ATMEGA261 datasheet(25/226 Pages) ATMEL

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7753E–AVR–06/10

ATtiny261/ATtiny461/ATtiny861

System Clock and Clock Options

7.1

Clock Systems and their Distribution

Figure 7-1 presents the principal clock systems in the AVR and their distribution. All of the clocks

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

not being used can be halted by using different sleep modes, as described in “Power Manage-

ment and Sleep Modes” on page 35. The clock systems are detailed below.

Figure 7-1.

Clock Distribution

7.1.1

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 Register and the

Data memory holding the Stack Pointer. Halting the CPU clock inhibits the core from performing

general operations and calculations.

7.1.2

I/O Clock – clk

I/O

The I/O clock is used by the majority of the I/O modules, like Timer/Counter. 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.

7.1.3

Flash Clock – clk

FLASH

The Flash clock controls operation of the Flash interface. The Flash clock is usually active simul-

taneously with the CPU clock.

7.1.4

ADC Clock – clk

ADC

The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks

in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion

results.

ATMEGA261 Datasheet(PDF) 25 Page - ATMEL Corporation