ISL12025 Datasheet, PDF (23/27 Pages) Intersil Corporation – Real-Time Clock/Calendar with I2C Bus and EEPROM

ISL12025
Backup Battery Operation
Many types of batteries can be used with the Intersil RTC
products. 3.0V or 3.6V Lithium batteries are appropriate, and
sizes are available that can power a Intersil RTC device for
up to 10 years. Another option is to use a Super Cap for
applications where V_DD may disappear intermittently for
short periods of time. Depending on the value of Super Cap
used, backup time can last from a few days to two weeks
(with >1F). A simple silicon or Schottky barrier diode can be
used in series with V_DD to charge the supercapacitor, which
is connected to the V_BAT pin. Try to use Schottky diodes with
very low leakages, <1µA desirable. Do not use the diode to
charge a battery (especially lithium batteries!)
There are two possible modes for battery backup operation;
Standard and Legacy Mode. In Standard Mode, there are no
operational concerns when switching over to battery backup
since all other devices functions are disabled. Battery drain
is minimal in Standard Mode and return to Normal V_DD
powered operations predictable. In Legacy modes, the V_BAT
pin can power the chip if the voltage is above V_DD and
V_TRIP. This makes it possible to generate alarms and
communicate with the device under battery backup, but the
supply current drain is much higher than the Standard Mode
and backup time is reduced. During initial power-up, the
default mode is the Legacy Mode.
I²C Communications During Battery Backup and
LVR Operation
Operation in Battery Backup Mode and LVR is affected by
the BSW and SBIB bits as described previously. These bits
allow flexible operation of the serial bus and EEPROM in
Battery Backup Mode, but certain operational details need to
be clear before utilizing the different modes. The most
significant detail is that once V_DD goes below V_RESET, then
I²C communications cease regardless of whether the device
is programmed for I²C operation in Battery Backup Mode.
Table 9 describes four different modes possible with using
the BSW and SBIB bits and how they are affect LVR and
battery backup operation.
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ANYTIME the device goes into Battery Backup Mode.
Regardless of the battery voltage, the I²C will work down
to the V_RESET voltage (see Figure 29).
• Mode B - In this mode the selection bits indicate
switchover to battery backup at V_DD<V_BAT, and I²C
communications in battery backup. In order to
communicate in Battery Backup Mode, the V_RESET
voltage must be
must be greater
less
than
than the V_BAT
V_RESET. Also,
vpoultlal-guepsAoNnDthVe_DID2C
bus pins must go to V_BAT to communicate. This mode is
the same as the normal operating mode of the X1227
device
• Mode C - In this mode the selection bits indicate a low
V_DD switchover combined with no communications in
battery backup. Operation is actually identical to Mode A
with I²C communications down to V_DD = V_RESET, then no
communications (see Figure 28).
• Mode D - In this mode the selection bits indicate
switchover to battery backup at V_DD < V_BAT, and no I²C
communications in battery backup. This mode is unique in
that there is I²C communication as long as V_DD is higher
than V_RESET
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is
when there is a Valid V_DD (see Figure 29).
2.7V TO 5.5V
V_DD
V_BAT
V_SS
SUPER CAP
FIGURE 28. SUPER CAPACITOR CHARGING CIRCUIT
.
TABLE 9. I²C, LV RESET, AND BATTERY BACKUP OPERATION SUMMARY (Shaded Row is same as X1227 operation)
BSW
MODE SBIB BIT BIT
VBAT
SWITCHOVER
VOLTAGE
I²C ACTIVE IN
BATTERY
BACKUP?
EE PROM WRITE/READ
IN BATTERY BACKUP?
NOTES
A
0
B
0
(X1227
Mode)
0
Standard Mode,
NO
V_TRIP = 2.2V typ
1
Legacy Mode,
YES, only if
V_DD < V_BAT
V_BAT > V_RESET
NO
YES
Operation of I²C bus down to
V_DD = V_RESET, then below that no
communications. Battery switchover at
V_TRIP.
Operation of I²C bus into Battery Backup
Mode, but only for
V_BAT > V_DD > V_RESET.
Bus must have pull-ups to V_BAT.
23
FN6371.3
August 13, 2008