Reading VCD Waveforms

What is a VCD file?

Value Change Dump (VCD) is a standard ASCII format for recording signal values over simulation time. iverilog produces VCD when you include $dumpfile / $dumpvars in your testbench.

initial begin
    $dumpfile("sim.vcd");
    $dumpvars(0, tb_and_gate);   // dump all signals in tb_and_gate
    // ... stimulus
    $finish;
end

VCD file structure

$timescale 1ns $end          // time unit for #N delays
$scope module tb_and_gate $end
  $var wire 1 ! a $end       // signal a, ID !
  $var wire 1 " b $end       // signal b, ID "
  $var wire 1 # y $end       // signal y, ID #
$upscope $end
$enddefinitions $end

#0                           // time = 0
0!  0"  0#                   // a=0, b=0, y=0

#10                          // time = 10ns
1!                           // a changes to 1

#20
1"                           // b changes to 1 → y goes to 1
1#

Reading the timeline

Each #N marks a time step. The lines following it show only the signals that changed (VCD is delta-encoded). A missing signal means its value is unchanged from the previous time step.

Scalar value changes

Syntax	Meaning
0!	signal with ID ! is 0
1!	signal with ID ! is 1
x!	unknown
z!	high-impedance

Vector (bus) value changes

b1010 $    // multi-bit signal with ID $ = 4'b1010

Mapping IDs back to signal names

The $var declarations at the top of the file map short IDs (like !, ", #) to human-readable signal names. Use the WaveformViewer in the SkillLift player to see this mapping automatically.

Common pitfalls

• No VCD output? Check that $dumpfile and $dumpvars are inside an initial block and that $finish is called.
• Signals show x at time 0? Normal — signals are uninitialized until driven.
• Missing signals? Increase the depth argument to $dumpvars(depth, module). Use 0 to dump all levels.
• VCD too large? Narrow the dump window: use $dumpon / $dumpoff around the region of interest.