Minimal Maximum-Level Programming-Combined Cell Mapping and Coding for Faster MLC Memory

In multi-level-cell memory, such as flash and phase-change memory, shrinking cell size and the growing number of levels per cell worsen the access rate to capacity ratio and even reduce access rate. We present minimal maximum-level programming, a scheme for expediting cell programming by sharing physical cells among multiple data sectors and exploiting the fact that making moderate changes to a cell's charge level is faster than making large ones. In particular, we encode the data such that in the k th writing of data to a cell, only the lowest k+1 levels are utilized. Unlike in previously proposed cell-sharing schemes, different same-size data sectors occupy different numbers of physical cells, and a cell may hold a fraction of a bit of a given data sector. Nevertheless, the exposed sector size remains unchanged. Data are encoded, but without redundancy. In a four-level cell example, we achieve up to 75% reduction in write latency. Read latency may be degraded, depending on the percentage of utilized capacity.