RCC: Enable LSE clock

Enable the low power oscillator to work with an external crystal and to work as input for the RTC peripehral, ( its main use )

#include "bsp.h"

int main( void )
{
    HAL_Init();     /*Init HAL library and call the HAL_MspInit function*/
    
    while(1)
    {
    }
}

msps.c

void Msp_Init( void )
{
    RCC_OscInitTypeDef        RCC_OscInitStruct = {0};
    RCC_PeriphCLKInitTypeDef  PeriphClkInitStruct = {0};

    __HAL_RCC_SYSCFG_CLK_ENABLE();
    __HAL_RCC_PWR_CLK_ENABLE();
    
    /*Eanlble backup domain*/
    HAL_PWREx_ControlVoltageScaling( PWR_REGULATOR_VOLTAGE_SCALE1 );
    HAL_PWR_EnableBkUpAccess();
    __HAL_RCC_LSEDRIVE_CONFIG( RCC_LSEDRIVE_LOW );

    /*reset previous RTC source clock*/
    PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_RTC;
    PeriphClkInitStruct.RTCClockSelection = RCC_RTCCLKSOURCE_NONE;
    HAL_RCCEx_PeriphCLKConfig( &PeriphClkInitStruct );
    
    /* Configure LSE/LSI as RTC clock source */
    RCC_OscInitStruct.OscillatorType =  RCC_OSCILLATORTYPE_LSI | RCC_OSCILLATORTYPE_LSE;
    RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
    RCC_OscInitStruct.LSEState = RCC_LSE_ON;    /*enable LSE*/
    RCC_OscInitStruct.LSIState = RCC_LSI_OFF;   /*disable LSI*/
    HAL_RCC_OscConfig( &RCC_OscInitStruct );

    /*Set LSE as source clock for the RTC*/
    PeriphClkInitStruct.RTCClockSelection = RCC_RTCCLKSOURCE_LSE;
    HAL_RCCEx_PeriphCLKConfig( &PeriphClkInitStruct );
      
    /* Peripheral clock enable for the RTC */
    __HAL_RCC_RTC_ENABLE();
    __HAL_RCC_RTCAPB_CLK_ENABLE();
}