anelok: power games

Ron K. Jeffries rjeffries at gmail.com
Mon Dec 16 12:01:07 EST 2013


I hope you choose to go with 1xAAA.
Much higher capacity, low cost, widely available.
The case gets thicker and the gadget weighs a bit more.
No biggie.


---
Ron K. Jeffries
805-567-4670






On Sun, Dec 15, 2013 at 8:51 PM, Werner Almesberger
<werner at almesberger.net>wrote:

> Now that I have a complete case, I tried to make Anelok run off
> battery power so that I can show it off also outside the lab.
>
> Alas, this turned out to be more difficult than expected. First
> of all, idle current (where "idle" means spinning in a while (1);
> loop) jumped from 4-5 mA after reset to well above 50 mA after
> initializing all the subsystems.
>
> This turned out to be caused by the solder pads of the OLED cable
> somehow shorting one GPIO to ground. The soldering looked fine,
> though. Re-soldering the pads made the problem go away for a
> moment, then return. It took cutting a groove between the pads
> to keep these two apart.
>
> With that issue gone, a pad further down the line decided to
> carry the torch and started to act up at about 20 mA. Well, also
> that issue fell victim to a hot iron and a sharp knife.
>
> These events suggest that the amount of rework I've done on that
> part of the PCB has about reached the board's limits. I call this
> "rework to death", where you eventually reach a point where every
> attempt to fix something just creates new problems.
>
>
> Anyway, so far that area seems to be under control. Then I
> optimized some of the GPIO initializations, saving some 400 uA in
> total. One surprising find was that setting GPIOs going to
> (unconnected) pads to "off" draws some 50 uA per pin more than
> setting them to output low.
>
> Alas, Anelok still didn't make it even to the PIN input dialog,
> with its rather moderate energy consumption (about 10 mA).
>
>
> So I kept on experimenting. I added CPU sleep and converted the
> timing delays (mdelay) to sleep with wakeup by interrupt (msleep).
> Surprisingly, this only reduced the power consumption from about
> 4 mA to about 3 mA, and the deeper sleep modes didn't seem to have
> any additional effect.
>
> And Anelok still wouldn't run from battery power alone.
>
>
> Today, I broke all those changes down into reasonably small chunks
> and added a few more uses of msleep. Lo and behold, after all
> this, it would suddenly be content with just the battery.
>
> Here's the proof - no cables attached:
>
> http://downloads.qi-hardware.com/people/werner/anelok/tmp/on-battery.jpg
>
> The case looks a bit odd because the two halves came apart during
> the experiment. Also, the temperature measurement is a bit too
> optimistic - ambient temperature was only in the low 30es.
>
> A quick measurement revealed that idle current can now be as low
> as about 1 mA, which would be reasonably close to the up to
> 500 uA the MCU alone can draw in the current sleep mode. (It can
> go much lower, but this needs more work.)
>
>
> There are more mysteries in power management, e.g., the board
> didn't want to start at 2.5 V (from a lab power supply) in one
> experiment, while even voltages as low as 1.5 V should be fine.
>
> While things look better now, the initial poor results may indicate
> that a stronger battery may be needed. The boost converter could
> also run from a single AAA cell (which would require only a
> moderate size increase) but the current unstabilized battery
> voltage rail (MCU and RF) would have to move past the/a converter,
> which means that the converter would be continuously active.
>
> As they say, research has shown that more research is needed.
>
> - Werner
>
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