DS1307 (RTC).pdf

DS1307

64 x 8 Serial Real-Time Clock

www.maxim-ic.com

FEATURES

PIN ASSIGNMENT

Real-time clock (RTC) counts seconds,

minutes, hours, date of the month, month, day

of the week, and year with leap-year

compensation valid up to 2100

V BAT

GND

V CC

SQW/OUT

SCL

SDA

56-byte, battery-backed, nonvolatile (NV)

RAM for data storage

DS1307 8-Pin DIP (300-mil)

Two-wire serial interface

Programmable squarewave output signal

V BAT

GND

V CC

SQW/OUT

SCL

SDA

Automatic power-fail detect and switch

circuitry

Consumes less than 500nA in battery backup

mode with oscillator running

DS1307 8-Pin SOIC (150-mil)

Optional industrial temperature range:

-40°C to +85°C

PIN DESCRIPTION

V CC - Primary Power Supply

X1, X2 - 32.768kHz Crystal Connection

V BAT - +3V Battery Input

GND - Ground

SDA - Serial Data

SCL - Serial Clock

SQW/OUT - Square Wave/Output Driver

Available in 8-pin DIP or SOIC

Underwriters Laboratory (UL) recognized

ORDERING INFORMATION

DS1307

8-Pin DIP (300-mil)

DS1307Z

8-Pin SOIC (150-mil)

DS1307N

8-Pin DIP (Industrial)

DS1307ZN

8-Pin SOIC (Industrial)

DESCRIPTION

The DS1307 Serial Real-Time Clock is a low-power, full binary-coded decimal (BCD) clock/calendar

plus 56 bytes of NV SRAM. Address and data are transferred serially via a 2-wire, bi-directional bus.

The clock/calendar provides seconds, minutes, hours, day, date, month, and year information. The end of

the month date is automatically adjusted for months with fewer than 31 days, including corrections for

leap year. The clock operates in either the 24-hour or 12-hour format with AM/PM indicator. The

DS1307 has a built-in power sense circuit that detects power failures and automatically switches to the

battery supply.

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TYPICAL OPERATING CIRCUIT

OPERATION

The DS1307 operates as a slave device on the serial bus. Access is obtained by implementing a START

condition and providing a device identification code followed by a register address. Subsequent registers

can be accessed sequentially until a STOP condition is executed. When V CC falls below 1.25 x V BAT the

device terminates an access in progress and resets the device address counter. Inputs to the device will

not be recognized at this time to prevent erroneous data from being written to the device from an out of

tolerance system. When V CC falls below V BAT the device switches into a low-current battery backup

mode. Upon power-up, the device switches from battery to V CC when V CC is greater than V BAT + 0.2V

and recognizes inputs when V CC is greater than 1.25 x V BAT . The block diagram in Figure 1 shows the

main elements of the serial RTC.

DS1307 BLOCK DIAGRAM Figure 1

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SIGNAL DESCRIPTIONS

V CC , GND – DC power is provided to the device on these pins. V CC is the +5V input. When 5V is

applied within normal limits, the device is fully accessible and data can be written and read. When a 3V

battery is connected to the device and V CC is below 1.25 x V BAT , reads and writes are inhibited. However,

the timekeeping function continues unaffected by the lower input voltage. As V CC falls below V BAT the

RAM and timekeeper are switched over to the external power supply (nominal 3.0V DC) at V BAT .

V BAT – Battery input for any standard 3V lithium cell or other energy source. Battery voltage must be

held between 2.0V and 3.5V for proper operation. The nominal write protect trip point voltage at which

access to the RTC and user RAM is denied is set by the internal circuitry as 1.25 x V BAT nominal. A

lithium battery with 48mAhr or greater will back up the DS1307 for more than 10 years in the absence of

power at 25ºC. UL recognized to ensure against reverse charging current when used in conjunction with a

lithium battery.

See “Conditions of Acceptability” at http://www.maxim-ic.com/TechSupport/QA/ntrl.htm .

