
7
8
9
Next Conversion
10
11
12
13
1
2
3
ADC Clock
ADSC
ADIF
ADCH
Sign and MSB of Result
ADCL
LSB of Result
Sample & Hold
Conversion
Complete
MUX and REFS
Update
MUX and REFS
Update
Figure 119. ADC Timing Diagram, Auto Triggered Conversion
One Conversion
Cycle Number
1
2
3
4
5
6
7
8
9
Next Conversion
10
11
12
13
1
2
ADC Clock
Trigger
Source
ADATE
ADIF
ADCH
Sign and MSB of Result
ADCL
LSB of Result
Prescaler
Reset
278
Sample &
Hold
Conversion
Complete
Prescaler
Reset
MUX and REFS
Update
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
Figure 120. ADC Timing Diagram, Free Running Conversion
One Conversion
Cycle Number
11
12
Next Conversion
13
1
2
3
4
ADC Clock
ADSC
ADIF
ADCH
Sign and MSB of Result
ADCL
LSB of Result
Sample & Hold
Conversion
Complete
MUX and REFS
Update
Table 123. ADC Conversion Time
Sample & Hold (Cycles
from Start of Conversion)
Conversion Time
(Cycles)
First conversion
13.5
25
Normal conversions, single ended
1.5
13
2
13.5
1.5/2.5
13/14
Condition
Auto Triggered conversions
Normal conversions, differential
Differential Channels
When using differential channels, certain aspects of the conversion need to be taken
into consideration.
Differential conversions are synchronized to the internal clock CKADC2 equal to half the
ADC clock. This synchronization is done automatically by the ADC interface in such a
way that the sample-and-hold occurs at a specific phase of CKADC2. A conversion initiated by the user (i.e., all single conversions, and the first free running conversion) when
CKADC2 is low will take the same amount of time as a single ended conversion (13 ADC
clock cycles from the next prescaled clock cycle). A conversion initiated by the user
when CKADC2 is high will take 14 ADC clock cycles due to the synchronization mechanism. In Free Running mode, a new conversion is initiated immediately after the
previous conversion completes, and since CKADC2 is high at this time, all automatically
started (i.e., all but the first) Free Running conversions will take 14 ADC clock cycles.
If differential channels are used and conversions are started by Auto Triggering, the
ADC must be switched off between conversions. When Auto Triggering is used, the
ADC prescaler is reset before the conversion is started. Since the stage is dependent of
a stable ADC clock prior to the conversion, this conversion will not be valid. By disabling
and then re-enabling the ADC between each conversion (writing ADEN in ADCSRA to
“0” then to “1”), only extended conversions are performed. The result from the extended
conversions will be valid. See “Prescaling and Conversion Timing” on page 277 for timing details.
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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. Continuous updating resumes in the last ADC
clock cycle before the conversion completes (ADIF in ADCSRA is set). Note that the
conversion starts on the following rising ADC clock edge after ADSC is written. The user
is thus advised not to write new channel or reference selection values to ADMUX until
one ADC clock cycle after ADSC is written.
If Auto Triggering is used, the exact time of the triggering event can be indeterministic.
Special care must be taken when updating the ADMUX Register, in order to control
which conversion will be affected by the new settings.
If both ADATE and ADEN is written to one, an interrupt event can occur at any time. If
the ADMUX Register is changed in this period, the user cannot tell if the next conversion
is based on the old or the new settings. ADMUX can be safely updated in the following
ways:
1. When ADATE or ADEN is cleared.
2. During conversion, minimum one ADC clock cycle after the trigger event.
3. After a conversion, before the Interrupt Flag used as trigger source is
cleared.
When updating ADMUX in one of these conditions, the new settings will affect the next
ADC conversion.
Special care should be taken when changing differential channels. Once a differential
channel has been selected, the stage may take as much as TBD µs to stabilize to the
new value. Thus conversions should not be started within the first TBD µs after selecting
a new differential channel. Alternatively, conversion results obtained within this period
should be discarded.
The same settling time should be observed for the first differential conversion after
changing ADC reference (by changing the REFS1:0 bits in ADMUX).
ADC Input Channels
When changing channel selections, the user should observe the following guidelines to
ensure that the correct channel is selected:
In Single Conversion mode, always select the channel before starting the conversion.
The channel selection may be changed one ADC clock cycle after writing one to ADSC.
However, the simplest method is to wait for the conversion to complete before changing
the channel selection.
In Free Running mode, always select the channel before starting the first conversion.
The channel selection may be changed one ADC clock cycle after writing one to ADSC.
However, the simplest method is to wait for the first conversion to complete, and then
change the channel selection. Since the next conversion has already started automatically, the next result will reflect the previous channel selection. Subsequent conversions
will reflect the new channel selection.
When switching to a differential gain channel, the first conversion result may have a
poor accuracy due to the required settling time for the automatic offset cancellation circuitry. The user should preferably disregard the first conversion result.
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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, internal 2.56V reference or external
AREF pin.
AVCC is connected to the ADC through a passive switch. The internal 1.1V reference is
generated from the internal bandgap reference (VBG) through an internal amplifier. In
either case, the external AREF pin is directly connected to the ADC, and the reference
voltage can be made more immune to noise by connecting a capacitor between the
AREF pin and ground. 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. The Internal 2.56V reference is generated from the 1.1V
reference.
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, 1.1V and 2.56V as reference selection. The first ADC conversion result
after switching reference voltage source may be inaccurate, and the user is advised to
discard this result.
If differential channels are used, the selected reference should not be closer to AVCC
than indicated in “ADC Characteristics – Preliminary Data” on page 374.
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 entering such sleep modes to avoid excessive power consumption.
If the ADC is enabled in such sleep modes and the user wants to perform differential
conversions, the user is advised to switch the ADC off and on after waking up from
sleep to prompt an extended conversion to get a valid result.
Analog Input Circuitry
The analog input circuitry for single ended channels is illustrated in Figure 121. An analog source applied to ADCn is subjected to the pin capacitance and input leakage of that
pin, regardless of whether that channel is selected as input for the ADC. When the channel is selected, the source must drive the S/H capacitor through the series resistance
(combined resistance in the input path).
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The ADC is optimized for analog signals with an output impedance of approximately
10 kΩ or less. If such a source is used, the sampling time will be negligible. If a source
with higher impedance is used, the sampling time will depend on how long time the
source needs to charge the S/H capacitor, with can vary widely. The user is 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 121. Analog Input Circuitry
IIH
ADCn
1..100 kΩ
CS/H= 14 pF
IIL
VCC/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 122.
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.
282
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 122. ADC Power Connections, ATmega1281/2561.
PA0
VCC
51
52
GND
53
(ADC7) PF7
54
(ADC6) PF6
55
(ADC5) PF5
56
(ADC4) PF4
57
(ADC3) PF3
58
(ADC2) PF2
59
(ADC1) PF1
60
(ADC0) PF0
61
AREF
62
10µΗ
GND
AVCC
63
64
100nF
1
PG5
Analog Ground Plane
Figure 123. ADC Power Connections, ATmega640/1280/2560
PJ7
79
VCC
80
GND
81
(ADC15/PCINT23) PK7
82
(ADC14/PCINT22) PK6
83
(ADC13/PCINT21) PK5
84
(ADC12/PCINT20) PK4
85
(ADC11/PCINT19) PK3
86
(ADC10/PCINT18) PK2
87
(ADC9/PCINT17) PK1
88
(ADC8/PCINT16) PK0
89
(ADC7/TDI) PF7
90
(ADC6/TDO) PF6
91
(ADC5/TMS) PF5
92
(ADC4/TCK) PF4
93
(ADC3) PF3
94
(ADC2) PF2
95
(ADC1) PF1
96
(ADC0) PF0
97
AREF
98
10µΗ
GND
99
AVCC
100
(OC0B) PG5
100nF
Analog Ground Plane
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2549A–AVR–03/05
Offset Compensation
Schemes
The stage has a built-in offset cancellation circuitry that nulls the offset of differential
measurements as much as possible. The remaining offset in the analog path can be
measured directly by selecting the same channel for both differential inputs. This offset
residue can be then subtracted in software from the measurement results. Using this
kind of software based offset correction, offset on any channel can be reduced below
one LSB.
ADC Accuracy Definitions
An n-bit single-ended ADC converts a voltage linearly between GND and 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 124. Offset Error
Output Code
Ideal ADC
Actual ADC
Offset
Error
•
VREF Input Voltage
Gain Error: After adjusting for offset, the Gain Error is found as the deviation of the
last transition (0x3FE to 0x3FF) compared to the ideal transition (at 1.5 LSB below
maximum). Ideal value: 0 LSB
Figure 125. Gain Error
Output Code
Gain
Error
Ideal ADC
Actual ADC
VREF Input Voltage
284
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
•
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 126. Integral Non-linearity (INL)
Output Code
INL
Ideal ADC
Actual ADC
VREF
•
Input Voltage
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 127. 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.
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2549A–AVR–03/05
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 IN ⋅ 1024
ADC = -------------------------V REF
where VIN is the voltage on the selected input pin and VREF the selected voltage reference (see Table 125 on page 287 and Table 126 on page 288). 0x000 represents
analog ground, and 0x3FF represents the selected reference voltage minus one LSB.
If differential channels are used, the result is
( V POS – V NEG ) ⋅ 512
ADC = ---------------------------------------------------V REF
where VPOS is the voltage on the positive input pin, VNEG the voltage on the negative
input pin, and VREF the selected voltage reference. The result is presented in two’s complement form, from 0x200 (-512d) through 0x1FF (+511d). Note that if the user wants to
perform a quick polarity check of the result, it is sufficient to read the MSB of the result
(ADC9 in ADCH). If the bit is one, the result is negative, and if this bit is zero, the result
is positive. Figure 128 shows the decoding of the differential input range.
Table 124 shows the resulting output codes if the differential input channel pair (ADCn ADCm) is selected with a gain of GAIN and a reference voltage of VREF.
Figure 128. Differential Measurement Range
Output Code
0x1FF
0x000
- VREF
0x3FF
0
VREF
Differential Input
Voltage (Volts)
0x200
286
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 124. Correlation Between Input Voltage and Output Codes
VADCn
Read Code
Corresponding Decimal Value
VADCm + VREF / GAIN
0x1FF
511
VADCm + 0.999 VREF / GAIN
0x1FF
511
VADCm + 0.998 VREF / GAIN
0x1FE
510
...
...
...
VADCm + 0.001 VREF / GAIN
0x001
1
VADCm
0x000
0
VADCm - 0.001 VREF / GAIN
0x3FF
-1
...
...
...
VADCm - 0.999 VREF / GAIN
0x201
-511
VADCm - VREF / GAIN
0x200
-512
Example:
ADMUX = 0xFB (ADC3 - ADC2, 10x gain, 2.56V reference, left adjusted result)
Voltage on ADC3 is 300 mV, voltage on ADC2 is 500 mV.
ADCR = 512 * 10 * (300 - 500) / 2560 = -400 = 0x270.
ADCL will thus read 0x00, and ADCH will read 0x9C. Writing zero to ADLAR right
adjusts the result: ADCL = 0x70, ADCH = 0x02.
ADC Multiplexer Selection
Register – ADMUX
Bit
7
6
5
4
3
2
1
0
REFS1
REFS0
ADLAR
MUX4
MUX3
MUX2
MUX1
MUX0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
ADMUX
• Bit 7:6 – REFS1:0: Reference Selection Bits
These bits select the voltage reference for the ADC, as shown in Table 125. 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 125. Voltage Reference Selections for ADC
Voltage Reference Selection(1)
REFS1
REFS0
0
0
AREF, Internal VREF turned off
0
1
AVCC with external capacitor at AREF pin
1
0
Internal 1.1V Voltage Reference with external capacitor at AREF pin
1
1
Internal 2.56V Voltage Reference with external capacitor at AREF pin
Note:
1. If 10x or 200x gain is selected, only 2.56 V should be used as Internal Voltage
Reference.
• 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
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2549A–AVR–03/05
adjusted. Changing the ADLAR bit will affect the ADC Data Register immediately,
regardless of any ongoing conversions. For a complete description of this bit, see “The
ADC Data Register – ADCL and ADCH” on page 292.
• Bits 4:0 – MUX4:0: Analog Channel and Gain Selection Bits
The value of these bits selects which combination of analog inputs are connected to the
ADC. See Table 126 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)
ADC Control and Status
Register B – ADCSRB
Bit
7
6
5
4
3
2
1
0
–
ACME
–
–
MUX5
ADTS2
ADTS1
ADTS0
Read/Write
R
R/W
R
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
ADCSRB
• Bit 3 – MUX5: Analog Channel and Gain Selection Bit
This bit is used together with MUX4:0 in ADMUX to select which combination in of analog inputs are connected to the ADC. See Table 126 for details. If this bit is changed
during a conversion, the change will not go in effect until this conversion is complete
(ADIF in ADCSRA is set).
This bit can only be used in ATmega640/1280/2560.
Table 126. Input Channel Selections
MUX5..0
Single Ended
Input
000000
ADC0
000001
ADC1
000010
ADC2
000011
ADC3
000100
ADC4
000101
ADC5
000110
ADC6
Positive Differential
Input
Negative Differential
Input
Gain
N/A
000111
288
ADC7
001000
(1)
ADC0
ADC0
10x
001001
(1)
ADC1
ADC0
10x
001010
(1)
ADC0
ADC0
200x
001011(1)
ADC1
ADC0
200x
001100
(1)
ADC2
ADC2
10x
001101
(1)
ADC3
ADC2
10x
001110
(1)
ADC2
ADC2
200x
001111(1)
ADC3
ADC2
200x
010000
ADC0
ADC1
1x
010001
ADC1
ADC1
1x
010010
ADC2
ADC1
1x
N/A
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 126. Input Channel Selections (Continued)
Positive Differential
Input
Negative Differential
Input
Gain
010011
ADC3
ADC1
1x
010100
ADC4
ADC1
1x
010101
ADC5
ADC1
1x
010110
ADC6
ADC1
1x
010111
ADC7
ADC1
1x
ADC0
ADC2
1x
011001
ADC1
ADC2
1x
011010
ADC2
ADC2
1x
011011
ADC3
ADC2
1x
011100
ADC4
ADC2
1x
011101
ADC5
ADC2
1x
MUX5..0
011000
Single Ended
Input
N/A
011110
1.1V (VBG)
011111
0V (GND)
100000
ADC8
100001
ADC9
100010
ADC10
100011
ADC11
100100
ADC12
100101
ADC13
100110
ADC14
100111
ADC15
N/A
N/A
101000(1)
ADC8
ADC8
10x
101001
(1)
ADC9
ADC8
10x
101010
(1)
ADC8
ADC8
200x
101011(1)
ADC9
ADC8
200x
101100
(1)
ADC10
ADC10
10x
101101
(1)
ADC11
ADC10
10x
101110
(1)
ADC10
ADC10
200x
ADC11
ADC10
200x
110000
ADC8
ADC9
1x
110001
ADC9
ADC9
1x
110010
ADC10
ADC9
1x
110011
ADC11
ADC9
1x
110100
ADC12
ADC9
1x
110101
ADC13
ADC9
1x
101111(1)
N/A
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2549A–AVR–03/05
Table 126. Input Channel Selections (Continued)
Positive Differential
Input
Negative Differential
Input
Gain
110110
ADC14
ADC9
1x
110111
ADC15
ADC9
1x
111000
ADC8
ADC10
1x
ADC9
ADC10
1x
111010
ADC10
ADC10
1x
111011
ADC11
ADC10
1x
111100
ADC12
ADC10
1x
111101
ADC13
ADC10
1x
MUX5..0
Single Ended
Input
111001
N/A
111110
Reserved
N/A
111111
Reserved
N/A
Note:
290
1. To reach the given accuracy, 10x or 200x Gain should not be used for operating voltage below 2.7V
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
ADC Control and Status
Register A – ADCSRA
Bit
7
6
5
4
3
2
1
0
ADEN
ADSC
ADATE
ADIF
ADIE
ADPS2
ADPS1
ADPS0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
ADCSRA
• 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 initialization of the ADC.
ADSC will read as one as long as a conversion is in progress. When the conversion is
complete, it returns to zero. Writing zero to this bit has no effect.
• Bit 5 – ADATE: ADC Auto Trigger Enable
When this bit is written to one, Auto Triggering of the ADC is enabled. The ADC will start
a conversion 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. Alternatively, ADIF is cleared by writing a logical one to the flag.
Beware that if doing a Read-Modify-Write on ADCSRA, a pending interrupt can be 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 Interrupt 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 127. ADC Prescaler Selections
ADPS2
ADPS1
ADPS0
Division Factor
0
0
0
2
0
0
1
2
0
1
0
4
0
1
1
8
1
0
0
16
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2549A–AVR–03/05
Table 127. ADC Prescaler Selections
ADPS2
ADPS1
ADPS0
Division Factor
1
0
1
32
1
1
0
64
1
1
1
128
The ADC Data Register –
ADCL and ADCH
ADLAR = 0
Bit
Read/Write
Initial Value
15
14
13
12
11
10
9
8
–
–
–
–
–
–
ADC9
ADC8
ADCH
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADC1
ADC0
ADCL
7
6
5
4
3
2
1
0
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
ADLAR = 1
Bit
Read/Write
Initial Value
15
14
13
12
11
10
9
8
ADC9
ADC8
ADC7
ADC6
ADC5
ADC4
ADC3
ADC2
ADCH
ADC1
ADC0
–
–
–
–
–
–
ADCL
7
6
5
4
3
2
1
0
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
When an ADC conversion is complete, the result is found in these two registers. If differential channels are used, the result is presented in two’s complement form.
When ADCL is read, the ADC Data Register is not updated until ADCH is read. Consequently, if the result is left adjusted and no more than 8-bit precision (7 bit + sign bit for
differential input channels) 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 286.
ADC Control and Status
Register B – ADCSRB
292
Bit
7
6
5
4
3
2
1
0
–
ACME
–
–
MUX5
ADTS2
ADTS1
ADTS0
Read/Write
R
R/W
R
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
ADCSRB
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
• 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 trigger 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 128. ADC Auto Trigger Source Selections
Digital Input Disable Register
0 – DIDR0
ADTS2
ADTS1
ADTS0
0
0
0
Free Running mode
0
0
1
Analog Comparator
0
1
0
External Interrupt Request 0
0
1
1
Timer/Counter0 Compare Match A
1
0
0
Timer/Counter0 Overflow
1
0
1
Timer/Counter1 Compare Match B
1
1
0
Timer/Counter1 Overflow
1
1
1
Timer/Counter1 Capture Event
Bit
Trigger Source
7
6
5
4
3
2
1
0
ADC7D
ADC6D
ADC5D
ADC4D
ADC3D
ADC2D
ADC1D
ADC0D
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
DIDR0
• 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
disabled. 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.
Digital Input Disable Register
2 – DIDR2
Bit
7
6
5
4
3
2
1
0
ADC15D
ADC14D
ADC13D
ADC12D
ADC11D
ADC10D
ADC9D
ADC8D
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
DIDR2
• Bit 7..0 – ADC15D..ADC8D: ADC15..8 Digital Input Disable
When this bit is written logic one, the digital input buffer on the corresponding ADC pin is
disabled. The corresponding PIN Register bit will always read as zero when this bit is
set. When an analog signal is applied to the ADC15..8 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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2549A–AVR–03/05
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ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
JTAG Interface and
On-chip Debug
System
Features
• JTAG (IEEE std. 1149.1 Compliant) Interface
• Boundary-scan Capabilities According to the IEEE std. 1149.1 (JTAG) Standard
• Debugger Access to:
– All Internal Peripheral Units
– Internal and External RAM
– The Internal Register File
– Program Counter
– EEPROM and Flash Memories
• Extensive On-chip Debug Support for Break Conditions, Including
– AVR Break Instruction
– Break on Change of Program Memory Flow
– Single Step Break
– Program Memory 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®
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 “Programming via the JTAG Interface” on page 353 and “IEEE 1149.1 (JTAG)
Boundary-scan” on page 301, respectively. The On-chip Debug support is considered
being private JTAG instructions, and distributed within ATMEL and to selected third
party vendors only.
Figure 129 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.
Test Access Port – TAP
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.
•
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.
295
2549A–AVR–03/05
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, the input TAP signals 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
monitored 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 129. Block Diagram
I/O PORT 0
DEVICE BOUNDARY
BOUNDARY SCAN CHAIN
TDI
TDO
TCK
TMS
JTAG PROGRAMMING
INTERFACE
TAP
CONTROLLER
AVR CPU
INSTRUCTION
REGISTER
ID
REGISTER
M
U
X
FLASH
MEMORY
Address
Data
BREAKPOINT
UNIT
BYPASS
REGISTER
INTERNAL
SCAN
CHAIN
PC
Instruction
FLOW CONTROL
UNIT
DIGITAL
PERIPHERAL
UNITS
ANALOG
PERIPHERIAL
UNITS
Analog inputs
BREAKPOINT
SCAN CHAIN
ADDRESS
DECODER
JTAG / AVR CORE
COMMUNICATION
INTERFACE
OCD STATUS
AND CONTROL
Control & Clock lines
I/O PORT n
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Figure 130. TAP Controller State Diagram
1
Test-Logic-Reset
0
0
Run-Test/Idle
1
Select-DR Scan
1
Select-IR Scan
0
0
1
1
Capture-DR
Capture-IR
0
0
Shift-DR
Shift-IR
0
1
Exit1-DR
0
Pause-DR
0
0
Pause-IR
1
1
0
Exit2-DR
Exit2-IR
1
1
Update-DR
TAP Controller
1
Exit1-IR
0
1
0
1
1
0
1
Update-IR
0
1
0
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 130 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.
•
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
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2549A–AVR–03/05
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 UpdateDR state. The Exit-DR, Pause-DR, and Exit2-DR states are only used for navigating
the state machine.
As shown in the state diagram, the Run-Test/Idle state need not be entered between
selecting JTAG instruction and using Data Registers, and some JTAG instructions may
select certain functions to be performed in the Run-Test/Idle, making it unsuitable as an
Idle state.
Note:
Independent of the initial state of the TAP Controller, the Test-Logic-Reset state can
always be entered by holding TMS high for five TCK clock periods.
For detailed information on the JTAG specification, refer to the literature listed in “Bibliography” on page 300.
Using the Boundaryscan Chain
A complete description of the Boundary-scan capabilities are given in the section “IEEE
1149.1 (JTAG) Boundary-scan” on page 301.
Using the On-chip Debug As shown in Figure 129, the hardware support for On-chip Debugging consists mainly of
System
• A scan chain on the interface between the internal AVR CPU and the internal
peripheral units.
•
Break Point unit.
•
Communication interface between the CPU and JTAG system.
All read or modify/write operations needed for implementing the Debugger are done by
applying AVR instructions via the internal AVR CPU Scan Chain. The CPU sends the
result to an I/O memory mapped location which is part of the communication interface
between the CPU and the JTAG system.
The Break Point Unit implements Break on Change of Program Flow, Single Step
Break, 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”).
A debugger, like the AVR Studio, may however use one or more of these resources for
its internal purpose, leaving less flexibility to the end-user.
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A list of the On-chip Debug specific JTAG instructions is given in “On-chip Debug Specific JTAG Instructions” on page 299.
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 Onchip 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 Corporation’s AVR Assembler and C programs compiled with third party vendors’ compilers.
AVR Studio runs under Microsoft® Windows® 95/98/2000 and Microsoft Windows NT®.
For a full description of the AVR Studio, please refer to the AVR Studio User Guide.
Only highlights 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.
On-chip Debug Specific
JTAG Instructions
The On-chip debug support is considered being private JTAG instructions, and distributed within ATMEL and to selected third party vendors only. Instruction opcodes are
listed for reference.
PRIVATE0; 0x8
Private JTAG instruction for accessing On-chip debug system.
PRIVATE1; 0x9
Private JTAG instruction for accessing On-chip debug system.
PRIVATE2; 0xA
Private JTAG instruction for accessing On-chip debug system.
PRIVATE3; 0xB
Private JTAG instruction for accessing On-chip debug system.
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On-chip Debug Related
Register in I/O Memory
On-chip Debug Register –
OCDR
Bit
7
6
5
4
3
2
1
MSB/IDRD
0
LSB
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
OCDR
The OCDR Register provides a communication channel from the running program in the
microcontroller to the debugger. The CPU can transfer a byte to the debugger by writing
to this location. At the same time, an internal flag; I/O Debug Register Dirty – IDRD – is
set to indicate to the debugger that the register has been written. When the CPU reads
the OCDR Register the 7 LSB will be from the OCDR Register, while the MSB is the
IDRD bit. The debugger clears the IDRD bit when it has read the information.
