The DS1321 Flexible Nonvolatile Controller with Lithium Battery Monitor is a CMOS circuit which
solves the application problem of converting CMOS SRAMs into nonvolatile memory. Incoming power
is monitored for an out-of-tolerance condition. When such a condition is detected, chip enable outputs are
inhibited to accomplish write protection and the battery is switched on to supply the SRAMs with
uninterrupted power. Special circuitry uses a low-leakage CMOS process which affords precise voltage
detection at extremely low battery consumption. One DS1321 can support as many as four SRAMs
arranged in any of three memory configurations.
In addition to battery-backup support, the DS1321 performs the important function of monitoring the
remaining capacity of the lithium battery and providing a warning before the battery reaches end-of-life.
Because the open-circuit voltage of a lithium backup battery remains relatively constant over the majority
of its life, accurate battery monitoring requires loaded-battery voltage measurement. The DS1321
performs such measurement by periodically comparing the voltage of the battery as it supports an internal
resistive load with a carefully selected reference voltage. If the battery voltage falls below the reference
voltage under such conditions, the battery will soon reach end-of-life. As a result, the Battery Warning
pin is activated to signal the need for battery replacement.
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