// watchdog.sv — FSM with a large timeout counter
module watchdog #(
  parameter int TW = 20
) (
  input  logic clk, rst, start, kick, stop,
  output logic bark                       // timeout expired
);
  typedef enum logic [1:0] {OFF, ARMED, BARK} st_t;
  st_t         st;
  logic [TW-1:0] cnt;
  logic        expired;

  assign expired = (cnt == '1);           // all ones = timeout
  assign bark    = (st == BARK);

  always_ff @(posedge clk) begin
    if (rst) begin
      st  <= OFF;
      cnt <= '0;
    end else begin
      case (st)
        OFF:     if (start) begin st <= ARMED; cnt <= '0; end
        ARMED:   if (stop)         st  <= OFF;
                 else if (kick)    cnt <= '0;
                 else if (expired) st  <= BARK;
                 else              cnt <= cnt + 1'b1;
        BARK:    if (stop)         st  <= OFF;
        default: st <= OFF;
      endcase
    end
  end
endmodule