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'[PIC]:PIC reseting due to relays'
2001\08\29@212258 by Jinx

face picon face
> Jinx,
>
> I noticed that you do not have a series resistor into the
> MCLR pin when using a capacitor like Mchip suggests.
> Not sure I actually understand their reasoning on this
> anyway, any comments?

I've never used a series R. My usual is 10k to 5V, 10n ceramic
to 0V. If the circuit requires a manual reset, then a pushbutton
switch + 100R is across the 10n

Microchip say in the manual "series resistor of 100 to
1000 ohms to limit any current flowing into Mclr from
external capacitor in the event of an Mclr breakdown
due to ESD or EOS"

I always assumed that if Mclr had broken down that was
the end of the chip. Yes ? No ? If Mclr has fried, who knows
what else has

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2001\08\29@213946 by Brent Brown

picon face
> > Jinx,
> >
> > I noticed that you do not have a series resistor into the
> > MCLR pin when using a capacitor like Mchip suggests.
> > Not sure I actually understand their reasoning on this
> > anyway, any comments?
>
> I've never used a series R. My usual is 10k to 5V, 10n ceramic
> to 0V. If the circuit requires a manual reset, then a pushbutton
> switch + 100R is across the 10n
>
> Microchip say in the manual "series resistor of 100 to
> 1000 ohms to limit any current flowing into Mclr from
> external capacitor in the event of an Mclr breakdown
> due to ESD or EOS"
>
> I always assumed that if Mclr had broken down that was
> the end of the chip. Yes ? No ? If Mclr has fried, who knows
> what else has

Yeah, one of those useless paragraphs that intends to communicate
some important piece of information but doesn't. Maybe the cap can
discharge into MCLR when Vdd drops and cause some damage?
Who knows.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  .....brent.brownKILLspamspam@spam@clear.net.nz

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2001\08\29@221235 by Jinx

face picon face
> > I always assumed that if Mclr had broken down that was
> > the end of the chip. Yes ? No ? If Mclr has fried, who knows
> > what else has
>
> Yeah, one of those useless paragraphs

What it tells you, in effect, is how to protect a chip that might
go loco on you. Re-arranging deckchairs on the Titanic ?

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2001\08\30@023339 by Dan Michaels

flavicon
face
Jinx wrote:
{Quote hidden}

Aha - you are still awake down there.

In my opinion - resulting have having experienced/researched
this type of failure, the external capacitor actually CAUSES
the failure by discharging into the /Mclr pin, and Mchp's msg
is really 1984-speak. You can find my critique here:

http://www.oricomtech.com/piclinks.htm#Alert2

- dan
==============

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2001\08\30@023357 by Dan Michaels

flavicon
face
Brent Brown wrote:
{Quote hidden}

Brent, BOTTA-BINGO!!! See my previous msg on this subject.

- dan
===============

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2001\08\30@032402 by Jinx

face picon face
> http://www.oricomtech.com/piclinks.htm#Alert2
>
> - dan

Ouch, getting a lesson at the school of hard knocks

I haven't a data sheet handy to check, but would the leakage
through a reverse diode be enough to satisfy Mclr ?

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2001\08\30@042657 by Brent Brown

picon face
> http://www.oricomtech.com/piclinks.htm#Alert2
>
> - dan

Mental note made - never put a cap directly to MCLR, add a series
resistor. Thanks for discovering this (and telling us).

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  EraseMEbrent.brownspam_OUTspamTakeThisOuTclear.net.nz

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2001\08\30@084549 by Alan B. Pearce

face picon face
>Yeah, one of those useless paragraphs that intends to communicate
>some important piece of information but doesn't. Maybe the cap can
>discharge into MCLR when Vdd drops and cause some damage?
>Who knows.

Consider what happens when you have a large capacitor (say 10uF for
arguments sake) and you turn off the power to the PIC system. What is the
lowest resistance for the capacitor to discharge through? The ESD protection
diode of course. Now what happens if you happen to have a 5V relay on the
same supply pulled in at power off time. Capacitor on MCLR tries to supply
any number of milliamps to relay coil for unknown number of milliseconds.

