Detecting over programmed memory

A non-volatile semiconductor memory system (or other type of memory system), a memory cell is programmed by changing the threshold voltage of the memory cell. Because of the difference in the programming speed of different memory cells in the system, there is a possibility that some of the memory cell is over programmed. That is, for example, the threshold voltage may be moved out of the range of the intended value or values. The invention includes determining whether the memory cell is over programmed.