A general question that I can't find an answer to so easily. Why is it that each digital input pin to a microcontroller doesn't require an extra pin for ground? Each analogue input requires two inputs , + and -. I would have thought that all inputs, be it digital and analogue would need a reference voltage (ground).
I am talking specifically about using a development prototyping board. The one I am using Atmel is Xplained pro for SAMD21 chip.
Thanks in advance.