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 128 kHz RC Oscillator 0011
Calibrated Internal RC Oscillator 0010
External Clock 0000
Reserved 0001
Note: For all fuses, '1' means unprogrammed while '0' means programmed.