Input

Button debouncer

debounce

The input passes a two-flop synchronizer; the output follows it only after it has held one value for STABLE_CYCLES clock cycles, so contact bounce and short glitches are ignored. rise and fall pulse for one cycle when the output changes.

Verified by Tapeo97 generic cells at the defaultsbutton inputswitch input

Instantiate it

Paste into your design and connect the signals.

systemverilog

debounce #(.STABLE_CYCLES(1000000)) u_button (
  .clk(clk), .rst(rst),
  .noisy(button_pin), .clean(button), .rise(button_pressed), .fall(button_released)
);

Parameters

Name	Default	Meaning
STABLE_CYCLES	1000000	Cycles the input must hold before the output changes (1,000,000 is 10 ms at 100 MHz).

Ports

Name	Direction	Width	Meaning
clk	input	1	Clock; everything changes on its rising edge.
rst	input	1	Synchronous reset, active high.
noisy	input	1	Button or switch input (asynchronous).
clean	output	1	Debounced level.
rise	output	1	Pulses when clean goes high (a press).
fall	output	1	Pulses when clean goes low (a release).

Add it to a project

It is copied to ip/debounce/debounce.sv with a README (ip/debounce/README.md). When your design needs it, the Architect (new designs) or the Architect (changes) suggests it and asks before using it. Or add it yourself:

In the workspace: the Tapeo Hub button in the project header, then Add to project.
In a terminal (in the project folder):
```
$ tapeo hub add debounce
```
In VS Code: Tapeo: Add from Tapeo Hub in the Command Palette.

How it is verified

Before every release Tapeo checks this block: its self-checking testbench passes in Icarus Verilog, Verilator -Wall reports nothing, and Yosys synthesizes it (97 generic cells at the default parameters). That shows it works for the tested cases; your copy is part of your project, and your own testbench checks it in your design.

Source

debounce.sv

// Button or switch debouncer. The input passes a two-flop synchronizer; the
// output follows it only after it has held one value for STABLE_CYCLES clock
// cycles (for example 1,000,000 for 10 ms at 100 MHz). `rise` and `fall`
// pulse for one cycle when the output changes. Tapeo Hub (verified by Tapeo).
module debounce #(
  parameter integer STABLE_CYCLES = 1000000
) (
  input  logic clk,
  input  logic rst,
  input  logic noisy,
  output logic clean,
  output logic rise,
  output logic fall
);
  localparam integer COUNT_WIDTH = (STABLE_CYCLES > 1) ? $clog2(STABLE_CYCLES) : 1;
  localparam logic [COUNT_WIDTH-1:0] LAST_TICK = COUNT_WIDTH'(STABLE_CYCLES - 1);

  logic [1:0] sync;
  logic [COUNT_WIDTH-1:0] stable;

  always_ff @(posedge clk) begin
    if (rst) sync <= 2'b00;
    else sync <= {sync[0], noisy};
  end

  always_ff @(posedge clk) begin
    if (rst) begin
      stable <= '0;
      clean <= 1'b0;
      rise <= 1'b0;
      fall <= 1'b0;
    end else begin
      rise <= 1'b0;
      fall <= 1'b0;
      if (sync[1] == clean) stable <= '0;
      else if (stable == LAST_TICK) begin
        stable <= '0;
        clean <= sync[1];
        rise <= sync[1];
        fall <= !sync[1];
      end else stable <= stable + 1'b1;
    end
  end
endmodule

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