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”. By programming the CKOPT Fuse, the user can enable an internal 36pF capacitor between XTAL1 and GND, and XTAL2 and GND.

Figure 1. External Clock Drive Configuration

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in the following table.

Table 1. Start-up Times for the External Clock Selection
SUT1:0	Start-up Time from  Power-down and  Power-save	Additional Delay  from Reset  (V_CC = 5.0V)	Recommended Usage
00	6 CK	–	BOD enabled
01	6 CK	4.1ms	Fast rising power
10	6 CK	65ms	Slowly rising power
11	Reserved

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. It is required to ensure that the MCU is kept in Reset during such changes in the clock frequency.