DS1321_04 Datasheet, PDF (4/12 Pages) Dallas Semiconductor – Flexible Nonvolatile Controller with Lithium Battery Monitor

DS1321
BATTERY VOLTAGE MONITORING
The DS1321 automatically performs periodic battery voltage monitoring at a factory-programmed time
interval of 24 hours. Such monitoring begins within t_REC after V_CCI rises above V_CCTP and is suspended
when power failure occurs.
After each 24-hour period (t_BTCN) has elapsed, the DS1321 connects V_BAT to an internal 1 MW test
resistor (R_INT) for one second (t_BTPW). During this one second, if V_BAT falls below the factory-
programmed battery voltage trip point (V_BTP), the battery warning output BW is asserted. While BW is
active, battery testing will be performed with period t_BTCW to detect battery removal and replacement.
Once asserted, BW remains active until the battery is physically removed and replaced by a fresh cell.
The battery is still retested after each V_CC power-up, however, even if BW was active on power-down. If
the battery is found to be higher than V_BTP during such testing, BW is deasserted and regular 24-hour
testing resumes. BW has an open-drain output driver.
Battery replacement following BW activation is normally done with V_CCI nominal so that SRAM data is
not lost. During battery replacement, the minimum time duration between old battery detachment and
new battery attachment (t_BDBA) must be met or BW will not deactivate following attachment of the new
battery. Should BW not deactivate for this reason, the new battery can be detached for t_BDBA and then re-
attached to clear BW .
NOTE: The DS1321 cannot constantly monitor an attached battery because such monitoring would
drastically reduce the life of the battery. As a result, the DS1321 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 DS1321 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 DS1321, 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 DS1321 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 DS1321
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
The DS1321 automatically detects out-of-tolerance power supply conditions and warns 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.
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