Hi there, if I have a sensor that produces 0V LO and 2.65V HI, is that valid for use on the I/O pins of a PIC16F877A? I looked through the datasheet but can't seem to find this anywhere.
It depends on the input and the supply voltage of the PIC.
For 4.5V ≤ VDD ≤ 5.5V:
Some inputs use TTL levels and for them, anything higher than 2V is considered a "high" Anything less than 0.8V is a low.
Other inputs are Schmitt Trigger types and for those a high would be 4V (Vdd * 0.8 = 4V) and a low 1V (Vdd * 0.2 = 1V) if Vdd = 5V.
You'll find this info under ELECTRICAL CHARACTERISTICS in the datasheet.