> Voltage on any pin with respect to VSS
> (except VDD, MCLR. and RA4)
>  -0.3V to (VDD + 0.3V)
>
> Input clamp current,
> IIK (VI < 0 or VI >VDD)
> +/- 20mA
>
> Now to me, these two specs seem to contradict each other.
>
> From the latter, for example I would think that a 15V signal thru a 100K
> resistor could safely be connected to the PIC, since the clamp current of
> the protection diode would only be (15V-5V)/10K = 0.01ma, which is orders
of
> magnitude below the 20mA specified.
>
> But of course, this connection would then drive the pin to one diode drop
> above Vdd, which exceeds the voltage spec.
>
> What gives? Why specify a nonzero maximum clamp current when the voltage
> spec implies that you can never apply  an input that forward biases the
> clamp (ie protection) diode?

Hmm.  Good point.  I think maybe they are trying to say that if you connect
a PIC pin to a voltage source with 0 impedence, don't go more than 300mV
outside the supply range (probably because the can be SURE that the diodes
won't conduct and therefore exceed current).  However, if you are going to
connect a current source, all is well as long as it is limited to +-20mA.

In other words, it is really a current spec, and the voltage spec is a
guaranteed limit within which the current won't be exceeded.

If this is indeed true (maybe it isn't), then I don't see the problem with
connecting a PIC pin to 170V via 5Mohm.  That's only 34uA.  The voltage at
the other end of the resistor is irrelevant (as long as the resistor can
handle it) because that voltage never shows up at the PIC.

I don't see Roman's point about not wanting to rely on the protection
diodes, as long as the relevant parameters are specified and guaranteed by
Microchip.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@embedinc.com, http://www.embedinc.com

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