ATmega64A datasheet(38/392 Pages) ATMEL

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8160C–AVR–07/09

ATmega64A

8.1.3

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.

8.1.4

Asynchronous Timer Clock – clk

ASY

The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly

from 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.

8.1.5

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.

8.2

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 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 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 8-

2. The frequency of the Watchdog Oscillator is voltage dependent as shown in the “Typical

Characteristics” on page 343.

8.3

Default Clock Source

The device is shipped with CKSEL = “0001” and SUT = “10”. The default clock source setting is

therefore the Internal RC Oscillator with longest startup time. This default setting ensures that all

users can make their desired clock source setting using an In-System or Parallel Programmer.

Table 8-1.

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

Table 8-2.

Number of Watchdog Oscillator Cycles

Typ Time-out (V

CC = 5.0V)

Typ Time-out (V

CC = 3.0V)

Number of Cycles

4.1 ms

4.3 ms

4K (4,096)

65 ms

69 ms

64K (65,536)

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