M48T35AY datasheet(10/25 Pages) STMICROELECTRONICS

10 / 25 page

M48T35AY, M48T35AV

10/25

Data Retention Mode

With valid VCC applied, the M48T35AY/V 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 (see Figure 15, Table 10, and Table

11., page 18). 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 M48T35AY/V 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.

When VCC drops below VSO, the control circuit

switches power to the internal battery which pre-

serves data and powers the clock. The internal

button cell will maintain data in the M48T35AY/V

for an accumulated period of at least 7 years when

VCC is less than VSO. As system power returns

and VCC rises above VSO, the battery is discon-

nected and the power supply is switched to exter-

nal VCC. Write protection continues until VCC

reaches VPFD (min) plus trec (min). E should be

kept high as VCC rises past VPFD (min) to prevent

inadvertent WRITE cycles prior to processor stabi-

lization. Normal RAM operation can resume trec af-

ter VCC exceeds VPFD (max).

Also, as VCC rises, the battery voltage is checked.

If the voltage is less than approximately 2.5V, 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.

Figure 10 illustrates how a BOK check routine

could be structured.

For more information on Battery Storage Life refer

to the Application Note AN1012.

Figure 10. 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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