Datasheet DS1307 (Maxim) - 6

link to page 4 DS1307 64 x 8, Serial, I2C Real-Time Clock
PIN DESCRIPTION PIN NAME FUNCTION
Connections for Standard 32.768kHz Quartz Crystal. The internal oscillator circuitry is 1 X1 designed for operation with a crystal having a specified load capacitance (CL) of 12.5pF. X1 is the input to the oscillator and can optionally be connected to an external 32.768kHz oscillator. The output of the internal oscillator, X2, is floated if an external oscillator is connected to X1. 2 X2
Note:
For more information on crystal selection and crystal layout considerations, refer to Application Note 58: Crystal Considerations with Dallas Real-Time Clocks. Backup Supply Input for Any Standard 3V Lithium Cell or Other Energy Source. Battery voltage must be held between the minimum and maximum limits for proper operation. Diodes in series between the battery and the VBAT pin may prevent proper operation. If a backup supply is not required, VBAT must be grounded. The nominal power-fail trip point 3 V (V BAT PF) voltage at which access to the RTC and user RAM is denied is set by the internal circuitry as 1.25 x VBAT nominal. A lithium battery with 48mAh or greater will back up the DS1307 for more than 10 years in the absence of power at +25°C. UL recognized to ensure against reverse charging current when used with a lithium battery. Go to: www.maxim-ic.com/qa/info/ul/. 4 GND Ground Serial Data Input/Output. SDA is the data input/output for the I2C serial interface. The 5 SDA SDA pin is open drain and requires an external pullup resistor. The pullup voltage can be up to 5.5V regardless of the voltage on VCC. Serial Clock Input. SCL is the clock input for the I2C interface and is used to synchronize 6 SCL data movement on the serial interface. The pullup voltage can be up to 5.5V regardless of the voltage on VCC. 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 7 SQW/OUT pin is open drain and requires an external pullup resistor. SQW/OUT operates with either VCC or VBAT applied. The pullup voltage can be up to 5.5V regardless of the voltage on VCC. If not used, this pin can be left floating. Primary Power Supply. When voltage is applied within normal limits, the device is fully accessible and data can be written and read. When a backup supply is connected to the 8 VCC device and VCC is below VTP, read and writes are inhibited. However, the timekeeping function continues unaffected by the lower input voltage.
DETAILED DESCRIPTION
The DS1307 is a low-power clock/calendar with 56 bytes of battery-backed SRAM. The clock/calendar provides seconds, minutes, hours, day, date, month, and year information. The date at the end of the month is automatically adjusted for months with fewer than 31 days, including corrections for leap year. The DS1307 operates as a slave device on the I2C 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 VCC falls below 1.25 x VBAT, 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 VCC falls below VBAT, the device switches into a low- current battery-backup mode. Upon power-up, the device switches from battery to VCC when VCC is greater than VBAT +0.2V and recognizes inputs when VCC is greater than 1.25 x VBAT. The block diagram in Figure 1 shows the main elements of the serial RTC. 6 of 14 Document Outline TYPICAL OPERATING CIRCUIT PIN CONFIGURATIONS ORDERING INFORMATION ABSOLUTE MAXIMUM RATINGS RECOMMENDED DC OPERATING CONDITIONS (TA = 0 C to +70 C, TA = -40 C to +85 C.) (Notes 1, 2) DC ELECTRICAL CHARACTERISTICS (VCC = 4.5V to 5.5V; TA = 0 C to +70 C, TA = -40 C to +85 C.) (Notes 1, 2) DC ELECTRICAL CHARACTERISTICS (VCC = 0V, VBAT = 3.0V; TA = 0 C to +70 C, TA = -40 C to +85 C.) (Notes 1, 2) AC ELECTRICAL CHARACTERISTICS (VCC = 4.5V to 5.5V; TA = 0 C to +70 C, TA = -40 C to +85 C.) CAPACITANCE (TA = +25 C) 64 x 8, Serial, I2C Real-Time Clock SYMBOL TIMING DIAGRAM CLOCK ACCURACY RTC AND RAM ADDRESS MAP CLOCK AND CALENDAR CONTROL REGISTER I2C DATA BUS RS1