Grumpy_Mike:
You can not use a referance voltage of less than 1V on an arduino.
I'm not convinced that limit applies to the analog comparator.
Trying a small test:
ISR (ANALOG_COMP_vect)
{
PIND = _BV (2); // toggle D2
}
void setup ()
{
ADCSRB = 0; // (Disable) ACME: Analog Comparator Multiplexer Enable
ACSR = _BV (ACI) // (Clear) Analog Comparator Interrupt Flag
| _BV (ACIE); // Analog Comparator Interrupt Enable
pinMode (2, OUTPUT);
} // end of setup
void loop () { }
With a pot on D7 to adjust the reference voltage I got it down to around 100 mV, and still the output pin (D2, yellow trace) toggled which means the analog comparator got a match:
