Nonvolatile memory system for multiple value storing

Abstract

A nonvolatile memory system for one of a multiple of values includes a memory cell having an input terminal, an output terminal, and a control terminal. The memory cell, which may be an EEPROM, stores nonvolatile electric charge, and establishes a voltage threshold between the input terminal and the output terminal which influences a current therebetween, the threshold having a level which is dependent upon the amount of the electric charge stored by the storing means. A writing circuit is connected to the input terminal and is responsive to an input data signal having a value selected from among at least three values for applying electric charge, in an amount corresponding to the value of the selected data signal, to the input terminal for storage in the memory cell. A reading circuit is provided to measure the value of the voltage threshold between the input and output terminals and to output a data signal having a value which corresponds to the measured value of the threshold voltage. In its preferred embodiments, the system is an electrically erasable programmable read only memory system which can store multiple values in a single memory cell.