You keep talking about the 170V as being at the PIC.  It's at the
    other end of a resistor whos job is to handle the high voltage.  What
    the PIC sees is essentially a current source at a few tens of microamps.

What bothers me about using the internal clamping diodes to limit the
voltage at a pin is that the energy you pump through them has to get
dissipated SOMEWHERE.  There seems to be a built-in assumption that the
overall circuit will have a high enough current consumption that it will be
dissapated throughout, and the voltage on the V+ rail will NOT creep up on
you into a range where it's out of spec for the whole circuit and
everything in it.  This is worrisome because the allowable current through
the clamping diodes is not that much smaller than what a "typical" PIC
circuit might draw.  I guess the idea is to keep the ACTUAL current through
the clamp diodes much smaller than that (I wonder if that means you need to
avoid the assorted power-saving modes?  Probably.)

You essentially wind up with a voltage divider, right?

Vline---RRR--->|-----V+-----RRR-----GND
+170V   big  clamp  "5V"    equv
             diode  rail    load

The ratio of Rbig to Rload needs to be very high, and/or the V+ regulator
has to handle "reverse" potentials...

BillW

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