At 01.38 26/02/2004 +1300, you wrote:
>> 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 with respect to Vss.
>
>Ah, OK. Wasn't quite what I imagined (one of *those* days, but at
>least you got me thinking)
>
>So it will conduct at Vss + 0.3V (although a zener may have a
>+/- 0.3V regulation range). If you have a true 0V signal coming in
>then that could cause excess power consumption if the zener
>conducts.

Yes, I'm using a 1K serie resistor to limit current (max 5 mA @ 5V).


>A signal might need to be > 0.6V < threshold for a "0"

Yes I know, that's why I keep the "0" in the 0.1V .. 0.2V range, to make
sure that the "0", although never negative, is as close to 0 volts as
possible anyway. Also, the "1" is in the 4.75V .. 4.85V range (Vdd is 5V),
for the same reason.


>> It works.. I checked it.. but I wish I had an oscilloscope to check
>> the transients as well
>
>Typical for a Philips 4V7 is ~300pF max @ 1MHz.  A BAT46
>is around 8pF @ 1MHz. I'm not sure if there's a spec stated for
>the PIC's internal diodes
>
>I think I still prefer the tighter specs of a Schottky

Yes, but that won't save you from getting -0.3V at the PIC pin..
may this cause problems? For MCLR it is definitely out of specs.

It isn't for I/O pins, but then again how do we know that component
tolerances don't make the PIC conduct instead of the Schottky diode
(both are rated -0.3V, but the former may be -0.3 and the latter
-0.31 because of tolerances.. who knows?). I/O clamp current is
+/- 20mA in the datasheet (18F6520) I'm checking, so if we're within
this current there should be no problem even without the Schottky
diode, right?

Again, for MCLR it is definitely out of specs anyway (the datasheet
says that voltage must be >= 0V for this pin).

Greets,
Andrea

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