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PICList Thread
'[PIC]: It's official: The Real-World Serial FAQ'
2002\10\11@125757 by Roman Black

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Mike Harrison wrote:

> Nonsense. The input clamp diodes ensure that the input voltage is not
> exceeded, as long as the series resistor limits the current within
> spec.


Actually this isn't true. Russell has already posted
a LONG retort, so i'll just add one more point and
try to keep it short. :o)

Diodes have a *turn on time*. This is one main
difference between a special high perf diode and the
puny crap diodes in a PIC chip. You cannot rely on
pin voltage being constrained by the PIC diode being
in the on state, especially when driven with a
fast squarewave at high (>5v) voltage.

In the period before total forward saturation of
the diode there is FULL input voltage applied to
the die inside the PIC. You MUST assume that for
all reliability purposes the PIC is exposed to the
full brunt of your input voltage as a resistor will
have no effect UNTIL the diode turns on and becomes
fully saturated. PSU fault spikes, static from
connectors etc have very fast time constants and
will be applied directly to the PIC silicon.

In effect the WORST CASE voltages are the ones LEAST
protected by the internal diodes.

Using an EXTERNAL clamp diode allows the voltage
input and turn-on time to be tested and examined etc,
and hopefully also snubbed in energy by external RC(Z)
as needed, *long before it gets near the PIC internals*.
:o)
-Roman

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