Analog comparator
ATmega328P.pdf ( Register summary on p 428 )
ATmega328/P has one analog comparator. The comparator has two inputs on pins AIN0(+)/PD6/D06 and AIN1(-)/PD7/D07. In addition, all other AD pins can also be used as comparator (-)input (if the AD converter is turned off). For comparator (+)input, an internal 1.1 V reference can be used. The comparator output can (only) be read programmatically, either as bit ACO or with interrupt (ANALOG_COMP_vect).

The test program examines the comparator with a potentiometer connected to AIN1/PD7 (-)input and the internal reference coupled to the (+)input. The comparator output is displayed with Arduino's built-in LED on the PB5/D13. The test program uses interrupt.
The full program is available here:
comparator.txt
(for Arduino use code stored as: comparator\comparator.ino)
William Sandqvist willsandqvist@gmail.com