STM32WL System Clock differs at boot/reset and after deepsleep about mbed-os HOT 2 OPEN

In the meanwhile, I found the reason

On init, clock is setup to 4MHz see STM32Cube_FW/system_stm32wlxx.c

and After deepsleep, ForceOscOutofDeepSleep() is called and on this one, the clock is set back to 48MHz with RCC_MSIRANGE_11

RCC_OscInitStruct.MSIClockRange = RCC_MSIRANGE_11;

So I don't think it's a bug or whatsoever, but worth mentioning because running at 4MHz can have some side effect see #15463

I tried to slow down frequency (8Mhz, 16MHz, 24MHz, ...) to lower consumption in ForceOscOutofDeepSleep() but in this case for example, I2C Secure Element is unable to wake (ATECC608a) from sleep in mbed-cryptoauthlib, only way is to set to 48MHz

So quick and dirty fix is going to deepsleep at startup.

int main()
{
  // Go go speed cruise, Don't go below 8ms 
  // because it could stay at 4MHz (time to setup ?)
  ThisThread::sleep_for(10ms);
 ....
}

from mbed-os.

cc @ARMmbed/team-st-mcd

from mbed-os.