mon "DC ground" for the PC,
   which is the same as the 0V on the PC power supply.  The button connection
   to GND can be physically wired to any of those points - it doesn't matter
   which because they're all connected together.
</p>
<p>
   In the <tt>buttons</tt> section, you configure a GPIO input via
   <tt>source.type</tt> "gpio":
</p>
<pre>
   {
      buttons: [
         // button #0
         {
            source: { type: "gpio", gpio: 5 },  // GPIO input on pin GP5
         },
      ],
   }
</pre>
