M41T82 datasheet(42/64 Pages) STMICROELECTRONICS

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Clock operation

M41T82, M41T83

42/64

DocID012578 Rev 19

3.10

Battery low warning

The M41T8x automatically checks the battery each time VCC powers up and each time the

clock rolls over at midnight.

VBAT is compared to VBL (approximately 2.5 V), then the battery low (BL) bit, D4 of flags

greater than VBL, 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 VCC 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 23, during the midnight rollover check, the M41T8x applies a load to the

battery, then compares VBAT to VBL and updates the BL bit accordingly. Because a load is

present, an open condition on the VBAT 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, VBAT will be floating and the battery check result will be

indeterminate.

Figure 23. Battery check

BAT

BL=2.5V

Only at

rollover

At power-up

and at rollover

AM03009v1

M41T82 Datenblatt(PDF) 42 Page - STMicroelectronics