Difference between NVRAM and EEPROM

[sunidrak]

Hi can anybody plz tell what is difference between NVRAM and EEPROM ??

NVRAM is non volatile RAM then why cant we consider it as Programmable ROM why it is not classified in ROM ??

 

[FvM]

An attempt to distinguish between NVRAM and other non-volatile memories in a FRAM (ferroelectric RAM) background paper:

The major difference (beside nonvolatility) between RAM and ROM type devices is the difficulty level of write-operations. Traditional nonvolatile memories derive from floating gate devices that are very difficult to write. Writing to an EEPROM, for example, involves pushing electrons through a glass barrier. This uses a large amount of power at high voltages and requires a relatively long time. Write times require milliseconds for EEPROM, while FRAM write access times are now under 70 ns.

In addition, ROM type devices allow very limited numbers of write operations. This is due to the high stress condition caused by a write. Floating-gate devices wear out after 100,000 or 1,000,000 write cycles. This is far too small a number for data collection applications. FRAM memories can be written over 10,000,000,000 times. This number is expected to increase dramatically in the coming years. Furthermore the floating-gate is mechanism contributes to system failures, since the glass breaks after a relatively small number of write cycles. FRAM memory write operation does not cause early system failure, since the write function produces no stress.

Other usual NVRAM designs are SRAM with built-in backup battery or SRAM with automatic backup to a flash memory.