There are actually 5529 pull-up / pull-down resistor enable register PXREN
While still do not know how to use, but used before when interrupted when doing IO port interrupt input, need to detect rising or falling, then it should be configured as an input port, and then enable interrupts, select trigger, usually we think that this is over, but not actually, you also need to enable the resistance register PXREN , then configure PXOUT register to control a pullup or pulldown.
Specifically what does it mean, when used in the study as well. . . Remember first