The Xicor X25401 is a serial 256 bit NOVRAM featuring a static RAM configured 16 x 16, overlaid bit-by-bit with a nonvolatile E2PROM array. The X25401 features a Serial Peripheral Interface (SPI) and software protocol allowing operation on a simple three-wire bus. The bus signals are a clock input (SCK) plus separate data in (SI) and data out (SO) lines. Access to the device is controlled through a chip select (CS) input, allowing any number of devices to share the same bus.