ATMEGA640 datasheet(39/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

10. System Clock and Clock Options

This section describes the clock options for the AVR microcontroller.

10.1 Overview

Figure 10-1 presents the principal clock systems in the AVR and their distribution. All of the clocks need not be

active at a given time. In order to reduce power consumption, the clocks to modules not being used can be halted

by using different sleep modes, as described in “Power Management and Sleep Modes” on page 50. The clock

systems are detailed below.

Figure 10-1. Clock Distribution.

10.2 Clock Systems and their Distribution

10.2.1

CPU Clock – clkCPU

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

ules 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.

10.2.2

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.

General I/O

Modules

Asynchronous

Timer/Counter

CPU Core

RAM

clk

I/O

clk

ASY

AVR Clock

Control Unit

clk

CPU

Flash and

EEPROM

clk

FLASH

Source clock

Watchdog Timer

Watchdog

Oscillator

Reset Logic

Clock

Multiplexer

Watchdog clock

Calibrated RC

Oscillator

Timer/Counter

Oscillator

Crystal

Oscillator

Low-frequency

Crystal Oscillator

External Clock

ADC

clk

ADC

System Clock

Prescaler

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