piclist-bounces@mit.edu wrote:
>> The spec is
>> Vss-0.6V. You say you put -8.2V on an input. Thats clearly out of
>> spec. The only reason you might be measuring -0.6V is because the
>> diodes are already conducting... 
> 
> No, I put -8.2V *through* 1k on the pin. Even so, I don't
> think that it's reasonable for the limit to be so stringent
> that a Schottky clamp must be used.


Hi Peter,

I think there's a fundamental misunderstanding here. I do understand that you're using a 1K resistor between the input and the pin. Let me try to explain: As you already know, the PIC input pin is high-impendance. This means that the voltage at the pin is going to be very close to -8.2V (actual value depends on the pin's impendance & characteristics of your input source). So there you are violating the specs. Now what happens is the protection diode conducts, which of course clamps the voltage at the pin. But that doesn't mean you are now suddenly within spec! 

If you use e.g. the external diode you mentioned, or even a resistive divider, then the pin would not have seen a voltage outside specified range, the protection diodes would not have conducted, and life is good. 

I hope this helps.

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