VCD Files

VCD (Value Change Dump) is a file format for recording signal transitions. nexsim can read and write VCD files, making it compatible with other waveform tools.

Exporting VCD

From the CLI

The CLI writes VCD by default:

nexsim design.vhd tb.vhd --top tb --run-time 1000
# → writes output.vcd

nexsim design.vhd tb.vhd --top tb --run-time 1000 --output result.vcd
# → writes result.vcd

From the web/desktop app

After a simulation, use the Export VCD action to download the results as a .vcd file.

Importing VCD

Into the web/desktop app

Drag and drop a .vcd file into the app, or use the file picker. nexsim parses the file and populates the waveform viewer — no simulation needed.

This is useful for viewing VCD files generated by other simulators (ModelSim, GHDL, Vivado, etc.).

Into other tools

nexsim’s VCD output is compatible with:

• GTKWave — open-source waveform viewer
• Surfer — modern waveform viewer
• Any tool that reads IEEE 1364-2001 VCD

VCD file structure

A VCD file contains three sections:

1. Header — date, version, timescale
2. Signal definitions — names, types, widths, organized by scope (module hierarchy)
3. Value changes — timestamped transitions

Example excerpt:

$timescale 1ns $end
$scope module mux_2x1_tb $end
$var wire 1 ! in0_tb $end
$var wire 1 " sel_tb $end
$var wire 1 # output_tb $end
$upscope $end
$enddefinitions $end

#0
0!
0"
0#
#10
1!
1#

Each #N line marks a new timestamp. The lines below it show which signals changed and their new values.