Sub-second timing taken from Nucleo STM32 F0 F3 F4 RTC

The RTC hardware on the STM32's is capable of reporting sub-second timing resolution. This is not supported by the mbed HAL & API's though. However, we can make a function analogous to mbed's rtc_read() which will return the current fractional second as well as the time_t whole second time/datestamp:

Modified rtc_read() function

void rtc_read_frac_s(time_t * t, float * secfrac)
    RTC_HandleTypeDef RtcHandle;
    RTC_DateTypeDef dateStruct;
    RTC_TimeTypeDef timeStruct;
    struct tm timeinfo;

    RtcHandle.Instance = RTC;

    HAL_RTC_GetTime(&RtcHandle, &timeStruct, FORMAT_BIN);
    HAL_RTC_GetDate(&RtcHandle, &dateStruct, 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
    *t = mktime(&timeinfo);

// Standard granularity is ~4ms (1/256sec)
// Import mbed-dev & modify rtc_api.c's rtc_init() with these values for ~1ms (1/1024sec) granularity:
//    RtcHandle.Init.AsynchPrediv   = 0x1f;
//    RtcHandle.Init.SynchPrediv    = (rtc_freq / 0x20) - 1;

    *secfrac = ((float)(timeStruct.SecondFraction-timeStruct.SubSeconds)) / ((float)(timeStruct.SecondFraction+1));


time_t time;
float frac_secs;
rtc_read_frac_s(&time, &frac_secs);

As an aside, it appears that the current Nucleos are being fitted with the LSE crystal, and the latest (as at 03/16) mbed library's RTC init has been fixed such that the time is not set to zero at every reboot.

To set the time, I've used code based on Kenji Arai's

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