> The "ideal" safe clamp is a pair of normally reverse biased schottky
diodes
> from PIC pin to Vcc and ground. These are relatively expensive compared to
> other solutions but cheap compared to malfunction in most cases. Be aware
> that an external voltage clamped to supply with diodes (whether external
> Schottky, silicon or PIC internal clamp diodes can "pump up" the PIC Vcc
> rail if the PIC and associated circuitry draws less than the clamped
current
> and the power supply has no provision for clamping over-voltage. This
would
> be an unusual situation but in some cases could lead to circuit
destruction.

In WLoader I use a clamp (for RS232) with a resistor, a 4V7 zener (limits
voltage at that point to -0.6 .. +5V) and a R divider from that point to Vcc
that raises the -0.6 V to just above 0 V. The first resistor is large enough
to avoid big currents through the zener. IMHO this should be safe, but from
what I heard it works 'almost always', which is maybe acceptable for a
hobby-design but of course not enough for serious work. Which goes to tell
that going to the extremes might save some components but adds a lot of
work...

Wouter

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