Datasheet

10.1.1. 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.

10.1.2. 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 the start condition detection in the USI module is

carried out asynchronously when clk

I/O

is halted, TWI address recognition in all sleep modes.

Note: 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.

10.1.3. Flash Clock – clk

FLASH

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

with the CPU clock.

10.1.4. Asynchronous Timer Clock – clk

ASY

The Asynchronous Timer clock allows Asynchronous Timer/Counters to be clocked directly from an

external clock or an external 32kHz clock crystal. The dedicated clock domain allows using this Timer/

Counter as a real-time counter even when the device is in sleep mode.

10.1.5. 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.

10.2. 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 10-1. Device Clocking Options Select

Device Clocking Option CKSEL[3:0]

Low Power Crystal Oscillator 1111 - 1000

Full Swing Crystal Oscillator 0111 - 0110

Low Frequency Crystal Oscillator 0101 - 0100

Internal 128kHz RC Oscillator 0011

Calibrated Internal RC Oscillator 0010

External Clock 0000

Reserved 0001

Note: For all fuses, '1' means unprogrammed while '0' means programmed.

Atmel ATmega644A [DATASHEET]

Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016