In some AVR devices, this register is shared with a standard I/O location. In this case,
the OCDR Register can only be accessed if the OCDEN Fuse is programmed, and the
debugger enables access to the OCDR Register. In all other cases, the standard I/O
location is accessed.
Refer to the debugger documentation for further information on how to use this register.
Using the JTAG
Programming
Capabilities
Programming of AVR parts via JTAG is performed via the 4-pin JTAG port, TCK, TMS,
TDI, and TDO. These are the only pins that need to be controlled/observed to perform
JTAG programming (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.
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 353.
Bibliography
300
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, AddisonWesley, 1992.
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
IEEE 1149.1 (JTAG)
Boundary-scan
Features
•
•
•
•
•
System Overview
The Boundary-scan chain has the capability of driving and observing the logic levels on
the digital 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 provides a mechanism for testing interconnections and integrity of components on Printed Circuits Boards by using the four TAP
signals only.
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
The four IEEE 1149.1 defined mandatory JTAG instructions IDCODE, BYPASS, SAMPLE/PRELOAD, and EXTEST, as well as the AVR specific public JTAG instruction
AVR_RESET can be used for testing the Printed Circuit Board. Initial scanning of the
Data Register path will show the ID-Code of the device, since IDCODE is the default
JTAG instruction. It may be desirable to have the AVR device in reset during test mode.
If not reset, inputs to the device may be determined by the scan operations, and the
internal software may be in an undetermined state when exiting the test mode. Entering
reset, the outputs of any port pin will instantly enter the high impedance state, making
the HIGHZ instruction redundant. If needed, the BYPASS instruction can be issued to
make the shortest possible scan chain through the device. The device can be set in the
reset state either by pulling the external RESET pin low, or issuing the AVR_RESET
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.
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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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 131 shows the structure of the Device Identification Register.
Figure 131. The Format of the Device Identification Register
LSB
MSB
Bit
31
Device ID
28
27
12
11
1
0
Version
Part Number
Manufacturer ID
1
4 bits
16 bits
11 bits
1-bit
Version
Version is a 4-bit number identifying the revision of the component. The JTAG version
number follows the revision of the device. Revision A is 0x0, revision B is 0x1 and so on.
Part Number
The part number is a 16-bit code identifying the component. The JTAG Part Number for
ATmega640/1280/1281/2560/2561 is listed in Table 129.
Table 129. AVR JTAG Part Number
Part Number
Manufacturer ID
JTAG Part Number (Hex)
ATmega640
0x9607
ATmega1280
0x9703
ATmega1281
0x9704
ATmega2560
0x9801
ATmega2561
0x9802
The Manufacturer ID is a 11-bit code identifying the manufacturer. The JTAG manufacturer ID for ATMEL is listed in Table 130.
Table 130. Manufacturer ID
Manufacturer
ATMEL
Reset Register
JTAG Manufactor ID (Hex)
0x01F
The Reset Register is a test Data Register used to reset the part. Since the AVR tristates 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 settings for the clock options, the part will remain reset for a reset time-out
period (see “Clock Sources” on page 40) 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 132.
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ATmega640/1280/1281/2560/2561
Figure 132. Reset Register
To
TDO
From Other Internal and
External Reset Sources
From
TDI
D
Q
Internal reset
ClockDR · AVR_RESET
Boundary-scan Chain
The Boundary-scan Chain has the capability of driving and observing the logic levels on
the digital 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 305 for a complete description.
Boundary-scan Specific
JTAG Instructions
The Instruction Register is 4-bit wide, supporting up to 16 instructions. Listed below are
the JTAG instructions useful for Boundary-scan operation. Note that the optional HIGHZ
instruction is not implemented, but all outputs with tri-state capability can be set in highimpedant state by using the AVR_RESET instruction, since the initial state for all port
pins is tri-state.
As a definition in this datasheet, the LSB is shifted in and out first for all Shift Registers.
The OPCODE for each instruction is shown behind the instruction name in hex format.
The text describes which Data Register is selected as path between TDI and TDO for
each instruction.
EXTEST; 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 contents 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:
IDCODE; 0x1
•
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.
Optional JTAG instruction selecting the 32 bit ID-Register as Data Register. The IDRegister consists of a version number, a device number and the manufacturer code
chosen by JEDEC. This is the default instruction after power-up.
The active states are:
•
Capture-DR: Data in the IDCODE Register is sampled into the Boundary-scan
Chain.
•
Shift-DR: The IDCODE scan chain is shifted by the TCK input.
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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:
AVR_RESET; 0xC
•
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.
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:
•
BYPASS; 0xF
Shift-DR: The Reset Register is shifted by the TCK input.
Mandatory JTAG instruction selecting the Bypass Register for Data Register.
The active states are:
•
Capture-DR: Loads a logic “0” into the Bypass Register.
•
Shift-DR: The Bypass Register cell between TDI and TDO is shifted.
Boundary-scan Related
Register in I/O Memory
MCU Control Register –
MCUCR
The MCU Control Register contains control bits for general MCU functions.
Bit
7
6
5
4
3
2
1
0
JTD
–
–
PUD
–
–
IVSEL
IVCE
Read/Write
R/W
R
R
R/W
R
R
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
MCUCR
• Bits 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.
MCU Status Register –
MCUSR
The MCU Status Register provides information on which reset source caused an MCU
reset.
Bit
304
7
6
5
4
3
2
1
0
–
–
–
JTRF
WDRF
BORF
EXTRF
PORF
Read/Write
R
R
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
MCUSR
See Bit Description
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
• 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.
Boundary-scan Chain
The Boundary-scan chain has the capability of driving and observing the logic levels on
the digital I/O pins, as well as the boundary between digital and analog logic for analog
circuitry having off-chip connection.
Scanning the Digital Port Pins
Figure 133 shows the Boundary-scan Cell for a bi-directional port pin. The pull-up function is disabled during Boundary-scan when the JTAG IC contains EXTEST or
SAMPLE_PRELOAD. The cell consists of a bi-directional pin cell that combines the
three signals Output Control - OCxn, Output Data - ODxn, and Input Data - IDxn, into
only a two-stage Shift Register. The port and pin indexes are not used in the following
description
The Boundary-scan logic is not included in the figures in the datasheet. Figure 134
shows a simple digital port pin as described in the section “I/O-Ports” on page 81. The
Boundary-scan details from Figure 133 replaces the dashed box in Figure 134.
When no alternate port function is present, the Input Data - ID - corresponds to the
PINxn Register value (but ID has no synchronizer), Output Data corresponds to the
PORT Register, Output Control corresponds to the Data Direction - DD Register, and
the Pull-up Enable - PUExn - corresponds to logic expression PUD · DDxn · PORTxn.
Digital alternate port functions are connected outside the dotted box in Figure 134 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. There is no scan chain on the
interface between the digital and the analog circuitry, but some digital control signal to
analog circuitry are turned off to avoid driving contention on the pads.
When JTAG IR contains EXTEST or SAMPLE_PRELOAD the clock is not sent out on
the port pins even if the CKOUT fuse is programmed. Even though the clock is output
when the JTAG IR contains SAMPLE_PRELOAD, the clock is not sampled by the
boundary scan.
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Figure 133. Boundary-scan Cell for Bi-directional Port Pin with Pull-up Function.
To Next Cell
ShiftDR
EXTEST
Pull-up Enable (PUE)
Vcc
0
1
Output Control (OC)
FF1
LD1
0
D
Q
D
Q
0
1
1
G
FF0
0
1
LD0
0
D
Q
D
1
Q
0
1
Port Pin (PXn)
Output Data (OD)
G
Input Data (ID)
From Last Cell
306
ClockDR
UpdateDR
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ATmega640/1280/1281/2560/2561
Figure 134. General Port Pin Schematic Diagram
See Boundary-scan
Description for Details!
PUExn
PUD
Q
D
DDxn
Q CLR
RESET
OCxn
WDx
Q
Pxn
ODxn
D
PORTxn
Q CLR
WRx
IDxn
DATA BUS
RDx
RESET
RRx
SLEEP
SYNCHRONIZER
D
Q
D
RPx
Q
PINxn
L
Q
Q
CLK I/O
PUD:
PUExn:
OCxn:
ODxn:
IDxn:
SLEEP:
Scanning the RESET Pin
PULLUP DISABLE
PULLUP ENABLE for pin Pxn
OUTPUT CONTROL for pin Pxn
OUTPUT DATA to pin Pxn
INPUT DATA from pin Pxn
SLEEP CONTROL
WDx:
RDx:
WRx:
RRx:
RPx:
CLK I/O :
WRITE DDRx
READ DDRx
WRITE PORTx
READ PORTx REGISTER
READ PORTx PIN
I/O CLOCK
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 135 is inserted for the 5V reset signal.
Figure 135. Observe-only Cell
To
Next
Cell
ShiftDR
From System Pin
To System Logic
FF1
0
D
Q
1
From
Previous
Cell
ClockDR
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ATmega640/1280/1281/25 Table 131 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
60/2561 Boundary-scan
scanned out. The scan order follows the pin-out order as far as possible. Therefore, the
Order
bits of Port A and Port K 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 133, PXn. Data corresponds to FF0, PXn. Control corresponds to
FF1, PXn. 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 131. ATmega640/1280/2560 Boundary-scan Order
308
Bit Number
Signal Name
Module
164
PG5.Data
Port G
163
PG5.Control
162
PE0.Data
161
PE0.Control
160
PE1.Data
159
PE1.Control
158
PE2.Data
157
PE2.Control
156
PE3.Data
155
PE3.Control
154
PE4.Data
153
PE4.Control
152
PE5.Data
151
PE5.Control
150
PE6.Data
149
PE6.Control
148
PE7.Data
147
PE7.Control
146
PH0.Data
145
PH0.Control
144
PH1.Data
143
PH1.Control
142
PH2.Data
141
PH2.Control
140
PH3.Data
139
PH3.Control
138
PH4.Data
137
PH4.Control
136
PH5.Data
Port E
Port H
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Table 131. ATmega640/1280/2560 Boundary-scan Order (Continued)
Bit Number
Signal Name
Module
135
PH5.Control
134
PH6.Data
133
PH6.Control
132
PB0.Data
131
PB0.Control
130
PB1.Data
129
PB1.Control
128
PB2.Data
127
PB2.Control
126
PB3.Data
125
PB3.Control
124
PB4.Data
123
PB4.Control
122
PB5.Data
121
PB5.Control
120
PB6.Data
119
PB6.Control
118
PB7.Data
117
PB7.Control
116
PH7.Data
115
PH7.Control
114
PG3.Data
113
PG3.Control
112
PG4.Data
111
PG4.Control
110
RSTT
Reset Logic (Observe Only)
109
PL0.Data
Port L
108
PL0.Control
107
PL1.Data
106
PL1.Control
105
PL2.Data
104
PL2.Control
103
PL3.Data
102
PL3.Control
101
PL4.Data
100
PL4.Control
Port B
Port H
Port G
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Table 131. ATmega640/1280/2560 Boundary-scan Order (Continued)
310
Bit Number
Signal Name
99
PL5.Data
98
PL5.Control
97
PL6.Data
96
PL6.Control
95
PL7.Data
94
PL7.Control
93
PD0.Data
92
PD0.Control
91
PD1.Data
90
PD1.Control
89
PD2.Data
88
PD2.Control
87
PD3.Data
86
PD3.Control
85
PD4.Data
84
PD4.Control
83
PD5.Data
82
PD5.Control
81
PD6.Data
80
PD6.Control
79
PD7.Data
78
PD7.Control
77
PG0.Data
76
PG0.Control
75
PG1.Data
74
PG1.Control
73
PC0.Data
72
PC0.Control
71
PC1.Data
70
PC1.Control
69
PC2.Data
68
PC2.Control
67
PC3.Data
66
PC3.Control
65
PC4.Data
64
PC4.Control
Module
Port D
Port G
Port C
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 131. ATmega640/1280/2560 Boundary-scan Order (Continued)
Bit Number
Signal Name
63
PC5.Data
62
PC5.Control
61
PC6.Data
60
PC6.Control
59
PC7.Data
58
PC7.Control
57
PJ0.Data
56
PJ0.Control
55
PJ1.Data
54
PJ1.Control
53
PJ2.Data
52
PJ2.Control
51
PJ3.Data
50
PJ3.Control
49
PJ4.Data
48
PJ4.Control
47
PJ5.Data
46
PJ5.Control
45
PJ6.Data
44
PJ6.Control
43
PG2.Data
42
PG2.Control
41
PA7.Data
40
PA7.Control
39
PA6.Data
38
PA6.Control
37
PA5.Data
36
PA5.Control
35
PA4.Data
34
PA4.Control
33
PA3.Data
32
PA3.Control
31
PA2.Data
30
PA2.Control
29
PA1.Data
28
PA1.Control
Module
Port J
Port G
Port A
311
2549A–AVR–03/05
Table 131. ATmega640/1280/2560 Boundary-scan Order (Continued)
312
Bit Number
Signal Name
27
PA0.Data
26
PA0.Control
25
PJ7.Data
24
PJ7.Control
23
PK7.Data
22
PK7.Control
21
PK6.Data
20
PK6.Control
19
PK5.Data
18
PK5.Control
17
PK4.Data
16
PK4.Control
15
PK3.Data
14
PK3.Control
13
PK2.Data
12
PK2.Control
11
PK1.Data
10
PK1.Control
9
PK0.Data
8
PK0.Control
7
PF3.Data
6
PF3.Control
5
PF2.Data
4
PF2.Control
3
PF1.Data
2
PF1.Control
1
PF0.Data
0
PF0.Control
Module
Port J
Port K
Port F
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 132. ATmega1281/2561 Boundary-scan Order
Bit Number
Signal Name
Module
100
PG5.Data
Port G
99
PG5.Control
98
PE0.Data
97
PE0.Control
96
PE1.Data
95
PE1.Control
94
PE2.Data
93
PE2.Control
92
PE3.Data
91
PE3.Control
90
PE4.Data
89
PE4.Control
88
PE5.Data
87
PE5.Control
86
PE6.Data
85
PE6.Control
84
PE7.Data
83
PE7.Control
82
PB0.Data
81
PB0.Control
80
PB1.Data
79
PB1.Control
78
PB2.Data
77
PB2.Control
76
PB3.Data
75
PB3.Control
74
PB4.Data
73
PB4.Control
72
PB5.Data
71
PB5.Control
70
PB6.Data
69
PB6.Control
68
PB7.Data
67
PB7.Control
66
PG3.Data
Port E
Port B
Port G
313
2549A–AVR–03/05
Table 132. ATmega1281/2561 Boundary-scan Order (Continued)
314
Bit Number
Signal Name
Module
65
PG3.Control
64
PG4.Data
63
PG4.Control
62
RSTT
Reset Logic (Observe Only)
61
PD0.Data
Port D
60
PD0.Control
59
PD1.Data
58
PD1.Control
57
PD2.Data
56
PD2.Control
55
PD3.Data
54
PD3.Control
53
PD4.Data
52
PD4.Control
51
PD5.Data
50
PD5.Control
49
PD6.Data
48
PD6.Control
47
PD7.Data
46
PD7.Control
45
PG0.Data
44
PG0.Control
43
PG1.Data
42
PG1.Control
41
PC0.Data
40
PC0.Control
39
PC1.Data
38
PC1.Control
37
PC2.Data
36
PC2.Control
35
PC3.Data
34
PC3.Control
33
PC4.Data
32
PC4.Control
31
PC5.Data
30
PC5.Control
Port G
Port C
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 132. ATmega1281/2561 Boundary-scan Order (Continued)
Boundary-scan
Description Language
Files
Bit Number
Signal Name
29
PC6.Data
28
PC6.Control
27
PC7.Data
26
PC7.Control
25
PG2.Data
24
PG2.Control
23
PA7.Data
22
PA7.Control
21
PA6.Data
20
PA6.Control
19
PA5.Data
18
PA5.Control
17
PA4.Data
16
PA4.Control
15
PA3.Data
14
PA3.Control
13
PA2.Data
12
PA2.Control
11
PA1.Data
10
PA1.Control
9
PA0.Data
8
PA0.Control
7
PF3.Data
6
PF3.Control
5
PF2.Data
4
PF2.Control
3
PF1.Data
2
PF1.Control
1
PF0.Data
0
PF0.Control
Module
Port G
Port A
Port F
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.
BSDL files are available for ATmega1281/2561 and ATmega640/1280/2560.
315
2549A–AVR–03/05
316
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Boot Loader Support
– Read-While-Write
Self-Programming
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 application 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 memory. The program code within
the Boot Loader section has the capability to write into the entire Flash, including the
Boot Loader memory. The Boot Loader can thus even modify itself, and it can also
erase itself from the code if the feature is not needed anymore. The size of the Boot
Loader memory is configurable with fuses and the Boot Loader has two separate sets of
Boot Lock bits which can be set independently. This gives the user a unique flexibility to
select different levels of protection.
Boot Loader Features
•
•
•
•
•
•
•
Read-While-Write Self-Programming
Flexible Boot Memory Size
High Security (Separate Boot Lock Bits for a Flexible Protection)
Separate Fuse to Select Reset Vector
Optimized Page(1) Size
Code Efficient Algorithm
Efficient Read-Modify-Write Support
Note:
1. A page is a section in the Flash consisting of several bytes (see Table 158 on page
340) used during programming. The page organization does not affect normal
operation.
Application and Boot
Loader Flash Sections
The Flash memory is organized in two main sections, the Application section and the
Boot Loader section (see Figure 137). The size of the different sections is configured by
the BOOTSZ Fuses as shown in Table 139 on page 330 and Figure 137. These two
sections can have different level of protection since they have different sets of Lock bits.
Application Section
The Application section is the section of the Flash that is used for storing the application
code. The protection level for the Application section can be selected by the application
Boot Lock bits (Boot Lock bits 0), see Table 134 on page 320. 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 software 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 135 on page
320.
Read-While-Write and No Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot
Loader software update is dependent on which address that is being programmed. In
Read-While-Write Flash
addition to the two sections that are configurable by the BOOTSZ Fuses as described
Sections
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 RWWand NRWW sections is given in Table 133 and Figure 136 on page 318. 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.
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2549A–AVR–03/05
Note that the user software can never read any code that is located inside the RWW
section during a Boot Loader software operation. The syntax “Read-While-Write 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.
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 load program memory, call, or jump instructions or an interrupt)
during programming, the software might end up in an unknown state. To avoid this, the
interrupts should either be disabled or moved to the Boot Loader section. The Boot
Loader section is always located in the NRWW section. The RWW Section Busy bit
(RWWSB) in the Store Program Memory Control and Status Register (SPMCSR) will be
read as logical one as long as the RWW section is blocked for reading. After a programming is completed, the RWWSB must be cleared by software before reading code
located in the RWW section. See “Store Program Memory Control and Status Register –
SPMCSR” on page 321. for details on how to clear RWWSB.
NRWW – No Read-While-Write
Section
The code located in the NRWW section can be read when the Boot Loader software is
updating a page in the RWW section. When the Boot Loader code updates the NRWW
section, the CPU is halted during the entire Page Erase or Page Write operation.
Table 133. Read-While-Write Features
Which Section does the Z-pointer
Address during the Programming?
Which Section can be
Read during Programming?
CPU Halted?
Read-While-Write
Supported?
RWW Section
NRWW Section
No
Yes
NRWW Section
None
Yes
No
Figure 136. Read-While-Write vs. No Read-While-Write
Read-While-Write
(RWW) Section
Z-pointer
Addresses RWW
Section
Z-pointer
Addresses NRWW
Section
No Read-While-Write
(NRWW) Section
CPU is Halted
During the Operation
Code Located in
NRWW Section
Can be Read During
the Operation
318
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 137. Memory Sections
Program Memory
BOOTSZ = '10'
Program Memory
BOOTSZ = '11'
0x0000
No Read-While-Write Section
Read-While-Write Section
Application Flash Section
End RWW
Start NRWW
Application Flash Section
Boot Loader Flash Section
End Application
Start Boot Loader
Flashend
No Read-While-Write Section
Read-While-Write Section
0x0000
Program Memory
BOOTSZ = '01'
Application Flash Section
End RWW
Start NRWW
Application Flash Section
End Application
Start Boot Loader
Boot Loader Flash Section
Flashend
Program Memory
BOOTSZ = '00'
No Read-While-Write Section
Boot Loader Lock Bits
Read-While-Write Section
Application Flash Section
End RWW
Start NRWW
Application Flash Section
End Application
Start Boot Loader
Boot Loader Flash Section
Flashend
Note:
0x0000
No Read-While-Write Section
Read-While-Write Section
0x0000
Application Flash Section
End RWW, End Application
Start NRWW, Start Boot Loader
Boot Loader Flash Section
Flashend
1. The parameters in the figure above are given in Table 139 on page 330.
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 134 and Table 135 for further details. The Boot Lock bits can be set in software and in Serial or Parallel Programming mode, but they can be cleared by a Chip
Erase command only. The general Write Lock (Lock Bit mode 2) does not control the
programming of the Flash memory by SPM instruction. Similarly, the general
Read/Write Lock (Lock Bit mode 1) does not control reading nor writing by
(E)LPM/SPM, if it is attempted.
319
2549A–AVR–03/05
Table 134. Boot Lock Bit0 Protection Modes (Application Section)(1)
BLB0 Mode
BLB02
BLB01
1
1
1
No restrictions for SPM or (E)LPM accessing the
Application section.
2
1
0
SPM is not allowed to write to the Application section.
3
0
0
SPM is not allowed to write to the Application section, and
(E)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.
4
0
1
(E)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:
Protection
1. “1” means unprogrammed, “0” means programmed
Table 135. Boot Lock Bit1 Protection Modes (Boot Loader Section)(1)
BLB1 Mode
BLB12
BLB11
1
1
1
No restrictions for SPM or (E)LPM accessing the Boot
Loader section.
2
1
0
SPM is not allowed to write to the Boot Loader section.
3
0
0
SPM is not allowed to write to the Boot Loader section,
and (E)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
(E)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:
Protection
1. “1” means unprogrammed, “0” means programmed
Entering the Boot Loader Entering the Boot Loader takes place by a jump or call from the application program.
This may be initiated by a trigger such as a command received via USART, or SPI interProgram
face. Alternatively, the Boot Reset Fuse can be programmed so that the Reset Vector is
pointing to the Boot Flash start address after a reset. In this case, the Boot Loader is
started after a reset. After the application code is loaded, the program can start executing the application code. Note that the fuses cannot be changed by the MCU itself. This
means that once the Boot Reset Fuse is programmed, the Reset Vector will always
point to the Boot Loader Reset and the fuse can only be changed through the serial or
parallel programming interface.
Table 136. Boot Reset Fuse(1)
BOOTRST
Note:
320
Reset Address
1
Reset Vector = Application Reset (address 0x0000)
0
Reset Vector = Boot Loader Reset (see Table 139 on page 330)
1. “1” means unprogrammed, “0” means programmed
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Store Program Memory
Control and Status Register –
SPMCSR
The Store Program Memory Control and Status Register contains the control bits
needed to control the Boot Loader operations.
Bit
7
6
5
4
3
2
1
0
SPMIE
RWWSB
SIGRD
RWWSRE
BLBSET
PGWRT
PGERS
SPMEN
Read/Write
R/W
R
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
SPMCSR
• 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
initiated, the RWWSB will be set (one) by hardware. When the RWWSB bit is set, the
RWW section cannot be accessed. The RWWSB bit will be cleared if the RWWSRE bit
is written to one after a Self-Programming operation is completed. Alternatively the
RWWSB bit will automatically be cleared if a page load operation is initiated.
• Bit 5 – SIGRD: Signature Row Read
If this bit is written to one at the same time as SPMEN, the next LPM instruction within
three clock cycles will read a byte from the signature row into the destination register.
see “Reading the Signature Row from Software” on page 326 for details. An SPM
instruction within four cycles after SIGRD and SPMEN are set will have no effect. This
operation is reserved for future use and should not be used.
• Bit 4 – RWWSRE: Read-While-Write Section Read Enable
When programming (Page Erase or Page Write) to the RWW section, the RWW section
is blocked for reading (the RWWSB will be set by hardware). To re-enable the RWW
section, the user software must wait until the programming is completed (SPMEN will be
cleared). Then, if the RWWSRE bit is written to one at the same time as SPMEN, the
next SPM instruction within four clock cycles re-enables the RWW section. The RWW
section cannot be re-enabled while the Flash is busy with a Page Erase or a Page Write
(SPMEN is set). If the RWWSRE bit is written while the Flash is being loaded, the Flash
load operation will abort and the data loaded will be lost.
