READING A FLASH MEMORY BY JOINT DECODING AND CELL VOLTAGE DISTRIBUTION TRACKING

• US 20090319868A1
• Filed: 03/19/2009
• Published: 12/24/2009
• Est. Priority Date: 06/23/2008
• Status: Active Grant

• Current Assignee: Ramot at Tel Aviv University Limited (Tel Aviv University)
• Original Assignee: Ramot at Tel Aviv University Limited (Tel Aviv University)
• Inventors: SHARON, Eran, LITSYN, Simon, ALROD, Idan
• Granted Patent: US 8,458,563 B2
• Time in Patent Office: Days
• US Class Current: 714/763
• CPC Class Codes: G06F 11/1068 in sector programmable memories, e.g. flash disk G06F11/1072 takes precedence
• CPC Class Codes: G11C 11/5642 Sensing or reading circuits; Data output circuits
• CPC Class Codes: G11C 16/26 Sensing or reading circuits; Data output circuits

Abstract

To read a plurality of memory cells, each cell is assigned to a respective cell population. A respective value of an operational parameter of each cell is measured. Each cell is assigned an a-priori metric based at least in part on one or more CVD parameter values of the cell'"'"'s population. The a-priori metrics are decoded. Based at least in part on the resulting a-posteriori metrics, the CVD parameter values are corrected, without repeating the measurements of the cell operational parameter values. The operational parameter values are indicative of bit patterns stored in the cells, and the correction of the CVD parameter values is constrained by requiring the bit patterns collectively to be a valid codeword.

Claims

1. A method of reading a plurality of memory cells, comprising:

(a) assigning each cell to a respective one of a plurality of cell populations;

(b) measuring a respective value of an operational parameter of each cell;

(c) providing a value of at least one first respective cell distribution model parameter of each population;

(d) assigning each cell a respective a priori metric, based at least in part on the respective at least one cell distribution model parameter value of the cell'"'"'s respective population;

(e) decoding the a priori metrics, thereby providing, for each cell, a respective a posteriori metric; and

(f) based at least in part on the a posteriori metrics, and without repeating the measuring of the respective values of the operational parameters;

estimating a value of at least one second respective cell distribution model parameter of each population.

2. The method of claim 1, wherein the operational parameters are threshold voltages of the cells.

3. The method of claim 1, wherein, for at least one population, the providing of the value of the at least one first respective cell distribution model parameter of that population is based at least in part on the measured values of the operational parameters of the cells of that population.

4. The method of claim 1, wherein, for each cell, the assigning of the a priori metric of that cell is based in part on the value of the operational parameter of that cell.

5. The method of claim 1, wherein a representation of an error correction codeword is stored in the cells, and wherein the estimating is contingent on the decoding failing to produce a valid error correction codeword.

6. The method of claim 1, wherein the decoding also provides, for each cell, a respective estimate of at least one bit that is stored in that cell.

7. The method of claim 1, wherein the a posteriori metrics are extrinsic a posteriori metrics.

8. The method of claim 1, wherein the metrics include log likelihood ratios.

9. The method of claim 1, wherein the metrics are cell state metrics.

10. The method of claim 1, wherein the metrics are bit metrics.

11. The method of claim 1, wherein the assigning of each cell to a respective population is at least in part according to estimated cross-coupling of the each cell with at least one neighbor of the each cell.

12. The method of claim 1, wherein the assigning of each cell to a respective population is at least in part according to estimated program disturb of the each cell by at least one neighbor of the each cell.

13. The method of claim 1, wherein the assigning of each cell to a respective population is at least in part according to a respective measured value of an operational parameter of at least one neighbor of the each cell.

14. The method of claim 1, wherein, for at least one population, the first and second cell distribution model parameters are identical.

15. The method of claim 1, wherein, for at least one population, the first and second cell distribution model parameters are different.

16. A memory device comprising,

(a) a plurality of cells;

(b) circuitry for measuring a respective value of an operational parameter of each cell; and

(c) a controller operative;

(i) to instruct the circuitry to measure the values of the operational parameters,

(ii) to assign each cell to a respective one of a plurality of cell populations,

(iii) to provide a value of at least one first respective cell distribution model parameter of each population,

(iv) to assign each cell a respective a priori metric, based at least in part on the respective at least one cell distribution model parameter value of the cell'"'"'s respective population;

(v) to decode the a priori metrics, thereby providing, for each cell, a respective a posteriori metric, and

(vi) to estimate a value of at least one second respective cell distribution model parameter of each population, based at least in part on the a posteriori metrics and without repeating the instructing of the circuitry to measure the operational parameter values.

