On Fri, Jan 06, 2012 at 07:20:08AM -0200, Isaac Marino Bavaresco wrote:
> Em 06/01/2012 06:34, William "Chops" Westfield escreveu:
> >>> Is there anything I can add to protect the chip from unwanted negativ=
e voltage?
> > Well, to start with, there should be a substantial resistor; about 10k.
> > This will rely on the internal protection diodes to clamp the voltages =
to levels that won't cause problems.  There is plenty of reason to assume t=
hat this is insufficient, but microchip does publish app notes where this i=
s done (apparently successfully.)
> > ...
>=20
>=20
> I prefer to use an external Zener diode plus two resistors to clamp the
> voltages, forming a "T" with the Zener as the "leg" of the "T".

What's the purpose of the resistor between the zener and the PIC input? For
quick and dirty testing, I use a 4.7K resistor in series with the RS-232
output and a 4.7V zener. For more permanent circuits I use this as the base
input to a NPN transistor inverter with a pullup resistor to protect the
PIC's input from the -0.7V the zener allows when the RS-232 output swings
negative.

The last time this discussion came up, Russel noted to me that allowing the
negative voltage to the PIC input, even though it's clamped by the
protection diodes, causes the part to operate outside of it's range
specified in the datasheet (-0.3V minimum). Hence the transistor in the
permanent circuit. I'm unsure what if any damage that 150 uA of current at
-0.7V can do to the input.

BAJ

>=20
> Isaac
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Byron A. Jeff
Department Chair: IT/CS/CNET
College of Information and Mathematical Sciences
Clayton State University
http://cims.clayton.edu/bjeff
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