M48Z08 datasheet(9/18 Pages) STMICROELECTRONICS

9 / 18 page

DATA RETENTION MODE

With valid VCC applied, the M48Z08/18 operates as

a conventional BYTEWIDE

™ static RAM. Should

the supply voltage decay, the RAM will automat-

ically power-fail deselect, write protecting itself

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

window. All outputs become high impedance, 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

content. 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 M48Z08/18 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 M48Z08/18 for

an accumulated period of at least 11 years when

VCC is less than VSO. As system power returns and

VCC rises above VSO, the battery is disconnected,

and the power supply is switched to external 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 system stabilization. Normal RAM

operation can resume tREC after VCC exceeds

VPFD(max).

For more information on Battery Storage Life refer

to the Application Note AN1012.

SYSTEM BATTERY LIFE

The useful life of the battery in the M48Z08/18 is

expected to ultimately come to an end for one of

two reasons: either because it has been dis-

charged while providing current to the RAM in the

battery back-up mode, or because the effects of

aging render the cell useless before it can actually

be completely discharged. The two effects are

virtually unrelated, allowing discharge or Capacity

Consumption, and the effects of aging or Storage

Life, to be treated as two independent but simulta-

neous mechanisms. The earlier occurring failure

mechanism defines the battery system life of the

M48Z08/18.

AI01399

TEMPERATURE (Degrees Celsius)

t50% (AVERAGE)

t1%

Figure 9. Predicted Battery Storage Life versus Temperature

9/18

M48Z08, M48Z18

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