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'[PIC]: Excessive PIC power at 32KHz'
2000\06\15@045357 by James Wilson

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We use PICs running on 3.7V lithium cells at 32KHz for long battery life.
Typically our circuits operate between 12 and 18uA. We have used a number of
processors 16LC58A, 16LC558, 923, 924 etc. Always LC versions for 32KHz and
Industrial temperature range.

The 58A and the 558 are products which Microchip are superseding with newer
parts: 58B, 622A (like a 558 but with comparators thrown in). However our
experience on Microchip's latest die shrinks is that the power consumption
at 32KHz has gone through the roof - typically x2 or x3.

Has anyone else come across this problem?
Has anyone got any ideas?

James Wilson

2000\06\15@055043 by Philippe

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At 09:52 15/06/00 +0100, you wrote:
>We use PICs running on 3.7V lithium cells at 32KHz for long battery life.
>Typically our circuits operate between 12 and 18uA. We have used a number of
>processors 16LC58A, 16LC558, 923, 924 etc. Always LC versions for 32KHz and
>Industrial temperature range.
>
>The 58A and the 558 are products which Microchip are superseding with newer
>parts: 58B, 622A (like a 558 but with comparators thrown in). However our
>experience on Microchip's latest die shrinks is that the power consumption
>at 32KHz has gone through the roof - typically x2 or x3.
>
>Has anyone else come across this problem?
>Has anyone got any ideas?
>
>James Wilson

Running at 32KHz is sometime just not the good solution even for current
saving. Another approach that save cost and power is to run your microcontroller
at maximum frequency using internal RC oscillator. As running the MCU
at 32Khz is surely to count time, you may generate an interrupt, say each 10ms
using an external circuitery running at 32Khz. A good and cheap one is a 4060
that is a counter, it integrate also an oscillator that might run with a 32Khz
crystal. So an interrupt can be geenrated at each edge of one of the counter
output, the rest of the time the MCU will enter in sleep mode saving maximum
of current.
Just make a little calcul for PIC16C622A RC osc. mode, 5V operation:
       Icc @ 4Mhz is 3,3 mA (max)
       Ipd        is  20 uA (max)

If you need, say, 1000 clock cycles to process your interrupt and do what
you have to do, you will need:
       1000 x 4 x 0,25us = 1ms @ 3,3mA
if an interrupt occur each 100ms then you will spend 1ms @ 3,3mA and 99ms in
power down mode @ 20uA, so finally the average will give:
       [1x3,3mA]/100 + [99x20uA]/100 = 33uA + 19,8uA = 52.8 uA

This average must be majored by the needed time for the oscillator to restart,
and then it need to add a "IccStart" value that can be quantified by experiment.
Do not forget to add the current needed to run the 4060 !

This just show a high speed can save power in some case and allow high
communication speed when really needed. Sure the calcul show we are still
far from 18uA, but we take only MAX value, an experiment will surely show
lowest current.

Regards,
       Philippe.

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2000\06\15@094317 by Jim Hartmann

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When switching from 622 to 622a we found that the crystal drive circuit
must have a series resistor ~100k added.  The xtal was being overdriven in
LP mode on the 622a, probably driving up current consumption as well.

:-Jim

2000\06\15@164421 by steve

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> power down mode @ 20uA, so finally the average will give:
>         [1x3,3mA]/100 + [99x20uA]/100 = 33uA + 19,8uA = 52.8 uA
>
> This average must be majored by the needed time for the oscillator to restart,
> and then it need to add a "IccStart" value that can be quantified by experiment.
> Do not forget to add the current needed to run the 4060 !

Which is about 40uA for an HC part @ 3V IIRC.
40uA + 53uA = 93uA vs. about 15uA in my application.
Assuming the batteries last that long, we get 3 years out of them.
With the 4060 solution that would be 6 months. Not much difference in
microamps but a big difference in the product market place.

It's a good technique but when you are at this level of power
consumption all the wrong things draw current. For example, any reset
generator will cost a year or more of product life.

Steve.
======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: .....stevebKILLspamspam@spam@tla.co.nz      fax +64 9 820-1929
======================================================

2000\06\15@164424 by steve
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> However our
> experience on Microchip's latest die shrinks is that the power consumption
> at 32KHz has gone through the roof - typically x2 or x3.
>
> Has anyone else come across this problem?
> Has anyone got any ideas?

