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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