ATMEGA644 datasheet(36/374 Pages) ATMEL

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2593L–AVR–02/07

ATmega644

7.8

External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in Figure

7-3. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.

Figure 7-3.

External Clock Drive Configuration

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in

Table 7-13.

When applying an external clock, it is required to avoid sudden changes in the applied clock fre-

quency to ensure stable operation of the MCU. A variation in frequency of more than 2% from

one clock cycle to the next can lead to unpredictable behavior. If changes of more than 2% is

required, ensure that the MCU is kept in Reset during the changes.

Note that the System Clock Prescaler can be used to implement run-time changes of the internal

clock frequency while still ensuring stable operation. Refer to ”System Clock Prescaler” on page

37 for details.

Table 7-12.

Crystal Oscillator Clock Frequency

Nominal Frequency

CKSEL3..0

0 - 20 MHz

0000

Table 7-13.

Start-up Times for the External Clock Selection

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (V

CC = 5.0V)

SUT1..0

BOD enabled

6 CK

14CK

Fast rising power

6 CK

14CK + 4.1 ms

Slowly rising power

6 CK

14CK + 65 ms

Reserved

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

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