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:

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

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

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

The VCD file is not written until dispose() is called. Always call dispose() when you are done — or use a try/finally block.

With Time-Based Simulation

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

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

• Writing Tests -- Simulator and Simulation patterns.