The solution depends on the '5volt signal'.
If grounds can be shared, and the signal can sink:
Use internal pull up on the pin, and a diode (1N4148) between pin and signal (cathode to signal).
If grounds can be shared, and the signal can source:
Use internal pull up on the pin, and an NPN transistor between pin (collector) and ground (emitter),
with a 100k resistor to the 5volt signal.
If grounds can't be shared:
Use internal pull up on the pin, and an opto coupler, with opto transistor between pin and ground,
with the opto LED driven by the signal via a (~5k) current limiting resistor.
Leo..