RTC smooth digital calibration in STM32F4 with HSE

[pierre-yves]

Hello,

I am taking the liberty of writing to you for information on the calibration of the RTC.

At present the RTC uses the HSE clock. The HSE clock is derived from a 25MHz quartz and then divided to be at 1MHz. This quartz has a tolerance of 20ppm.

The microcontroller is a STM32F4.

I want to calibrate the RTC to cancel the quartz drift but I can't do it.

Here is the configuration of the RTC on the clock side:

   PeriphClkInitStruct.PeriphClockSelection   = RCC_PERIPHCLK_RTC;

   PeriphClkInitStruct.RTCClockSelection      = RCC_RTCCLKSOURCE_HSE_DIV25;

   RTC_handle.Init.AsynchPrediv   = 124;

   RTC_handle.Init.SynchPrediv    = 7999;

With this configuration the RTC clock is at 1Hz (ck_spre).

To quantify the error on the HSE clock, I count the number of milliseconds of the timer TIM1 which is at 1ms

For a period of 32 seconds, so I have 32.293s, an error of 293ms.

Is the method good?

How to calculate the compensation ?

How to use the HAL_RTCEx_SetSmoothCalib function?

Thank you in advance

Best Regards

[A Maq]

If you look at the last section of the STM32F7 Reference Manual RTC synchronization that tells you about how to advance or delay adding into SSR using SUB bits of SHIFTR.

Let me know if you need any more help.