Datasheet

48
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 10-12.
10.9 External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
10-4. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Figure 10-4. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 10-15 on page 51.
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.
Table 10-12. Start-up Times for the 128kHz Internal Oscillator
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset SUT1:0
BOD enabled 6CK 14CK 00
Fast rising power 6CK 14CK + 4ms 01
Slowly rising power 6CK 14CK + 64ms 10
Reserved 11
Table 10-13. Crystal Oscillator Clock Frequency
Nominal Frequency CKSEL3:0
0 - 16MHz 0000
Table 10-14. 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 6CK 14CK 00
Fast rising power 6CK 14CK + 4.1ms 01
Slowly rising power 6CK 14CK + 65ms 10
Reserved 11
NC
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND