ATtiny44 datasheet(25/236 Pages) ATMEL

25 / 236 page

8006H–AVR–10/09

ATtiny24/44/84

6.1.2

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.

6.1.3

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.

6.1.4

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.

6.2

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.

Note:

1. For all fuses “1” means unprogrammed and “0” means programmed.

The various choices for each clocking option is given in the following sections. When the CPU

wakes up from Power-down the selected clock source is used to time the start-up, ensuring sta-

ble Oscillator operation before instruction execution starts. When the CPU starts from reset,

there is an additional delay allowing the power to reach a stable level before commencing nor-

mal operation. The Watchdog Oscillator is used for timing this real-time part of the start-up time.

The number of WDT Oscillator cycles used for each time-out is shown in Table 6-2.

Table 6-1.

Device Clocking Options Select

Device Clocking Option

CKSEL3:0

(1)

External Clock (see page 26)

0000

Reserved

0001

Calibrated Internal 8 MHz Oscillator (see page 26)

0010

Reserved

0011

Internal 128 kHz Oscillator (see page 27)

0100

Reserved

0101

Low-Frequency Crystal Oscillator (see page 28)

0110

Reserved

0111

Crystal Oscillator / Ceramic Resonator (see page 28)

1000-1111

Table 6-2.

Number of Watchdog Oscillator Cycles

Typ Time-out

Number of Cycles

4 ms

512

64 ms

8K (8,192)

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