3.2.4 What is ``pull up''?

Consider ``pull up'' as ``a weak tendency to default to a high voltage for an input pin''. This means that if a pin, configured as input and with ``pull up'', is not connected to anything, it senses a high voltage.

The ``weak tendency'' is important, because we want the sensed voltage of this input pin change as soon as some component (such as a sensor) drives the voltage low. Most sensors that are transistor based or labeled ``open collector'' only have the ability to drive low but not drive high, which means it is important for the input pin to have the ability to ``default'' to a high voltage.

To make this more clear, there are only two situations with an input pin that is configured to ``pull up'':

• The external component is not driving low. The input pin defaults to a high voltage and reports a 1.
• The external component drives low, overrides the default voltage, the input pin senses a low voltage and reports 0.

If there is a ``pull up'' configuration, is there a ``pull down'' configuration? The answer is yes. However, ``pull down'' is less commonly used, so MCUs typically do not have built-in abilities to configure for ``pull down''. One can still configure ``pull down'' with circuits external to the MCU.