I have an encoder in form of a hall sensor. It works when I use the Interrupt pins 18,19 on the mega. Using quadrature I get the speed of the motor. But when I attachInterrupt(digitalPinToInterrupt(interruptPin), blink, CHANGE); with pins 2,3,20,or 21 instead of 18,19 nothing works anymore. Any clues? Do I have to treat those pins differently?