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PICList Thread
'ESD protection'
1996\07\08@145059 by Mike Jones

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Hello PIC experts,

I have couple of simple questions, which I haven't seen covered in any
data sheets:-

1) What ESD precautions should I take when designing PIC application
circuits? Do I tie unused i/o pins to VSS (or VDD) via a resistor, or
is it safe to leave the pins disconnected and rely on the on-chip ESD
protection?

2) Should I think about putting rf bypass/decoupling capacitors on
the power supply lines near the chip? If so, what sort of values
should I use?



Thanks in advance for your help.

Mike

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1996\07\08@214022 by Steve Hardy

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> From: Mike Jones <mikespamKILLspamnewjay.win-uk.net>
>
> Hello PIC experts,
>
> I have couple of simple questions, which I haven't seen covered in any
> data sheets:-
>
> 1) What ESD precautions should I take when designing PIC application
> circuits? Do I tie unused i/o pins to VSS (or VDD) via a resistor, or
> is it safe to leave the pins disconnected and rely on the on-chip ESD
> protection?

If you are leaving the pins as _inputs_, tie directly to VDD.  If the
power supply is a bit dodgy, use a 4K7 resistor.  Only one resistor may
be used for all unused pins.  If pins are left as _input_ open
circuits, they may just float at a value which causes excessive current
drain.  This is a no-no for battery powered circuits.  However, the
sensible thing to do (which avoids all these problems) is to define
unused pins as ouputs.  They may then be safely left open-circuit.

>
> 2) Should I think about putting rf bypass/decoupling capacitors on
> the power supply lines near the chip? If so, what sort of values
> should I use?
>

Yes.  Use a 0.1uF monolithic or ceramic cap, and place as close as
possible to the device's VSS and VDD pins.  Note that if using one of
the PIC devices with more than one VDD/VSS pin make sure there is a
power net connection to _all_ such pins.  E.g. the 16C74 has two each
of VDD and VSS.  I run a thick (50mil) trace directly between each pair
and put one of those 0.1uF monocaps right up against one side.  This
pretty much case-hardens it against ground bounce and other problems.

Regards,
SJH
Canberra, Australia

1996\07\08@215449 by Clyde Smith-Stubbs

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hardy@sweng.stortek.com (Steve Hardy) wrote:

> sensible thing to do (which avoids all these problems) is to define
> unused pins as ouputs.  They may then be safely left open-circuit.

Not necessarily - if the output pin is an open-drain type, then you
must also drive it low - an open-drain output is high-impedance when
high, and then acts like an input.

Clyde

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'esd protection'
1997\12\13@172031 by Rob Aerts
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part 0 1090 bytes
!DOCTYPE HTML PUBLIC "-//W3C//DTD W3 HTML//EN">
hello,
 
I have one question for all you guys/girls
 
In a project with a PIC16C84 the inputs can be touched bij fingers,
my question : how can i protect the i/o against ESD charges ?
 
Kind regards,
 
Rob Aerts
 
 
 
 

1997\12\15@104448 by Justin Crooks
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Well, how about grounding the person touching the pins?  If this isn't
feasable, you could use an RC filter...

----------
From: Rob Aerts <rob.aertsspamspam_OUTTIP.NL>
To: @spam@PICLISTKILLspamspamMITVMA.MIT.EDU
Subject: esd protection
Date: Thursday, December 11, 1997 3:13 PM

hello,

I have one question for all you guys/girls

In a project with a PIC16C84 the inputs can be touched bij fingers,
my question : how can i protect the i/o against ESD charges ?

Kind regards,

Rob Aerts

1997\12\15@113649 by Peter Lorand Peres

picon face
It is VERY bad to ground the person, it involves risk of electric shock
from anything not connected through a fault current breaker, including the
project's power supply. As far as I know, the only place where you can
ground the operator safely is in a production environment where the units
are not powered....

There are chips that protect IO pins. Check MAXIM MAX336 and co.

