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VCD Waveform Output ​

Celox can write Value Change Dump (VCD) files for viewing waveforms in tools like GTKWave or Surfer.

Basic Usage ​

Pass vcd with the output file path when creating a simulator:

typescript
const sim = Simulator.fromSource(SOURCE, "Top", {
  vcd: "./dump.vcd",
});

Then call sim.dump(timestamp) at each point in time you want recorded:

typescript
sim.dut.a = 10n;
sim.dump(0);      // record initial state at t=0

sim.dut.a = 20n;
sim.tick();
sim.dump(10);     // record state at t=10

sim.dispose();    // flushes and closes the file

WARNING

VCD output is buffered. Always call dispose() when you are done so the remaining bytes are written — or use a try/finally block.

In Rust, call Simulator::flush_vcd() or Simulation::flush_vcd() to make buffered output visible before dropping the simulator and to handle write errors. Standalone VcdWriter instances provide flush() and into_inner().

The VCD performance notes describe change tracking, shared-memory behavior, and reproducible benchmarks.

With Time-Based Simulation ​

With Simulation, sim.time() gives you the current time to pass to dump():

typescript
const sim = Simulation.fromSource(SOURCE, "Top", {
  vcd: "./dump.vcd",
});
sim.addClock("clk", { period: 10 });

sim.reset("rst");
sim.dump(sim.time());

sim.dut.en = 1n;
sim.runUntil(100);
sim.dump(sim.time());

sim.dispose();

Further Reading ​