Nonvolatile semiconductor memory device and method of rewriting data thereof

1. A nonvolatile semiconductor memory device, comprising:

a memory cell array having a plurality of memory cells; and

a writing controller for writing data to said memory cell array, wherein

said memory cell array stores data in a nonvolatile state with respect to each memory cell pair composed of two memory cells, the data storing being executed based on a difference in memory information between said two memory cells,

said writing controller is capable of individually setting said memory information for each of said memory cells of said memory cell array,

each of said memory cells of said memory cell array is a memory cell transistor having a control gate and a floating gate,

said writing controller sets said memory information of said memory cell by setting a threshold voltage of said memory cell of said memory cell array, and

said nonvolatile semiconductor memory device further comprising a plurality of differential sense amplifiers, each of which for amplifying an output difference of said two memory cells included in the corresponding memory cell pair for outputting, wherein

said writing controller executes writing data of the same value to plural pairs of said memory cell pairs, and

said nonvolatile semiconductor memory device further comprises a decision element, wherein

said decision element obtains a majority logic based on outputs from differential sense amplifiers among said plurality of differential sense amplifiers, each of said differential sense amplifiers being provided in a correspondence relationship to each of said plural pairs of said memory cell pairs, thereby to decide the data written to said plural pairs of said memory cell pairs for outputting.

2. The nonvolatile semiconductor memory device according to claim 1 , wherein said writing controller is capable of setting the threshold voltage of said memory cells of said memory cell array to three or more levels.

3. A nonvolatile semiconductor memory device, comprising:

a memory cell array having a plurality of memory cells; and

a writing controller for writing data to said memory cell array, wherein

said memory cell array stores data in a nonvolatile state with respect to each memory cell pair composed of two memory cells, the data storing being executed based on a difference in memory information between said two memory cells,

said writing controller is capable of individually setting said memory information for each of said memory cells of said memory cell array,

each of said memory cells of said memory cell array is a memory cell transistor having a control gate and a floating gate,

said writing controller sets said memory information of said memory cell by setting a threshold voltage of said memory cell of said memory cell array, and

said nonvolatile semiconductor memory device further comprising a plurality of differential sense amplifiers, each of which for amplifying an output difference of said two memory cells included in the corresponding memory cell pair for outputting,

the nonvolatile semiconductor memory device further comprising:

a reading controller for reading data from said memory cell array; and

a detector for detecting levels of a first voltage and a second voltage in comparison with a reference voltage set for respective first and second voltages, wherein

said first voltage is a voltage supplied to said plurality of differential sense amplifiers as a supply voltage,

said second voltage is a voltage obtained by boosting said first voltage and is a preceding voltage of a gate voltage supplied to said control gate upon reading data from said memory cell array,

said detector detects whether both said first voltage and second voltage increase above said respective reference voltages, and further detects whether at least one of said first voltage and second voltage drops below said respective reference voltages, and

said reading controller newly executes data reading from said memory cell array when said detector detects that at least one of said first voltage and second voltage drops below said respective reference voltages while data is read from said memory cell array, and detects at a later time that both said first voltage and second voltage increase above said respective reference voltages.

4. The nonvolatile semiconductor memory device according to claim 3 , wherein

said detector comprises a voltage detector for detecting whether said first voltage level increases above said reference voltage and further detecting whether said first voltage level drops below said reference voltage, and

said voltage detector is supplied with said second voltage as a supply voltage.

5. The nonvolatile semiconductor memory device according to claim 3 , wherein said writing controller is capable of setting the threshold voltage of said memory cells of said memory cell array to three or more levels.

6. A method of rewriting data of a nonvolatile semiconductor memory device, said nonvolatile semiconductor memory device comprising a memory cell array having a plurality of memory cells, and a writing controller for writing data to said memory cell array, wherein

said memory cell array stores data in a nonvolatile state with respect to each memory cell pair composed of two memory cells, the data storing being executed based on a difference in memory information between said two memory cells,

each of the memory cells of said memory cell array is a memory cell transistor having a control gate and a floating gate,

said writing controller is capable of setting a threshold voltage of each of said memory cells of said memory cell array to three or more levels,

said writing controller sets said memory information of each of said memory cells individually by setting the threshold voltage of each of said memory cells of said memory cell array, and

said nonvolatile semiconductor memory device further comprising a plurality of differential sense amplifiers for amplifying an output difference of two memory cells included in the corresponding memory cell pair for outputting,

said method of rewriting data comprises the steps of:

(a) writing data to said memory cell pair; and

(b) rewriting said data written in said step (a), wherein in step (a), the threshold voltage of one memory cell included in said memory cell pair is set to a lower level than that of the threshold voltage of the other memory cell in said memory cell pair, while said threshold voltage of said other memory cell is set to a level chosen from said three or more levels except the greatest one, and

in step (b), the threshold voltage of said one memory cell is increased to a level greater than the threshold voltage of said other memory cell.