anelok: more hardware testing

Felix sucotronic at gmail.com
Mon Oct 14 10:38:10 EDT 2013


On Sun, Oct 13, 2013 at 11:53 AM, Werner Almesberger <werner at almesberger.net
> wrote:

> I've now placed all the components and tested some more of them.
> This is what the complete board looks like:
>
> http://downloads.qi-hardware.com/people/werner/anelok/tmp/brd0-fnt.jpg


It look gorgeous!!


>
> - boost converter: works but I noticed that, when disabled, Vout =
>   Vin. This seems to be a low resistance path, which would mean that
>   high leakage of anything on the 3.3 V rail could drain the battery.
>   In this case, a FET to cut off that part or the entire rail would
>   be needed.
>
>   The 3.3 V rail feeds the display and the memory card. The memory
>   card already has a FET (but see below). For the OLED, the design
>   relies on its own power-saving capabilities. Early tests with the
>   MCU-less board have shown a substantial standby current (114 uA)
>   into the OLED, well above the 1-3 uA the data sheet specifies. If
>   further testing confirms this, the FET may be needed.
>
> - speaking of FETs, when the battery is low, the voltage difference
>   between the MCU's supply and the 3.3 V rail should be large enough
>   to open the memory card's FET. In other words, it will stop working
>   as an off switch.
>
>   The easiest way to fix this is probably by adding an n-FET and a
>   pull-up resistor. Again, nothing major.
>
>
It seems you cannot rely on power saving features on weird screen
controllers.
FETs are the hard way to teach them to stop eating precious energy :P


> Then, power needs a closer look. The things to check there are:
> - operation under USB power,
> - operating current at 3.0 V (nominal battery voltage),
> - operating current at 2.0 V (battery cut-off voltage),
> - standby current at 3.0 V,
> - standby current at 2.0 V.
>
>
If you're only testing it with a workbench supply, don't forget to test it
with a real coin battery. It would be a pity to produce next prototype and
discover that it would need bigger capacitor ;)
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