Why do some logic devices provide schmitt trigger inputs?

Question

Why do some logic devices provide schmitt trigger inputs?

Answer 1

A Schmitt trigger provides hysteresis. This is important when an input might be noisy or transition slowly.

For example, suppose you want to build a voltage divider with a thermistor. Your logic threshold is 2.5V so you set the divider so that at the temperature of interest and above you get 2.5V. You won't get good results because as the divider exactly hits 2.5V the gate will flip back and forth between 0 and 1 until you get significantly over 2.5V.

With a Schmitt trigger, the threshold for a 0 to 1 might be 2.5V and the threshold from a 1 to 0 might be 1.5V (check the data sheet). So as the temperature rises and hits 2.5V, there will be a solid 0 to 1 transition. The gate will stay at 1 until the voltage drops below 1.5V.

Another common place to see this is when reading an RC circuit charging for the same reason. As the voltage ramps up, you spend some time right at the "corner" of the threshold and a Schmitt will take the uncertainly out of the equation.