DS1308 Datasheet, PDF (3/14 Pages) Dallas Semiconductor – 64 X 8 Serial Real Time Clock

DS1307/1308
SIGNAL DESCRIPTIONS
V_CC, GND - DC power is provided to the device on these pins. V_CC is the +5 volt input. When 5 volts is
applied within normal limits, the device is fully accessible and data can be written and read. When a
3-volt 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 3-volt lithium cell or other energy source. Battery voltage must be
held between 2.0 and 3.5 volts for proper operation. The nominal write protect trip point voltage at which
access to the real time clock and user RAM is denied is set by the internal circuitry as 1.25 x V_BAT
nominal. A lithium battery with 48 mAhr or greater will back up the DS1307/DS1308 for more than 10
years in the absence of power at 25 degrees C.
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 (1 Hz, 4 kHz, 8 kHz, 32 kHz). The SQW/OUT pin is open
drain which requires an external pullup resistor. SQW/OUT will operate with either Vcc or Vbat applied.
NOTE: X1, X2 are not applicable for the DS1308 or DS1308N.
X1, X2 - Connections for a standard 32.768 kHz quartz crystal. The internal oscillator circuitry is
designed for operation with a crystal having a specified load capacitance (CL) of 12.5 pF.
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 32.768 kHz oscillator. In this configuration, the X1 pin is connected to the external oscillator
signal and the X2 pin is floated.
Please review Application Note 95, “Interfacing the DS1307/DS1308 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/DS1308 is shown in Figure 2. The real
time clock 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.
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