Yeah, I brain farted on the resistor values. I have a big box of 4.7k that I
use when I need such a thing. Truth is, Paul is right FOR THE PIC in that I
program unused pins for output and set them low. I DO always leave space for
a pad or a resistor for unexpected additions (especially post-production)
and if I were working with chips that I wasn't absolutely sure were going to
survive a short, I would use the resistors.

I also worry about my designs freaking on power-up before I can set the
unused pins for output. I haven't had the problems that Tony mentions with
erratic operation but if a production part started exhibiting that effect, I
would have a space on the board for the resistors to be added as a fix.

So, when answering a newbie question, I always advise the resistors. I still
think its the safest single answer.

I was asked off list if its ok to tie the pins together and use just one
resistor. My response was:
The safest thing to do is to use a separate resistor to a rail for each pin.
The next safest thing to do is to not use the resistors at all and program
the pins as outputs.
The least safest thing would be to tie the pins together and use one
resistor to a rail as it would be very easy to get two pins programmed as
outputs and one set high while the other was set low.

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-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Dave VanHorn
Sent: Wednesday, October 27, 1999 3:22 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: Unused Ports


> 4. By using very large value resistors (as I recommended), the current
draw,
> even if the pin is incorrectly set, will be negligible. A 100M resistor at
5
> volts is 50nA IF set incorrectly. I have seen power consumption increase
by
> 50mA when the pull-ups were removed from an unused port on an old Z80/PIO

This is a useless value for a pullup.
Electric field effects will dominate, and your "pulled up" input will be
bouncing all over the place.  Try something sane like 1-10k, which will
survive even near field E and H field events and usually remain in whatever
state you pulled it to.

Pullups on cmos inputs don't draw appreciable current.
NMOS Z80s are not CMOS, and they do draw current.

The safest, least cost method to secure unused pins is as outputs, low or
high.
Making them inputs adds ways for the code to have problems.
Shorting an output pin to the rails is not going to kill a chip. They will
simply output their max current into the load, and get slightly warm.

That being said, I would still put a resistor to ground on the schematic,
and not populate it, so that later, you can add things to those pins, using
the resistor pads.