DS1307 register map

I2C real-time clock with 56 bytes of battery-backed NVRAM — Analog Devices

8 registers I2C I²C 0x68 8-bit reg addresses 75% bit coverage not yet human-verified

I²C address conflict. On its default address(es) this part collides with DS3231 (0x68); MPU6050 (0x68); PCA9685 (0x68). You cannot put them on the same bus without changing a strap pin, using an I²C multiplexer, or moving one to a second bus. See the full address map.

Get it as a C header

pip install regforge
regforge gen DS1307 --out ./src --driver --tests

Writes ds1307_regs.h with every address, mask and shift below, a bus-agnostic driver skeleton, and a file of compile-time assertions that make your compiler check the bit arithmetic. No API key needed — this part ships inside the package.

Not yet human-verified. Transcribed and mechanically validated — no overlapping fields, no out-of-range bits, reset values consistent with their fields — but not yet checked line-by-line against the datasheet. Verifying it is a useful half hour.

Registers

Addr	Register	Access	Width	Reset	Description
0x00	SECONDS	RW	8	—	Seconds in BCD, plus the clock halt bit
0x01	MINUTES	RW	8	—	Minutes in BCD, 00-59
0x02	HOURS	RW	8	—	Hours in BCD. Layout depends on the 12/24 bit.
0x03	DAY	RW	8	—	Day of week, 1-7, defined by the user
0x04	DATE	RW	8	—	Day of month in BCD, 01-31
0x05	MONTH	RW	8	—	Month in BCD, 01-12
0x06	YEAR	RW	8	—	Year within the century in BCD, 00-99
0x07	CONTROL	RW	8	0x03	Square-wave output control

Bit layouts

Drawn MSB-first, the way the datasheet draws them. [7:5] means shift 5, width 3, mask 0xE0 — this is the notation that costs people a day when they read it backwards.

SECONDS @ 0x00 — 3 fields

CH[7]

SECONDS_10[6:4]

SECONDS_1[3:0]

Bits	Field	Access	Mask	Reset	Description
[7]	CH	RW	0x80	—	Clock halt. 1 stops the oscillator. Set on a new part, so clear it before the clock will run.
[6:4]	SECONDS_10	RW	0x70	—	Tens of seconds, 0-5
[3:0]	SECONDS_1	RW	0x0F	—	Units of seconds, 0-9

MINUTES @ 0x01 — 2 fields

—7

MINUTES_10[6:4]

MINUTES_1[3:0]

Bits	Field	Access	Mask	Reset	Description
[6:4]	MINUTES_10	RW	0x70	—	Tens of minutes, 0-5
[3:0]	MINUTES_1	RW	0x0F	—	Units of minutes, 0-9

HOURS @ 0x02 — 4 fields

—7

MODE_12_24[6]

AM_PM_HOUR_20[5]

HOURS_10[4]

HOURS_1[3:0]

Bits	Field	Access	Mask	Reset	Description
[6]	MODE_12_24	RW	0x40	—	0 = 24-hour mode, 1 = 12-hour mode
[5]	AM_PM_HOUR_20	RW	0x20	—	In 12-hour mode: 0 = AM, 1 = PM. In 24-hour mode: the 20-hour bit.
[4]	HOURS_10	RW	0x10	—	The 10-hour bit
[3:0]	HOURS_1	RW	0x0F	—	Units of hours, 0-9

HOURS.MODE_12_24 values

Value	Name	Meaning
0x00	MODE_24H	24-hour format
0x01	MODE_12H	12-hour format, bit 5 is AM/PM

DAY @ 0x03 — 1 fields

—7

—6

—5

—4

—3

DAY[2:0]

Bits	Field	Access	Mask	Reset	Description
[2:0]	DAY	RW	0x07	—	Day of week, 1-7

DATE @ 0x04 — 2 fields

—7

—6

DATE_10[5:4]

DATE_1[3:0]

Bits	Field	Access	Mask	Reset	Description
[5:4]	DATE_10	RW	0x30	—	Tens of the date, 0-3
[3:0]	DATE_1	RW	0x0F	—	Units of the date, 0-9

MONTH @ 0x05 — 2 fields

—7

—6

—5

MONTH_10[4]

MONTH_1[3:0]

Bits	Field	Access	Mask	Reset	Description
[4]	MONTH_10	RW	0x10	—	Tens of the month
[3:0]	MONTH_1	RW	0x0F	—	Units of the month, 0-9

YEAR @ 0x06 — 2 fields

YEAR_10[7:4]

YEAR_1[3:0]

Bits	Field	Access	Mask	Reset	Description
[7:4]	YEAR_10	RW	0xF0	—	Tens of the year
[3:0]	YEAR_1	RW	0x0F	—	Units of the year

