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<section id="waveforms-with-gtkwave">
<h1>Waveforms With GTKWave<a class="headerlink" href="#waveforms-with-gtkwave" title="Link to this heading"></a></h1>
<p>GTKWave is a VCD waveform viewer based on the GTK library. This viewer support
VCD and LXT formats for signal dumps. GTKWAVE is available on github
<a class="reference external" href="https://github.com/gtkwave/gtkwave">here</a>. Most Linux distributions already
include gtkwave prepackaged.</p>
<img alt="../_images/GTKWave_Example2.png" src="../_images/GTKWave_Example2.png" />
<p>Generating VCD/FST files for GTKWAVE ————————————
Waveform dumps are written by the Icarus Verilog runtime program vvp. The user
uses $dumpfile and $dumpvars system tasks to enable waveform dumping, then the
vvp runtime takes care of the rest. The output is written into the file
specified by the $dumpfile system task. If the $dumpfile call is absent, the
compiler will choose the file name dump.vcd or dump.lxt or dump.fst, depending
on runtime flags. The example below dumps everything in and below the test
module:</p>
<div class="highlight-verilog notranslate"><div class="highlight"><pre><span></span><span class="c1">// Do this in your test bench</span>
<span class="k">initial</span>
<span class="k">begin</span>
<span class="w"> </span><span class="n">$dumpfile</span><span class="p">(</span><span class="s">&quot;test.vcd&quot;</span><span class="p">);</span>
<span class="w"> </span><span class="n">$dumpvars</span><span class="p">(</span><span class="mh">0</span><span class="p">,</span><span class="n">test</span><span class="p">);</span>
<span class="k">end</span>
</pre></div>
</div>
<p>By default, the vvp runtime will generate VCD dump output. This is the default
because it is the most portable. However, when using gtkwave, the FST output
format is faster and most compact. Use the “-fst” extended argument to
activate LXT output. For example, if your compiled output is written into the
file “foo.vvp”, the command:</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">% </span>vvp<span class="w"> </span>foo.vvp<span class="w"> </span>-fst<span class="w"> </span>&lt;other-plusargs&gt;
</pre></div>
</div>
<p>will cause the dumpfile output to be written in FST format. Absent any
specific $dumpfile command, this file will be called dump.fst, which can be
viewed with the command:</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">% </span>gtkwave<span class="w"> </span>dump.fst
</pre></div>
</div>
<section id="a-working-example">
<h2>A Working Example<a class="headerlink" href="#a-working-example" title="Link to this heading"></a></h2>
<p>First, the design itself:</p>
<div class="highlight-verilog notranslate"><div class="highlight"><pre><span></span><span class="k">module</span><span class="w"> </span><span class="n">counter</span><span class="p">(</span><span class="n">out</span><span class="p">,</span><span class="w"> </span><span class="n">clk</span><span class="p">,</span><span class="w"> </span><span class="n">reset</span><span class="p">);</span>
<span class="w"> </span><span class="k">parameter</span><span class="w"> </span><span class="n">WIDTH</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mh">8</span><span class="p">;</span>
<span class="w"> </span><span class="k">output</span><span class="w"> </span><span class="p">[</span><span class="n">WIDTH</span><span class="o">-</span><span class="mh">1</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="mh">0</span><span class="p">]</span><span class="w"> </span><span class="n">out</span><span class="p">;</span>
<span class="w"> </span><span class="k">input</span><span class="w"> </span><span class="n">clk</span><span class="p">,</span><span class="w"> </span><span class="n">reset</span><span class="p">;</span>
<span class="w"> </span><span class="kt">reg</span><span class="w"> </span><span class="p">[</span><span class="n">WIDTH</span><span class="o">-</span><span class="mh">1</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="mh">0</span><span class="p">]</span><span class="w"> </span><span class="n">out</span><span class="p">;</span>
<span class="w"> </span><span class="kt">wire</span><span class="w"> </span><span class="n">clk</span><span class="p">,</span><span class="w"> </span><span class="n">reset</span><span class="p">;</span>
<span class="w"> </span><span class="k">always</span><span class="w"> </span><span class="p">@(</span><span class="k">posedge</span><span class="w"> </span><span class="n">clk</span><span class="p">)</span>
<span class="w"> </span><span class="n">out</span><span class="w"> </span><span class="o">&lt;=</span><span class="w"> </span><span class="n">out</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mh">1</span><span class="p">;</span>
<span class="w"> </span><span class="k">always</span><span class="w"> </span><span class="p">@</span><span class="n">reset</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">reset</span><span class="p">)</span>
