I'm currently examining the reference design schematics from ST, available at en.MB1389-WL55JC-highband-E02_Schematic. In the schematics, the EXTERNAL HSE CLK with TCXO configuration is linked to the MCUs F6 (PB0_VDD_TCXO) and G9 (OSC_IN, net name XTA) Pins.

From my understanding, it is feasible to connect the EXTERNAL HSE CLK (XO back-up) to G9 and F8. If I choose not to use a backup crystal, what happens to the OSC_OUT pin? Does it remain unused? enter image description here

  • \$\begingroup\$ Please add data sheet links to the main parts. \$\endgroup\$
    – Andy aka
    Nov 17, 2023 at 10:03
  • \$\begingroup\$ Updated with respective schematics link \$\endgroup\$
    – Atrahasis
    Nov 18, 2023 at 10:12

1 Answer 1


The circuit you show is not backup low-power and low-frequency clock, but it is main precise clock refered as HSE (High-Speed External). While MCU itself can work without it, the RF module use it as a precise clock for generating RF freqeuncies. As long as you want to use SubGHz Radio, you need it. G9 and F8 are identifiers (coordinates) of BGA pins and they are not easily accessible directly.

The kit allow you to change type of oscilator used for generating this clock. You can choose crystal or TCXO oscilator, but you always should use one oscilator. Oscilator type is selectable using jumpers and basicaly it is highligheted on following diagram 18 from User Manual (UM2592, rev. 2). The same manual describe which jumpers are used for reconfiguration.

STM32WL55 RF Block Diagram

If you want to use none of crystal oscilator nor TCXO oscilator, you need to provide precise (at least 10ppm) 32MHz clock signal to pad connected to OSC_IN pin and rework board for removing unnescessary components. STM32WL55 Reference Manual (RM0453, rev. 5) allows this configuration and show diagram how should all three clocking options look like. In case of TCXO and external clock option OSC_OUT pin remain not connected.

Since it requires desoldering several components, removing metal shield and exposes clock input only on small pads, I do not recommend doing so and recommends using just default TCXO mode.

STM32WL55 Clocking Options

  • \$\begingroup\$ This is the most committed, thoroughly professional, and comprehensive reply I've come across on any forum. I sincerely appreciate your dedication and in-depth exploration of my question, providing a detailed and thorough response. Thank you very much. Respect!!! \$\endgroup\$
    – Atrahasis
    Nov 24, 2023 at 12:34

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