by toggling SCL until SDA is observed to be at a high

level. At that point the microcontroller should pull SDA

low while SCL is high, generating a START condition.

Clock and Calendar

The time and calendar information is obtained by read-

ing the appropriate register bytes. Figure 1 illustrates the

RTC registers. The time and calendar data are set or ini-

tialized by writing the appropriate register bytes. The con-

tents of the time and calendar registers are in the

binary-coded decimal (BCD) format. The DS3231 can be

run in either 12-hour or 24-hour mode. Bit 6 of the hours

register is defined as the 12- or 24-hour mode select bit.

When high, the 12-hour mode is selected. In the 12-hour

mode, bit 5 is the AM/PM bit with logic-high being PM. In

the 24-hour mode, bit 5 is the second 10-hour bit (20–23

hours). The century bit (bit 7 of the month register) is tog-

gled when the years register overflows from 99 to 00.

The day-of-week register increments at midnight.

Values that correspond to the day of week are user-

defined but must be sequential (i.e., if 1 equals

Sunday, then 2 equals Monday, and so on). Illogical

time and date entries result in undefined operation.

When reading or writing the time and date registers, sec-

ondary (user) buffers are used to prevent errors when

the internal registers update. When reading the time and

date registers, the user buffers are synchronized to the

internal registers on any START and when the register

pointer rolls over to zero. The time information is read

from these secondary registers, while the clock contin-

ues to run. This eliminates the need to reread the regis-

ters in case the main registers update during a read.

RTC/TCXO/Crystal

____________________________________________________________________

11

Figure 1. Timekeeing Registers

Note: Unless otherwise specified, the registers’ state is not defined when power is first applied.

ADDRESS

BIT 7

MSB

BIT 6

BIT 5

BIT 4

BIT 3

BIT 2

BIT 1

BIT 0

LSB

FUNCTION

RANGE

00H

0

10 Seconds

Seconds

Seconds

00–59

01H

0

10 Minutes

Minutes

Minutes

00–59

AM/PM

02H

0

12/24

10 Hour

10 Hour

Hour

Hours

1–12 + AM/PM

00–23

03H

0

0

0

0

0

Day

Day

1–7

04H

0

0

10 Date

Date

Date

00–31

05H

Century

00

10 Month

Month

Month/

Century

01–12 +

Century

06H

10 Year

Year

Year

00–99

07H

A1M1

10 Seconds

Seconds

Alarm 1 Seconds

00–59

08H

A1M2

10 Minutes

Minutes

Alarm 1 Minutes

00–59

AM/PM

09H

A1M3

12/24

10 Hour

10 Hour

Hour

Alarm 1 Hours

1–12 + AM/PM

00–23

Day

Alarm 1 Day

1–7

0AH

A1M4

DY/DT

10 Date

Date

Alarm 1 Date

1–31

0BH

A2M2

10 Minutes

Minutes

Alarm 2 Minutes

00–59

AM/PM

0CH

A2M3

12/24

10 Hour

10 Hour

Hour

Alarm 2 Hours

1–12 + AM/PM

00–23

Day

Alarm 2 Day

1–7

0DH

A2M4

DY/DT

10 Date

Date

Alarm 2 Date

1–31

0EH

EOSC

BBSQW

CONV

RS2

RS1

INTCN

A2IE

A1IE

Control

—

0FH

OSF

00

0

EN32kHz

BSY

A2F

A1F

Control/Status

—

10H

SIGN

DATA

DATA

DATA

DATA

DATA

DATA

DATA

Aging Offset

—

11H

SIGN

DATA

DATA

DATA

DATA

DATA

DATA

DATA

MSB of Temp

—

12H

DATA

DATA

00

00

00

LSB of Temp

—