The safest thing to do is to tie all unused pins to either ground or the
positive supply voltage using resistors, and leave them tri-stated (if they
are programmable i/o). Anything from about 1K to about 10K will work fine.
This way, if the pins get set to an unintended state (due to software bugs
or electrical noise or whatever), the chip will not be damaged because the
current will be limited by the resistor.

You could get rid of the resistors by just leaving the pins unconnected and
set as outputs, (high or low) but this leaves the possibility of the pins
floating while the chip is initializing, or in a fault situation in which
the pins are tristated unintentionally. It also introduces the (low but
real) possibility of the pin being shorted to the other supply rail and
causing functional failure or damage to the chip or supply.

The important thing is to avoid having a tri-stated (input) pin floating,
and to avoid having an output pin driven against an external connection so
that it exceeds its rated source/sink capability (or the combined
source/sink capability of the chip). Pull up or pull down resistors
eliminate both possibilities in all conditions. The worst than can happen is
accidentally driving an output against a pull up/down and thus wasting
power.

James Newton mailto:jamesnewton@geocities.com phone:1-619-652-0593
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-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of HJ Simpson
Sent: Tuesday, October 26, 1999 11:13 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Unused Ports


Does anyone have an pinion on what to do with unused ports?
Make them outputs and leave floating, and set or clear?
Make them inputs and pull up or pull down?
And.... speaking of inputs, what do think is best, pull up and input as
a clear, or pull down and input as a set?
Opinions gratefully received.
Regards Howard.