14.7 External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in the Figure below. To run the device on an external clock, the CKSEL Fuses must be programmed to '0000':
Frequency | CKSEL[3:0] |
---|---|
0 - 20MHz | 0000 |
When this clock source is selected, start-up times are determined by the SUT Fuses:
Power Conditions | Start-Up Time from Power-down and Power-save | Additional Delay from Reset (VCC = 5.0V) | SUT[1:0] |
---|---|---|---|
BOD enabled | 6 CK | 19CK | 00 |
Fast rising power | 6 CK | 19CK + 4.1ms | 01 |
Slowly rising power | 6 CK | 19CK + 65ms | 10 |
Reserved | 11 |
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.
The System Clock Prescaler can be used to implement run-time changes of the internal clock frequency while still ensuring stable operation.