M41T82_10 Datasheet, PDF (39/61 Pages) STMicroelectronics – Serial I2C bus RTC with battery switchover

M41T82-M41T83
Clock operation
3.10
Battery low warning
The M41T8x automatically checks the battery each time V_CC powers up and each time the
clock rolls over at midnight.
V_BAT is compared to V_BL (approximately 2.5 V), then the battery low (BL) bit, D4 of flags
register 0Fh, is set if the battery voltage is found to be less than V_BL. Similarly, if V_BAT is
greater than V_BL, the BL bit is cleared during battery check.
The BL bit retains its state until the next battery check occurs. This means the BL bit will not
clear immediately upon battery replacement, but only after the next battery check occurs at
the next power-up or midnight rollover.
If a battery low is generated during a power-up sequence, this indicates that the battery is
below approximately 2.5 volts and may not be able to maintain data integrity. Clock data
should be considered suspect and verified as correct. A fresh battery should be installed.
If a battery low indication is generated during the 24-hour interval check, this indicates that
the battery is near end of life. However, data is not compromised due to the fact that a
nominal V_CC is supplied. In order to ensure data integrity during subsequent periods of
battery backup mode, the battery should be replaced.
Midnight rollover check
As shown in Figure 22, during the midnight rollover check, the M41T8x applies a load to the
battery, then compares V_BAT to V_BL and updates the BL bit accordingly. Because a load is
present, an open condition on the V_BAT pin will result in the BL bit being set. After the check
is performed, the RTC removes the load.
Power-up battery check
During the power-up check, no load is applied to the battery under the assumption the
battery has already been stressed to its working level by having powered the RTC in backup
mode. If no battery is present, V_BAT will be floating and the battery check result will be
indeterminate.
Figure 22. Battery check
V_BAT
At power-up
and at rollover
Only at
rollover
R_L
V_BL=2.5V
S BL
Q
FF
R
AM03009v1
Doc ID 12578 Rev 12
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