One thing ppl. keep forgetting when using PICs with external connections,
is the fact that the same bulk and protection diodes that allow a PIC to
be powered by its IO pins (featured elsewhere in this list), still work
when there is power applied to the Vcc and GND pins as it should. Since
most regulators cannot sink current, this can easily lead to a PIC being
powered by its (incorrectly designed) load, which can bring the VCC RAIL
TO FAR MORE THAN +5v. The cure is a zener connected between Vcc and GND
after the regulator, directly at the PIC power pins (Z5V6 or such), AND
the usual limiting diodes/networks at every concerned IO pin... else the
limiting diodes only supply power to the PIC...

I've seen many such designs in mags, and it's scary. You have this PIC
running at 5 V from a 78L05, and an input or output going through a
resistor to +12V or whatever. The resistor is dimensioned to limit the
current into the PIC input pin. However, if the PIC sleeps then the design
current flowing through the resistor raises the VCC to almost the level of
the higher voltage, due to the low current drain of the part.... the
regulator opposes absolutely no resistance to this.

here's my 2 bits.


On Mon, 15 Dec 1997, Justin Crooks wrote:

{Quote hidden}

Peter (plp)

1997\12\15@125933 by John Payson

picon face
> It is VERY bad to ground the person, it involves risk of electric shock
> from anything not connected through a fault current breaker, including the
> project's power supply. As far as I know, the only place where you can
> ground the operator safely is in a production environment where the units
> are not powered....

The common thing is a wrist strap connected to ground through a 1 meg resis-
tor.  This will let through enough current to prevent electrostatic charge
from building up, but will limit current to well under a miliamp in case of
accidental line contact (even with 250VAC, the RMS current is only 1/4mA).

> There are chips that protect IO pins. Check MAXIM MAX336 and co.

Such devices can be useful in some cases; in many cases they can be avoided
with careful design.  I personally like optoes for protecting I/O, but they
aren't free; pretty oops-proof, though.

> One thing ppl. keep forgetting when using PICs with external connections,
> is the fact that the same bulk and protection diodes that allow a PIC to
> be powered by its IO pins (featured elsewhere in this list), still work
> when there is power applied to the Vcc and GND pins as it should. Since
> most regulators cannot sink current, this can easily lead to a PIC being
> powered by its (incorrectly designed) load, which can bring the VCC RAIL
> TO FAR MORE THAN +5v.

This can be a problem; even in cases where the PIC doesn't mind the extra
voltage, other devices may squawk and cause bad things to happen.  Also, an
overly-high VDD is one of the few ways I've seen to destroy PICs while run-
ning close to their specification (I accidentally ran a PIC 16F84 off 7.5
volts; oddly, it now works at higher voltages but won't run or program at
5V any more).

>                        The cure is a zener connected between Vcc and GND
> after the regulator, directly at the PIC power pins (Z5V6 or such), AND
> the usual limiting diodes/networks at every concerned IO pin... else the
> limiting diodes only supply power to the PIC...

Depending upon the application and the number of I/O pins, I sorta like using
a 5.1V zener on the I/O pins.  That one component provides both ground and
VDD clamping while avoiding the overvoltage-VDD problem.  Otherwise, I just
use a series resistor on inputs and make sure VDD goesn't get pulled too high
(ex. power enough stuff from VDD that a minimal current draw is assured; on
at least one occasion I used a dumping resistor from VDD to ground).

By the way, does anyone have any idea how much current it takes to damage a
PIC with excess VDD?  For example, if a PIC were connected to a +24 supply
through a 1meg resistor, would it fry or would it safely clamp the voltage
to something reasonable?

> I've seen many such designs in mags, and it's scary. You have this PIC
> running at 5 V from a 78L05, and an input or output going through a
> resistor to +12V or whatever. The resistor is dimensioned to limit the
> current into the PIC input pin. However, if the PIC sleeps then the design
> current flowing through the resistor raises the VCC to almost the level of
> the higher voltage, due to the low current drain of the part.... the
> regulator opposes absolutely no resistance to this.

Some regulators do offer some sinking capability; others don't.  Any design
which relies upon that behavior, however, may fail when a "drop-in replace-
ment" regulator is substituted.

1997\12\15@133540 by Peter Lorand Peres

picon face
>5.1V zener
Way too low for Vdd=5V. The leakage current at 5V will cause problems so
close to the zener voltage.

