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  • The X-CUBE-CLKTRIM Expansion Package illustrates the trimming of an HSE crystal oscillator to reach the high-accuracy frequency required by RF applications.

    The STM32WB Series microcontrollers support low-power RF standards such as Bluetooth® Low Energy and IEEE 802.15.4. Bluetooth® Low Energy requires a minimum ±50 ppm RF clock accuracy while IEEE 802.15.4 accuracy requirement is ±40 ppm.
    The STM32WL Series microcontrollers support sub-GHz RF standards such as LoRaWAN® and meet the requirements of a wide range of wireless applications for low-power wide area networks (LPWAN). For LoRaWAN® and for other LPWAN applications, the highest the RF clock accuracy, the best the RF performance.
    STM32WB Series and STM32WL Series offer an efficient architecture with internal load capacitors that allow the fine tuning of the crystal frequency (32 MHz), saving the extra cost of additional external capacitors. Internal capacitance values are register controlled. They depend on various factors such as process variation, crystal, and PCB design and must therefore be set for each device individually.
    The X-CUBE-CLKTRIM Expansion Package and associated application note (AN5042) propose three different trimming procedures (manual, automatic, and with STM32CubeMonitor-RF (STM32CubeMonRF)) for the once-for-all storage of the right capacitance values in OTP bytes. Each proposes two configurations: the trimming procedure itself and an implementation example of HSE clock initialization in RF applications.
    The manual trimming procedure consists in measuring the RF clock with a precision frequency meter (better than 0.1 ppm). The RF clock frequency value is controlled through the user push-buttons so that the best clock accuracy can be calibrated step by step.
    The automatic trimming procedure consists in providing a precision clock reference to the STM32 so that it can trim its RF clock frequency itself, without any user intervention needed.
    The STM32CubeMonitor-RF trimming procedure consists in measuring the RF clock with a precision frequency meter (better than 0.1 ppm). The RF clock frequency value is controlled by means of STM32CubeMonitor-RF configurable scripts for calibrating the best clock accuracy step by step.
    It is not mandatory to use these trimming methods on each board in a production chain. The Precise HSE frequency trimming using STM32 wireless MCUs application note (AN5042) proposes a procedure to determine a reliable trimming parameter on a dozen test boards and use it on every other board produced.
    X-CUBE-CLKTRIM firmware is built on the HAL drivers and runs on the Nucleo boards of the STM32 Wireless MCUs (STM32WB Series only).

    Key Features

    • High-speed external (HSE) crystal oscillator trimming
    • Allows best RF performance for Bluetooth® Low Energy, IEEE 802.15.4, LoRaWAN® and LPWAN wireless applications
    • Trimming parameters stored in user OTP Flash area
    • Trimming in production procedure suggestion

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