• Bit 3 – BLBSET: Boot Lock Bit Set
If this bit is written to one at the same time as SPMEN, the next SPM instruction within
four clock cycles sets Boot Lock bits, according to the data in R0. The data in R1 and
the address in the Z-pointer are ignored. The BLBSET bit will automatically be cleared
upon completion of the Lock bit set, or if no SPM instruction is executed within four clock
cycles.
An (E)LPM instruction within three cycles after BLBSET and SPMEN are set in the
SPMCSR Register, will read either the Lock bits or the Fuse bits (depending on Z0 in
the Z-pointer) into the destination register. See “Reading the Fuse and Lock Bits from
Software” on page 325 for details.
• Bit 2 – PGWRT: Page Write
If this bit is written to one at the same time as SPMEN, the next SPM instruction within
four clock cycles executes Page Write, with the data stored in the temporary buffer. The
321
2549A–AVR–03/05
page address is taken from the high part of the Z-pointer. The data in R1 and R0 are
ignored. The PGWRT bit will auto-clear upon completion of a Page Write, or if no SPM
instruction is executed within four clock cycles. The CPU is halted during the entire
Page Write operation if the NRWW section is addressed.
• Bit 1 – PGERS: Page Erase
If this bit is written to one at the same time as SPMEN, the next SPM instruction within
four clock cycles executes Page Erase. The page address is taken from the high part of
the Z-pointer. The data in R1 and R0 are ignored. The PGERS bit will auto-clear upon
completion of a Page Erase, or if no SPM instruction is executed within four clock
cycles. The CPU is halted during the entire Page Write operation if the NRWW section is
addressed.
• Bit 0 – SPMEN: Store Program Memory Enable
This bit enables the SPM instruction for the next four clock cycles. If written to one
together with either RWWSRE, BLBSET, PGWRT’ or PGERS, the following SPM
instruction will have a special meaning, see description above. If only SPMEN is written,
the following SPM instruction will store the value in R1:R0 in the temporary page buffer
addressed by the Z-pointer. The LSB of the Z-pointer is ignored. The SPMEN bit will
auto-clear upon completion of an SPM instruction, or if no SPM instruction is executed
within four clock cycles. During Page Erase and Page Write, the SPMEN bit remains
high until the operation is completed.
Writing any other combination than “10001”, “01001”, “00101”, “00011” or “00001” in the
lower five bits will have no effect.
Note:
Addressing the Flash
During SelfProgramming
Only one SPM instruction should be active at any time.
The Z-pointer is used to address the SPM commands. The Z pointer consists of the Zregisters ZL and ZH in the register file, and RAMPZ in the I/O space. The number of bits
actually used is implementation dependent. Note that the RAMPZ register is only implemented when the program space is larger than 64K bytes.
Bit
23
22
21
20
19
18
17
15
14
13
12
11
10
9
16
8
RAMPZ
RAMPZ7
RAMPZ6
RAMPZ5
RAMPZ4
RAMPZ3
RAMPZ2
RAMPZ1
RAMPZ0
ZH (R31)
Z15
Z14
Z13
Z12
Z11
Z10
Z9
Z8
ZL (R30)
Z7
Z6
Z5
Z4
Z3
Z2
Z1
Z0
7
6
5
4
3
2
1
0
Since the Flash is organized in pages (see Table 158 on page 340), 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 138. Note that the Page Erase
and Page Write operations are addressed independently. Therefore it is of major importance that the Boot Loader software addresses the same page in both the Page Erase
and Page Write operation. Once a programming operation is initiated, the address is
latched and the Z-pointer can be used for other operations.
The (E)LPM instruction use the Z-pointer to store the address. Since this instruction
addresses the Flash byte-by-byte, also bit Z0 of the Z-pointer is used.
322
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 138. Addressing the Flash During SPM(1)
BIT
15
ZPCMSB
ZPAGEMSB
Z - REGISTER
1 0
0
PCMSB
PROGRAM
COUNTER
PAGEMSB
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:
Self-Programming the
Flash
1. The different variables used in Figure 138 are listed in Table 141 on page 330.
The program memory is updated in a page by page fashion. Before programming a
page with the data stored in the temporary page buffer, the page must be erased. The
temporary page buffer is filled one word at a time using SPM and the buffer can be filled
either before the Page Erase command or between a Page Erase and a Page Write
operation:
Alternative 1, fill the buffer before a Page Erase
•
Fill temporary page buffer
•
Perform a Page Erase
•
Perform a Page Write
Alternative 2, fill the buffer after Page Erase
•
Perform a Page Erase
•
Fill temporary page buffer
•
Perform a Page Write
If only a part of the page needs to be changed, the rest of the page must be stored (for
example in the temporary page buffer) before the erase, and then be rewritten. When
using alternative 1, the Boot Loader provides an effective Read-Modify-Write feature
which allows the user software to first read the page, do the necessary changes, and
then write back the modified data. If alternative 2 is used, it is not possible to read the
old data while loading since the page is already erased. The temporary page buffer can
be accessed in a random sequence. It is essential that the page address used in both
the Page Erase and Page Write operation is addressing the same page. See “Simple
Assembly Code Example for a Boot Loader” on page 328 for an assembly code
example.
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2549A–AVR–03/05
Performing Page Erase by
SPM
Filling the Temporary Buffer
(Page Loading)
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.
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.
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.
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 69.
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 SelfProgramming
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 69, or the interrupts must be disabled. Before addressing the RWW section after the programming is
completed, the user software must clear the RWWSB by writing the RWWSRE. See
“Simple Assembly Code Example for a Boot Loader” on page 328 for an example.
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ATmega640/1280/1281/2560/2561
Setting the Boot Loader Lock
Bits by SPM
To set the Boot Loader Lock bits, write the desired data to R0, write “X0001001” to
SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The only
accessible Lock bits are the Boot Lock bits that may prevent the Application and Boot
Loader section from any software update by the MCU.
Bit
7
6
5
4
3
2
1
0
R0
1
1
BLB12
BLB11
BLB02
BLB01
1
1
See Table 134 and Table 135 for how the different settings of the Boot Loader bits affect
the Flash access.
If bits 5..2 in R0 are cleared (zero), the corresponding Boot Lock bit will be programmed
if an SPM instruction is executed within four cycles after BLBSET and SPMEN are set in
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, 6, 1, and 0 in R0 to “1”
when writing the Lock bits. When programming the Lock bits the entire Flash can be
read during the operation.
EEPROM Write Prevents
Writing to 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 (EEPE)
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 (E)LPM instruction 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 (E)LPM instruction is executed within three CPU cycles or no SPM
instruction is executed within four CPU cycles. When BLBSET and SPMEN are cleared,
(E)LPM will work as described in the Instruction set Manual.
Bit
7
6
5
4
3
2
1
0
Rd
–
–
BLB12
BLB11
BLB02
BLB01
LB2
LB1
The algorithm for reading the Fuse Low 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 (E)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 152 on page 337 for a detailed description and mapping of the Fuse Low
byte.
Bit
7
6
5
4
3
2
1
0
Rd
FLB7
FLB6
FLB5
FLB4
FLB3
FLB2
FLB1
FLB0
Similarly, when reading the Fuse High byte, load 0x0003 in the Z-pointer. When an
(E)LPM instruction 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 151 on page 337 for detailed
description and mapping of the Fuse High byte.
Bit
7
6
5
4
3
2
1
0
Rd
FHB7
FHB6
FHB5
FHB4
FHB3
FHB2
FHB1
FHB0
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When reading the Extended Fuse byte, load 0x0002 in the Z-pointer. When an (E)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 150 on page 336 for detailed description
and mapping of the Extended Fuse byte.
Bit
7
6
5
4
3
2
1
0
Rd
–
–
–
–
–
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.
Reading the Signature Row
from Software
To read the Signature Row from software, load the Z-pointer with the signature byte
address given in Table 137 on page 326 and set the SIGRD and SPMEN bits in
SPMCSR. When an LPM instruction is executed within three CPU cycles after the
SIGRD and SPMEN bits are set in SPMCSR, the signature byte value will be loaded in
the destination register. The SIGRD and SPMEN bits will auto-clear upon completion of
reading the Signature Row Lock bits or if no LPM instruction is executed within three
CPU cycles. When SIGRD and SPMEN are cleared, LPM will work as described in the
Instruction set Manual.
Table 137. Signature Row Addressing
Signature Byte
Device Signature Byte 1
0x0000
Device Signature Byte 2
0x0002
Device Signature Byte 3
0x0004
RC Oscillator Calibration Byte
0x0001
Note:
Preventing Flash Corruption
Z-Pointer Address
All other addresses are reserved for future use.
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 voltage 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 prevent the CPU from attempting to decode and execute instructions, effec-
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ATmega640/1280/1281/2560/2561
tively protecting the SPMCSR Register and thus the Flash from unintentional
writes.
Programming Time for Flash
when Using SPM
The calibrated RC Oscillator is used to time Flash accesses. Table 138 shows the typical programming time for Flash accesses from the CPU.
Table 138. SPM Programming Time
Symbol
Min Programming Time
Max Programming Time
Flash write (Page Erase, Page Write,
and write Lock bits by SPM)
3.7 ms
4.5 ms
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Simple Assembly Code
Example for a Boot Loader
;-the routine writes one page of data from RAM to Flash
; the first data location in RAM is pointed to by the Y pointer
; the first data location in Flash is pointed to by the Z-pointer
;-error handling is not included
;-the routine must be placed inside the Boot space
; (at least the Do_spm sub routine). Only code inside NRWW section
can
; be read during Self-Programming (Page Erase and Page Write).
;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24),
; loophi (r25), spmcrval (r20)
; storing and restoring of registers is not included in the routine
; register usage can be optimized at the expense of code size
;-It is assumed that either the interrupt table is moved to the
Boot
; loader section or that the interrupts are disabled.
.equ PAGESIZEB = PAGESIZE*2
;PAGESIZEB is page size in BYTES, not
words
.org SMALLBOOTSTART
Write_page:
; Page Erase
ldi spmcrval, (1<<PGERS) | (1<<SPMEN)
call Do_spm
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
; transfer data from RAM to Flash page buffer
ldi looplo, low(PAGESIZEB)
;init loop variable
ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256
Wrloop:
ld
r0, Y+
ld
r1, Y+
ldi spmcrval, (1<<SPMEN)
call Do_spm
adiw ZH:ZL, 2
sbiw loophi:looplo, 2
;use subi for PAGESIZEB<=256
brne Wrloop
; execute Page Write
subi ZL, low(PAGESIZEB)
;restore pointer
sbci ZH, high(PAGESIZEB)
;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)
;restore pointer
sbci YH, high(PAGESIZEB)
Rdloop:
elpm r0, Z+
ld
r1, Y+
cpse r0, r1
jmp Error
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ATmega640/1280/1281/2560/2561
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
; If RWWSB is set, the RWW section is not
ready yet
ret
; re-enable the RWW section
ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN)
call Do_spm
rjmp Return
Do_spm:
; check for previous SPM complete
Wait_spm:
in
temp1, 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, EEPE
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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ATmega640 Boot Loader
Parameters
In Table 139 through Table 141, the parameters used in the description of the Self-Programming are given.
Boot Reset Address
(Start Boot Loader
Section)
End Application
Section
Boot Loader
Flash Section
Appli-cation
Flash Section
Pages
Boot Size
BOOTSZ0
BOOTSZ1
Table 139. Boot Size Configuration, ATmega640(1)
1
1
512
words
4
0x0000 0x7DFF
0x7E00 0x7FFF
0x7DFF
0x7E00
1
0
1024
words
8
0x0000 0x7BFF
0x7C00 0x7FFF
0x7BFF
0x7C00
0
1
2048
words
16
0x0000 0x77FF
0x7800 0x7FFF
0x77FF
0x7800
0
0
4096
words
32
0x0000 0x6FFF
0x7000 0x7FFF
0x6FFF
0x7000
Note:
1. The different BOOTSZ Fuse configurations are shown in Figure 137.
Table 140. Read-While-Write Limit, ATmega640
Section(1)
Pages
Address
Read-While-Write section (RWW)
224
0x0000 - 0x6FFF
No Read-While-Write section (NRWW)
32
0x7000 - 0x7FFF
Note:
1. For details about these two section, see “NRWW – No Read-While-Write Section” on
page 318 and “RWW – Read-While-Write Section” on page 318.
Table 141. Explanation of different variables used in Figure 138 and the mapping to the
Z-pointer, ATmega640
Corresponding
Z-value(2)
Variable
PCMSB
330
14
Description(1)
Most significant bit in the Program Counter.
(The Program Counter is 15 bits PC[14:0])
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 141. Explanation of different variables used in Figure 138 and the mapping to the
Z-pointer, ATmega640
Corresponding
Z-value(2)
Variable
PAGEMSB
Most significant bit which is used to address the
words within one page (128 words in a page
requires seven bits PC [6:0]).
ZPCMSB
Z15
Bit in Z-pointer that is mapped to PCMSB.
Because Z0 is not used, the ZPCMSB equals
PCMSB + 1.
ZPAGEMSB
Z7
Bit in Z-pointer that is mapped to PCMSB.
Because Z0 is not used, the ZPAGEMSB
equals PAGEMSB + 1.
PCPAGE
PC[14:7]
Z15:Z8
Program Counter page address: Page select,
for Page Erase and Page Write
PCWORD
PC[6:0]
Z7:Z1
Program Counter word address: Word select,
for filling temporary buffer (must be zero during
Page Write operation)
Notes:
ATmega1280/1281 Boot
Loader Parameters
6
Description(1)
1. Z0: should be zero for all SPM commands, byte select for the (E)LPM instruction.
2. See “Addressing the Flash During Self-Programming” on page 322 for details about
the use of Z-pointer during Self-Programming.
In Table 142 through Table 143, the parameters used in the description of the Self-Programming are given.
Boot Reset Address
(Start Boot Loader
Section)
End Application
Section
Boot Loader
Flash Section
Appli-cation
Flash Section
Pages
Boot Size
BOOTSZ0
BOOTSZ1
Table 142. Boot Size Configuration, ATmega1280/1281(1)
1
1
512
words
4
0x0000 0xFDFF
0xFE00 0xFFFF
0xFDFF
0xFE00
1
0
1024
words
8
0x0000 0xFBFF
0xFC00 0xFFFF
0xFBFF
0xFC00
0
1
2048
words
16
0x0000 0xF7FF
0xF800 0xFFFF
0xF7FF
0xF800
0
0
4096
words
32
0x0000 0xEFFF
0xF000 0xFFFF
0xEFFF
0xF000
Note:
1. The different BOOTSZ Fuse configurations are shown in Figure 137.
Table 143. Read-While-Write Limit, ATmega1280/1281
Section(1)
Pages
Address
Read-While-Write section (RWW)
480
0x0000 - 0xEFFF
No Read-While-Write section (NRWW)
32
0xF000 - 0xFFFF
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2549A–AVR–03/05
Note:
1. For details about these two section, see “NRWW – No Read-While-Write Section” on
page 318 and “RWW – Read-While-Write Section” on page 318.
Table 144. Explanation of different variables used in Figure 138 and the mapping to the
Z-pointer, ATmega1280/1281
Corresponding
Z-value(2)
Variable
PCMSB
15
Most significant bit in the Program Counter.
(The Program Counter is 16 bits PC[15:0])
PAGEMSB
6
Most significant bit which is used to address the
words within one page (128 words in a page
requires seven bits PC [6:0]).
Z16(3)
ZPCMSB
ZPAGEMSB
Z7
Bit in Z-pointer that is mapped to PCMSB.
Because Z0 is not used, the ZPCMSB equals
PCMSB + 1.
Bit in Z-pointer that is mapped to PCMSB.
Because Z0 is not used, the ZPAGEMSB
equals PAGEMSB + 1.
PCPAGE
PC[15:7]
Z16(3):Z8
PCWORD
PC[6:0]
Z7:Z1
Notes:
ATmega2560/2561 Boot
Loader Parameters
Description(1)
Program Counter page address: Page select,
for Page Erase and Page Write
Program Counter word address: Word select,
for filling temporary buffer (must be zero during
Page Write operation)
1. Z0: should be zero for all SPM commands, byte select for the (E)LPM instruction.
2. See “Addressing the Flash During Self-Programming” on page 322 for details about
the use of Z-pointer during Self-Programming.
3. The Z-register is only 16 bits wide. Bit 16 is located in the RAMPZ register in the I/O
map.
In Table 145 through Table 147, the parameters used in the description of the Self-Programming are given.
332
Boot Reset Address
(Start Boot Loader
Section)
End Application
Section
Boot Loader
Flash Section
Appli-cation
Flash Section
Pages
Boot Size
BOOTSZ0
BOOTSZ1
Table 145. Boot Size Configuration, ATmega2560/2561(1)
1
1
512
words
4
0x00000 0x1FDFF
0x1FE00 0x1FFFF
0x1FDFF
0x1FE00
1
0
1024
words
8
0x00000 0x1FBFF
0x1FC00 0x1FFFF
0x1FBFF
0x1FC00
0
1
2048
words
16
0x00000 0x1F7FF
0x1F800 0x1FFFF
0x1F7FF
0x1F800
0
0
4096
words
32
0x00000 0x1EFFF
0x1F000 0x1FFFF
0x1EFFF
0x1F000
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
Note:
1. The different BOOTSZ Fuse configurations are shown in Figure 137.
Table 146. Read-While-Write Limit, ATmega2560/2561
Section(1)
Pages
Address
Read-While-Write section (RWW)
992
0x00000 - 0x1EFFF
No Read-While-Write section (NRWW)
32
0x1F000 - 0x1FFFF
Note:
1. For details about these two section, see “NRWW – No Read-While-Write Section” on
page 318 and “RWW – Read-While-Write Section” on page 318.
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Table 147. Explanation of different variables used in Figure 138 and the mapping to the
Z-pointer, ATmega2560/2561
Corresponding
Z-value(2)
Variable
PCMSB
16
Most significant bit in the Program Counter.
(The Program Counter is 17 bits PC[16:0])
PAGEMSB
6
Most significant bit which is used to address the
words within one page (128 words in a page
requires seven bits PC [6:0]).
Z17:Z16(3)
ZPCMSB
ZPAGEMSB
Z7
PCPAGE
PC[16:7]
Z17(3):Z8
PCWORD
PC[6:0]
Z7:Z1
Notes:
334
Description(1)
Bit in Z-pointer that is mapped to PCMSB.
Because Z0 is not used, the ZPCMSB equals
PCMSB + 1.
Bit in Z-pointer 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
Program Counter word address: Word select,
for filling temporary buffer (must be zero during
Page Write operation)
1. Z0: should be zero for all SPM commands, byte select for the (E)LPM instruction.
2. See “Addressing the Flash During Self-Programming” on page 322 for details about
the use of Z-pointer during Self-Programming.
3. The Z-register is only 16 bits wide. Bit 16 is located in the RAMPZ register in the I/O
map.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Memory
Programming
Program And Data
Memory Lock Bits
The ATmega640/1280/1281/2560/2561 provides six Lock bits which can be left unprogrammed (“1”) or can be programmed (“0”) to obtain the additional features listed in
Table 149. The Lock bits can only be erased to “1” with the Chip Erase command.
Table 148. Lock Bit Byte(1)
Lock Bit Byte
Description
Default Value
7
–
1 (unprogrammed)
6
–
1 (unprogrammed)
BLB12
5
Boot Lock bit
1 (unprogrammed)
BLB11
4
Boot Lock bit
1 (unprogrammed)
BLB02
3
Boot Lock bit
1 (unprogrammed)
BLB01
2
Boot Lock bit
1 (unprogrammed)
LB2
1
Lock bit
1 (unprogrammed)
LB1
0
Lock bit
1 (unprogrammed)
Note:
Bit No
1. “1” means unprogrammed, “0” means programmed
Table 149. Lock Bit Protection Modes(1)(2)
Memory Lock Bits
Protection Type
LB Mode
LB2
LB1
1
1
1
No memory lock features enabled.
0
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)
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)
2
1
3
0
0
BLB0 Mode
BLB02
BLB01
1
1
1
No restrictions for SPM or (E)LPM accessing the
Application section.
2
1
0
SPM is not allowed to write to the Application section.
0
SPM is not allowed to write to the Application section, and
(E)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.
1
(E)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
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Table 149. Lock Bit Protection Modes(1)(2) (Continued)
Memory Lock Bits
BLB1 Mode
BLB12
BLB11
1
1
1
No restrictions for SPM or (E)LPM accessing the Boot
Loader section.
2
1
0
SPM is not allowed to write to the Boot Loader section.
0
SPM is not allowed to write to the Boot Loader section,
and (E)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.
1
(E)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
4
Notes:
Fuse Bits
Protection Type
0
1. Program the Fuse bits and Boot Lock bits before programming the LB1 and LB2.
2. “1” means unprogrammed, “0” means programmed
The ATmega640/1280/1281/2560/2561 has four Fuse bytes. Table 150 - Table 152
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 150. Extended Fuse Byte
Fuse Low Byte
Description
Default Value
–
7
–
1
–
6
–
1
–
5
–
1
–
4
–
1
–
3
–
1
(1)
2
Brown-out Detector trigger level
1 (unprogrammed)
(1)
1
Brown-out Detector trigger level
1 (unprogrammed)
(1)
0
Brown-out Detector trigger level
1 (unprogrammed)
BODLEVEL2
BODLEVEL1
BODLEVEL0
Note:
336
Bit No
1. See Table 24 on page 60 for BODLEVEL Fuse decoding.
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
Table 151. Fuse High Byte
Fuse High Byte
Bit No
Description
Default Value
Enable OCD
1 (unprogrammed, OCD
disabled)
Enable JTAG
0 (programmed, JTAG
enabled)
OCDEN(4)
7
JTAGEN
6
SPIEN(1)
5
Enable Serial Program and Data
Downloading
0 (programmed, SPI prog.
enabled)
WDTON(3)
4
Watchdog Timer always on
1 (unprogrammed)
EESAVE
3
EEPROM memory is preserved
through the Chip Erase
1 (unprogrammed,
EEPROM not preserved)
BOOTSZ1
2
Select Boot Size (see Table 153
for details)
0 (programmed)(2)
BOOTSZ0
1
Select Boot Size (see Table 153
for details)
0 (programmed)(2)
BOOTRST
0
Select Reset Vector
1 (unprogrammed)
Note:
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 139 on
page 330 for details.
3. See “Watchdog Timer Control Register - WDTCSR” on page 67 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.
Table 152. Fuse Low Byte
Fuse Low Byte
Description
Default Value
7
Divide clock by 8
0 (programmed)
CKOUT(3)
6
Clock output
1 (unprogrammed)
SUT1
5
Select start-up time
1 (unprogrammed)(1)
SUT0
4
Select start-up time
0 (programmed)(1)
CKSEL3
3
Select Clock source
0 (programmed)(2)
CKSEL2
2
Select Clock source
0 (programmed)(2)
CKSEL1
1
Select Clock source
1 (unprogrammed)(2)
CKSEL0
0
Select Clock source
0 (programmed)(2)
(4)
CKDIV8
Note:
Bit No
1. The default value of SUT1..0 results in maximum start-up time for the default clock
source. See Table 23 on page 58 for details.
2. The default setting of CKSEL3..0 results in internal RC Oscillator @ 8 MHz. See
Table 7 on page 40 for details.
3. The CKOUT Fuse allow the system clock to be output on PORTE7. See “Clock Output Buffer” on page 48 for details.
4. See “System Clock Prescaler” on page 48 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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Latching of Fuses
The fuse values are latched when the device enters programming mode and changes of
the fuse values will have no effect until the part leaves Programming mode. This does
not apply to the EESAVE Fuse which will take effect once it is programmed. The fuses
are also latched on Power-up in Normal mode.
Signature Bytes
All Atmel microcontrollers have a three-byte signature code which identifies the device.
This code can be read in both serial and parallel mode, also when the device is locked.
The three bytes reside in a separate address space.
ATmega640 Signature Bytes:
1. 0x000: 0x1E (indicates manufactured by Atmel).
2. 0x001: 0x96 (indicates 64K bytes Flash memory).
3. 0x002: 0x07 (indicates ATmega640 device when 0x001 is 0x96).
ATmega1280 Signature Bytes:
1. 0x000: 0x1E (indicates manufactured by Atmel).
2. 0x001: 0x97 (indicates 128K bytes Flash memory).
3. 0x002: 0x03 (indicates ATmega1280 device when 0x001 is 0x97).
ATmega1281 Signature Bytes:
1. 0x000: 0x1E (indicates manufactured by Atmel).
2. 0x001: 0x97 (indicates 128K bytes Flash memory).
3. 0x002: 0x04 (indicates ATmega1281 device when 0x001 is 0x97).
