Patent ID: 8687421

Claim:
A method of operating a non-volatile memory system, the memory system including a controller circuit and one or more non-volatile memory circuits having a plurality of physical units of one or more blocks, the method comprising: maintaining a set of read parameters for use in reading operations of memory cells of the memory circuits, the set of read parameters including read parameters for a default read case and read parameters for a plurality of distinct, shifted read cases; for each of the physical units, maintaining a correspondence between the physical unit and a which of the read cases to use when reading the physical unit; performing a read operation for a page of data using the corresponding read case for the physical unit to which page belongs; determining whether the read operation was successful or yielded a fail result; in response to a read operation yielding a fail result, selecting a shifted read case at which the page can be read; and based on the shifted read case selected, determining whether to refresh the block to which the page belongs.