File format:vcd

vcd

Name	Value change dump
Status	supported
Source code (in)	vcd.c
Source code (out)	vcd.c
Common extension(s)	.vcd, .wave
MIME type	—
ASCII format	yes
Compression	yes (only stores transitions)
Website	wikipedia.org

vcd is an ASCII-based file format that...

It can be opened and processed e.g. in a waveform viewer software.

Format

See Wikipedia for a short overview.

Implementation

These are the input module's options and their default values:

 $ sigrok-cli -I vcd --show 
 ID: vcd
 Name: VCD
 Description: Value Change Dump data
 Options:
   numchannels: The number of (logic) channels in the data (default 0)
   skip: Skip samples until the specified timestamp; < 0: Skip until first timestamp listed; 0: Don't skip (default -1)
   downsample: Downsample, i.e. divide the samplerate by the specified factor (default 1)
   compress: Compress idle periods longer than the specified value (default 0)

Interpretation:

• The number of channels gets auto-determined by the software, but can get overridden by user specs (limited to a smaller number).
• By default all input data gets processed, users can specify that a leading part of the input shall get skipped.
• The sigrok software will use the input file's sample count and timescale values, which _can_ result in excessive memory consumption. Some generators love to specify insane timescales which results in large sample numbers. The downsample option allows to workaround that implementation detail.
• The sigrok software will provide lots of samples for extended periods of time when the input signal has these idle periods. Users can request to have these periods shortened.

There are some limitations in the current VCD input module implementation:

• Support for nested name spaces is missing ('module' keyword).
• Exclusive support for single-bit logic data ('wire 1' or 'reg 1'). No support for bit vectors, integer or real numbers.
• The total number of channels may be limited (single letter names, which results in some 94 supported names or so).

Examples

$date Fri Aug  1 23:46:38 2014 $end
$version libsigrok 0.3.0 $end
$comment
  Acquisition with 2/8 channels at 500 kHz
$end
$timescale 1 us $end
$scope module libsigrok $end
$var wire 1 ! sda $end
$var wire 1 " scl $end
$upscope $end
$enddefinitions $end
#0 1! 0"
#40 0!
#72 1"
#102 1!
#144 0!
#174 0"
#204 1!
#246 0!
#276 1"
#306 1!
#348 0!
#380 0"
#410 1!
#452 0!
#514 1!
#566 0!
#628 1!
#668 0!
#730 1!
#770 0!
#832 1!
#872 0!
#938 1!
#978 0! 1"
#1010 0"
#1040 1!
[...]
#106772 1!
#106812 0!
#106844 0"
#106874 1!
#106914 1"
#160000

Related/alternative formats

The [GTKWave] project also supports extensions to the VCD format, as well as alternative formats which avoid VCD's limitations, and/or improve performance. The [GTKWave manual] discusses the formats and conversion utilities.

• EVCD: (TODO Find the name and short description for this format.)
• FST: Fast Signal Trace. This format is a block-based variant of IDX which is designed for very fast sequential and random access.
• IDX: VCD Recoder Index File. This format is written by GTKWave when instructed to generate fastload files.
• GHW: GHDL Wave file. This is a nine state (“01XZHUWL-”) file format written by the VHDL simulator GHDL.

Resources

• Wikipedia: Value change dump
• beyondttl.com: Value Change Dump (VCD)