Anelok: first capacitive sensor results

Werner Almesberger werner at almesberger.net
Sun Aug 10 01:55:22 UTC 2014


I merged the firmware trees for Anelok 2013, Anelok 2014, and Y-Box,
got USB device to work on Anelok 2014, and added full DFU support
(boot loader and reset-to-DFU in the application) for Anelok 2014.
The merge is messy and needs some more thinking and cleanup, but
it'll do for now.

USB didn't yield easily and I tought the FLL jitter may be just too
big for it to work, but in the end it seems to be fine. Still need
to have a closer look at that jitter, though - it does look pretty
wild:

http://downloads.qi-hardware.com/people/werner/anelok/tmp/jitter-fll-xtal.png

This shows the 24 MHz bus block 2 us (i.e., 24 Full-Speed USB bit
times) after the trigger.

I also found another mystery: when DFU'ing, the received data is
passed with size 0. But the full 64 bytes are there. Strange. I
wonder if that's my punishment for having started to use
--gc-sections ...


With DFU, updating the firmware is very easy. I put this to good use
and added capacitive sensing. This is a first result, without any
tuning (using the parameters from the experiments on Y-Box) and with
idle plates floating:

http://downloads.qi-hardware.com/people/werner/anelok/tmp/cap-20140809.png

The red and green curves show the two channels, the blue curve their
difference. The five patterns are, from left to right:

- gently pressing the sensor (covered with 1 mm of plastic) in the
  middle,
- pressing it on the left side,
- pressing it on the right side,
- sliding the finger from left to right,
- and back from right to left.

This look reasonably good for a first try. Handling positions near
the ends of the sensor area may get a bit tricky, but we'll see. I
also need to make the plastic cover fit a bit better - right now
one end sits on the vias and thus has a little air gap.

- Werner



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