17. A memory controller, for a memory that includes a plurality of cells, the memory controller comprising:

(a) a measurement functional block for obtaining from the memory, for each cell, a measurement of a respective value of an operational parameter of the cell;

(b) an assignment functional block for assigning each cell to a respective one of a plurality of cell populations;

(c) an a priori metric functional block for assigning each cell a respective a priori metric;

(d) a decoder for decoding the a priori metrics, thereby providing, for each cell, a respective a posteriori metric; and

(e) a cell distribution model functional block for;

(i) providing a value of at least one first respective cell distribution model parameter of each population, the a priori metric of each cell then being based at least in part on the respective at least one cell distribution model parameter value of the cell'"'"'s respective population, and

(ii) estimating a value of at least one second respective cell distribution model parameter of each population, based at least in part on the a posteriori metrics and without repeating the obtaining of the operational parameter value measurements from the memory.

18. A system for managing a first memory that includes a plurality of cells, the system comprising:

(a) a second memory for storing code for;

(i) obtaining, from the first memory, for each cell, a measurement of a respective value of an operational parameter of the cell;

(ii) assigning each cell to a respective one of a plurality of cell populations,

(iii) providing a value of at least one first respective cell distribution model parameter of each population,

(iv) assigning each cell a respective a priori metric, based at least in part on the respective at least one cell distribution model parameter value of the cell'"'"'s respective population,

(v) decoding the a priori metrics, thereby providing, for each cell, a respective a posteriori metric, and

(vi) estimating a value of at least one second respective cell distribution model parameter of each population, based at least in part on the a posteriori metrics, and without repeating the obtaining of the operational parameter value measurements from the first memory; and

(b) a processor for executing the code.

19. A computer readable storage medium having computer readable code embodied on the computer readable storage medium, the computer readable code for managing a memory that includes a plurality of cells, the computer readable code comprising:

(a) program code for obtaining from the memory, for each cell, a measurement of a respective value of an operational parameter of the cell;

(b) program code for assigning each cell to a respective one of a plurality of cell populations;

(c) program code for assigning each cell a respective a priori metric;

(d) program code for decoding the a priori metrics, thereby providing, for each cell, a respective a posteriori metric; and

(e) program code for;

(i) providing a value of at least one first respective cell distribution model parameter of each population, the a priori metric of each cell then being based at least in part on the respective at least one cell distribution model parameter value of the cell'"'"'s respective population, and

(ii) estimating a value of at least one second respective cell distribution model parameter of each population, based at least in part on the a posteriori metrics and without repeating the obtaining of the operational parameter value measurements from the memory.

31. The method of claim 19, further comprising the step of:

(f) based at least in part on the at least one first cell distribution model parameter value, correcting the respective values of the operational parameters for program disturb;

and wherein the assigning of the respective a priori metrics is based at least in part on the corrected values of the operational parameters.

32. The method of claim 19, further comprising the step of:

(f) for each cell, correcting the respective value of the operational parameter of the each cell, based at least in part on the at least one first cell distribution parameter value and on a respective measured value of an operational parameter of at least one neighbor of the each cell;

and wherein the assigning of the respective a priori metrics is based at least in part on the corrected respective values of the operational parameters.

20. A method of reading a plurality of memory cells, comprising:

(a) measuring a respective value of an operational parameter of each cell;

(b) providing a value of at least one first cell distribution model parameter of the plurality of memory cells;

(c) assigning each cell a respective a priori metric, based at least in part on the at least one first cell distribution model parameter value;

(d) decoding the a priori metrics, thereby providing, for each cell, a respective a posteriori metric; and

(e) based at least in part on the a posteriori metrics, and without repeating the measuring of the respective values of the operational parameters;

estimating a value of at least one second cell distribution model parameter of the plurality of memory cells.

21. The method of claim 20, wherein the operational parameters are threshold voltages of the cells.

22. The method of claim 20, wherein the providing of the value of the at least one first cell distribution model parameter is based at least in part on the measured values of the operational parameters.

23. The method of claim 20, wherein, for each cell, the assigning of the a priori metric of that cell is based in part on the value of the operational parameter of that cell.

24. The method of claim 20, wherein a representation of an error correction codeword is stored in the cells, and wherein the estimating is contingent on the decoding failing to produce a valid error correction codeword.

25. The method of claim 20, wherein the decoding also provides, for each cell, a respective estimate of at least one bit that is stored in that cell.

