The resistor (usually to VCC) is called a pullup resistor, not a
current-limiting resistor.  It is probably better for noise immunity to put
the resistor closer to the PIC pin.  It is conventional to use a pullup to
VCC and switch to ground, partly for historical reasons (TTL works better
that way), and partly because then wherever the switch is located, all it
needs is its signal and ground.  Usually ground is readily available in lots
of locations (such as a metal chassis) where it would take an extra wire to
carry VCC to the switch.

You are, of course, configuring your I/O port for input, so it is in the
high-impedence state, and negligible current is actually flowing in or out
of the PIC.  In a very power-critical situation (i.e. sleeping for an
extended period), you will use less current when the switch is open, and not
drawing any current through the resistor.

The CMOS logic with which the PIC's are implemented is pretty much
symmetrical with respect to power and ground, so it will work just as will
to use a pulldown resistor to ground and connect the switch between the
input and VCC.  Either way, it is arbitrary and up to you to decide whether
you'd rather consider the switch to be "on" when it's open or when it's
closed.

-----Original Message-----
From: David E. Olson [mailto:dolson@PROGRESS.COM]
Sent: Friday, January 07, 2000 12:20 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: [SOT] Currently limiting resistor location


I've got a series of three switches that are located remotely (about 3ft)
from my PIC. On the switch block, I've got the current limiting resistors
for each switch. I had a synapse misfire that makes we wonder if there were
a short in the wire(s) that I could be pulling the port low with a higher
the expected current (my power supply is capable of 1.5a). Can I locate the
switch resistors on the PIC board so that if that happens, I'm safe(r).

Also, wouldn't I be better off setting the port high for an on (instead of
pulling low) condition so that if there is a wiring ground fault, I don't
have an unexpected port change? I assume I'd have to then worry about over
sourcing...which should be predictable since the load is a switch.

-DO