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• Decoding the Smoke Signals

  skytoastar • 08/11/2018 at 14:26 • 1 comment

  It's surprising sometimes how much information a company hands over to the FCC. Our ID of interest is 2AI67-TX1300CH. If you look at the exhibits list you'll find internal photos (what's that I see? a NRF24L01+ radio?) and a handy test report (you say it broadcasts on Channels 10, 40, and 70?) and surprisingly even a "FREQUENCE AND HOPPING SYSTEM" report that describes how it broadcasts on those three channels (every ~16 seconds it broadcasts on each of the three channels). I didn't even have to take my device apart. There's also no report for the portable display that comes with the thermometer so we know the portable display doesn't transmit. Thanks FCC!

  That being said, there are still a number of NRF24L01+ parameters we still don't know. It is using ShockBurst? CRC length? Address length? Address value? Payload length? Data rate? (Someone smarter than me would have figured out data rate from the FCC test report. Someone like me ends up using that info to make an incorrect judgement and going down a dead end for several days.)

  I assumed the radio was using a 1 Mbps rate based on the test report. First I used an Arduino and NRF24L01+ to try to sniff for packets as described on Yveaux's blog and using an address of 0x0055 (the preamble). After many fruitless attempts I decided this was a good excuse to buy an ADALM-PLUTO SDR. (I thought about using a downconverter with an RTL-SDR but after considering the cost, an ADALM-PLUTO made more sense.)

  An entire project log needs to be dedicated to just setting up the ADALM-PLUTO SDR. Analog Device's wiki is sufficient but lacking, in my opinion, if you're running Windows. But for now I'll cut to the chase and say it is actually using 250 kbps. Finding that out reminded my face what my palm feel like. (Using this data rate with Yveaux's software was much more successful, though not needed after decoding with the SDR.)

  My Smoke was transmitting with an address of 0x36BD529F51. Does every Smoke use this address? I doubt it. BBQ competitions would be a problematic place to use a Smoke if that were the case. I don't have two Smoke thermometers to know for sure but if I had to guess they use the first three bytes (0x36BD52) for all units and then choose a random number for the last two bytes for each device. NRF24L01+ radios are designed to sync onto 3, 4, or 5 byte addresses. So a receiver sync process could be designed as follows:

  1. Listen always for a packet with: Address (0x36BD52)
  2. Take the first two "payload" bytes and append them to the base address.
  3. Sleep for ~16 seconds.
  4. Wake and listen for the next transmission, listening for the full 5 byte address.
  5. Repeat steps 3 and 4.

  This is speculation. It could be just one byte of uniquely identifying information. Or maybe it's zero bytes and it relies on the long sleep times to avoid collisions. Or maybe it's actually much more complicated than that. Having more information from more Smoke units would help clarify this information.

  Here's a brain teaser: With two bytes of uniquely identifying information, what's the likelihood of a collision? (Left as an exercise for the reader. Hint: birthday problem.)

  Here are the full details of the transmission:

  • 5 byte address
  • No ShockBurst
  • 21 byte payload
  • 2 byte CRC
  • Channels 10, 40, and 70
  • 250 kbps
  • 20 transmissions per channel every 16 seconds or so.
  • ~500ms per channel to perform the 20 transmissions.
  • ~15 seconds from the end of last transmission to the beginning of the next.

  And the payload:

  • Little endian byte order:
  • 2: Probe 1 Temp
  • 2: Probe 1 MAX Alarm Temp
  • 2: Probe 1 MIN Alarm Temp
  • 2: Probe 2 Temp 
  • 2: Probe 2 MAX Alarm Temp
  • 2: Probe 2 MIN Alarm Temp
  • 1: Alarm 1 ON(1)/OFF(0) 
  • 1: Probe 1 Inserted(0)/Not(3) 
  • 1: Alarm 2 ON(1)/OFF(0) 
  • 1: Probe 2 Inserted(0)/Not(3) 
  • 1: Units of C(0) or F(1) 
  • 1: Sync timing offset?
  • 3: Unknown (constant value of 0x009600)

  All two-byte temperatures are signed 16-bit integers and are in...

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