Hello,

I have been searching all over the net and can not find an answer for this. It is possible I just missed it or maybe there simply isn't a single answer to be had. In any case, what is the value of the internal resistors on an input pin? If I set a pin's mode to pullup, what resistance is it offering?

They don't seem to be specified anywhere, but there are some numbers for the pull-up/pull-down current depending in the input voltage. See the LPC17xx datasheet, "Electrical pin characteristics".

Yeah I figured they pretty much just weren't listed. Thanks for the help :) Those charts though lol...I look at those things and my mind melts into a pile of goo.

Take a look at Fig. 34 (page 60) in the datasheet. Mosfets are used to provide the pull-up/pull-down ability with no reference to the resistance these provide. You can always measure it with a miltimeter. I would guess they would be no more than 1k considering the supply voltage of 3.3v.

You can calculate the pin's impedance based on the voltage & current of the charts. But that does not gives you the pullup resistance value. Is there a reason why you need to know the internal pullup resistance value ?

I was just adding a new IC that required a 5K pullup. I was just wondering if it were possible to avoid doing this myself. I am thinking it isn't. As far as I could figure out from the data sheet, most resistances are something like 33Ohms.

If said a pullup is required, it is better to add it. Generaly, pullups can be from 1K-10K, depending how much current require by the i/o. It serve also to have a defined state on the pin at power on/reset & signal stability.