I have a board that uses a 47k resistor as input protection to PC5 but I am having trouble getting the voltage to go low enough for PC5 to be low. The internal pull-up is enabled and when I ground the input to the resistor, PC5 goes from 3V3 to 1V8 which is not low enough.
The board has places for PC5 to be pulled-up to 3V3 and pulled-down to ground. If I install a 10k pull-down resistor, PC5 goes from 0V8 to 0V6 (IIRC) when the input gets grounded. So I then installed a 10k pull-up in addition to the pull-down but the voltages are something like 2V4 and 1V5. Sorry I cannot remember the exact voltages, I got distracted.
I have STFF and STFW (until I got distracted) and found a real input protection circuit but that requires a new PCB.
I am afraid to lower the input resistor lower than 20k since its sole purpose in life (I think) is for over-voltage protection. I believe the inputs are supposed to survive 14V.
Another thought was to replace the resistor with a Schottky diode. Will this work?