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'[PIC] Battery powered PICs'
2006\05\29@202944 by William Chops Westfield

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So I have an actual project that actually got built; a battery powered
12f675.  But it's flakey; sometimes it has to be power cycled several
times before it seems to enter the correct operating state :-(
It's a simple circuit with some LEDs (current limited by resistors),
and a couple bypass caps.  (It implements a "Sims" cursor as a costume,
fading back and forth between red (sad Sim) and green (happy Sim))
It's powered from 3x alkaline or 4xNiMH cells (4.5 to 5V nominal; both
version have been flakey.)  The MCLR pin is configured as an input and
tied high with a 33k resistor (possible future remote control.)

I'm assuming that most of my problems are due to the bouncy nature
of power-on in this circuit (cheap switch or cheaper battery clip.)
The 12f675 has several robustness features (brown out detect, power
on timer, watchdog), all of which I have turned off at the moment,
under the assumption that the basic internal reset would be sufficient.
Apparently not.  What sort of configuration settings and/or external
circuitry do people find useful or necessary to ensure clean startup
in battery-powered circuits subject to messy poweron?

Thanks
Bill W

2006\05\29@210840 by Marcel duchamp

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William Chops Westfield wrote:
{Quote hidden}

Is there a power switch? Or does it power up wrong as you insert the
battery?  If there is no switch, you could try adding one.  This will be
aided by some capacitance on the Vcc line, in case you don't have enough
already.

One thing to try is to get it in the non-working mode and then using a
clip lead, apply a reset pulse.  If that gets it running, then you could
try a cap to ground from the reset pin.

2006\05\29@214916 by Russell McMahon

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>> The 12f675 has several robustness features (brown out detect, power
>> on timer, watchdog), all of which I have turned off at the moment,
>> under the assumption that the basic internal reset would be
>> sufficient.
>> Apparently not.

The features are there because they are often enough needed in real
world applications. Watchdog can be a pain or useless if not driven
well but brownout detect is utterly invaluable and invisible in most
cases and power on timer usually does little harm.



       RM

2006\05\29@220443 by Matthew Miller

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Hi Bill,

On Mon, May 29, 2006 at 05:29:43PM -0700, William Chops Westfield wrote:
>
> I'm assuming that most of my problems are due to the bouncy nature
> of power-on in this circuit (cheap switch or cheaper battery clip.)
> The 12f675 has several robustness features (brown out detect, power
> on timer, watchdog), all of which I have turned off at the moment,
> under the assumption that the basic internal reset would be sufficient.
> Apparently not.  What sort of configuration settings and/or external
> circuitry do people find useful or necessary to ensure clean startup
> in battery-powered circuits subject to messy poweron?

I can't say this with absolute certainty, but I would bet that if you enable
the power on reset feature that you problem may be solved. My last battery
powered circuit uses a 12f629 with POR and I've not had any power-up
problems. Inserting batteries or connecting a battery connector is an
operation that is going to bounce just as a switch would.

I can't think of any reason to not use POR in your situation. A change to
the config word will be all that you need to make and your problem may be
solved.

Take care.

Matthew

--
8)   Use common sense in routing cable.  Avoid wrapping coax around sources of
    strong electric or magnetic fields.  Do not wrap the cable around
    flourescent light ballasts or cyclotrons, for example.
-- Ethernet Headstart Product, Information and Installation Guide,
  Bell Technologies, pg. 11

2006\05\30@001649 by Bob Axtell

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William Chops Westfield wrote:
{Quote hidden}

The power on timer is critical, because it ensures that the processor is
held in reset long enough.

The brown-out detector helps only when the battery is low.

The WDT should be on because sometimes the power on reset by using
battery clip might be to flakey
to allow the chip to reset. The WDT will quickly reset and bring things
up properly.

The only time the WDT is off is when I am using the ICD2.

--Bob

2006\05\30@010353 by Russell McMahon

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> The brown-out detector helps only when the battery is low.

Indeed. But the battery may "go low" due to sudden current drain
caused by device operation. WHY it's needed may vary. THAT it's needed
is less uncertain.


       RM

2006\05\30@015529 by Bob Axtell

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Russell McMahon wrote:
>> The brown-out detector helps only when the battery is low.
>>    
>
> Indeed. But the battery may "go low" due to sudden current drain
> caused by device operation. WHY it's needed may vary. THAT it's needed
> is less uncertain.
>
>
>         RM
>
>  
I agree. I use it rarely, myself.

I was hinting that it will be of no effect for his stated problem.

--Bob

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