26. The method of claim 20, wherein the a posteriori metrics are extrinsic a posteriori metrics.

27. The method of claim 20, wherein the metrics include log likelihood ratios.

28. The method of claim 20, wherein the metrics are cell state metrics.

29. The method of claim 20, wherein the metrics are bit metrics.

30. The method of claim 20, further comprising the step of:

(f) based at least in part on the at least one first cell distribution model parameter value, correcting the respective values of the operational parameters for cross coupling;

and wherein the assigning of the respective a priori metrics is based at least in part on the corrected values of the operational parameters.

33. The method of claim 20, wherein the first and second cell distribution model parameters are identical.

34. The method of claim 20, wherein the first and second cell distribution model parameters are different.

35. A memory device comprising:

(a) a plurality of cells;

(b) circuitry for measuring a respective value of an operational parameter of each cell; and

(c) a controller operative;

(i) to instruct the circuitry to measure the values of the operational parameters,

(ii) to provide a value of at least one first cell distribution model parameter of the plurality of cells,

(iii) to assign each cell a respective a priori metric, based at least in part on the at least one cell distribution model parameter value;

(iv) to decode the a priori metrics, thereby providing, for each cell, a respective a posteriori metric, and

(v) to estimate a value of at least one second respective cell distribution model parameter of the plurality of cells, based at least in part on the a posteriori metrics and without repeating the instructing of the circuitry to measure the operational parameter values.

36. A memory controller, for a memory that includes a plurality of cells, the memory controller comprising:

(a) a measurement functional block for obtaining from the memory, for each cell, a measurement of a respective value of an operational parameter of the cell;

(b) an a priori metric functional block for assigning each cell a respective a priori metric;

(c) a decoder for decoding the a priori metrics, thereby providing, for each cell, a respective a posteriori metric; and

(d) a cell distribution model functional block for;

(i) providing a value of at least one first cell distribution model parameter of the plurality of cells, the a priori metric of each cell then being based at least in part on the at least one cell distribution model parameter value, and

(ii) estimating a value of at least one second respective cell distribution model parameter of the plurality of cells, based at least in part on the a posteriori metrics and without repeating the obtaining of the operational parameter value measurements from the memory.

37. A system for managing a first memory that includes a plurality of cells, the system comprising:

(a) a second memory for storing code for;

(i) obtaining, from the first memory, for each cell, a measurement of a respective value of an operational parameter of the cell;

(ii) providing a value of at least one first cell distribution model parameter of the plurality of cells,

(iii) assigning each cell a respective a priori metric, based at least in part on the at least one cell distribution model parameter value,

(iv) decoding the a priori metrics, thereby providing, for each cell, a respective a posteriori metric, and

(v) estimating a value of at least one second respective cell distribution model parameter of the plurality of cells, based at least in part on the a posteriori metrics, and without repeating the obtaining of the operational parameter value measurements from the first memory; and

(b) a processor for executing the code.

38. A computer readable storage medium having computer readable code embodied on the computer readable storage medium, the computer readable code for managing a memory that includes a plurality of cells, the computer readable code comprising:

(a) program code for obtaining from the memory, for each cell, a measurement of a respective value of an operational parameter of the cell;

(b) program code for assigning each cell a respective a priori metric;

(c) program code for decoding the a priori metrics, thereby providing, for each cell, a respective a posteriori metric; and

(d) program code for;

(i) providing a value of at least one first cell distribution model parameter of the plurality of cells, the a priori metric of each cell then being based at least in part on the at least one cell distribution model parameter value, and

(ii) estimating a value of at least one second cell distribution model parameter, based at least in part on the a posteriori metrics and without repeating the obtaining of the operational parameter value measurements from the memory.

39. A method of reading a plurality of memory cells comprising:

(a) for each cell;

(i) measuring a respective value, of an operational parameter of the cell, that is indicative of a bit pattern stored in the cell, and

(ii) assigning the cell to a respective one of a plurality of cell populations;

(b) estimating a value of at least one respective cell distribution model parameter of each cell population; and

(c) constraining the estimating by requiring the bit patterns collectively to be a valid error correction codeword without repeating the measuring of the respective values of the operational parameters of any of the cells.

40. The method of claim 39, wherein the operational parameters are threshold voltages of the cells.

41. The method of claim 39, wherein, for at least one population, the estimating of the value of the at least one respective cell distribution model parameter of that population is based at least in part on the measured values of the operational parameters of the cells of that population.

42. A memory device comprising:

(a) a plurality of cells;

(b) circuitry for measuring a respective value, of an operational parameter of each cell, that is indicative of a bit pattern stored in the cell; and

(c) a controller operative;

(i) to instruct the circuitry to measure the values of the operational parameters,

(ii) to assign each cell to a respective one of a plurality of cell populations, and

(iii) to estimate a value of at least one respective cell distribution model parameter of each cell population, the estimating being constrained by requiring the bit patterns collectively to be a valid error correction codeword without repeating the instructing of the circuitry to measure the operational parameter values.

