-- debounce is used to eliminate the debounce of input signal
-- it is a ASM
LIBRARY ieee;
USE ieee.std_logic_1164.all;

ENTITY debounce IS
PORT
    (
        signal SW : IN    STD_LOGIC;
        signal done : IN    STD_LOGIC;
        signal clk : IN    STD_LOGIC;
        signal reset: IN STD_LOGIC;
        signal START: OUT STD_LOGIC;
        signal SWdown : OUT STD_LOGIC
    );
end debounce;

ARCHITECTURE debounce_architecture OF debounce IS
    constant S0 :STD_LOGIC_VECTOR(1 downto 0):="00";  --declareation of states
    constant S1 :STD_LOGIC_VECTOR(1 downto 0):="01";
    constant S2 :STD_LOGIC_VECTOR(1 downto 0):="10";
    constant S3 :STD_LOGIC_VECTOR(1 downto 0):="11";
    signal p_state, n_state: STD_LOGIC_VECTOR(1 downto 0);
BEGIN
    state: PROCESS(clk,reset)
    BEGIN
        if(reset='0') then                  --set the initial state
            p_state<=S0;
        elsif(clk'EVENT AND clk='1') then
            p_state<=n_state;
        end if;
    end PROCESS state;
   
    comb: PROCESS(SW,done,p_state)
    BEGIN
        SWdown<='0';                       
        START<='0';
        n_state<=p_state;
        if(p_state=S0) then       
            SWdown<='1';                   
            START<='0';
            if(SW='1') then
                n_state<=S0;
            elsif(SW='0') then
                n_state<=S1;
            end if;
        end if;
       
        if(p_state=S1) then       
            SWdown<='1';           
            START<='1';        --start the counter.
            if(done='1') then
                n_state<=S2;
            elsif(done='0' and SW='1') then
                n_state<=S0;
            elsif(done='0' and SW='0') then
                n_state<=S1;
            end if;
        end if;

        if(p_state=S2) then               
            SWdown<='0';                   
            START<='0';                   
            if(SW='1') then
                n_state<=S3;
            elsif(SW='0') then
                n_state<=S2;
            end if;
        end if;
       
        if(p_state=S3) then               
            SWdown<='0';               
            START<='1';            --start the counter
            if(done='1') then
                n_state<=S0;
            elsif(done='0' and SW='1') then
                n_state<=S3;
            elsif(done='0' and SW='0') then
                n_state<=S2;
            end if;
        end if;
    end process comb;
end debounce_architecture;