On Sat, 18 Aug 2001, Andy Shaw wrote:

> Hi Folks,
> A quick question about RA4 when used for output. On most devices it is open
> collector. What is the max voltage I can pull this up to? For example if I
> wanted to drive a MOSFET from it could I use it to pull down to zero volts
> and have a resistor hooked up to say a 12V supply to turn the FET on? I have
> a feeling this is not allowed but I thought I would ask. I seem to remember
> something in the spec for 16F87X products saying 8V or so, but I don't have
> the spec handy and I can't get to Microchips web site at the moment!

Data sheet says Vdd max, absolute max is Vdd + 0.6V.  I have heard
otherwise secondhand, but anything higher would be out of spec.  You may
need to use RA4 to drive a transistor to turn on the FET, or use a logic
level FET.

> On the same subject what about other common logic families TTL/CMOS etc. Do
> they have chips that take PIC level logic inputs and have open collector
> outputs that could be used in this way. Or would I be better of using some
> sort of driver chip that is intended for the job?

There are TTL family OC drivers.  Some will withstand some relatively high
voltages, like 40V or so, I think.

Dale
--
A train stops at a train station.  A bus stops at a bus station.
On my desk I have a workstation...

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