> Evil AVR app note that uses microcontroller body diodes to zero cross
> detect  mains.
> Insanity writ large.
>
>       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 =~ 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.
>
>       ww1.microchip.com/downloads/en/DeviceDoc/chapter%208.pdf
>
>
>     Russell McMahon
>
> _____________________
>
> Copy and paste from my stack exchange rant here
>
>   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
> survive without damage. Correct operation is not guaranteed.
>
> The PIC concerned allows Vdd + 0.3V on its pins as an absolute maximum
> rating. Operation is not guaranteed during this condition.
>
> Most data sheets clearly specify that during normal operation input
> voltages 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 "inh 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
> iject currents into floating nodes that have no formal output path.
> Thse may act as gates for FETs - formal or accidental ones, that tuirn
> on or off semirandom 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. Atmel's 1 megohm off mains is
> an accident waiting to happen. Suit yourself.

> -----Original Message-----
> From: .....piclist-bouncesKILLspam@spam@MIT.EDU [piclist-bouncesKILLspamMIT.EDU] On Behalf
> Of .....alan.b.pearceKILLspam.....stfc.ac.uk
> Sent: 26 January 2012 16:54
> To: EraseMEpiclistspam_OUTTakeThisOuTMIT.EDU
> Subject: RE: [EE]:: Body diode conduction insanity
> > > Short of using Schottky diodes for the external diode, in order to
> guarantee they
> > conduct first, what else can be done?
> > For the cost of another couple of diodes, a resistor, and slightly more
> current draw, the clamp diodes could have their own power supply, with the
> two diodes forward biased, and the resistor between them.
> > i.e.
> Vcc
> Forward biased diode
> Clamp voltage going to top protection diode
> Resistor (10k or so?)
> Clamp voltage to bottom protection diode
> Forward biased diode
> GND