Sean Alcorn wrote...

>I am just about to embark on my biggest PIC project to date. I am just =
in
>the hardware planning stages, and I am going to need every I/O available=
 on
>the 40/44 Pin version.
>
>I have seen a lot of discussion on this list regarding RA4 being an =
"Open
>Drain" type.
>
>Can somebody please advise the concerns when using this pin - either as =
an
>input or an output and point out the traps and pitfalls that can be =
avoided.

AFAIK, there's nothing unusual about the RA4 pin except for the fact
that it's open-drain in output mode.  It can be an ordinary port input
pin, an output pin, or an external clock input to Timer 0 if configured
as such.

"Open drain" outputs can be thought of as "the lower half of a normal
CMOS output".  Whereas the latter have an N-channel MOSFET to pull the
output down to ground when outputting a LOW and a P-channel MOSFET to
pull the output up to Vcc when outputting a HIGH, an open-drain output
omits the P-channel device.  It can pull down, but but it can't pull up.

In certain kinds of circuits this can have advantages; but if you want
to use RA4 as a normal output, you simply connect a pullup resistor
between RA4 and Vcc to do the "pulling up" part.  Depending on what
you're trying to do, anything from 1K ohms to 10K ohms should do just
fine.

If you're using RA4 as an input, there are no special considerations.
It acts like any other input.

Hope this helps...

Dave

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