Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as shown below. The clock from the selected source is input to the AVR clock generator, and routed to the appropriate modules.

Table 1. Device Clocking Options Select
Device Clocking Option	CKSEL[3:0]
Low Power Crystal Oscillator	1111 - 1000
Low Frequency Crystal Oscillator	0101 - 0100
Internal 128kHz RC Oscillator	0011
Calibrated Internal RC Oscillator	0010
External Clock	0000
Reserved	0001

Note: For all fuses, '1' means unprogrammed while '0' means programmed.