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Thread: Good Design versus what (usually) works - Tutorial / was ...
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picon face BY : Russell McMahon email (remove spam text)



>This is an important question - if there are Vcc problems as opposed to
>internal issues, that means that your external clamping circuit has
>limitations as well - in particular, the external schottky diode scheme
>Russell suggested isn't a good idea.


Yes - axtra protection may be needed. I specifically mentioned (was it in
that post or the one before?) the risk of this happening and noted that a
power supply zener at eg 5v6 for a 5 volt system would largely overcome
this. The engineer's assistant that MChip was promoting had just such a Vcc
pump up risk and my rough calculations indicated that it was a real danger
under some input conditions. Addition of a zener there would have been very
wise. If you can't afford ANY supply pump up at all then you need to limit
the excess potentials before they exceed supply +/- one diode drop (less a
little bit).

When I experienced this problem with a PIC I don't believe it was due to Vcc
pump-up.
I think it was a genuine internal chip malfunction.
I don't blame MChip for this - although it would be nice if it didn't
happen.
I suspect this problem is common to almost all commonly available
microprocessors.

>    I wouldn't attempt to interface the Inputs and Outputs of ANY
>    micro-controller or microprocessor directly to the outside world.  ...


Agree fully. But this problem may occur if resistors alone are used.

>    The very very minute cost of either diodes, transistors, or buffers
>    ICs is just not worth destroying your project.


Yes. As someone noted, such costs may be not insignificant in volume
production but must still be considered.



     Russell McMahon
_____________________________

>From another world - http://www.easttimor.com

What can one man* do?
Help the hungry at no cost to yourself!
at  http://www.thehungersite.com/

(* - or woman, child or internet enabled intelligent entity :-))

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