Typically each pic has:
2 power lines
2 clock lines
1 reset line
The remainder being regular i/o.

An exception would be the 12cXXX series, which have 2 power lines, and
two lines which can be clock or i/o, for a total of 4 or 6 i/o.

So the 16c84, which has 18 lines uses 5 for clock/power/reset, and has
13 available as i/o.

You should check the data sheets for more specific info and some caveats
(one line on the 16f84 will only drain current, not source it, etc.)

-Adam

Mike Tennent wrote:
>
> Hi:
>
> I've been lurking here for a bit and have checked the various web sites for
> information but I need a little help.
>
> I've worked with the Stamp2 and BasicX-24 but would like to convert one of my
> projects to a PIC based board with the intent to actually produce a product.
>
> What I'm doing is about as plain vanilla as you can imagine. Over a 45 minute
> period, the program simply switches various I/O pins HIGH or LOW in a programmed
> sequence that doesn't vary. (The pins activate/deactivate 12V light
> circuits/relays through a ULN2803.)  Each circuit stays on for a set amount of
> time and then turns off.  Simple timing loops are used to stretch out the
> sequence to 45 minutes. All the pins end up LOW at the end.
>
> What I want is the maximum number of I/O pins possible to drive the circuits.
> The more the merrier.  What I'm unclear of is how many of the I/O pins on the
> various family of chips have to get used for other overhead uses such as clock,
> etc. (If any?)
>
> In other words, the 16C5xx chips have 13 I/O pins, the 16C77x have 16, 22, and
> 33 I/O pins, but how many are actually available on a functioning board? The
> number of useable pins is the primary thing I'm looking for.
>
> Thanks for any help you can offer.
>
> Mike Tennent