Do CoolRunner devices have hysteresis on the inputs?
CoolRunner XPLA3 devices have 50 mV of hysteresis on all inputs.
CoolRunner-II devices have a minimum of 500mV hysteresis that can be selected on each pin (via the Schmitt_trigger attribute). This can be done via the User Constraints File (UCF) or through the Assign Package Pins process in ISE.
Net datain schmitt_trigger;