Two years ago Microchip introduced serial EEPROMs with UNI/O. The speciality is the single pin data interface, so the memory chip needs only 3 pins including power. This can be great help for the smallest 6-pin microcontrollers like PIC10F or ATtiny10 series which doesn’t have any non-volatile memory writable from application itself. Memory densities range from 1 Kb to 16 Kb. It competes with 1-wire EEPROMs from Maxim but at significantly lower prices and lower working voltages.
UNI/O bus uses Manchester encoding to combine clock and data to single line. ACK bits at the end of each byte are similar to I2C bus, but there are always two: one from master and one from slave. Addressing and write protection is all managed by software. Currently there are no other UNI/O devices than EEPROMs but the bus manual suggest that there should be others like temperature sensors, A/D converters or port expanders. Application notes with code examples are available for several different MCUs.