Constrained Flash memory programming

In NAND Flash memory featuring multi-level cells (MLC), the width of threshold voltage distributions about their nominal values affects the permissible number of levels and thus storage capacity. Unfortunately, inter-cell coupling causes a cell´s charge to affect its neighbors´ sensed threshold voltage, resulting in an apparent broadening of these distributions. We present a novel approach, whereby the data written to Flash is constrained, e.g., by forbidding certain adjacent-cell level combinations, so as to limit the maximum voltage shift and thus narrow the distributions. To this end, we present a new family of constrained codes. Our technique can serve for capacity enhancement (more levels) or for improving endurance, retention and bit error rate (wider guard bands between adjacent levels). It may also be combined with various programming order techniques that mitigate the inter-cell coupling effects and with decoding techniques that compensate for them.