Wear Leveling Techniques for Flash Memory EEPROM Systems

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Wear Leveling Techniques for Flash Memory EEPROM
Systems

• Flash Memory
• 2006. 2
• Lofgren et al.

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Contents

• Motivation
• Background
• Wear Leveling Technique
• Summary

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1. Motivation
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Motivation

• flash EEPROM (electrically erasable and
  programmable read only memory)
• Advantage
• non-volatility
• speed
• ease of erasure and reprogramming
• small physical size and similar factors
• Disadvantage
• limited life time(10,000)
• Goal
• maximize the service lifetime of an entire mass
  storage EEPROM system

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Summary of the Invention

• EEPROM array of cell
• is devided two or more interchangeable banks of
  cells
• bank has one or more blocks of cells
• Interchange (wear leveling)
• allow for extending overall memory system
  lifetime
• without having to provide such replacement groups
  of memory cells

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2. Background
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General Computer System

• include flash EEPROM memory

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Memory Operation Technique
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Logical/Physical Memory Bank Char. Usage

• the total number of blocks of memory with which
  the computer system is working at the moment
• record may be maintained of the number of times
  that the block was written
• a total number of block writing cycles that have
  been initiated
• the total number of cycles experienced by the
  interchangeable banks
• maintain an identification of the banks having
  the minimum and the maximum number of cycles

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3. Wear Leveling Technique
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Wear Leveling Operation

• be triggered by that difference exceeding a
  certain magnitude
• when wear leveling is accomplished,
• data is transferred between the most heavily used
  and least used banks
• ex) when bank 0 is very larger number of erase
  and rewrite cycles than bank 1, both can be
  swapped
• the address translation table is updated for bank
  1 and bank 0

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Outline of Wear Leveling System

• Memory Controller and EEPROM Banks

the memory system address translation table
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The Nature of each Block

• a field 73 includes a count of number of time
  that the block has been erased and rewritten

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The Process Flow Diagram
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Example of Swap Banks
spare bank
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Example of Swap Blocks (Cont.)
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Example of Swap Blocks (Cont.)
spare bank
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4. Summary
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Summary

• the header of each erase unit includes an erase
  counter
• spare erase unit
• when the difference of counter exceeds a
  threshold, wear leveling technique swap between
  the most worn-out unit and the least worn-out
  unit