Posted on January 03, 2018 at 10:22Hi,
I'm using a STM32L496VG, with an external LSE of 32768 Hz. I configure the RTC this way:
/* Enable Power Control clock */
__HAL_RCC_PWR_CLK_ENABLE();
/* Enable writing RTC back-up registers */
HAL_PWR_EnableBkUpAccess();
/* Configure RTC */
RtcHandle.Instance = RTC;
RtcHandle.Init.HourFormat = RTC_HOURFORMAT_24;
RtcHandle.Init.AsynchPrediv = RTC_ASYNCH_PREDIV;
RtcHandle.Init.SynchPrediv = RTC_SYNCH_PREDIV;
RtcHandle.Init.OutPut = RTC_OUTPUT_DISABLE;
RtcHandle.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
RtcHandle.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;
HAL_RTC_Init(&RtcHandle);
and:
/* Configure the RTC clock source */
RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSE;
RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
RCC_OscInitStruct.LSEState = RCC_LSE_ON;
HAL_RCC_OscConfig(&RCC_OscInitStruct);
/* Select LSE as RTC clock source */
PeriphClkInitStruct.PeriphClockSelection = RCC_PERIPHCLK_RTC;
PeriphClkInitStruct.RTCClockSelection = RCC_RTCCLKSOURCE_LSE;
HAL_RCCEx_PeriphCLKConfig(&PeriphClkInitStruct);
/* Enable RTC Clock */
__HAL_RCC_RTC_ENABLE();
In order to compensate the LSE drift, we output the RTC clock using HAL_RTCEx_SetCalibrationOutPut() function.
Using RTC_CALIBOUTPUT_1HZ we obtain 0.999951 Hz, which are 49 ppm drift, which is an expected value. But using RTC_CALIBOUTPUT_512HZ we obtain 508.022 Hz, which are 7769 ppm.
Also using an oscilloscope the output using RTC_CALIBOUTPUT_512HZ seems to be very unstable.
What's happening? Why there is such difference?
Thank you!
#rtc-calibration
