Electrically erasable and programmable nonvolatile semiconductor memory device having data holding function and data holding method

A nonvolatile semiconductor memory device comprising:

• nonvolatile memory cells capable of storing data;
• an address generating circuit for generating a row address and column address based on address data;
• a decoder for selecting a memory cell which is to be subjected to the read/write operation based on the row address and column address;
• a readout circuit for reading out data from the memory cell selected by said decoder;
• a write circuit for writing data into the memory cell selected by said decoder;
• a voltage control circuit for controlling potentials of word lines connected to said memory cells;
• a verify data latch circuit for latching data read out by said readout circuit by use of a word line potential at the program verify time set by said voltage control circuit;
• a data comparing circuit for comparing data latched by said verify data latch circuit with data read out by said readout circuit by use of the word line potential which is set by said voltage control circuit to be lower than that at the program verify time and higher than that at the read time;
• and
• an operation control circuit for controlling whether or not the write operation by said write circuit is to be effected according to the result of comparison by said data comparing circuit.

A data holding method of a nonvolatile semiconductor memory device, for holding data stored in nonvolatile memory cells comprising:

• a first readout step of reading out data of the memory cell by use of a first word line potential;
• a second readout step of reading out data of the memory cell by use of a second word line potential;
• a comparing step of comparing data read out in said first readout step with data read out in said second readout step;
• and
• a step of writing data into the memory cell according to the result of comparison in said comparing step.

A data readout method of a nonvolatile semiconductor memory device having a data holding method for holding data stored in nonvolatile memory cells, comprising the steps of:

• starting the data readout method;
• determining whether or not a preset period of time has elapsed after the operation of the data readout method is started;
• starting the data holding method when it is determined by said determining step that the preset period of time has elapsed;
• and
• restoring the data readout method after execution of the data holding method is terminated.

A storage medium on which a data holding program for holding data stored in memory cells constructing a nonvolatile semiconductor memory device and which can be read by a computer, the data holding program comprising:

• a first readout step of reading out data of the memory cell by use of a first word line potential;
• a second readout step of reading out data of the memory cell by use of a second word line potential;
• a comparing step of comparing data read out in said first readout step with data read out in said second readout step;
• and
• a step of writing data into the memory cell according to the result of comparison in said comparing step.