RTC Time value

[GUIRAT.Youssef]

Posted on November 28, 2016 at 10:59

Hi, 

I'm using STM32L476RG microcontroller and I'm configuring the RTC to get the instant time value. I'm using LSE with 32,786 KHz as a clock source for my RTC . I have followed this application note (

http://www.st.com/content/ccc/resource/technical/document/application_note/group0/71/b8/5f/6a/8e/d5/45/0a/DM00226326/files/DM00226326.pdf/jcr:content/translations/en.DM00226326.pdf

) to set correctly the initialization process as well as the PREDIV_SYNCH and PREDIV_ASYNCH values (which are respectively in my case 0x00FF and 0x7F ) . However, when I let my application run for too long ( 24 Hours ) and then I check the RTC Time value , I found a gap of 10 seconds ( difference between my RTC time and the desktop time ). This difference is so huge for me because I'm working on medical app. 

Here's my Init function:

&sharpdefine RTC_ASYNCH_PREDIV  0x7F

&sharpdefine RTC_SYNCH_PREDIV   0x00FF

  BSP_RTC_Status Status= RTC_OK;

 /**** Configure the RTC peripheral ******************/

  /* Configure RTC prescaler and RTC data registers */

  /* RTC configured as follow:

      - Hour Format    = Format 12

      - Asynch Prediv  = Value according to source clock

      - Synch Prediv   = Value according to source clock

      - OutPut         = Output Disable

      - OutPutPolarity = High Polarity

      - OutPutType     = Open Drain */

  __HAL_RTC_RESET_HANDLE_STATE(&RtcHandle);

  RtcHandle.Instance = RTC;

  RtcHandle.Init.HourFormat     = RTC_HOURFORMAT_12;

  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;

  if(HAL_RTC_Init(&RtcHandle)!= HAL_OK)

  {

    Status = RTC_ERROR;

  }

  __HAL_RTC_WRITEPROTECTION_DISABLE(&RtcHandle);

  RtcHandle.Instance->CR |= RTC_CR_BYPSHAD;

  /* Enable the write protection for RTC registers */

  __HAL_RTC_WRITEPROTECTION_ENABLE(&RtcHandle);

Any ideas please ?

#rtc #l4 #time #stm32

[waclawek.jan]

Posted on November 28, 2016 at 18:17

This is around 115ppm. More than one would expect from a common 32kHz crystal, but not that much that couldn't be explained by poor design of the oscillator - improper burden capacitors, long traces imposing further parasitics. The oscillator circuit is a low-power one, and can be easily thrown wrong even by flux residues.

Review AN2867.

Also, RTCs in most STM32 have ''fine tuning'' provisions, read the RTC chapter of RM.

JW

[Walid FTITI_O]

Posted on November 29, 2016 at 10:05

Hi Elliot, 

As mentionned in the same application note that you mentioned AN4759, you should perform a dogital smooth calibration to compensate the RTC clock deviation which causes a time deviation. You would refer to following parts:

- 1.4 Digital Smooth Calibration

- 5 STM32L4 API and digital smooth calibrationapplication example

-Hannibal-

[GUIRAT.Youssef]

Posted on November 30, 2016 at 15:32

Thanks for both of you !

I will do the smooth calibration and hope that I will be back with good news :))

[damh]

Posted on June 20, 2017 at 17:07

a) disconnect debugger (or set DBGMCU_APB1FZR1 appropriate (DBG_RTC_STOP=0))

b) check, if your PC is the problem (most windows PCs without automatic RTC sync have this problem, difference of ~10-15s is normal)

c) check your ambient temperature (RTCs are working only well at ~20°C)