debounce.v

/* debounce.v - a debounced push button by karttu, 12.12.2004.

// See:
// http://groups-beta.google.com/group/comp.lang.vhdl/messages/06dceec75f8ca08e,93d2ccf324a87543,ba3d805dcf093614,afeaec4c5fdc6004,bb912762f254318b?thread_id=1a429c2092c1b3e3&view=thread&noheader=1
// Especially the message from Rick Collins:
//
// The second way is to use a single pole single throw switch and any fast
// (>100 Hz) clock and require that the switch be in a given state for some
// minimum amount of time before changing the output. The idea is to wait
// out the bouncing time before changing the output. For example if you
// were trying to detect when the user pushes a momentary push button, you
// might use a 1 KHz clock and a seven bit counter. The counter is reset
// any time the button is not pushed. When the button is pushed, the
// counter is allowed to run, getting reset when ever the switch bounces.
// After the switch has stopped bouncing for 128 counts, (you can assume
// that it won't bounce any more) and the counter reaches max count. The
// counter then outputs a terminal count signal and stops counting. When
// the button switch is released, the counter is reset and the TC output
// goes away. If this is not long enough to debounce the switch, use a
// slower clock or a longer counter.

 */

module debounced_button(CLK,PB_IN,DEB_OUT);
input CLK;
input PB_IN;
output DEB_OUT;

reg DEB_OUT = 0;

parameter msb = 19;
// 1048576 x 20 ns = 20971520 ns ~ 21 ms. (~ 1/50 s).
reg [msb:0] delay_counter = 1;

always @(posedge CLK)
  begin
   if(~PB_IN)
    begin
     DEB_OUT <= 0;
     delay_counter <= 1;
    end
   else if(delay_counter)
    begin
     DEB_OUT <= 0;
     delay_counter <= delay_counter + 1;
    end
   else // delay_counter has wrapped over,
    begin // so the button has settled to ON state.
     DEB_OUT <= 1;
    end
  end

endmodule