Bob Ammerman wrote:
>
> > There is a SPEC for the absolute max voltage allowable to...

> > I don't want to hear about internal clamp diodes, I
> > feel strongly about this like Bob does about overclocking
> > and Olin does about comments.
> >
> > -Roman
>
> Roman,
>
> As you note re: overclocking, I am a stickler for sticking to the specs.
> Just out of curiosity I scanned the datasheet for a couple popular PICs: the
> 16F87x and 16F84A. In both of them, I found the following in the 'absolute
> maximum ratings' section:
>
> 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?


I have no idea what gives, but I still would not
exceed the first spec. For the cost of a 5.1v zener,
either 1w or 400mW types only cost a few cents.
They are A LOT more rugged than the 20mA spec
internal diodes, and can be tested separately.
Even the tiny 1N4148 glass diodes are 100mA and
these are too weak and unreliable to be a good
choice as input circuit protectors (for a size
example). All things weighed up, I can't see why
someone would not use external zener. It is the
only way to stay within all specs.

My opinion is that the internal clamp diodes are
a nice quality feature that Microchip added to
help protect the PIC from BADLY DESIGNED CIRCUITS...
:o)
-Roman

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