Hardware Reference
In-Depth Information
WARNING
By default, many of the Bone's GPIO pins are configured as inputs, such as we would want in
this circuit. Later, we'll show you that they can be configured as outputs, and thus you should
take care to avoid shorting an output pin. If the switches were connected as shown in
Fig-
ure 1-21
and the GPIO pins were configured as outputs, having the buttons active would
cause the GPIO pin to be shorted to 3.3 V. If the pin were outputting a low or 0, it would try
to sink all the current it could to make the pin go low. The result is that the pin would burn
out.
NOTE
In addition to the GPIO pins being able to be either inputs or outputs, there are also internal
pull-up and pull-down resistors on the Bone's GPIO pins. It is necessary to use the pull-down
mode of the GPIO pins for this circuit to work predictably. Otherwise, when the switches are
open, the input pins are not connected electrically and will be in an unknown state.
