> >
> > > The clamping diodes are useful when working with large input voltages
> > > needing only an external current limiting resistor for this purpose.
> > >
> >
> >This comment is likely to reignite a religious war here on PICLIST. There
> >are those that state that you must have more than a simple series
resistor.
> >Others claim that this technique is valid.
>
> Most devices have similar clamping diodes.
> The problem is that most manufacturers won't specify how much current you
> can put into a pin in this manner. So, if you end up depending on this,
> you're into a grey area as far as a dependable production design.
>
> If you get it wrong, you risk SCR latchup on the die, and the only way out
> is to power down, and let the device cool. (assuming it hasn't been
toasted)
>
> >2: in the dc parameters the maximum current is limited to (for some
typical
> >chips) 20 ma.
>
> Output current rating, not related to how much you can safely dump through
> the protection diodes.
>

For the '876 line at least, there are specs in the 'absolute maximum
ratings' section that state:

Input clamp current, IIK (VI < 0 or VI > Vdd)  == +/- 20mA
Output clamp current, IOK (VO < 0 or VO > Vdd) == +/- 20mA

Don't get me wrong: you gotta build to be in specs. But you should know what
the specs really are, and not just design  based on old-wives-tales and the
like.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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