Configurable ECC Scheme for Nonvolatile Memory Bad Column Management

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Solution Overview

Problem

Nonvolatile memory devices face challenges in efficiently utilizing defective bit lines, leading to uncorrectable bit error rates and resulting in unusable blocks due to the limitations of existing Error Correction Code (ECC) schemes, especially when the number of bad columns or bit lines exceeds the number of spare columns or bit lines.

Innovation Solution

Implementing a configurable ECC scheme that dynamically adjusts page sizes based on the number of defective columns, applying a first ECC scheme for fewer defects and a second ECC scheme with smaller pages when defects are more severe, allowing for continued data storage by optimizing error correction capacity and redundancy.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a first ECC scheme is applied to user data, then data storage capacity is maximized, but blocks with high numbers of bad columns cannot be used

Engineering Contradiction:

Improveerror correction capabilityVSAvoidusable block capacity

Core Design Contradiction:

ReliabilityVSProductivity

Solution Approach 1:

The patent applies a second ECC scheme dynamically when the number of unreplaced bad columns exceeds a threshold, making the ECC scheme adaptive to the actual defect conditions rather than using a fixed first ECC scheme for all blocks

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent changes the ECC scheme parameters (encoding method, page size, parity bytes) based on the number of bad columns detected, switching from a first ECC scheme to a second ECC scheme with different parameters to accommodate high-defect blocks

Inventive Principle:

Principle #35Parameter changes

2Reliability

If a second ECC scheme with smaller pages is applied to blocks with many bad columns, then error correction capacity increases, but unused space increases

Engineering Contradiction:

Improveerror correction capacityVSAvoidunused storage space

Core Design Contradiction:

ReliabilityVSLoss of substance

Solution Approach 1:

The patent applies the second ECC scheme with smaller pages locally only to the portions of blocks that contain unreplaced bad columns, while other portions can still use the first ECC scheme, thereby limiting the unused space to only where necessary

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent uses partial action by applying the second ECC scheme only when needed (when bad column threshold is exceeded) and only to the extent necessary to correct errors, rather than applying it universally to all data

Inventive Principle:

Principle #16Partial or excessive action

3Reliability

If spare columns are used to replace bad columns, then bit error rate decreases, but the number of replaceable blocks is limited

Engineering Contradiction:

Improvebit error rateVSAvoidblock usability

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The patent converts the harmful effect of having many bad columns (which would normally make a block unusable) into a benefit by detecting the high defect count and automatically switching to the second ECC scheme with higher error correction capacity, thereby making previously unusable blocks usable

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Data Source

PatentUS9792175B2Bad column management in nonvolatile memory

Publication Date: 2017.10.17 SANDISK TECHNOLOGIES LLC

AI summary

When the number of bad columns in a memory or plane is less than a threshold number then a first Error Correction Code (ECC) scheme encodes user data in first pages of a first size. If the number of bad columns is greater than the threshold number then a second ECC scheme encodes the user data in second pages of a second size that is smaller than the first size.