SCL (Serial Clock Input) – SCL is used to synchronize data movement on the serial interface.

SDA (Serial Data Input/Output) – SDA is the input/output pin for the 2-wire serial interface. The SDA

pin is open drain which requires an external pullup resistor.

SQW/OUT (Square Wave/Output Driver) – When enabled, the SQWE bit set to 1, the SQW/OUT pin

outputs one of four square wave frequencies (1Hz, 4kHz, 8kHz, 32kHz). The SQW/OUT pin is open

drain and requires an external pull-up resistor. SQW/OUT will operate with either Vcc or Vbat applied.

X1, X2 – Connections for a standard 32.768kHz quartz crystal. The internal oscillator circuitry is

designed for operation with a crystal having a specified load capacitance (CL) of 12.5pF.

For more information on crystal selection and crystal layout considerations, please consult Application

Note 58, “Crystal Considerations with Dallas Real-Time Clocks.” The DS1307 can also be driven by an

external 32.768kHz oscillator. In this configuration, the X1 pin is connected to the external oscillator

signal and the X2 pin is floated.

RECOMMENDED LAYOUT FOR CRYSTAL

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CLOCK ACCURACY

The accuracy of the clock is dependent upon the accuracy of the crystal and the accuracy of the match

between the capacitive load of the oscillator circuit and the capacitive load for which the crystal was

trimmed. Additional error will be added by crystal frequency drift caused by temperature shifts. External

circuit noise coupled into the oscillator circuit may result in the clock running fast. See Application Note

58, “Crystal Considerations with Dallas Real-Time Clocks” for detailed information.

Please review Application Note 95, “Interfacing the DS1307 with a 8051-Compatible Microcontroller”

for additional information.

RTC AND RAM ADDRESS MAP

The address map for the RTC and RAM registers of the DS1307 is shown in Figure 2. The RTC registers

are located in address locations 00h to 07h. The RAM registers are located in address locations 08h to

3Fh. During a multi-byte access, when the address pointer reaches 3Fh, the end of RAM space, it wraps

around to location 00h, the beginning of the clock space.

DS1307 ADDRESS MAP Figure 2

00H

SECONDS

MINUTES

HOURS

DAY

DATE

MONTH

YEAR

CONTROL

RAM

56 x 8

07H

08H

3FH

CLOCK AND CALENDAR

The time and calendar information is obtained by reading the appropriate register bytes. The RTC

registers are illustrated in Figure 3. The time and calendar are set or initialized by writing the appropriate

is the clock halt (CH) bit. When this bit is set to a 1, the oscillator is disabled. When cleared to a 0, the

oscillator is enabled.

Please 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- 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).

On a 2-wire START, the current time is transferred to a second set of registers. The time information is

read from these secondary registers, while the clock may continue to run. This eliminates the need to re-

read the registers in case of an update of the main registers during a read.

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DS1307

DS1307 TIMEKEEPER REGISTERS Figure 3

CONTROL REGISTER

The DS1307 control register is used to control the operation of the SQW/OUT pin.

BIT 7

BIT 6

BIT 5

BIT 4

BIT 3

BIT 2

BIT 1

BIT 0

OUT

SQWE

RS1

RS0

OUT (Output control): This bit controls the output level of the SQW/OUT pin when the square wave

output is disabled. If SQWE = 0, the logic level on the SQW/OUT pin is 1 if OUT = 1 and is 0 if

OUT = 0.

SQWE (Square Wave Enable): This bit, when set to a logic 1, will enable the oscillator output. The

frequency of the square wave output depends upon the value of the RS0 and RS1 bits. With the square

wave output set to 1Hz, the clock registers update on the falling edge of the square wave.

RS (Rate Select): These bits control the frequency of the square wave output when the square wave

output has been enabled. Table 1 lists the square wave frequencies that can be selected with the RS bits.

SQUAREWAVE OUTPUT FREQUENCY Table 1

RS1

RS0

SQW OUTPUT FREQUENCY

1Hz

4.096kHz

8.192kHz

32.768kHz

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