DS1307 64 x 8, Serial, I

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CLOCK AND CALENDAR

The time and calendar information is obtained by reading the appropriate register bytes. Table 2 shows

the RTC registers. The time and calendar are set or initialized by writing the appropriate register bytes.

The contents of the time and calendar registers are in the BCD format. 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. Bit 7 of Register 0 is the clock halt (CH) bit. When this bit is set to 1, the

oscillator is disabled. When cleared to 0, the oscillator is enabled.

Note that the initial power-on state of all registers is not defined. Therefore, it is important to

enable the oscillator (CH bit = 0) during initial configuration.

The DS1307 can be run in either 12-hour or 24-hour mode. Bit 6 of the hours register is defined as the

12-hour 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 to

23 hours). The hours value must be re-entered whenever the 12/24-hour mode bit is changed.

When reading or writing the time and date registers, secondary (user) buffers are used to prevent errors

when the internal registers update. When reading the time and date registers, the user buffers are

secondary registers while the clock continues to run. This eliminates the need to re-read the registers in

case the internal registers update during a read. The divider chain is reset whenever the seconds register is

to avoid rollover issues, the remaining time and date registers must be written within one second.

Table 2. Timekeeper Registers

ADDRESS

BIT 7

BIT 6

BIT 5

BIT 4

BIT 3

BIT 2

BIT 1

BIT 0

FUNCTION

RANGE

00H

CH

10 Seconds

Seconds

Seconds

00–59

01H

0

10 Minutes

Minutes

Minutes

00–59

12

10

Hour

02H

0

24

PM/

AM

10

Hour

Hours

Hours

1–12

+AM/PM

00–23

03H

0

0

0

0

0

DAY

Day

01–07

04H

0

0

10 Date

Date

Date

01–31

05H

0

0

0

10

Month

Month

Month

01–12

06H

10 Year

Year

Year

00–99

07H

OUT

0

0

SQWE

0

0

RS1

RS0

Control

—

08H-3FH

RAM

56 x 8

00H–FFH

0 = Always reads back as 0.