Real button inputs: synchronization, debounce and one event

One press can contain many transitions

A mechanical contact can bounce, and its transitions are asynchronous to the system clock. Synchronization and debouncing^{idebouncing Filtering repeated mechanical contact transitions to obtain a stable button state. This is separate from synchronizing an asynchronous input.} solve different problems. A two-flop synchronizer^{isynchronizer A circuit for safely handling a signal in the receiving clock domain. A multi-stage single-bit synchronizer is not by itself a coherent multi-bit transfer protocol. Learn more} does not automatically remove contact bounce.

This pattern assumes a single-bit level held long enough to observe. Short pulses and coherent multi-bit transfers need other protocols.

Require N consecutive samples

Let q be the accepted state, x the synchronized input and count the number of consecutive samples differing from q. If x=q, clear count. Otherwise increment it; on the Nth differing sample, accept q=x and clear count.

For N=3 and initial q=0, inputs 0,1,0,1,1,1,1,0,1 produce counts 0,1,0,1,2,0,0,1,0. State changes at sample 5 only.

Each column is a sampled synchronized input, not an analog contact waveform.

Specify latency and boundaries

For sample period $T_s$, phase-dependent acceptance delay is roughly $(N-1)T_s$ to $NT_s$, plus preceding synchronization delay. A conservative stable-time requirement must consider the interval between the first and last samples. Short transitions between samples can be missed: equal samples do not prove the physical input never changed. Specify switch characteristics and sampling assumptions together.

Storing counts 0 through N−1 needs $\max(1,\lceil\log_2 N\rceil)$ bits. N=1 must accept the first differing sample. To emit an event on the acceptance sample, use $rise=\overline q\,q_{next}$. Comparing separately registered current and delayed states can shift event timing; define when the consumer observes it.