library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.numeric_std.all;

entity debounce is
port( clk, reset : in std_logic;
      sw : in std_logic;
      db_level, db_tick :out std_logic);
end debounce;

architecture Behavioral of debounce is
constant N: integer := 21;
type state_type is (zero, wait0, one, wait1);
signal state_reg, state_next: state_type;
signal q_reg, q_next : unsigned (N-1 downto 0);

begin
process (clk, reset)
begin
if reset = '1' then
   state_reg <= zero;
   q_reg <= (others => '0');
   elsif (clk'event and clk = '1') then
   state_reg <= state_next;
   q_reg <= q_next;
   end if;
   end process;

   process (state_reg, q_reg, sw, q_next)
begin
     state_next <= state_reg;
     q_next <= q_reg;
     db_tick <= '0';
     case state_reg is
        when zero =>
             db_level <= '0';
             if (sw='1') then
                state_next <= wait1;
                q_next <= (others=> '1');
             end if;
           when wait1 =>
                db_level <= '0';
                if (sw = '1') then
                q_next <= q_reg - 1;
                   if (q_next=0) then
                      state_next <= one;
                      db_tick <= '1';
                   end if;
                else state_next <= zero;
                end if;

        when one =>
             db_level <= '1';
             if (sw='0') then
                state_next <= wait0;
                q_next <= (others=>'1');
             end if;
        when wait0 =>
            db_level <= '1';
            if (sw='0') then
            q_next <= q_reg - 1;
               if (q_next=0) then
                  state_next <= zero;
               end if;
            else state_next <= one;
            end if;
      end case;
   end process;



end Behavioral;