Semiconductor memory device for storing multivalued data
Data storage circuits are connected to the bit lines in a one-to-one correspondence. A write circuit writes the data on a first page into a plurality of first memory cells selected simultaneously by a word line. Thereafter, the write circuit writes the data on a second page into the plurality of first memory cell. Then, the write circuit writes the data on the first and second pages into second memory cells adjoining the first memory cells in the bit line direction.
1. A semiconductor memory device comprising:
a memory cell array configured by arranging a plurality of memory cells in a matrix, each of, which is capable of having one of four threshold voltages and storing two bits of data;
a plurality of bit lines, each of which is connected to said plurality of memory cells arranged in the column direction;
a plurality of word lines, each of which is connected to said plurality of memory cells arranged in the row direction;
a first flag cell and a second flag cell which are selected at the same time when the memory cells are selected by each of the word lines; and
a control section which, when writing the data in a second page composed of the two bits into the memory cells selected by the word line, writes specific data into the first and second flag cells and which, when reading the data in a first page from the memory cells, determines from the data read from the first flag cell whether the second page has been written into the memory cells and which, when reading the data in the second page from the memory cells, determines from the data read from the second flag cell whether the second page has been written into the memory cells.
2. The semiconductor memory device according to claim 1 , wherein said plurality of memory cells arranged in the column direction are connected in series to configure a NAND cell.
3. The semiconductor memory device according to claim 1 , wherein the first and the second flag cells include a plurality of cells, respectively; and
the control section causes the plurality of cells to store the same logic level in a write operation and decides the output of the first and the second flag cells by a majority of the outputs of the plurality of cells.
4. The device according to claim 1 , wherein the control section changes the threshold voltage of the memory cell from a first threshold voltage to one of the first threshold voltage and a second threshold voltage (the first threshold voltage<the second threshold voltage) by a first page write operation, and
when the threshold voltage of the memory cell is the first threshold voltage, the control section changes the threshold voltage of the memory cell to one of the first threshold voltage and a third threshold voltage (the first threshold voltage<the third threshold voltage) by a second page write operation, and
when the threshold voltage of the memory cell is the second threshold voltage, the control section changes the threshold voltage of the memory cell to one of a fourth threshold voltage and a fifth threshold voltage (the second threshold voltage≦the fourth threshold voltage, the fourth threshold voltage<the fifth threshold voltage) by a second page write operation.
5. The device according to claim 4 , wherein when data is written into the second page during the second page operation, the control section changes a threshold voltage of the first flag cell from a first threshold voltage to the fourth threshold voltage and which changes a threshold voltage of the second flag cell from the first threshold voltage to the third threshold voltage.
6. The device according to claim 5 , wherein when data is read from the first page, the control section reads data in the memory cell by a first read voltage (the third threshold voltage<the first read voltage≦the fourth threshold voltage), and when the threshold voltage of the first flag cell is the first threshold voltage, the control section reads the data in the memory cell by the second read voltage (the first threshold voltage<the second read voltage≦the second threshold voltage).
7. The device according to claim 6 , wherein when data is read from the second page, the control section reads data in the memory cell by a third read voltage and a fourth read voltage (the fourth threshold voltage<the third read voltage≦the fifth threshold voltage)(the first threshold voltage<the fourth read voltage≦the third threshold voltage), and when the threshold voltage of the second flag cell is the first threshold voltage, the control section changes the read data of the memory cell to be the same as data obtained when the threshold voltage of the memory cell is the first threshold voltage.