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'[PIC] survivability of short circuited output pins'
2010\01\25@114551 by Matt Rhys-Roberts

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I can't find anything in the datasheets about whether a PIC (say 18F
series) can withstand a short circuit on any of its pins when configured
as outputs. Clearly this would draw more than the 25mA spec limit, but
would it survive? Is there any known internal protection?

Thanks,
Matt

2010\01\25@121617 by Wouter van Ooijen

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Matt Rhys-Roberts wrote:
> I can't find anything in the datasheets about whether a PIC (say 18F
> series) can withstand a short circuit on any of its pins when configured
> as outputs. Clearly this would draw more than the 25mA spec limit, but
> would it survive? Is there any known internal protection?

it is not guaranteed to survive. People have reported chips to survive
such an experiment, but no guarantees.

Of course, there is no guarantee either that it will not survive. Or in
which manner it will die.

--

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu

2010\01\25@122313 by Ariel Rocholl

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In fact the datasheet does indicate what will happen when (if) you exceed
"Absolute Maximum Ratings" in this case 25ma / IO pin either sourced or
sunk:

*NOTICE: Stresses above those listed under “Absolute Maximum Ratings” may
cause permanent damage to the device. This is a stress rating only and
functional operation of the device at those or any other conditions above
those indicated in the operation listings of this specification is not
implied. Exposure to maximum rating conditions for extended periods may
affect device reliability.*

2010/1/25 Matt Rhys-Roberts <spam_OUTmattTakeThisOuTspamnu-ins.com>

> I can't find anything in the datasheets about whether a PIC (say 18F
> series) can withstand a short circuit on any of its pins when configured
> as outputs. Clearly this would draw more than the 25mA spec limit, but
> would it survive? Is there any known internal protection?
>
> Thanks,
> Matt
>

2010\01\25@152150 by peter green

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Ariel Rocholl wrote:
> In fact the datasheet does indicate what will happen
Did you actually read that block of text before copying it? it says may
not will

> when (if) you exceed
> "Absolute Maximum Ratings" in this case 25ma / IO pin either sourced or
> sunk:
>
> *NOTICE: Stresses above those listed under “Absolute Maximum Ratings” may
> cause permanent damage to the device. This is a stress rating only and
> functional operation of the device at those or any other conditions above
> those indicated in the operation listings of this specification is not
> implied. Exposure to maximum rating conditions for extended periods may
> affect device reliability.*
>

2010\01\25@152354 by peter green

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Wouter van Ooijen wrote:
> Matt Rhys-Roberts wrote:
>  
>> I can't find anything in the datasheets about whether a PIC (say 18F
>> series) can withstand a short circuit on any of its pins when configured
>> as outputs. Clearly this would draw more than the 25mA spec limit, but
>> would it survive? Is there any known internal protection?
>>    
>
> it is not guaranteed to survive. People have reported chips to survive
> such an experiment, but no guarantees.
>  
No gaurantees but if short duration shorts frequently resulted in
failure i'd think we would know about it by now. During prototyping
short circuits, misconnections etc are very common.

2010\01\25@154207 by M. Adam Davis

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On Mon, Jan 25, 2010 at 11:42 AM, Matt Rhys-Roberts <.....mattKILLspamspam@spam@nu-ins.com> wrote:
> I can't find anything in the datasheets about whether a PIC (say 18F
> series) can withstand a short circuit on any of its pins when configured
> as outputs.

They do not specify correct operation outside of listed maximums.  In
other words, you can't find this text because they do not gaurantee
it.

> Clearly this would draw more than the 25mA spec limit, but
> would it survive?

It depends completely on the circumstances.  CMOS naturally limits its
own current under some circumstances (as the chip heats up, the
junction resistance increases, which decreases the current, etc, etc).
Further the chips are generally overengineered to some small degree
so that process variations won't eliminate a whole batch of chips that
end up at the wrong end of the variance.

Some hobbyists use this to current limit LEDs - leave out the
resistor, and just let the CMOS take the heat.

> Is there any known internal protection?

