Question

How do I connect a TCXO to an EFR32 Series 1 SoC?

Answer

A TCXO clock source can be used to supply the EFR32 main clock.  Reconfiguration of the HFXO register settings in the Clock Management Unit will be required for operation with a TCXO.  Consult the EFR32 Reference Manual for additional details regarding the following settings:

• CMU_HFXOCTRL - HFXO Control Register
  • HFXO Mode
    • Set this bit to 1 in order to allow the single-ended TCXO instead of the oscillator.
    • This is set to 0 by default, which selects the crystal oscillator.
  • CMU_OSCENCMD - Oscillator Enable/Disable Command Register
    • HFXOEN
      • Set this bit to 1 in order for the HFXC Mode register setting to take effect.
      • This is set to 0 by default, which disables the HFXO.
    • HFXODIS
      • Set this bit to 1 to reset the oscillator setting to default if / when not using the TXCO or crystal oscillator.
      • This is set to 0 by default.

• CMU_HFXOSTARTUPCTRL - HFXO Startup Control / CMU_HFXOSTEADYSTATECTRL - HFXO Steady State Control
  • CTUNE
    • To compensate the extra load, CTUNE will probably need to be lowered or zeroed out compared to a crystal.

Be sure to physically connect the TCXO to the HFXTAL_N pin for the above register settings.  The HFXTAL_P pin should be no-connect.