What clock source does CPU2 select coming out of CStop on the STM32WB55 if the HSE is not enabled?
I am trying to get to the lowest possible power consumption on a WB55 NUCLEO board while the BLE is advertising every three seconds and waking up on the RTC every second. I have the NUCLEO booting up on the HSE clock, and the stm32_lpm_if.c code switches from HSE to HSI and back as it enters and exits STOP2. Re-enabling HSE takes about 24 ms and consumes significant current, giving me an average of about 62 uA in this mode. I’ve found that if don’t enable the HSE, but stay on the HSI the current drops to about 10.5 uA. The BLE radio functions normally, and I can connect and disconnect and return to the 10.5 uA advertising mode. Is CPU2 somehow choosing the LSE clock to run on when it exits STOP2 and the HSE is not available? What is going on? The documentation is adamant about needing 32MHz for the system clock when running the radio.
