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Hello, and welcome to The Logic Minute. In this video, we will look at a simple method to debounce a signal with logic. When a mechanical switch is actuated, it often produces multiple electrical signals due to physical bouncing of the metallic contacts. The signal to the right shows a typical example of the output of a mechanical switch. Many devices can detect even a one-nanosecond pulse and would be triggered multiple times by this signal. Zooming out a little bit lets us see the bounces on both the rising and falling edges of the signal. We will keep the signal in the background to compare to the debounced signals.

Just by adding a capacitor we have the simplest debounced circuit. This will work in most systems, but we can see that the initial discharge is not slowed at all, so we're really counting on the switch's first bounce to discharge the capacitor fully. To be absolutely certain that the signal is debounced in both directions, a second resistor can be added. Here you can see that the bounces are filtered out of both the rising and falling edge.

One side effect of this circuit is that the rising and falling edges are now very slow, and sometimes non-monotonic, as can be seen here and here. The signal rebounds slightly, which could cause double triggering on a normal CMOS input if it just so happened to fall at the threshold level. The solution is to use a logic device with added input hysteresis like this Schmitt trigger inverter. This solves two problems at once, providing separation between the rising and falling edge trigger levels to eliminate issues from non-monotonic behavior and preventing excess current from the slow input edges.

Thank you for watching. Please explore the other videos and training material on our site. And if you have any questions, come over to the E2E forums to ask us directly.

This video is part of a series