>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.

>-----Original Message-----
>From: Omega Software [RemoveMEomegasoftwareTakeThisOuTTISCALI.IT]
>Sent: 25 February 2004 09:56
>To: PICLISTEraseME.....MITVMA.MIT.EDU
>Subject: Re: [EE]: -0.75V on a I/O pin, limited current
>
>
>At 15.59 25/02/2004 +1300, you wrote:
>>> Good idea the cap and the resistor, that should kill much of the
>>>overshoot.. but wouldn't it be better another zener (this one
>>>connected to Vdd, not Vss) instead of the Schottky diode? I  mean a
>>>two zeners solution, one zener to Vss and the other to Vdd
>>
>>Wasn't "undershoot" the problem ? R+Z will limit V on the PIC pin to
>>4V7, but adding another ZD to Vdd won't do anything to stop sub-0V
>>transitions. A zener has the same Vf as a normal P-N diode (like the
>>PIC's internal ones), whereas the Schottky's metal-N
>composition has a
>>lower Vf and will conduct first, which is what you need
>
>If you put a 4.7V zener, anode to your I/O pin and cathode to
>Vdd, if will conduct when the I/O pin is below -4.7V respect
>to Vdd, i.e. when it's below ~0.2V respect to Vss.
>
>It works.. I checked it.. but I wish I had an oscilloscope to
>check the transients as well. For this application though (low
>frequency) the rise/fall time is so slow that it's not going
>to be a problem.. with (and probably also without) a cap. But
>I like the idea of the double 4V7 zeners (one to Vss and one
>to Vdd) and was wondering how they behave with transients, at
>high frequences.

>At 15.59 25/02/2004 +1300, you wrote:
>>> Good idea the cap and the resistor, that should kill much of the
>>> overshoot.. but wouldn't it be better another zener (this one
>>> connected to Vdd, not Vss) instead of the Schottky diode? I
>>> mean a two zeners solution, one zener to Vss and the other to
>>>Vdd
>>
>>Wasn't "undershoot" the problem ? R+Z will limit V on the PIC
>>pin to 4V7, but adding another ZD to Vdd won't do anything to
>>stop sub-0V transitions. A zener has the same Vf as a normal
>>P-N diode (like the PIC's internal ones), whereas the Schottky's
>>metal-N composition has a lower Vf and will conduct first, which
>>is what you need
>
>If you put a 4.7V zener, anode to your I/O pin and cathode to Vdd,
>if will conduct when the I/O pin is below -4.7V respect to Vdd, i.e.
>when it's below ~0.2V respect to Vss.
>
>It works.. I checked it.. but I wish I had an oscilloscope to check
>the transients as well. For this application though (low frequency)
>the rise/fall time is so slow that it's not going to be a problem..
>with (and probably also without) a cap. But I like the idea of the
>double 4V7 zeners (one to Vss and one to Vdd) and was wondering how
>they behave with transients, at high frequences.
>
>Greets,
>Andrea
>
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