They are, I believe, P-channel MOSFETs and should therefore behave just
like real resistors for your purpose. The tolerance is however fairly
broad, but you ought to be able to parallel a bunch of input port pins
to do it. To measure the resistance, simply enable the weak pullups,
then put the pins in parallel and connect a 47K resistor to ground. The
voltage across the resistor can then be measured. Use Ohms law to work
out the current, then the voltage Vcc-Vx must be developed across the
pullups, which will have the same current through them as the 47K
resistor and voila, you can calculate the resistance.

Note that you are also presenting quite a bit of capacitance to your
open-drain output, though. It seems like a strange idea....

> ----------
> From:         Marc Heuler[SMTP:marc@AARGH.MAYN.DE]
> Reply To:     pic microcontroller discussion list
> Sent:         Thursday, January 29, 1998 1:09 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      16C84 PortB "weak pullups" in parallel?
>
> Hi.
>
> I'm trying to build a zero-components pullup resistor for an open
> drain
> output.  I have plenty of port pins left on the C84.
>
> Somewhere in the data sheet (can't find where) I read the "weak
> pullups" of
> PortB are equivalent to 470kohm pullup resistors.  Can I parallel
> them,
> like real resistors?  Ie when paralleling 8x 470kohm, I'd get 58kohm.
>
> Do I get 58kohm with 8 PortB pins in pullup mode?
>