Ramtron FRAM memories and FRAM MCU can retain data without power and without of erasing and rewriting. However these parts are still developing and form only small portion of the memory market. Their works looks well pointed towards future to replace nVSRAM, FLASH and EEPROM memories completely.
www.ramtron.com/
Their featured product
FM3130: Processor Companion with 64Kb FRAM and
Enhanced RTC
in a Tiny Package looks very good to me.
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Prof. (Dr.) Shyam, Managing Director for Sensors Technology Private Limited. Gwalior, MP474001, India.