Datasheet

DS1746/DS1746P Y2K-Compliant, Nonvolatile Timekeeping RAMs
7 of 16
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.