Method and system for adaptively finding reference voltages for reading data from a MLC flash memory

1. A method for adaptively finding reference voltages for reading data from a MLC flash memory, the method comprising:

providing a first total number of cells of the flash memory above a first threshold voltage in a shifted threshold voltage distribution;

searching to find a second threshold voltage in an initial threshold voltage distribution such that a second total number of the cells above the second threshold voltage is approximate to the first total number, the second threshold voltage being different from the first threshold voltage; and

shifting an initial reference voltage or voltages of the initial threshold voltage distribution with an amount approximate to a voltage difference between the second threshold voltage and the first threshold voltage, thereby resulting in a new reference voltage or voltages for reading the data from the MLC flash memory.

2. The method of claim 1 , further comprising a step of subjecting the flash memory to a predetermined number of program or erase cycles or for a predetermined data retention elapsed time.

3. The method of claim 1 , wherein the second threshold voltage is a program verify voltage during a program phase.

4. The method of claim 1 , wherein the searching step comprises:

(1) setting a first word line;

(2) retrieving <N,V> data, wherein N represents a total number of cells above a threshold voltage V corresponding to the first word line;

(3) comparing the total number of cells N with the first total number to obtain a cell difference; and

(4) setting a next word line and repeating the steps (2) and (3) corresponding to the associated word line, until the cell difference is less than a predetermined value, thereby finding the second threshold voltage.

5. The method of claim 1 , wherein the initial reference voltage is shifted toward the shifted reference voltage that has a value less than the initial reference voltage.

6. The method of claim 1 , wherein the initial reference voltages are shifted respectively with the same voltage difference.

7. The method of claim 1 , before providing the first total number, further comprising:

requesting the flash memory by a read command.