Module 28 min

A Simple Testbench

Drive inputs, check outputs, and make it self-checking

Before any fancy methodology, you must understand the plain idea: a testbench wraps your design, feeds it inputs, and checks the outputs. Everything else in this path is just a better-organized version of this.

The DUT we will test

Say the design is a simple adder: out = a + b. The spec is one line, but it is enough to show the whole pattern.

verilog

module adder (
  input  [7:0] a,
  input  [7:0] b,
  output [8:0] out   // 9 bits so the carry fits
);
  assign out = a + b;
endmodule

A first testbench - just print

systemverilog

module adder_tb;
  logic [7:0] a, b;
  logic [8:0] out;

  adder dut (.a(a), .b(b), .out(out));

  initial begin
    a = 8'd10; b = 8'd20; #1;
    $display("10 + 20 = %0d", out);

    a = 8'd200; b = 8'd100; #1;
    $display("200 + 100 = %0d", out);

    $finish;
  end
endmodule

This works, but it makes you read the numbers and judge them yourself. That does not scale, and tired eyes miss bugs.

A self-checking testbench - the real upgrade

Instead of printing, compute what the answer should be and compare. Let the computer judge, and shout only when something is wrong.

systemverilog

initial begin
  for (int i = 0; i < 100; i++) begin
    a = $random;          // random value, truncated to 8 bits on assignment
    b = $random;
    #1;                    // let the result settle
    if (out !== a + b)     // the golden expected value
      $error("FAIL: %0d + %0d gave %0d", a, b, out);
  end
  $display("All checks done");
  $finish;
end

Pro tip

The line if (out !== a + b) $error(...) is the heart of all verification. You have a reference for the correct answer (here, a + b) and you compare automatically. Everything in UVM later is an elaborate, reusable way to do exactly this: generate stimulus, predict the answer, compare.

Watch out

Use !== and === (the case-equality operators) when checking, not != and ==. The triple version also catches x and z values, so an uninitialized output cannot silently pass your check. This single habit catches a surprising number of real bugs.