STM805L datasheet(14/43 Pages) STMICROELECTRONICS

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Operation

STM690A/692A/703/704/802/805/817/818/819

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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 VOUT. With a backup battery installed with voltage VBAT, the devices

automatically switch the SRAM to the backup supply when VCC falls.

Note:

When the battery is first connected without VCC power applied, the device does not

immediately provide backup battery voltage on VOUT. Only after VCC exceeds VRST 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 VBAT and

VOUT to VCC .

Whenever VCC falls below the switchover voltage, VSO, VOUT is connected to VBAT through a

100

Ω switch. VSO is the lesser of VBAT and VRST. Choosing the lesser allows the device to

be powered by VCC for as long as possible before switching over thereby maximizing the

battery life.

Assuming VBAT > 2.0 V, switchover at VSO ensures that battery backup mode is entered

before VOUT gets too close to the 2.0 V minimum required to reliably retain data in most

external SRAMs. When VCC recovers, hysteresis is used to avoid oscillation around the VSO

point. VOUT is connected to VCC through a 3 Ω PMOS power switch.

Note:

The backup battery may be removed while VCC is valid, assuming VBAT is adequately

decoupled (0.1 µF typ), without danger of triggering a reset.

Table 4.

I/O status in battery backup

VOUT

Connected to VBAT through internal switch

VCC

Disconnected from VOUT

PFI

Disabled

PFO

Logic low

High impedance

ECON

Logic high

WDI

Watchdog timer is disabled

Disabled

RST

Logic low

RST

Logic high

VBAT

Connected to VOUT

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