Odd/Even Invert Coding for PCM Thermal Crosstalk

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Solution Overview

Problem

Phase change memory (PCM) faces reliability issues due to limited write endurance and thermal crosstalk, which degrades performance and reduces its widespread adoption.

Innovation Solution

The odd/even invert coding algorithm addresses these issues by using a cost model that captures write asymmetries and thermal crosstalk, employing data encoding/decoding to minimize write activities and extend PCM lifetime through selective inverting of data bits, requiring only two bits for storage overhead.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If PCM cells are continuously set/reset to maintain functionality, then the memory can be rewritten, but the write endurance degrades and reliability decreases

Engineering Contradiction:

ImproverewritabilityVSAvoidwrite endurance

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The encoder performs preliminary analysis of the data transition patterns before writing to PCM. By predicting the number of bit flips and thermal crosstalk effects in advance, the system selects the optimal encoding scheme (even invert, odd invert, or no invert) to minimize write operations and extend PCM lifetime before the actual write occurs.

Inventive Principle:

Principle #10Preliminary action

2Speed

If high-current reset pulses are used to program PCM cells, then the programming speed increases, but thermal crosstalk affects adjacent cells and causes state changes

Engineering Contradiction:

Improveprogramming speedVSAvoidthermal crosstalk

Core Design Contradiction:

SpeedVSObject-affected harmful factors

Solution Approach 1:

The invention converts the harmful thermal crosstalk effect into a beneficial factor by using it to predict and prevent unwanted state changes. The encoder analyzes potential thermal crosstalk impacts and proactively adjusts the encoding of adjacent cells to compensate for expected thermal interference, thereby maintaining data integrity despite the presence of thermal crosstalk from high-speed programming operations.

Inventive Principle:

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

3Ease of operation

If data is written directly to PCM without encoding, then the write operation is simple, but write activities increase and PCM lifetime is reduced

Engineering Contradiction:

Improvewrite operation simplicityVSAvoidPCM lifetime

Core Design Contradiction:

Ease of operationVSDuration of action of stationary object

Solution Approach 1:

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system dynamically changes the encoding parameter (inversion type) based on the data characteristics. The encoder analyzes the input data and transitions between different encoding modes (no invert, even invert, odd invert) to optimize the number of write operations, thereby extending PCM lifetime while maintaining ease of operation through automated selection.

Inventive Principle:

Principle #35Parameter changes

4Duration of action of stationary object

If selective inverting encoding is implemented to reduce write activities, then PCM lifetime is extended, but additional storage overhead is required

Engineering Contradiction:

ImprovePCM lifetimeVSAvoidstorage overhead

Core Design Contradiction:

Duration of action of stationary objectVSQuantity of substance

Solution Approach 1:

The encoding scheme applies different inversion operations to different portions of the data based on local characteristics. Instead of inverting all data uniformly, the system selectively inverts only the necessary portions (even bits or odd bits) to achieve the desired reduction in write activities, thereby minimizing the overhead while maximizing the lifetime extension benefit.

Inventive Principle:

Principle #3Local quality

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach effectively reduces write activities and extends the lifetime of PCM by optimizing data encoding based on transition costs and thermal crosstalk considerations, improving the reliability and performance of PCM cells.

Implementation Method 1

In phase change memory (PCM), a phase change material, such as Germanium-Antimony-Telluride (GST) or other chalcogenide material, is used

Methodology Applied

Scientific EffectPhase change: Phase Change

Implementation Method 2

a critical reliability issue caused by PCM cell down scaling is thermal crosstalk, which leads to the change of a cell's state while its adjacent cells are being programmed by a high-current reset pulse

Methodology Applied

Scientific EffectThermal crosstalk: Conduction (thermal)

Data Source

PatentUS9891843B2Odd/even invert coding for phase change memory with thermal crosstalk

Publication Date: 2018.02.13 KUWAIT UNIV

AI summary

The odd/even invert coding for phase change memory with thermal crosstalk devises a cost model that captures Phase Change Memory (PCM) SET/RESET write asymmetries, as well as write disturbs caused by thermal crosstalk. The cost is computed by counting the different types of transitions between the old and the new data to be written to PCM. An Odd/Even Invert data encoding/decoding algorithm makes intelligent decisions based on a cost model by taking into account the number of bit flips, write asymmetry, as well as write disturbs. The data encoding algorithm recodes the data on the fly based on selective inverting (even, odd, or full invert) to search for a minimum cost solution with aim at reducing write activities and extending the PCM lifetime. A hardware architecture for the present encoding/decoding algorithm is presented that requires only two bits storage overhead for coding, regardless of the width of data.