SOFT-DECISION COMPENSATION FOR FLASH CHANNEL VARIATION

• US 20150169402A1
• Filed: 10/06/2014
• Published: 06/18/2015
• Est. Priority Date: 08/04/2012
• Status: Active Grant

• Current Assignee: Seagate Technology LLC (Seagate Technology Holdings Plc)
• Original Assignee: Seagate Technology LLC (Seagate Technology Holdings Plc)
• Inventors: WU, Yunxiang, COHEN, Earl T.
• Granted Patent: US 9,483,347 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G06F 11/1068 in sector programmable memories, e.g. flash disk G06F11/1072 takes precedence
• CPC Class Codes: G11C 29/52 Protection of memory contents; Detection of errors in memory contents

Abstract

In an SSD controller reading from flash memory, subsequent to failure of an initial soft-decision decoding attempt based on a nominal LLR, soft-decision re-decoding attempts are made using compensated LLR soft-decision information sets, pre-calculated at respective read-equilibrium points corresponding to mean shifts and variance change in the actual charge-state distributions of the flash memory channel. According to embodiment, soft-decision re-decoding attempts are performed without a retry read, or overlapped with one or more retry reads. By overlapping re-decoding with one or more retry reads, the probability of successful decoding increases, the need for further retry reads diminishes, and throughput is improved. The LLR compensation becomes very effective over a large number of retry reads, improving decoding reliability and achieving close to optimal bit error rates, even in the presence of large channel variation.

Claims

1. A method, comprising:

performing an initial read and an initial attempted ECC decoding, for each of a plurality of flash memory locations;

performing up to a first plurality of ECC soft-decision redecoding attempts based on the initial read, subsequent to failure of the initial attempted ECC decoding, for at least some of the locations; and

wherein each ECC soft-decision redecoding attempt uses a selected one of a plurality of log likelihood ratio (LLR) soft-decision information sets corresponding to a respective one of a plurality of read-equilibrium points.