anelok: battery measurements

Werner Almesberger werner at almesberger.net
Mon Jan 6 09:23:53 EST 2014


One of the big questions I still aim to answer with the first board
is whether a) the CR2032 battery is up to the task and b) if there
are any changes that should be made to improve power consumption.

I built a little test rig that tests a real battery:

http://downloads.qi-hardware.com/people/werner/anelok/tmp/bat-test-rig.jpg

The aligator clips go to a multimeter to measure static current,
the two probes are for battery voltage and the LED which acts as
trigger/marker signal.

The signals captured look like this:

http://downloads.qi-hardware.com/people/werner/anelok/tmp/brd0-display-on.jpg

This shows the response to pressing the button:

- the MCU detects the button press and very briefly flashes the LED
  (about 8-9 mA, causes a slight drop in battery voltage), then

- resets the display (the prolonged drop of about 300 mV is caused
  by the display reset function busy-looping in mdelay instead of
  waiting in msleep. Need to change this.)

- after the reset, the display is activated. This causes a sharp
  drop of almost 1 V that could reset the device if it was a little
  deeper and/or if the battery was a little weaker.

- after that, the display is erased, I generate another LED flash
  to indicate that display_on has returned, display the "-" PIN
  entry prompt, and wait for further user input.

The drop seems to be caused by inrush current of the display's
DC-DC converter. This is what it looks like when zooming in:

http://downloads.qi-hardware.com/people/werner/anelok/tmp/brd0-display-on-drop.jpg

This should be fixable by adding a hopefully not too large
capacitor on Vsys.


The screenshots show something else: my scope has lost the ability
to make digital screendumps :-( This may be related to the USB
excinction event that killed two powered hubs last year. I'm still
not sure what did it and how.

Without proper screenshots, analyzing battery behaviour and
especially documenting it will be messy. So I'm trying to resove
the half-dead scope situation first ...

- Werner



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