DS1339 Datasheet, PDF (8/18 Pages) Dallas Semiconductor – Serial Real-Time Clock

DS1339 I²C Serial Real-Time Clock
DETAILED DESCRIPTION
The DS1339 serial real-time clock (RTC) is a low-power clock/date device with two programmable time-of-day
alarms and a programmable square-wave output. Address and data are transferred serially through an I²C bus.
The clock/date 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
clock operates in either the 24-hour or 12-hour format with AM/PM indicator. The DS1339 has a built-in power-
sense circuit that detects power failures and automatically switches to the backup supply, maintaining time and
date operation.
OPERATION
The DS1339 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 data. Subsequent registers can be accessed sequentially
until a STOP condition is executed. The device is fully accessible and data can be written and read when V_CC is
greater than V_PF. However, when V_CC falls below V_PF, the internal clock registers are blocked from any access. If
V_PF is less than V_BACKUP, the device power is switched from V_CC to V_BACKUP when V_CC drops below V_PF. If V_PF is
greater than V_BACKUP, the device power is switched from V_CC to V_BACKUP when V_CC drops below V_BACKUP. The
registers are maintained from the V_BACKUP source until V_CC is returned to nominal levels. The block diagram in
Figure 3 shows the main elements of the serial real-time clock.
OSCILLATOR CIRCUIT
The DS1339 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or
capacitors to operate. Table 1 specifies several crystal parameters for the external crystal. Figure 4 shows a
functional schematic of the oscillator circuit. The startup time is usually less than 1 second when using a crystal
with the specified characteristics.
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