DS1340/DS1340C pdf datasheet (

The DS1340 is a real-time clock (RTC)/calendar that is
pin compatible and functionally equivalent to the ST
M41T00, including the software clock calibration. The
device additionally provides trickle-charge capability
on the VBACKUP pin, a lower timekeeping voltage, and
an oscillator STOP flag. Block access of the register
map is identical to the ST device. Two additional registers,
which are accessed individually, are required for
the trickle charger and flag. The clock/calendar provides
seconds, minutes, hours, day, date, month, and
year information. A built-in power-sense circuit detects
power failures and automatically switches to the backup
supply. Reads and writes are inhibited while the
clock continues to run. The device is programmed serially
through an I2C bidirectional bus.
Applications
Portable Instruments
Point-of-Sale Equipment
Medical Equipment
Telecommunications