Write circuits for analog memory

Abstract

Circuits and processes write and read analog signals in non-volatile memory cells such as EPROM and flash EPROM cells. One write circuit to a control gate of a memory cell a voltage which after a long write time would saturate the threshold voltage of the memory cell near a target threshold voltage being written. One such write circuit includes a voltage shifter which generates the voltage to be applied to the control gate of the memory cell from an analog voltage representing a value to be written in the memory cell. A verify feedback circuit terminates the write when the target threshold voltage is reached. The write circuit uses variable write pulse widths, voltages, and loadline resistances to reduce write time and further improve control of writing. The fast write time of EPROM and flash EPROM cells simplifies control of write processes and therefore reduces chip size and cost in applications such as sound recording. Reading a memory cell's threshold voltage uses substantially the same circuit as used in the verify feedback during a write. One read process determines a memory cell's threshold voltage by slowly ramping the control gate voltage and sensing when the cell conducts. Read and write processes disclosed also apply to binary memory cells, multi-level digital memory cells, and other applications which require precise reading or writing of threshold voltages.