Datasheet

13.1.3 Flash Clock – clk

FLASH

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

with the CPU clock.

13.1.4 Asynchronous Timer Clock – clk

ASY

The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly from 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. The Asynchronous Timer/Counter uses the same XTAL

pins as the CPU main clock but requires a CPU main clock frequency of more than four times the

Oscillator frequency. Thus, asynchronous operation is only available while the chip is clocked on the

Internal Oscillator.

13.1.5 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.

13.2 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 13-1. Device Clocking Options Select

(1)

Device Clocking Option CKSEL3:0

External Crystal/Ceramic Resonator 1111 - 1010

External Low-frequency Crystal 1001

External RC Oscillator 1000 - 0101

Calibrated Internal RC Oscillator 0100 - 0001

External Clock 0000

Note: 1. For all fuses “1” means unprogrammed while “0” means programmed.

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

from Power-down or Power-save, the selected clock source is used to time the start-up, ensuring stable

Oscillator operation before instruction execution starts. When the CPU starts from reset, there is as an

additional delay allowing the power to reach a stable level before commencing normal 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 the table below. The frequency of the Watchdog

Oscillator is voltage dependent as shown in Typical Characteristics – TA = -40°C to 85°C. The device is

shipped with CKSEL = “0001” and SUT = “10” (1MHz Internal RC Oscillator, slowly rising power).

Table 13-2. Number of Watchdog Oscillator Cycles

Typical Time-out (V

= 5.0V) Typical Time-out (V

= 3.0V) Number of Cycles

4.1ms 4.3ms 4K (4,096)

65ms 69ms 64K (65,536)