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Hi

I don't think the input of the pic should ever be stressed. Maybe you can
get away with it on the bench with a JDM progger, but if your device is out
in the field, it's too risky.
If your inputs are going to be spiked, something like the attached GIF
should be used.
- the input is first current restricted by the first 10K
- then it is clamped by the schottky
- then it is subject to clamping by the pic after second 10K. At this
stage, it should only be a few mV
- a 5V1 zener should be used on the Vcc incase the input is pulled high for
long periods, and could pull the Vcc up if your PIC cct is low power.
- bypass the 5V1 zener with a small cap for transients (back to ground), at
the point of the zener. You could even further isolate the Vcc shown here
with a small R from the main Vcc
- keep all clamping relative to gnd only, with a good ground plane
- you could put tiny caps at the pic inputs too, depending on whether
rise/fall times are important.

Regards
Roland

At 10:56 AM 25/02/04 +0100, you wrote:
>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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>

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Regards
Roland

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