>1Meg, high Vdd, damage, when
The real problem is, that the decoupling capacitor that is obligatory in
parallel with the chip Vdd will build up enough energy to damage almost
anything as the chip fails in thyristor mode (voltage raises until it
trips, then you have full current through the chip). You can compute how
many Joules from 24V to 0.6V (assuming it is not fried already) given
W=C*U^2/2, the stored energy (actually only half of it goes into the chip,
but that's another story).

The worst part is, that this almost never happens during normal operation,
only when sleeping. Then, you nicely debug your thing, it works, you have
it sleep, you make 20 of them, you test them, and 10 never wake up
again... Go figure.

>one stuck, works only with high voltage
Various things are known to go wrong with PICs that were exposed to er...
unhealthy conditions. They mostly boil down to charges injected into the
bulk at unexpected locations. Or so I am told. Anyway, it happened to me
too, even after 'overprogramming' an EPROM type. The cure was, to put the
chip in a normal toaster oven at 80 degrees C overnight (NOT a microwawe
please, and do this at your own risk of course). Then it was erased for 2
hours under a mercury vapor lamp (500W) (DON'T do this without eye
protection and ozone evacuation etc.). This made it as good as new. Both
procedures are based on known ways to accelerate charge evacuation from
semiconductor bulks...

Peter (plp)

1997\12\15@133603 by Tim Kerby

picon face
<x-rich>Hi

All digital I/O on PICs have diode clamps on the inputs to protect them
for this sort of damage.  What you have to watch is where the PIC has
inbuilt serial or analogue modules where they are not protected and can
be hit by ESD.  I found this out after blowing the serial and analogue
modules in three 16C74 chips (JW to a total of £80) through not taking
adequate static precautions in UV erasing the chips (as UV builds up
charge on objects).



Tim Kerby



At 23:13 11/12/97 +0100, you wrote:

>>>>

<excerpt><fontfamily><param>Arial</param><smaller>hello,

</smaller></fontfamily>  

<fontfamily><param>Arial</param><smaller>I have one question for all you
guys/girls

</smaller></fontfamily>  

<fontfamily><param>Arial</param><smaller>In a project with a PIC16C84 the
inputs can be touched bij fingers,

my question : how can i protect the i/o against ESD charges ?

</smaller></fontfamily>  

<fontfamily><param>Arial</param><smaller>Kind regards,

</smaller></fontfamily>  

<fontfamily><param>Arial</param><smaller>Rob Aerts

</smaller></fontfamily>  

 

 

 

</excerpt><<<<<<<<







------------------------------------------------------------------

Personal Web Pages: http://web.ukonline.co.uk/members/tim.kerby/

Email: spamBeGonetim.kerbyspamBeGonespamukonline.co.uk

------------------------------------------------------------------
</x-rich>

1997\12\15@142605 by Peter Lorand Peres

picon face
The pins that DO NOT have clamps are very very very clearly specified in
the data sheet. Example: MCLR has only one clamp. 10 big red ponits for
good documentation to Microchip (I did NOT say that it was well structured
though).

On Sun, 14 Dec 1997, Tim Kerby wrote:

{Quote hidden}

Peter Lorand Peres
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1997\12\15@150152 by John Payson

picon face
> The pins that DO NOT have clamps are very very very clearly specified in
> the data sheet. Example: MCLR has only one clamp. 10 big red ponits for
> good documentation to Microchip (I did NOT say that it was well structured
> though).

The open-collector output pins (e.g. RA4 on 16C84) are not clamped to VDD,
and can safely take a voltage above VDD; one of the Microchip guys quoted
14 volts.  Unfortunately, Microchip has not in most/any of their data sheets
mentioned this feature.  Without a documented ability to go above VDD that
pin would be more useful with a clamp diode; with that ability, however, it
can be used for controlling higher voltages than would be possible without.

1997\12\15@170945 by XYGAX

picon face
In a message dated 15/12/97  20:52:46, you write:

<<
The open-collector output pins (e.g. RA4 on 16C84) are not clamped to VDD,
and can safely take a voltage above VDD; one of the Microchip guys quoted
14 volts.  >>
As this pin doubles as the Vpp does this not force the pic into program mode
at 13v and stop program opperation if so it would be unwise to use this pin in
this mode of opperation unless you want to write self modifing code ???

