Anelok: the journey towards V1

Werner Almesberger werner at almesberger.net
Sun Jun 1 22:16:54 EDT 2014


I'm working towards the V1 board which will, among smaller changes,
replace the wheel with a capacitive sensor and the AT86RF231 with the
CC2543.

I tried a few different component placements for this. An overview
drawing can be found here:
http://downloads.qi-hardware.com/people/werner/anelok/tmp/v1s.pdf


First, I tried the approach where the CC2543 helps the KL26 with its
GPIOs, so that a smaller package with fewer pins can be used with the
KL26. This is in the branch "puppet". The memory card would be
ejected into the battery compartment.

This is nice and compact, but routing the control signals from the RF
SoC promises to be difficult. I also had second thoughts about having
the two chips interact so tightly, especially since the CC2543 would
not only have to handle control outputs (power on, etc.), but also
some interrupt lines.


I then tried to see what would happen if I just adapted the version 0
design. I also gave the KL26 its own crystal (more about this in a
separate post).

A first try - branch "large" - looked somewhat promising but I ended
up with very long traces and a few problems with the pin assignment.


Next was a more radical attempt to put all major components on the
top layer, to avoid vias. This is in the "coplanar" branch. It looked
pretty good, with one problem: with components as tall as about 2-2.5
mm on the top, the FPC of the OLED would get bent quite a bit.

I also moved the battery to the center, to allow for better holders.
More about this later. Last but not least, I tentatively added the
"hard" rfkill switch.

I almost made this board, but then I chickened out, fearing it might
become too much of an FPC killer.


The next attempt, "split", moved everything but the OLED FPC and the
LED to the bottom. I could make it all fit but had to share some
signals, had other pin assignments that were far from optimal, and
the traces to the capacitive sensor took a rather epic journey.

Naw, too many concessions.


Then I decided to face my fears and made a design that wouldn't try
to avoid vias. It's in the "dftv" branch, "Don't Fear The Vias".

This one places the large chips on the top layer and tall components
(USB, switch, memory card holder) on the bottom. It also places the
memory card such that it ejects into the battery compartment again.

This worked surprisingly well and I could assign all the signals to
"good" pins and keep delicate traces short. Since the card holder
eats up a lot of space, there was very little room for the USB side,
which also has some of the power supply circuit. Things got too
crowded with the OLED FPC in the same area, so I flipped the OLED and
the FPC now connects at the top edge.


I think this looks good enough to try. I therefore merged "dftv" into
the master branch. If anyone wants to have a look, the schematics are
here:
http://downloads.qi-hardware.com/people/werner/anelok/tmp/anelok-20140601.pdf

The layout is here:
http://downloads.qi-hardware.com/people/werner/anelok/tmp/anelok-20140601.png


More details about the various designs can be found in
https://gitorious.org/anelok/anelok/source/hw/NOTES

- Werner



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