14.2 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 14-1. Device Clocking Options Select
Device Clocking Option CKSEL[3:0]
Low Power Crystal Oscillator1111 - 1000
Low Frequency Crystal Oscillator0101 - 0100
Internal 128kHz RC Oscillator0011
Calibrated Internal RC Oscillator0010
External Clock0000
Reserved0001
Note: For all fuses, '1' means unprogrammed while '0' means programmed.