ATmega2560 Signature Bytes:
1. 0x000: 0x1E (indicates manufactured by Atmel).
2. 0x001: 0x98 (indicates 256K bytes Flash memory).
3. 0x002: 0x01 (indicates ATmega2560 device when 0x001 is 0x98).
ATmega2561 Signature Bytes:
1. 0x000: 0x1E (indicates manufactured by Atmel).
2. 0x001: 0x98 (indicates 256K bytes Flash memory).
3. 0x002: 0x02 (indicates ATmega2561 device when 0x001 is 0x98).
Calibration Byte
The ATmega640/1280/1281/2560/2561 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.
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
ATmega640/1280/1281/2560/2561. Pulses are assumed to be at least 250 ns unless
otherwise noted.
Signal Names
In this section, some pins of the ATmega640/1280/1281/2560/2561 are referenced by
signal names describing their functionality during parallel programming, see Figure 139
and Table 153. 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 156.
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When pulsing WR or OE, the command loaded determines the action executed. The different commands are shown in Table 157.
Figure 139. Parallel Programming(1)
+5V
RDY/BSY
PD1
OE
PD2
WR
PD3
BS1
PD4
XA0
PD5
XA1
PD6
PAGEL
PD7
+12 V
VCC
+5V
AVCC
PB7 - PB0
DATA
RESET
BS2
PA0
XTAL1
GND
Note:
1. Unused Pins should be left floating.
Table 153. Pin Name Mapping
Signal Name in
Programming Mode
Pin Name
I/O
Function
RDY/BSY
PD1
O
0: Device is busy programming, 1: Device is
ready for new command.
OE
PD2
I
Output Enable (Active low).
WR
PD3
I
Write Pulse (Active low).
BS1
PD4
I
Byte Select 1.
XA0
PD5
I
XTAL Action Bit 0
XA1
PD6
I
XTAL Action Bit 1
PAGEL
PD7
I
Program Memory and EEPROM data Page Load.
BS2
PA0
I
Byte Select 2.
DATA
PB7-0
I/O
Bi-directional Data bus (Output when OE is low).
Table 154. BS2 and BS1 Encoding
BS2
BS1
Flash /
EEPROM
Address
Flash Data
Loading /
Reading
Fuse
Programming
Reading Fuse
and Lock Bits
0
0
Low Byte
Low Byte
Low Byte
Fuse Low Byte
0
1
High Byte
High Byte
High Byte
Lockbits
1
0
Extended High
Byte
Reserved
Extended Byte
Extended Fuse
Byte
1
1
Reserved
Reserved
Reserved
Fuse High Byte
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,
Table 155. Pin Values Used to Enter Programming Mode
Pin
Symbol
Value
PAGEL
Prog_enable[3]
0
XA1
Prog_enable[2]
0
XA0
Prog_enable[1]
0
BS1
Prog_enable[0]
0
Table 156. XA1 and XA0 Enoding
XA1
XA0
Action when XTAL1 is Pulsed
0
0
Load Flash or EEPROM Address (High or low address byte
determined by BS2 and BS1).
0
1
Load Data (High or Low data byte for Flash determined by BS1).
1
0
Load Command
1
1
No Action, Idle
Table 157. Command Byte Bit Encoding
Command Byte
Command Executed
1000 0000
Chip Erase
0100 0000
Write Fuse bits
0010 0000
Write Lock bits
0001 0000
Write Flash
0001 0001
Write EEPROM
0000 1000
Read Signature Bytes and Calibration byte
0000 0100
Read Fuse and Lock bits
0000 0010
Read Flash
0000 0011
Read EEPROM
Table 158. No. of Words in a Page and No. of Pages in the Flash
Flash Size
Page Size
PCWORD
No. of
Pages
PCPAGE
PCMSB
128K words (256K bytes)
128 words
PC[6:0]
1024
PC[16:7]
16
Table 159. No. of Words in a Page and No. of Pages in the EEPROM
340
EEPROM Size
Page Size
PCWORD
No. of
Pages
PCPAGE
EEAMSB
4K bytes
8 bytes
EEA[2:0]
512
EEA[11:3]
11
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
Parallel Programming
Enter Programming Mode
The following algorithm puts the device in parallel programming mode:
1. Apply 4.5 - 5.5V between VCC and GND.
2. Set RESET to “0” and toggle XTAL1 at least six times.
3. Set the Prog_enable pins listed in Table 155 on page 340 to “0000” and wait at
least 100 ns.
4. Apply 11.5 - 12.5V to RESET. Any activity on Prog_enable pins within 100 ns
after +12V has been applied to RESET, will cause the device to fail entering programming mode.
5. Wait at least 50 µs before sending a new command.
Considerations for Efficient
Programming
Chip Erase
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.
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.
Programming the Flash
The Flash is organized in pages, see Table 158 on page 340. When programming the
Flash, the program data is latched into a page buffer. This allows one page of program
data to be programmed simultaneously. The following procedure describes how to program the entire Flash memory:
A. Load Command “Write Flash”
1. Set XA1, XA0 to “10”. This enables command loading.
2. Set BS1 to “0”.
3. Set DATA to “0001 0000”. This is the command for Write Flash.
4. Give XTAL1 a positive pulse. This loads the command.
B. Load Address Low byte (Address bits 7..0)
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1. Set XA1, XA0 to “00”. This enables address loading.
2. Set BS2, BS1 to “00”. This selects the address low byte.
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 141 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 140 on page 343. 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 (Address bits15..8)
1. Set XA1, XA0 to “00”. This enables address loading.
2. Set BS2, BS1 to “01”. This selects the address high byte.
3. Set DATA = Address high byte (0x00 - 0xFF).
4. Give XTAL1 a positive pulse. This loads the address high byte.
H. Load Address Extended High byte (Address bits 23..16)
1. Set XA1, XA0 to “00”. This enables address loading.
2. Set BS2, BS1 to “10”. This selects the address extended high byte.
3. Set DATA = Address extended high byte (0x00 - 0xFF).
4. Give XTAL1 a positive pulse. This loads the address high byte.
I. Program Page
1. Set BS2, BS1 to “00”
2. Give WR a negative pulse. This starts programming of the entire page of data.
RDY/BSY goes low.
3. Wait until RDY/BSY goes high (See Figure 141 for signal waveforms).
J. Repeat B through I until the entire Flash is programmed or until all data has been
programmed.
K. 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 140. Addressing the Flash Which is Organized in Pages(1)
PCMSB
PROGRAM
COUNTER
PAGEMSB
PCPAGE
PCWORD
PAGE ADDRESS
WITHIN THE FLASH
WORD ADDRESS
WITHIN A PAGE
PROGRAM MEMORY
PAGE
PAGE
PCWORD[PAGEMSB:0]:
00
INSTRUCTION WORD
01
02
PAGEEND
Note:
1. PCPAGE and PCWORD are listed in Table 158 on page 340.
Figure 141. Programming the Flash Waveforms(1)
F
DATA
A
B
C
D
E
0x10
ADDR. LOW
DATA LOW
DATA HIGH
XX
B
ADDR. LOW
C
D
DATA LOW
DATA HIGH
E
XX
G
H
ADDR. HIGH ADDR. EXT.H
I
XX
XA1
XA0
BS1
BS2
XTAL1
WR
RDY/BSY
RESET +12V
OE
PAGEL
Note:
Programming the EEPROM
1. “XX” is don’t care. The letters refer to the programming description above.
The EEPROM is organized in pages, see Table 159 on page 340. 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 341 for details on
Command, Address and Data loading):
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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 BS2, BS1 to “00”.
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
142 for signal waveforms).
Figure 142. Programming the EEPROM Waveforms
K
DATA
A
G
0x11
ADDR. HIGH
B
ADDR. LOW
C
DATA
E
XX
B
ADDR. LOW
C
DATA
E
L
XX
XA1
XA0
BS1
XTAL1
WR
RDY/BSY
RESET +12V
OE
PAGEL
BS2
Reading the Flash
The algorithm for reading the Flash memory is as follows (refer to “Programming the
Flash” on page 341 for details on Command and Address loading):
1. A: Load Command “0000 0010”.
2. H: Load Address Extended Byte (0x00- 0xFF).
3. G: Load Address High Byte (0x00 - 0xFF).
4. B: Load Address Low Byte (0x00 - 0xFF).
5. Set OE to “0”, and BS1 to “0”. The Flash word low byte can now be read at DATA.
6. Set BS to “1”. The Flash word high byte can now be read at DATA.
7. Set OE to “1”.
Reading the EEPROM
The algorithm for reading the EEPROM memory is as follows (refer to “Programming the
Flash” on page 341 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).
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4. Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at
DATA.
5. Set OE to “1”.
Programming the Fuse Low
Bits
The algorithm for programming the Fuse Low bits is as follows (refer to “Programming
the Flash” on page 341 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 341 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 BS2, BS1 to “01”. This selects high data byte.
4. Give WR a negative pulse and wait for RDY/BSY to go high.
5. Set BS2, BS1 to “00”. This selects low data byte.
Programming the Extended
Fuse Bits
The algorithm for programming the Extended Fuse bits is as follows (refer to “Programming the Flash” on page 341 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 BS2, BS1 to “10”. This selects extended data byte.
4. 4. Give WR a negative pulse and wait for RDY/BSY to go high.
5. 5. Set BS2, BS1 to “00”. This selects low data byte.
Figure 143. Programming the FUSES Waveforms
Write Fuse Low byte
DATA
A
C
0x40
DATA
XX
Write Fuse high byte
A
C
0x40
DATA
XX
Write Extended Fuse byte
A
C
0x40
DATA
XX
XA1
XA0
BS1
BS2
XTAL1
WR
RDY/BSY
RESET +12V
OE
PAGEL
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Programming the Lock Bits
The algorithm for programming the Lock bits is as follows (refer to “Programming the
Flash” on page 341 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.
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 341 for details on Command loading):
1. A: Load Command “0000 0100”.
2. Set OE to “0”, and BS2, BS1 to “00”. The status of the Fuse Low bits can now be
read at DATA (“0” means programmed).
3. Set OE to “0”, and BS2, BS1 to “11”. The status of the Fuse High bits can now be
read at DATA (“0” means programmed).
4. Set OE to “0”, and BS2, BS1 to “10”. The status of the Extended Fuse bits can
now be read at DATA (“0” means programmed).
5. Set OE to “0”, and BS2, BS1 to “01”. The status of the Lock bits can now be read
at DATA (“0” means programmed).
6. Set OE to “1”.
Figure 144. Mapping Between BS1, BS2 and the Fuse and Lock Bits During Read
0
Fuse Low Byte
0
Extended Fuse Byte
1
DATA
BS2
0
Lock Bits
1
Fuse High Byte
1
BS1
BS2
Reading the Signature Bytes
The algorithm for reading the Signature bytes is as follows (refer to “Programming the
Flash” on page 341 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 BS to “0”. The selected Signature byte can now be read at
DATA.
4. Set OE to “1”.
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Reading the Calibration Byte
The algorithm for reading the Calibration byte is as follows (refer to “Programming the
Flash” on page 341 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”.
Parallel Programming
Characteristics
Figure 145. Parallel Programming Timing, Including some General Timing
Requirements
tXLWL
tXHXL
XTAL1
tDVXH
tXLDX
Data & Contol
(DATA, XA0/1, BS1, BS2)
tPLBX t BVWL
tBVPH
PAGEL
tWLBX
tPHPL
tWLWH
WR
tPLWL
WLRL
RDY/BSY
tWLRH
Figure 146. Parallel Programming Timing, Loading Sequence with Timing
Requirements(1)
LOAD ADDRESS
(LOW BYTE)
LOAD DATA LOAD DATA
(HIGH BYTE)
LOAD DATA
(LOW BYTE)
t XLXH
tXLPH
LOAD ADDRESS
(LOW BYTE)
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 145 (i.e., tDVXH, tXHXL, and tXLDX) also apply
to loading operation.
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Figure 147. 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
tBVDV
BS1
tOLDV
OE
tOHDZ
DATA
ADDR0 (Low Byte)
ADDR1 (Low Byte)
DATA (High Byte)
DATA (Low Byte)
XA0
XA1
Note:
1. The timing requirements shown in Figure 145 (i.e., tDVXH, tXHXL, and tXLDX) also apply
to reading operation.
Table 160. Parallel Programming Characteristics, VCC = 5V ± 10%
Symbol
Parameter
Min
VPP
Programming Enable Voltage
11.5
IPP
Programming Enable Current
tDVXH
Data and Control Valid before XTAL1 High
67
ns
tXLXH
XTAL1 Low to XTAL1 High
200
ns
tXHXL
XTAL1 Pulse Width High
150
ns
tXLDX
Data and Control Hold after XTAL1 Low
67
ns
tXLWL
XTAL1 Low to WR Low
0
ns
tXLPH
XTAL1 Low to PAGEL high
0
ns
tPLXH
PAGEL low to XTAL1 high
150
ns
tBVPH
BS1 Valid before PAGEL High
67
ns
tPHPL
PAGEL Pulse Width High
150
ns
tPLBX
BS1 Hold after PAGEL Low
67
ns
tWLBX
BS2/1 Hold after WR Low
67
ns
tPLWL
PAGEL Low to WR Low
67
ns
tBVWL
BS2/1 Valid to WR Low
67
ns
tWLWH
WR Pulse Width Low
150
ns
tWLRL
WR Low to RDY/BSY Low
tWLRH
348
(1)
WR Low to RDY/BSY High
(2)
tWLRH_CE
WR Low to RDY/BSY High for Chip Erase
tXLOL
XTAL1 Low to OE Low
Typ
Max
Units
12.5
V
250
µA
0
1
µs
3.7
4.5
ms
7.5
9
ms
0
ns
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 160. Parallel Programming Characteristics, VCC = 5V ± 10% (Continued)
Symbol
Parameter
tBVDV
BS1 Valid to DATA valid
tOLDV
tOHDZ
Notes:
Serial Downloading
Serial Programming Pin
Mapping
Min
Max
Units
250
ns
OE Low to DATA Valid
250
ns
OE High to DATA Tri-stated
250
ns
0
Typ
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.
Both the Flash and EEPROM memory arrays can be programmed using a serial programming bus while RESET is pulled to GND. The serial programming interface
consists of pins SCK, PDI (input) and PDO (output). After RESET is set low, the Programming Enable instruction needs to be executed first before program/erase
operations can be executed. NOTE, in Table 161 on page 349, the pin mapping for
serial programming is listed. Not all packages use the SPI pins dedicated for the internal
Serial Peripheral Interface - SPI.
Table 161. Pin Mapping Serial Programming
Symbol
Pins
(TQFP-100)
Pins
(TQFP-64)
I/O
Description
PDI
PB2
PE0
I
Serial Data in
PDO
PB3
PE1
O
Serial Data out
SCK
PB1
PB1
I
Serial Clock
Figure 148. Serial Programming and Verify(1)
+1.8 - 5.5V
VCC
+1.8 - 5.5V(2)
PDI
AVCC
PDO
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
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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
Serial Programming
Algorithm
When writing serial data to the ATmega640/1280/1281/2560/2561, data is clocked on
the rising edge of SCK.
When reading data from the ATmega640/1280/1281/2560/2561, data is clocked on the
falling edge of SCK. See Figure 149 for timing details.
To program and verify the ATmega640/1280/1281/2560/2561 in the serial programming
mode, the following sequence is recommended (See four byte instruction formats in
Table 163):
1. Power-up sequence:
Apply power between VCC and GND while RESET and SCK are set to “0”. In
some systems, the programmer can not guarantee that SCK is held low during
power-up. In this case, RESET must be given a positive pulse of at least two
CPU clock cycles duration after SCK has been set to “0”.
2. Wait for at least 20 ms and enable serial programming by sending the Programming Enable serial instruction to pin PDI.
3. The serial programming instructions will not work if the communication is out of
synchronization. When in sync. the second byte (0x53), will echo back when
issuing the third byte of the Programming Enable instruction. Whether the echo
is correct or not, all four bytes of the instruction must be transmitted. If the 0x53
did not echo back, give RESET a positive pulse and issue a new Programming
Enable command.
4. The Flash is programmed one page at a time. The memory page is loaded one
byte at a time by supplying the 7 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 address lines 15..8. Before issuing this command, make sure the instruction Load Extended Address Byte has been used to
define the MSB of the address. The extended address byte is stored until the
command is re-issued, i.e., the command needs only be issued for the first page,
and when crossing the 64KWord boundary. If polling (RDY/BSY) is not used, the
user must wait at least tWD_FLASH before issuing the next page. (See Table 162.)
Accessing the serial programming interface before the Flash write operation
completes can result in incorrect programming.
5. The EEPROM array is programmed one byte at a time by supplying the address
and data together with the appropriate Write instruction. An EEPROM memory
location is first automatically erased before new data is written. If polling is not
used, the user must wait at least tWD_EEPROM before issuing the next byte. (See
Table 162.) In a chip erased device, no 0xFFs in the data file(s) need to be
programmed.
6. Any memory location can be verified by using the Read instruction which returns
the content at the selected address at serial output PDO. When reading the
350
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
Flash memory, use the instruction Load Extended Address Byte to define the
upper address byte, which is not included in the Read Program Memory instruction. The extended address byte is stored until the command is re-issued, i.e.,
the command needs only be issued for the first page, and when crossing the
64KWord boundary.
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.
Table 162. Minimum Wait Delay Before Writing the Next Flash or EEPROM Location
Symbol
Minimum Wait Delay
tWD_FLASH
4.5 ms
tWD_EEPROM
9.0 ms
tWD_ERASE
9.0 ms
Figure 149. Serial Programming Waveforms
SERIAL DATA INPUT
(MOSI)
MSB
LSB
SERIAL DATA OUTPUT
(MISO)
MSB
LSB
SERIAL CLOCK INPUT
(SCK)
SAMPLE
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Table 163. Serial Programming Instruction Set
Instruction Format
Instruction
Byte 1
Byte 2
Byte 3
Byte4
Programming Enable
1010 1100
0101 0011
xxxx xxxx
xxxx xxxx
Enable Serial Programming after
RESET goes low.
Chip Erase
1010 1100
100x xxxx
xxxx xxxx
xxxx xxxx
Chip Erase EEPROM and Flash.
Load Extended Address Byte
0100 1101
0000 0000
cccc cccc
xxxx xxxx
Defines Extended Address Byte for
Read Program Memory and Write
Program Memory Page.
Read Program Memory
0010 H000
aaaa aaaa
bbbb bbbb
oooo oooo
Read H (high or low) data o from
Program memory at word address
c:a:b.
Load Program Memory Page
0100 H000
xxxx xxxx
xxbb bbbb
iiii iiii
Write H (high or low) data i to Program
Memory page at word address b. Data
low byte must be loaded before Data
high byte is applied within the same
address.
Write Program Memory Page
0100 1100
aaaa aaaa
bbxx xxxx
xxxx xxxx
Write Program Memory Page at
address c:a:b.
Read EEPROM Memory
1010 0000
0000 aaaa
bbbb bbbb
oooo oooo
Read data o from EEPROM memory at
address a:b.
Write EEPROM Memory
1100 0000
0000 aaaa
bbbb bbbb
iiii iiii
Write data i to EEPROM memory at
address a:b.
Load EEPROM Memory
Page (page access)
1100 0001
0000 0000
0000 00bb
iiii iiii
Load data i to EEPROM memory page
buffer. After data is loaded, program
EEPROM page.
Write EEPROM Memory
Page (page access)
1100 0010
0000 aaaa
bbbb bb00
xxxx xxxx
Write EEPROM page at address a:b.
Read Lock bits
0101 1000
0000 0000
xxxx xxxx
xxoo oooo
Read Lock bits. “0” = programmed, “1”
= unprogrammed. See Table 148 on
page 335 for details.
Write Lock bits
1010 1100
111x xxxx
xxxx xxxx
11ii iiii
Write Lock bits. Set bits = “0” to
program Lock bits. See Table 148 on
page 335 for details.
Read Signature Byte
0011 0000
000x xxxx
xxxx xxbb
oooo oooo
Read Signature Byte o at address b.
Write Fuse bits
1010 1100
1010 0000
xxxx xxxx
iiii iiii
Set bits = “0” to program, “1” to
unprogram. See Table 124 on page
287 for details.
Write Fuse High bits
1010 1100
1010 1000
xxxx xxxx
iiii iiii
Set bits = “0” to program, “1” to
unprogram. See Table 123 on page
279 for details.
Write Extended Fuse Bits
1010 1100
1010 0100
xxxx xxxx
iiii iiii
Set bits = “0” to program, “1” to
unprogram. See Table 150 on page
336 for details.
Read Fuse bits
0101 0000
0000 0000
xxxx xxxx
oooo oooo
Read Fuse bits. “0” = programmed, “1”
= unprogrammed. See Table 124 on
page 287 for details.
352
Operation
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
Table 163. Serial Programming Instruction Set (Continued)
Instruction Format
Instruction
Byte 1
Byte 2
Byte 3
Byte4
Read Fuse High bits
0101 1000
0000 1000
xxxx xxxx
oooo oooo
Read Fuse High bits. “0” = programmed, “1” = unprogrammed. See
Table 123 on page 279 for details.
Read Extended Fuse Bits
0101 0000
0000 1000
xxxx xxxx
oooo oooo
Read Extended Fuse bits. “0” = programmed, “1” = unprogrammed. See
Table 150 on page 336 for details.
Read Calibration Byte
0011 1000
000x xxxx
0000 0000
oooo oooo
Read Calibration Byte
Poll RDY/BSY
1111 0000
0000 0000
xxxx xxxx
xxxx xxxo
If o = “1”, a programming operation is
still busy. Wait until this bit returns to
“0” before applying another command.
Note:
Operation
a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in,
x = don’t care
Serial Programming
Characteristics
For characteristics of the Serial Programming module see “SPI Timing Characteristics”
on page 372.
Programming via the
JTAG Interface
Programming through the JTAG interface requires control of the four JTAG specific
pins: TCK, TMS, TDI, and TDO. Control of the reset and clock pins is not required.
To be able to use the JTAG interface, the JTAGEN Fuse must be programmed. The
device is default shipped with the fuse programmed. In addition, the JTD bit in MCUCSR
must be cleared. Alternatively, if the JTD bit is set, the external reset can be forced low.
Then, the JTD bit will be cleared after two chip clocks, and the JTAG pins are available
for programming. This provides a means of using the JTAG pins as normal port pins in
Running mode while still allowing In-System Programming via the JTAG interface. Note
that this technique can not be used when using the JTAG pins for Boundary-scan or Onchip Debug. In these cases the JTAG pins must be dedicated for this purpose.
During programming the clock frequency of the TCK Input must be less than the maximum frequency 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 datasheet, the LSB is shifted in and out first of all Shift Registers.
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 150.
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2549A–AVR–03/05
Figure 150. State Machine Sequence for Changing the Instruction Word
1
Test-Logic-Reset
0
0
Run-Test/Idle
1
Select-DR Scan
1
Select-IR Scan
0
0
1
1
Capture-DR
Capture-IR
0
0
0
Shift-DR
1
1
Exit1-DR
0
0
Pause-DR
0
Pause-IR
1
1
0
Exit2-DR
Exit2-IR
1
1
Update-DR
AVR_RESET (0xC)
1
Exit1-IR
0
1
0
Shift-IR
1
0
1
Update-IR
0
1
0
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:
•
PROG_ENABLE (0x4)
354
Shift-DR: The Reset Register is shifted by the TCK input.
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.
ATmega640/1280/1281/2560/2561
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ATmega640/1280/1281/2560/2561
PROG_COMMANDS (0x5)
PROG_PAGELOAD (0x6)
PROG_PAGEREAD (0x7)
Data Registers
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
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.
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.
The Data Registers are selected by the JTAG instruction registers described in section
“Programming Specific JTAG Instructions” on page 353. The Data Registers relevant for
programming operations are:
•
Reset Register
•
Programming Enable Register
•
Programming Command Register
•
Flash Data Byte Register
355
2549A–AVR–03/05
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 40) 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 132 on page 303.
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 contents of the register is equal to the programming enable signature, programming via the JTAG port is enabled. The register is reset
to 0 on Power-on Reset, and should always be reset when leaving Programming mode.
