Anelok: one more MCU - maybe an idea for later
Werner Almesberger
werner at almesberger.net
Thu Dec 25 17:28:47 UTC 2014
I wrote:
> - two MCUs, even if both are 32-QFN, cost more than a single
> one in 48-QFN,
Let me put some numbers: the current BOM cost (1) of Anelok is
about USD 20.30 for 100 units. (2) Today's Digi-Key prices for 100
MCUs are:
Chip Package Flash Product code Unit price (USD)
------- ------- ----- ------------ ----------------
KL26 48-QFN 128 kB MKL26Z128VFT4 2.795
KL26 32-QFN 128 kB MKL26Z128VFM4 2.652
KL27 32-QFN 256 kB MKL27Z256VFM4 3.185
KL16 32-QFN 128 kB MKL16Z128VFM4 2.314
We also need another crystal (3), in the range 8-32 MHz, preferably
small (the 32 MHz crystal I currently have for RF is 3.2 x 2.5 mm).
Here's one that looks nice and is cheap:
Murata XRCGB32M000F2P00R0, 32 MHz +/- 20 ppm, 6 pF, 2.0 x 1.6 mm,
USD 0.399 @ 100 units.
So we get ...
Chips(s) Total Flash Sum of costs (USD)
------------------------------------- ----------- ------------------
MKL26Z128VFT4 128 kB 2.795
MKL26Z128VFM4 + MKL16Z128VFM4 + XTAL 256 kB 5.365
MKL27Z256VFM4 + MKL16Z128VFM4 + XTAL 384 kB 5.898
To this, add a few cents for extra capacitors and maybe a resistor
or two. Regarding total cost, we then get ...
MCUs BOM cost (1) Comparison
--------------- ------------ ----------
KL26 alone 20.30 100%
KL26 + KL16 22.90 113%
KL27 + KL16 23.45 116%
Not too bad.
(1) Components only. To obtain total manufacturing costs, add PCB,
case, yield, factory fees, labour, shipping, taxes, etc.
(2) For some parts I don't have volume discounts above 100 units.
For, say, 10k units, I would expect the component cost to drop
to about half the 100 units figure.
(3) That is, unless we do some crystal sharing. One option would be
to let the cMCU run from a crystal-governed clock provided by
the sMCU, but that would have the accuracy issue discussed in
[1].
Another option would be to coopt the RF MCU and make it share
its own (precise) crystal clock. This would of course mean that
one couldn't use USB during rfkill. But in this scenario,
rfkill would at least not affect timekeeping by the sMCU.
One could also mix and match: have a cMCU clock line that
connects to RF MCU and sMCU and pick whatever we like. I'd
begin to get a bit worried about EMI, with high-speed clock
traces crawling all around the board, though.
Last but not least, if picking a KL27 instead of a KL26 for the
cMCU, one could use the former's improved RC oscillator for USB
device mode. (4) One would still have to find a solution for
USB host, though.
But for the sake of simplicity let's just assume that the cMCU
will have its own crystal.
(4) You may wonder whether we shouldn't use the KL27 instead of the
KL26 even in a single-MCU scenario. The improved oscillator
would indeed be nice to have and the KL27 comes with 256 kB of
Flash while the KL26 maxes out at 128 kB. Alas, Freescale
dropped capacitive sensing in the KL27 (there's still the "GPIO
method" but I don't know how well that would work), so it would
be less suitable for Anelok in this regard than a KL26. Maybe
they'll some day make a KL28 with the best of KL26 and KL27 ...
[1] http://lists.en.qi-hardware.com/pipermail/discussion/2014-July/010676.html
- Werner
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