On Tue, Mar 26, 2002 at 10:29:59AM -0500, Chris Loiacono wrote:
> I am doing a board with a PIC18, on which I'm planning what I'm calling an
> expansion connector.
> Basically I want to bring all the unused I/O, along with some other pins to
> a connector that will allow me to make various daughter-boards that can be
> plugged-in as future options.

Always a good idea.

>
> Normally, I tie all unused I/O to Gnd. DOing this will be somewhat
> challenging, since any of these will be made useless for future use if this
> is done permanently. The options I can foresee are either shunt-jumpers to
> Gnd, or perhaps setting all the unused pins as outputs. The latter is
> something I have never tried. I'm hoping one of you has some insight to
> share on this.

Well you missed a third option. Actually your first option of tying directly to
GND is a somewhat dangerous proposistion. If for any reason such a pin was
ever set to an output and high, you'd blow it sky high.

The third option is to use a very light pullup/pulldown resistor. Gives the
same effect by forcing the input pin to a known state, with the additional
advantage that it can be easily overridden either as an output or input.
I like it best because it's not subject to programmatic failure. However it'll
cost you a bit of board real estate and a bit of power consumption. Also it
can hinder very high speed bus operation, But since a PIC isn't going to
be able to twiddle I/O lines any faster that once every 200ns, it probably
won't be a problem. 22Kohms is probably a good value for starters. Also
places like Digikey carry resistor packs where 15 resistors with a common
pin in a 16 pin package. Two of those should be able to cover the entire bus.

Setting I/O pins to outputs is the next logical choice. However don't trust
your code to set and forget. PIC code can sometimes flip bits in the output
latches especially using read-modify-write instructions.

BAJ

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