downcntr.v

vcs tutorial lab1

module DOWNCNTR(COUNT, MODE, COUNTMSB, COUNTLSB, LOADCNT, CLK, GATE, OUT);

  input         CLK,
                OUT,
                GATE,
                LOADCNT;
  input   [3:0] MODE;
  input   [7:0] COUNTMSB,
                COUNTLSB;

  output [15:0] COUNT;

  reg           LOAD,
                CLRLOAD;
  reg    [15:0] COUNT;

  function [15:0] BCDDOWN;
    input [1:0] VALUE;

    begin

      BCDDOWN = COUNT;

      if (COUNT[3:0] < VALUE)
        begin

          BCDDOWN[3:0] = COUNT[3:0] + 10 - VALUE;

          if (!COUNT[7:4])
            begin

              BCDDOWN[7:4] = 9;

              if (!COUNT[11:8])
                begin

                  BCDDOWN[11:8] = 9;

                  if (!COUNT[15:12])
                    BCDDOWN[15:12] = 9;
                  else
                    BCDDOWN[15:12] = COUNT[15:12] - 1;

                end
              else
                BCDDOWN[11:8] = COUNT[11:8] - 1;

            end
          else
            BCDDOWN[7:4] = COUNT[7:4] - 1;

        end
      else
        BCDDOWN[3:0] = COUNT[3:0] - VALUE;

    end

  endfunction

  // Counter

  always @(posedge CLK)
    if (GATE || (MODE[3:1] == 1) || (MODE[3:1] == 5))
      if (LOAD)
        begin

          // Load Counter From Count Register

          COUNT = {COUNTMSB,COUNTLSB};

          // Clear Load Flag

          CLRLOAD = 1;

        end
      else
        begin

          // Decrement Counter

          if (MODE[3:1] == 3)
            if (MODE[0])
              if (OUT)
                COUNT = BCDDOWN(1);
              else
                COUNT = BCDDOWN(3);
            else
              if (COUNT[0])
                if (OUT)
                  COUNT = COUNT - 1;
                else
                  COUNT = COUNT - 3;
              else 
                COUNT = COUNT - 2;
          else
            if (MODE[0])
              COUNT = BCDDOWN(1);
            else
              COUNT = COUNT - 1;

          // Allow Counter To Be Loaded

          CLRLOAD = 0;
 
        end

  // Reload Counter On Rising GATE In Modes 1, 2 and 5

  always @(posedge GATE)
    if ((MODE[3:1] == 1) || (MODE[3:1] == 2) || (MODE[3:1] == 5))
      LOAD = 'b1;
    else 
      LOAD = 'b0;
 
  // Set LOAD Until Cleared By Next Rising Clock Edge

  always @(LOADCNT)
    if (LOADCNT)
      begin
        assign LOAD = 'b1;
        assign CLRLOAD = 'b0;
      end
    else
      begin
        deassign LOAD;
        deassign CLRLOAD;
      end
 
  always @(CLRLOAD)
    if (CLRLOAD)
      assign LOAD = 'b0;
    else
      deassign LOAD;

endmodule