DS1556 Datasheet, PDF (5/20 Pages) Dallas Semiconductor – 1M, Nonvolatile, Y2K-Compliant Timekeeping RAM

DS1556 1M, Nonvolatile, Y2K-Compliant Timekeeping RAM
BATTERY LONGEVITY
The DS1556 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 DS1556 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 DS1556 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 DS1556 will be much longer than 10 years since no internal battery energy is
consumed when V_CC is present.
INTERNAL BATTERY MONITOR
The DS1556 constantly monitors the battery voltage of the internal battery. The Battery Low Flag (BLF)
bit of the Flags Register (B4 of 1FFF0h) is not writable 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.
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