Programming: ADC (Analog to Digital Converter) Establishing the Refrence Voltage from the AVCC Pin

The reference voltage for the ADC is the top threshold voltage that it will consider when making a conversion. That is to say, if the range of voltages on the ADC pin will be between 0 and 3.3 volts, then the reference voltage must be 3.3 volts rather than the voltage of the circuit. In this case, the AVCC pin will provide us the reference voltage since the top threshold voltage will match the circuit's voltage. The REFS1 and REFS0 bits in the ADMUX register will control where the reference voltage will be derived. If the REFS0 bit is set, then the AVCC will provide this reference.