Datasheet

Operation STM690A/692A/703/704/802/805/817/818/819
14/43 Doc ID 10522 Rev 10
2.4 Backup battery switchover
In the event of a power failure, it may be necessary to preserve the contents of external
SRAM through V
OUT
. With a backup battery installed with voltage V
BAT
, the devices
automatically switch the SRAM to the backup supply when V
CC
falls.
Note: When the battery is first connected without V
CC
power applied, the device does not
immediately provide backup battery voltage on V
OUT
. Only after V
CC
exceeds V
RST
will the
switchover operate as described below. This mode allows a battery to be attached during
manufacturing but not used until after the system has been activated for the first time. As a
result, no battery power is consumed by the device during storage and shipment. For the
STM81x devices, the battery freshness seal can be initiated again by following the
procedure outlined in Section 2.12. If the backup battery is not used, connect both V
BAT
and
V
OUT
to V
CC
.
Whenever V
CC
falls below the switchover voltage, V
SO
, V
OUT
is connected to V
BAT
through a
100 Ω switch. V
SO
is the lesser of V
BAT
and V
RST
. Choosing the lesser allows the device to
be powered by V
CC
for as long as possible before switching over thereby maximizing the
battery life.
Assuming V
BAT
> 2.0 V, switchover at V
SO
ensures that battery backup mode is entered
before V
OUT
gets too close to the 2.0 V minimum required to reliably retain data in most
external SRAMs. When V
CC
recovers, hysteresis is used to avoid oscillation around the V
SO
point. V
OUT
is connected to V
CC
through a 3 Ω PMOS power switch.
Note: The backup battery may be removed while V
CC
is valid, assuming V
BAT
is adequately
decoupled (0.1 µF typ), without danger of triggering a reset.
Table 4. I/O status in battery backup
V
OUT
Connected to V
BAT
through internal switch
V
CC
Disconnected from V
OUT
PFI Disabled
PFO
Logic low
E
High impedance
E
CON
Logic high
WDI Watchdog timer is disabled
MR
Disabled
RST
Logic low
RST Logic high
V
BAT
Connected to V
OUT