26

2570L–AVR–08/07

ATmega325/3250/645/6450

9.1.4

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.

9.1.5

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.

9.2

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:

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 an additional delay allowing the power to reach a stable level before com-

mencing 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 Table 9-2

on page 26. The frequency of the Watchdog Oscillator is voltage dependent as shown in “Typi-

cal Characteristics” on page 305.

9.2.1

Default Clock Source

The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default

clock source setting is the Internal RC Oscillator with longest start-up time and an initial system

clock prescaling of 8. This default setting ensures that all users can make their desired clock

source setting using an In-System or Parallel programmer.

Table 9-1.

Device Clocking Options Select

(Note:)

Device Clocking Option

CKSEL3..0

External Crystal/Ceramic Resonator

1111 - 1000

External Low-frequency Crystal

0111 - 0110

Calibrated Internal RC Oscillator

0010

External Clock

0000

Reserved

0011, 0001, 0101, 0100

Table 9-2.

Number of Watchdog Oscillator Cycles

Typ Time-out (V

Typ Time-out (V

Number of Cycles

4.1 ms

4.3 ms

4K (4,096)

65 ms

69 ms

64K (65,536)