Figure 151. Programming Enable Register
TDI
D
A
T
A
0xA370
=
D
Q
Programming Enable
ClockDR & PROG_ENABLE
TDO
Programming Command
Register
356
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 164. The
state sequence when shifting in the programming commands is illustrated in Figure 153.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 152. Programming Command Register
TDI
S
T
R
O
B
E
S
A
D
D
R
E
S
S
/
D
A
T
A
Flash
EEPROM
Fuses
Lock Bits
TDO
357
2549A–AVR–03/05
Table 164. JTAG Programming Instruction
Set a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in, x =
don’t care
Instruction
TDI Sequence
TDO Sequence
1a. Chip Erase
0100011_10000000
0110001_10000000
0110011_10000000
0110011_10000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
1b. Poll for Chip Erase Complete
0110011_10000000
xxxxxox_xxxxxxxx
2a. Enter Flash Write
0100011_00010000
xxxxxxx_xxxxxxxx
2b. Load Address Extended High Byte
0001011_cccccccc
xxxxxxx_xxxxxxxx
2c. Load Address High Byte
0000111_aaaaaaaa
xxxxxxx_xxxxxxxx
2d. Load Address Low Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
2e. Load Data Low Byte
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
2f. Load Data High Byte
0010111_iiiiiiii
xxxxxxx_xxxxxxxx
2g. Latch Data
0110111_00000000
1110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
2h. Write Flash Page
0110111_00000000
0110101_00000000
0110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
2i. Poll for Page Write Complete
0110111_00000000
xxxxxox_xxxxxxxx
(2)
3a. Enter Flash Read
0100011_00000010
xxxxxxx_xxxxxxxx
3b. Load Address Extended High Byte
0001011_cccccccc
xxxxxxx_xxxxxxxx
3c. Load Address High Byte
0000111_aaaaaaaa
xxxxxxx_xxxxxxxx
3d. Load Address Low Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
3e. Read Data Low and High Byte
0110010_00000000
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
xxxxxxx_oooooooo
4a. Enter EEPROM Write
0100011_00010001
xxxxxxx_xxxxxxxx
4b. Load Address High Byte
0000111_aaaaaaaa
xxxxxxx_xxxxxxxx
4c. Load Address Low Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
4d. Load Data Byte
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
4e. Latch Data
0110111_00000000
1110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
4f. Write EEPROM Page
0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
4g. Poll for Page Write Complete
0110011_00000000
xxxxxox_xxxxxxxx
(2)
358
Notes
(2)
(10)
(10)
Low byte
High byte
(10)
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 164. JTAG Programming Instruction (Continued)
Set (Continued) a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out, i
= data in, x = don’t care
Instruction
TDI Sequence
TDO Sequence
5a. Enter EEPROM Read
0100011_00000011
xxxxxxx_xxxxxxxx
5b. Load Address High Byte
0000111_aaaaaaaa
xxxxxxx_xxxxxxxx
5c. Load Address Low Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
5d. Read Data Byte
0110011_bbbbbbbb
0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
6a. Enter Fuse Write
0100011_01000000
xxxxxxx_xxxxxxxx
6b. Load Data Low Byte(6)
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
(3)
6c. Write Fuse Extended Byte
0111011_00000000
0111001_00000000
0111011_00000000
0111011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
6d. Poll for Fuse Write Complete
0110111_00000000
xxxxxox_xxxxxxxx
(2)
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
(3)
6f. Write Fuse High Byte
0110111_00000000
0110101_00000000
0110111_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
6g. Poll for Fuse Write Complete
0110111_00000000
xxxxxox_xxxxxxxx
(2)
6h. Load Data Low Byte(7)
0010011_iiiiiiii
xxxxxxx_xxxxxxxx
(3)
6i. Write Fuse Low Byte
0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
6j. Poll for Fuse Write Complete
0110011_00000000
xxxxxox_xxxxxxxx
(2)
7a. Enter Lock Bit Write
0100011_00100000
xxxxxxx_xxxxxxxx
7b. Load Data Byte
0010011_11iiiiii
xxxxxxx_xxxxxxxx
(4)
7c. Write Lock Bits
0110011_00000000
0110001_00000000
0110011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
(1)
7d. Poll for Lock Bit Write complete
0110011_00000000
xxxxxox_xxxxxxxx
(2)
8a. Enter Fuse/Lock Bit Read
0100011_00000100
xxxxxxx_xxxxxxxx
8b. Read Extended Fuse Byte
0111010_00000000
0111011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
8c. Read Fuse High Byte(7)
0111110_00000000
0111111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
8d. Read Fuse Low Byte(8)
0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
6e. Load Data Low Byte
(7)
(9)
(6)
Notes
(10)
359
2549A–AVR–03/05
Table 164. JTAG Programming Instruction (Continued)
Set (Continued) a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out, i
= data in, x = don’t care
Instruction
TDI Sequence
TDO Sequence
Notes
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxoooooo
(5)
8f. Read Fuses and Lock Bits
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
9a. Enter Signature Byte Read
0100011_00001000
xxxxxxx_xxxxxxxx
9b. Load Address Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
9c. Read Signature Byte
0110010_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
10a. Enter Calibration Byte Read
0100011_00001000
xxxxxxx_xxxxxxxx
10b. Load Address Byte
0000011_bbbbbbbb
xxxxxxx_xxxxxxxx
10c. Read Calibration Byte
0110110_00000000
0110111_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_oooooooo
11a. Load No Operation Command
0100011_00000000
0110011_00000000
xxxxxxx_xxxxxxxx
xxxxxxx_xxxxxxxx
8e. Read Lock Bits
Notes:
360
(9)
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 150 on page 336
7. The bit mapping for Fuses High byte is listed in Table 151 on page 337
8. The bit mapping for Fuses Low byte is listed in Table 152 on page 337
9. The bit mapping for Lock bits byte is listed in Table 148 on page 335
10. Address bits exceeding PCMSB and EEAMSB (Table 158 and Table 159) are don’t care
11. All TDI and TDO sequences are represented by binary digits (0b...).
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 153. State Machine Sequence for Changing/Reading the Data Word
1
Test-Logic-Reset
0
0
Run-Test/Idle
1
Select-DR Scan
1
Select-IR Scan
0
0
1
1
Capture-DR
Capture-IR
0
0
Shift-DR
Shift-IR
0
1
Exit1-DR
0
Pause-DR
0
0
Pause-IR
1
1
0
Exit2-DR
Exit2-IR
1
1
Update-DR
Flash Data Byte Register
1
Exit1-IR
0
1
0
1
1
0
1
Update-IR
0
1
0
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 register. 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 UpdateDR 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 Capture-DR encountered after entering the PROG_PAGEREAD
command. The Program Counter is post-incremented after reading each high byte,
361
2549A–AVR–03/05
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 154. Flash Data Byte Register
STROBES
TDI
State
Machine
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 Register 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.
Programming Algorithm
All references below of type “1a”, “1b”, and so on, refer to Table 164.
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
Programming Enable Register.
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
programming Enable Register.
4. Enter JTAG instruction AVR_RESET and shift 0 in the Reset Register.
Performing Chip Erase
1. Enter JTAG instruction PROG_COMMANDS.
2. Start Chip Erase using programming instruction 1a.
3. Poll for Chip Erase complete using programming instruction 1b, or wait for
tWLRH_CE (refer to Table 160 on page 348).
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Programming the Flash
Before programming the Flash a Chip Erase must be performed, see “Performing Chip
Erase” on page 362.
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash write using programming instruction 2a.
3. Load address Extended High byte using programming instruction 2b.
4. Load address High byte using programming instruction 2c.
5. Load address Low byte using programming instruction 2d.
6. Load data using programming instructions 2e, 2f and 2g.
7. Repeat steps 5 and 6 for all instruction words in the page.
8. Write the page using programming instruction 2h.
9. Poll for Flash write complete using programming instruction 2i, or wait for tWLRH
(refer to Table 160 on page 348).
10. Repeat steps 3 to 9 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, 2c and 2d.
PCWORD (refer to Table 158 on page 340) 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 Register 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 2h.
8. Poll for Flash write complete using programming instruction 2i, or wait for tWLRH
(refer to Table 160 on page 348).
9. Repeat steps 3 to 8 until all data have been programmed.
Reading the Flash
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash read using programming instruction 3a.
3. Load address using programming instructions 3b, 3c and 3d.
4. Read data using programming instruction 3e.
5. Repeat steps 3 and 4 until all data have been read.
A more efficient data transfer can be achieved using the PROG_PAGEREAD
instruction:
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Flash read using programming instruction 3a.
3. Load the page address using programming instructions 3b, 3c and 3d.
PCWORD (refer to Table 158 on page 340) is used to address within one page
and must be written as 0.
4. Enter JTAG instruction PROG_PAGEREAD.
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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.
Programming the EEPROM
Before programming the EEPROM a Chip Erase must be performed, see “Performing
Chip Erase” on page 362.
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 160 on page 348).
9. Repeat steps 3 to 8 until all data have been programmed.
Note that the PROG_PAGELOAD instruction can not be used when programming the
EEPROM.
Reading the EEPROM
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable EEPROM read using programming instruction 5a.
3. Load address using programming instructions 5b and 5c.
4. Read data using programming instruction 5d.
5. Repeat steps 3 and 4 until all data have been read.
Note that the PROG_PAGEREAD instruction can not be used when reading the
EEPROM.
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 160 on page 348).
6. Load data low byte using programming instructions 6e. A “0” will program the
fuse, a “1” will unprogram the fuse.
7. Write Fuse low byte using programming instruction 6f.
8. Poll for Fuse write complete using programming instruction 6g, or wait for tWLRH
(refer to Table 160 on page 348).
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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
corresponding 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 160 on page 348).
Reading the Fuses and Lock
Bits
1. Enter JTAG instruction PROG_COMMANDS.
2. Enable Fuse/Lock bit read using programming instruction 8a.
3. To read all Fuses and Lock bits, use programming instruction 8e.
To only read Fuse High byte, use programming instruction 8b.
To only read Fuse Low byte, use programming instruction 8c.
To only read Lock bits, use programming instruction 8d.
Reading the Signature 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.
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.
4. Read the calibration byte using programming instruction 10c.
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ATmega640/1280/1281/2560/2561
Electrical Characteristics
Absolute Maximum Ratings*
Operating Temperature.................................. -55°C to +125°C
*NOTICE:
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
Stresses beyond those listed under “Absolute
Maximum Ratings” may cause permanent damage 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.
Maximum Operating Voltage ............................................ 6.0V
DC Current per I/O Pin ............................................... 40.0 mA
DC Current VCC and GND Pins................................ 200.0 mA
DC Characteristics
TA = -40°C to 85°C, VCC = 1.8V to 5.5V (unless otherwise noted)
Symbol
Parameter
Condition
Min.(5)
Typ.
Max.(5)
Units
(1)
VIL
Input Low Voltage,Except
XTAL1 and Reset pin
VCC = 1.8V - 2.4V
VCC = 2.4V - 5.5V
-0.5
-0.5
0.2VCC
0.3VCC(1)
V
VIL1
Input Low Voltage,
XTAL1 pin
VCC = 1.8V - 5.5V
-0.5
0.1VCC(1)
V
VIL2
Input Low Voltage,
RESET pin
VCC = 1.8V - 5.5V
-0.5
0.1VCC(1)
V
VIH
Input High Voltage,
Except XTAL1 and
RESET pins
VCC = 1.8V - 2.4V
VCC = 2.4V - 5.5V
0.7VCC(2)
0.6VCC(2)
VCC + 0.5
VCC + 0.5
V
VIH1
Input High Voltage,
XTAL1 pin
VCC = 1.8V - 2.4V
VCC = 2.4V - 5.5V
0.8VCC(2)
0.7VCC(2)
VCC + 0.5
VCC + 0.5
V
VIH2
Input High Voltage,
RESET pin
VCC = 1.8V - 5.5V
0.9VCC(2)
VCC + 0.5
V
VOL
Output Low Voltage(3),
IOL = 10mA, VCC = 5V
IOL = 5mA, VCC = 3V
0.7
0.5
V
VOH
Output High Voltage(4),
IOH = -20mA, VCC = 5V
IOH = -10mA, VCC = 3V
IIL
Input Leakage
Current I/O Pin
VCC = 5.5V, pin low
(absolute value)
1
µA
IIH
Input Leakage
Current I/O Pin
VCC = 5.5V, pin high
(absolute value)
1
µA
RRST
Reset Pull-up Resistor
30
60
kΩ
RPU
I/O Pin Pull-up Resistor
20
50
kΩ
4.2
2.3
V
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2549A–AVR–03/05
TA = -40°C to 85°C, VCC = 1.8V to 5.5V (unless otherwise noted) (Continued)
Symbol
Parameter
Power Supply Current(6)
Max.(5)
Units
Active 1MHz, VCC = 2V
(ATmega640/1280/256
0/1V)
0.8
mA
Active 4MHz, VCC = 3V
(ATmega640/1280/256
0/1L)
5
mA
Active 8MHz, VCC = 5V
(ATmega640/1280/128
1/2560/2561)
18
mA
0.75
mA
Idle 4MHz, VCC = 3V
(ATmega640/1280/256
0/1L)
2.2
mA
Idle 8MHz, VCC = 5V
(ATmega640/1280/128
1/2560/2561)
8
mA
Condition
Idle 1MHz, VCC = 2V
(ATmega640/1280/256
0/1V)
ICC
Power-down mode
Typ.
0.4
WDT enabled, VCC = 3V
<10
20
µA
WDT disabled, VCC = 3V
<1
3
µA
<10
40
mV
50
nA
VACIO
Analog Comparator
Input Offset Voltage
VCC = 5V
Vin = VCC/2
IACLK
Analog Comparator
Input Leakage Current
VCC = 5V
Vin = VCC/2
tACID
Analog Comparator
Propagation Delay
VCC = 2.7V
VCC = 4.0V
Note:
Min.(5)
-50
750
500
ns
1. "Max" means the highest value where the pin is guaranteed to be read as low
2. "Min" means the lowest value where the pin is guaranteed to be read as high
3. Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state
conditions (non-transient), the following must be observed:
ATmega1281/2561:
1.)The sum of all IOL, for ports A0-A7, G2, C4-C7 should not exceed 100 mA.
2.)The sum of all IOL, for ports C0-C3, G0-G1, D0-D7 should not exceed 100 mA.
3.)The sum of all IOL, for ports G3-G5, B0-B7, E0-E7 should not exceed 100 mA.
4.)The sum of all IOL, for ports F0-F7 should not exceed 100 mA.
ATmega640/1280/2560:
1.)The sum of all IOL, for ports J0-J7, A0-A7, G2 should not exceed 200 mA.
2.)The sum of all IOL, for ports C0-C7, G0-G1, D0-D7, L0-L7 should not exceed 200 mA.
3.)The sum of all IOL, for ports G3-G4, B0-B7, H0-B7 should not exceed 200 mA.
4.)The sum of all IOL, for ports E0-E7, G5 should not exceed 100 mA.
5.)The sum of all IOL, for ports F0-F7, K0-K7 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 (20mA at VCC = 5V, 10mA at VCC = 3V) under steady
state conditions (non-transient), the following must be observed:
ATmega1281/2561:
1)The sum of all IOH, for ports A0-A7, G2, C4-C7 should not exceed 100 mA.
2)The sum of all IOH, for ports C0-C3, G0-G1, D0-D7 should not exceed 100 mA.
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ATmega640/1280/1281/2560/2561
3)The sum of all IOH, for ports G3-G5, B0-B7, E0-E7 should not exceed 100 mA.
4)The sum of all IOH, for ports F0-F7 should not exceed 100 mA.
ATmega640/1280/2560:
1)The sum of all IOH, for ports J0-J7, G2, A0-A7 should not exceed 200 mA.
2)The sum of all IOH, for ports C0-C7, G0-G1, D0-D7, L0-L7 should not exceed 200 mA.
3)The sum of all IOH, for ports G3-G4, B0-B7, H0-H7 should not exceed 200 mA.
4)The sum of all IOH, for ports E0-E7, G5 should not exceed 100 mA.
5)The sum of all IOH, for ports F0-F7, K0-K7 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. All DC Characteristics contained in this datasheet are based on simulation and characterization of other AVR microcontrollers manufactured in the same process technology. These values are preliminary values representing design targets, and
will be updated after characterization of actual silicon
6. Values with “Power Reduction Register 1 - PRR1” disabled (0x00).
External Clock Drive
Waveforms
Figure 155. External Clock Drive Waveforms
V IH1
V IL1
External Clock Drive
Table 165. 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
1/tCLCL
Oscillator
Frequency
0
2
0
8
0
16
MHz
tCLCL
Clock Period
500
125
62.5
ns
tCHCX
High Time
200
50
25
ns
tCLCX
Low Time
200
50
25
ns
tCLCH
Rise Time
2.0
1.6
0.5
µs
tCHCL
Fall Time
2.0
1.6
0.5
µs
∆tCLCL
Change in period
from one clock
cycle to the next
2
2
2
%
Note:
All DC Characteristics contained in this datasheet are based on simulation and characterization of other AVR microcontrollers manufactured in the same process technology.
These values are preliminary values representing design targets, and will be updated
after characterization of actual silicon.
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2549A–AVR–03/05
Maximum speed vs. VCC
Maximum frequency is depending on VCC. As shown in Figure 156 and Figure 157, the
Maximum Frequency vs. VCC curve is linear between 1.8V < VCC < 2.7V and between
2.7V < VCC < 4.5V.
Figure 156. Maximum Frequency vs. VCC, ATmegaXXX1V/XXX0V
8 MHz
Safe Operating Area
4 MHz
1.8V
2.7V
5.5V
Figure 157. Maximum Frequency vs. VCC, ATmega640/1280/1281/2560/2561
16 MHz
8 MHz
Safe Operating Area
2.7V
370
4.5V
5.5V
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
2-wire Serial Interface Characteristics
T a b l e 1 6 6 d e s c r i b e s t h e r e q u i r e m e n t s f o r d ev i c e s c o n n e c t e d t o t h e 2 - w i r e S e r i a l B u s . T h e
ATmega640/1280/1281/2560/2561 2-wire Serial Interface meets or exceeds these requirements under the noted
conditions.
Timing symbols refer to Figure 158.
Table 166. 2-wire Serial Bus Requirements
Symbol
Parameter
VIL
VIH
Vhys
(1)
VOL(1)
Min
Max
Units
Input Low-voltage
-0.5
0.3 VCC
V
Input High-voltage
0.7 VCC
VCC + 0.5
V
–
V
Hysteresis of Schmitt Trigger Inputs
Output Low-voltage
tr(1)
Rise Time for both SDA and SCL
tof(1)
Output Fall Time from VIHmin to VILmax
tSP(1)
Spikes Suppressed by Input Filter
Ii
Input Current each I/O Pin
Ci(1)
Capacitance for each I/O Pin
fSCL
SCL Clock Frequency
Rp
tHD;STA
0.05 VCC
3 mA sink current
10 pF < Cb < 400 pF(3)
Hold Time (repeated) START Condition
Low Period of the SCL Clock
tHIGH
High period of the SCL clock
tSU;STA
Set-up time for a repeated START condition
tHD;DAT
Data hold time
tSU;DAT
Data setup time
tSU;STO
Setup time for STOP condition
tBUF
Bus free time between a STOP and START
condition
(2)
0
0.4
V
(3)(2)
300
ns
20 + 0.1Cb(3)(2)
250
ns
(2)
ns
20 + 0.1Cb
0
0.1VCC < Vi < 0.9VCC
50
-10
10
µA
–
10
pF
fCK(4) > max(16fSCL, 250kHz)(5)
0
400
kHz
fSCL ≤ 100 kHz
V CC – 0,4V
---------------------------3mA
1000ns
------------------Cb
Ω
fSCL > 100 kHz
V CC – 0,4V
---------------------------3mA
300ns
---------------Cb
Ω
fSCL ≤ 100 kHz
4.0
–
µs
fSCL > 100 kHz
Value of Pull-up resistor
tLOW
Notes:
Condition
0.6
–
µs
(6)
4.7
–
µs
fSCL > 100 kHz(7)
1.3
–
µs
fSCL ≤ 100 kHz
4.0
–
µs
fSCL > 100 kHz
0.6
–
µs
fSCL ≤ 100 kHz
4.7
–
µs
fSCL > 100 kHz
0.6
–
µs
fSCL ≤ 100 kHz
0
3.45
µs
fSCL > 100 kHz
0
0.9
µs
fSCL ≤ 100 kHz
250
–
ns
fSCL > 100 kHz
100
–
ns
fSCL ≤ 100 kHz
4.0
–
µs
fSCL > 100 kHz
0.6
–
µs
fSCL ≤ 100 kHz
4.7
–
µs
fSCL > 100 kHz
1.3
–
µs
fSCL ≤ 100 kHz
1. In ATmega640/1280/1281/2560/2561, this parameter is characterized and not 100% tested.
2. Required only for fSCL > 100 kHz.
3. Cb = capacitance of one bus line in pF.
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2549A–AVR–03/05
4. fCK = CPU clock frequency
5. This requirement applies to all ATmega640/1280/1281/2560/2561 2-wire Serial Interface operation. Other devices connected to the 2-wire Serial Bus need only obey the general fSCL requirement.
6. The actual low period generated by the ATmega640/1280/1281/2560/2561 2-wire Serial Interface is (1/fSCL - 2/fCK), thus
fCK must be greater than 6 MHz for the low time requirement to be strictly met at fSCL = 100 kHz.
7. The actual low period generated by the ATmega640/1280/1281/2560/2561 2-wire Serial Interface is (1/fSCL - 2/fCK), thus
the low time requirement will not be strictly met for fSCL > 308 kHz when fCK = 8 MHz. Still,
ATmega640/1280/1281/2560/2561 devices connected to the bus may communicate at full speed (400 kHz) with other
ATmega640/1280/1281/2560/2561 devices, as well as any other device with a proper tLOW acceptance margin.
Figure 158. 2-wire Serial Bus Timing
tof
tHIGH
tLOW
tr
tLOW
SCL
tSU;STA
tHD;STA
tHD;DAT
tSU;DAT
tSU;STO
SDA
tBUF
SPI Timing
Characteristics
See Figure 159 and Figure 160 for details.
Table 167. SPI Timing Parameters
Description
Mode
1
SCK period
Master
See Table 96
2
SCK high/low
Master
50% duty cycle
3
Rise/Fall time
Master
TBD
4
Setup
Master
10
5
Hold
Master
10
6
Out to SCK
Master
0.5 • tsck
7
SCK to out
Master
10
8
SCK to out high
Master
10
9
SS low to out
Slave
15
10
SCK period
Slave
4 • tck
11
SCK high/low(1)
Slave
2 • tck
12
Rise/Fall time
Slave
13
Setup
Slave
10
14
Hold
Slave
tck
15
SCK to out
Slave
16
SCK to SS high
Slave
17
SS high to tri-state
Slave
18
Note:
372
Min
Typ
Max
ns
TBD
15
20
10
SS low to SCK
Slave
20
1. In SPI Programming mode the minimum SCK high/low period is:
- 2 tCLCL for fCK < 12 MHz
- 3 tCLCL for fCK > 12 MHz
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 159. SPI Interface Timing Requirements (Master Mode)
SS
6
1
SCK
(CPOL = 0)
2
2
SCK
(CPOL = 1)
4
MISO
(Data Input)
5
3
MSB
...
LSB
8
7
MOSI
(Data Output)
MSB
...
LSB
Figure 160. SPI Interface Timing Requirements (Slave Mode)
SS
10
9
16
SCK
(CPOL = 0)
11
11
SCK
(CPOL = 1)
13
MOSI
(Data Input)
14
12
MSB
...
LSB
15
MISO
(Data Output)
MSB
17
...
