Posted on May 17, 2011 at 11:59 Good question. These ports have pull-up, see page 49 of the datasheet. At page 147 the input low current is specified to be at least -10uA and at most -50uA. That means the pull-value value is between 100K and 500K. Compare this to the standard 50K (typical) of a traditional 8051 machine. If you are designing a board and you need pull-ups on some input, you should better place external resistors and not rely too much on these internal pull-ups: the lower the resistance the greater will be RFI immunity (at the expense of more current).
Daniel
