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PICList Thread
'pic lock-up'
1998\01\26@150807 by wouter van ooijen

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I have read somewhere that a PIC can lock up (latch-up?) beyond reset,
needing a power-off to get back into a normal mode.
For my (in-circuit) 16c84 programmer I'm considering to include a
'power-short' feature for this kind of situation.
Questions:
- has anyone experienced such a lock-up where a reset is not sufficient
- what caused it
- does this situation ocur often enough to warrant a few extra components
in my programmer

regards,
Wouter.

1998\01\26@160726 by Micheal Yano

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The lock up problem I have found is documented in the MicroChip data book,
basically if the MCLR does not stay in sync with Vdd then the chip will lockup.
I use an MC3064 reset chip to ensure proper operation, as well as using the
watchdog.

At 08:26 PM 1/26/98 +0100, you wrote:
{Quote hidden}

Thanks.


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1998\01\26@220907 by Andy Kunz

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>For my (in-circuit) 16c84 programmer I'm considering to include a
>'power-short' feature for this kind of situation.
>Questions:
>- has anyone experienced such a lock-up where a reset is not sufficient
>- what caused it
>- does this situation ocur often enough to warrant a few extra components
>in my programmer

Oh, my.  I have spent the past two weeks working with this one.  I've put
hardware together to fix the problem, basically a daughterboard for the PIC
with a reset chip on it.

This situation, in our instance, is caused by a PIC running at 20MHz
(that's legal) at 5V most of the time.  The problem is that, during an AC
brownout, it is possible for the 5V rail to drop below 4.5V (very bad).
This will put the chip into a mode where the osc still runs, but the
outputs are locked in their last set state and no processing is being done.

Worse yet, the WDT won't trip either, so we have a totally useless device
out there.

And the brown-out detect circuitry (which may or may not be active,
depending upon which guy burned them :-( won't trip until the voltage hits
about 3.8V for a long-enough period of time.

To remedy the situation, we are requiring all future units to have a Dallas
DS1233-5 reset chip installed.  I investigated other chips (such as
Panasonic's from Digi-Key) and found it to have the best parameters.  It
trips very reliably at 4.5V (+/- 5% is the spec, but reality is much
tighter), generates a reset pulse a minimum of 300mS wide, and stays low
until power is up about 4.5V for 300mS.

It is very important that this mod be installed in our case.  We are having
to visit sites scattered around a several-hours-drive-wide area to
basically power cycle a unit to revive it.

A word to the wise.  Put the reset chip in.

Andy

==================================================================
Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
         Hardware & Software for Industry & R/C Hobbies
       "Go fast, turn right, and keep the wet side down!"
==================================================================

1998\01\27@131446 by Harold Hallikainen

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On Mon, 26 Jan 1998 22:06:58 -0500 Andy Kunz <montanaspamKILLspamFAST.NET> writes:

{Quote hidden}

       This is really too bad.  To me, the whole purpose of the WDT is
to recover from stuff like this...  And you say the WDT dies!  Wonderful!

       On using an external reset generator...  sounds like an external
WDT could be more useful.  Under what other conditions does the WDT get
messed up?
       Back in my 6802 days (I still have a few hundred systems out
there running those), I had a watchdog timer on the processor board.  The
original WDT was level triggered instead of edge triggered.  A WDT reset
consisted of setting a line low, then back high again.  Of course, if the
machine crashed with the line low, the WDT was continuously reset and
never timed out.  I had to drive 100 miles in Kansas to press a reset
button because of this.  We changed to edge triggered WDT.


Harold

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1998\01\27@170602 by Andy Kunz

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>        This is really too bad.  To me, the whole purpose of the WDT is
>to recover from stuff like this...  And you say the WDT dies!  Wonderful!

I can demonstrate it repeatedly.  Had to show it to my boss - he didn't
believe it either!

>        On using an external reset generator...  sounds like an external
>WDT could be more useful.  Under what other conditions does the WDT get
>messed up?

This is the only one I know of, and I didn't see more in the errata.  (Or
this one either, btw).

>button because of this.  We changed to edge triggered WDT.

Smart move!

Andy


==================================================================
Andy Kunz - Statistical Research, Inc. - Westfield, New Jersey USA
==================================================================

1998\01\27@215559 by Charles Hoss

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Andy Kunz wrote:
>
> >        This is really too bad.  To me, the whole purpose of the WDT is
> >to recover from stuff like this...  And you say the WDT dies!  Wonderful!
>
> I can demonstrate it repeatedly.  Had to show it to my boss - he didn't
> believe it either!
>

the only time I saw such things with a
pic 16f84 was when I
forgot to put nano level capacitors on
the powerline

it took us 18 hours to find out that the
power was faulty
not the "new" f series .

try it - it possible that at high freq.
you need a little more
interference precautions .

bye
charley

1998\01\28@065540 by Tom Handley

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re: Brown-out

  Andy, I also got `bit' by the PIC brown-out circuitry. In my case, I'm
using Maxim devices. The Dallas products are also a good option.

  - Tom

At 10:06 PM 1/26/98 -0500, you wrote:
[snip]
>Oh, my.  I have spent the past two weeks working with this one.  I've put
>hardware together to fix the problem, basically a daughterboard for the PIC
>with a reset chip on it.
[snip]
>To remedy the situation, we are requiring all future units to have a Dallas
>DS1233-5 reset chip installed.  I investigated other chips (such as
>Panasonic's from Digi-Key) and found it to have the best parameters.  It
>trips very reliably at 4.5V (+/- 5% is the spec, but reality is much
>tighter), generates a reset pulse a minimum of 300mS wide, and stays low
>until power is up about 4.5V for 300mS.
[snip]
>==================================================================
>Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
>          Hardware & Software for Industry & R/C Hobbies
>        "Go fast, turn right, and keep the wet side down!"
>==================================================================

1998\01\28@091716 by Philip Cowley

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Does anyone know of a more generic electronics mailing list I might
join?.... I seem to be moving away from a PIC as a solution to my problem
and I don't want to clutter up this mailing list with "Off Topic" postings.

