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STM32F446 RTC Accuracy

Question asked by sahin.gokhan on Apr 16, 2018
Latest reply on Apr 16, 2018 by waclawek.jan

Hi everyone,

I have been trying to set 1ppm RTC clock for two days.I have read the datasheet and manual. They are recommended to use 2 timers and an external oscillator. Also, with using HAL_RTCEx_SetSmoothCalib function, says that we can calibrate it easily. I set the RTC routed output as calibration 512Hz, but the frequency of it is 511.96Hz. After that, I read a manual that is CD00167326 for STM32F1s. As it says, I have calculated the ppm error. 

512-511.96=0.04Hz => 0.04/511.96=7.81e-5 => ppm=78.13, the manual recommends the using 83 for the calibration value.

And then called the smooth calibration function this value. Thus;

HAL_RTCEx_SetSmoothCalib(&hrtc, RTC_SMOOTHCALIB_PERIOD_32SEC, RTC_SMOOTHCALIB_PLUSPULSES_RESET, 83);

However, although the calibration value has been changed, the routed value is still the 511.96Hz. Why doesn't it change? 

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