Adaptive PCM Encoding for Partially-Defective Multi-Level Cells
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Solution Overview
Problem
Multi-level phase change memory (PCM) technology faces challenges in writing multiple resistance levels due to process variability and cell-to-cell variability, leading to limitations in writing all nominal levels in partially-defective cells, which restricts multilevel coding and affects storage capacity.
Innovation Solution
A method and encoder/decoder system that adaptively encode data words as either standard or excess codewords based on the state of cells, using q nominal levels or (q-d) levels, depending on the presence of partially-defective cells, to ensure reliable writing and decoding in PCM cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multi-level cell functionality is implemented to increase storage capacity, then cost per bit is reduced, but process variability and cell-to-cell variability cause deviations from intended resistance levels
Solution Approach 1:
The system dynamically adapts the coding scheme based on the actual state of the PCM cells. The encoder determines whether cells are fully functional or partially-defective and selects between standard codewords (for fully functional cells) and excess codewords (for partially-defective cells), making the storage system flexible and adaptive to manufacturing variations
Solution Approach 2:
The invention changes the parameter of codeword symbol values from q values to (q-d) values when dealing with partially-defective cells. This parameter adjustment allows the system to accommodate cells that cannot achieve all nominal resistance levels while maintaining reliable data storage
2Reliability
If adaptive iterative write scheme is used to compensate for variability, then reliability is improved, but partially-defective cells cannot support all nominal levels
Solution Approach 1:
The invention segments the codeword types into two categories: standard codewords for fully functional cells and excess codewords for partially-defective cells. This segmentation allows the system to treat different cell types differently, ensuring that each cell type uses an appropriate coding scheme that matches its capabilities
Solution Approach 2:
The encoder acts as an intermediary that mediates between the data to be stored and the PCM cells. It determines the appropriate codeword type based on cell state and selects the suitable coding scheme, bridging the gap between ideal multi-level storage and the reality of partially-defective cells
3Quantity of substance
If standard codewords with q symbol values are used, then storage capacity is maximized, but partially-defective cells cannot be utilized
Solution Approach 1:
The encoding system is designed to be universal by supporting both standard codewords and excess codewords. This multi-functionality allows the same storage system to handle both fully functional cells and partially-defective cells, maximizing the utilization of available storage resources while maintaining reliability
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 increases the storage capacity of solid state memory by utilizing partially-defective cells and improves reliability by using excess codewords to write data in cells that cannot support all nominal levels, enhancing the robustness of multi-level recording.
Implementation Method 1
Phase transformation, i.e., memory programming, can be enabled by Joule heating. In this regard, Joule heating can be controlled by a programming current or voltage pulse.
Implementation Method 2
PCM is a non-volatile solid state memory technology that exploits a reversible, thermally-assisted switching of certain chalcogenide and non-chalcogenide compounds between certain states of different electrical conductivity.
Data Source
AI summary
A method for encoding a data word for writing an encoded data word in N cells of a solid state memory. Each of the N cells can be programmed in one of q nominal levels. The method includes encoding the data word as a codeword of a first codeword type having q symbol values or as a codeword of a second codeword type having (q−d) symbol values, dε[1, . . . , q−1], depending on a state of the N cells.