Yes. I had exactly the same problem with 54's and you'll also notice
that the minimum operating voltage tends to creep up with die
shrinks too. Not what I was expecting.
I talked to one of the Microchip production guys about it at ESC last
year. I can't remember the exact reasoning but it was quite
reasonable. The upshot was that a die shrink gives them less
production tolerance so they have to adjust parameters to make
sure the yield stays within economic reality. Most devices are used
at 5V so those specs are the ones that must stay the same or get
better. If something has to suffer then it is better (for them) that
it is the applications on the edge of the operating envelope, than
those in the middle.
The good news is that they don't discontinue the earlier versions
although they cost more than the later ones.

Steve.


======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: stevebspamKILLspamtla.co.nz      fax +64 9 820-1929
======================================================

2000\06\15@201456 by Dan Michaels

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Steve Baldwin wrote:
>> However our
>> experience on Microchip's latest die shrinks is that the power consumption
>> at 32KHz has gone through the roof - typically x2 or x3.
>>
>> Has anyone else come across this problem?
>> Has anyone got any ideas?
>
>Yes. I had exactly the same problem with 54's and you'll also notice
>that the minimum operating voltage tends to creep up with die
>shrinks too. Not what I was expecting.
>I talked to one of the Microchip production guys about it at ESC last
>year. I can't remember the exact reasoning but it was quite
>reasonable. The upshot was that a die shrink gives them less
>production tolerance so they have to adjust parameters to make
>sure the yield stays within economic reality. Most devices are used
..........


The other thing you will notice is that the Vdd max rating is
also moving in - was 6v, is 5.5v. So you have to watch it on that
side, too. [2 Li batteries - no, no].

I just talked to Mchp about this 2 days ago, and they used much the
same rationale as stated above - shrinking dies, "traces closer
together", blah/blah/blah, ....

- DanM
======

2000\06\15@205105 by Brent Brown

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<x-rich>Steve Baldwin wrote:

<color><param>0000,0000,0000</param>> It's a good technique but when you are at this level of power

> consumption all the wrong things draw current. For example, any reset

> generator will cost a year or more of product life.


</color>Sorry, can't help on the current consumption problem but Seiko have some
extremely low current parts that make good reset generators. Check these out.


http://www.seiko-usa-ecd.com/intcir/html/power/s808.html


<color><param>0100,0100,0100</param>á Very-low current consumption:

: 1.3µA typ. (VDD=1.5V)
When detection voltage is 1.4Vtyp. or less.

: 0.8µA typ. (VDD=3.5V)
When detection voltage is 1.5Vtyp. or more.

á Accuracy: ±2.0%

á Operating voltage range: : 0.7V to 5.0V
When detection voltage is 1.4Vtyp. or less.

: 0.95V to 10.0V
When detection voltage is 1.5Vtyp. or more.

á Hysteresis characteristics 5% typ.

á Detection voltage : 0.8V to 6.0V (0.1V step)

á Output forms:

: Nch open drain active "L" output

: CMOS active "L" output

á Super-small package SC-82AB, TO-92, SOT-89-3, SOT-23-5</color>


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

</x-rich>

2000\06\16@161113 by Oliver Broad

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I'd want to try measuring the current with the PIC clocked from an external
source/osc. This might prove whether the change is inherent or whether it
was caused by some change in the osc parameters that could be fixed.

Oliver.

2000\06\16@164413 by Erik Reikes

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I currently have a product that uses the Watchdog sleep mode.  So we wake
up every 2.5 seconds for about a ms or so (we have a 4MHz oscillator) and
decide whether we really want to be awake or not.  It works well.  We are
getting a projected several years out of our lithium cells.  I believe I
set my steady state power budget at 100uA, with a bucnh of external circuitry.

At 11:46 AM 6/15/00 +0200, you wrote:
{Quote hidden}

Erik Reikes
Senior Software Engineer
Xsilogy, Inc.

ereikesspamspam_OUTxsilogy.com
ph : (858) 535-5113
fax : (858) 535-5163
cell : (858) 663-1206

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