I added debouncing for a switch to >> timer input of a F876 back in the day.

I tried using a simple R/C network but found that the slow rise time of the signal as it was charging the capacitor back up violated the maximum rise time for the pic I/O.

I ended up using the R/C filter but fed it into a schmidtt trigger prior to sending it to the timer I/O.

This took care of things, I routinely test my devices by having another pic activate a relay thousands of times and test to see that the receiving pic counted correctly.

fwiw