At 10.09 25/02/2004 +1300, you wrote:
>> 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
>
>Don't rely on the internal diodes, the whole chip may latch up.
>If it does you can probably kiss at least that pin goodbye. I've
>had it happen. 12V signal (through 1k with a 4V7 zener to 0V)
>had noise on it that went below ground to the tune of ~0.8V. The
>PIC locked up and fried. I added a BAT46 Schottky diode and
>10n cap to 0V, no more problems

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.

I did it and now the voltages on the pin range from ~0.1V to
~4.9V.. while Vss is 0.0V and Vdd is 5.0V. Per contra, the
Schottky diode would make it -0.3V to 4.9V.

Unfortunately I don't have a digital (nor analogue) oscilloscope
to check the amount (if any) of the overshoot.. but the zeners
should be quite fast even without the cap, right?

Greets,
Andrea

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body