Patent ID: 7236405

Claim:
A method for setting erasing pulses by determining the number of or time for applications of the erasing pulses in a block erasing operation where a target block which consists a plurality of memory cells to be erased in a nonvolatile memory is erased by applying the erasing pulses a plurality of times for erasing its block data together, the method comprising, in the process of applying the erasing pulses the plurality of times: an extraction step of extracting at least two pairs (Np 1 , Re 1 ) and (Np 2 , Re 2 ) of the accumulated number of the erasing pulses Np and the ratio Re of the number of erased memory cells in the target block to be erased after the accumulated number of the erasing pulses Np has been applied; a first conversion step of converting the two ratios Re 1 and Re 2 extracted in the extraction step and defined as two levels of a normal distribution probability into at least two normalized variables S(Re 1 ) and S(Re 2 ) respectively through normalizing random variables of the normal distribution probabilities with standard deviations; a second conversion step of converting the two accumulated numbers of the erasing pulses Np extracted in the extraction step into common logarithms Log(Np) respectively; an extrapolation step of calculating a common logarithm Log(Nt) through a point [Log(Nt), S(T)] including predetermined target normalized variables S(T) larger than the two normalized variables S(Re 1 ) and S(Re 2 ) from two sets of coordinates [Log(Np 1 ), S(Re 1 )] and [Log(Np 2 ), S(Re 2 )] determined in the first and second conversion steps; a third conversion step of converting the common logarithm Log(Nt) determined in the extrapolation step into an extrapolation erasing pulses number Nt; and a determination step of determining the number of or time for applications of the remaining erasing pulses so that the extrapolation erasing pulse number Nt determined in the third conversion step is a target accumulated number of applications of erasing pulses.