DS1746_10 Datasheet, PDF (7/16 Pages) Maxim Integrated Products – Y2K-Compliant, Nonvolatile Timekeeping RAMs

DS1746/DS1746P Y2K-Compliant, Nonvolatile Timekeeping RAMs
DATA-RETENTION MODE
The 5V device is fully accessible and data can be written or read only when V_CC is greater than V_PF.
However, when V_CC is below the power-fail point, V_PF, (point at which write protection occurs) the
internal clock registers and SRAM are blocked from any access. At this time the power fail reset output
signal (RST) is driven active and will remain active until V_CC returns to nominal levels. When V_CC falls
below the battery switch point V_SO (battery supply level), device power is switched from the V_CC pin to
the backup battery. RTC operation and SRAM data are maintained from the battery until V_CC is returned
to nominal levels. The 3.3V device is fully accessible and data can be written or read only when V_CC is
greater than V_PF. When V_CC falls below the power fail point, V_PF, access to the device is inhibited. At this
time the power fail reset output signal (RST) is driven active and will remain active until V_CC returns to
nominal levels. If V_PF is less than V_SO, the device power is switched from V_CC to the backup supply
(V_BAT) when V_CC drops below V_PF. If V_PF is greater than V_SO, the device power is switched from V_CC to
the backup supply (V_BAT) when V_CC drops below V_SO. RTC operation and SRAM data are maintained
from the battery until V_CC is returned to nominal levels. 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 DS1746 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 DS1746 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 DS1746 is shipped from Maxim 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 DS1746 will
be much longer than 10 years since no lithium battery energy is consumed when V_CC is present.
BATTERY MONITOR
The DS1746 constantly monitors the battery voltage of the internal battery. The Battery Flag bit (bit 7) of
the day register is used to indicate the voltage level range of the battery. This bit is not writable and
should always be a one when read. If a zero is ever present, an exhausted lithium energy source is
indicated and both the contents of the RTC and RAM are questionable.
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