The value depends on the circuit, especially if there also is capacitance a=
nd=20
frequency involved.

Since you need a pull-up, the driver is only driving the input low, for exa=
mple=20
an open colector/drain transistor output in some sort of digital sensor. Th=
e=20
resistor is then responsible of pulling the input high after the transistor=
 is=20
switched off. If there is some capacitance in the circuit, it will take som=
e=20
time for the voltage to raise to a high input level. The higher the resisto=
r,=20
the longer the time. If the output varies with a high frequency then the=20
resistor needs to be low enough to pull up the input fast enough.

I2C is a good example. Since this can operate at 100kHz or higher, a relati=
vely=20
low value resistor is needed. Another example is a 1-wire interface which=20
operates with pulsewidth in the us region.

If you, on the other hand, are going to monitor a button relatively close t=
o=20
the PIC, a 27k resistor or even higher for a battery operated circuit, woul=
d be=20
fine.=20

Another factor to take into account is noise. If you have relative long wir=
es=20
connected to an input and too high value for the resistor, the input might=
=20
become sensetive to radiated electromagnetic noise.

I always try to use as high value as possible for the given circuit in orde=
r to=20
draw as little current as possible when the input is driven low.=20

/Ruben


>=20
> -----Original Message-----
> >From: Allen Mulvey <allen@amulvey.com>
> >Sent: Apr 3, 2013 12:46 PM
> >To: PIC List <PICList@mit.edu>
> >Subject: [PIC] Pull Up Resistors
> >
> >Sorry, this is my first post. I forgot to tag the subject
> >line.
> >
> >=20
> >
> >Is there a way to compute the optimum value of pull up (or
> >
> >pull down) resistors on PIC digital inputs?
> >
> >=20
> >
> >Most example circuits I have seen seem to use 10k. My
> >
> >EasyPIC7 board uses 4k7. I have been experimenting and 20k
> >
> >resistors produce unreliable results. I don't see anything
> >
> >in the data sheet to help with this.=20
> >
> >=20
> >
> >I am using a pic16f877a. The leakage current is one
> >
> >micro-amp. Sampling the port must draw considerably more. My
> >
> >current project is not battery operated but, if it were, I
> >
> >would like to use the largest resistors possible.
> >
> >=20
> >
> >Thanks,
> >
> >=20
> >
> >Allen
> >
> >=20
> >
> >--=20
> >http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> >View/change your membership options at
> >http://mailman.mit.edu/mailman/listinfo/piclist
>=20
>=20
> --=20
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>=20
>=20
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2013.0.2904 / Virus Database: 2641/6215 - Release Date: 03/30/13
>=20

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.