ISL12029_1012 Datasheet, PDF (25/29 Pages) Intersil Corporation – Real Time Clock/Calendar with I2C Bus™ and EEPROM

ISL12029, ISL12029A
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 VDD goes below VRESET, then
I²C communications cease regardless of whether the device
is programmed for I²C operation in battery backup mode.
Table 11 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 VDD
switchover combined with I²C operation in battery backup
mode. In actuality the VDD 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 VBAT 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
VDD 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).
• 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 VDD is higher
than V_RESET or V_BAT, whichever is greater. This mode is
the safest for guaranteeing I²C communications only when
there is a Valid V_DD (see Figure 30).
TABLE 11. 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 EE PROM WRITE/
BATTERY READ IN BATTERY
BACKUP?
BACKUP?
FREQ/IRQ
ACTIVE?
NOTES
A
0
B
0
(X1228
mode)
C
1
D
1
0
Standard Mode,
NO
V_TRIP = 2.2V typ
Default for
ISL12029A
1
Legacy Mode, YES, only if
V_DD < V_BAT
Default for
V_BAT > V_RESET
ISL12029
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. No
nonvolatile writes with V_BAT>V_DD
Operation of I²C bus down to
V_DD = V_RESET, then below that no
communications. Battery switchover
at V_TRIP.
Operation of I²C bus down to V_RESET
or V_BAT, whichever is higher.
25
FN6206.10
December 16, 2010