Datasheet
Clock operation M41T93
36/56 DocID12615 Rev 8
3.10 Battery low warning
The M41T93 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 17,during the midnight rollover check, the M41T93 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 17. Battery check
V
BAT
V
BL
=2.5V
Only at
rollover
BL
FF
Q
S
R
At power-up
and at rollover
R
L
AM03009v1