Content originally posted in LPCWare by nerd herd on Tue Jul 21 10:43:00 MST 2015 Hi drvrh,
To enable the internal pull-up resistor on a specific pin, make sure the pin you are interested in has an internal pull up (check UM, chapter 6), and that you are modifying the correct bit. For instance, if you want to enable the pull up on PIO0_4, you need to set bit 4, for that pin's specific register.