Russell McMahon wrote:
> Also, pins that are utilised are far less likely to be overlooked when
> changes are made at some time after initial design, whereas unused
> pins have no "champion" and if something happens to change their
> status it may not be noticed. For example, a port may contain a
> certain mix of inputs and outputs and have a DDR mask set accordingly.
> Some pins may be unused and set as outputs and left unconnected or as
> inputs and connected to say Vdd. If at some later stage the DDR mask
> is changed to reflect some new usage of the existing pins the change
> may also affect the unused pins.

This issue can be dealt with by good software design.  I use /INBIT and
/OUTBIT preprocessor directives to declare all I/O pins.  Any pins not
explicitly declared are automatically set as outputs driven low by the
standard PORT module.  If pins are moved around or added or deleted, the
/INBIT and /OUTBIT pin declarations have to change by necessity.  I would
have to go out of my way to do something stupid for unused pins not to be
handled correctly automatically.

I've done somewhere between 50 and 100 PIC projects and never had a problem
with this.  A few times it's been handy to connect something to a previously
unused pin after the fact.  On designs that aren't that cost sensitive, I
usually bring each unused I/O pin to its own labeled pad to aid rework in
case it becomes necessary.


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