NAND256-A datasheet(30/57 Pages) STMICROELECTRONICS

30 / 57 page

NAND128-A, NAND256-A, NAND512-A, NAND01G-A

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SOFTWARE ALGORITHMS

This section gives information on the software al-

gorithms 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 val-

ue) and it is recommended to implement Garbage

Collection, a Wear-Leveling Algorithm and an Er-

ror Correction Code, to extend the number of pro-

gram and erase cycles and increase the data

retention.

To help integrate a NAND memory into an applica-

tion ST Microelectronics can provide:

■

File System OS Native reference software,

which supports the basic commands of file

management.

Contact the nearest ST Microelectronics sales of-

fice for more details.

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 be-

cause it is isolated from the bit line and common

source line by a select transistor.

The devices are supplied with all the locations in-

side valid blocks erased (FFh). The Bad Block In-

formation is written prior to shipping. Any block

where the 6th Byte/ 1st Word 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 Informa-

tion may be erased. For the system to be able to

recognize the Bad Blocks based on the original in-

formation it is recommended to create a Bad Block

table following the flowchart shown in Figure 20.

Block Replacement

Over the lifetime of the device additional Bad

Blocks may develop. 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-program-

ming 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 the “Copy Back Program” section for more de-

tails.

Refer to Table 13. for the recommended proce-

dure to follow if an error occurs during an opera-

tion.

Table 13. Block Failure

Figure 20. Bad Block Management Flowchart

Operation

Recommended Procedure

Erase

Block Replacement

Program

Block Replacement or ECC

Read

ECC

AI07588C

START

END

YES

Block Address =

Block 0

Data

= FFh?

Last

block?

Increment

Block Address

Update

Bad Block table

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