In SX Microcontrollers, SX/B Compiler and SX-Key Tool, bean wrote:

Brody,
  It is probably the noise that is causing false edges to be detected.

  In TTL mode the threshold level is 1.4V. So for the short amount of time that the signal is going from 0 to 5 (or 5 to 0) and is at the 1.4V level any amount of noise will be seen as a bunch of edges.

  The Schmitt trigger mode is the way to avoid this. 

  Also the gain in the op-amp circuit is probably causing the noise to be amplified too. If you could run the SX at 3.3V you could avoid using the op-amp, but that still may not solve the problem. Maybe you could use a schmitt triggered gate instead ?

Bean.

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