LSB
X
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2549A–AVR–03/05
ADC Characteristics – Preliminary Data
Table 168. ADC Characteristics
Symbol
Parameter
Min(1)
Typ(1)
Max(1)
Units
Single Ended Conversion
10
Differential Conversion
Gain = 1x or 20x
8
Differential Conversion
Gain = 200x
7
Single Ended Conversion
VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
2
Single Ended Conversion
VREF = 4V, VCC = 4V,
ADC clock = 1 MHz
4.5
LSB
Single Ended Conversion
VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
Noise Reduction Mode
2
LSB
Single Ended Conversion
VREF = 4V, VCC = 4V,
ADC clock = 1 MHz
Noise Reduction Mode
4.5
LSB
Integral Non-Linearity (INL)
Single Ended Conversion
VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
0.5
LSB
Differential Non-Linearity (DNL)
Single Ended Conversion
VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
0.25
LSB
Gain Error
Single Ended Conversion
VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
2
LSB
Offset Error
Single Ended Conversion
VREF = 4V, VCC = 4V,
ADC clock = 200 kHz
2
LSB
Conversion Time
Free Running Conversion
13
260
µs
Clock Frequency
Single Ended Conversion
50
1000
kHz
VCC - 0.3
VCC + 0.3
V
Single Ended Conversion
1.0
AVCC
V
Differential Conversion
1.0
AVCC - 0.5
V
Single ended channels
GND
VREF
V
Differential Conversion
0
AVCC
V
Resolution
Absolute accuracy (Including
INL, DNL, quantization error,
gain and offset error)
AVCC
Analog Supply Voltage
VREF
Reference Voltage
VIN
Condition
Input Voltage
Single Ended Channels
Bits
Bits
Bits
2.5
LSB
38,5
kHz
4
kHz
Input Bandwidth
Differential Channels
VINT1
374
Internal Voltage Reference
1.1V
1.0
1.1
1.2
V
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 168. ADC Characteristics (Continued)
Symbol
Min(1)
Typ(1)
Max(1)
Units
2.4
2.56
2.8
V
Parameter
Condition
VINT2
Internal Voltage Reference
2.56V
RREF
Reference Input Resistance
32
kΩ
RAIN
Analog Input Resistance
100
MΩ
Notes:
1. Values are guidelines only. Actual values are TBD
375
2549A–AVR–03/05
External Data Memory Timing
Table 169. External Data Memory Characteristics, 4.5 - 5.5 Volts, No Wait-state
8 MHz Oscillator
Min
Max
Variable Oscillator
Symbol
Parameter
Min
Max
Unit
0
1/tCLCL
Oscillator Frequency
0.0
16
MHz
1
tLHLL
ALE Pulse Width
115
1.0tCLCL-10
ns
2
tAVLL
Address Valid A to ALE Low
57.5
0.5tCLCL-5(1)
ns
3a
tLLAX_ST
Address Hold After ALE Low,
write access
5
5
3b
tLLAX_LD
Address Hold after ALE Low,
read access
5
5
ns
ns
(1)
4
tAVLLC
Address Valid C to ALE Low
57.5
0.5tCLCL-5
ns
5
tAVRL
Address Valid to RD Low
115
1.0tCLCL-10
ns
6
tAVWL
Address Valid to WR Low
115
1.0tCLCL-10
ns
7
tLLWL
ALE Low to WR Low
47.5
8
tLLRL
ALE Low to RD Low
9
tDVRH
Data Setup to RD High
10
tRLDV
Read Low to Data Valid
11
tRHDX
Data Hold After RD High
12
tRLRH
RD Pulse Width
47.5
67.5
0.5tCLCL-15(2)
0.5tCLCL+5(2)
ns
67.5
(2)
(2)
ns
40
0.5tCLCL-15
0.5tCLCL+5
40
ns
75
1.0tCLCL-50
0
0
115
1.0tCLCL-10
ns
ns
13
tDVWL
Data Setup to WR Low
42.5
14
tWHDX
Data Hold After WR High
115
1.0tCLCL-10
ns
15
tDVWH
Data Valid to WR High
125
1.0tCLCL
ns
16
tWLWH
WR Pulse Width
115
1.0tCLCL-10
ns
Notes:
0.5tCLCL-20
ns
(1)
ns
1. This assumes 50% clock duty cycle. The half period is actually the high time of the external clock, XTAL1.
2. This assumes 50% clock duty cycle. The half period is actually the low time of the external clock, XTAL1.
Table 170. External Data Memory Characteristics, 4.5 - 5.5 Volts, 1 Cycle Wait-state
8 MHz Oscillator
Symbol
Parameter
0
1/tCLCL
Oscillator Frequency
10
tRLDV
Read Low to Data Valid
12
tRLRH
RD Pulse Width
240
2.0tCLCL-10
ns
15
tDVWH
Data Valid to WR High
240
2.0tCLCL
ns
16
tWLWH
WR Pulse Width
240
2.0tCLCL-10
ns
376
Min
Max
Variable Oscillator
Min
Max
Unit
0.0
16
MHz
200
2.0tCLCL-50
ns
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table 171. External Data Memory Characteristics, 4.5 - 5.5 Volts, SRWn1 = 1, SRWn0 = 0
4 MHz Oscillator
Min
Max
Variable Oscillator
Symbol
Parameter
Min
Max
Unit
0
1/tCLCL
Oscillator Frequency
0.0
16
MHz
10
tRLDV
Read Low to Data Valid
12
tRLRH
RD Pulse Width
365
3.0tCLCL-10
ns
15
tDVWH
Data Valid to WR High
375
3.0tCLCL
ns
16
tWLWH
WR Pulse Width
365
3.0tCLCL-10
ns
325
3.0tCLCL-50
ns
Table 172. External Data Memory Characteristics, 4.5 - 5.5 Volts, SRWn1 = 1, SRWn0 = 1
4 MHz Oscillator
Min
Max
Variable Oscillator
Symbol
Parameter
Min
Max
Unit
0
1/tCLCL
Oscillator Frequency
0.0
16
MHz
10
tRLDV
Read Low to Data Valid
12
tRLRH
RD Pulse Width
365
3.0tCLCL-10
ns
14
tWHDX
Data Hold After WR High
240
2.0tCLCL-10
ns
15
tDVWH
Data Valid to WR High
375
3.0tCLCL
ns
16
tWLWH
WR Pulse Width
365
3.0tCLCL-10
ns
325
3.0tCLCL-50
ns
Table 173. External Data Memory Characteristics, 2.7 - 5.5 Volts, No Wait-state
4 MHz Oscillator
Min
Max
Variable Oscillator
Symbol
Parameter
Min
Max
Unit
0
1/tCLCL
Oscillator Frequency
0.0
8
MHz
1
tLHLL
ALE Pulse Width
235
tCLCL-15
ns
2
tAVLL
Address Valid A to ALE Low
115
0.5tCLCL-10(1)
ns
3a
tLLAX_ST
Address Hold After ALE Low,
write access
5
5
3b
tLLAX_LD
Address Hold after ALE Low,
read access
5
5
ns
(1)
4
tAVLLC
Address Valid C to ALE Low
115
5
tAVRL
Address Valid to RD Low
235
1.0tCLCL-15
6
tAVWL
Address Valid to WR Low
235
1.0tCLCL-15
7
tLLWL
ALE Low to WR Low
115
8
tLLRL
ALE Low to RD Low
115
9
tDVRH
Data Setup to RD High
45
10
tRLDV
Read Low to Data Valid
11
tRHDX
Data Hold After RD High
0.5tCLCL-10
ns
130
130
0.5tCLCL-10
(2)
0.5tCLCL-10
(2)
ns
ns
0.5tCLCL+5
(2)
ns
0.5tCLCL+5
(2)
ns
45
190
0
ns
ns
1.0tCLCL-60
0
ns
ns
377
2549A–AVR–03/05
Table 173. External Data Memory Characteristics, 2.7 - 5.5 Volts, No Wait-state (Continued)
4 MHz Oscillator
12
Symbol
Parameter
Min
tRLRH
RD Pulse Width
235
Max
Variable Oscillator
Min
Max
Unit
1.0tCLCL-15
ns
(1)
13
tDVWL
Data Setup to WR Low
105
14
tWHDX
Data Hold After WR High
235
1.0tCLCL-15
ns
15
tDVWH
Data Valid to WR High
250
1.0tCLCL
ns
16
tWLWH
WR Pulse Width
235
1.0tCLCL-15
ns
Notes:
0.5tCLCL-20
ns
1. This assumes 50% clock duty cycle. The half period is actually the high time of the external clock, XTAL1.
2. This assumes 50% clock duty cycle. The half period is actually the low time of the external clock, XTAL1.
Table 174. External Data Memory Characteristics, 2.7 - 5.5 Volts, SRWn1 = 0, SRWn0 = 1
4 MHz Oscillator
Min
Max
Variable Oscillator
Symbol
Parameter
Min
Max
Unit
0
1/tCLCL
Oscillator Frequency
0.0
8
MHz
10
tRLDV
Read Low to Data Valid
12
tRLRH
RD Pulse Width
485
2.0tCLCL-15
ns
15
tDVWH
Data Valid to WR High
500
2.0tCLCL
ns
16
tWLWH
WR Pulse Width
485
2.0tCLCL-15
ns
440
2.0tCLCL-60
ns
Table 175. External Data Memory Characteristics, 2.7 - 5.5 Volts, SRWn1 = 1, SRWn0 = 0
4 MHz Oscillator
Min
Max
Variable Oscillator
Symbol
Parameter
Min
Max
Unit
0
1/tCLCL
Oscillator Frequency
0.0
8
MHz
10
tRLDV
Read Low to Data Valid
12
tRLRH
RD Pulse Width
735
3.0tCLCL-15
ns
15
tDVWH
Data Valid to WR High
750
3.0tCLCL
ns
16
tWLWH
WR Pulse Width
735
3.0tCLCL-15
ns
690
3.0tCLCL-60
ns
Table 176. External Data Memory Characteristics, 2.7 - 5.5 Volts, SRWn1 = 1, SRWn0 = 1
4 MHz Oscillator
Symbol
Parameter
0
1/tCLCL
Oscillator Frequency
10
tRLDV
Read Low to Data Valid
12
tRLRH
RD Pulse Width
735
3.0tCLCL-15
ns
14
tWHDX
Data Hold After WR High
485
2.0tCLCL-15
ns
15
tDVWH
Data Valid to WR High
750
3.0tCLCL
ns
16
tWLWH
WR Pulse Width
735
3.0tCLCL-15
ns
378
Min
Max
Variable Oscillator
Min
Max
Unit
0.0
8
MHz
690
3.0tCLCL-60
ns
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Figure 161. External Memory Timing (SRWn1 = 0, SRWn0 = 0
T1
T2
T3
T4
System Clock (CLKCPU )
1
ALE
4
A15:8
7
Prev. addr.
Address
15
3a
DA7:0
Prev. data
Address
13
XX
Data
14
16
6
Write
2
WR
9
3b
DA7:0 (XMBK = 0)
11
Data
5
Read
Address
10
8
12
RD
Figure 162. External Memory Timing (SRWn1 = 0, SRWn0 = 1)
T1
T2
T3
T4
T5
System Clock (CLKCPU )
1
ALE
4
A15:8
7
Prev. addr.
Address
15
DA7:0
Prev. data
3a
Address
13
Data
XX
14
16
6
Write
2
WR
9
3b
Address
11
Data
5
Read
DA7:0 (XMBK = 0)
10
8
12
RD
379
2549A–AVR–03/05
Figure 163. External Memory Timing (SRWn1 = 1, SRWn0 = 0)
T1
T2
T3
T5
T4
T6
System Clock (CLKCPU )
1
ALE
4
A15:8
7
Address
Prev. addr.
15
3a
DA7:0
Prev. data
Address
13
XX
Data
14
16
6
Write
2
WR
9
3b
DA7:0 (XMBK = 0)
Address
11
5
Read
Data
10
8
12
RD
Figure 164. External Memory Timing (SRWn1 = 1, SRWn0 = 1)()
T1
T2
T3
T4
T6
T5
T7
System Clock (CLKCPU )
1
ALE
4
A15:8
7
Address
Prev. addr.
15
3a
DA7:0
Prev. data
Address
13
XX
Data
14
16
6
Write
2
WR
9
3b
Address
11
Data
5
Read
DA7:0 (XMBK = 0)
10
8
12
RD
The ALE pulse in the last period (T4-T7) is only present if the next instruction accesses the RAM
(internal or external).
380
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
ATmega640/1280/128
1/2560/2561 Typical
Characteristics –
Preliminary Data
TBD
The following charts show typical behavior. These figures are not tested during manufacturing. All current consumption measurements are performed with all I/O pins
configured as inputs and with internal pull-ups enabled. A sine wave generator with railto-rail output is used as clock source.
All Active- and Idle current consumption measurements are done with all bits in the PRR
registers set and thus, the corresponding I/O modules are turned off. Also the Analog
Comparator is disabled during these measurements. Table 177 on page 381 and Table
178 on page 382 show the additional current consumption compared to ICC Active and
ICC Idle for every I/O module controlled by the Power Reduction Register. See “Power
Reduction Register” on page 54 for details.
The power consumption in Power-down mode is independent of clock selection.
The current consumption is a function of several factors such as: operating voltage,
operating frequency, loading of I/O pins, switching rate of I/O pins, code executed and
ambient temperature. The dominating factors are operating voltage and frequency.
The current drawn from capacitive loaded pins may be estimated (for one pin) as
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 current drawn by the Watchdog Timer.
Supply Current of IO
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 “Power Reduction
Register” on page 54 for details.
Table 177.
Additional Current Consumption for the different I/O modules (absolute values)
PRR bit
Typical numbers
VCC = 2V, F = 1MHz
VCC = 3V, F = 4MHz
VCC = 5V, F = 8MHz
PRUSART3
8.0 uA
51 uA
220 uA
PRUSART2
8.0 uA
51 uA
220 uA
PRUSART1
8.0 uA
51 uA
220 uA
PRUSART0
8.0 uA
51 uA
220 uA
PRTWI
12 uA
75 uA
315 uA
PRTIM5
6.0 uA
39 uA
150 uA
PRTIM4
6.0 uA
39 uA
150 uA
PRTIM3
6.0 uA
39 uA
150 uA
PRTIM2
11 uA
72 uA
300 uA
PRTIM1
6.0 uA
39 uA
150 uA
381
2549A–AVR–03/05
Table 177.
Additional Current Consumption for the different I/O modules (absolute values)
(Continued)
PRR bit
Typical numbers
VCC = 2V, F = 1MHz
VCC = 3V, F = 4MHz
VCC = 5V, F = 8MHz
PRTIM0
4.0 uA
24 uA
100 uA
PRSPI
15 uA
95 uA
400 uA
PRADC
12 uA
75 uA
315 uA
Table 178.
Additional Current Consumption (percentage) in Active and Idle mode
PRR bit
Additional Current consumption
compared to Active with external
clock
Additional Current consumption
compared to Idle with external
clock
PRUSART3
3.0%
17%
PRUSART2
3.0%
17%
PRUSART1
3.0%
17%
PRUSART0
3.0%
17%
PRTWI
4.4%
24%
PRTIM5
1.8%
10%
PRTIM4
1.8%
10%
PRTIM3
1.8%
10%
PRTIM2
4.3%
23%
PRTIM1
1.8%
10%
PRTIM0
1.5%
8.0%
PRSPI
3.3%
18%
PRADC
4.5%
24%
It is possible to calculate the typical current consumption based on the numbers from
Table 177 for other VCC and frequency settings than listed in Table 178.
Example 1
Calculate the expected current consumption in idle mode with USART0, TIMER1, and
TWI enabled at VCC = 3.0V and F = 1MHz. From Table 177, third column, we see that
we need to add 18% for the USART0, 26% for the TWI, and 11% for the TIMER1 module. Reading from Figure XXXX, we find that the idle current consumption is ~0,075mA
at VCC = 3.0V and F = 1MHz. The total current consumption in idle mode with USART0,
TIMER1, and TWI enabled, gives:
I CC total ≈ 0.075mA • ( 1 + 0.18 + 0.26 + 0.11 ) ≈ 0.116mA
Example 2
Same conditions as in example 1, but in active mode instead. From Table 178 second
column we see that we need to add 3.3% for the USART0, 4.8% for the TWI, and 2.0%
for the TIMER1 module. Reading from Figure XXXX, we find that the active current consumption is ~0,42mA at VCC = 3.0V and F = 1MHz. The total current consumption in idle
mode with USART0, TIMER1, and TWI enabled, gives:
I CC total ≈ 0.42mA • ( 1 + 0.033 + 0.048 + 0.02 ) ≈ 0.46mA
382
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Example 3
All I/O modules should be enabled. Calculate the expected current consumption in
active mode at VCC = 3.6V and F = 10MHz. We find the active current consumption without the I/O modules to be ~ 4.0mA (from Figure XXXX). Then, by using the numbers
from Table 178 - second column, we find the total current consumption:
I CC total ≈ 5.0mA • ( 1 + 0.03 + 0.03 + 0.03 + 0.03 + 0.044 + 0.018 + 0.018 + 0.018 + 0.043 + 0.018 + 0.015 + 0.033 + 0.045 ) ≈ 6.9mA
383
2549A–AVR–03/05
384
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Register Summary
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
(0x1FF)
Reserved
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
...
Reserved
(0x13F)
Reserved
(0x13E)
Reserved
(0x13D)
Reserved
(0x13C)
Reserved
(0x13B)
Reserved
(0x13A)
Reserved
(0x139)
Reserved
(0x138)
Reserved
(0x137)
Reserved
(0x136)
UDR3
(0x135)
UBRR3H
(0x134)
UBRR3L
(0x133)
Reserved
-
-
-
-
-
-
-
-
(0x132)
UCSR3C
UMSEL31
UMSEL30
UPM31
UPM30
USBS3
UCSZ31
UCSZ30
UCPOL3
(0x131)
UCSR3B
RXCIE3
TXCIE3
UDRIE3
RXEN3
TXEN3
UCSZ32
RXB83
TXB83
(0x130)
UCSR3A
RXC3
TXC3
UDRE3
FE3
DOR3
UPE3
U2X3
MPCM3
Page
USART3 I/O Data Register
-
USART3 Baud Rate Register High Byte
USART3 Baud Rate Register Low Byte
(0x12F)
Reserved
-
-
-
-
-
-
-
-
(0x12E)
Reserved
-
-
-
-
-
-
-
-
(0x12D)
OCR5CH
-
Timer/Counter5 - Output Compare Register C High Byte
(0x12C)
OCR5CL
Timer/Counter5 - Output Compare Register C Low Byte
(0x12B)
OCR5BH
Timer/Counter5 - Output Compare Register B High Byte
(0x12A)
OCR5BL
Timer/Counter5 - Output Compare Register B Low Byte
(0x129)
OCR5AH
Timer/Counter5 - Output Compare Register A High Byte
(0x128)
OCR5AL
Timer/Counter5 - Output Compare Register A Low Byte
(0x127)
ICR5H
Timer/Counter5 - Input Capture Register High Byte
(0x126)
ICR5L
Timer/Counter5 - Input Capture Register Low Byte
(0x125)
TCNT5H
Timer/Counter5 - Counter Register High Byte
(0x124)
TCNT5L
(0x123)
Reserved
-
-
-
-
-
-
-
(0x122)
TCCR5C
FOC5A
FOC5B
FOC5C
-
-
-
-
-
(0x121)
TCCR5B
ICNC5
ICES5
-
WGM53
WGM52
CS52
CS51
CS50
Timer/Counter5 - Counter Register Low Byte
(0x120)
TCCR5A
COM5A1
COM5A0
COM5B1
COM5B0
COM5C1
COM5C0
WGM51
WGM50
(0x11F)
Reserved
-
-
-
-
-
-
-
-
(0x11E)
Reserved
-
-
-
-
-
-
-
-
(0x11D)
Reserved
-
-
-
-
-
-
-
-
(0x11C)
Reserved
-
-
-
-
-
-
-
-
(0x11B)
Reserved
-
-
-
-
-
-
-
-
(0x11A)
Reserved
-
-
-
-
-
-
-
-
(0x119)
Reserved
-
-
-
-
-
-
-
-
(0x118)
Reserved
-
-
-
-
-
-
-
-
(0x117)
Reserved
-
-
-
-
-
-
-
-
(0x116)
Reserved
-
-
-
-
-
-
-
-
(0x115)
Reserved
-
-
-
-
-
-
-
-
(0x114)
Reserved
-
-
-
-
-
-
-
-
(0x113)
Reserved
-
-
-
-
-
-
-
-
(0x112)
Reserved
-
-
-
-
-
-
-
-
(0x111)
Reserved
-
-
-
-
-
-
-
-
(0x110)
Reserved
-
-
-
-
-
-
-
-
(0x10F)
Reserved
-
-
-
-
-
-
-
-
(0x10E)
Reserved
-
-
-
-
-
-
-
-
(0x10D)
Reserved
-
-
-
-
-
-
-
-
(0x10C)
Reserved
-
-
-
-
-
-
-
-
(0x10B)
PORTL
PORTL7
PORTL6
PORTL5
PORTL4
PORTL3
PORTL2
PORTL1
PORTL0
(0x10A)
DDRL
DDL7
DDL6
DDL5
DDL4
DDL3
DDL2
DDL1
DDL0
(0x109)
PINL
PINL7
PINL6
PINL5
PINL4
PINL3
PINL2
PINL1
PINL0
(0x108)
PORTK
PORTK7
PORTK6
PORTK5
PORTK4
PORTK3
PORTK2
PORTK1
PORTK0
(0x107)
DDRK
DDK7
DDK6
DDK5
DDK4
DDK3
DDK2
DDK1
DDK0
(0x106)
PINK
PINK7
PINK6
PINK5
PINK4
PINK3
PINK2
PINK1
PINK0
(0x105)
PORTJ
PORTJ7
PORTJ6
PORTJ5
PORTJ4
PORTJ3
PORTJ2
PORTJ1
PORTJ0
(0x104)
DDRJ
DDJ7
DDJ6
DDJ5
DDJ4
DDJ3
DDJ2
DDJ1
DDJ0
(0x103)
PINJ
PINJ7
PINJ6
PINJ5
PINJ4
PINJ3
PINJ2
PINJ1
PINJ0
(0x102)
PORTH
PORTH7
PORTH6
PORTH5
PORTH4
PORTH3
PORTH2
PORTH1
PORTH0
385
2549A–AVR–03/05
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
(0x101)
DDRH
DDH7
DDH6
DDH5
DDH4
DDH3
DDH2
DDH1
DDH0
(0x100)
PINH
PINH7
PINH6
PINH5
PINH4
PINH3
PINH2
PINH1
PINH0
(0xFF)
Reserved
-
-
-
-
-
-
-
-
(0xFE)
Reserved
-
-
-
-
-
-
-
-
(0xFD)
Reserved
-
-
-
-
-
-
-
-
(0xFC)
Reserved
-
-
-
-
-
-
-
-
(0xFB)
Reserved
-
-
-
-
-
-
-
-
(0xFA)
Reserved
-
-
-
-
-
-
-
-
(0xF9)
Reserved
-
-
-
-
-
-
-
-
(0xF8)
Reserved
-
-
-
-
-
-
-
-
(0xF7)
Reserved
-
-
-
-
-
-
-
-
(0xF6)
Reserved
-
-
-
-
-
-
-
-
(0xF5)
Reserved
-
-
-
-
-
-
-
-
(0xF4)
Reserved
-
-
-
-
-
-
-
-
(0xF3)
Reserved
-
-
-
-
-
-
-
-
(0xF2)
Reserved
-
-
-
-
-
-
-
-
(0xF1)
Reserved
-
-
-
-
-
-
-
-
(0xF0)
Reserved
-
-
-
-
-
-
-
-
(0xEF)
Reserved
-
-
-
-
-
-
-
-
(0xEE)
Reserved
-
-
-
-
-
-
-
-
(0xED)
Reserved
-
-
-
-
-
-
-
-
(0xEC)
Reserved
-
-
-
-
-
-
-
-
(0xEB)
Reserved
-
-
-
-
-
-
-
(0xEA)
Reserved
-
-
-
-
-
-
-
-
(0xE9)
Reserved
-
-
-
-
-
-
-
-
(0xE8)
Reserved
-
-
-
-
-
-
-
-
(0xE7)
Reserved
-
-
-
-
-
-
-
(0xE6)
