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STM32F2 RTC coarse calibration seems to have no effect

Question asked by Mr_M_from_G on Feb 11, 2016
Latest reply on Oct 14, 2016 by button.benjamin
on a STM32F207 I want to use coarse calibration but it seems to have no effect.
- I changed PRER to PREDIV_A = 63 and PREDIV_S = 511 and print it out on a UART after reset. I changed to these values to have a better calibration resolution. This is how I understand chapter 22.3.9 of STM32F205 207 ref manual.
- Still I have 512 Hz at the calibration output, though chapter 22.3.12 in ref manu implies that the frequency depends on PREDIV_A, following this I would expect 1024 Hz
- for several weeks I changed CALIBR but always have the RTC running about 20 ppm too fast (eg. 12 sec in 6.8 days). Now I reached CALIBR = 0x9F, I also print this out after reset. As far a I understood I should have a slow down of the RTC of 31 ppm with these settings, but no success.
Is there any setting I need to make to enable coarse calibration? Or do I need the standard values in PRER? Any other ideas?

Thanks a lot