>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.

I think the elastic explanation was quite good (Wagner Lipnharski; subject:
"Re: Err, this is SO confusing =(").
I had a problem using a NO pushbutton which shorted an input pin to the
ground without using a pull-up resistor.

Input pin ---------NOPB---------GND

When the pushbutton was (normally) open, the input pin wasn4t connected to
anything.  It had little resistance to interference and was jumping randomly
between HIGH and LOW causing interrupts every time.

When i added a pull-up the input pin was pulled to Vdd while the PB was
open, but when the PB closed the input pin was pulled to ground (causing an
interrupt) and when it opened again the pin was again pulled to Vdd (also
causing an interrupt).

Pin-------NOPB-------GND
  |
  |
Pull-up-----Vdd

Hope this helps,
Johann Geir Jonsson

PS.  Instead of the pull-up resistor, I could have used the PIC internal
weak pull-ups (I think; I haven't used them yet).