Memory reliability improvements based on maximized error-correcting codes

Error-correcting codes (ECC) offer an efficient way to improve the reliability and yield of memory subsystems. ECC-based protection is usually provided on a memory word basis such that the number of data-bits in a codeword corresponds to the amount of information that can be transferred during a single memory access operation. Consequently, the codeword length is not the maximum allowed by a certain check-bit number since the number of data-bits is constrained by the width of the memory data interface. This work investigates the additional error correction opportunities offered by the absence of a perfect match between the numbers of data-bits and check-bits in some of the most used ECCs. A method is proposed for the selection of multiple-bit errors which can become correctable with a minimal impact on decoder latency. Reliability improvements are evaluated for memories in which all errors affecting the same number of bits in a codeword are equally probable. It is shown that the application of the proposed methods to standard double-bit ECCs can improve the mean-time-to-failure (MTTF) of memories with up to 100%.