Switch Debouncer

Medium · Counters & Timers · Sequential Logic

Mechanical switches "bounce": for a short time after being pressed or released, the contact opens and closes rapidly. A debouncer filters this out. Its output only follows the input once the input has held a new value for long enough.

Design a debouncer whose output changes only after the input has been sampled at the new value on N consecutive rising edges.

Parameters

Parameter	Default	Meaning
`N`	4	Number of consecutive samples required, `1 <= N <= 255`. Your design must work for every value in this range: the hidden tests also use non-default values, including the smallest and the largest.

Ports

Port	Dir	Width	Meaning
`clk`	input	1	Clock.
`rst`	input	1	Synchronous, active-high reset (takes effect only at a rising edge of `clk`).
`din`	input	1	Noisy input (already synchronized to `clk`), sampled at every rising edge.
`dout`	output	1	Debounced output (registered).

Behavior

The debouncer keeps a count c of how many consecutive samples have differed from the current dout. At every rising edge:

If rst = 1: dout <= 0 and c <= 0. (The sample taken at a reset edge never counts.)
Else if din == dout: c <= 0 (any sample equal to dout restarts the count).
Else (din != dout): the count increases. If this is the N-th consecutive

differing sample, dout takes the value of din at this edge, and the count restarts from 0 (the next samples are compared against the new dout).

So if din changes and then stays stable, with its first new-value sample at edge k, dout changes at edge k + N − 1 and is visible in the cycle after that edge. With N = 1, dout is simply din delayed by one register.

Example (`N = 4`)

edge	0	1	2	3	4	5	6	7	8	9	10	11
`rst`	1	0	0	0	0	0	0	0	0	0	0	0
`din` sampled	0	1	1	0 (bounce)	1	1	1	1	1	0	0	0
count after the edge	0	1	2	0	1	2	3	0	0	1	2	3
`dout` after the edge	0	0	0	0	0	0	0	1	1	1	1	1

Switch Debouncer

Parameters

Ports

Behavior

Example (N = 4)

More Counters & Timers problems

Example (`N = 4`)