The X-CUBE-CLKTRIM expansion software illustrates the trimming of an HSE crystal oscillator in order to reach the high-accuracy frequency required by RF applications.
The STM32WB Series microcontrollers support low-power RF standards such as Bluetooth Low Energy (BLE) and IEEE 802.15.4. BLE requires a minimum ±20 ppm-accurate RF clock while IEEE 802.15.4 accuracy requirement is ±40 ppm.
STM32WB Series microcontrollers offer an efficient architecture with internal load capacitances that allow the fine tuning of the crystal frequency (32 MHz), saving the extra cost of additional external capacitances. 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 software expansion for STM32CubeWB proposes a trimming procedure that allows the once-for-all storage of the right capacitance values in OTP bytes.
X-CUBE-CLKTRIM is composed of a single project with two configurations. The first is the trimming procedure itself. The second is an implementation example of HSE clock initialization in RF applications.
The firmware is built on the HAL drivers for the STM32WB Series.
X-CUBE-CLKTRIM runs on Nucleo boards for the STM32WB Series.
The 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.
- High-speed external (HSE) crystal oscillator trimming
- Allows best RF performance for BLE and IEEE 802.15.4
- Trimming parameters stored in user OTP Flash area