> I was quite seriously considering such a technique in my next project.
> Can I assume that you do not think this is wise?

I don't see a problem with connecting a PIC input to mains power via a
suitably large resistor (several Mohm), but you should note the following:

1  -  This does not apply to analog inputs.  I have noticed small Vdd
protection diode current cause significant errors on the remaining analog
channels.  I'm guessing this is because the internal reference voltage
gets messed up.

2  -  I would keep the protection diode current well under the specified
maximum.

3  -  This topology should never be used if any part of the PIC circuit
goes anywhere near where humans can touch.  It would never get regulatory
approval, and is safety hazard in any case (imagine a little dirt or
moisture on the resistor).  It is fine if this circuits in encapsulated
inside a grounded metal box on the primary side of a power supply, for
example.

4  -  Unless this is a very high volume product where every cent matters,
I would spend a few extra resistors and diodes to make it just a little
more failsafe.


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