Rainbow Electronics DS1747P - Manual (Page 6)

DS1747/DS1747P
6 of 18
DATA RETENTION MODE
The 5-volt 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 failing 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 3.3 volt 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 Vso
,
the device power is switched from
V
CC
to the backup supply (V
BAT
) when V
CC
drops below Vso
.
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 DS1747 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 DS1747 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 DS1747 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 DS1747 will be much longer than 10 years since no lithium battery energy is
consumed when V
CC
is present.
BATTERY MONITOR
The DS1747 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.