ATmega165 datasheet(23/336 Pages) ATMEL

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ATmega165/V

2573G–AVR–07/09

System Clock and

Clock Options

Clock Systems and their

Distribution

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

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

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

described in “Power Management and Sleep Modes” on page 32. The clock systems

are detailed below.

Figure 11. Clock Distribution

CPU Clock – clk

CPU

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

core. Examples of such modules are the General Purpose Register File, the Status Reg-

ister and the data memory holding the Stack Pointer. Halting the CPU clock inhibits the

core from performing general operations and calculations.

I/O Clock – clk

I/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 clk

I/O is halted, enabling USI start condition

detection in all sleep modes.

Flash Clock – clk

FLASH

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

active simultaneously with the CPU clock.

Asynchronous Timer Clock –

clk

ASY

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.

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

System Clock

Prescaler

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