VCD Viewer Online
Drop a .vcd file from Icarus Verilog, Verilator, Vivado, ModelSim or GHDL to see its waveforms. Pick signals by scope, zoom and drag through time, read every value at the cursor and measure between two markers. No install, no account and no upload.
VCD Viewer & Waveform Viewer for VS Code and Cursor, free
- Click a .vcd file in VS Code: no signal limit, and a 1.07 GB dump with 20,000 signals shows its first waves after 4.3 s.
- Group signals into a bus, rename and move rows, search with words such as axi data, and your signal list is kept for each file.
- Values are checked against a separate reference reader on 16 sample files, including 4-state values, reals, strings and $dumpoff.
Optional Zovo Pro, USD 19 once: export signals to CSV, compare two VCD files, and a signal activity report. 3 free tries for each.
Drop a .vcd file here
or
Value Change Dump (.vcd) from Icarus Verilog, Verilator, Vivado xsim, ModelSim, Questa, GHDL and others, up to 1.5 GB.
Click to place the cursor. Drag or Shift+wheel moves in time, Ctrl or Cmd+wheel zooms at the mouse, the wheel scrolls the rows. Keys: F fit, + and - zoom, A and B set markers, Left and Right jump to the previous or next change of the selected signal. Undefined values are red, high-impedance values yellow.
Open this in VS Code or Cursor
The free VCD Viewer & Waveform Viewer extension opens a .vcd file with a click inside VS Code and Cursor, next to your testbench. It has no signal limit, reads a 1 GB dump without loading it whole, groups signals into a bus, renames and reorders rows, and keeps your signal list for each file.
Free, MIT license. About VCD Viewer & Waveform Viewer for VS Code.
Zovo Pro
CSV export, compare two runs and a signal activity report. Zovo Pro, USD 19 once.
In the VCD Viewer & Waveform Viewer extension, Zovo Pro exports the signals you pick to CSV (one row per change, or per clock edge with the values a flip-flop captures), compares this run with a golden run (every signal that differs and the first time it does), and gives one activity table for the whole file: edges, high time, shortest pulses and clock frequency of each signal. Each Pro tool works 3 times for free. One payment covers the Pro tools in 22 Zovo extensions. Everything that is free stays free.
After a card payment you get a key on the page. With GitHub Sponsors you sign in from the extension. Details on the Zovo Pro page.
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Questions
Is my VCD file uploaded anywhere?
No. The page reads the file inside this tab. Its Content Security Policy sets connect-src to none, so the page cannot send your design anywhere, and it loads no library. Open the Network tab of your browser tools and you will see no request when a file opens.
How do I make a VCD file?
In a Verilog testbench add $dumpfile("wave.vcd"); $dumpvars(0, tb); and run it with Icarus Verilog (iverilog and vvp). Verilator writes one with its trace option, GHDL with its vcd option, and Vivado, ModelSim and Questa have a VCD dump command too.
Which signals show first?
The first 64 signals of the file. Tick more in the list on the left, press + next to a scope to add all of its signals, or Show all. The search box matches every word, so axi data finds tb.dut.s_axi.wdata. Only the rows on screen are drawn, so many signals stay fast.
How do I move and measure?
Drag the waves or use Shift and the wheel to move in time, Ctrl or Cmd and the wheel to zoom at the mouse, and F to fit the whole run. Click to place the cursor: the Value column shows every signal at that time. Press A and B to set two markers, and the line under the waves shows B - A in the best time unit.
How are x and z values shown?
Undefined values are red and high-impedance values yellow, in the waves and in the Value column. In hex, a digit is x or z when all its 4 bits are, and X or Z when only some are. VHDL values U, W, L, H and - from GHDL show as they are. Short vectors are extended to their width the way the VCD standard says.
How big a file can it open?
The page reads the whole file into the tab, so dumps of a few hundred MB work on a normal computer, up to 1.5 GB. The VS Code extension reads files of many GB without loading them whole.
What is free and what is Zovo Pro?
Everything on this page is free. In the VS Code extension the viewer has no signal limit and is free too. CSV export, comparing two VCD files and the signal activity report are Zovo Pro, with 3 free tries each. One USD 19 payment covers the Pro tools in every Zovo extension.