ATmega169P
7.7
External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure
7-4. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”.
Figure 7-4. 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 7-12.
Table 7-11. Crystal Oscillator Clock Frequency
CKSEL3..0
Frequency Range
0000
0 - 16 MHz
Table 7-12. Start-up Times for the External Clock Selection
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (VCC = 5.0V)
SUT1..0
00
Recommended Usage
BOD enabled
6 CK
6 CK
6 CK
14CK
01
14CK + 4.1 ms
14CK + 65 ms
Reserved
Fast rising power
Slowly rising power
10
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 2ꢀ from
one clock cycle to the next can lead to unpredictable behavior. It is required to ensure that the
MCU is kept in Reset during such changes in the clock frequency.
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
36 for details.
35
8018A–AVR–03/06
ATMEL [ ATMEL ]
