Usually, pull-ups are relatively weak (low amounts of resistance) so the processor can
drive a pulled-up I/O line to be low. The pull-up means that the signal on that line is
defined to be high even when the processor isn't driving it. A processor may have internal
pull-ups so that inputs to the processor have a default state even when unconnected.
Note that there are also pull-downs, which means resistors to ground. All of this applies to
them except that their default logic level is low instead of high.
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