Unless the input pin of interest does not have any internal protection circuitry, which can easily found out by having a look into datasheet, the maximum input voltage should not exceed power supply rails, means Vcc and 0V, by more than the voltage drop of a forward biased pn-junction. This is about 0.5V. But take care, some manufacturers specifiy even a smaller value, only 0.3V. Have a look at actual datasheet.

If you cannot exclude the possibility of overranging a port pin, than you must apply some external protection circuitry. This can be a set of low ohmic Schottky diodes from port pin to supply rails in combination with a current limiting resistor or the use of an OPamp which is driven by the same supply voltage as your micro. Then, by design, potential at port pin cannot exceed supply rails.

Take care, that potential at port pins does NEVER exceed allowed values, even not for a very short period of time! Serious damage can occur due to invoking of internal parasitic thyristor latch-up. It can also cause a loss of information in the code memory if you use a FLASH micro. So, take this seriously.

Kai