> Van: Sebastián Dols <sdols@ESEGI.ES>
> Aan: PICLIST@MITVMA.MIT.EDU
> Onderwerp: basic question
> Datum: dinsdag 6 april 1999 16:37
> 
> 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?

The simplest answer I could think of was this one :

1) a line should never be in a un-defined state.
2) a line that is in a un-defined state should be forced in a
defined-state.

Solutions:
 
A) Connect the above line to either VCC or GND. Dis-allowing any other
source to manipulate the line. (the source will short-circuit to either VCC
or GND)
B) Connect the above line to either VCC or GND using a resistor. This way
an other source can pull the line any way it want's, when it takes the
increased current (thru the resistor) into count.  If the source is _not_
applied (three-stated for example) the line is still in a defined state.

A resistor connected to VCC is mostly called a Pull-Up,  if connected to
GND it's sometimes reffered to as a Pull-Down.

Greetz,
  Rudy Wieser