> P.S.  It nags at me that the "absolute max" ratings specify Vcc+0.3V:  I
wonder
> if this is the forward drop of the internal clamping diodes, or an
arbitrary (x
> 0.5) safety-factor applied to the usual 0.6V?


I'd guess it was a 0.5 factor after they had some problems at 0.6v.

Putting a series diode in the line reduces the voltage BUT floats the pin
when  the diode is non-conducting.
Placing a resistor to ground from the pin should allow this to work OK.

Schottky's work (but cost a bit more).

A 5v1 zener can be driven to more than 5v1 depending on current and the
supply may be below 5v.
A 4vX zener may be OK depending on circumstance.
Note that zeners may have slightly higher diode direction (non zener)
conduction than standard diodes.



            Russell McMahon

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