That would provide some interesting results if possable !
Just my .02 worth
Cheers Steve,,,,,

1997\12\15@192256 by Andrew Mayo

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       [snip]
{Quote hidden}

It is not uncommon to connect a reverse-polarity diode across the
regulator in order that any bypass capacitors connected to the regulated
side don't discharge through the regulator when power is turned off. If
the regulator is powered from a transformer and a bridge, then this will
sink voltage quite nicely.

The protective diodes ought to be able to handle several milliamps quite
nicely without a problem so a 1M resistor to 24V will not even remotely
damage them. In fact, there are mains-powered circuits which connect
4.7M from 240V to the PIC - this will cause around 50uA of current which
is well within specification.

1997\12\15@205431 by Rob Zitka

flavicon
face
I think the best thing would to design a way that the pins wouldn't be
touched by a persons fingers.  I don't know the situation you are in, so
maybe a better explanation of why the pins would be touched would help.  As
for esd to pins, there are surge arrestor circuits available that could be
implemented.  My company a well as others must be able to design products
to withstand esd.  I can maybe look to see what kind of circuits we use if
it helps.

Rob

{Quote hidden}

1997\12\15@212910 by Dwayne Reid

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face
>
>In a message dated 15/12/97  20:52:46, you write:
>
><<
> The open-collector output pins (e.g. RA4 on 16C84) are not clamped to VDD,
> and can safely take a voltage above VDD; one of the Microchip guys quoted
> 14 volts.  >>
>As this pin doubles as the Vpp does this not force the pic into program mode
>at 13v and stop program opperation if so it would be unwise to use this pin in
>this mode of opperation unless you want to write self modifing code ???

I'll have to double check, but I'm positive that the VPP pin / program mode
pin is MCLR, not RA4.  As far as RA4 and higher voltages is concerned, a
Microchip FAE has repeatedly told me that it is OK up to about 14 Vdc.

Dwayne Reid   <dwaynerEraseMEspam.....planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, Alberta, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

1997\12\16@093954 by wwl

picon face
On Mon, 15 Dec 1997 19:33:10 -0700, you wrote:

>>
>>In a message dated 15/12/97  20:52:46, you write:
>>
>><<
>> The open-collector output pins (e.g. RA4 on 16C84) are not clamped to VDD,
>> and can safely take a voltage above VDD; one of the Microchip guys quoted
>> 14 volts.  >>
>>As this pin doubles as the Vpp does this not force the pic into program mode
>>at 13v and stop program opperation if so it would be unwise to use this pin in
>>this mode of opperation unless you want to write self modifing code ???
>
>I'll have to double check, but I'm positive that the VPP pin / program mode
>pin is MCLR, not RA4.  As far as RA4 and higher voltages is concerned, a
>Microchip FAE has repeatedly told me that it is OK up to about 14 Vdc.
I think the reason RA4 hasn't got a diode is that a high voltage input
is used for factory testing. I'd be wary about using it at >Vdd in a
product, as it's an undocumented feature, which might flip the chip
into some funny mode!
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1997\12\16@095220 by Mike Keitz

picon face
On Mon, 15 Dec 1997 19:33:10 -0700 Dwayne Reid <RemoveMEdwaynerEraseMEspamEraseMEPLANET.EON.NET>
writes:
>  As far as RA4 and higher voltages is concerned, a Microchip FAE > has
repeatedly told me that it is OK up to about 14 Vdc.

Doesn't this risk putting the chip into the (undocumented) test mode?  It
does on a 16C5X.  The "absolute maximum rating" is indeed 14V, but the
chip may not work properly there.

1997\12\16@100655 by Justin Crooks

flavicon
face
> By the way, does anyone have any idea how much current it takes to damage
a
> PIC with excess VDD?  For example, if a PIC were connected to a +24
supply
> through a 1meg resistor, would it fry or would it safely clamp the
voltage
> to something reasonable?
>
My boss had a +-12 volt RS232 signal coming in to a PIC through a 22k
resistor.  It did not seem to damage the pic, but I redesigned it anyway.
It is possible, but personally I don't recommend it.  There are internal
diodes to Vss and Vdd on certain ports.  If you try this, use one of those
ports.  For the PIC 16C74, these are ports B,C,D, and A with the exception
of A4.

