| 摘要 |
<P>PROBLEM TO BE SOLVED: To further improve the reliability of a nonvolatile semiconductor memory device. <P>SOLUTION: In a memory part, a plurality of memory cells are arranged in which at least four data states are defined according to the difference in a threshold level and a value composed of a plurality of bits is assigned to each of the plurality of data states. A controller sets at least one bit out of the plurality of bits, which are stored by each of the memory cells, as an error correction bit indicating the "normal" or "abnormal" state; and sets the other bits as data bits that store data. In four consecutive data states which are obtained when the plurality of data states are arranged in the order of the size of the threshold level, "normal" is assigned for error correction bits of data states having the lowest and highest threshold levels; and "abnormal" is assigned for error correction bits of two data states having intermediate threshold levels. Data of the error correction bits of the memory cells is periodically read; and if a read value is "abnormal", an error correction bit of the corresponding memory cell is reset to "normal". <P>COPYRIGHT: (C)2013,JPO&INPIT |
