This register is write protected. The write access procedure is described in RTC register write protection on page9.
| SUBFS | Subtract a fraction of a second These bits are write only and is always read as zero. Writing to this bit has no effect when a shift operation is pending (when SHPF=1, in RTC_ISR). The value which is written to SUBFS is added to the synchronous prescaler counter. Since this counter counts down, this operation effectively subtracts from (delays) the clock by: Delay (seconds) = SUBFS / (PREDIV_S + 1) A fraction of a second can effectively be added to the clock (advancing the clock) when the ADD1S function is used in conjunction with SUBFS, effectively advancing the clock by: Advance (seconds) = (1 - (SUBFS / (PREDIV_S + 1))). Note: Writing to SUBFS causes RSF to be cleared. Software can then wait until RSF=1 to be sure that the shadow registers have been updated with the shifted time. |
| ADD1S | Add one second This bit is write only and is always read as zero. Writing to this bit has no effect when a shift operation is pending (when SHPF=1, in RTC_ISR). This function is intended to be used with SUBFS (see description below) in order to effectively add a fraction of a second to the clock in an atomic operation. |