Dump only what you need

$dumpfile names the VCD output, and $dumpvars with a depth and a scope selects how much hierarchy to record. Depth 0 records the scope and everything below it, while depth 1 records only the scope's own signals.

Dump scope is the largest lever on run time and disk use. Full-design dumps slow runs and flood disks, so regressions run dump-free and reruns record a window instead.

Commercial flows record FSDB or vendor equivalents instead of VCD. The scoping ideas match, and the compression is far better, through tool-specific tasks.

A window around the failure

$dumpoff and $dumpon gate recording at run time. The window edges below come from plusargs, so a failing seed reruns with waves only around the failure.

The listing names DUT hierarchy, so it is not run in this course's checks. $test$plusargs matches prefixes, so the window knobs are named +WIN_START and +WIN_STOP: WAVES is not a prefix of either, and only +WAVES turns dumping on.

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// not-run: needs a testbench harness with a matching DUT hierarchy
`timescale 1ns/1ps
time dump_start = 0;
time dump_stop = 0;   // 0 means run to the end

initial begin
  if ($test$plusargs("WAVES")) begin
    void'($value$plusargs("WIN_START=%d", dump_start));
    void'($value$plusargs("WIN_STOP=%d", dump_stop));
    $dumpfile("waves.vcd");
    $dumpvars(0, tb_top.dut);
    if (dump_start > 0) begin
      $dumpoff;                       // armed but not recording
      #(dump_start) $dumpon;          // open the window
    end
    if (dump_stop > dump_start)
      #(dump_stop - dump_start) $dumpoff;   // close it
  end
end

// Clean run:        simv +TESTNAME=stress
// Rerun the fault:  simv +TESTNAME=stress +WAVES +WIN_START=9000000 +WIN_STOP=9400000

The bottom rows show the window, the fault and an 8-bit counter. The fault lands inside the open window.

• Slot 0: win is 0 and data reads 10, before the window.

• Slot 1: data reads 11, still outside the window.

• Slot 2: win rises and data reads 12, so recording starts.

• Slot 3: win stays high and data reads 13.

• Slot 4: fault pulses and data reads ee, the signature inside the window.

• Slot 5: win falls and data reads 14, so recording stops.

DUMP WINDOW STRATEGY

  full dump        ██████████████████████████████  large file, slow run
                   0ms                        10ms

  windowed dump    ░░░░░░░░░░░░░░░░░░░░░░░░██░░░  small file, fast run
                                          ▲  ▲
                                  window open   failure inside
                                  before fault  window closes after

  regression default: no +WAVES, so zero wave cost
  failure rerun:      +WAVES with a window around the failing time

End the run and tune the noise

Three calls end a run three ways. $finish terminates normally and runs final checks, $stop pauses into the debugger, and $fatal terminates with an error status for scripts.

Verbosity is one more run-time knob. A log helper compares each message level against it, so rerunning louder never recompiles.

Every testbench also carries a timeout guard. The guard below has a time unit, so it is a plain illustration rather than a checked block.

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// snippet
`timescale 1ns/1ps
initial begin
  #50ms;
  $fatal(1, "TIMEOUT: test did not complete");
end

The testbench below proves the verbosity filter: with the default of 1, two of three messages show.

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module tb_verbosity;
  integer verbosity = 1;   // 0 shows errors only, 1 is normal, 2 is debug
  integer shown = 0;
  integer errors = 0;
  integer matched;

  task automatic log_msg(input integer level);
    if (level <= verbosity) shown = shown + 1;
  endtask

  initial begin
    matched = $value$plusargs("VERBOSITY=%d", verbosity);
    if (matched != 0) errors++;   // no plusargs in this run, so 0 is expected
    log_msg(0);
    log_msg(1);
    log_msg(2);
    if (shown != 2) errors++;     // levels 0 and 1 pass the default filter

    if (errors == 0) $display("PASS: verbosity 1 showed 2 of 3 messages");
    else $fatal(1, "FAIL: %0d verbosity checks did not hold", errors);
    $finish;
  end
endmodule

Common mistakes

Related topics

• Regression flow

• Performance and memory