Anelok: CC2543 vs. nRF51822

Werner Almesberger werner at almesberger.net
Sun Feb 7 15:44:05 UTC 2016


Paul Boddie wrote:
> but you could probably feel "validated" by 
> choosing it if all these other products influence you in some way.

Yes, the Nordic chips are rather popular. This also means that
nicely matched baluns are readily available :) Alas, not two with
the same package, so the RF balun will become a part without second
source.

I've now updated the schematics:
http://downloads.qi-hardware.com/people/werner/anelok/tmp/anelok-20160207.pdf

There may still be loose ends (e.g., if nRF51 and cMCU end up on the
same side of the PCB, I may want t swap RF_CRRT and RF_CTRR), but
this would by and large be the design.

There are a number of options in the RF part (sheet 10):

- on the left side, we have components for a DC-DC converter to
  provide the internal 1.8 V supply from the system's 3.3 V.

  This should reduce power consumption by maybe up to 30% while
  the radio interface is active (i.e., listening or sending). Not
  sure if this is actually worth the effort, but it can't hurt to
  have the option for now.

  If the DC-DC converter turns out to be overkill, we can use the
  same layout for the LDO setup, by removing C24, L2, and L3, and
  (optionally) reducing C25 to 100 nF.

- for "wall-clock" (RTC) timekeeping, there are a few options, too:

  1) use a low-power 32768 Hz crystal (0.4 uA),
  2) use the voracious 16 MHz crystal (15 uA, which is high, but
     still surprisingly low given that the 16 MHz oscillator draws
     hundreds of uA in other modes),
  3) use an external oscillator, or
  4) maybe use an external trigger ?

  For 4), we already have an RTC in the sMCU, so that one could
  provide an occasional trigger, provided that

  a) some jitter is allowed (this would have to go from sMCU to
     cMCU and then to RF),
  b) it's okay to sometimes lose a trigger (e.g., if some other
     communication is in progress at the time),

  This option needs further study.

  For 3), we could use a clock output from the sMCU. Drawbacks of
  this approach would include having a permanently active "long"
  trace on the PCB, the need to coordinate rfkill with the sMCU
  (to avoid driving the hypothetical clock signal while VRF is
  shorted to ground), and the need to free up one GPIO.

  Avoiding the crystal would mean a potential BOM cost saving of
  about 40 US-cents at 1000 units, 30 at 10k. For further study.

- I also increased the capacitance of some decoupling capacitors
  (C30, C32), to match similar parts used elsewhere. This is to
  keep the number of different parts down.

- the nRF51822 itself comes with various memory configurations,
  with 128 or 256 kB Flash and 16 or 32 kB RAM. I've specified the
  biggest (and most expensive), to be sure to have enough room.
  This can be optimized later.

Speaking of cost, the component cost (without PCB, case, battery
contacts, etc.) is currently at USD 25.40 @ 100 units,
USD 22.51 @ 500 units. Some published quantity discounts don't go
beyond 100 units, so prices for 500 units and beyond may go down
a little.

This is a bit on the high side of the acceptable range. The
relatively high cost is caused less by fancy chips but by the
price of the  OLED jumping up from USD 4.85 @ 100 (2014-2015) to
USD 8.26 @ 100 in 2016.

http://downloads.qi-hardware.com/people/werner/anelok/tmp/anelok-pie-usd-500-20160207.pdf

Maybe it's time to look for second sources, e.g.,

- Shineworld Innovations GLO13-D-M2001
  http://www.alibaba.com/product-detail/1-3-Graphic-Blue-Color-128x64_60018968100.html?spm=a2700.7724857.29.1.kwI3WO&s=p

  It says "blue" but looks white-ish. In any case, it seem to exist
  in white:
  http://p.globalsources.com/IMAGES/PDT/SPEC/239/K1077987239.pdf

  Note: different controller, apparently not entirely compatible:
  https://forum.arduino.cc/index.php?topic=259186.0

- Formike KWH0130UL01
  http://www.formike.com.cn/product/60244849708-219498402/Formike_1_3_inch_flexible_OLED128x64_KWH0130UL01.html

In any case, this type of OLED is very common, so if there's a
problem with Eastrising, it shouldn't be hard to find a drop-in
replacement.

- Werner



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