PCB is missing the LSE and not connecting over BLE - what are our options?

Question

Hi everyone,When my group was designing our PCB for the STM32WB55RG, we read page 38 of the datasheet (https://www.st.com/resource/en/datasheet/stm32wb55rg.pdf) and thought we only needed the HSE for BLE functionality. However, after soldering our board and testing it, we can successfully advertise but not connect. From this post https://community.st.com/s/question/0D50X0000BbMebpSQC/stm32wb-ble-application-using-the-lsi2-as-source-of-rf-wakeup, it looks like we do need an LSE after all if we want to connect over Bluetooth to our board. In our project's .ioc file, I also tried using the LSI for the RTC/LCD Source Mux and RFWKP Clock Mux, but it also didn't work.Our project is due within a couple days, so we can't manufacture a new PCB. We think our best option at this point is purchasing and externally connecting a 32.768 LSE to the relevant MCU pins, but does anyone know of any cheaper/easier ways to get around this? We would really appreciate any help you can give.

Tesla DeLorean · Answer

STM32 parts historically do not have the LSI in the low-power domain, as it draws too much power. Sleep/Standby typically uses LSE to maintain clock in RTC.

Wiring things up externally might be difficult as the crystal here is sensitive to loading and circuit characteristics. Perhaps use an XO source instead of a crystal, or an enclosed RTC unit with a 32.768 KHz output.

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