Data Storage Time Sensitive ECC Schemes for MLC NAND Flash Memories

Errors in MLC NAND Flash can be classified into retention errors and program interference (PI) errors. While retention errors are dominant when the data storage time is greater than 1 day, PI errors are dominant for short data storage times. Furthermore these two types of errors have different probabilities of 0-> 1 or 1-> 0 bit flips. We utilize the characteristics of the two types of errors in the development of ECC schemes for applications that have different storage times. In both cases, we first apply Gray coding and 2-bit interleaving. The corresponding most significant bit (MSB) and least significant bit (LSB) sub-page has only one type of dominating error (0-> 1 or 1-> 0). Next we form a product code using linear block code along rows and even parity check along columns to detect all the possible error locations. We develop an algorithm to choose errors among the possible error locations based on the dominant error type. Performance simulation and hardware implementation results show that the proposed solutions have the same performance as BCH codes with larger error correction capability but with significantly lower hardware overhead. For instance, for a 2KB MLC Flash used in long storage time applications, the proposed ECC scheme has 50% lower energy and 60% lower decoding latency compared to the BCH scheme.