A pullup 'PULLS' a signal to the Positive supply rail keeping
it in a known state of LOGIC HIGH until it is driven low by
some means.  The means may be the chip itself or it could be
an external signal to the chip.   The point is, the PULLUP
keeps that pin in a known and stable state until it is changed on purpose.
This is done to prevent a pin from changing on
it's own from an unrelated or unconnected signal to that pin.  For instance,
you have a gate package that has 4 gates in it.  Three are used, but the forth
is extra.  If you don't use a pullup on the input of the unused gate, it's outpu
t
could attempt to follow one of the other used gates as it is switching.  This
causes the power dissipated by the package to go up.  Also, you don't want this
extra gate switching anyway.  Therefore, pullup the input to Vdd through a resis
tor
(ie 10K for instance), to prevent this.  The sympathy drive from an adjacent
signal will not have enough ooomph to overcome the 10K
pullup current, but it can be (overcome) driven by a legit logic signal.  This
is kind of a long winded explanation, but I tried
to speak it in a way that you could easily understand what I'm talking about.
 BTW, the same thing goes for a PULLDOWN resistor, except that now you have
to source current rather than sink it, and the pin is in a known and stable
logic low state.


Did I acheive success?   Hope this helps.

                                 Regards,

                                   Jim





>Well, pretty simple question. I've read about the pullups in a PIC in this

>list, even something that explains the pullup resistors as a elastic gum.
>But the problem is that I don't understand the utility of the pullups and
>for what and when are useful. Could someone of yours explain me the basic
>uses and advantadges of the pullups in a PIC?
>
>thanks in advance.
>