DS1644L-120 Ver la hoja de datos (PDF) - Dallas Semiconductor -> Maxim Integrated

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DS1644L-120

Nonvolatile Timekeeping RAM

Dallas Semiconductor -> Maxim Integrated 

DS1644LPM

DATA RETENTION MODE

When VCC is within nominal limits (VCC > 4.5 volts) the

DS1644L can be accessed as described above with

read or write cycles. However, when VCC is below the

power fail point VPF (point at which write protection oc-

curs) the internal clock registers and RAM are blocked

from access. This is accomplished internally by inhibit-

ing access via the CE signal. At this time the power–fail

output signal (PFO) will be driven active low and will

remain active until VCC returns to nominal levels. When

VCC falls below the level of the internal battery supply,

power input is switched from the VCC pin to the internal

battery and clock activity, RAM, and clock data are

maintained from the battery until VCC is returned to

nominal level.

INTERNAL BATTERY LONGEVITY

The DS1644L has a self contained lithium power source

that is designed to provide energy for clock activity, and

clock and RAM data retention when the VCC supply is

not present. The capability of this internal power supply

is sufficient to power the DS1644L continuously for the

life of the equipment in which it is installed. For specifi-

cation purposes, the life expectancy is 10 years at 25°C

with the internal clock oscillator running in the absence

of VCC power. The DS1644L is shipped from Dallas

Semiconductor with the clock oscillator turned off, so

the expected life should be considered to start from the

time the clock oscillator is first turned on. Actual life ex-

pectancy of the DS1644L will be much longer than 10

years since no internal lithium battery energy is con-

sumed when VCC is present. In fact, in most applica-

tions, the life expectancy of the DS1644L will be approx-

imately equal to the shelf life (expected useful life of the

lithium battery with no load attached) of the lithium bat-

tery which may prove to be as long as 20 years.

041697 5/11

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