Patent ID: 7382660

Claim:
A method for accessing a multilevel nonvolatile NAND memory device, comprising: multi-level programming the memory device by writing a first and a second bit in a selected cell of said memory device, the multi-level programming including the steps of: programming the first bit in said selected cell, said step of programming the first bit including shifting said selected cell from a first threshold voltage to a second threshold voltage; and separately programming the second bit in said selected cell, said step of separately programming further including the steps of: preliminarily reading the selected cell to determine whether said first bit has been modified, performing a first writing by bringing said selected cell to a third threshold voltage if said first bit has been modified, and performing a second writing by bringing said selected cell to a fourth threshold voltage different from said third threshold voltage, if said first bit has not been modified, wherein said memory device comprises: a first portion including cells storing plural threshold voltages corresponding to a plurality of bits; and a second portion including cells storing two threshold voltages corresponding to a single bit, wherein said step of preliminary reading said selected cell includes a step of forcing a reading result to correspond to a threshold voltage when said selected cell belongs to said second portion.