M48T254V datasheet(18/24 Pages) STMICROELECTRONICS

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M48T254V

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Battery Low

The M48T254V automatically performs battery

voltage monitoring upon power-up, and at factory-

programmed time intervals of at least 24 hours.

The Battery Low (BL) signal will be asserted if the

battery voltage is found to be less than approxi-

mately 2.5V. The BL signal will remain asserted

until completion of battery replacement and sub-

sequent battery low monitoring tests, either during

the next power-up sequence or the next scheduled

24-hour interval.

If a battery low is generated during a power-up se-

quence, this indicates that one of the batteries is

below 2.5V and may not be able to maintain data

integrity in the SRAM. Data should be considered

suspect,

and

verified

correct.

All

three

SNAPHAT® tops should be replaced.

If a battery low indication is generated during the

24-hour interval check, this indicates that one of

the batteries is near end of life. However, data is

not compromised due to the fact that a nominal

VCC is supplied. In order to insure data integrity

during subsequent periods of battery back-up

mode, the batteries should be replaced. The

SNAPHAT top should be replaced with valid VCC

applied to the device.

The M48T254V only monitors the batteries when

a nominal VCC is applied to the device. Thus appli-

cations which require extensive durations in the

battery back-up mode should be powered-up peri-

odically (at least once every few months) in order

for this technique to be beneficial. Additionally, if a

battery low is indicated, data integrity should be

verified upon power-up via a checksum or other

technique. The BL signal is an open drain output

and an appropriate pull-up resistor should be cho-

sen to control the rise time.

Note: The BL signal is available only for the exter-

nal SRAM, not for the Real-Time Clock.

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