A Stepwise Rate-Compatible LDPC and Parity Management in NAND Flash Memory-Based Storage Devices

Abstract

The storage capacity of the NAND flash memory has increased rapidly, and accordingly, the error rate for data writing and reading to the flash memory cell has also escalated. Error-correcting code (ECC) modules, such as low-density parity-check (LDPC), have been applied to flash controllers for error recovery. However, since the error rate increases rapidly, compared to the aging factor and program/erase (P/E) cycle, fixed ECCs and parities are inappropriate methods for resolving this proliferating error, according to the P/E cycle. Therefore, the design of a dynamic ECC scheme and a proper ECC parity management system to increase the lifespan of flash memory storage devices remains in great demand. Herein, an LDPC encoding and decoding scheme is designed to obtain a step-by-step code rate according to the P/E cycle by applying a stepwise rate-compatible LDPC. In addition, an ECC parity management scheme for the increasingly excessive stage-wise ECC is proposed to reduce management and read/write operational overheads. The ECC management scheme also includes the ECC cache system. The proposed LDPC, as well as its management system, will improve the recovery ability of the NAND flash storage device according to the P/E cycle, while it can reduce system read and write overheads due to additional parity data growth.