DS1643P-70+【PDF 14/17 页】IC RAM TIMEKEEP NV 70NS 34-PCM

DS1643/DS1643P

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DATA RETENTION MODE

When VCC is within nominal limits (VCC > 4.5V) the DS1643 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 occurs) the internal clock registers and RAM are blocked from access. This is accomplished

internally by inhibiting access via the CE signal. At this time the power-on reset output signal ( RST ) 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. The RST signal is an open drain output and requires a pull up. Except for the RST , all control, data,

and address signals must be powered down when VCC is powered down.

BATTERY LONGEVITY

The DS1643 has a 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 DS1643 continuously for the life of the equipment in which it is installed. For

specification purposes, the life expectancy is 10 years at 25

C with the internal clock oscillator running in

the absence of VCC power. Each DS1643 is shipped from Dallas Semiconductor with its lithium energy

source disconnected, guaranteeing full energy capacity. When VCC is first applied at a level greater than

VPF, the lithium energy source is enabled for battery backup operation. Actual life expectancy of the

Ds1643 will be much longer than 10 years since no lithium battery energy is consumed when VCC is

present.

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