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'[PIC]: Small battery backup?'
2002\07\09@111848 by 9-1?B?QnJhbmtvIEthcmFrbGFqaeY=?=

Hello PicListians,

      Does anyone know way to do a small battery backup for my
project. I have +5V from 78L05 and I need backup when that power
failed. Project is RTC without specialized RTC chip. Only 16F876
running on 6.5536MHz. And please no answer that said 'Take RTC chip!'
because project is budget limited!

Thanx in advance

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2002\07\09@113150 by Attila Muhi

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Hi,

Can you allow a small voltage drop ? If so, why not two diodes, one in series with the power supply and one with the battery.

Regards

Attila - SM4RAN


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2002\07\10@015050 by Michael Rigby-Jones

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> -----Original Message-----
> From: Branko Karaklajiæ [SMTP:branekspamspam_OUTeunet.yu]
> Sent: Tuesday, July 09, 2002 4:17 PM
> To:   @spam@PICLISTKILLspamspamMITVMA.MIT.EDU
> Subject:      [PIC]: Small battery backup?
>
> Hello PicListians,
>
>        Does anyone know way to do a small battery backup for my
> project. I have +5V from 78L05 and I need backup when that power
> failed. Project is RTC without specialized RTC chip. Only 16F876
> running on 6.5536MHz. And please no answer that said 'Take RTC chip!'
> because project is budget limited!
>
> Thanx in advance
>
> How long must this battery last in the absence of of main supply?  Running a
PIC at such a high clock speed is going to give a high current consumption
(for an RTC).  A 32.768KHz watch crystal gives very low power and exactly
divides down to seconds.  Also using the 16LF876 and running at low voltage
(down to 2 volts) will reduce power.  The standard 'F' part will only
officialy work down to 4 volts.

Regards

Mike

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2002\07\10@021606 by Andrei B.

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> (down to 2 volts) will reduce power.  The standard 'F' part will only
> officialy work down to 4 volts.
>

Though it works at lower voltages.
I managed to run several 16F84 @4MHz down to about 2.4V and stable.
(datasheet says for 16F84 (not LF) operation down to 4V and 4.5V with
HS osc)


=====
ing. Andrei Boros
Centrul pt. Tehnologia Informatiei
Societatea Romana de Radiodifuziune

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2002\07\10@080915 by Olin Lathrop

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> > (down to 2 volts) will reduce power.  The standard 'F' part will only
> > officialy work down to 4 volts.
> >
>
> Though it works at lower voltages.

Don't go there.

> I managed to run several 16F84 @4MHz down to about 2.4V and stable.
> (datasheet says for 16F84 (not LF) operation down to 4V and 4.5V with
> HS osc)

Maybe that chip, that day, at that temperature.  Maybe it workd correctly or
maybe it just seemed to work right and would have failed after the next
firmware change or different use of peripherals or whatever.

Please don't advise people to do this.

If you need low voltage operation, check out the 16LF628.  It can run at
4MHz down to 2V, 10MHz at 3V, and 20MHz at 4.5V.


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2002\07\10@112419 by 9-1?B?QnJhbmtvIEthcmFrbGFqaeY=?=

Hello Michael,

Tuesday, July 09, 2002, 5:46:34 PM, you wrote:


>> Hello PicListians,
>>
>>        Does anyone know way to do a small battery backup for my
>> project. I have +5V from 78L05 and I need backup when that power
>> ...blah..blah


Michael> How long must this battery last in the absence of of main supply?  Running a
Michael> PIC at such a high clock speed is going to give a high current consumption
Michael> (for an RTC).  A 32.768KHz watch crystal gives very low power and exactly
Michael> divides down to seconds.  Also using the 16LF876 and running at low voltage
Michael> (down to 2 volts) will reduce power.  The standard 'F' part will only
Michael> officialy work down to 4 volts.

        Yes, 32.768KHz is better but now 6.5536MHz is used, and there
is no way to change that. The solution I use is 2 diodes and 9V
battery before 7805. I think I will have some 2-5 hours of work for
RTC. Current consumption is high but it's time sliced (once in second
for 60ms) because of FlipDot displays.

thanx to everyone who helped me!

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2002\07\10@115713 by Pic Dude

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Looking at the specs & prices for the LF vs F parts, I'm not sure
what the advantage is in picking up F parts.  Seems like the LF
will do everything the F will, but has the advantage of lower
power, lower voltage, etc.  So why would they even bother to make
the F parts?



{Original Message removed}

2002\07\10@121802 by mike

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F and LF are the same silicon.
LF parts are tested and guaranteed to run at low voltages.
On Wed, 10 Jul 2002 10:52:59 -0500, you wrote:

>Looking at the specs & prices for the LF vs F parts, I'm not sure
>what the advantage is in picking up F parts.  Seems like the LF
>will do everything the F will, but has the advantage of lower
>power, lower voltage, etc.  So why would they even bother to make
>the F parts?
>
>
>
>{Original Message removed}

2002\07\10@132323 by Paul Hutchinson

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The F parts are about 5% cheaper than the LF versions. Probably not
significant for a lot of projects but, if it's a volume production run a few
% savings can add up to a lot of $.

