In Anelok the radio chip does more than just talk to the aether. I already mentioned the clock sharing. That is optional (we could just give the MCU its own crystal, at the price of one of the somewhat scarce interrupt-capable GPIOs and increased board crowding) but desirable.
There is also the option of reversing the roles and sending a clock from the MCU to the radio chip. Once upon a long ago I tried to do that with atben, hoping to need no new crystal at all. The result was a disastrously dirty spectrum since some minor impurities in the clock signal caused the RF chip to radiate also outside the intended channel. That struggle has been recorded for posteriority. The results of various attempts of eliminating potential sources of contamination and of "smoothing" the clock signal can be seen here: http://downloads.qi-hardware.com/people/werner/wpan/rework-20110112/ http://downloads.qi-hardware.com/people/werner/wpan/rework-20110113/ After accepting the utter futility of my resistance and adding a crystal to atben, I got this: http://downloads.qi-hardware.com/people/werner/wpan/redesign-xtal/ In the case of Anelok we may be able to output the bus clock which is 48/n MHz with 2 <= n <= 8. That would cost us another GPIO. The bus clock is currently at its maximum, 24 MHz, but it probably wouldn't hurt to run it a bit slower. Given the atben experience, I wouldn't have a lot of hope for this yielding an acceptable result, though. More importantly, the AT86RF232 also has a random number generator. We need a source of high-quality randomness for the cryptography. Of the BTLE candidates I looked at, only the CC2543 has a true random number generator (based on receiving RF noise, which may make it susceptible to tampering.) Another thing to consider are export restrictions. Some of the chips have built-in AES support, but - according to Digi-Key - it seems that Uncle Sam thinks they're all harmless enough to let us play with them. (I couldn't check the CYRF8935, though.) Summary so far: Chip Package Clock RNG Export Unit @ 1000 Comments --------------- ------- ------- ---- ------- ----------- ------- A7105 20-QFN yes no - 1.48 ADF7242 32-QFN no no ok 3.05 also 802.15.4 AT86RF231 32-QFN yes yes ok 2.75 802.15.4 only AT86RF232 32-QFN (yes) yes ok 2.03-2.30 idem CC2540F128 40-QFN no? no ok 2.81 BTLE, 2 x xtal, USB CC2543 32-QFN no yes ok 2.52 CYRF8935 24-QFN (no) no ? 1.22 unbalanced output TRC104 24-QFN no? no ok 1.80 "naked" bitstream I've also included the AT86RF23x chips for comparison purposes. The AT86RF232 has two prices at Digi-Key with the "tray" version being a little cheaper (and "Non-Stock") than the "cut tape" version. "(yes)" means that the chip doesn't officially support a suitable clock output but that the ones I have do implement the corresponding setting. "(no)" means that the CYRF8935 has a clock output pad ... but only on the bare die, not bonded in the package. Duh. "no?" means that I haven't found anything but didn't spend enough time with the data sheet to be sure that there wouldn't be some clock output option. Of these chips, I would drop the CC2540F128 because it's not open, and the TRC104 because having to work at the bit level is a bit too hardcore even for my taste. This leaves four candidates, each with a unique strength: - A7105: small yet has clock output - ADF7242: most flexible RF which includes 802.15.4 - CC2543: popular and has a true random number generator - CYRF8935: small and has the simplest RF circuit (I hate baluns. The integrated ones are expensive and a latent sourcing risk. The discrete ones - if you can get the design information - are a component zoo.) A lot of fun can be had with a flexible RF interface, as can be seen here: http://www.remote-exploit.org/articles/keykeriki_v2_0__8211_2_4ghz/index.html They use an A7125 (and others). The slides of the CanSecWest 2010 talk are worth reading. Going back to the possibility of using the MCU clock (instead of a local crystal) for a moment, the clocking options of the four survivors are: - A7105: seems to be pretty flexible and all the supported frequencies (6, 8, 12, 16, and 24 MHz) can be generated by our KL25. The data sheet explicitly mentions the possibility of using an external clock generator. (Note: the AT86RF23x manuals do, too. And we know how that went.) - ADF7242: requires 26 MHz (which is a frequency we can't provide) and the data sheet doesn't even mention the possibility of using a digital clock. - C2543: requires 32 MHz (which we can't provide either) and no mention of not using a crystal. - CYRF8935: uses 12 MHz (which we can provide) but the data sheet doesn't mention using anything but a crystal and doesn't specify the crystal input voltage. Next: tentative roadmap. (Would that be a "toadmap" ?) - Werner _______________________________________________ Qi Hardware Discussion List Mail to list (members only): [email protected] Subscribe or Unsubscribe: http://lists.en.qi-hardware.com/mailman/listinfo/discussion
