DS1388
I
2
C RTC/Supervisor with Trickle Charger
and 512 Bytes EEPROM
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Detailed Description
The DS1388 I
2
C RTC, supervisor, and EEPROM is a
multifunction device that provides a clock/calendar,
programmable watchdog timer, power-supply monitor
with reset, and 512 bytes of EEPROM. The clock pro-
vides hundredths of seconds, seconds, minutes, and
hours, and operates in 24-hour or 12-hour format with
an AM/PM indicator. The calendar provides 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. A
watchdog timer provides a reset for an unresponsive
microprocessor. It is programmable in 10ms intervals
from 0.01 to 99.99 seconds. A temperature-compensat-
ed voltage reference and comparator circuit monitors
the status of V
CC
. If a primary power failure is detected,
the device automatically switches to the backup supply
and drives the reset output to the active state. When
V
CC
returns to nominal levels, the reset is held low for a
period to allow the power supply and processor to sta-
bilize. The device also has a pushbutton reset con-
troller, which debounces a reset input signal. The
device is accessed through an I
2
C serial interface.
Operation
The DS1388 operates as a slave device on the I
2
C bus.
Access is obtained by implementing a START condition
and providing a device identification code followed by
data. Subsequent registers can be accessed sequen-
tially until a STOP condition is executed. See the
Block
Diagram
, which 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 regis-
ters 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 RST goes high (Figure 1).
On first application of power to the device, the time and
date registers are set to 00/00/00 00 00:00:00
(MM/DD/YY DOW HH:MM:SS). The user should initial-
ize all RTC registers to valid date and time settings.
Oscillator Circuit
The DS1388 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. Using a
crystal with the specified characteristics, the startup
time is usually less than one second.
