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%META:TOPICINFO{author="blundar" date="1077907080" format="1.0" version="1.1"}%
%META:TOPICPARENT{name="RealTimeProgramming"}%
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 SRAM to EEPROM. 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 (???) - chip contains electromagnetic storage. Someone describe more.
- 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
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