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DS1312

Nonvolatile Controller with Lithium Battery Monitor

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DS1312

Nonvolatile Controller with Lithium

Battery Monitor

MEMEORY BACKUP

The DS1312 performs all the circuit functions required to provide battery-backup for an SRAM. First, the device provides a switch

to direct power from the battery or the system power supply (VCCI). Whenever VCCI is less than the switch point VSW and VCCI is

less than the battery voltage VBAT, the battery is switched in to provide backup power to the SRAM. This switch has voltage drop of

less than 0.2 volts.

Second, the DS1312 handles power failure detection and SRAM write-protection. VCCI is constantly monitored, and when the

supply goes out of tolerance, a precision comparator detects power failure and inhibits chip enable output ( ) in order to

write-protect the SRAM. This is accomplished by holding

to within 0.2 volts of Vcco when VCCI is out of tolerance. If

is

(active) low at the time that power failure is detected, the

signal is kept low until

is brought high again. Once

is

brought high,

is taken high and head high until after VCCI has returned to its nominal voltage level. If

is not brought high

by 1.5µs after power failure is detected,

is forced high at that time. This specific scheme for delaying write protection for up

to 1.5µs guarantees that any memory access in progress when power failure occurs will complete properly. Power failure

detection occurs in the range of 4.75 to 4.5 volts (5% tolerance) when the TOL pin is wired to GND or in the range of 4.5 to 4.25

volts (10% tolerance) when TOL is connected to Vcco.

BATTERY VOLTAGE MONITORING

The DS1312 automatically performs periodic battery voltage monitoring at a factory-programmed time interval of 24 hours. Such

monitoring begins within tREC after VCCI rises above VCCTP, and is suspended when power failure occurs.

After each 24-hour period (tBTCN) has elapsed, the DS1312 connects VBAT to an internal 1.2MΩ test resistor (RINT) for one second

(tBTPW). During this one second, if VBAT falls below the factory-programmed battery voltage trip point (VBTP), the battery warning

output is asserted. While is active battery testing will be performed with period tBTCW to detect battery removal and

replacement. Once asserted, remains active until the battery is physically removed and replaced by a fresh cell. The battery

is still retested after each Vcc power-up, however, even if was active on power-down. If the battery is found to be higher than

VBTP during such testing, is deasserted and regular 24-hour testing resumes. has an open-drain output driver.

Battery replacement following activation is normally done with VCCI nominal so that SRAM data is not lost. During battery

replacement, the minimum time duration between old battery detachment and new battery attachment (tBDBA) must be met or

will not deactivate following attachment of the new battery. Should not deactivate for this reason, the new battery can be

detached for tBDBA and then reattached to clear .

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 ) is removed between battery tests, the

DS1312 may not immediately sense immediately sense the removal and may not activate 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 Vcc 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) is order

to perform battery testing. Furthermore, anytime is activated on the first battery test after a power-up, data integrity should be

checked via checksum or other technique.

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