Apparatus and method for refreshing a flash memory unit

摘要

In a Flash memory unit, the storage of a logic signal in the memory cells is determined by performing a READ operation. The NORMAL READ operation requires that the floating gate store an amount of charge QNR above which a logic "0" is identified and below which a logic "1" is identified as being stored in the memory cell. A second level of charge QTR stored on the floating gate is used in a TEST READ operation. The stored charge QTR is greater than the stored charge QNR, but less than the charge stored on the floating gate as the result of a WRITE operation. The result of a TEST READ operation is compared with a NORMAL READ operation of a memory cell. When the logic state identified by the TEST READ operation and the NORMAL READ operation are not the same, the charge on the cell is determined to have decayed below a prescribed level and the memory cell is refreshed to the level that is present during a WRITE operation.