Thanks

Phil

1998\01\28@203702 by Andrew Mayo

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Does anyone know whether the discrete brownout circuit that MChip
recommend i.e the transistor and zener circuit, is significantly less
reliable than the Dallas chip, which I suppose will be much more
precise. How much do they cost?

{Quote hidden}

1998\01\28@204748 by Jens.Madsen

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The topic was covered some time ago on the PICLIST. One had a problem
when Vcc fall, and then rise again, but not fall to zero. Then both WDT
and reset hang and it was needed to power off the chip. Even external
reset did not work.

I think that the way to do it must be an external WDT to power of and on
the chip. It may cause problems with remembering data, but they may be
stored in the flash. Another had problems with similar behavior of the
PIC16F84 chip. It erased data in it's flash if the power was turned of
and up (slowly?). Maybee you need a checksum too?

Several years ago, I had a project with a LCD-display. It worked
perfect, but after days or weeks it hang. I needed to power off and
power up.
I decided to always make it possible to power up and off by software,
and e.g. verify the data on the display and do an automatic reset when
it hang, or to reset it automatic with some intervals.

Maybee a lot of chips has similar behavior if noise, or other transient
on Vcc occeurs?

Regards,
Jens Dyekjfr Madsen

1998\01\29@062915 by Andy Kunz

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>the only time I saw such things with a
>pic 16f84 was when I
>forgot to put nano level capacitors on
>the powerline

This unit is very well supported.  It's very clean.  I can generate the
response AT WILL with clean AC simply by varying its amplitude.

Andy

==================================================================
Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
         Hardware & Software for Industry & R/C Hobbies
       "Go fast, turn right, and keep the wet side down!"
==================================================================

1998\01\29@090114 by Andy Kunz

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At 09:03 AM 1/29/98 +1300, you wrote:
>Does anyone know whether the discrete brownout circuit that MChip
>recommend i.e the transistor and zener circuit, is significantly less
>reliable than the Dallas chip, which I suppose will be much more
>precise. How much do they cost?

Low voltage zeners are not especially accurate, and the cost is probably
less of a problem.  I have a quote here for 858 pieces DS1233-5 (best
accuracy, TO-92) at 1.01 each, and that wasn't pressing the issue at all.

For the reliability it was a bargain.

Andy

==================================================================
Andy Kunz - Statistical Research, Inc. - Westfield, New Jersey USA
==================================================================

1998\01\29@112709 by Madsen, Jens

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Andy Kunz wrote:
>
> At 09:03 AM 1/29/98 +1300, you wrote:
> >Does anyone know whether the discrete brownout circuit that MChip
> >recommend i.e the transistor and zener circuit, is significantly less
> >reliable than the Dallas chip, which I suppose will be much more
> >precise. How much do they cost?
>
> Low voltage zeners are not especially accurate, and the cost is probably
> less of a problem.  I have a quote here for 858 pieces DS1233-5 (best
> accuracy, TO-92) at 1.01 each, and that wasn't pressing the issue at all.
>
> For the reliability it was a bargain.
>
> Andy
>
> ==================================================================
> Andy Kunz - Statistical Research, Inc. - Westfield, New Jersey USA
> ==================================================================

I'm not sure if a brownout circuit will cover all aspects of using a
watch-dog. Only errors caused by too low rise time is covered. I guess
that it is much better to add an external watch-dog timer, by using e.g
LM555 to pull off and on the power to the chips. The internal watch-dog
may be used, and the external only if the internal watchdog not
'answer'. If the current is limited, then latch-up is protected too.

Regards,
Jens Dyekjfr Madsen

1998\01\30@222401 by Dwayne Reid

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I've been following this thread with interest.  One of my earliest PIC
designs (16c71) exhibited strange behaviour with slow power supplies - much
as has been described here.  The chip locked up with all outputs in a HIGH
state, which led to a potentially unsafe condition.  Since then, all but one
of my projects using PICs have had external watchdogs plus a timer which
monitors the watchdog and disables critical outputs if the watchdog trips.
I used to use the MAX705 but have been using a Xicor part for the past
couple of years.

The Xicor part (X25043) has a couple of nice things about it.  It includes:
power-up-reset, power supply supervisor, watchdog, and 512 bytes eeprom.
Its not too expensive (~3.00 Cdn) and has been utterly reliable.  One of the
nice features NOT in the data sheet is that the low voltage dropout detector
threshold value can be changed using a simple programming jig (it is not
possible to change in circuit).  This means that I only have to stock one
part number for the different supply voltages.

I mention that I disable critical outputs if the watchdog trips.  This is
just a simple 2 second timing network and comparitor tied to /MCLR via a
diode.  The timing cap is discharged anytime /MCLR goes low.  The time
constant is long enough to insure that the outputs remain disabled during
sucessive watchdog resets.

The X25043 that I use is in an 8 pin DIP package.  I use RB5,6,7 to talk to
the chip.  The rest of the circuit is arranged so that high voltage on /MCLR
will not cause damage.  The whole idea is that I can leave the watchdog off
the board until test time and use the watchdog socket as my In Circuit
Programming connector.  It has worked well for the last two and a half years.

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

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