At 01:24 PM 2/24/2004 +0100, you wrote:
>At 13.16 24/02/2004 +0100, you wrote:
> >> What happens if one puts -0.75V on a PIC's I/O pin, with
> >> limited current (say max 10 mA)?
> >
> >This is within the 'absolute maxima', so you chip will not die.
> >
> >But it is outside the 'normal operating conditions', so the chip is not
> >guaranteed to behave conform the datasheet.
> >
> >If you feel like taking a chance you could try and see. But such a test
> >has limited scope: one temperature, one Vcc, one chip, one program, etc.
> >
> >One thing that has been reported: even a very small current through the
> >protection diodes can seriously affect the A/D accuracy.
> >
> >NB this has been discussed over and over, but there are two vieuwpoints:
> >1- propper operation is not guaranteed so don't do it
> >2- it will probably work so why not (but IIRC 10mA is a pretty high
> >current, even for the proponents of this viewpoint)
>
>Thanks for the insights.
>
>I also just noticed a pretty obvious thing that was causing problems:
>if we let the internal protection diodes of the chip kick in, that will
>create a low impedance path to Vdd or Vss, and thus may cause problems
>if one expects the PIC input to behave as high impedance.
>
>I'll go the zener route now.

A Schottky diode to Vss might be more predictable. For example, if
the signal is applied when Vdd = 0. A 1N5819, for example, will ensure
that little current flows through the internal diode. You could also
use a regular diode (eg. 1N4148) and add some series resistance to
the input (eg. a few k ohms) so that again very little current flows
into the PIC when Vin < 0.

You can also make a high/low clamp with a couple of dual complimentary
transistors (eg. XN0560100L) and a resistor (3 components, or 5 if you
use discretes).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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