Posted on January 20, 2016 at 17:58
There is another issue that users and ST should be aware of, and that is the inability to properly run off of HSE_RTC when using STM32CubeMX to initialize the hardware without adding user code. It will work on power-up, but not in a debugger or off of a reset.
The problem lies in SystemClock_Config, and specifically in the HAL_RCCex_PeriphCLKConfig function. In this function, a check is made to see if the RTC clock source has changed from what is already in the RCC_BDCR register. Only if it has changed will the backup domain be reset. In the case of a reset, the RCC_BDCR register will always match the RTC clock source selection auto-generated by theCube because the RTCSEL value is retained on reset. However,a reset also causes the HSE_RTC prescaler to be reset to0 (an invalid value), causing the RTC clock to not work. Therefore the RTC will never work out of reset because the prescaler is only set if the _HAL_RCC_RTC_CONFIG macro is called, and that doesn't happen due to the RTC clock source selection being unchanged. The only remedy to this that won't get wiped out when updating your Cube code is to reset the backup domain in the User Code area of HAL_MspInit as follows:
/**
* Initializes the Global MSP.
*/
void HAL_MspInit(void)
{
/* USER CODE BEGIN MspInit 0 */
/* USER CODE END MspInit 0 */
HAL_NVIC_SetPriorityGrouping(NVIC_PRIORITYGROUP_4);
/* System interrupt init*/
/* SysTick_IRQn interrupt configuration */
HAL_NVIC_SetPriority(SysTick_IRQn, 15, 0);
/* USER CODE BEGIN MspInit 1 */
/* Enable Power Clock*/
__HAL_RCC_PWR_CLK_ENABLE();
/* Enable write access to Backup domain */
PWR->CR1 |= PWR_CR1_DBP;
/* RTC Clock selection can be changed only if the Backup Domain is reset */
__HAL_RCC_BACKUPRESET_FORCE();
__HAL_RCC_BACKUPRESET_RELEASE();
/* USER CODE END MspInit 1 */
}
I think an argument could be made that the backup domain should automatically be reset in theHAL_RCCex_PeriphCLKConfig routine if the RTC clock source is HSE_RTC and the prescaler value located in RCC_CFGR is invalid.Or, perhaps the backup domain should ALWAYS be reset in the HAL_RCCex_PeriphCLKConfig function if the clock source is HSE_RTC.After all, why else would the function be calledif not to initialize the RTC clock?
