The absolute maximum ratings state that the input to ANY pin may not be less than -0.5V relative to Vss. That implies when you apply a more negative voltage you'll destroy something. For sure!
When there is an AC signal to input to a PSoC you ought to use a level shifter.
A negative voltage to a cmos pin can trigger an internal parasitic SCR which
shorts Vdd to Vss and generally will blow out the supply die internal bond wire.
You can generate a negative voltage using a PWM or Square wave to a pin followed
by a diode multiplier, see http://www.gyraf.dk/schematics/Voltage_multipliers_with_CMOS_gates.pdf
If you need to level shift a signal into CM range of an input, here is a simple
method using just 2 R's -
121236.xls 12.0 K