debounce.v

// Listing 6.2
module debounce
   (
    input wire clk, reset,
    input wire sw,
    output reg db_level, db_tick
   );

   // symbolic state declaration
   localparam  [1:0]
               zero  = 2'b00,
               wait0 = 2'b01,
               one   = 2'b10,
               wait1 = 2'b11;

   // number of counter bits (2^N * 20ns = 40ms)
   localparam N=21;

   // signal declaration
   reg [N-1:0] q_reg, q_next;
   reg [1:0] state_reg, state_next;

   // body
   // fsmd state & data registers
    always @(posedge clk, posedge reset)
       if (reset)
          begin
             state_reg <= zero;
             q_reg <= 0;
          end
       else
          begin
             state_reg <= state_next;
             q_reg <= q_next;
          end

   // next-state logic & data path functional units/routing
   always @*
   begin
      state_next = state_reg;   // default state: the same
      q_next = q_reg;           // default q: unchnaged
      db_tick = 1'b0;           // default output: 0
      case (state_reg)
         zero:
            begin
               db_level = 1'b0;
               if (sw)
                  begin
                     state_next = wait1;
                     q_next = {N{1'b1}}; // load 1..1
                  end
            end
         wait1:
            begin
               db_level = 1'b0;
               if (sw)
                  begin
                     q_next = q_reg - 1;
                     if (q_next==0)
                        begin
                           state_next = one;
                           db_tick = 1'b1;
                        end
                  end
               else // sw==0
                  state_next = zero;
            end
         one:
            begin
               db_level = 1'b1;
               if (~sw)
                  begin
                     state_next = wait0;
                     q_next = {N{1'b1}}; // load 1..1
                  end
            end
         wait0:
            begin
               db_level = 1'b1;
               if (~sw)
                  begin
                     q_next = q_reg - 1;
                     if (q_next==0)
                        state_next = zero;
                  end
               else // sw==1
                  state_next = one;

            end
         default: state_next = zero;
      endcase
   end

endmodule