DS1307
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 Real Time Clock.
DS1307 BLOCK DIAGRAM
Figure 1
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 for more than 10 years in
the absence of power at 25 degrees C.
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DALLAS [ DALLAS SEMICONDUCTOR ]