In any other scenario name your load, and see if the current out of the
capacitor exceeds the maximum current through the ESD diode :)

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2001\08\30@085618 by Alan B. Pearce

face picon face
>Consider what happens when you have a large capacitor (say 10uF for
>arguments sake) and you turn off the power to the PIC system. What is the
>lowest resistance for the capacitor to discharge through? The ESD
protection

Whoops foot in mouth time - of course there is no ESD diode to Vcc, because
the programming voltage goes in here (RED) :) (/RED)

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2001\08\30@095817 by Olin Lathrop

face picon face
> Consider what happens when you have a large capacitor (say 10uF for
> arguments sake) and you turn off the power to the PIC system. What is the
> lowest resistance for the capacitor to discharge through? The ESD
protection
> diode of course.

I don't think MCLR has a diode to Vcc due to the high voltage programming
feature.  However, that just means all kinds of other unspecified and
possibly damaging things can happen with a cap on MCLR.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamspam_OUTembedinc.com, http://www.embedinc.com

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2001\08\30@105242 by Spehro Pefhany

picon face
At 01:40 PM 8/30/01 +0100, you wrote:
>>Yeah, one of those useless paragraphs that intends to communicate
>>some important piece of information but doesn't. Maybe the cap can
>>discharge into MCLR when Vdd drops and cause some damage?
>>Who knows.
>
>Consider what happens when you have a large capacitor (say 10uF for
>arguments sake) and you turn off the power to the PIC system. What is the
>lowest resistance for the capacitor to discharge through? The ESD protection
>diode of course. Now what happens if you happen to have a 5V relay on the
>same supply pulled in at power off time. Capacitor on MCLR tries to supply
>any number of milliamps to relay coil for unknown number of milliseconds.
>
>In any other scenario name your load, and see if the current out of the
>capacitor exceeds the maximum current through the ESD diode :)

Yes, this scenario applies to any capacitor connected to any pin but Vdd.

With small capacitors, the dVdd/dt *may* be small enough to protect
the diodes but it's worth considering if you have high current loads on
the Vdd bus.

Consider also the scenario that has power "blip" down by a volt or two
and immediately return- if there is any load on the Vdd bus of more than
(typically) about 80mA, the current injected into the protection diode
can trigger the suicide SCR (tm) and the chip will rapidly self-destruct
(melting of bonding wires or whatever). I believe this is the situation
that Microchip is alluding to.

Best regards,
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2001\08\30@122141 by Spehro Pefhany

picon face
At 01:56 PM 8/30/01 +0100, you wrote:
>
>Whoops foot in mouth time - of course there is no ESD diode to Vcc, because
>the programming voltage goes in here (RED) :) (/RED)

Mmm... good point, they specifically refer to voltage spikes below Vss.

Perhaps they are concerned about parasitics in long wires running to a
reset switch or something along that line. I regularly tie the output of
the BOR circuit directly to /MCLR with a very short conductor along a
ground plane. Can't see any reason to add the extra resistor...

Best regards,


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2001\08\30@124746 by Alan B. Pearce

face picon face
>Mmm... good point, they specifically refer to voltage spikes below Vss.
>
>Perhaps they are concerned about parasitics in long wires running to a
>reset switch or something along that line. I regularly tie the output of
>the BOR circuit directly to /MCLR with a very short conductor along a
>ground plane. Can't see any reason to add the extra resistor...

I think it has more to do with driving the programming circuitry when it
should not. Consider that under programming conditions you have 13V on this
pin, i.e. 8V higher than Vcc. Now if you have 5V on this pin due to large
capacitor, and no other supply to the chip, what is happening inside the
programming enable circuitry? I suspect some large currents can flow because
devices are not biased correctly and things get destroyed in the manner
described by someone else.

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2001\08\30@125928 by Spehro Pefhany

picon face
At 05:46 PM 8/30/01 +0100, you wrote:

>I think it has more to do with driving the programming circuitry when it
>should not. Consider that under programming conditions you have 13V on this
>pin, i.e. 8V higher than Vcc. Now if you have 5V on this pin due to large
>capacitor, and no other supply to the chip, what is happening inside the
>programming enable circuitry? I suspect some large currents can flow because
>devices are not biased correctly and things get destroyed in the manner
>described by someone else.

They don't say anything about a capacitor.. here's the exact quote:

>Voltage spikes below VSS at the MCLR pin, inducing currents greater than
>80 mA, may cause latch-up. Thus, a series resistor of 50-100 should be used
>when applying a  low  level to the MCLR pin, rather than pulling this pin
>directly to VSS.