Paul

> {Original Message removed}

2002\07\10@143756 by Mark J. Dulcey

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Pic Dude wrote:
> Looking at the specs & prices for the LF vs F parts, I'm not sure
> what the advantage is in picking up F parts.  Seems like the LF
> will do everything the F will, but has the advantage of lower
> power, lower voltage, etc.  So why would they even bother to make
> the F parts?

The LF chips are identical to the F parts, except that they passed their
tests all the way down to 2 volts. Parts sold as F chips may have failed
some low voltage tests, or may have never been tested at low voltage.

Not all PICs make the grade at the low voltage; they may fail
completely, or only intermittently. Also, the additional low voltage
testing takes additional time, which slightly increases the production
cost. Only a few customers are willing to pay a premium for low voltage
parts, so if Microchip has already filled their quota for LF parts, and
they have more PICs available, the additional chips probably won't be
tested at the full voltage range.

Similarly, the 4 MHz parts are likely to be chips that failed testing at
20 MHz. But some of them will simply never have been tested at 20 MHz,
or might have even passed! There is a demand for minimum-cost parts,
especially from high-volume customers; if Microchip doesn't have enough
parts that fail the 20 MHz testing to meet demand for 4 MHz chips, they
may simply mark some perfectly good 20 MHz chips as 4 MHz and sell them
at the lower price, as it is better than not selling them at all.

The same logic also applies to the commercial (0 to 70C), industrial
(-40 to 85C), and extended (-40 to 125C) temperature grades. Commercial
grade chips may have failed at high or low temperature, but they also
might have passed. Chip makers have been doing this kind of quality
grading for many years; it's not unique to Microchip.

It would appear that Microchip's processes have improved; some of the
newest PICs (16F877A, the 18F series) don't come in low-speed grades,
suggesting that they no longer have a significant number of chips that
fail to work at full speed. Similarly, the 18F parts don't come in
commercial temperature grade; only industrial and extended.

What this means is that some of the chips sold as lower grade parts (4
MHz parts or not LF) will work perfectly at low voltage or full speed.
Microchip just won't guarantee that they will work. If you're building a
prototype, and you don't have the right grade of chip on hand, go right
ahead and try the wrong one; it's likely to work. But if you're going
into production, use the right parts; you'll be happier in the long run.

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2002\07\10@145741 by Pic Dude

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Damn dude!  Awesome answer.  Thanks!

I had asked since I run my 16F872's at 4Mhz and found
that the F's were $2.83 (actually 20Mhz versions), and
the LF's were $2.98.  Each in 25-piece qty's from Digi-
key.  $0.16 seemed like a no brainer case for the LF's,
but now it all makes sense.

Cheers,
-Neil.



{Original Message removed}

2002\07\11@233959 by Michael Rigby-Jones

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> -----Original Message-----
> From: Pic Dude [SMTP:EraseMEpicdudespamPILOTTOOLS.COM]
> Sent: Wednesday, July 10, 2002 7:53 PM
> To:   RemoveMEPICLISTEraseMEspamEraseMEMITVMA.MIT.EDU
> Subject:      Re: [PIC]: Small battery backup?
>
> Damn dude!  Awesome answer.  Thanks!
>
> I had asked since I run my 16F872's at 4Mhz and found
> that the F's were $2.83 (actually 20Mhz versions), and
> the LF's were $2.98.  Each in 25-piece qty's from Digi-
> key.  $0.16 seemed like a no brainer case for the LF's,
> but now it all makes sense.
>
> Cheers,
> -Neil.
>
Be aware of one of he most annoying parts of using PIC's at low voltages.
The brown out detector in the 16F series has a fixed threshold of around 4
volts, which means you have to disable it to use the PIC at lower voltages.
That's not too bad a problem, you can buy cheap 3 pin supervisor chips in
virtualy any reasonable voltage range.  However, if you want to use high
voltage programming at the same time (e.g. yoiu are fresh out of port pins
and RB4 is too valuable to give up), then you have to arange something to
stop the supervisor being fed with 13 volts.  It all works out to be a
little messy because AFAICT there are no 3 pin open collector supervisor
IC's that can tolerate 13volts.  Even MChips own devices won't.  I guess
most people wouldn't think twice about adding a current limiting resistor
and a schotky diode to clamp the voltage, but when you are really pushed for
board space even a single 0402 component can be a problem.

The 18 series have a programmable brownout voltage, hopefully I'll be
migrating to them soon!

Regards

Mike

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