Understanding Flash memory

Damien Col, Hyperstone
eeNews Europe (December 18, 2017)

Flash memory is currently the most widely used type of non-volatile memory. NAND Flash is optimised for file storage, to replace traditional disk drives. This article provides an overview of how NAND Flash technology works, and the role of the controller to optimise the performance and lifetime of the Flash memory.

Inherent weaknesses of Flash technology

One problem with Flash memory is that it has a limited number of write-erase cycles. The high voltages used cause a small amount of damage to the cells, with the result that they become harder to program and erase over time. Damage to the insulating layers around the floating gate also reduces the time that data can be retained. This limits the useful lifetime of the Flash memory to about 100,000 cycles, or fewer for MLC Flash.

The lifetime of the Flash array can be maximised by ensuring that the number of write-erase cycles is the same for all blocks, a process called wear-levelling. This requires the Flash controller to keep track of which blocks have been used, and select the best block to move data to when new data is written.

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