Two Bit Overlap: A Class of Double Error Correction One Step Majority Logic Decodable Codes

Error Correction Codes (ECCs) are commonly used to protect memories against soft errors with an impact on memory area and delay. For large memories, the area overhead is mostly due to the additional cells needed to store the parity check bits. In terms of delay, the overhead is mostly needed to detect and correct errors when the data is read from the memory. Most ECCs that can correct more than one error have a complex decoding process and so are limited in high speed memory applications. One exception is One Step Majority Logic Decodable (OS-MLD) codes for which decoding can be done in parallel at high speed. Unfortunately, there are only a few OS-MLD codes that provide a limited choice in terms of block sizes, error correction capabilities and code rate. Therefore, there is considerable interest in a novel construction of OS-MLD codes to provide additional choices for protecting memories. In this paper, a new method to construct Double Error Correction (DEC) OS-MLD codes is presented. This method is based on the use of parity check matrices in which two bits have at most two parity check equations in common; the proposed method provides codes that require a smaller number of parity check bits than existing codes like Orthogonal Latin Square (OLS) codes. The drawback of the proposed Two Bit Overlap (TBO) codes is that they require slightly more complex decoding than OLS codes. Therefore, they provide an intermediate solution between OLS and non OS-MLD codes in terms of decoding delay and number of parity check bits. The proposed TBO codes have been implemented for some block sizes and compared to both OLS and BCH codes to illustrate the trade off in delay and memory overhead. Finally, this paper discusses the generalization of the proposed scheme to codes with larger error correction capabilities.