CONTROL @ 0x07 — 3 fields

OUT[7]

—6

—5

SQWE[4]

—3

—2

RS[1:0]

Bits	Field	Access	Mask	Reset	Description
[7]	OUT	RW	0x80	0x00	Output level on SQW/OUT when SQWE is 0
[4]	SQWE	RW	0x10	0x00	Square-wave enable
[1:0]	RS	RW	0x03	0x03	Square-wave rate select

CONTROL.RS values

Value	Name	Meaning
0x00	HZ_1	1 Hz
0x01	KHZ_4_096	4.096 kHz
0x02	KHZ_8_192	8.192 kHz
0x03	KHZ_32_768	32.768 kHz (default)

Generated C header

What regforge gen DS1307 writes for HOURS:

/* ------------------------------------------------------------------------ */
/* HOURS @ 0x02u  rw  8-bit */
/* Hours in BCD. Layout depends on the 12/24 bit. */
/* ------------------------------------------------------------------------ */

#define DS1307_HOURS                                   0x02u
#define DS1307_HOURS_WIDTH                             8u

/*   MODE_12_24 bits [6] rw -- 0 = 24-hour mode, 1 = 12-hour mode */
#define DS1307_HOURS_MODE_12_24_SHIFT                  6u
#define DS1307_HOURS_MODE_12_24_WIDTH                  1u
#define DS1307_HOURS_MODE_12_24_MASK                   0x40u
#define DS1307_HOURS_MODE_12_24_GET(reg)               (((uint8_t)(reg) & 0x40u) >> 6u)
#define DS1307_HOURS_MODE_12_24_SET(reg, val)          (((uint8_t)(reg) & (uint8_t)~0x40u) | (((uint8_t)(val) << 6u) & 0x40u))
#define DS1307_HOURS_MODE_12_24_MODE_24H               0x00u  /* 24-hour format */
#define DS1307_HOURS_MODE_12_24_MODE_12H               0x01u  /* 12-hour format, bit 5 is AM/PM */

/*   AM_PM_HOUR_20 bits [5] rw -- In 12-hour mode: 0 = AM, 1 = PM. In 24-hour mode: the 20-hour bit. */
#define DS1307_HOURS_AM_PM_HOUR_20_SHIFT               5u
#define DS1307_HOURS_AM_PM_HOUR_20_WIDTH               1u
#define DS1307_HOURS_AM_PM_HOUR_20_MASK                0x20u
#define DS1307_HOURS_AM_PM_HOUR_20_GET(reg)            (((uint8_t)(reg) & 0x20u) >> 5u)
#define DS1307_HOURS_AM_PM_HOUR_20_SET(reg, val)       (((uint8_t)(reg) & (uint8_t)~0x20u) | (((uint8_t)(val) << 5u) & 0x20u))

/*   HOURS_10 bits [4] rw -- The 10-hour bit */
#define DS1307_HOURS_HOURS_10_SHIFT                    4u
#define DS1307_HOURS_HOURS_10_WIDTH                    1u
#define DS1307_HOURS_HOURS_10_MASK                     0x10u
#define DS1307_HOURS_HOURS_10_GET(reg)                 (((uint8_t)(reg) & 0x10u) >> 4u)
#define DS1307_HOURS_HOURS_10_SET(reg, val)            (((uint8_t)(reg) & (uint8_t)~0x10u) | (((uint8_t)(val) << 4u) & 0x10u))

/*   HOURS_1 bits [3:0] rw -- Units of hours, 0-9 */
#define DS1307_HOURS_HOURS_1_SHIFT                     0u
#define DS1307_HOURS_HOURS_1_WIDTH                     4u
#define DS1307_HOURS_HOURS_1_MASK                      0x0Fu
#define DS1307_HOURS_HOURS_1_GET(reg)                  (((uint8_t)(reg) & 0x0Fu) >> 0u)
#define DS1307_HOURS_HOURS_1_SET(reg, val)             (((uint8_t)(reg) & (uint8_t)~0x0Fu) | (((uint8_t)(val) << 0u) & 0x0Fu))

/* ------------------------------------------------------------------------ */
/* DAY @ 0x03u  rw  8-bit */
/* Day of week, 1-7, defined by the user */
/* ------------------------------------------------------------------------ */

#define DS1307_DAY                                     0x03u

Compared with

DS1307 vs DS3231

Related parts

Source

Transcribed from Maxim/ADI DS1307 datasheet (hand-transcribed) (hand-transcribed). Hand-authored seed entry, not machine-extracted. Timekeeping registers carry no reset value because the datasheet states none -- they hold whatever the backup battery preserved. The 56 bytes of NVRAM at 0x08-0x3F are plain memory rather than registers and are not modelled. Check against the datasheet before trusting it, then mark it verified.

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