As a learning exercise, I'm trying to reverse engineer the radio protocol of a 27MHz wireless mouse. The device currently transmits on the 27.045MHz channel and uses 2FSK. I have an IQ recording of the data transmitted during a left button press here (32-bit float 1MHz sample rate).

Using the cursors in inspectrum, I can see that the chip rate is 5k, there is a preamble at the start (not sure what the extra few chips before are), followed by the data presumably.

Inspectrum, no cursors

Inspectrum, with cursors

Judging by the lack of long runs of 1s and 0s, the data is probably line encoded, however I'm not sure what encoding is being used here. In the data section after the preamble, the gaps between transitions vary between 1, 1.5, 2 and 2.5 chips. Looking at common line encodings such as Manchester, the gaps between transitions are either 1 or 2 chips. It's not obvious what line encoding is being used here.

Any thoughts?

  • 1
    $\begingroup$ See this answer for a few pointers on how to get technical information about how your device operates. $\endgroup$
    – MBaz
    Mar 17 '19 at 19:02
  • $\begingroup$ @MBaz Thank you. I had looked at the FCC information for the device (C3K1056), but other than the frequencies and modulation method, I wasn't able to find anything regarding the line encoding. $\endgroup$
    – Amr Bekhit
    Mar 17 '19 at 19:10
  • 1
    $\begingroup$ The information in this is presentation would suggest that it is Miller encoding: remote-exploit.org/content/keykeriki_ph7d9.pdf . The presentation also has information on MicroSoft's variant of Miller encoding. FWIW, here is Miller's now expired patent from 1960: patents.google.com/patent/US3108261 $\endgroup$
    – Andy Walls
    Mar 17 '19 at 21:03
  • $\begingroup$ @AndyWalls Thank you - this is probably it. The 2.5 chip delays at the start and end are probably start and end delimiters. If you write up an answer, I can mark it as the correct one. $\endgroup$
    – Amr Bekhit
    Mar 18 '19 at 12:35

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