Problem with STM32F407 using RTC_SmoothCalibConfig

- DBP bit (PWR_CR) is set to enable write access
- RTC_WPR is written key-sequenze (by function itself) to enable write 
access
- call of RTC_SmoothCalibConfig(...) does not change Register RTC->CALR
- return of RTC_SmoothCalibConfig(...) is SUCSESS, nevetheless RTC->CALR 
remains always 0x0000
- I find no Remarks in Errata sheet about this problem