UM1653 datasheet(11/45 Pages) STMICROELECTRONICS

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UM1653

NAND driver firmware modules

The NAND driver library has the following modules:

Garbage collection

Wear leveling

Bad block management

ECC check

LUT

FAT file system

2.1

Garbage collection

The Garbage Collection software copies the valid data into a new (free) area and erases the

original invalid data.

Garbage Collection is performed when a virtual block is full or the number of free pages in

the whole device is lower than a specified threshold value.

The basic operations involved in Garbage Collection are the following:

The virtual blocks meeting the conditions are selected for erasure.

The valid physical pages are copied into a free area.

The selected physical blocks are erased.

As virtual blocks can contain more than one physical block, the Garbage Collection may

erase more than one physical block.

2.2

Wear leveling

Wear leveling is a technique to increase the lifetime of NAND Flash memory. The number of

reliable write cycles in NAND Flash is 100,000 erase/write cycles. If some of the blocks are

written repeatedly, wearing of these blocks will happen earlier than other blocks. To balance

the erase cycles over all the blocks, a wear leveling technique is introduced.

All new data is written to the empty blocks. The memory controller selects the new empty

block based on the number of write / erase cycles it has experienced.

After the new data is written, the controller updates the LUT to point to the position of the

selected physical block. The block containing the old data is erased and the number of

write/erase cycles increments.

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