Datasheet

38
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
9.8 External clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
9-4. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Figure 9-4. External clock drive configuration.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 9-15.
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
39 for details.
Table 9-14. Crystal oscillator clock frequency.
Nominal frequency CKSEL3..0
0 - 20MHz 0000
Table 9-15. Start-up rimes 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