Codes for Limited Magnitude Error Correction in Multilevel Cell Memories

Multilevel cell (MLC) memories have been advocated for increasing density at low cost in next generation memories. However, the feature of several bits in a cell reduces the distance between levels; this reduced margin makes such memories more vulnerable to defective phenomena and parameter variations, leading to an error in stored data. These errors typically are of limited magnitude, because the induced change causes the stored value to exceed only a few of the level boundaries. To protect these memories from such errors and ensure that the stored data is not corrupted, Error Correction Codes (ECCs) are commonly used. However, most existing codes have been designed to protect memories in which each cell stores a bit and thus, they are not efficient to protect MLC memories. In this paper, an efficient scheme that can correct up to magnitude-3 errors is presented and evaluated. The scheme is based by combining ECCs that are commonly used to protect traditional memories. In particular, Interleaved Parity (IP) bits and Single Error Correction and Double Adjacent Error Correction (SEC-DAEC) codes are utilized; both these codes are combined in the proposed IP-DAEC scheme to efficiently provide a strong coding function for correction, thus exceeding the capabilities of most existing coding schemes for limited magnitude errors. The SEC-DAEC code is used to detect the cell in error and correct some bits, while the IP bits identify the remaining erroneous bits in the memory cell. The use of these simple codes results in an efficient implementation of the decoder compared to existing techniques as shown by the evaluation results presented in this paper. The proposed scheme is also competitive in terms of number of parity check bits and memory redundancy. Therefore, the proposed IP-DAEC scheme is a very efficient alternative to protect and correct MLC memories from limited magnitude errors.