Toshiba TMP91C815F Data Book page 211

(2) Writing timer data
When there is carry on the way of write operation , expecting data can not be wrote exactly.
Therefore, in order to write in data exactly please follow the below way.
① Reset for a divider
Inside of RTC, there is 15-stage divider which generates 1Hz clock from 32,768KHz. Carry of a timer is not done for
one second when reset this divider. So write in data during this interval.
START
PAGER<PA0>='0'
Select PAGE0
RESTR<RSTTMR>='1'
Divider reset
Write the timer data
END
Figure 3.13(4) Flowchart of data write
91C815-
208
TMP91C815
(note):
This period is within
0.5 second.