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CC1125: Frequency programming with FS_SPARE register

Jan Swanepoel
Prodigy 20 points

Part Number: CC1125

I have recently started with the CC1125 on a custom board for the 434MHz band. I worked my way through the datasheet and user guide to get it up and running. After some time I managed to transmit some variable length packets but only on one frequency (around 379MHz). No matter how I changed the FREQ and LO Divider registers it did not change. Auto FS calibration when going to TX mode and SCAL strobe also did not help. Finally I wrote 0xAC to FS_SPARE (after seeing it in SmartRF studio) and the problem was solved!

My question now is what does the FS_SPARE register do? The datasheet says, "For test purposes only, use values from SmartRF Studio", and makes no metion that it is required for frequency programming. So does this mean that there is something wrong with my chip or should the register always be set to 0xAC?

  • 0
    TI__Mastermind 21630 points
    Hi Jan,
    It is recommended to test the RF link at your desired frequency using smartRF studio and then export the raido settings to your development project.
    Try that for 434MHz.
  • 0 in reply to FI
    Prodigy 20 points
    Thanks, I understand that but if a register (like FS_SPARE) is required to be set to some value which is not discussed in the documentation then using SmartRF is not really a recommendation but rather a compulsary step to get it to work.

    Anyhow, since this is my first time working with the cc1125 I guess I just want to know that the chip and board is fine and setting FS_SPARE to 0xAC is normal procedure? I think it's the "For test purpose only..." part that makes me wonder if something is wrong, because it is working but why is a test register required to make it work...
  • 0 in reply to Jan Swanepoel
    TI__Mastermind 21630 points
    Hi Jan,
    When i export the settings from SmartRF studio the FS_SPARE register is set to 0xAC. Can you do the same and see if that is the case?
    Usually registers that are marked as "For test proposes only..." should not be altered by user as the SmartRF studio automatically sets their value.