DS1307/1308
DESCRIPTION
The DS1307 Serial Real Time Clock is a low power, full BCD clock/calendar plus 56 bytes of
nonvolatile SRAM. Address and data are transferred serially via a 2-wire bi-directional bus. The
clock/calendar provides seconds, minutes, hours, day, date, month, and year information. The end of the
month date is automatically adjusted for months with less than 31 days, including corrections for leap
year. The clock operates in either the 24-hour or 12-hour format with AM/PM indicator. The DS1307
has a built-in power sense circuit which detects power failures and automatically switches to the battery
supply.
The DS1308 incorporates the DS1307 chip with a 32.768 kHz crystal in a surface mountable, 36-pin ball
grid array package (BGA). The close proximity of the embedded crystal to the high impedance crystal
input pins on the DS1307 minimizes capacitive loading and noise injection problems associated with
many other oscillator designs. The total area required for installation is less than that of one United States
dime: thus, minimizing PCB space required.
OPERATION
The DS1307/1308 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 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 Real Time Clock.
DS1307/DS1308 BLOCK DIAGRAM Figure 1
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DALLAS [ DALLAS SEMICONDUCTOR ]
