The Read And Write Voltage Optimization And Programming Study Of NAND Flash Memory

NAND flash memory has been widely used due to the advantages such as high read/write speed,low power consumption,small size,and portability.With the increasing integration of flash memory devices and the development of multi-level cell storage technology,the information stored in flash memory is increasingly subject to interference.This thesis discusses ways to improve the reliability of flash storage data from different perspectives.First of all,a new method for cell-to-cell interference in flash memory is proposed.This method writes row by row to a flash memory cell that is on an odd word line,and then uses the index modulation to perform a write operation on the even word line cell.The index modulation divides the to-be-written bit sequence into two parts;one part is used to determine the position of the flash memory cell activated on the word line,and the other part determines the voltage states of activated flash cells.The simulation results show that the performance of the proposed index modulation method is greatly improved compared with the traditional method of writing,and to a certain extent,it can achieve unequal error protection of stored data.Secondly,an optimization method of programming voltage is proposed for the existence of random telephone noise and retention process noise in flash memory.This method first calculates the bit error rate expression of the flash storage data through the flash channel model,and optimizes the programming voltage through the one-dimension blind-walking optimization method.Simulations show that the error rate of a flash memory channel with optimized programming voltage is better than that of a flash memory channel without optimized programming voltage.Finally,to deal with long-term storage of flash information,a read voltage optimization method is proposed.Firstly,setting the input probability of each state information be identical by balanced coding,and then obtain the overall voltage distribution function of the flash memory according to the voltage distribution function of the erased state and each programming state given by flash channel model.Then,according to the dynamic threshold voltage obtained by the balance encoding,the data retention time of the flash information is estimated,and the optimized read voltage is obtained through the equal relationship between the two adjacent write state distribution functions.The simulation results show that the reliability of the flash memory to store information for a long time is greatly improved by the proposed method.