DS1744/DS1744P 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 remains 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 V
PF
access to the device is inhibited. At this time the power-fail
reset-output signal ( RST ) is driven active and remains 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 pullup. Except for the
RST , all control, data, and address signals must be powered down when V
CC
is powered down.
BATTERY LONGEVITY
The DS1744 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 DS1744 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 DS1744 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
DS1744 is much longer than 10 years since no lithium battery energy is consumed when V
CC
is present.
BATTERY MONITOR
The DS1744 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 1 when read. If a 0 is ever present, an exhausted lithium energy source is indicated, and both
the contents of the RTC and RAM are questionable.
6 of 18
DALLAS [ DALLAS SEMICONDUCTOR ]
