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'[PIC]: PIC reseting due to relays'
2001\08\28@204735 by Drew Vassallo

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This is similar to a thread that I started a few months back.  Similar
suggestions were made back then, too.  You might want to search through the
PICLIST message archives if you want more information.  However, I guess I
never really understood exactly WHY the chip would reset if the /MCLR pin
was pulled up to a solid +5VDC source independent of the motor's supply.

Is there some mechanism for some PICs whereby the output pins themselves can
"feed back" into the chip and cause a reset, or is it simply that the supply
lines get spikes that cause a small brownout?  I noticed that the 16C71 (no
onboard BOR) is not susceptible to this effect in my circuit, whereas simply
replacing it with a 16C715 (onboard BOR disabled) causes the chip to reset
frequently.

Any further information may clarify things for me.

--Andrew

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2001\08\28@205817 by Tony Nixon

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One thing to check is for floating inputs, especially if the relay
contacts arc when the motor is turned off. PICs don't like being near
EMI.

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2001\08\28@212539 by David VanHorn

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>
>Is there some mechanism for some PICs whereby the output pins themselves can
>"feed back" into the chip and cause a reset, or is it simply that the supply
>lines get spikes that cause a small brownout?  I noticed that the 16C71 (no
>onboard BOR) is not susceptible to this effect in my circuit, whereas simply
>replacing it with a 16C715 (onboard BOR disabled) causes the chip to reset
>frequently.

The input protection diodes dump excess current on the inputs into your
main supply, and that is what they are designed to do. If your supply
impedance is high enough relative to the current source, you may take VCC
outside the spec limits. There may also be problems with droop, immediately
after the transient where the regulator was shut down, and now has to turn
back on quickly.

I don't know your configuration, but series resistance in each I/O lead
will help prevent this from happening.

Another method to reset a processor, is to induce current into traces on
the PCB with rapidly varying magnetic fields. So far, I haven't found
anything in the normal world that will upset a bare processor (loop area
very small) but once you add real traces on a pcb, you get into the area
where EMI can cause induced current as above.

Also, weak oscillator design can get you there. It's very hard to diagnose
without special equipment.

For starters, take a small photo-strobe, and set it off repeatedly, near
the project. Move around till you find the part of the project that is most
sensitive to this disturbance.  If you need more flux, add a couple turns
of wire, about 1-2 inches diameter, in series with the flash
tube.  Basically, you're using the flash tube as a switch to discharge a
few joules into the indctance of it's wiring, or your inductor, and
creating a pulsed magnetic field.

Beware strobe mods, charged strobe caps can kill. Assume all strobe caps
are charged.

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2001\08\28@213612 by Tom Messenger

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At 08:24 PM 8/28/01 -0500, you wrote:
>Beware strobe mods, charged strobe caps can kill. Assume all strobe caps
>are charged.
>
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Yes, they can. I witnessed the death of a FET h bridge via strobe once.

Setup: pic chip controlling a motor in a typical h bridge. PIC just
happened to be an EPROM variety compleat with window.

System was on and running, customer said "Looks cool, gotta get a photo for
the folks back home". He grabbed his camera, click, flash, bang, smoke jets
out, motor stops, shows over.

Seems the flash latched up the photo sensitive pic and caused just the
right outputs to go on to fire top and bottom of the h bridge simultaneously.

Tom M.

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2001\08\28@215357 by Jeff DeMaagd

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----- Original Message -----
From: Tom Messenger <kristspamspam_OUTTHEGRID.NET>

> Setup: pic chip controlling a motor in a typical h bridge. PIC just
> happened to be an EPROM variety compleat with window.
>
> System was on and running, customer said "Looks cool, gotta get a
photo for
> the folks back home". He grabbed his camera, click, flash, bang, smoke
jets
> out, motor stops, shows over.
>
> Seems the flash latched up the photo sensitive pic and caused just the
> right outputs to go on to fire top and bottom of the h bridge
simultaneously.

Um....  wow.  I suppose no one thought to cover the window for proper
testing?

Jeff

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2001\08\28@215616 by David VanHorn

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>
>Setup: pic chip controlling a motor in a typical h bridge. PIC just
>happened to be an EPROM variety compleat with window.
>
>System was on and running, customer said "Looks cool, gotta get a photo for
>the folks back home". He grabbed his camera, click, flash, bang, smoke jets
>out, motor stops, shows over.
>
>Seems the flash latched up the photo sensitive pic and caused just the
>right outputs to go on to fire top and bottom of the h bridge simultaneously.


ROTFLMAO!!!  They DO recommend that you cover those windoes!

I've never had THAT experience, but I did have an ericsson 3717 crater,
while I was sitting talking to the rep about excessive cratering. It was
just sitting idle, power on.  Turns out there is a max switching frequency
on the choppers, which is poorly documented, and apparently none of their
tech guys were aware of, that we were exceeding. So, in some parts, one
part of the bridge was not quite off when the other part was turning on.

Good timing though, we had been talking for maybe 15 minutes about this
very problem, and suddenly smoke is pouring out of the product, little
flame jets inside, but the case masked that from view.

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2001\08\29@170742 by Peter L. Peres

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> ROTFLMAO!!!  They DO recommend that you cover those windoes!

I know people who had problems with 8751 with paper labels on the window
in light control devices (disco/party lights, including strobes).

Strobes are really bad because they contain a healthy proportion of UV.
One of the stories involved a 8751 that got 'amnesia' after every about 2
months in the application above. It took a year to figure it out. It was a
120 Ws strobe (or more).

Peter

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2001\08\30@195628 by Benjamin Bromilow

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> Beware strobe mods, charged strobe caps can kill. Assume all strobe caps
> are charged.
>

Ain't that the truth! The trigger voltage just makes your muscles contract,
the full voltage is pure pain!! I speak from experience, luckily only on a
low power device....

Ben

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