VCD Viewer & Waveform Viewer (Verilog, No Signal Limit)

Zovo

3 installs

VCD viewer and waveform viewer for Verilog, SystemVerilog and VHDL simulations. Free with no signal limit: signal tree with search, cursor and two markers, bus grouping, hex, binary, signed and ASCII radix, and VCD files of 1 GB and more. A GTKWave-style viewer inside VS Code that works offline.

Installation

Launch VS Code Quick Open (Ctrl+P), paste the following command, and press enter.

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VCD Viewer & Waveform Viewer (Verilog, No Signal Limit)

Click a .vcd file from Icarus Verilog, Verilator, Vivado, ModelSim or GHDL and see its waveforms in VS Code. Free, no sign-in, works offline, and there is no limit on how many signals you show.

Real output from VS Code on macOS. The file is axi_lite_bus.vcd, made by scripts/make_samples.py. scripts/e2e-vscode.mjs installed the packaged extension in a clean VS Code profile, opened the file, placed the markers with real key presses (A and B, after jumping to the edges with the arrow keys) and saved the window through the debug port.

There is no signal limit. Add one signal, a whole scope or all 20,000 signals of a file. The rows are drawn only where you look, so 250 rows scroll as fast as 5.
The values are checked. Every release is checked in a real VS Code against a separate reference reader: all signals of 16 sample files at 5 cursor times, including 4-state values, short vectors that need left extension, real numbers, strings and $dumpoff.
Scrolling works the way you expect. The wheel scrolls the signal list. Shift+wheel, a trackpad swipe, a drag or the bar under the waves moves through time. Ctrl or Cmd+wheel zooms at the mouse.
Large files open fast. A 1.07 GB VCD with 20,000 signals showed its first waves after 4.3 s. A signal that changes now and then appeared 0.06 s after you added it, and one that changes all the time in 0.73 s. The file is never read into memory as a whole.

Quick start

Install VCD Viewer & Waveform Viewer.
In your testbench, dump a VCD: $dumpfile("wave.vcd"); $dumpvars(0, tb); (Verilog), --vcd=wave.vcd (GHDL) or --trace (Verilator).
Click the .vcd file in the Explorer. The first 64 signals show right away.

Double-click a signal in the tree to add it, or press + next to a scope to add every signal in it. If another viewer opens the file, right-click it and pick "Open in VCD Waveform Viewer". "VCD Waveform Viewer: Get Started" in the Command Palette opens a short tour.

Find your way in time

Do this	To
Ctrl or Cmd + wheel, or pinch	Zoom at the mouse
Drag, Shift + wheel, a trackpad swipe, or the bar under the waves	Move through time
Shift + drag, or drag on the time ruler	Zoom to that range
`F` or Fit	Show the whole simulation
Click	Put the cursor there. The Value column shows every signal at the cursor.
Left and Right arrow (or `P` and `N`)	Jump to the previous or next change of the selected signal
`A` and `B`	Put marker A or B at the cursor. The toolbar shows the time between them.
Zoom A–B	Zoom to the range between the markers

Times show in the unit that fits the zoom (fs, ps, ns, µs, ms or s), from the file's $timescale.

Organize the signals

The soc_256_signals.vcd file with all 259 signals added. The rows list clk, rst_n, irq and the valid and data signals of each core, with hex values in the Value column. Names that appear in more than one scope get their scope in front, such as fetch.valid_0.

Do this	To
Radix menu in the toolbar	Show the selected signals in hex, binary, unsigned, signed or ASCII. Integer variables start as signed.
Ctrl/Cmd + click, Shift + click	Select several signals
Bus, or `G`	Group the selected signals into one bus. The top one is the most significant bit. Press again to split it.
Double-click a name, or `F2`	Rename a signal. The real name stays in the tooltip.
Drag a row, or Alt + Up/Down	Move signals
`Delete`	Remove the selected rows from the view
Double-click a value	Copy it
Type in the box above the tree	Search. Every word must match, so `axi data` finds `tb.dut.s_axi.wdata`.

Undefined values are red and high-impedance values are yellow, in the waves and in the Value column. A hex digit is x or z when all 4 bits are, and X or Z when only some are. VHDL values U, W, L, H and - from GHDL are shown too.

Your signal list is kept

The viewer remembers the signals, radixes, names, zoom, cursor and markers for each file, on this computer only. When the simulator writes the file again, the viewer reads it again and keeps your list. Export saves the list as a .signals.json file you can commit next to the testbench, and Import loads it into another file.

Large files

The file is read in blocks by a background thread. The first pass builds an index of which block holds changes of which signal. When you add a signal, only those blocks are read again, by up to 8 threads at once. Measured in a real VS Code on a 1.07 GB file with 20,000 signals (Apple silicon, other programs running):

Step	Time
Signal tree shown	0.13 s
First waves shown	4.3 s
Add a signal with 2 changes	0.06 s
Add a signal with 81,173 changes spread over the whole file	0.73 s
Memory of the VS Code extension host at its peak	552 MB

While many other programs were busy on the same computer, the same steps took up to 13 s for the first waves and 4.5 s for the busy signal. A progress bar shows while the file is read. Files larger than the vcdViewer.maxFileMB setting (16 GB) show a message instead.

Files that are not quite right

The viewer opens damaged files and tells you what is wrong, in a bar above the waves:

a file cut off in the middle of a value change (a simulation that was stopped),
a timestamp that goes back in time,
a signal declared twice, or a value change for an identifier that no $var declares.

A file that is not a VCD at all shows a short message instead of a blank page.

A file whose timestamps go back. The note bar reads: Timestamp #25 at byte 243 is earlier than #30. Changes after it are drawn at the later time (2 times). The waves of clk and n are drawn below it.

Settings

Setting	Default	What it does
`vcdViewer.defaultRadix`	hex	Radix for vector signals when you add them
`vcdViewer.initialSignals`	64	Signals shown when a file opens for the first time (16 at most for files over 200 MB)
`vcdViewer.autoReload`	true	Read the file again when the simulator writes it
`vcdViewer.rowHeight`	normal	compact, normal or large rows
`vcdViewer.maxFileMB`	16384	Larger files show a message and are not read
`vcdViewer.askForRating`	true	Ask one time for a Marketplace rating after a few days of use
`vcdViewer.showSupportLinks`	true	Show the small Sponsor and More free viewers links

Dark, light and high-contrast themes are followed, including the colours of the waves.

The same AXI-Lite file in the Light Modern theme, with the scope tree open on the left and markers A and B on the first write

Privacy and safety

No network calls, no telemetry, no account. The webview has a content security policy of default-src 'none', and the file is opened read-only: the viewer never writes to your VCD. The signal list is kept in VS Code's own storage. The install is about 180 KB.

Limits

VCD only for now. FST, GHW and LXT files are not read yet.
Real values are drawn as values in a bus shape, not as an analog curve.
GTKWave .gtkw save files are not read. Use Export and Import for signal lists.

Support Zovo

Zovo extensions are free, with no ads and no tracking. They are made by theluckystrike on GitHub. If this one saves you time, you can support the work:

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License

MIT