Fortunately, there already is a project that tries to solve specifically the problem of doing BT in an open-friendly way, and that already has done a great amount of trailblazing. It's called "Ubertooth":
http://ubertooth.sourceforge.net Their motivation is that they want to be able to sniff BT traffic and to inject artificial traffic, so they need a lot of flexibility. The same flexibility can of course also be used to just implement a regular stack. They use a TI CC2400 transceiver chip. Now, looking at that chip [1, 2], three things catch the eye: 1) it's HUGE, 48-QFN, 7x7 mm. That's as big as the MCU. For comparison, the AT86RF232 comes in a 32-QFN package of only 5x5 mm and we'll see later on that transceivers can be even smaller. 2) it's expensive, USD 4.73 @ 1000 units. AT86RF232: USD 2.03-2.30 @ 1000. 3) the produce page warns "Not Recommended for New Designs". [1] http://www.ti.com/product/cc2400 [2] http://www.digikey.com/product-detail/en/CC2400RSUR/296-21512-1-ND/1304552 While it is quite possible to make products with obsolete chips, doing so has the risk of sourcing getting increasingly messy. So let's try to steer our Titanic on a southernly route instead of relying on the strength of our lifeboats. If there is one chip that can do it, there are usually more. So what would they look like ? In other words, what are the radio requirements for BT ? First of all, there's a number of different requirements. BT 1.2 and the new shooting star BTLE are fairly similar at that level, so let's focus on them: - Carrier frequency: 2.4 GHz band, channels in 1 MHz steps - Modulation: GFSK - Frequency deviation (modulation depth): +/- ~170 kHz (BT 1.2) or +/- 250 kHz (BTLE). - Data rate: 1 Mbps According to the experts, BTLE also has a considerably simpler protocol architecture than regular BT. The downsides are that it is still relatively new and not nearly as widely supported as BT and that it's security architecture is a joke - just what we want for a password safe. Here is a nice presentation that explains the basics of BTLE and also the nearly Adobesque blunder they did with the key exchange: https://www.usenix.org/conference/woot13/bluetooth-low-energy-comes-low-security The good news is that BTLE will improve with time, that radio hardware that can talk BTLE is usually also capable of talking BT 1.2 (if someone gets around to implementing the stack. For the higher layers, BlueZ may be of some use but there may be considerable evil below it.), and that by doing most of the processing in software the risk of getting cut off by some future protocol change is very low. Next: chips ! - Werner _______________________________________________ Qi Hardware Discussion List Mail to list (members only): [email protected] Subscribe or Unsubscribe: http://lists.en.qi-hardware.com/mailman/listinfo/discussion