43. A memory controller, for a memory that includes a plurality of cells, the memory controller comprising:

(a) a measurement functional block for obtaining from the memory, for each cell, a measurement of a respective value, of an operational parameter of the cell, that is indicative of a bit pattern stored in the cell;

(b) an assignment functional block for assigning each cell to a respective one of a plurality of cell populations; and

(c) an estimation functional block for estimating a value of at least one respective cell distribution model parameter of each cell population, the estimating being constrained by requiring the bit patterns collectively to be a valid error correction codeword without repeating the measurement of the operational parameter values.

44. A system for managing a first memory that includes a plurality of cells, the system comprising:

(a) a second memory for storing code for;

(i) obtaining, from the first memory, for each cell, a measurement of a respective value, of an operational parameter of the cell, that is indicative of a bit pattern stored in the cell;

(ii) assigning each cell to a respective one of a plurality of cell populations, and

(iii) estimating a value of at least one respective cell distribution model parameter of each cell population, the estimating being constrained by requiring the bit patterns collectively to be a valid error correction codeword without repeating the instructing of the circuitry to measure the operational parameter values; and

(b) a processor for executing the code.

45. A computer readable storage medium having computer readable code embodied on the computer readable storage medium, the computer readable code for managing a memory that includes a plurality of cells, the computer readable code comprising:

(a) program code for obtaining from the memory, for each cell, a measurement of a respective value, of an operational parameter of the cell, that is indicative of a bit pattern stored in the cell;

(b) program code for assigning each cell to a respective one of a plurality of cell populations; and

(c) program code for estimating a value of at least one respective cell distribution model parameter of each cell population, the estimating being constrained by requiring the bit patterns collectively to be a valid error correction codeword without repeating the measurement of the operational parameter values.

46. A method of reading a plurality of memory cells, comprising:

(a) for each cell, measuring a respective value, of an operational parameter of the cell, that is indicative of a bit pattern stored in the cell;

(b) estimating a value of at least one cell voltage distribution parameter of the plurality of memory cells; and

(c) constraining the estimating by requiring the bit patterns collectively to be a valid error correction codeword without repeating the measuring of the respective values of the operational parameters of any of the cells.

47. The method of claim 46, wherein the operational parameters are threshold voltages of the cells.

48. The method of claim 46, wherein the estimating of the value of the at least one cell distribution model parameter is based at least in part on the measured values of the operational parameters.

49. A memory device comprising:

(a) a plurality of cells;

(b) circuitry for measuring a respective value, of an operational parameter of each cell, that is indicative of a bit pattern stored in the cell; and

(c) a controller operative;

(i) to instruct the circuitry to measure the values of the operational parameters, and

(ii) to estimate a value of at least one cell distribution model parameter of the plurality of cells, the estimating being constrained by requiring the bit patterns collectively to be a valid error correction codeword without repeating the instructing of the circuitry to measure the operational parameter values.

50. A memory controller, for a memory that includes a plurality of cells, the memory controller comprising:

(a) a measurement functional block for obtaining from the memory, for each cell, a measurement of a respective value, of an operational parameter of the cell, that is indicative of a bit pattern stored in the cell; and

(b) an estimation functional block for estimating a value of at least one cell distribution model parameter of the plurality of cells, the estimating being constrained by requiring the bit patterns collectively to be a valid error correction codeword without repeating the measurement of the operational parameter values.

51. A system for managing a first memory that includes a plurality of cells, the system comprising:

(a) a second memory for storing code for;

(i) obtaining, from the first memory, for each cell, a measurement of a respective value, of an operational parameter of the cell, that is indicative of a bit pattern stored in the cell; and

(ii) estimating a value of at least one cell distribution model parameter of the plurality of cells, the estimating being constrained by requiring the bit patterns collectively to be a valid error correction codeword without repeating the measurement of the operational parameter values; and

(b) a processor for executing the code.

52. A computer readable storage medium having computer readable code embodied on the computer readable storage medium, the computer readable code for managing a memory that includes a plurality of cells, the computer readable code comprising:

(a) program code for obtaining from the memory, for each cell, a measurement of a respective value, of an operational parameter of the cell, that is indicative of a bit pattern stored in the cell; and

(b) program code for estimating a value of at least one cell distribution model parameter of the plurality of cells, the estimating being constrained by requiring the bit patterns collectively to be a valid error correction codeword without repeating the measurement of the operational parameter values.