DS1312 Datasheet, PDF (3/11 Pages) Dallas Semiconductor – Nonvolatile Controller with Lithium Battery Monitor

DS1312
NOTE: The DS1312 cannot constantly monitor an attached battery because such monitoring would
drastically reduce the life of the battery. As a result, the DS1312 only tests the battery for one second out
of every 24 hours and does not monitor the battery in any way between tests. If a good battery (one that
has not been previously flagged with BW ) is removed between battery tests, the DS1312 may not
immediately sense the removal and may not activate BW until the next scheduled battery test. If a battery
is then reattached to the DS1312, the battery may not be tested until the next scheduled test.
NOTE: Battery monitoring is only a useful technique when testing can be done regularly over the entire
life of a lithium battery. Because the DS1312 only performs battery monitoring when V_CC is nominal,
systems which are powered-down for excessively long periods can completely drain their lithium cells
without receiving any advanced warning. To prevent such an occurrence, systems using the DS1312
battery monitoring feature should be powered–up periodically (at least once every few months) in order
to perform battery testing. Furthermore, anytime BW is activated on the first battery test after a power-up,
data integrity should be checked via checksum or other technique.
POWER MONITORING
DS1312S and DS1312E varieties have an additional reset pin. These varieties detect out-of-tolerance
power supply conditions and warn a processor-based system of impending power failure. When V_CCI falls
below the trip point level defined by the TOL pin (V_CCTP), the V_CCI comparator activates the reset signal
RST . Reset occurs in the range of 4.75 to 4.5 volts (5% tolerance) when the TOL pin is connected to
GND or in the range of 4.5 to 4.25 volts (10% tolerance) when TOL is connected to V_CCO.
RST also serves as a power-on reset during power-up. After V_CCI exceeds V_CCTP, RST will be held active
for 200 ms nominal (t_RPU). This reset period is sufficiently long to prevent system operation during
power-on transients and to allow t_REC to expire. RST has an open-drain output driver.
FRESHNESS SEAL MODE
When the battery is first attached to the DS1312 without V_CC power applied, the device does not
immediately provide battery-backup power on V_CCO. Only after V_CCI exceeds V_CCTP and later falls below
both V_SW and V_BAT will the DS1312 leave Freshness Seal Mode and provide battery-backup power. This
mode allows a battery to be attached during manufacturing but not used until after the system has been
activated for the first time. As a result, no battery energy is drained during storage and shipping.
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