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Hardware debounce and power-on reset

Digital logic · Intermediate · about 15 minutes · part of Electronics Technician

A push button’s contacts bounce for a few milliseconds, so a counter sees five presses instead of one. An RC slows the edge and a Schmitt-trigger input, with separate up and down thresholds, turns it back into one clean step. The same RC-plus-Schmitt holds a reset line low for a moment after power-up.

Hardware debounce and power-on reset: the circuit on the simulator's bench

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How it works

Debounce: τ = 10 kΩ × 1 µF = 10 ms, longer than the 4–8 ms of bounce, so the capacitor never crosses back over the Schmitt thresholds (0.55 and 0.37 × V_CC) during a bounce: one edge per press.

Reset: C_por charges through 100 kΩ (τ = 1 s); the inverter’s RESET output stays high until the input passes 0.55 × 5 V, after t = τ·ln(1 ÷ (1 − 0.55)) = 0.799 s.

Try it on the bench

  1. Click and hold the button on the bench, then release: watch TP1 chatter and TP2 step once.

Parts on the bench

In the course

Electronics Technician, module “Digital and logic levels”.

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