Toshiba TLCS-900/L1 Series Manual page 211

(3) Writing Clock 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 1 Hz clock from
32.768 kHz. Carry of a Clock is not done for 0.5 second when reset this divider.
So write in data during this interval.
PAGER<PAGE> = "0"
RESTR<RSTTMR> = "1"
Figure 3.13.4 Flowchart of Data Write
Start
Select PAGE0
Divider reset
Write the clock data
End
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Note: This period is within 0.5 [s].
2008-02-20