Datasheet

46
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
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 49.
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.
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