Datasheet

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

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

shown in Table 23.

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

clock frequency 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 47 for details.

Clock Output Buffer The device can output the system clock on the CLKO pin. To enable the output, the

CKOUT Fuse has to be programmed. This mode is suitable when the chip clock is used

to drive other circuits on the system. The clock also will be output during reset, and the

normal operation of I/O pin will be overridden when the fuse is programmed. Any clock

source, including the internal RC Oscillator, can be selected when the clock is output on

CLKO. If the System Clock Prescaler is used, it is the divided system clock that is

output.

Timer/Counter Oscillator The device can operate its Timer/Counter2 from an external 32.768 kHz watch crystal or

a external clock source. See Figure 21 on page 40 for crystal connection.

Applying an external clock source to TOSC1 requires EXCLK in the ASSR Register writ-

ten to logic one. See “Asynchronous Operation of Timer/Counter2” on page 188 for

further description on selecting external clock as input instead of a 32 kHz crystal.

Table 21. Crystal Oscillator Clock Frequency

Nominal Frequency CKSEL3:0

0 - 16 MHz 0000

Table 22. Start-up Times for the External Clock Selection

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (V

= 5.0V) SUT1:0

BOD enabled 6 CK 14CK 00

Fast rising power 6 CK 14CK + 4.1 ms 01

Slowly rising power 6 CK 14CK + 65 ms 10

Reserved 11