AN3060

Battery low detect

Doc ID 16253 Rev 2

13/23

6

Battery low detect

6.1

How does the battery low bit work? Can it detect a missing

battery?

Many ST serial real-time clocks include a battery monitor function which sets the BL bit

when the battery is low. The basic functionality is to periodically compare the battery to the

internal 2.5 V reference and set the BL bit whenever the battery voltage is less than 2.5 V.

Figure 8.

Battery monitor function

Table 2.

M41T81S register map

Addr

Function/range

BCD format

D7

D6

D5

D4

D3

D2

D1

D0

00h

0.1 seconds

0.01 seconds

Seconds

00-99

01h

ST

10 seconds

Seconds

Seconds

00-59

02h

0

10 minutes

Minutes

Minutes

00-59

03h

CEB

CB

10 Hours

Hours (24-hour format)

Century/hours

0-1/00-23

04h

0

0

0

0

0

Day of week

Day

01-7

05h

0

0

10 date

Date: day of month

Date

01-31

06h

0

0

0

10M

Month

Month

01-12

07h

10 years

Year

Year

00-99

08h

OUT

FT

S

Calibration

Calibration

09h

OFIE

BMB4

BMB3

BMB2

BMB1 BMB0

RB1

RB0

Watchdog

0Ah

AFE

SQWE

ABE

Al 10M

Alarm month

Al month

01-12

0Bh

RPT4

RPT5

AI 10 date

Alarm date

Al date

01-31

0Ch

RPT3

HT

AI 10 hour

Alarm hour

Al hour

00-23

0Dh

RPT2

Alarm 10 minutes

Alarm minutes

Al min

00-59

0Eh

RPT1

Alarm 10 seconds

Alarm seconds

Al sec

00-59

0Fh

WDF

AF

0

BL

0

OF

0

0

Flags

10h

0

0

0

0

0

0

0

0

Reserved

11h

0

0

0

0

0

0

0

0

Reserved

12h

0

0

0

0

0

0

0

0

Reserved

13h

RS3RS2

RS1RS0

0

0

0

0

SQW

AT ROLLOVER

AT ROLLOVER &

AT POWER-UP

2.5V

Q

AT ROLLOVER &

AT POWER-UP

DBL

AM04726v1