8-bit AVR Microcontrollers

External Clock

To drive the device from an external clock source, EXTCLK 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 1. External Clock Frequency
Frequency CKSEL[3:0]
0 - 20 MHz 0000
Figure 1. External Clock Drive Configuration

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

Table 2. Start-Up Times for the External Clock Selection - SUT
Power Conditions Start-Up Time from Power-Down and Power-Save Additional Delay from Reset (VCC = 5.0V) SUT[1:0]
BOD enabled 6CK 19CK 00
Fast rising power 6CK 19CK + 4.1 ms 01
Slowly rising power 6CK 19CK + 65 ms 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% are 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.