M48T02 datasheet(10/19 Pages) STMICROELECTRONICS

10 / 19 page

M48T02, M48T12

10/19

Data Retention Mode

With valid VCC applied, the M48T02/12 operates

as a conventional BYTEWIDE™ static RAM.

Should the supply voltage decay, the RAM will au-

tomatically power-fail deselect, write protecting it-

self when VCC falls within the VPFD (max), VPFD

(min) window. All outputs become high imped-

ance, and all inputs are treated as “don't care.”

Note: A power failure during a WRITE cycle may

corrupt data at the currently addressed location,

but does not jeopardize the rest of the RAM's con-

tent. At voltages below VPFD (min), the user can be

assured the memory will be in a write protected

state, provided the VCC fall time is not less than tF.

The M48T02/12 may respond to transient noise

spikes on VCC that reach into the deselect window

during the time the device is sampling VCC. There-

fore, decoupling of the power supply lines is rec-

ommended.

The power switching circuit connects external VCC

to the RAM and disconnects the battery when VCC

rises above VSO. As VCC rises, the battery voltage

is checked. If the voltage is too low, an internal

Battery Not OK (BOK) flag will be set. The BOK

flag can be checked after power up. If the BOK flag

is set, the first WRITE attempted will be blocked.

The flag is automatically cleared after the first

WRITE, and normal RAM operation resumes. Fig-

ure 9 illustrates how a BOK check routine could be

structured.

For more information on a Battery Storage Life re-

fer to the Application Note AN1012.

Figure 9. Checking the BOK Flag Status

READ DATA

AT ANY ADDRESS

AI00607

IS DATA

COMPLEMENT

OF FIRST

READ?

(BATTERY OK)

POWER-UP

YES

WRITE DATA

COMPLEMENT BACK

TO SAME ADDRESS

READ DATA

AT SAME

ADDRESS AGAIN

NOTIFY SYSTEM

OF LOW BATTERY

(DATA MAY BE

CORRUPTED)

WRITE ORIGINAL

DATA BACK TO

SAME ADDRESS

(BATTERY LOW)

CONTINUE

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