Flexible product code-based ECC schemes for MLC NAND Flash memories

Abstract

Error control coding (ECC) is essential for correcting soft errors in Flash memories. In such memories, as the number of erase/program cycles increases over time, the number of errors increases. In this paper we propose a flexible product code based ECC scheme that can support ECC of higher strength when needed. Specifically, we propose product codes which use Reed-Solomon (RS) codes along rows and Hamming codes along columns. When higher ECC is needed, the Hamming code along columns is replaced by two shorter Hamming codes. For instance, when the raw bit error rate increases from 2.2*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> to 4.0*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , the proposed ECC scheme migrates from RS(127, 121) along rows and Hamming(72,64) along columns to RS(127, 121) along rows and two Hamming(39, 32) along columns to achieve the same BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> . While the resulting implementation has 12% higher decoding latency, it increases the lifetime of the device significantly.