Simple counter used for learning test bench features and workflow

comparator.v

// A simple comparator used for learning simulation workflows

// comparator - comparator_tb.v
//
// Description:
// Testbench for comparator module.
//
`timescale 1ns/10ps

module comparator #(
   parameter WIDTH = 8      // data width
   ) (
   input [WIDTH-1:0] a, b,  // values to compare
   output            lt,    // high when a < b
   output            eq,    // high when a = b
   output            gt     // high when a > b
   );
   assign lt = (a < b) ? 1'b1 : 1'b0;
   assign eq = (a == b) ? 1'b1 : 1'b0;
   assign gt = (a > b) ? 1'b1 : 1'b0;
endmodule  // comparator

comparator_tb.v

// --------------------------------------------------------------------------
// Test bench
// --------------------------------------------------------------------------
module comparator_tb;
   parameter WIDTH = 2;   // data width
   integer i, j;          // for-loop variables
   reg [WIDTH-1:0] a, b;  // input values to compare
   wire lt, eq, gt;       // output comparison status

   initial begin
      $dumpfile("comparator_tb.vcd");  // waveforms file
      $dumpvars;  // save waveforms
      $display("%d %m: Starting testbench simulation...", $stime);
      $monitor("%d %m: MONITOR - a = %d, b = %d, lt = %d, eq = %d, gt = %d.", $stime, a, b, lt, eq, gt);
      #1;
      for (i = 0; i < 2 ** WIDTH; i = i + 1) begin
         for (j = 0; j < 2 ** WIDTH; j = j + 1) begin
            #1 a = i; b = j;
            #1;
            if (a < b && (!lt || eq || gt)) begin
               $display("%d %m: ERROR - Status flags lt (%d) eq (%d) gt (%d) are not correct for a (%d) less than b (%d).", $stime, lt, eq, gt, a, b);
               $finish;
            end
            if (a == b && (lt || !eq || gt)) begin
               $display("%d %m: ERROR - Status flags lt (%d) eq (%d) gt (%d) are not correct for a (%d) equal to b (%d).", $stime, lt, eq, gt, a, b);
               $finish;
            end
            if (a > b && (lt || eq || !gt)) begin
               $display("%d %m: ERROR - Status flags lt (%d) eq (%d) gt (%d) are not correct for a (%d) greater than b (%d). ", $stime, lt, eq, gt, a, b);
               $finish;
            end
         end
      end
      #1 $display("%d %m: Testbench simulation PASSED.", $stime);
      $finish;  // end simulation
   end

   // Instances
   comparator #(.WIDTH(WIDTH)) comparator_1(.a(a), .b(b), .lt(lt), .eq(eq), .gt(gt));

endmodule  // comparator_tb

Make sure you have both files in the same folder otherwise you will need to add an include directive:

`include "folder/..."

If you are using APIO then you can use the following command to simulate the counter:

apio sim

You should get an output in your console:

0 comparator_tb: Starting testbench simulation...
0 comparator_tb: MONITOR - a = x, b = x, lt = x, eq = x, gt = x.
2 comparator_tb: MONITOR - a = 0, b = 0, lt = 0, eq = 1, gt = 0.
4 comparator_tb: MONITOR - a = 0, b = 1, lt = 1, eq = 0, gt = 0.
6 comparator_tb: MONITOR - a = 0, b = 2, lt = 1, eq = 0, gt = 0.
8 comparator_tb: MONITOR - a = 0, b = 3, lt = 1, eq = 0, gt = 0.
10 comparator_tb: MONITOR - a = 1, b = 0, lt = 0, eq = 0, gt = 1.
12 comparator_tb: MONITOR - a = 1, b = 1, lt = 0, eq = 1, gt = 0.
14 comparator_tb: MONITOR - a = 1, b = 2, lt = 1, eq = 0, gt = 0.
16 comparator_tb: MONITOR - a = 1, b = 3, lt = 1, eq = 0, gt = 0.
18 comparator_tb: MONITOR - a = 2, b = 0, lt = 0, eq = 0, gt = 1.
20 comparator_tb: MONITOR - a = 2, b = 1, lt = 0, eq = 0, gt = 1.
22 comparator_tb: MONITOR - a = 2, b = 2, lt = 0, eq = 1, gt = 0.
24 comparator_tb: MONITOR - a = 2, b = 3, lt = 1, eq = 0, gt = 0.
26 comparator_tb: MONITOR - a = 3, b = 0, lt = 0, eq = 0, gt = 1.
28 comparator_tb: MONITOR - a = 3, b = 1, lt = 0, eq = 0, gt = 1.
30 comparator_tb: MONITOR - a = 3, b = 2, lt = 0, eq = 0, gt = 1.
32 comparator_tb: MONITOR - a = 3, b = 3, lt = 0, eq = 1, gt = 0.
34 comparator_tb: Testbench simulation PASSED.

APIO will also attempt to launch gtkwave if you have it installed: