Thanks Paul for your quick reply. Is the stm32l4xx_hal_rcc.h file the only place to change the LSE timeout? I have changed it to 500mS and 5sec and neither one helps startup the LSE for me.

1. define RCC_LSE_TIMEOUT_VALUE ((uint32_t)5000)

Yes, using the latest MBED-DEV library, I have mine set to 5000mS.

Below is the corrected RTC code, you could copy and replace this in yours, not sure if it will effect the LSE start up. I have this running on both Nucleo and Disco boards + prototype board. I have fitted the 8MHz crystal and use this, but sure it works without. I also tend to use mine HSE clocked at 80MHz (without USB), it does run quite quick. But as I mentioned Interrupts are all over the place, causes Ticker issues as well and RTOS won't run. I have messaged ST and they are looking at this, just have to wait :(

/* mbed Microcontroller Library
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 *    may be used to endorse or promote products derived from this software
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 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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 */
#include "rtc_api.h"

#if DEVICE_RTC

#include "mbed_error.h"

#if DEVICE_RTC_LSI 
 static int rtc_inited = 0; 
#endif 

static RTC_HandleTypeDef RtcHandle;

void rtc_init(void)
{
    RCC_OscInitTypeDef RCC_OscInitStruct = {0};
    RCC_PeriphCLKInitTypeDef PeriphClkInitStruct = {0};
    uint32_t rtc_freq = 0;
    
#if DEVICE_RTC_LSI  
    rtc_inited = 1;  
#endif  

    RtcHandle.Instance = RTC;

    // Enable Power clock
    __HAL_RCC_PWR_CLK_ENABLE();

    // Enable access to Backup domain
    HAL_PWR_EnableBkUpAccess();

    // Reset Backup domain
    __HAL_RCC_BACKUPRESET_FORCE();
    __HAL_RCC_BACKUPRESET_RELEASE();


    // Enable LSE Oscillator
    RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSI | RCC_OSCILLATORTYPE_LSE;
    RCC_OscInitStruct.PLL.PLLState   = RCC_PLL_NONE; // Mandatory, otherwise the PLL is reconfigured!
    RCC_OscInitStruct.LSEState       = RCC_LSE_ON; // External 32.768 kHz clock on OSC_IN/OSC_OUT
    RCC_OscInitStruct.LSIState       = RCC_LSI_OFF;
    if (HAL_RCC_OscConfig(&RCC_OscInitStruct) == HAL_OK) { // Check if LSE has started correctly
        // Connect LSE to RTC
        PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_RTC;
        PeriphClkInitStruct.RTCClockSelection = RCC_RTCCLKSOURCE_LSE;
        HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct);
        rtc_freq = LSE_VALUE;
    }
    else {
      error("RTC error: LSE clock initialization failed.");
    }
#else
        // Enable LSI clock
        RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSI | RCC_OSCILLATORTYPE_LSE;
        RCC_OscInitStruct.PLL.PLLState   = RCC_PLL_NONE; // Mandatory, otherwise the PLL is reconfigured!
        RCC_OscInitStruct.LSEState       = RCC_LSE_OFF;
        RCC_OscInitStruct.LSIState       = RCC_LSI_ON;
        if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) {
            error("Cannot initialize RTC with LSI\n");
        }
        // Connect LSI to RTC
        PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_RTC;
        PeriphClkInitStruct.RTCClockSelection = RCC_RTCCLKSOURCE_LSI;
        if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct) != HAL_OK) {
            error("Cannot initialize RTC with LSI\n");
        }
        // This value is LSI typical value. To be measured precisely using a timer input capture for example.
        rtc_freq = 40000;
    }
#endif

    // Enable RTC
    __HAL_RCC_RTC_ENABLE();

    RtcHandle.Init.HourFormat     = RTC_HOURFORMAT_24;
    RtcHandle.Init.AsynchPrediv   = 127;
    RtcHandle.Init.SynchPrediv    = (rtc_freq / 128) - 1;
    RtcHandle.Init.OutPut         = RTC_OUTPUT_DISABLE;
    RtcHandle.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
    RtcHandle.Init.OutPutType     = RTC_OUTPUT_TYPE_OPENDRAIN;

    if (HAL_RTC_Init(&RtcHandle) != HAL_OK) {
        error("Cannot initialize RTC\n");
    }
}

void rtc_free(void)
{
    // Enable Power clock
    __HAL_RCC_PWR_CLK_ENABLE();

    // Enable access to Backup domain
    HAL_PWR_EnableBkUpAccess();

    // Reset Backup domain
    __HAL_RCC_BACKUPRESET_FORCE();
    __HAL_RCC_BACKUPRESET_RELEASE();

    // Disable access to Backup domain
    HAL_PWR_DisableBkUpAccess();

