5v input on ATmega328p running on 3.3v The ATmega328p can run on 5v but will it cause problems if it runs on 3.3 v and one input signal is 5v?
The voltage at any input must be less than VCC+0.3V unless it is restricted to only a few milliamperes (so it doesn't burn out the clamping diode). Use a proper voltage translator if it is available, otherwise use a 1kohm or greater resistor (at the cost of life expectancy for the MCU).