Flash memory device with physical cell value deterioration accommodation and methods useful in conjunction therewith
View Patent ↗A method for determining thresholds useful for converting cell physical levels into cell logical values in an array of digital memory cells storing physical levels which diminish over time, the method comprising determining extent of deterioration of the physical levels and determining thresholds accordingly for at least an individual cell in said array; and reading the individual cell including reading a physical level in the cell and converting the physical level into a logical value using the thresholds, wherein the determining comprises storing predefined physical levels rather than data-determined physical levels in each of a plurality of cells and computing extent of deterioration by determining deterioration of the predefined physical levels.
1. A system for determining thresholds useful for converting cell physical levels into cell logical values in an array of memory cells storing physical levels which diminish over time, the system comprising: deterioration determining apparatus determining extent of deterioration of the physical levels and determining thresholds accordingly for at least an individual cell in said array; and reading circuitry operative to read said individual cell including reading a physical level in said cell and converting said physical level into a logical value using said thresholds, wherein said deterioration determining apparatus is operative to store predefined physical levels rather than data-determined physical levels in each of a plurality of cells and to compute extent of deterioration by determining deterioration of said predefined physical levels wherein said plurality of cells comprises a number of cells selected to as to achieve a level of reading errors which does not exceed a predetermined acceptable level; wherein said reading comprises using error correction code to eliminate errors and wherein said predetermined acceptable level comprises a predetermined maximum proportion of pages containing so many errors as to be uncorrectable by said error correction code.
2. The system of claim 1 wherein said predefined physical levels are maximum physical levels.
3. The system of claim 1 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming that said deterioration is a known function of said physical levels.
4. The system of claim 3 wherein said known function comprises a linear function.
5. The system of claim 1 wherein said storing comprises storing said predefined physical levels in cells whose reliability is no different than the reliability of those cells in the array which are not used for storing said predefined physical levels.
6. The system of claim 1 wherein said array comprises more than one erase sector, each of which includes a subset of cells which together undergo cycles affecting said extent of deterioration; and wherein said determining of said extent of deterioration is performed no more than once per erase sector.
7. The system of claim 1 wherein said array of digital memory cells comprises a plurality of pages characterized in that the cells within each page are written onto as a single operation and wherein said determining is performed no more than once per page.
8. The system of claim 1 wherein said determining comprises computing extent of deterioration by determining deterioration of said predefined physical levels in each of a plurality of cells.
9. The system of claim 1 wherein said reading comprises binary reading and wherein said predefined physical levels comprise an ascending sequence of physical levels.
10. The system of claim 9 wherein said ascending sequence is evenly spaced.
11. The system of claim 1 wherein said physical levels comprise charge levels.
12. The system of claim 1 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming a known function of deterioration as a function of program/erase cycle number.
13. The system of claim 1 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming a known function of deterioration as a function of address of said predefined physical levels.
14. A method for determining thresholds useful for converting cell physical levels into cell logical values in an array of memory cells storing physical levels which diminish over time, the method comprising: determining extent of deterioration of the physical levels and systematically determining thresholds accordingly for at least an individual cell in said array; and reading said individual cell including reading a physical level in said cell and converting said physical level into a logical value using said thresholds; wherein said systematic determination of thresholds comprises computation of thresholds; wherein said computation does not assume linearity of deterioration throughout the range of physical levels stored in individual cells in said array.
15. A method according to claim 14 wherein said systematic determination of thresholds comprises use of a look-up table.
16. A system for determining thresholds useful for converting cell physical levels into cell logical values in an array of memory cells storing physical levels which diminish over time, the system comprising: deterioration determining apparatus determining extent of deterioration of the physical levels and systematically determining thresholds accordingly for at least an individual cell in said array; and reading circuitry operative to read said individual cell including reading a physical level in said cell and converting said physical level into a logical value using said thresholds; wherein said systematic determination of thresholds comprises computation of thresholds; wherein said computation does not assume linearity of deterioration throughout the range of physical levels stored in individual cells in said array.
17. A method for converting a measured physical level of a cell into a logical value, in an array of memory cells storing physical levels which diminish over time, the method comprising: determining extent of deterioration of the physical levels and determining thresholds accordingly for at least an individual cell in the array; and reading the individual cell including reading a physical level in said cell and converting said physical level into a logical value using at least some of said thresholds, wherein said determining extent of deterioration comprises storing predefined physical levels rather than data-determined physical levels in each of a plurality of cells and determining extent of deterioration by computing deterioration of said predefined physical levels; wherein said plurality of cells comprises a number of cells selected to as to achieve a level of reading errors which does not exceed a predetermined acceptable level; wherein said reading comprises using error correction code to eliminate errors and wherein said predetermined acceptable level comprises a predetermined maximum proportion of pages containing so many errors as to be uncorrectable by said error correction code.
18. A method according to claim 17 wherein said predefined physical levels are maximum physical levels.
19. A method according to claim 17 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming that said deterioration is a known function of said physical levels.
20. A method according to claim 19 wherein said known function comprises a linear function.
21. A method according to claim 17 wherein said storing comprises storing said predefined physical levels in cells whose reliability is no different than the reliability of those cells in the array which are not used for storing said predefined physical levels.
22. A method according to claim 17 wherein said array comprises more than one erase sector, each of which includes a subset of cells which together undergo cycles affecting said extent of deterioration; and wherein said determining of said extent of deterioration is performed no more than once per erase sector.
23. A method according to claim 17 wherein said array of digital memory cells comprises a plurality of pages characterized in that the cells within each page are written onto as a single operation and wherein said determining is performed no more than once per page.
24. A method according to claim 17 wherein said determining comprises computing extent of deterioration by determining deterioration of said predefined physical levels in each of a plurality of cells.
25. A method according to claim 17 wherein said reading comprises binary reading and wherein said predefined physical levels comprise an ascending sequence of physical levels.
26. A method according to claim 25 wherein said ascending sequence is evenly spaced.
27. A method according to claim 17 wherein said physical levels comprise charge levels.
28. A method according to claim 17 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming a known function of deterioration as a function of program/erase cycle number.
29. A method according to claim 17 wherein said determining extent of deterioration comprises determining deterioration of said predefined physical levels and assuming a known function of deterioration as a function of address of said predefined physical levels.