8.8
External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
8-4 on page 36. To run the device on an external clock, the CKSEL Fuses must be programmed
to “0000” (see Table 8-12).
Table 8-12. Crystal Oscillator Clock Frequency
Frequency
CKSEL3..0
0 - 20 MHz
0000
Figure 8-4. External Clock Drive Configuration
NC
XTAL2
EXTERNAL
CLOCK
XTAL1
GND
SIGNAL
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-13.
Table 8-13. Start-up Times for the External Clock Selection
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
Power Conditions
BOD enabled
SUT1..0
00
6 CK
6 CK
14CK
Fast rising power
14CK + 4.1 ms
14CK + 65 ms
01
Slowly rising power
6 CK
10
Reserved
11
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.
8.9
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 cir-
cuits 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
36
ATmega48/88/168
2545M–AVR–09/07
ATMEL [ ATMEL ]
