> | 7. Port pins have a backwards diode to the power supply pin. Overdriving
a
> | port pin can feed energy back to the power supply.  This can be useful
if
> | you understand how it works, maddening if you don't.
> |
>
>
> 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.

The datasheets give two specifications that seem to apply:

1: in the absolute maximum ratings section: pin voltage is limited to
Vss-0.3 to Vdd+0.3

2: in the dc parameters the maximum current is limited to (for some typical
chips) 20 ma.

The 'series-resistor-is-enough' camp believe that the series resistor will
handle the current limiting, and the on-chip clamping diodes will limit the
pin voltage.

The 'something-more-is-needed' group insist that you cannot depend on the
clamping diodes to do the voltage limiting.

There are several app notes from mChip that seem to endorse the
'series-resistor-is-enough' line of thinking.

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

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