Dump only what is needed

Wave files grow with scope and time. Dumping a full design for a long run slows simulation and buries the event.

Dump the suspect hierarchy, and only for runs that need waves. Heavy dumping stays behind a plusarg or a compile define.

Gate dumps behind plusargs

The gate below dumps nothing by default. Adding +WAVE_ON to the command line enables the dump without recompiling.

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// snippet
`timescale 1ns/1ps
initial begin
  if ($test$plusargs("WAVE_ON")) begin
    $dumpfile("trig_cnt.vcd");
    $dumpvars(0, tb_trig_cnt);
  end
end

Arm triggers on violations

Rare errors drown in long logs. A trigger watches a protocol condition and prints a snapshot when it fires.

The testbench below arms a trigger on count 10 and prints one line. The trigger reads the DUT's count port; a hierarchical reference such as dut.count would also work, but only in testbench code, never in synthesizable RTL.

Testbench

A counter with a trigger at count 10. With +WAVE_ON, dump activity is enabled only from count 8 through count 12, preserving two cycles of context on each side. This is a deterministic replay window: first reproduce/log the failure cycle, then rerun with a window starting before it. An unpredictable first-run trigger needs simulator circular/pre-trigger capture or retained transaction history; $dumpon cannot recover events from the past.

Read the timeline left to right along the count axis. The shaded band is the only stretch written to the wave file.

• Counts 0 to 7: $dumpoff keeps the file to its header.

• Counts 8 to 12: $dumpon records two counts of context before and after the event.

• Count 10: the trigger prints one snapshot; the bench fails if it fires more than once.

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`timescale 1ns/1ps
module trig_cnt (
  input  logic       clk, rst_n,
  output logic [7:0] count
);
  always_ff @(posedge clk) begin
    if (!rst_n) count <= 8'd0;
    else        count <= count + 8'd1;
  end
endmodule

Related topics

• Waveform bug patterns: latch and race