1997\12\16@125440 by Bruce Cannon

flavicon
face
At 08:00 AM 12/16/97 -0700, you wrote:
>> By the way, does anyone have any idea how much current it takes to damage
>a
>> PIC with excess VDD?  For example, if a PIC were connected to a +24
>supply
>> through a 1meg resistor, would it fry or would it safely clamp the
>voltage
>> to something reasonable?
>>
>My boss had a +-12 volt RS232 signal coming in to a PIC through a 22k
>resistor.  It did not seem to damage the pic, but I redesigned it anyway.
>It is possible, but personally I don't recommend it.  There are internal
>diodes to Vss and Vdd on certain ports.  If you try this, use one of those
>ports.  For the PIC 16C74, these are ports B,C,D, and A with the exception
>of A4.

Microchip discusses this in AN521, Interfacing to AC Power Lines.

Bruce

1997\12\16@165600 by Mike Ghormley

flavicon
face
Justin Crooks wrote:

>Well, how about grounding the person touching the pins?

Your basic idea is correct, but needs a little clarification to keep
newbies from taking your literally.

It is *never* a good idea to directly ground yourself when handling
electronics, yes?  IMHO, it would be best to ground yourself through a
100k ohm resistor.

Just a thought...

Michael

When the way of the Tao is forgotten, kindness and ethics must be taught.
Men must learn to pretend to be wise and good.  --  Lao Tzu

1997\12\17@105029 by Justin Crooks

flavicon
face
       Yes, ground yourself through a 1M or 100K.  Bad idea otherwise. :) Didn'
t
mean to confuse anyone...  I can't understand an application where the user
directly touches the pins... that seems like an unreliable input source...
but hey, who knows?  Thanks for the clarification.
       How about a ckt.  that measures capacitance at inputs, rather than a
direct connection to the pins?  Or make the user's skin a (very low
current) contact b/w two points?

----------
{Quote hidden}

1997\12\17@164157 by Dwayne Reid

flavicon
face
>
>On Mon, 15 Dec 1997 19:33:10 -0700 Dwayne Reid <EraseMEdwaynerspamspamspamBeGonePLANET.EON.NET>
>writes:
>>  As far as RA4 and higher voltages is concerned, a Microchip FAE > has
>repeatedly told me that it is OK up to about 14 Vdc.
>
>Doesn't this risk putting the chip into the (undocumented) test mode?  It
>does on a 16C5X.  The "absolute maximum rating" is indeed 14V, but the
>chip may not work properly there.

You could very well be right!  I was not aware of this test mode (it being
undocumented and all) but I would have hoped that the FAE would know about
it.  He was discussing 14 bit core parts ('61, '71, '73, '84) at the time.

Dwayne Reid   <RemoveMEdwaynerKILLspamspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, Alberta, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

1997\12\20@111044 by Rob Aerts

flavicon
face
>        Yes, ground yourself through a 1M or 100K.  Bad idea otherwise. :)
Didn't
>mean to confuse anyone...  I can't understand an application where the user
>directly touches the pins... that seems like an unreliable input source...
>but hey, who knows?  Thanks for the clarification.

suppose the pins are connected to a connector. users can touch those pins,
so how can i protect my pic against any esd charges ?

Grtz Rob

1997\12\22@061701 by Russell McMahon

picon face
Grounding the user may be acceptable if the user is in an environment where
they expect to be grounded randomly.

Some associates of mine had made some airline counter equipment associated
with luggage scales and under certain conditions people putting luggage onto
the scales were inputting vv high esd levels into the equipment. Normal esd
protection helped but did not totally eliminate the problem. Finally they
provided a "wiping contact" which caused the high voltage party to be
grounded if they came withing touching distance of the appropriate
equipment.

The grounding resistance in such cases can be quite high - 1 megohm or more.
You will find that commercial wrist-straps have a 1 megohm resistor
(possibly more) in their grounding lead to avoid (or at least reduce the
probability of) electrocution of the user should they choose to touch a live
contact.


{Original Message removed}

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