RM440 Rev 5 says : "SS can be larger than PREDIV_S only after a shift operation. In that case, the correct time/date is one second less than as indicated by RTC_TR/RTC_DR"
Low Layer Driver documentation says: "
==> Seconds fraction ratio * time_unit= [(SecondFraction-SubSeconds)/(SecondFraction+1)]
This conversion can be performed only if no shift operation is pending (ie. SHFP=0) when PREDIV_S >= SS. "
