As stated in Data Sheet:

bit 7-6 ADCS1:ADCS0: A/D Conversion Clock Select bits
00 = FOSC/2
01 = FOSC/8
10 = FOSC/32
11 = FRC (clock derived from the internal A/D module RC oscillator)


If these are complex things to play with then use the following DEFINEs instead:


For FOSC/2:
DEFINE ADC_CLOCK 0

For FOSC/8:
DEFINE ADC_CLOCK 1

FOSC/32:
DEFINE ADC_CLOCK 2

For A/D RC oscillator:
DEFINE ADC_CLOCK 3




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