debounce.vhd

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-- File      : debounce.vhd
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-- Author     : Peter Samarin <peter.samarin@gmail.com>
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-- Copyright (c) 2019 Peter Samarin
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-- Description: debouncing circuit that forwards only
-- signals that have been stable for the whole duration of
-- the counter
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library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
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entity debounce is
  generic (
    WAIT_CYCLES : integer := 5);
  port (
    signal_in  : in  std_logic;
    signal_out : out std_logic;
    clk        : in  std_logic);
end entity debounce;
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architecture arch of debounce is
  type state_t is (idle, check_input_stable);
  signal state_reg     : state_t                          := idle;
  signal out_reg       : std_logic                        := signal_in;
  signal signal_in_reg : std_logic;
  signal counter       : integer range 0 to WAIT_CYCLES-1 := 0;
begin

  process (clk) is
  begin
    if rising_edge(clk) then
      case state_reg is
        when idle =>
          if out_reg /= signal_in then
            signal_in_reg <= signal_in;
            state_reg     <= check_input_stable;
            counter       <= WAIT_CYCLES-1;
          end if;

        when check_input_stable =>
          if counter = 0 then
            if signal_in = signal_in_reg then
              out_reg <= signal_in;
            end if;
            state_reg <= idle;
          else
            if signal_in /= signal_in_reg then
              state_reg <= idle;
            end if;
            counter <= counter - 1;
          end if;
      end case;
    end if;
  end process;

  -- output
  signal_out <= out_reg;

end architecture arch;