There is no intentional internal protection.  Such protection is not
needed most of the time, and costs more in terms of semiconductor
size.  Further, the protection needed depends on the situation - a
simple short in a low current situation isn't hard to protect against,
but a static shock takes more to protect it.  How far do you go before
that protection is enough, and should you make all your customers pay
for it, when only a small portion of customers would actually need it?

You should understand, though, that even though the chip will not
always be destroyed by going outside the limits, it may be damaged in
ways that are not entirely obvious.

The point of static workstations is not because chips kept dying on
the assembly line - they kept dying under 'normal' stresses out in the
field.  Turns out that static directly to the chip didn't kill it, but
weakened it enough that it failed later.

The chips are cheap enough that it's rarely worth your time to keep
using a chip that's been abused - you'll waste far more time debugging
a stupid problem with a bad chip when it finally fails, and you won't
recognize the problem until you give up and swap with a new chip.

-Adam

2010\01\25@161440 by Bob Blick

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On Mon, 25 Jan 2010 16:42:53 +0000, "Matt Rhys-Roberts"
<mattspamKILLspamnu-ins.com> said:
> I can't find anything in the datasheets about whether a PIC (say 18F
> series) can withstand a short circuit on any of its pins when configured
> as outputs. Clearly this would draw more than the 25mA spec limit, but
> would it survive? Is there any known internal protection?


Hi Matt,

In my experience, PICs can survive a longterm single pin short circuit
pretty well at lower supply voltages. If you get over 4.5 volts I
wouldn't count on it surviving. You will lose that pin, and maybe more
of the chip as well.

But an intermittent short during development is not likely to do it any
damage.

In a production device, never allow a short circuit by design!

Cheerful regards,

Bob

--
http://www.fastmail.fm - Choose from over 50 domains or use your own

2010\01\25@164224 by Herbert Graf

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On Mon, 2010-01-25 at 16:42 +0000, Matt Rhys-Roberts wrote:
> I can't find anything in the datasheets about whether a PIC (say 18F
> series) can withstand a short circuit on any of its pins when configured
> as outputs. Clearly this would draw more than the 25mA spec limit, but
> would it survive? Is there any known internal protection?

It's not speced to survive.

In practice it often does.

Since it's not speced to survive, assume it won't.

TTYL

2010\01\25@185512 by Ariel Rocholl

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2010/1/25 peter green <.....plugwashKILLspamspam.....p10link.net>

> Ariel Rocholl wrote:
> > In fact the datasheet does indicate what will happen
> Did you actually read that block of text before copying it? it says may
> not will
>

You're right, it is a language thing. It says "may", I am used to safety
critical requirementes where shall/will/may makes no difference.



--
Ariel Rocholl
Madrid, Spain

2010\01\25@202351 by Jon Chandler

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Where the data sheet says "may fail" in the absolute maximums section, it
MAY fail catastrophically, in which case you may be lucky.  It also might
sort of work most of the time, or not.  As is the nature of such things, it
will probably fail at the worst possible time or during a demo test.  If you
exceed maximum ratings, there's no telling what will happen in the real
world.

Jon



On Mon, Jan 25, 2010 at 3:54 PM, Ariel Rocholl <EraseMEforosspam_OUTspamTakeThisOuTarocholl.com> wrote:

{Quote hidden}

> -

2010\01\26@021350 by Wouter van Ooijen

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> No gaurantees but if short duration shorts frequently resulted in
> failure i'd think we would know about it by now. During prototyping
> short circuits, misconnections etc are very common.

But thorough testing (including full temperature, power supply, and
clock ranges) is very rare during prototyping, and problems are easily
blamed on other issues. So no conclusions can be drawn except that the
chip often appears to survive.

--

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu

2010\01\26@063310 by Marechiare

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Matt Rhys-Roberts wrote:
> I can't find anything in the datasheets about whether a PIC (say 18F
> series) can withstand a short circuit on any of its pins when configured
> as outputs. Clearly this would draw more than the 25mA spec limit...

Not necessarily this would draw enough current to kill by the current
density. It may kill the output by power dissipation by even 25mA when
it gets short-circuited under 5V or even under much lesser voltage
when other outputs are heavily loaded.

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