Chapter 10 Memory Subsystem.pdf

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Below each mapping diagram, a write (encoding) and read (decoding: syndrome generation) sequence is
given, with the number of the active buffers at each point in time (yellow). In the inactive zones (grey), no
computing is taking place but the data counter is still active.
A table on the left summarizes the mode, size0, size1 parameters to program for respectively write and
read processing of a page, with the given mapping, where :
• P is the size of spare byte section Protected by the ECC (in nibbles)
• U is the size of spare byte section Unprotected by the ECC (in nibbles)
• E is the size of the ECC itself (in nibbles)
• S is the number of Sectors per page (2 in the current diagrams)
Each time the processing of a BCH block is complete (ECC calculation for write/encoding, syndrome
generation for read/decoding, indicated by red arrows), the update pointer is pulsed. Note that the
processing for block 0 can be the first or the last to complete, depending on the NAND page mapping and
operation (read or write). All examples show a page size of 1kByte + spares; that is, S = 2 sectors of 512
bytes. The same principles can be extended to larger pages by adding more sectors.
The actual BCH codeword size is used during the error location work to restrict the search range: by
definition, errors can only happen in the codeword that was actually written to the NAND, and not in the
mathematical codeword of n = 2 13 - 1 = 8191 bits. That codeword (higher-order bits) is all-zero and implicit
during computations.
The actual BCH codeword size depends on the mode, programmed sizes, and sector number (all sizes in
nibbles):
• Spares mapped and protected per sector (below: see M1-M2-M3-M9-M10):
– all sectors: (512) + P + E
• Spares pooled and protected by sector 0 (below: see M5-M6):
– sector 0 codeword: (512) + P + E
– other sectors: (512) + E
• Unprotected spares (below: see M4-M7-M8-M11-M12):
– all codewords (512) + E
10.1.5.14.3.2.3.1 Per-Sector Spare Mappings
In the schemes in Figure 10-33, each 512-byte sector of the main area has its own dedicated section of
the spare area. The spare area of each sector is composed of :
• ECC, which must be located after the data it protects
• Other data, which may or may not be protected by the sector ECC
2158 Memory Subsystem SPRUGN4L–May 2010–Revised June 2011
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