This works fine in an app without SM but when I run this from a non-secure app together with SM, the last step fails. This is the code:
// Enable access to RTC and backup domain
HAL_PWR_EnableBkUpAccess();
if (__HAL_RCC_GET_FLAG(RCC_FLAG_LSERDY) == RESET)
{
// Reset the backup domain (equivalent to LL_RCC_ForceBackupDomainReset)
__HAL_RCC_BACKUPRESET_FORCE();
__HAL_RCC_BACKUPRESET_RELEASE();
// Ensure LSE is OFF and NOT in bypass mode
CLEAR_BIT(RCC->BDCR, RCC_BDCR_LSEON | RCC_BDCR_LSEBYP);
// Set drive strength for startup (must be done while LSE is OFF)
__HAL_RCC_LSEDRIVE_CONFIG(RCC_LSEDRIVE_MEDIUMHIGH);
// Enable LSE
__HAL_RCC_LSE_CONFIG(RCC_LSE_ON);
// Wait until LSE is ready, with timeout
if (SYSTIMER_WaitFlag(&RCC->BDCR, RCC_BDCR_LSERDY, true, 20000000))
{
}
else
{
// Disable LSE
__HAL_RCC_LSE_CONFIG(RCC_LSE_OFF);
// LSE activation error, handle as appropriate
status = HAL_TIMEOUT;
}
// Select LSE as RTC clock source
__HAL_RCC_RTC_CONFIG(RCC_RTCCLKSOURCE_LSE);
}
When entering this code, the BDCR register in RCC is 0 so completely clear, nothing is enabled. I noticed that when I repeat the process, the __HAL_RCC_BACKUPRESET_FORCE() (i.e. setting of the VSWRST bit) doesn't have any effect but that's not the issue. I can enable LSEON and LSERDY is being set so the LSE is working but the last step, setting RTCSEL to 1 to select the LSE as RTC clock is not working at all no matter what I try. RTC_SECCFGR is 0x0808 so LSESEC is 0 and RCC_PRIVCFGR is 0x0.
I am stumped, this has worked perfectly fine without SM. I couldn't find any configuration options for SM that relate to this.
