wave2tb-generator

Generate a cycle-based SystemVerilog testbench from a captured VCD waveform and DUT RTL.

Why This Project Matters

• Small design team using Chisel can keep their verification written in svsim.
• Outsourced backend/PD teams usually need plain, static SV testbenches for GLS handoff.
• This tool bridges both worlds by converting captured VCD behavior into deterministic, standalone SV TB code (no Chisel runtime dependency).
• It reduces manual TB rewrite effort and lowers handoff risk between frontend and backend flows.

Workflow

1. Parse DUT ports from RTL in Python (tree-sitter-verilog).
2. Read VCD waveform and auto-match one DUT scope.
3. Build a JSON IR containing per-cycle input stimulus and expected outputs.
4. Emit a standalone SystemVerilog testbench that:
   • drives the same per-cycle stimulus (only emits changed input assignments),
   • steps the DUT clock,
   • checks expected outputs every cycle.

Install

uv sync

After uv sync, run CLI commands with uv run (recommended), for example:

uv run wave2tb --help

If you prefer not to use uv run each time, activate the venv first:

source .venv/bin/activate
wave2tb --help

CLI

Generate testbench in one step from VCD + RTL:

uv run wave2tb vcd-to-tb \
  --vcd test/out/reference.vcd \
  --rtl test/data/sample_dut.sv \
  --top-module sample_dut \
  --clock clk \
  --tb-out test/out/generated_tb.sv \
  --ir-out test/out/generated.ir.json

Generate IR and testbench (legacy two-output command):

uv run wave2tb from-vcd \
  --vcd test/out/reference.vcd \
  --rtl test/data/sample_dut.sv \
  --top-module sample_dut \
  --clock clk \
  --ir-out test/out/generated.ir.json \
  --tb-out test/out/generated_tb.sv

Generate testbench from an existing IR:

uv run wave2tb ir-to-tb \
  --ir test/out/generated.ir.json \
  --tb-out test/out/generated_tb.sv

Optional external equivalence verification:

uv run wave2tb vcd-to-tb \
  --vcd test/out/reference.vcd \
  --rtl test/data/sample_dut.sv \
  --top-module sample_dut \
  --clock clk \
  --tb-out test/out/generated_tb.sv \
  --verify-with-verilator

End-to-End Test

Run:

bash test/run_e2e.sh

Note: the optional --verify-with-verilator flow and test/run_e2e.sh require external Verilator.

Generated Output Examples

generated.ir.json (delta IR)

generated.ir.json uses encoding: "delta" and is the intermediate format consumed by the toolchain. Core rules:

• Top-level metadata includes schema_version, encoding, top_module, scope, timescale, clock_name, and clock_edge.
• The ports array stores the DUT port list; each item has direction, name, and width.
• The cycles array is time-ordered; each entry has index, time, inputs, and outputs.
• inputs / outputs only store signals that changed relative to the previous cycle; an empty object means no changes in that direction for this cycle.
• When loading IR, the tool performs carry-forward expansion to reconstruct full per-cycle vectors before TB generation.

generated_tb.sv (generated SystemVerilog TB)

Below is the key excerpt copied from the real test artifact test/out/generated_tb.sv: the statements inside initial begin ... end.

clk = CLK_IDLE;
rst_n = '0;
clear = '0;
en = '0;
din = '0;
mode = '0;

// cycle 0, source_time=5 (1ps)
step_to_edge();
`CHECK_EQ(accum, 6'b000000, 0, "accum");
`CHECK_EQ(dout, 4'b0000, 0, "dout");
`CHECK_EQ(valid, 1'b0, 0, "valid");
`CHECK_EQ(parity, 1'b0, 0, "parity");
step_to_idle();

// cycle 1, source_time=15 (1ps)
rst_n = 1'b1;
en = 1'b1;
din = 4'b0011;
step_to_edge();
`CHECK_EQ(accum, 6'b000011, 1, "accum");
`CHECK_EQ(dout, 4'b0011, 1, "dout");
`CHECK_EQ(valid, 1'b1, 1, "valid");
`CHECK_EQ(parity, 1'b1, 1, "parity");
step_to_idle();

$display("PASS: %0d cycles checked for sample_dut from scope reference_wave_tb.u_dut", NUM_CYCLES);
$finish;