ISL12028 Datasheet, PDF (24/28 Pages) Intersil Corporation – Real Time Clock/Calendar with EEPROM

ISL12028
TABLE 10. I²C, LV RESET, AND BATTERY BACKUP OPERATION SUMMARY (SHADED ROW IS SAME AS X1228 OPERATION)
MODE
SBIB
BIT
BSW
BIT
V_BAT
SWITCHOVER
VOLTAGE
I²C ACTIVE IN
BATTERY
BACKUP?
EE PROM WRITE/
READ IN BATTERY
BACKUP?
FREQ/IRQ
ACTIVE?
NOTES
A
0
B
0
(X1228
mode)
C
1
D
1
0 Standard Mode,
NO
V_TRIP = 2.2V typ
1
Legacy Mode, YES, only if
V_DD < V_BAT
V_BAT>V_RESET
0 Standard Mode,
NO
V_TRIP = 2.2V typ
1
Legacy Mode,
NO
V_DD < V_BAT
NO
YES
NO
NO
YES
YES
YES
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.
Operation of I²C bus down to V_DD =
V_RESET, then below that no
communications. Battery switchover
at V_TRIP.
Operation of I²C busdown to V_RESET
or V_BAT, whichever is higher.
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 capacitor for
applications where V_DD may disappear intermittently for short
periods of time. Depending on the value of super capacitor
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 super capacitor, 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. In this case if alarms are used
in backup mode, the IRQ/F_OUT pull up resistor must be
connected to V_BAT voltage source. During initial power up
the default mode is the Standard mode.
2.7-5.5V
V_CC
V_back
V_SS
Supercapacitor
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 earlier. 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 VRESET, then
I²C communications cease regardless of whether the device
is programmed for I²C operation in battery backup mode.
Table 10 describes 4 different modes possible with using the
BSW and SBIB bits, and how they are affect LVR and battery
backup operation.
• Mode A - In this mode selection bits indicate a low V_DD
switchover combined with I²C operation in battery backup
mode. In actuality the V_DD will go below V_RESET before
switching to battery backup, which will disable I²C
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 less than the V_BAT voltage AND V_DD must be
greater than V_RESET. Also, pull-ups on the I²C bus pins
must go to V_BAT to communicate. This mode is the same
as the normal operating mode of the X1228 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 29.
FIGURE 28. SUPERCAPACITOR CHARGING CIRCUIT
24
FN8233.5
October 18, 2006