Reserved
-
-
-
-
-
-
-
-
(0xE5)
Reserved
-
-
-
-
-
-
-
-
(0xE4)
Reserved
-
-
-
-
-
-
-
-
(0xE3)
Reserved
-
-
-
-
-
-
-
(0xE2)
Reserved
-
-
-
-
-
-
-
(0xE1)
Reserved
-
-
-
-
-
-
-
(0xE0)
Reserved
-
-
-
-
-
-
-
(0xDF)
Reserved
-
-
-
-
-
-
-
-
(0xDE)
Reserved
-
-
-
-
-
-
-
-
(0xDD)
Reserved
-
-
-
-
-
-
-
(0xDC)
Reserved
-
-
-
-
-
-
-
-
(0xDB)
Reserved
-
-
-
-
-
-
-
-
(0xDA)
Reserved
-
-
-
-
-
-
-
-
(0xD9)
Reserved
-
-
-
-
-
-
-
(0xD8)
Reserved
-
-
-
-
-
-
-
-
(0xD7)
Reserved
-
-
-
-
-
-
-
-
(0xD6)
UDR2
(0xD5)
UBRR2H
(0xD4)
UBRR2L
-
USART2 I/O Data Register
-
-
-
-
USART2 Baud Rate Register High Byte
USART2 Baud Rate Register Low Byte
(0xD3)
Reserved
-
-
-
-
-
-
-
-
(0xD2)
UCSR2C
UMSEL21
UMSEL20
UPM21
UPM20
USBS2
UCSZ21
UCSZ20
UCPOL2
(0xD1)
UCSR2B
RXCIE2
TXCIE2
UDRIE2
RXEN2
TXEN2
UCSZ22
RXB82
TXB82
(0xD0)
UCSR2A
RXC2
TXC2
UDRE2
FE2
DOR2
UPE2
U2X2
MPCM2
-
-
-
-
-
-
-
-
(0xCF)
Reserved
(0xCE)
UDR1
(0xCD)
UBRR1H
(0xCC)
UBRR1L
USART1 I/O Data Register
-
-
-
-
USART1 Baud Rate Register High Byte
USART1 Baud Rate Register Low Byte
(0xCB)
Reserved
-
-
-
-
-
-
-
-
(0xCA)
UCSR1C
UMSEL11
UMSEL10
UPM11
UPM10
USBS1
UCSZ11
UCSZ10
UCPOL1
(0xC9)
UCSR1B
RXCIE1
TXCIE1
UDRIE1
RXEN1
TXEN1
UCSZ12
RXB81
TXB81
(0xC8)
UCSR1A
RXC1
TXC1
UDRE1
FE1
DOR1
UPE1
U2X1
MPCM1
(0xC7)
Reserved
-
-
-
-
-
-
-
-
(0xC6)
UDR0
(0xC5)
UBRR0H
(0xC4)
UBRR0L
386
Page
USART0 I/O Data Register
-
-
-
-
USART0 Baud Rate Register High Byte
USART0 Baud Rate Register Low Byte
(0xC3)
Reserved
-
-
-
-
-
-
-
-
(0xC2)
UCSR0C
UMSEL01
UMSEL00
UPM01
UPM00
USBS0
UCSZ01
UCSZ00
UCPOL0
(0xC1)
UCSR0B
RXCIE0
TXCIE0
UDRIE0
RXEN0
TXEN0
UCSZ02
RXB80
TXB80
(0xC0)
UCSR0A
RXC0
TXC0
UDRE0
FE0
DOR0
UPE0
U2X0
MPCM0
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
(0xBF)
Reserved
-
-
-
-
-
-
-
-
(0xBE)
Reserved
-
-
-
-
-
-
-
(0xBD)
TWAMR
TWAM6
TWAM5
TWAM4
TWAM3
TWAM2
TWAM1
TWAM0
-
(0xBC)
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
-
TWIE
(0xBB)
TWDR
(0xBA)
TWAR
TWA6
TWA5
TWA4
TWA3
TWA2
TWA1
TWA0
TWGCE
(0xB9)
TWSR
TWS7
TWS6
TWS5
TWS4
TWS3
-
TWPS1
TWPS0
(0xB8)
TWBR
(0xB7)
Reserved
-
-
-
-
-
-
-
-
(0xB6)
ASSR
-
EXCLK
AS2
TCN2UB
OCR2AUB
OCR2BUB
TCR2AUB
TCR2BUB
(0xB5)
Reserved
-
-
-
-
-
-
-
-
(0xB4)
OCR2B
Timer/Counter2 Output Compare Register B
(0xB3)
OCR2A
Timer/Counter2 Output Compare Register A
(0xB2)
TCNT2
(0xB1)
TCCR2B
FOC2A
FOC2B
-
-
WGM22
CS22
CS21
CS20
(0xB0)
TCCR2A
COM2A1
COM2A0
COM2B1
COM2B0
-
-
WGM21
WGM20
Page
2-wire Serial Interface Data Register
2-wire Serial Interface Bit Rate Register
Timer/Counter2 (8 Bit)
(0xAF)
Reserved
-
-
-
-
-
-
-
-
(0xAE)
Reserved
-
-
-
-
-
-
-
-
(0xAD)
OCR4CH
-
Timer/Counter4 - Output Compare Register C High Byte
(0xAC)
OCR4CL
Timer/Counter4 - Output Compare Register C Low Byte
(0xAB)
OCR4BH
Timer/Counter4 - Output Compare Register B High Byte
(0xAA)
OCR4BL
Timer/Counter4 - Output Compare Register B Low Byte
(0xA9)
OCR4AH
Timer/Counter4 - Output Compare Register A High Byte
(0xA8)
OCR4AL
Timer/Counter4 - Output Compare Register A Low Byte
(0xA7)
ICR4H
Timer/Counter4 - Input Capture Register High Byte
(0xA6)
ICR4L
Timer/Counter4 - Input Capture Register Low Byte
(0xA5)
TCNT4H
Timer/Counter4 - Counter Register High Byte
(0xA4)
TCNT4L
(0xA3)
Reserved
-
-
-
-
-
-
-
(0xA2)
TCCR4C
FOC4A
FOC4B
FOC4C
-
-
-
-
-
(0xA1)
TCCR4B
ICNC4
ICES4
-
WGM43
WGM42
CS42
CS41
CS40
(0xA0)
TCCR4A
COM4A1
COM4A0
COM4B1
COM4B0
COM4C1
COM4C0
WGM41
WGM40
(0x9F)
Reserved
-
-
-
-
-
-
-
-
(0x9E)
Reserved
-
-
-
-
-
-
-
-
(0x9D)
OCR3CH
-
Timer/Counter4 - Counter Register Low Byte
Timer/Counter3 - Output Compare Register C High Byte
(0x9C)
OCR3CL
Timer/Counter3 - Output Compare Register C Low Byte
(0x9B)
OCR3BH
Timer/Counter3 - Output Compare Register B High Byte
(0x9A)
OCR3BL
Timer/Counter3 - Output Compare Register B Low Byte
(0x99)
OCR3AH
Timer/Counter3 - Output Compare Register A High Byte
(0x98)
OCR3AL
Timer/Counter3 - Output Compare Register A Low Byte
(0x97)
ICR3H
Timer/Counter3 - Input Capture Register High Byte
(0x96)
ICR3L
Timer/Counter3 - Input Capture Register Low Byte
(0x95)
TCNT3H
Timer/Counter3 - Counter Register High Byte
(0x94)
TCNT3L
(0x93)
Reserved
-
-
-
-
-
-
-
(0x92)
TCCR3C
FOC3A
FOC3B
FOC3C
-
-
-
-
-
(0x91)
TCCR3B
ICNC3
ICES3
-
WGM33
WGM32
CS32
CS31
CS30
Timer/Counter3 - Counter Register Low Byte
(0x90)
TCCR3A
COM3A1
COM3A0
COM3B1
COM3B0
COM3C1
COM3C0
WGM31
WGM30
(0x8F)
Reserved
-
-
-
-
-
-
-
-
(0x8E)
Reserved
-
-
-
-
-
-
-
-
(0x8D)
OCR1CH
-
Timer/Counter1 - Output Compare Register C High Byte
(0x8C)
OCR1CL
Timer/Counter1 - Output Compare Register C Low Byte
(0x8B)
OCR1BH
Timer/Counter1 - Output Compare Register B High Byte
(0x8A)
OCR1BL
Timer/Counter1 - Output Compare Register B Low Byte
(0x89)
OCR1AH
Timer/Counter1 - Output Compare Register A High Byte
(0x88)
OCR1AL
Timer/Counter1 - Output Compare Register A Low Byte
(0x87)
ICR1H
Timer/Counter1 - Input Capture Register High Byte
(0x86)
ICR1L
Timer/Counter1 - Input Capture Register Low Byte
(0x85)
TCNT1H
Timer/Counter1 - Counter Register High Byte
(0x84)
TCNT1L
(0x83)
Reserved
-
-
-
-
-
-
-
(0x82)
TCCR1C
FOC1A
FOC1B
FOC1C
-
-
-
-
-
(0x81)
TCCR1B
ICNC1
ICES1
-
WGM13
WGM12
CS12
CS11
CS10
WGM10
Timer/Counter1 - Counter Register Low Byte
(0x80)
TCCR1A
COM1A1
COM1A0
COM1B1
COM1B0
COM1C1
COM1C0
WGM11
(0x7F)
DIDR1
-
-
-
-
-
-
AIN1D
AIN0D
(0x7E)
DIDR0
ADC7D
ADC6D
ADC5D
ADC4D
ADC3D
ADC2D
ADC1D
ADC0D
387
2549A–AVR–03/05
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
(0x7D)
DIDR2
ADC15D
ADC14D
ADC13D
ADC12D
ADC11D
ADC10D
ADC9D
ADC8D
(0x7C)
ADMUX
REFS1
REFS0
ADLAR
MUX4
MUX3
MUX2
MUX1
MUX0
(0x7B)
ADCSRB
-
ACME
-
-
MUX5
ADTS2
ADTS1
ADTS0
(0x7A)
ADCSRA
ADEN
ADSC
ADATE
ADIF
ADIE
ADPS2
ADPS1
ADPS0
(0x79)
ADCH
-
ADC Data Register High byte
(0x78)
ADCL
(0x77)
Reserved
-
-
-
ADC Data Register Low byte
-
-
-
-
(0x76)
Reserved
-
-
-
-
-
-
-
-
(0x75)
XMCRB
XMBK
-
-
-
-
XMM2
XMM1
XMM0
(0x74)
XMCRA
SRE
SRL2
SRL1
SRL0
SRW11
SRW10
SRW01
SRW00
(0x73)
TIMSK5
-
-
ICIE5
-
OCIE5C
OCIE5B
OCIE5A
TOIE5
(0x72)
TIMSK4
-
-
ICIE4
-
OCIE4C
OCIE4B
OCIE4A
TOIE4
(0x71)
TIMSK3
-
-
ICIE3
-
OCIE3C
OCIE3B
OCIE3A
TOIE3
(0x70)
TIMSK2
-
-
-
-
-
OCIE2B
OCIE2A
TOIE2
(0x6F)
TIMSK1
-
-
ICIE1
-
OCIE1C
OCIE1B
OCIE1A
TOIE1
(0x6E)
TIMSK0
-
-
-
-
-
OCIE0B
OCIE0A
TOIE0
(0x6D)
PCMSK2
PCINT23
PCINT22
PCINT21
PCINT20
PCINT19
PCINT18
PCINT17
PCINT16
(0x6C)
PCMSK1
PCINT15
PCINT14
PCINT13
PCINT12
PCINT11
PCINT10
PCINT9
PCINT8
(0x6B)
PCMSK0
PCINT7
PCINT6
PCINT5
PCINT4
PCINT3
PCINT2
PCINT1
PCINT0
(0x6A)
EICRB
ISC71
ISC70
ISC61
ISC60
ISC51
ISC50
ISC41
ISC40
(0x69)
EICRA
ISC31
ISC30
ISC21
ISC20
ISC11
ISC10
ISC01
ISC00
(0x68)
PCICR
-
-
-
-
-
PCIE2
PCIE1
PCIE0
(0x67)
Reserved
-
-
-
-
-
-
-
-
(0x66)
OSCCAL
Oscillator Calibration Register
(0x65)
PRR1
-
-
PRTIM5
PRTIM4
PRTIM3
PRUSART3
PRUSART2
PRUSART1
(0x64)
PRR0
PRTWI
PRTIM2
PRTIM0
-
PRTIM1
PRSPI
PRUSART0
PRADC
(0x63)
Reserved
-
-
-
-
-
-
-
-
(0x62)
Reserved
-
-
-
-
-
-
-
-
(0x61)
CLKPR
CLKPCE
-
-
-
CLKPS3
CLKPS2
CLKPS1
CLKPS0
WDP0
(0x60)
WDTCSR
WDIF
WDIE
WDP3
WDCE
WDE
WDP2
WDP1
0x3F (0x5F)
SREG
I
T
H
S
V
N
Z
C
0x3E (0x5E)
SPH
SP15
SP14
SP13
SP12
SP11
SP10
SP9
SP8
0x3D (0x5D)
SPL
SP7
SP6
SP5
SP4
SP3
SP2
SP1
SP0
0x3C (0x5C)
EIND
-
-
-
-
-
-
-
EIND0
0x3B (0x5B)
RAMPZ
-
-
-
-
-
-
RAMPZ1
RAMPZ0
0x3A (0x5A)
Reserved
-
-
-
-
-
-
-
-
0x39 (0x59)
Reserved
-
-
-
-
-
-
-
-
0x38 (0x58)
Reserved
-
-
-
-
-
-
-
-
0x37 (0x57)
SPMCSR
SPMIE
RWWSB
SIGRD
RWWSRE
BLBSET
PGWRT
PGERS
SPMEN
0x36 (0x56)
Reserved
-
-
-
-
-
-
-
-
0x35 (0x55)
MCUCR
JTD
-
-
PUD
-
-
IVSEL
IVCE
0x34 (0x54)
MCUSR
-
-
-
JTRF
WDRF
BORF
EXTRF
PORF
0x33 (0x53)
SMCR
-
-
-
-
SM2
SM1
SM0
SE
0x32 (0x52)
Reserved
-
-
-
-
-
-
-
-
0x31 (0x51)
OCDR/
MONDR
OCDR7
OCDR6
OCDR5
OCDR4
OCDR3
OCDR2
OCDR1
OCDR0
0x30 (0x50)
ACSR
ACD
ACBG
ACO
ACI
ACIE
ACIC
ACIS1
ACIS0
0x2F (0x4F)
Reserved
-
-
-
-
-
-
-
-
0x2E (0x4E)
SPDR
Monitor Data Register
SPI Data Register
0x2D (0x4D)
SPSR
SPIF
WCOL
-
-
-
-
-
SPI2X
0x2C (0x4C)
SPCR
SPIE
SPE
DORD
MSTR
CPOL
CPHA
SPR1
SPR0
0x2B (0x4B)
GPIOR2
-
-
-
General Purpose I/O Register 2
0x2A (0x4A)
GPIOR1
0x29 (0x49)
Reserved
0x28 (0x48)
OCR0B
Timer/Counter0 Output Compare Register B
0x27 (0x47)
OCR0A
Timer/Counter0 Output Compare Register A
0x26 (0x46)
TCNT0
0x25 (0x45)
TCCR0B
FOC0A
FOC0B
-
-
WGM02
CS02
CS01
CS00
0x24 (0x44)
TCCR0A
COM0A1
COM0A0
COM0B1
COM0B0
-
-
WGM01
WGM00
0x23 (0x43)
GTCCR
TSM
-
-
-
-
-
PSRASY
PSRSYNC
0x22 (0x42)
EEARH
-
-
-
-
0x21 (0x41)
EEARL
0x20 (0x40)
EEDR
0x1F (0x3F)
EECR
0x1E (0x3E)
GPIOR0
0x1D (0x3D)
EIMSK
388
Page
General Purpose I/O Register 1
-
-
-
-
-
Timer/Counter0 (8 Bit)
EEPROM Address Register High Byte
EEPROM Address Register Low Byte
EEPROM Data Register
-
-
EEPM1
EEPM0
EERIE
EEMPE
EEPE
EERE
INT2
INT1
INT0
General Purpose I/O Register 0
INT7
INT6
INT5
INT4
INT3
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
0x1C (0x3C)
EIFR
INTF7
INTF6
INTF5
INTF4
INTF3
INTF2
INTF1
INTF0
0x1B (0x3B)
PCIFR
-
-
-
-
-
PCIF2
PCIF1
PCIF0
0x1A (0x3A)
TIFR5
-
-
ICF5
-
OCF5C
OCF5B
OCF5A
TOV5
0x19 (0x39)
TIFR4
-
-
ICF4
-
OCF4C
OCF4B
OCF4A
TOV4
0x18 (0x38)
TIFR3
-
-
ICF3
-
OCF3C
OCF3B
OCF3A
TOV3
0x17 (0x37)
TIFR2
-
-
-
-
-
OCF2B
OCF2A
TOV2
0x16 (0x36)
TIFR1
-
-
ICF1
-
OCF1C
OCF1B
OCF1A
TOV1
0x15 (0x35)
TIFR0
-
-
-
-
-
OCF0B
OCF0A
TOV0
PORTG0
0x14 (0x34)
PORTG
-
-
PORTG5
PORTG4
PORTG3
PORTG2
PORTG1
0x13 (0x33)
DDRG
-
-
DDG5
DDG4
DDG3
DDG2
DDG1
DDG0
0x12 (0x32)
PING
-
-
PING5
PING4
PING3
PING2
PING1
PING0
0x11 (0x31)
PORTF
PORTF7
PORTF6
PORTF5
PORTF4
PORTF3
PORTF2
PORTF1
PORTF0
0x10 (0x30)
DDRF
DDF7
DDF6
DDF5
DDF4
DDF3
DDF2
DDF1
DDF0
0x0F (0x2F)
PINF
PINF7
PINF6
PINF5
PINF4
PINF3
PINF2
PINF1
PINF0
PORTE0
0x0E (0x2E)
PORTE
PORTE7
PORTE6
PORTE5
PORTE4
PORTE3
PORTE2
PORTE1
0x0D (0x2D)
DDRE
DDE7
DDE6
DDE5
DDE4
DDE3
DDE2
DDE1
DDE0
0x0C (0x2C)
PINE
PINE7
PINE6
PINE5
PINE4
PINE3
PINE2
PINE1
PINE0
0x0B (0x2B)
PORTD
PORTD7
PORTD6
PORTD5
PORTD4
PORTD3
PORTD2
PORTD1
PORTD0
0x0A (0x2A)
DDRD
DDD7
DDD6
DDD5
DDD4
DDD3
DDD2
DDD1
DDD0
0x09 (0x29)
PIND
PIND7
PIND6
PIND5
PIND4
PIND3
PIND2
PIND1
PIND0
0x08 (0x28)
PORTC
PORTC7
PORTC6
PORTC5
PORTC4
PORTC3
PORTC2
PORTC1
PORTC0
0x07 (0x27)
DDRC
DDC7
DDC6
DDC5
DDC4
DDC3
DDC2
DDC1
DDC0
0x06 (0x26)
PINC
PINC7
PINC6
PINC5
PINC4
PINC3
PINC2
PINC1
PINC0
0x05 (0x25)
PORTB
PORTB7
PORTB6
PORTB5
PORTB4
PORTB3
PORTB2
PORTB1
PORTB0
0x04 (0x24)
DDRB
DDB7
DDB6
DDB5
DDB4
DDB3
DDB2
DDB1
DDB0
0x03 (0x23)
PINB
PINB7
PINB6
PINB5
PINB4
PINB3
PINB2
PINB1
PINB0
0x02 (0x22)
PORTA
PORTA7
PORTA6
PORTA5
PORTA4
PORTA3
PORTA2
PORTA1
PORTA0
0x01 (0x21)
DDRA
DDA7
DDA6
DDA5
DDA4
DDA3
DDA2
DDA1
DDA0
0x00 (0x20)
PINA
PINA7
PINA6
PINA5
PINA4
PINA3
PINA2
PINA1
PINA0
Note:
Page
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 $00 - $1F are directly bit-accessible using the SBI and CBI instructions. In these registers, the value of single bits can be checked by using the SBIS and SBIC instructions.
3. Some of the status flags are cleared by writing a logical one to them. Note that the CBI and SBI instructions will operate on
all bits in the I/O register, writing a one back into any flag read as set, thus clearing the flag. The CBI and SBI instructions
work with registers 0x00 to 0x1F only.
4. When using the I/O specific commands IN and OUT, the I/O addresses $00 - $3F must be used. When addressing I/O registers as data space using LD and ST instructions, $20 must be added to these addresses. The
ATmega640/1280/1281/2560/2561 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 $60 - $1FF in SRAM,
only the ST/STS/STD and LD/LDS/LDD instructions can be used.
389
2549A–AVR–03/05
Instruction Set Summary
Mnemonics
Operands
Description
Operation
Flags
#Clocks
ARITHMETIC AND LOGIC INSTRUCTIONS
ADD
Rd, Rr
Add two Registers
Rd ← Rd + Rr
Z,C,N,V,H
ADC
Rd, Rr
Add with Carry two Registers
Rd ← Rd + Rr + C
Z,C,N,V,H
1
ADIW
Rdl,K
Add Immediate to Word
Rdh:Rdl ← Rdh:Rdl + K
Z,C,N,V,S
2
SUB
Rd, Rr
Subtract two Registers
Rd ← Rd - Rr
Z,C,N,V,H
1
SUBI
Rd, K
Subtract Constant from Register
Rd ← Rd - K
Z,C,N,V,H
1
SBC
Rd, Rr
Subtract with Carry two Registers
Rd ← Rd - Rr - C
Z,C,N,V,H
1
SBCI
Rd, K
Subtract with Carry Constant from Reg.
Rd ← Rd - K - C
Z,C,N,V,H
1
SBIW
Rdl,K
Subtract Immediate from Word
Rdh:Rdl ← Rdh:Rdl - K
Z,C,N,V,S
2
AND
Rd, Rr
Logical AND Registers
Rd ← Rd • Rr
Z,N,V
1
ANDI
Rd, K
Logical AND Register and Constant
Rd ← Rd • K
Z,N,V
1
OR
Rd, Rr
Logical OR Registers
Rd ← Rd v Rr
Z,N,V
1
ORI
Rd, K
Logical OR Register and Constant
Rd ← Rd v K
Z,N,V
1
EOR
Rd, Rr
Exclusive OR Registers
Rd ← Rd ⊕ Rr
Z,N,V
1
1
COM
Rd
One’s Complement
Rd ← 0xFF − Rd
Z,C,N,V
1
NEG
Rd
Two’s Complement
Rd ← 0x00 − Rd
Z,C,N,V,H
1
SBR
Rd,K
Set Bit(s) in Register
Rd ← Rd v K
Z,N,V
1
CBR
Rd,K
Clear Bit(s) in Register
Rd ← Rd • (0xFF - K)
Z,N,V
1
INC
Rd
Increment
Rd ← Rd + 1
Z,N,V
1
DEC
Rd
Decrement
Rd ← Rd − 1
Z,N,V
1
TST
Rd
Test for Zero or Minus
Rd ← Rd • Rd
Z,N,V
1
CLR
Rd
Clear Register
Rd ← Rd ⊕ Rd
Z,N,V
1
SER
Rd
Set Register
Rd ← 0xFF
None
1
MUL
Rd, Rr
Multiply Unsigned
R1:R0 ← Rd x Rr
Z,C
2
MULS
Rd, Rr
Multiply Signed
R1:R0 ← Rd x Rr
Z,C
2
MULSU
Rd, Rr
Multiply Signed with Unsigned
R1:R0 ← Rd x Rr
Z,C
2
FMUL
Rd, Rr
Fractional Multiply Unsigned
R1:R0 ← (Rd x Rr) <<
1
R1:R0 ← (Rd x Rr) << 1
R1:R0 ← (Rd x Rr) << 1
Z,C
2
Z,C
2
Z,C
2
2
FMULS
Rd, Rr
Fractional Multiply Signed
FMULSU
Rd, Rr
Fractional Multiply Signed with Unsigned
BRANCH INSTRUCTIONS
Relative Jump
PC ← PC + k + 1
None
IJMP
Indirect Jump to (Z)
None
2
EIJMP
Extended Indirect Jump to (Z)
PC ← Z
PC ←(EIND:Z)
None
2
RJMP
k
JMP
k
Direct Jump
PC ← k
None
3
RCALL
k
Relative Subroutine Call
PC ← PC + k + 1
None
4
ICALL
Indirect Call to (Z)
4
Extended Indirect Call to (Z)
PC ← Z
PC ←(EIND:Z)
None
EICALL
None
4
Direct Subroutine Call
PC ← k
None
5
RET
Subroutine Return
PC ← STACK
None
5
RETI
Interrupt Return
PC ← STACK
I
5
CALL
k
CPSE
Rd,Rr
Compare, Skip if Equal
if (Rd = Rr) PC ← PC + 2 or 3
None
CP
Rd,Rr
Compare
Rd − Rr
Z, N,V,C,H
1
CPC
Rd,Rr
Compare with Carry
Rd − Rr − C
Z, N,V,C,H
1
CPI
Rd,K
Compare Register with Immediate
Rd − K
Z, N,V,C,H
SBRC
Rr, b
Skip if Bit in Register Cleared
if (Rr(b)=0) PC ← PC + 2 or 3
None
1/2/3
1/2/3
1
SBRS
Rr, b
Skip if Bit in Register is Set
if (Rr(b)=1) PC ← PC + 2 or 3
None
1/2/3
SBIC
P, b
Skip if Bit in I/O Register Cleared
if (P(b)=0) PC ← PC + 2 or 3
None
1/2/3
SBIS
P, b
Skip if Bit in I/O Register is Set
if (P(b)=1) PC ← PC + 2 or 3
None
1/2/3
BRBS
s, k
Branch if Status Flag Set
if (SREG(s) = 1) then PC←PC+k + 1
None
1/2
BRBC
s, k
Branch if Status Flag Cleared
if (SREG(s) = 0) then PC←PC+k + 1
None
1/2
BREQ
k
Branch if Equal
if (Z = 1) then PC ← PC + k + 1
None
1/2
BRNE
k
Branch if Not Equal
if (Z = 0) then PC ← PC + k + 1
None
1/2
BRCS
k
Branch if Carry Set
if (C = 1) then PC ← PC + k + 1
None
1/2
BRCC
k
Branch if Carry Cleared
if (C = 0) then PC ← PC + k + 1
None
1/2
BRSH
k
Branch if Same or Higher
if (C = 0) then PC ← PC + k + 1
None
1/2
BRLO
k
Branch if Lower
if (C = 1) then PC ← PC + k + 1
None
1/2
BRMI
k
Branch if Minus
if (N = 1) then PC ← PC + k + 1
None
1/2
BRPL
k
Branch if Plus
if (N = 0) then PC ← PC + k + 1
None
1/2
BRGE
k
Branch if Greater or Equal, Signed
if (N ⊕ V= 0) then PC ← PC + k + 1
None
1/2
BRLT
k
Branch if Less Than Zero, Signed
if (N ⊕ V= 1) then PC ← PC + k + 1
None
1/2
BRHS
k
Branch if Half Carry Flag Set
if (H = 1) then PC ← PC + k + 1
None
1/2
BRHC
k
Branch if Half Carry Flag Cleared
if (H = 0) then PC ← PC + k + 1
None
1/2
BRTS
k
Branch if T Flag Set
if (T = 1) then PC ← PC + k + 1
None
1/2
BRTC
k
Branch if T Flag Cleared
if (T = 0) then PC ← PC + k + 1
None
1/2
390
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Mnemonics
Operands
Description
Operation
Flags
#Clocks
BRVS
k
Branch if Overflow Flag is Set
if (V = 1) then PC ← PC + k + 1
None
1/2
BRVC
k
Branch if Overflow Flag is Cleared
if (V = 0) then PC ← PC + k + 1
None
1/2
BRIE
k
Branch if Interrupt Enabled
if ( I = 1) then PC ← PC + k + 1
None
1/2
BRID
k
Branch if Interrupt Disabled
if ( I = 0) then PC ← PC + k + 1
None
1/2
BIT AND BIT-TEST INSTRUCTIONS
SBI
P,b
Set Bit in I/O Register
I/O(P,b) ← 1
None
2
CBI
P,b
Clear Bit in I/O Register
I/O(P,b) ← 0
None
2
LSL
Rd
Logical Shift Left
Rd(n+1) ← Rd(n), Rd(0) ← 0
Z,C,N,V
1
LSR
Rd
Logical Shift Right
Rd(n) ← Rd(n+1), Rd(7) ← 0
Z,C,N,V
1
ROL
Rd
Rotate Left Through Carry
Rd(0)←C,Rd(n+1)← Rd(n),C←Rd(7)
Z,C,N,V
1
ROR
Rd
Rotate Right Through Carry
Rd(7)←C,Rd(n)← Rd(n+1),C←Rd(0)
Z,C,N,V
1
ASR
Rd
Arithmetic Shift Right
Rd(n) ← Rd(n+1), n=0..6
Z,C,N,V
1
SWAP
Rd
Swap Nibbles
Rd(3..0)←Rd(7..4),Rd(7..4)←Rd(3..0)
None
1
BSET
s
Flag Set
SREG(s) ← 1
SREG(s)
1
BCLR
s
Flag Clear
SREG(s) ← 0
SREG(s)
1
BST
Rr, b
Bit Store from Register to T
T ← Rr(b)
T
1
BLD
Rd, b
Bit load from T to Register
Rd(b) ← T
None
1
SEC
Set Carry
C←1
C
1
CLC
Clear Carry
C←0
C
1
SEN
Set Negative Flag
N←1
N
1
CLN
Clear Negative Flag
N←0
N
1
SEZ
Set Zero Flag
Z←1
Z
1
CLZ
Clear Zero Flag
Z←0
Z
1
SEI
Global Interrupt Enable
I←1
I
1
CLI
Global Interrupt Disable
I←0
I
1
SES
Set Signed Test Flag
S←1
S
1
CLS
Clear Signed Test Flag
S←0
S
1
SEV
Set Twos Complement Overflow.
