DS1307 Datasheet, PDF (6/14 Pages) Dallas Semiconductor – 64 X 8 Serial Real Time Clock

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 (C_L) 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 V_BAT pin may prevent proper operation. If a
backup supply is not required, V_BAT must be grounded. The nominal power-fail trip point
3
V_BAT
(V_PF) 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 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 V_CC.
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 V_CC.
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
V_CC or V_BAT applied. The pullup voltage can be up to 5.5V regardless of the voltage on
V_CC. If not used, this pin can be left floating.
Primary Power Supply. When voltage is applied within normal limits, the device is fully
8
V_CC
accessible and data can be written and read. When a backup supply is connected to the
device and V_CC is below V_TP, 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 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.
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