The Maxim DS1321S/T&R is a RAM Memory Controller. The DS1321 Flexible Nonvolatile Controller with Lithium Battery Monitor is a CMOS circuit whichsolves the application problem of converting CMOS SRAMs into nonvolatile memory. Incoming poweris monitored for an out-of-tolerance condition. When such a condition is detected, chip enable outputs areinhibited to accomplish write protection and the battery is switched on to supply the SRAMs withuninterrupted power. Special circuitry uses a low-leakage CMOS process which affords precise voltagedetection at extremely low battery consumption. One DS1321 can support as many as four SRAMsarranged in any of three memory configurations. In addition to battery-backup support, the DS1321 performs the important function of monitoring theremaining 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 majorityof its life, accurate battery monitoring requires loaded-battery voltage measurement. The DS1321performs such measurement by periodically comparing the voltage of the battery as it supports an internalresistive load with a carefully selected reference voltage. If the battery voltage falls below the referencevoltage under such conditions, the battery will soon reach end-of-life. As a result, the Battery Warningpin is activated to signal the need for battery replacement.
