Read Eddystone TLM advertisement data

[Ripper346]

Hi, after struggling I was able to make the scanner working and now I wanted to grab the advertisement of some eddystone beacons. I thought that the returning AdvertisementData object when something is discovered included it but it doesn't seem so. For example this is an advertisement I received

AdvertisementData(service_data={'0000feaa-0000-1000-8000-00805f9b34fb': b' \x00\x0b\xa1\x14\x90\x00\xa0FV\x0f\xe3Ut'}, service_uuids=['0000feaa-0000-1000-8000-00805f9b34fb'], rssi=-76)

I am 100% sure that the beacon is sending eddystone tlm packets but the only thing I am seeing is that one service data that it is too short to be it. Is there a way to see the data (even raw) received? Thank you

[dlech]

What other data were you expecting? I see the service UUID and service data that is described at https://github.com/google/eddystone/blob/master/protocol-specification.md and the service data is 14 bytes as described at https://github.com/google/eddystone/blob/master/eddystone-tlm/tlm-plain.md.

[Ripper346]

Oh, so you are just assuming that that uuid corresponds to an eddystone tlm package. Ok, however the unpacking is wrong, using beacontools the example package says voltage 2977 and temperature 205625, but the unpacking says 41227 and temperature 32788 (I don't have 33°C in the room, it would be quite unpleasant 🥵). I think there is something wrong but I have never used the struct package, I need to check the documentation to understand the string you wrote

[dlech]

Also check https://github.com/google/eddystone/blob/master/eddystone-tlm/tlm-plain.md#field-notes. It looks like it uses some weird encodings that I didn't take into account. You will probably have to manipulate the unpacked values since I don't think there are any struct formats that correspond to some of the stuff used.

[Ripper346]

OK, thank you very much!

[Ripper346]

Ok, I looked into beacontools library and the code below gives the correct values. I write it so if anyone land on the question has a complete answer.

from construct import Struct, Int16ub, Int32ub, GreedyRange, Byte

UnencryptedTLMFrame = Struct(
    "frame_type" / Byte,
    "tlm_version" / Byte,
    "voltage" / Int16ub,
    "temperature" / Int16ub,
    "advertising_count" / Int32ub,
    "seconds_since_boot" / Int32ub,
)
frame = GreedyRange(UnencryptedTLMFrame)
parsed = frame.parse(b' \x00\x0b\xa3\x14\x80\x00\xc5\xf9\x03\x10\t\x08!')
parsed[0].voltage /= float(1000)
parsed[0].temperature /= float(256)

with struct it is (in the end you only missed the >)

frame_type, version, voltage, temperature, count, time = struct.unpack(">BBHHII", b' \x00\x0b\xa3\x14\x80\x00\xc5\xf9\x03\x10\t\x08!')
voltage /= 1000
temperature /= 256