Methods and apparatus for providing error correction to unwritten pages and for identifying unwritten pages in flash memory

Abstract

Provided are methods for error correction coding (ECC) for flash memory pages which have been erased but have not been programmed. In one method, each ECC code word is bitwise inverted before being programmed into a page, and bitwise inverted again after being read back from the page before entering the decoder. Thus an unwritten page, whose bits are all ones when random errors are absent, appears to the decoder as all zeros, which form a valid code word(s) in linear block codes. In another method, in both page programming and page read, the parity section of each ECC code word is bitwise XORed with the complement of a parity calculated from a message whose bits are all ones. Thus an unwritten page appears to the decoder as a valid ECC code word(s) when random errors are absent. Further provided is an apparatus for determining after a page read whether or not the read page has been programmed by comparing the number of read code word symbols which do not have the default erased value and the maximum number of symbol errors correctable by the ECC.