piclist 2002\10\17\171229a >
Thread: Foolproof input line
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face BY : Russell McMahon email (remove spam text)



> *>>  With few mA it couldn't raise the +5v power suply.

> *>OK, so how about just one part - a series resistor, high in value. If
you
> *>need to drive with open collector, add a pullup resistor before the
series
> *>resistor.

> *>There's no universal way to do this. There are too many PICs, too many
> *>different pins on the PICs, etc...

> Yes there is. The clamping circuit I posted on the piclist before will do
> this, will not rise the Vcc line, will maintain the pic inputs within
> manufacturer specs at all times.

I assumed he meant that there is no single solution that would be cheapest
in every case (or close to cheapest) and meet spec and that it is better to
design to the current requirement than to try to propose a single solution
for all cases. .

One could conceive of a relatively complex circuit that met essentially
every need but it would be liable to be overkill in almost every case. I
assume you are referring to your clamp cct attached as a PDF to an August
email. This has 4 transistors which puts it outside the class of what was
desired and still needs extra components for the input resistor and default
O/C bias. Which is not to say it is a bad cct - just still not the universal
solution.

Maybe we need a FAQuette on input systems?


           Russell McMahon

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