    // Disable LSI and LSE clocks
    RCC_OscInitTypeDef RCC_OscInitStruct;
    RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSI | RCC_OSCILLATORTYPE_LSE;
    RCC_OscInitStruct.PLL.PLLState   = RCC_PLL_NONE;
    RCC_OscInitStruct.LSIState       = RCC_LSI_OFF;
    RCC_OscInitStruct.LSEState       = RCC_LSE_OFF;
    HAL_RCC_OscConfig(&RCC_OscInitStruct);
#if DEVICE_RTC_LSI
    rtc_inited = 0;
#endif

}

int rtc_isenabled(void)
{
#if DEVICE_RTC_LSI
    return rtc_inited;
#else
  if ((RTC->ISR & RTC_ISR_INITS) ==  RTC_ISR_INITS) return 1;
  else return 0;
#endif

}

/*
 RTC Registers
   RTC_WeekDay 1=monday, 2=tuesday, ..., 7=sunday
   RTC_Month   1=january, 2=february, ..., 12=december
   RTC_Date    day of the month 1-31
   RTC_Year    year 0-99
 struct tm
   tm_sec      seconds after the minute 0-61
   tm_min      minutes after the hour 0-59
   tm_hour     hours since midnight 0-23
   tm_mday     day of the month 1-31
   tm_mon      months since January 0-11
   tm_year     years since 1900
   tm_wday     days since Sunday 0-6
   tm_yday     days since January 1 0-365
   tm_isdst    Daylight Saving Time flag
*/
time_t rtc_read(void)
{
    RTC_DateTypeDef dateStruct;
    RTC_TimeTypeDef timeStruct;
    struct tm timeinfo;

    RtcHandle.Instance = RTC;

    // Read actual date and time
    // Warning: the time must be read first!
    HAL_RTC_GetTime(&RtcHandle, &timeStruct, RTC_FORMAT_BIN);
    HAL_RTC_GetDate(&RtcHandle, &dateStruct, RTC_FORMAT_BIN);

    // Setup a tm structure based on the RTC
    timeinfo.tm_wday = dateStruct.WeekDay;
    timeinfo.tm_mon  = dateStruct.Month - 1;
    timeinfo.tm_mday = dateStruct.Date;
    timeinfo.tm_year = dateStruct.Year + 100;
    timeinfo.tm_hour = timeStruct.Hours;
    timeinfo.tm_min  = timeStruct.Minutes;
    timeinfo.tm_sec  = timeStruct.Seconds;

    // Convert to timestamp
    time_t t = mktime(&timeinfo);

    return t;
}

void rtc_write(time_t t)
{
    RTC_DateTypeDef dateStruct;
    RTC_TimeTypeDef timeStruct;

    RtcHandle.Instance = RTC;
    
     // Enable Power clock
    __HAL_RCC_PWR_CLK_ENABLE();

    // Enable access to Backup domain
    HAL_PWR_EnableBkUpAccess();

    // Convert the time into a tm
    struct tm *timeinfo = localtime(&t);

    // Fill RTC structures
    dateStruct.WeekDay        = timeinfo->tm_wday;
    dateStruct.Month          = timeinfo->tm_mon + 1;
    dateStruct.Date           = timeinfo->tm_mday;
    dateStruct.Year           = timeinfo->tm_year - 100;
    timeStruct.Hours          = timeinfo->tm_hour;
    timeStruct.Minutes        = timeinfo->tm_min;
    timeStruct.Seconds        = timeinfo->tm_sec;
    timeStruct.TimeFormat     = RTC_HOURFORMAT12_PM;
    timeStruct.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;
    timeStruct.StoreOperation = RTC_STOREOPERATION_RESET;

    // Change the RTC current date/time
    HAL_RTC_SetDate(&RtcHandle, &dateStruct, RTC_FORMAT_BIN);
    HAL_RTC_SetTime(&RtcHandle, &timeStruct, RTC_FORMAT_BIN);
}

You need to add this line in device.h file, under line 50 :

#define DEVICE_RTC_LSI          0 

Thanks Paul, the timeout value did work. The LSE on this processor is a lot more sensitive to my scope probe then the L152. After double checking it does look like it is starting.

As for the interupt problems for the L476, I switched to a offline complier (GCC) and they have gone away so far. It might be related to the errata that ST posted. Problem 1.1 - http://www.st.com/st-web-ui/static/active/en/resource/technical/document/errata_sheet/DM00111498.pdf

Kevin, there is now a fix for the interrupt issue, NVIC_RAM_VECTOR_ADDRESS is set to 0x20000000 instead of 0x10000000 in the cmsis_nvic.c file.

I have corrected my MBED-DEV library and the interrupts now work as expected.

Using the corrected RTC and NVIC code, deep sleep and RTC wake up are working well @ 6uA deep sleep with rapid and accurate wake up.

Paul