DS1339A Datasheet, PDF (8/19 Pages) Maxim Integrated Products – Low-Current, I2C, Serial Real-Time Clock

DS1339A
Low-Current, I²C, Serial Real-Time Clock
Detailed Description
The DS1339A 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 DS1339A has a built-
in power-sense circuit that detects power failures and
automatically switches to the backup supply, maintaining
time, date, and alarm operation.
Table 1. Power Control
SUPPLY CONDITION
V_CC < V_PF, V_CC < V_BACKUP
V_CC < V_PF, V_CC > V_BACKUP
V_CC > V_PF, V_CC < V_BACKUP
V_CC > V_PF, V_CC > V_BACKUP
READ/
WRITE
ACCESS
No
No
Yes
Yes
POWERED
BY
V_BACKUP
V_CC
V_CC
V_CC
Table 2. Crystal Specifications*
PARAMETER SYMBOL MIN TYP MAX UNITS
Nominal
Frequency
f_O
32.768
kHz
Series Resistance ESR
60 kI
Load
Capacitance
C_L
6
pF
*The crystal, traces, and crystal input pins should be isolated
from RF generating signals. Refer to Application Note 58:
Crystal Considerations for Dallas Real-Time Clocks for addi-
tional specifications.
Operation
The DS1339A 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 Functional Diagram shows the main elements
of the serial real-time clock.
Power Control
The power-control function is provided by a precise,
temperature-compensated voltage reference and a
comparator circuit that monitors the V_CC level. 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 (Table 1). After V_CC returns above V_PF, read and
write access is allowed after t_REC (Figure 2). On the first
application of power to the device the time and date
registers are reset to 01/01/00 01 00:00:00 (DD/MM/YY
DOW HH:MM:SS).
Oscillator Circuit
The DS1339A uses an external 32.768kHz crystal. The
oscillator circuit does not require any external resistors
or capacitors to operate. Table 2 specifies several
crystal parameters for the external crystal. The Functional
Diagram shows a basic 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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