The DS1218 Nonvolatile Controller Chip supplies circuitry required to render standard CMOS RAM nonvolatile. VCC is monitored using a precise comparator that detects power-fail and inhibits chip-enable holding a write-protect status until power supplies return to nominal levels. The DS1218 switches active power supplies between VCC and a battery source, whichever is greater. The switch has a voltage drop of less than 0.2V.