On Tue, 7 May 2002, Olin Lathrop wrote:

> > I've always done an external PUP on input pins. This might sound rather
> > dumb,
>
> Yup.

Nope, not really.

> > but if you enable the weak pup on PORTB, can you actually see them
> > pulled EXTERNAL to the pin (as in using a logic probe), or is it just
> > internal?
>
> THINK about it.  What use would an internal pullup be that isn't reflected
> on the input pin?  If the input is high, you want the bit to read 1.  If the
> input is low, what would you want it to do?  If it reads 0 then the pullup
> has no effect.  If it reads 1, then the input is useless.

I think you misunderstood his question.  He was asking whether an
*internal* pullup on the PIC would affect the circuit *outside* the PIC -
IOW, if the internal pullup would be detectable from outside the PIC
itself.  The answer is yes.  If you have an input with no internal pullup
it will float, with the internal pullup ON it will assume a high level
until a low signal is applied (with lower impedance than the internal
pullup).

You can see why by looking at the PIC datasheet.  In the port section is
an equivalent circuit for the I/O pins -- the pullup is a CMOS FET just
inboard of the I/O pin.  When it's turned on, you have in effect a
resistor to Vdd.

Dale

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