>-----Original Message-----
>From: Omega Software [mailto:omegasoftware@TISCALI.IT]
>Sent: 25 February 2004 09:56
>To: PICLIST@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.

Beware when using zeners to clamp low level signals!  Low voltage zeners
have a very soft "knee", and the zeners rated voltage is defined at an
appreciable current (usualy around 5-20mA).  At low current levels the zener
voltage can be significantly lower! e.g at 1mA a typical 4.7v zener may only
have a potential of (say) 3.7v across it.

Regards

Mike




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