From: RussellMc <apptechnz@gmail.com>
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Subject: [EE]:: Body diode conduction insanity
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Repost of a 2012 rant re not using body diodes as "protection" against
overvoltage on device pins.
Seems as worth reading now as it was then.
How worthwhile that is I leave to the individual (except those who argue
strenuously against the argument).

 Russell

From: RussellMc <apptechnz@gmail.com>
Date: Fri, 27 Jan 2012 at 02:40
Subject: [EE]:: Body diode conduction insanity
To: Microcontroller discussion list - Public. <piclist@mit.edu>




*Evil AVR app note that uses microcontroller body diodes to zero
cross detect  mains. Insanity writ large.*
      http://www.atmel.com/dyn/resources/prod_documents/doc2508.pdf

The AVR IC designers must tear their hair out when their app note
writers commit such lunacy.
They effectively use 2 megohm input resistance (although they are not
obviously aware of this from in-text comments).
Diode currents of around +/- 240 x 1.414 / 2 m =3D~ 150 uA+ can be expected=
..
Well above the "microamp range" that Microchip recommend in their not
quite as bad but still stupid foray in figs 10-1 and 10-2 here.

      http://ww1.microchip.com/downloads/en/DeviceDoc/chapter%208.pdf


    Russell McMahon

_____________________

* Copy and paste from my stack exchange rant here*


http://electronics.stackexchange.com/questions/25691/measuring-frequency-of=
-a-signal-above-5v-with-a-microcontroller

PROTECTION DIODES:

Many people are unaware of or just ignore the datasheet distinction between
"Absolute maximum" ratings and recommended operating conditions.

Absolute maximum ratings are those which the device is guaranteed to surviv=
e
without damage. Correct operation is not guaranteed.

The PIC concerned allows Vdd + 0.3V on its pins as an absolute maximum rati=
ng.
Operation is not guaranteed during this condition.

Most data sheets clearly specify that during normal operation input voltage=
s
should not exceed the ground to Vdd range.
This datasheet may or may not ro so in its several hundred pages. It is
still wrong to do so.

Many people have thought that concerns about protection diode currents
are baseless. Only some of them have rued the day they thought so and
most have probably lived to rue it or not :-).

Note that the (evil) Atmel application note here uses a 1 megohm
resistor (connected to AC mains ! ) and the Microchip app note here -
figs 10-1 10-2 at least has the grace to say " ... The current through
the clamp diodes should be kept small (in the micro amp range). If the
current through the clamping diodes gets too large, then you risk the
part latching up." Atmels hundreds of uA is NOT "in the microamp
range".

BUT latch up is the least of your problems. IF you latchup the part
(SCR action triggered by currents into IC substrate) the IC often
turns into a smoking ruin and you realise that something may possibly
be wrong.

The problem with body diode currents is when you do NOT get an
immediate smoking ruin. What happens is that the IC was never designed
to accept current between input pin and substrate - the layer hat the
IC is laid down on. When you raise Vin > Vdd the current effectively
flows out of the ICV proper into a phantom fairyland that the iC is
unaware of and that the designer did not and usually cannot design
for. Once there you have small potential;s set up that are never
normally there and current can flow back into adjacent circuit modes,
of not quite adjacent nodes or even into locations some distance away
depending on how big the currents are and what voltages are set up.
The reason this is hard to describe and pin down is because it is
totally undesigned and essentially undesignable. One effect is to
inject currents into floating nodes that have no formal output path.
These may act as gates for FETs - formal or accidental ones, that turn
on or off semi-random parts of your circuit. Which parts ? When? How
often? How long? How hard? Answer - who can tell / nobody can tell -
its undesigned an undesignable.

Q: Does this actually happen? A: Oh yes! Q: Have I seen it happen? A: Yes.

I started what has now proved to be a 1+ decade crusade to make people
aware of this (even though I should have been well aware of it) after
being very badly bitten by it.
I had a relatively simple async serial circuit that caused me no end
of strife. Processor operation was intermittent or semi random. Code
faulted sometimes and not other times. Nothing was stable. The
problem? Body diode conduction, of course. I had copied a simple
circuit from an application note supplied with a product and away we
went.

If you do this without due care it WILL bite you.
If you do it with care and intelligence and design it may well not
bite you. But may.
This is akin to pulling over the centre line into ongoing traffic to
overtake - done with care and not too often and leaving what may be
good enough margins you will usually not die. If you do you will
probably not be surprised :-). So it is with body diode conduction.
Microchips 'microamp range" may be OK. Or not.
Atmel's 1 megohm off mains is an accident waiting to happen.
Suit yourself.
--=20
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