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':

Table 14-14. Crystal Oscillator Clock Frequency
Frequency CKSEL[3:0]
0 - 20MHz0000
Figure 14-3. External Clock Drive Configuration

When this clock source is selected, start-up times are determined by the SUT Fuses:

Table 14-15. Start-Up Times for the External Clock Selection - SUT
Power ConditionsStart-Up Time from Power-down and Power-saveAdditional Delay from Reset (VCC = 5.0V)SUT[1:0]
BOD enabled6 CK19CK00
Fast rising power6 CK19CK + 4.1ms01
Slowly rising power6 CK19CK + 65ms10
Reserved11

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.