They are concerned about undershoot. Why /MCLR, since every pin should be
similarly sensitive to latchup due to undershoot? Maybe that's your
programming circuitry - connected by long wires to the in-circuit
programmer. It still doesn't explain it completely, because the other
pins should be similarly sensitive and require similar resistors.

Best regards,

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2001\08\30@131125 by Dan Michaels

flavicon
face
Alan B. Pearce wrote:
>>Mmm... good point, they specifically refer to voltage spikes below Vss.
>>
>>Perhaps they are concerned about parasitics in long wires running to a
>>reset switch or something along that line. I regularly tie the output of
>>the BOR circuit directly to /MCLR with a very short conductor along a
>>ground plane. Can't see any reason to add the extra resistor...
>

>I think it has more to do with driving the programming circuitry when it
>should not. Consider that under programming conditions you have 13V on this
>pin, i.e. 8V higher than Vcc. Now if you have 5V on this pin due to large
>capacitor, and no other supply to the chip, what is happening inside the
>programming enable circuitry? I suspect some large currents can flow because
>devices are not biased correctly and things get destroyed in the manner
>described by someone else.
>


Alan, precisely my conclusions, as indicated on my page, since
the problems I saw were inscrutable phenomena associated with
erasing/verifying/re-programming the chips. HVP programming
circuitry is precisely what is tied to that particular pin
and is not tied to any other.

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2001\08\30@180853 by Jinx

face picon face
> can trigger the suicide SCR (tm) and the chip will rapidly
> self-destruct (melting of bonding wires or whatever). I believe
> this is the situation that Microchip is alluding to

A technical manual should do more than "allude" IMHO

I note that descriptions of Mclr on the F87x and F628 say
that the pin has a noise filter (although the block diagram
of the reset circuitry is the same for the 84/87x/628), which
increases the time for a valid Mclr from 1us (F84) to 2us.

Has the Mclr problem now been addressed then or is the
filter not enough ? If a resistor is considered mandatory
then couldn't it be added to the die ?

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2001\08\30@204817 by Spehro Pefhany

picon face
At 09:41 AM 8/31/01 +1200, you wrote:
>> can trigger the suicide SCR (tm) and the chip will rapidly
>> self-destruct (melting of bonding wires or whatever). I believe
>> this is the situation that Microchip is alluding to
>
>A technical manual should do more than "allude" IMHO

You'll find the semiconductor manufacturers get very
cagey when there are problems. The liability issues are huge.
It used to be like pulling teeth to even get official errata.

Since the Pentium I floating point fiasco ca. 1994, things have
gotten better.

>I note that descriptions of Mclr on the F87x and F628 say
>that the pin has a noise filter (although the block diagram
>of the reset circuitry is the same for the 84/87x/628), which
>increases the time for a valid Mclr from 1us (F84) to 2us.
>
>Has the Mclr problem now been addressed then or is the
>filter not enough ? If a resistor is considered mandatory
>then couldn't it be added to the die ?

Good question.

On a similar, but non-PIC note, has anyone been following the
class-action lawsuit against Palm- the argument is that the
cradle (connected to the serial port, I assume) conducted static
to (IMHO poorly designed) motherboards, which failed. They want
millions from Palm. I could see arguing for the plaintiffs
pretty easily.. "and *why* Mr. Engineer did you leave out the
2 cents worth of resistors that would have protected the
millions of dollars worth of my clients valuable equipment that
they need to run their businesses.. etc. ad nauseam.

Best regards,

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2001\08\31@120714 by Peter L. Peres

picon face
> They are concerned about undershoot. Why /MCLR, since every pin should
> be similarly sensitive to latchup due to undershoot? Maybe that's your
> programming circuitry - connected by long wires to the in-circuit
> programmer. It still doesn't explain it completely, because the other
> pins should be similarly sensitive and require similar resistors.

The programming enable circuitry probably eventually leads to the gate of
a large P-FET that is part of the charge pump or internal Vpp switching
system. That could definitely go thyristor and drive enough current to
destroy the chip. (a shot in the dark from a man who once destroyed a
12C509JW by touching it MCLR first).

Peter

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