DS1643 Datasheet, PDF (6/17 Pages) Dallas Semiconductor – Nonvolatile Timekeeping RAM

DS1643/DS1643P
DATA RETENTION MODE
When V_CC is within nominal limits (V_CC > 4.5V) the DS1643 can be accessed as described above with
read or write cycles. However, when V_CC is below the power-fail point V_PF (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 V_CC returns to nominal levels. When V_CC falls below the
level of the internal battery supply, power input is switched from the V_CC pin to the internal battery and
clock activity, RAM, and clock data are maintained from the battery until V_CC 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 V_CC 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 V_CC 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 V_CC power. Each DS1643 is shipped from Dallas Semiconductor with its lithium energy
source disconnected, guaranteeing full energy capacity. When V_CC is first applied at a level greater than
V_PF, 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 V_CC is
present.
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