More on (ab)use of PIC protection diodes & bad design - please explain ...
Russell McMahon email (remove spam text)
Rudy said -
>The story below tells me that the protection-diodes should be protected
>Normally I would discard such a story as an hoax. But you seem to be
>serious about it. Could you try to explain ? (in as simple as terms as
>possible please ...)
This is my perspective - others should by all means correct me or improve on
my descriptions if I seem to be in error.
Protection diodes are intended to protect against electrostatic voltages
which may be applied to i/o pins. They are not intended to ensure normal IC
operation while the diodes are conducting, so -
Protection diodes may be purposefully integrated into the product as
discrete devices on the main die or may be "parasitic" diodes which occur
when silicon and metallization is laid down on the basic IC substrate.
Generally the latter will only be input-to-ground diodes. Others here will
be able to describe this in much more detail than I.
However, whether purposefully or incidentally provided on chip diodes which
are intended for protection against electrostatic or out of spec voltages
will most probably be integrated into the IC "where they occur" - ie it is
likely that the shortest path to Vcc or ground will be taken from the pin
which the diode is protecting. I'll refer only to diodes from input to Vcc
from here on but the same applies to diodes to ground. When such diodes do
conduct the voltages at the "common" point will be about 0.6 volts above
Vcc. If they are in fact in close proximity to other connections to the
common Vcc "plane" these voltages will be available for "injection" to other
unrelated parts of the circuitry. Any impedance in the common Vcc circuit
will increase the chance that such stray voltages will couple into an
unintended circuit point. (A discrete circuit analog equivalent is
"motorboating" - an oscillation which occurs when input and output circuitry
of a gain stage have a common power supply impedance in their power supply
paths.) When the currents involved are very small it is likely that digital
circuitry will NOT be affected adversely or at least, not to a noticeable
extent. Where currents are large, strange things WILL happen. BUT, even when
currents are small strange things MAY happen. If the currents are injected
into analog portions of the circuitry (eg oscillator section) all bets are
off as to what will happen. A lot of the time you can get away with using
the protection diodes this way. Microchip's data sheets (and all other
reasonable manufacturers datasheets) specifically warn you that operation
like this MAY cause problems.
Why are discrete catch diodes different?
If diodes are provided to eg Vcc external to the PIC etc the currents are
connected at what should be a very low impedance point (the supply pin) and
appear in the same way as supply noise. Potentials which occur on the chip
due to them will always have a positive gradient (ie they will be greater
than the nominal positive supply) and the chip maker will have designed to
have noise at this point have as little affect as possible. It is possible
to design external catch diode circuits so that they have almost zero affect
on Vcc noise and the results can be fully designed. Injecting the currents
into the IC at randomly chosen points in the midst of the IC can not be
designed for - the IC maker does not provide details on what goes on inside
at this level of detail.
If a manufacturer wanted to include catch diodes to clamp input pins to
supply voltage DURING NORMAL OPERATION the could design the IC such that
injected currents and potential rises were isolated from random internal IC
points. As this is not what the diodes are meant for this is not usually
what the manufacturer does.
My practical experience indicates that the strange performance expected from
using the protection diodes to limit pin voltages during operation does in
fact sometimes occur.
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See also: www.piclist.com/techref/microchip/devices.htm?key=pic
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