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'[PIC]: Low-power consumption.'
2003\06\20@152350 by Picdude

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Hi all,

Got my timer/clock working and tuned down for low power, but from various archive posts, I feel like I should be able to go lower.  In a nutshell, think of it as a digital clock w/3 switches, 4 LED digits and a buzzer.  Normal operation is internal clock at 4Mhz, AND external osc (Timer 1) at 32.768kHz.  There are 2 modes -- normal and sleep.

In normal operation, I get ~52mA.  No prob.

In sleep mode, the internal osc is shut down and all LED's are off, and only Timer 1 wakes up every 1/2 second (which wakes up the internal osc), runs about 50 instructions and goes to sleep.  During all this time, RA4 (open drain) is such that it draws no power during the brief wake up.  In this mode, I get 0.12mA average.

Is lower possible?  Or am I asking too much?  I don't *need* it to be less, but would be nice to know/learn if any other optimizations are possible.

Cheers,
-Neil.

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2003\06\20@162302 by Bob Axtell

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We get best power comsumption when the main osc runs slow, i.e. 32K watch
crystal. We use about 20-50uA but are not displaying anything, and it never
goes to sleep at all. We have several designs at 32K and one at 100K. The
batteries last for months. The PIC is a PICLC711-04/P.

--Bob

At 02:24 PM 6/20/2003 -0500, you wrote:
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2003\06\20@162303 by Alex Kilpatrick

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A normal PIC without any peripherals is suppsed to be 1 uA in sleep.  I
was experimenting with a PIC using the internal 4 MHz clock and an
external 32.768 KHz clock tied to Timer1, as you have.  Nothing else was
active.  During a 2 second sleep, I got about 40 uA, or about 1/3 of
what you are getting.  That was just the current to the PIC; I didn't
measure the overall current.

The only difference I can see between what you are doing and what I did
was that I left Timer1 as-is, so I was getting woken up every two
seconds.

I would try working your way up.  Start with a PIC with nothing
connected to it.  See if you get 1 uA on sleep.  Then add stuff one at a
time to see where your current is going.

I agree, it is fun to experiment with getting the absolute lowest power,
even if you don't need it.

Alex


> {Original Message removed}

2003\06\20@180450 by Olin Lathrop

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> In sleep mode, the internal osc is shut down and all LED's are off, and
> only Timer 1 wakes up every 1/2 second (which wakes up the internal
> osc), runs about 50 instructions and goes to sleep.  During all this
> time, RA4 (open drain) is such that it draws no power during the brief
> wake up.  In this mode, I get 0.12mA average.

120uA is awfully high for what should be just timer 1 oscillator running
with 32,768Hz crystal.  Is the watchdog timer off?  Is BOR off?  Are port
B pullups disabled or disconnected or floating?  Is the A/D off?  It would
help to see the schematic.


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2003\06\23@041840 by Nigel Orr

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pic microcontroller discussion list <> wrote on Friday, June 20, 2003 8:24
PM:

{Quote hidden}

Is the A/D off?  Is the voltage reference off?  Is the WDT off?  Check each
pin with a resistive pull to each rail, if the pin value changes, it isn't
set as an output!

If it's a '628 with dual speed clock, you could set it to 37kHz for lower
power when on, obviously it would be on for longer though- has anyone
measured the relative power drain in this sort of application?  I guess it
isn't a simple calculation because of oscillator startup time etc.

One other thing, how are you measuring the current?  Could it be the meter
at fault?  You could try a largish resistor in series with the power and
measure voltage across that- aim to drop, say, 100mV across the resistor.

Nigel
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2003\06\23@174459 by Picdude

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More info that I should've added before ...
- PIC 16F628.
- Internal oscillator at 4Mhz.
- Also 32.768khz oscillator on Timer 1.
- Comparator off.
- WDT off.
- Timer 0 automatically off during sleep
- Timer 1 oscillator on during sleep
- Timer 2 off (by default)
- CCP off (by default)
- V.Ref off (by default)
- UART off (by default)
- PWRT on
- BOD off
- MCLR disabled (I'm using RA5 as an input)
- Port B pull-ups explicitly set off.
- 5V supply.  I'm measuring current at the output of the 7805.

BTW, I'm using a super-duper high-class Radio-Shack mutltimeter, so the 0.12mA figure may have a noticeable tolerance.  I expect this to be significant at these lower mA values, but hopefully the error is not in the hundreds of percent!  =8-0

Schematic ...
  http://www.narwani.org/neil/stuff/WST1.00_revA.gif

Before the sleep instruction, RA4 is explicitly set to "1", and all other outputs are set to 0.  RA5 is the only input, and externally pulled to ground.

Cheers,
-Neil.




On Friday 20 June 2003 14:24, Picdude scribbled:
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2003\06\23@231929 by Matt Pobursky

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On Mon, 23 Jun 2003 16:45:46 -0500, Picdude wrote:
> BTW, I'm using a super-duper high-class Radio-Shack mutltimeter, so
> the 0.12mA figure may have a noticeable tolerance.  I expect this to
> be significant at these lower mA values, but hopefully the error is
> not in the hundreds of percent!  =8-0

Back in my former life as a consumer electronics designer, I routinely
had to measure the operating current of devices that ran on a single
button cell for years (i.e. maximum supply current was ~20uA or less).
I found out quickly that almost any general purpose test instrument
introduced significant errors. In some cases it was 100's of percent!
Scope inputs, meter inputs, etc. all caused problems.

We solved the problem by making an active buffer circuit for the 'scope
using a battery powered high input impedance, ultra-low input bias
current op-amp. We also made an active current to voltage converter
using a similar technique. Accurately measuring micropower circuits is
tricky business.
Long ago I read an excellent application note by Jim Williams of Linear
Technology that dealt with the practical aspects of measurements in
micropower circuits. I think Maxim also has a similar application note.

Matt Pobursky
Maximum Performance Systems

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