NAND512R3A2C datasheet(28/51 Pages) STMICROELECTRONICS

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Software algorithms

NAND512-A2C

28/51

Software algorithms

This section gives information on the software algorithms that ST recommends to implement

to manage the Bad Blocks and extend the lifetime of the NAND device.

NAND Flash memories are programmed and erased by Fowler-Nordheim tunneling using a

high voltage. Exposing the device to a high voltage for extended periods can cause the

oxide layer to be damaged. For this reason, the number of program and erase cycles is

limited (see Table 14 for value) and it is recommended to implement Garbage Collection, a

Wear-Leveling Algorithm and an Error Correction Code, to extend the number of program

and erase cycles and increase the data retention.

To help integrate a NAND memory into an application ST Microelectronics can provide:

●

File System OS Native reference software, which supports the basic commands of file

management.

Contact the nearest ST Microelectronics sales office for more details.

7.1

Bad Block management

Devices with Bad Blocks have the same quality level and the same AC and DC

characteristics as devices where all the blocks are valid. A Bad Block does not affect the

performance of valid blocks because it is isolated from the bit line and common source line

by a select transistor.

The devices are supplied with all the locations inside valid blocks erased (FFh). The Bad

Block Information is written prior to shipping. Any block where the 6th byte (x8 device) / 1st

word (x16 device) in the spare area of the 1st page does not contain FFh is a Bad Block.

The Bad Block Information must be read before any erase is attempted as the Bad Block

Information may be erased. For the system to be able to recognize the Bad Blocks based on

the original information it is recommended to create a Bad Block table following the

flowchart shown in Figure 13.

7.2

NAND Flash memory failure modes

Over the lifetime of the device additional Bad Blocks may develop.

To implement a highly reliable system, all the possible failure modes must be considered:

●

Program/Erase failure: in this case the block has to be replaced by copying the data to

a valid block. These additional Bad Blocks can be identified as attempts to program or

erase them will give errors in the Status Register.

As the failure of a Page Program operation does not affect the data in other pages in

the same block, the block can be replaced by re-programming the current data and

copying the rest of the replaced block to an available valid block. The Copy Back

Program command can be used to copy the data to a valid block. See Section 6.4:

Copy Back Program for more details.

●

Read failure: in this case, ECC correction must be implemented. To efficiently use the

memory space, it is recommended to recover single-bit error in read by ECC, without

replacing the whole block.

Refer to Table 13 for the procedure to follow if an error occurs during an operation.

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