NEIL GANDLER wrote:
>
>  In a an automotive powered circuit I am designing. I am concerned about
> interfacing a switched positive power rail to an I/O port. For example.
> When a stereo system provides a switched 12vdc rail, I want the PIC
> to sense this, a very easy task. I assume I could simply use a
> 2 resistor-zener diode circuit to bring the 12v to 5v. Would such a simple
> circuit provide the spike protection required to protect the pic?
> I would appreciate any advice on interfacing such a power rail
> to an I/O port. Thanks.

You don't need a Zener diode, a large resistor is sufficient. The port-pins
of the PICs have protection diodes to Gnd and Vcc. You only have to ensure,
that the current is lower than (I am not sure) 100 Micro-Amps. There
is an application note by Microchip, that tells something about that.
They even connect the PIC to 120 VAC.

Wolfram

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! Wolfram Liebchen, liebchen@ipserv.ffo.fgan.de !
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