Write this in your "Things must know about microprocessors" book.

        **** Ignoring unused pins is bad engineering. ****

It's asking for trouble.
Sooner or later you will be sorry.
You may already be sorry and wonder why :-)
This applies to almost every microprocessor - it's NOT a PIC specific
problem.

The only situation where it wouldn't matter (much) would be if the default
port setting was inputs with pullups enabled.

If an input floats it will likely spend a significant part of its time near
the middle of the voltage range. Near is a relative term depending somewhat
on processor. Unless you input is Schmitt triggered, having it float mid
supply will do BAD things. A Schmitt will draw less extra current (but still
more than normal). Regardless, it will not only draw higher current but MAY
noise up A2Ds, make the processor act funny like, create EMI and more. Often
it mightn't. Sometimes it will. Don't do it.

Choices in approximate order of decreasing goodness (IMHO) BUT this will
vary with your level of discipline and documentation.

1 Input with internal pullup.
2 Output set high or low.
3 Input with external pull up/down
4 Input tied to ground.supply

1 is zero cost but can be destroyed by software change.

2 is zero cost but has a pin doing something - not usually a problem unless
someone equates unused with don care.

3 Costs the pull up/down but is well defined.
This is the safest option but annoying.

4 Is free BUT can cause bad problems if software is changed.

        Russell McMahon

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