ST72681 STMicroelectronics, ST72681 Datasheet - Page 11

• Manufacturer Part Number: ST72681
• Description: USB 2.0 HIGH-SPEED 8-BIT MCU FLASH DRIVE CONTROLLER
• Manufacturer: STMicroelectronics

ST72681
4.2
4.2.1
4.2.2
4.3
4.3.1
NAND error correction
No NAND Flash memory arrays are guaranteed by manufacturers to be error-free. Error
occurrence depends on the Flash cell type (MLC or SLC).
The ST72681 embeds hardware and firmware mechanisms to correct the errors.
Hardware error correction
The ST72681 embeds a Reed-Solomon algorithm-based hardware cell. This cell directly
manages 512-byte data packets on the NAND Flash I/O system.
Based on a data packet contents, the cell generates an 80-bit Error Correction Code (ECC)
consisting of 8 words each containing 10 bits.
During write operations to NAND memory, the 512-bytes of data and the ECC are stored
together in the same page. The ECC is stored in the corresponding Redundant Area (RA),
using 10 bytes.
During read operations, the 512-bytes of data and the 8 ECC words are read back and are
passed through the Reed-Solomon cell for decoding. The cell allows the correction of 4
symbols in this 520-symbol packet (512 symbols from data + 8 symbols from ECC).
The hardware cell gives 3 possible results:
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Firmware error management
The firmware defines the error correction possibilities with the corrected data packet.
When data is not repairable, the block is considered as bad and replaced by another one.
See below for further information.
Management of bad NAND Flash blocks
NAND Flash device manufacturers deliver their products with factory-marked bad blocks.
This marking depends on the manufacturer and the NAND Flash type (page size, memory
technology, etc.). The ST72681 supports all bad block markings currently available on the
market.
Bad block identification
During firmware initialization, the MCU scans the entire NAND Flash configuration to
identify bad blocks.
A bad block is defined as follows:
●
●
No error detected: the data packet can be used as it is.
Correctable error detected: the corrected data are available in a specific 512-byte buffer
in the Reed-Solomon cell and are ready to use.
Uncorrectable error detected: data corruption is not repairable.
5 different Block Status bytes are considered: 4 Status bytes from page 0 and 1 from an
other page (page 127 for MLC NAND Flash memory; page 1 for SLC NAND Flash
memory).
The considered block is declared bad if 1 of these 5 bytes contains 4 bits or more at 0.
NAND Flash interface
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