RTC running fast on STM32L433

[Scott Dev]

Hi

I have setup the RTC on the STM32L433 and the clock is running about 5 second fast in 24 hours. I am using a 32.768KHz crystal, which is directly next to the processor . I have set the

RTC->PREDIV_A to 0x 7f, and RTC->PREDIV_A to 0x ff, and selected the LSE clock, cant think what it can be, anyone help?

Thanks

Scott

[Tesla DeLorean]

Double check the measured frequency. Should be able to benchmark via a connected TIM, or if available on an MCO pin.

Check loading, check trim settings.

[Uwe Bonnes]

5 second in a day is about 60 ppm. Crystal tolerance is oftne 50 ppm plus some load capacitance mismatch and your number can be explained.

[Tesla DeLorean]

You can use TIM16

Example here for measuring LSI here, but can select LSE

STM32Cube_FW_L4_V1.13.0\Projects\NUCLEO-L432KC\Examples\RTC\RTC_LSI\Src\main.c

The math here is broken, destroys accuracy

  /* Frequency computation */

  uwLsiFreq = (uint32_t) SystemCoreClock / uwPeriodValue;

  uwLsiFreq *= 8;

Use

  uwLsiFreq = (SystemCoreClock * 😎 / uwPeriodValue;

[Peter Mather]

Use HAL_RTCEx_SetSmoothCalib to tune the frequency. Here is my clock set up. Note Prediv setting are different to allow sub second reading

The calibration value (Option.RTC_calibrate in my code) can be between -511 and + 512. n specifies the number of extra clock pulses to be "created" or "removed" every 2^20 real clock pulses. If my maths are correct therefore a change of +/-1 should equate to about 0.0824 seconds per day.

/* RTC init function */
static void myMX_RTC_Init(void)
{
int up=RTC_SMOOTHCALIB_PLUSPULSES_RESET;
int calibrate= -Option.RTC_Calibrate;
  /* USER CODE BEGIN RTC_Init 0 */
 
  /* USER CODE END RTC_Init 0 */
 
  RTC_TimeTypeDef sTime;
  RTC_DateTypeDef sDate;
 
  /* USER CODE BEGIN RTC_Init 1 */
 
  /* USER CODE END RTC_Init 1 */
 
    /**Initialize RTC Only
    */
  hrtc.Instance = RTC;
  hrtc.Init.HourFormat = RTC_HOURFORMAT_24;
  hrtc.Init.AsynchPrediv = 7;
  hrtc.Init.SynchPrediv = 4095;
  hrtc.Init.OutPut = RTC_OUTPUT_DISABLE;
  hrtc.Init.OutPutRemap = RTC_OUTPUT_REMAP_NONE;
  hrtc.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
  hrtc.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;
  if (HAL_RTC_Init(&hrtc) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }
  if (HAL_RTC_GetTime(&hrtc, &sTime, RTC_FORMAT_BIN) != HAL_OK)
  {
  _Error_Handler(__FILE__, __LINE__);
  }
 
  if (HAL_RTC_GetDate(&hrtc, &sDate, RTC_FORMAT_BIN) != HAL_OK)
  {
  _Error_Handler(__FILE__, __LINE__);
  }
  RtcGetTime();
  if(sDate.Year<18){
 
    /**Initialize RTC and set the Time and Date
    */
	  sTime.Hours = 0x0;
	  sTime.Minutes = 0x0;
	  sTime.Seconds = 0x0;
	  sTime.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;
	  sTime.StoreOperation = RTC_STOREOPERATION_RESET;
	  if (HAL_RTC_SetTime(&hrtc, &sTime, RTC_FORMAT_BCD) != HAL_OK)
	  {
		  _Error_Handler(__FILE__, __LINE__);
	  }
 
	  sDate.WeekDay = RTC_WEEKDAY_MONDAY;
	  sDate.Month = RTC_MONTH_JANUARY;
	  sDate.Date = 0x1;
	  sDate.Year = 0x0;
 
	  if (HAL_RTC_SetDate(&hrtc, &sDate, RTC_FORMAT_BCD) != HAL_OK)
	  {
		  _Error_Handler(__FILE__, __LINE__);
	  }
  }
  if(Option.RTC_Calibrate>0){
	  up=RTC_SMOOTHCALIB_PLUSPULSES_SET;
	  calibrate=512-Option.RTC_Calibrate;
  }
  HAL_RTCEx_SetSmoothCalib(&hrtc, RTC_SMOOTHCALIB_PERIOD_32SEC, up, calibrate);
 
}

[Scott Dev]

Thanks Clive.

I will have to download the examples, do I have to install the latest cubemx , which wil instalkl the examples?

Scott

[Scott Dev]

Thanks Peter

I will look into this. I am only using the 32.768KHz as the external crystal, what does it calibrate it against? Wouldnt it need a known frequency to calibrate against?

Scott

[Peter Mather]

"Wouldnt it need a known frequency to calibrate against?"

The real world 🙂

"the clock is running about 5 second fast in 24 hours"

You would start with a value of -5/0.0824  = -61

this converts to

HAL_RTCEx_SetSmoothCalib(&hrtc, RTC_SMOOTHCALIB_PERIOD_32SEC, RTC_SMOOTHCALIB_PLUSPULSES_RESET, 61);

I you were 5 seconds slow it would be

HAL_RTCEx_SetSmoothCalib(&hrtc, RTC_SMOOTHCALIB_PERIOD_32SEC, RTC_SMOOTHCALIB_PLUSPULSES_SET, 512-61);

[Tesla DeLorean]

Typically an HSE supplied source, or you measuring it via MCO pin output on a test fixture.

[Tesla DeLorean]

They should be in the repository if you have previously installed the L4 libraries. You can alternately pull the CubeL4 ZIP file an unpack and use that.