next up previous contents
Next: The Data Bus Up: Digital Circuits Previous: Multiplexers and Decoders

Schmitt Trigger

A noisy input signal to a logic gate could cause unwanted state changes near the voltage threshold. Schmitt trigger logic reduces this problem by using two voltage thresholds: a high threshold to switch the circuit during low-to-high transitions and a lower threshold to switch the circuit during high-to-low transitions. Such a trigger scheme is immune to noise as long as the peak-to-peak amplitude of the noise is less than the difference between the threshold voltages. A gate with the Schmitt trigger feature has a small hysteresis curve drawn inside the gate symbol. Schmitt triggers are mostly used in inverters or simple gates to condition slow or noisy signals before passing them to more critical parts of the logic circuit.

Doug Gingrich
Tue Jul 13 16:55:15 EDT 1999