NAND Flash memory interface

ST72682

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4.2

Bad block management

NAND memory manufacturers deliver their devices with factory-marked bad blocks. This

marking depends on the manufacturer and the NAND memory type (page size, memory

technology, etc.). The ST72682 supports all bad block markings currently available on the

market.

4.2.1

Bad Block identification

During firmware initialization, the MCU scans the entire NAND memory configuration to

identify bad blocks.

A bad block is defined as follows:

●

Five different block status bytes are considered: 4 status bytes from page 0 and 1 from

an other page (page 127 for MLC NAND; page 1 for SLC NAND).

●

The considered block is marked as a bad block if one out of these five bytes contains at

least four bits set to ‘0’.

4.2.2

Bad block replacement

The firmware works on groups of 1024 blocks, called zones. A complete NAND

configuration can contain several zones:

●

Each zone is described in a Look Up Table (LUT) containing 1024 entries. A LUT is

composed of 3 parts: used blocks, free blocks and bad blocks.

●

The “bad blocks” part contains as many entries as the number of bad blocks identified

in that zone.

●

The “used blocks” part can have a size of 1000, 900 or 500 entries. This size is

configurable and also depends on the number of identified bad blocks.

●

The “free blocks” part contains the remaining entries.

The used blocks part is used to do a correspondence between NAND blocks and logical

address ranges.

This system allows all bad blocks to be masked from the Host. As a result, bad blocks are

never seen. Only a range of logical addresses are visible which correspond to the sum of

the used blocks part of all zones.

4.2.3

Late Fail block

During normal application life, defects may appear in the NAND memory. Under certain

conditions, these defects are not correctable and the corresponding block is declared as

“bad”.

In this case, new bad blocks are identified in the bad blocks part of the LUT and replaced by

new blocks from the “free blocks” part.