FLASH MEMORY

FLASH MEMORY

ABSTRACT

 Flash memory is a non-volatile memory similar to EEPROM differing in the principle of in-circuit wiring used to erase the entire chip or blocks of defined size in it. Flash memory stores charge (information) in the array of floating gate transistors called ‘cells’ storing one bit of information. Presence of charge determines whether channel will conduct or not. In NOR During read cycle a ‘1’ at output corresponds to channel being in ON state. Control gate is used to charge gate capacitance during ‘write’ cycle using Fowler- Nordheim tunneling. NAND flash uses tunnel injection for writing and tunnel release for erasing. Only NOR flash memory offers execute in place (XIP) capability for read operation. Common flash interface commands are defined for write/erase operations by block handling. In NAND virtual memory strategies are used for read/write/erase operations. Factors for considering NAND/NOR reliability are Bit flipping, Bad block handling, Life span (Number of Erase/Write cycles). EDC/ECC algorithms need to be used while using NAND type flash. NAND also needs to be scanned for bad blocks and mapping them all out. NOR offers 10 times the number of erase/write cycles than NOR. The improvements that took place in conventional flash memory design are MLC (Multi level cell) NAND, Raw NAND, NAND with external controller, NAND with Chipset controller, Mirror bit flash technology. The advantages of Flash memory are Improved Data protection, Security, Capacity, Highest chip density, Fabrication compatible with CMOS process. Flash memory has an edge over other emerging non-volatile memories with DiskonchipG3 and Mirror Bit Flash technology the most promising technologies today.

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