Mixed data encoding EEPROM system

Claim 1

For an array of electrically erasable and programmable read only memory cells, each cell capable of being programmed to a "programmed" state or erased to an "erased" state, said array being organized into groups of cells, and each group having data programmed therein according to either a first data encoding or a second data encoding, said first data encoding having first and second logical states of data represented respectively by the "erased" state and the "programmed" state, and said second data encoding having the representations of first and second logical states reversed relative to the "programmed" and "erased" states of the first data encoding, a method for determining in a read operation on a group whether the data therein has been programmed according to the first or the second data encoding comprising the steps of: when data is being programmed into the group, encoding and storing an error correction code ("ECC") for at least a portion of the data; in a subsequent read operation on the group, retrieving said at least a portion of the data and the ECC therefor by assuming either the first or the second data encoding; decoding the ECC for said at least a portion of the data; and determining the data in the group has been programmed using the assumed data encoding when a first condition is true that an ECC error thereof is absent.