The absolute maximum voltage allowed on PSoC GPIO pins is 0.5 V above the GPIO pin supply or 0.5 V below ground. If there is any chance of the input voltage to exceed this, it is recommended to protect the PSoC device by using a series resistor and Schottky diode arrangement as shown in the following diagram.
The diodes D_1 and D_2 are low forward drop Schottky diodes such as the NSR05F20NXT5G Schottky Barrier Diode from ON Semiconductor. See the datasheet at https://www.onsemi.com/pub/Collateral/NSR05F20.PDF
A R_3 resistor value of 100 Ω will keep the voltage on the GPIO pin below ± 0.4 V (over VDD/below Gnd) for Vinput values up to ± 50 V. Therefore, the PSoC device will remain unaffected because it can stand voltages up to ± 0.5 V (over VDD/below Gnd) on its GPIO pins.