<span class="w"> </span><span class="k">assign</span><span class="w"> </span><span class="n">out</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mh">0</span><span class="p">;</span>
<span class="w"> </span><span class="k">else</span>
<span class="w"> </span><span class="k">deassign</span><span class="w"> </span><span class="n">out</span><span class="p">;</span>
<span class="k">endmodule</span><span class="w"> </span><span class="c1">// counter</span>
</pre></div>
</div>
<p>Then the simulation file:</p>
<div class="highlight-verilog notranslate"><div class="highlight"><pre><span></span><span class="k">module</span><span class="w"> </span><span class="n">test</span><span class="p">;</span>
<span class="w"> </span><span class="cm">/* Make a reset that pulses once. */</span>
<span class="w"> </span><span class="kt">reg</span><span class="w"> </span><span class="n">reset</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mh">0</span><span class="p">;</span>
<span class="w"> </span><span class="k">initial</span><span class="w"> </span><span class="k">begin</span>
<span class="w"> </span><span class="n">$dumpfile</span><span class="p">(</span><span class="s">&quot;test.vcd&quot;</span><span class="p">);</span>
<span class="w"> </span><span class="n">$dumpvars</span><span class="p">(</span><span class="mh">0</span><span class="p">,</span><span class="n">test</span><span class="p">);</span>
<span class="w"> </span><span class="p">#</span><span class="w"> </span><span class="mh">17</span><span class="w"> </span><span class="n">reset</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mh">1</span><span class="p">;</span>
<span class="w"> </span><span class="p">#</span><span class="w"> </span><span class="mh">11</span><span class="w"> </span><span class="n">reset</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mh">0</span><span class="p">;</span>
<span class="w"> </span><span class="p">#</span><span class="w"> </span><span class="mh">29</span><span class="w"> </span><span class="n">reset</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mh">1</span><span class="p">;</span>
<span class="w"> </span><span class="p">#</span><span class="w"> </span><span class="mh">5</span><span class="w"> </span><span class="n">reset</span><span class="w"> </span><span class="o">=</span><span class="mh">0</span><span class="p">;</span>
<span class="w"> </span><span class="p">#</span><span class="w"> </span><span class="mh">513</span><span class="w"> </span><span class="nb">$finish</span><span class="p">;</span>
<span class="w"> </span><span class="k">end</span>
<span class="w"> </span><span class="cm">/* Make a regular pulsing clock. */</span>
<span class="w"> </span><span class="kt">reg</span><span class="w"> </span><span class="n">clk</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mh">0</span><span class="p">;</span>
<span class="w"> </span><span class="k">always</span><span class="w"> </span><span class="p">#</span><span class="mh">1</span><span class="w"> </span><span class="n">clk</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">!</span><span class="n">clk</span><span class="p">;</span>
<span class="w"> </span><span class="kt">wire</span><span class="w"> </span><span class="p">[</span><span class="mh">7</span><span class="o">:</span><span class="mh">0</span><span class="p">]</span><span class="w"> </span><span class="n">value</span><span class="p">;</span>
<span class="w"> </span><span class="n">counter</span><span class="w"> </span><span class="n">c1</span><span class="w"> </span><span class="p">(</span><span class="n">value</span><span class="p">,</span><span class="w"> </span><span class="n">clk</span><span class="p">,</span><span class="w"> </span><span class="n">reset</span><span class="p">);</span>
<span class="w"> </span><span class="k">initial</span>
<span class="w"> </span><span class="nb">$monitor</span><span class="p">(</span><span class="s">&quot;At time %t, value = %h (%0d)&quot;</span><span class="p">,</span>
<span class="w"> </span><span class="nb">$time</span><span class="p">,</span><span class="w"> </span><span class="n">value</span><span class="p">,</span><span class="w"> </span><span class="n">value</span><span class="p">);</span>
<span class="k">endmodule</span><span class="w"> </span><span class="c1">// test</span>
</pre></div>
</div>
<p>Compile, run, and view waveforms with these commands:</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">% </span>iverilog<span class="w"> </span>-o<span class="w"> </span>dsn<span class="w"> </span>counter_tb.v<span class="w"> </span>counter.v
<span class="gp">% </span>vvp<span class="w"> </span>dsn
<span class="gp">% </span>gtkwave<span class="w"> </span>test.vcd<span class="w"> </span><span class="p">&amp;</span>
</pre></div>
</div>
<p>Click on the ‘test’, then ‘c1’ in the top left box on GTKWAVE, then drag the
signals to the Signals box. You will be able to add signals to display,
scanning by scope.</p>
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