Rainbow Electronics DS1554 - Manual (Page 5)

DS1554
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BATTERY LONGEVITY
The DS1554 has a lithium power source that is designed to provide energy for the 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 DS1554 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
. Each DS1554 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 DS1554 will be much longer than 10 years since no internal battery energy is
consumed when V
CC
is present.
INTERNAL BATTERY MONITOR
The DS1554 constantly monitors the battery voltage of the internal batter. The Battery Low Flag (BLF)
bit of the Flags Register (B4 of 7FFF0h) is not writeable and should always be a 0 when read. If a 1 is
ever present, an exhausted lithium energy source is indicated and both the contents of the RTC and RAM
are questionable.
POWER-ON RESET
A temperature compensated comparator circuit monitors the level of V
CC
. When V
CC
falls to the power
fail trip point, the RST signal (open drain) is pulled low. When V
CC
returns to nominal levels, the
RST signal continues to be pulled low for a period of 40 ms to 200 ms. The power-on reset function is
independent of the RTC oscillator and thus is operational whether or not the oscillator is enabled.
CLOCK OPERATIONS
Table 2 and the following paragraphs describe the operation of RTC, alarm, and watchdog functions.