NVSRAM

Non Volatile SRAM - fast, byte addressible, byte write, RAM that retains its contents after power is lost.

NVSRAM usually takes one of several forms:

• Battery backed SRAM (Dallas DS1220,1230, etc. TI Benchmarq chips, ST ...) - chip contains SRAM, a battery and power sense circuitry. When power fails, the battery backup kicks in and is used to provide power to the RAM preventing loss of data. Pros: looks just like a SRAM electrically, old technology. Cons: cost, large physical size, battery can die after several years
• EEPROM/SRAM hybrids (ZMD, Simtek chips, others?) - chip contains EEPROM, SRAM and power sense circuitry. On powerup, data is copied from EEPROM to SRAM. When a powerfail condition is detected, the data in SRAM is written to EEPROM. Often require large(r) capacitors on the power line to ensure a gradual drop on the power line occurs. Pros: cheaper, small packages available. Cons: long startup time after poweron can require electrical / logical modification of existing designs.
• FRAM (Ramtron) - Ferroelectric RAM. Uses a ferroelectric crystal to store the bit. Has the read/write speed of RAM (~1nS) with the feature of permanently retaining data. Near unlimited write cycles. Pros: Available in many packages. Very high endurance cycles. Read/Write speeds the same as RAM. Cons: Availability. Minor pinout differences from standard 27C256. Detailed information on Ramtron's site: http://www.ramtron.com/doc/AboutFRAM/technology.asp
• MRAM (Cypress) - chip contains next generation electromagnetic storage. Supposedly overcomes the shortcomings of EEPROM/SRAM hybrids (powerup time) and the form factor / battery life issues of battery backed SRAM without introducing the peculiar electrical/logical characteristics of FRAM. Overall: looks to be the perfect technology for Real Time Programming of devices not designed for it. Hope to see real parts instead of vaporware sometime Q2 '04