M41T93_11 Datasheet, PDF (19/51 Pages) STMicroelectronics – Serial SPI bus real-time clock with battery switchover

M41T93
Clock operation
3.2.1
Power-down time stamp
Some applications may need to determine the amount of time spent in backup mode. That
can be calculated if the time of power-down and the time of power-up are known. The latter
is straightforward to obtain. But the time of power-down is only available if an access
occurred just prior to power-down. That is, if there was an access of the device just prior to
power-down, the time of the access would have been frozen in the buffer transfer registers
and thus the approximate time of power-down could be obtained.
If an application requires the time of power-down, the best way to implement it is to set up
the software to do frequent reads of the clock, such as once every 1 or 5 seconds. That
way, at power-up, the buffer-transfer registers will contain a time value within 1 (or 5)
seconds of the actual time of power-down.
Doc ID 12615 Rev 6
19/51