An Improved ECC Decoding Method for Flash Memory Utilizing Uncorrected Error Information

Abstract

With the ever-increasing requirement for higher performance of system and memory with a huge storage capacity, multi-level cell (MLC) flash memory holds a dominant position in flash memory. Various error correction techniques (e.g. Hamming, BCH, LDPC) have been adopted to maintain the reliability of data stored in flash memory. However, more errors are observed in flash memory as storage size increases. Retention error, which occurs in data not accessed for a long period after storage in flash memory, is the largest contributor of flash memory error. This paper presents a safe system-level reprogramming scheme to optimally correct retention errors. Furthermore, information about the uncorrected errors is efficiently used to improve the performance of the LDPC decoder. Through an experiment that simulates real NAND flash products, safe reprogramming scheme at system level is explicitly demonstrated along with a LDPC decoder with an improved correction ratio and reduced number average iteration.