V←1
V
1
CLV
Clear Twos Complement Overflow
V←0
V
1
SET
Set T in SREG
T←1
T
1
CLT
Clear T in SREG
T←0
T
1
SEH
CLH
Set Half Carry Flag in SREG
Clear Half Carry Flag in SREG
H←1
H←0
H
H
1
1
Rd ← Rr
Rd+1:Rd ← Rr+1:Rr
None
1
None
1
1
DATA TRANSFER INSTRUCTIONS
MOV
Rd, Rr
Move Between Registers
MOVW
Rd, Rr
Copy Register Word
LDI
Rd, K
Load Immediate
Rd ← K
None
LD
Rd, X
Load Indirect
Rd ← (X)
None
2
LD
Rd, X+
Load Indirect and Post-Inc.
Rd ← (X), X ← X + 1
None
2
2
LD
Rd, - X
Load Indirect and Pre-Dec.
X ← X - 1, Rd ← (X)
None
LD
Rd, Y
Load Indirect
Rd ← (Y)
None
2
LD
Rd, Y+
Load Indirect and Post-Inc.
Rd ← (Y), Y ← Y + 1
None
2
LD
Rd, - Y
Load Indirect and Pre-Dec.
Y ← Y - 1, Rd ← (Y)
None
2
LDD
Rd,Y+q
Load Indirect with Displacement
Rd ← (Y + q)
None
2
LD
Rd, Z
Load Indirect
Rd ← (Z)
None
2
LD
Rd, Z+
Load Indirect and Post-Inc.
Rd ← (Z), Z ← Z+1
None
2
LD
Rd, -Z
Load Indirect and Pre-Dec.
Z ← Z - 1, Rd ← (Z)
None
2
LDD
Rd, Z+q
Load Indirect with Displacement
Rd ← (Z + q)
None
2
LDS
Rd, k
Load Direct from SRAM
Rd ← (k)
None
2
ST
X, Rr
Store Indirect
(X) ← Rr
None
2
ST
X+, Rr
Store Indirect and Post-Inc.
(X) ← Rr, X ← X + 1
None
2
ST
- X, Rr
Store Indirect and Pre-Dec.
X ← X - 1, (X) ← Rr
None
2
ST
Y, Rr
Store Indirect
(Y) ← Rr
None
2
ST
Y+, Rr
Store Indirect and Post-Inc.
(Y) ← Rr, Y ← Y + 1
None
2
ST
- Y, Rr
Store Indirect and Pre-Dec.
Y ← Y - 1, (Y) ← Rr
None
2
STD
Y+q,Rr
Store Indirect with Displacement
(Y + q) ← Rr
None
2
ST
Z, Rr
Store Indirect
(Z) ← Rr
None
2
ST
Z+, Rr
Store Indirect and Post-Inc.
(Z) ← Rr, Z ← Z + 1
None
2
ST
-Z, Rr
Store Indirect and Pre-Dec.
Z ← Z - 1, (Z) ← Rr
None
2
STD
Z+q,Rr
Store Indirect with Displacement
(Z + q) ← Rr
None
2
STS
k, Rr
Store Direct to SRAM
(k) ← Rr
None
2
Load Program Memory
R0 ← (Z)
None
3
LPM
LPM
Rd, Z
Load Program Memory
Rd ← (Z)
None
3
LPM
Rd, Z+
Load Program Memory and Post-Inc
Rd ← (Z), Z ← Z+1
None
3
ELPM
ELPM
Rd, Z
Extended Load Program Memory
R0 ← (RAMPZ:Z)
None
3
Extended Load Program Memory
Rd ← (Z)
None
3
391
2549A–AVR–03/05
Mnemonics
ELPM
Operands
Rd, Z+
SPM
Description
Operation
Flags
#Clocks
Extended Load Program Memory
Rd ← (RAMPZ:Z), RAMPZ:Z ←RAMPZ:Z+1
None
Store Program Memory
(Z) ← R1:R0
None
-
Rd ← P
None
1
3
IN
Rd, P
In Port
OUT
P, Rr
Out Port
P ← Rr
None
1
PUSH
Rr
Push Register on Stack
STACK ← Rr
None
2
POP
Rd
Pop Register from Stack
Rd ← STACK
None
2
MCU CONTROL INSTRUCTIONS
NOP
No Operation
None
1
SLEEP
Sleep
(see specific descr. for Sleep function)
None
1
WDR
BREAK
Watchdog Reset
Break
(see specific descr. for WDR/timer)
For On-chip Debug Only
None
None
1
N/A
Note:
392
EICALL and EIJMP do not exist in ATmega640/1280/1281.
ELPM does not exist in ATmega640.
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Ordering Information
ATmega1281/2561
Speed (MHz)(2)
8
16
Notes:
Ordering Code
Package(1)
Operation Range
1.8 - 5.5V
ATmega1281/2561V-8AI
ATmega1281/2561V-8AU(3)
ATmega1281/2561V-8MI
ATmega1281/2561V-8MU(3)
64A
64A
64M1
64M1
Industrial
(-40°C to 85°C)
4.5 - 5.5V
ATmega1281/2561-16AI
ATmega1281/2561-16AU(3)
ATmega1281/2561-16MI
ATmega1281/2561-16MU(3)
64A
64M1
64A
64M1
Industrial
(-40°C to 85°C)
Power Supply
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. See “Maximum speed vs. VCC” on page 370.
3. Pb-free packaging alternative, complies to the European Directive for Restriction of Hazardous Substances (RoHS directive). Also Halide free and fully Green.
Package Type
64A
64-Lead, Thin (1.0 mm) Plastic Gull Wing Quad Flat Package (TQFP)
64M1
64-pad, 9 x 9 x 1.0 mm body, lead pitch 0.50 mm, Micro Lead Frame Package (QFN/MLF)
100A
100-Lead, Thin (1.0 mm) Plastic Gull Wing Quad Flat Package (TQFP)
393
2549A–AVR–03/05
ATmega640/1280/2560
Speed (MHz)(2)
8
16
Notes:
Ordering Code
Package(1)
1.8 - 5.5V
ATmega640/1280/2560V-8AI
ATmega640/1280/2560V-8AU(3)
100A
100A
Industrial (-40°C to 85°C)
4.5 - 5.5V
ATmega640/1280/2560-16AI
ATmega640/1280/2560-16AU(3)
100A
100A
Industrial (-40°C to 85°C)
Power Supply
Operation Range
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. See “Maximum speed vs. VCC” on page 370.
3. Pb-free packaging alternative, complies to the European Directive for Restriction of Hazardous Substances (RoHS directive). Also Halide free and fully Green.
Package Type
64A
64-Lead, Thin (1.0 mm) Plastic Gull Wing Quad Flat Package (TQFP)
64M1
64-pad, 9 x 9 x 1.0 mm body, lead pitch 0.50 mm, Micro Lead Frame Package (QFN/MLF)
100A
100-Lead, Thin (1.0 mm) Plastic Gull Wing Quad Flat Package (TQFP)
394
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Packaging Information
100A
PIN 1
B
PIN 1 IDENTIFIER
E1
e
E
D1
D
C
0˚~7˚
A1
A2
A
L
COMMON DIMENSIONS
(Unit of Measure = mm)
MIN
NOM
MAX
A
–
–
1.20
A1
0.05
–
0.15
SYMBOL
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.
3. Lead coplanarity is 0.08 mm maximum.
A2
0.95
1.00
1.05
D
15.75
16.00
16.25
D1
13.90
14.00
14.10
E
15.75
16.00
16.25
E1
13.90
14.00
14.10
B
0.17
–
0.27
C
0.09
–
0.20
L
0.45
–
0.75
e
NOTE
Note 2
Note 2
0.50 TYP
10/5/2001
R
2325 Orchard Parkway
San Jose, CA 95131
TITLE
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)
DRAWING NO.
100A
REV.
C
395
2549A–AVR–03/05
64A
PIN 1
B
PIN 1 IDENTIFIER
E1
e
E
D1
D
C
0˚~7˚
A1
A2
A
L
COMMON DIMENSIONS
(Unit of Measure = mm)
Notes:
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.
3. Lead coplanarity is 0.10 mm maximum.
SYMBOL
MIN
NOM
MAX
A
–
–
1.20
A1
0.05
–
0.15
A2
0.95
1.00
1.05
D
15.75
16.00
16.25
D1
13.90
14.00
14.10
E
15.75
16.00
16.25
E1
13.90
14.00
14.10
B
0.30
–
0.45
C
0.09
–
0.20
L
0.45
–
0.75
e
NOTE
Note 2
Note 2
0.80 TYP
10/5/2001
R
396
2325 Orchard Parkway
San Jose, CA 95131
TITLE
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)
DRAWING NO.
REV.
64A
B
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
64M1
D
Marked Pin# 1 ID
E
C
SEATING PLANE
A1
TOP VIEW
A
K
0.08 C
L
Pin #1 Corner
D2
1
2
3
Option A
SIDE VIEW
Pin #1
Triangle
COMMON DIMENSIONS
(Unit of Measure = mm)
E2
Option B
Pin #1
Chamfer
(C 0.30)
SYMBOL
MIN
NOM
MAX
A
0.80
0.90
1.00
–
0.02
0.05
0.23
0.25
0.28
A1
b
D
K
Option C
b
e
D2
Pin #1
Notch
(0.20 R)
BOTTOM VIEW
Note: JEDEC Standard MO-220, (SAW Singulation) Fig. 1, VMMD.
9.00 BSC
5.20
E
E2
NOTE
5.40
5.60
9.00 BSC
5.20
e
5.40
5.60
0.50 BSC
L
0.35
0.40
0.45
K
0.20
–
–
8/19/04
R
2325 Orchard Parkway
San Jose, CA 95131
TITLE
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)
DRAWING NO.
64M1
REV.
D
397
2549A–AVR–03/05
Errata
ATmega640 rev. A
No errata.
ATmega1280 rev. A
No errata.
ATmega1280 rev. A
No errata.
ATmega2560 rev. A
• Non-Read-While-Write area of flash not functional
• Part does not work under 2.0 Volts
1.
Non-Read-While-Write area of flash not functional
The Non-Read-While-Write area of the flash is not working as expected. The problem is related to the speed of the part when reading the flash of this area.
Problem Fix/Workaround
- Only use the first 248K of the flash.
- If boot functionality is needed, run the code in the Non-Read-While-Write area at
maximum 1/4th of the maximum frequency of the device at any given voltage. This
is done by writing the CLKPR register before entering the boot section of the code
2. Part does not work under 2.0 Volts
The part does not execute code correctly below 2.0 Volts
Problem Fix/Workaround
Do not use the part at voltages below 2.0 Volts.
ATmega2561 rev. A
• Non-Read-While-Write area of flash not functional
• Part does not work under 2.0 Volts
1.
Non-Read-While-Write area of flash not functional
The Non-Read-While-Write area of the flash is not working as expected. The problem is related to the speed of the part when reading the flash of this area.
Problem Fix/Workaround
- Only use the first 248K of the flash.
- If boot functionality is needed, run the code in the Non-Read-While-Write area at
maximum 1/4th of the maximum frequency of the device at any given voltage. This
is done by writing the CLKPR register before entering the boot section of the code
2. Part does not work under 2.0 Volts
The part does not execute code correctly below 2.0 Volts
Problem Fix/Workaround
Do not use the part at voltages below 2.0 Volts.
398
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
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.
Rev. 2549A-12/04
1.
Initial version.
399
2549A–AVR–03/05
400
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Table of Contents
Features................................................................................................ 1
Pin Configurations............................................................................... 2
Disclaimer ............................................................................................................. 3
Overview............................................................................................... 4
Block Diagram ...................................................................................................... 4
Comparison Between ATmega2561 and ATmega2560 ....................................... 5
Pin Descriptions.................................................................................................... 6
About Code Examples......................................................................... 8
AVR CPU Core ..................................................................................... 9
Introduction ........................................................................................................... 9
Architectural Overview.......................................................................................... 9
ALU – Arithmetic Logic Unit................................................................................ 10
Status Register ................................................................................................... 10
General Purpose Register File ........................................................................... 12
Stack Pointer ...................................................................................................... 13
Instruction Execution Timing............................................................................... 14
Reset and Interrupt Handling.............................................................................. 15
AVR ATmega256/2560 Memories ..................................................... 18
In-System Reprogrammable Flash Program Memory ........................................
SRAM Data Memory...........................................................................................
EEPROM Data Memory......................................................................................
I/O Memory .........................................................................................................
External Memory Interface..................................................................................
18
19
21
27
28
System Clock and Clock Options .................................................... 37
Clock Systems and their Distribution ..................................................................
Clock Sources.....................................................................................................
Low Power Crystal Oscillator..............................................................................
Full Swing Crystal Oscillator ...............................................................................
Low Frequency Crystal Oscillator .......................................................................
Calibrated Internal RC Oscillator ........................................................................
128 kHz Internal Oscillator..................................................................................
External Clock.....................................................................................................
Clock Output Buffer ............................................................................................
Timer/Counter Oscillator.....................................................................................
System Clock Prescaler......................................................................................
37
38
39
41
43
43
45
45
46
46
46
Power Management and Sleep Modes............................................. 49
Idle Mode ............................................................................................................ 50
ADC Noise Reduction Mode............................................................................... 50
Power-down Mode.............................................................................................. 50
i
2549A–AVR–03/05
Power-save Mode...............................................................................................
Standby Mode.....................................................................................................
Extended Standby Mode ....................................................................................
Power Reduction Register ..................................................................................
Minimizing Power Consumption .........................................................................
50
51
51
52
53
System Control
and Reset............................................................................................ 55
Internal Voltage Reference ................................................................................. 60
Watchdog Timer ................................................................................................. 61
Interrupts ............................................................................................ 66
Interrupt Vectors in ATmega256/2560................................................................ 66
I/O-Ports.............................................................................................. 72
Introduction ......................................................................................................... 72
Ports as General Digital I/O ................................................................................ 73
Alternate Port Functions ..................................................................................... 77
Register Description for I/O-Ports..................................................................... 103
External Interrupts........................................................................... 107
8-bit Timer/Counter0 with PWM...................................................... 112
Overview...........................................................................................................
Timer/Counter Clock Sources...........................................................................
Counter Unit......................................................................................................
Output Compare Unit........................................................................................
Compare Match Output Unit .............................................................................
Modes of Operation ..........................................................................................
Timer/Counter Timing Diagrams.......................................................................
8-bit Timer/Counter Register Description .........................................................
112
113
113
114
115
116
121
123
16-bit Timer/Counter (Timer/Counter1, Timer/Counter3, Timer/Counter4 and Timer/Counter5) .................................................. 129
Overview...........................................................................................................
Accessing 16-bit Registers ...............................................................................
Timer/Counter Clock Sources...........................................................................
Counter Unit......................................................................................................
Input Capture Unit.............................................................................................
Output Compare Units ......................................................................................
Compare Match Output Unit .............................................................................
Modes of Operation ..........................................................................................
Timer/Counter Timing Diagrams.......................................................................
16-bit Timer/Counter Register Description .......................................................
129
131
135
135
136
138
140
141
148
151
Timer/Counter0, Timer/Counter1, Timer/Counter3, Timer/Counter4,
ii
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
and Timer/Counter5 Prescalers...................................................... 163
Output Compare Modulator (OCM1C0A) ....................................... 165
Overview........................................................................................................... 165
Description........................................................................................................ 165
8-bit Timer/Counter2 with PWM and Asynchronous Operation .. 167
Overview...........................................................................................................
Timer/Counter Clock Sources...........................................................................
Counter Unit......................................................................................................
Output Compare Unit........................................................................................
Compare Match Output Unit .............................................................................
Modes of Operation ..........................................................................................
Timer/Counter Timing Diagrams.......................................................................
8-bit Timer/Counter Register Description .........................................................
Asynchronous operation of the Timer/Counter .................................................
Timer/Counter Prescaler...................................................................................
167
168
168
169
171
172
176
178
183
187
Serial Peripheral Interface – SPI..................................................... 188
SS Pin Functionality.......................................................................................... 193
Data Modes ...................................................................................................... 195
USART .............................................................................................. 197
Overview...........................................................................................................
Clock Generation ..............................................................................................
Frame Formats .................................................................................................
USART Initialization..........................................................................................
Data Transmission – The USART Transmitter .................................................
Data Reception – The USART Receiver ..........................................................
Asynchronous Data Reception .........................................................................
Multi-processor Communication Mode .............................................................
USART Register Description ............................................................................
Examples of Baud Rate Setting........................................................................
197
199
202
203
204
206
210
213
214
219
USART in SPI Mode ......................................................................... 223
Overview...........................................................................................................
Clock Generation ..............................................................................................
SPI Data Modes and Timing.............................................................................
Frame Formats .................................................................................................
Data Transfer....................................................................................................
USART MSPIM Register Description ...............................................................
AVR USART MSPIM vs.
AVR SPI............................................................................................................
223
223
223
224
226
228
230
2-wire Serial Interface...................................................................... 232
Features............................................................................................................ 232
iii
2549A–AVR–03/05
2-wire Serial Interface Bus Definition................................................................
Data Transfer and Frame Format .....................................................................
Multi-master Bus Systems, Arbitration and Synchronization ............................
Overview of the TWI Module ............................................................................
TWI Register Description..................................................................................
Using the TWI ...................................................................................................
Transmission Modes.........................................................................................
Multi-master Systems and Arbitration...............................................................
232
233
236
238
240
243
247
260
Analog Comparator ......................................................................... 262
Analog Comparator Multiplexed Input .............................................................. 264
Analog to Digital Converter ............................................................ 265
Features............................................................................................................
Operation ..........................................................................................................
Starting a Conversion .......................................................................................
Prescaling and Conversion Timing ...................................................................
Changing Channel or Reference Selection ......................................................
ADC Noise Canceler.........................................................................................
ADC Conversion Result....................................................................................
265
266
267
268
271
272
277
JTAG Interface and On-chip Debug System ................................. 285
Overview...........................................................................................................
Test Access Port – TAP....................................................................................
TAP Controller ..................................................................................................
Using the Boundary-scan Chain .......................................................................
Using the On-chip Debug System ....................................................................
On-chip Debug Specific JTAG Instructions ......................................................
On-chip Debug Related Register in I/O Memory ..............................................
Using the JTAG Programming Capabilities ......................................................
Bibliography ......................................................................................................
285
285
287
288
288
289
290
290
290
IEEE 1149.1 (JTAG) Boundary-scan .............................................. 291
Features............................................................................................................
System Overview..............................................................................................
Data Registers ..................................................................................................
Boundary-scan Specific JTAG Instructions ......................................................
Boundary-scan Related Register in I/O Memory ..............................................
Boundary-scan Chain .......................................................................................
ATmega256/2560 Boundary-scan Order..........................................................
Boundary-scan Description Language Files .....................................................
291
291
291
293
294
295
298
305
Boot Loader Support – Read-While-Write Self-Programming ..... 306
Boot Loader Features ....................................................................................... 306
Application and Boot Loader Flash Sections .................................................... 306
Read-While-Write and No Read-While-Write Flash Sections........................... 306
iv
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATmega640/1280/1281/2560/2561
Boot Loader Lock Bits.......................................................................................
Entering the Boot Loader Program ...................................................................
Addressing the Flash During Self-Programming ..............................................
Self-Programming the Flash .............................................................................
308
309
311
312
Memory Programming..................................................................... 320
Program And Data Memory Lock Bits ..............................................................
Fuse Bits...........................................................................................................
Signature Bytes ................................................................................................
Calibration Byte ................................................................................................
Parallel Programming Parameters, Pin Mapping, and Commands ..................
Parallel Programming .......................................................................................
Serial Downloading...........................................................................................
Serial Programming Pin Mapping .....................................................................
Programming via the JTAG Interface ...............................................................
320
321
323
323
323
326
334
334
338
Electrical Characteristics................................................................ 350
Absolute Maximum Ratings*.............................................................................
DC Characteristics............................................................................................
External Clock Drive Waveforms ......................................................................
External Clock Drive .........................................................................................
Maximum speed vs. VCC ...................................................................................
2-wire Serial Interface Characteristics ..............................................................
SPI Timing Characteristics ...............................................................................
ADC Characteristics – Preliminary Data...........................................................
External Data Memory Timing ..........................................................................
350
350
352
352
353
354
355
357
359
ATmega256/2560 Typical Characteristics – Preliminary Data..... 364
Supply Current of IO modules .......................................................................... 364
Register Summary ........................................................................... 367
Instruction Set Summary ................................................................ 372
Ordering Information....................................................................... 375
ATmega2561 .................................................................................................... 375
ATmega2560 .................................................................................................... 376
Packaging Information .................................................................... 377
100A ................................................................................................................. 377
64A ................................................................................................................... 378
64M1................................................................................................................. 379
Table of Contents ................................................................................. i
v
2549A–AVR–03/05
vi
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
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