Systems and methods for storing, retrieving, and adjusting read thresholds in flash memory storage system

Abstract

A method, system and computer-readable medium are provided for reading information from a memory unit. A read instruction may be received to read information from a set of memory cells in the memory unit. A data structure storing sets of read thresholds may be searched for a set of read thresholds based on one or more characteristic value(s) of the set of memory cells. If the set of read thresholds is found, the set of memory cells may be read to execute the read instruction using the found set of read thresholds. The set of read thresholds may be thresholds which were previously used to successfully read a set of cells having the same or similar characteristic value(s).

Description

REFERENCE TO RELATED APPLICATIONS[0001]This patent application claims priority from U.S. provisional patent application Ser. No. 61 / 361,626 filed Jul. 6, 2010, which is incorporated herein by reference in its entirety.BACKGROUND OF THE INVENTION[0002]Flash memory devices may store information in the form of an electrical charge which may be programmed into cells by injecting electrons between electrically isolated floating-gates of the cells, where the electrons may be trapped by the insulating properties of the floating-gates. In single-level cell (SLC) devices, each cell may either be programmed with charge or may remain un-programmed (erased), thus effectively defining two binary states to store one bit of information per cell. Multi level cell (MLC) devices may store more than one bit per cell by applying electrical charge to the floating gates in one of multiple (n) levels. Thus, an (n)-level flash cell, where n=2̂k, may store k bits of information, where each combination of k ...

AI Technical Summary

Benefits of technology

[0007]An embodiment of the invention may provide a method, system and computer-readable medium for reading information from a memory unit. A read instruction may be received to read information from a set of memory cells in the memory unit. A data structure storing sets of read thresholds may be searched for a set of read thresholds based on one or more c

Problems solved by technology

During the life span of a flash memory device, the insulating properties of the floating-gates may degrade and the charge levels of cells may shift to other values, which may be referred to as “charge loss.” Charge loss or other errors in the charge stored in cells may result from programming errors (e.g., either intentional or not), age, storage temperature, repeated use of program / erase cycles, retention and various other reasons.

Charge loss may cause the cell voltage to decrease causing a spontaneous change in their programming levels generating errors in the flash memory device.

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