> Under what circumstances would one need to use RA4 as an open-drain?  And
what would be the difference between the open drain pin another pin?

RA4 is open drain by necessity and you use it as such because that's how
they made it :-)

HOWEVER it can be useful. The pin can have voltages applied to above Vdd, as
there is no upper protection diode as their is on "normal" I/O pins. As long
as you don't exceed the rated operating voltage for this pin (12v from
memory) you can switch "high side" loads. For example, a PIC run from a 9
volt battery via a 5 volt regulator could use RA4 to drive the base of a
transistor operating from the battery positive rail. No other pin could do
this as the protection diodes would conduct when the voltage rose above Vdd.

A disadvantage is that this pin is not protected against electrostatic
damage due to its absent protection diode.

Open drain is low impedance when low and (very) high impedance when driven
high. This could be used to clamp an analog line to ground when low and to
"ignore" it when high. The same result could be obtained with a normal pin
by switching it between output/low and input with no pullup.

The reason of the lack of a diode is that the pin is used for the high
voltage programming supply connection.



        Russell McMahon

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