SYSTEMS AND METHODS FOR DETERMINING LOGICAL VALUES OF COUPLED FLASH MEMORY CELLS

• US 20100211724A1
• Filed: 09/17/2008
• Published: 08/19/2010
• Est. Priority Date: 09/20/2007
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: DensBits Technologies Ltd. (Broadcom, Inc.)
• Inventors: Weingarten, Hanan
• Granted Patent: US 8,650,352 B2
• Time in Patent Office: Days
• US Class Current: 711/103
• CPC Class Codes: G11C 11/5642 Sensing or reading circuits; Data output circuits
• CPC Class Codes: G11C 2211/5634 Reference cells

Abstract

Systems and methods for determining program levels useful for reading cells of a flash memory, such as but not limited to detecting charge levels for the cells, obtaining joint conditional probability densities for a plurality of combinations of program levels of the cells; and determining program levels for the cells respectively such that an aggregated joint probability value of the joint conditional probability densities is maximized.

Claims

1. A method for reading cells of a flash memory, said method comprising:

comparing a charge level of a first individual cell to a first plurality of charge level boundary points, thereby to determine a first program level of said first individual cell, and subsequently comparing a second charge level of a second individual cell to a second plurality of charge level boundary points,

wherein said first plurality of boundary points depends on at least one charge level of at least one neighbor of said first cell and said second plurality of boundary points depends on at least one charge level of at least one neighbor of said second cell such that said first and second pluralities of boundary points differ.

2. A method according to claim 1, further comprising:

obtaining joint conditional probability densities for a plurality of combinations of program levels of said first individual cell; and

determining program levels for said first individual cell and at least one neighbor of said first cell such that an aggregated joint probability value of said joint conditional probability densities is maximized.

3. A method according to claim 1, further comprising:

a) obtaining a linear dependency function which refers to at least the individual cell and at least one neighbor of said first cell;

b) computing an estimated charge level for the first individual cell based on the linear dependency function; and

c) determining program level for said first individual cell and at least one neighbor of said first cell such that the aggregated standard variation between the estimated charge level of the first individual cell and a detected charge level of said first individual cell is minimized.

4. A method according to claim 1, wherein said comparing further comprising:

a) obtaining initial charge level boundary point values; and

b) comparing a charge level of the first individual cell to said initial charge level boundary point values.

5. A method according to claim 4, further comprising:

determining program levels for said first individual cell and at least one neighbor of said first cell based on charge levels, including, for each of at least one individual cell of said first individual cell and at least one neighbor of said first cell, perform iteratively, at least once;

i) computing new charge level boundary point values for the measurement of charge level of said individual cell based on the previous detected charge level of said individual cell and on previous determined program levels of said individual cell and of at least one neighbor;

ii) detecting charge levels for said individual cell based on said new threshold values; and

iii) determining a new program level for said individual cell based on the last detected charge levels of said individual cell.

6. A method according to claim 1, wherein said comparing further comprising:

a) obtaining initial charge level boundary point values; and

b) comparing a charge level of the first individual cell to said initial charge level boundary point values.

7. A method according to claim 6, further comprising:

a) determining voltage regions for said first individual cell and at least one neighbor of said first cell based on charge levels, including, for each of at least one individual cell of said first individual cell and at least one neighbor of said first cell, performing the following operations iteratively, at least once;

i) computing new charge level boundary point values for the measurement of charge levels of said individual cell based on the determined voltage regions of said individual cell and at least one neighbors;

ii) detecting charge levels for said individual cell based on said new charge level boundary point values; and

iii) determining a new voltage region for at least said individual cell based on the last determined charge level of said individual cell and the determined voltage regions of at least one other cell, and

b) determining program level of at least one individual cell based on the last determined voltage regions.

46. A method according to claim 5 wherein said detecting comprises comparing several cells simultaneously against a group of thresholds, said group including one or more of said new threshold values.

8. A method for determining program levels useful for reading cells of a flash memory, said method comprising:

a) detecting charge levels for said cells;

b) obtaining joint conditional probability densities for a plurality of combinations of program levels of said cells; and

c) determining program levels for said cells respectively such that an aggregated joint probability value of said joint conditional probability densities is maximized.

9. A method according to claim 8 wherein determining comprises:

a) determining programmed levels for each cell base on predefined thresholds; and

b) iteratively, until a criterion is met;

for each cell determine a program level for which the probability value of said cell'"'"'s joint conditional probability density is maximized, given the program levels of the other cells.

10. A method according to claim 8 wherein the joint conditional probability density for at least one individual cell comprises a probability that said cell contains at least one particular charge level given that a particular n-tuplet of program levels were respectively programmed into a corresponding n-tuplet of cells, said n-tuplet including said at least one individual cell and at least one neighboring cell in its vicinity.

11. A method according to claim 10 wherein the joint conditional probability density for a plurality of individual cells comprises a joint probability that said plurality of cells respectively contain a plurality of particular charge levels given that a particular n-tuplet of program levels were respectively programmed into a corresponding n-tuplet of cells, said n-tuplet including said plurality of cells and at least one neighboring cell in a vicinity of at least one of said plurality of cells.

12. A method according to claim 9 wherein the criterion includes a limitation on the number of iterations.

13. A method for determining program levels useful for reading cells of a flash memory, said method comprising:

a) detecting charge levels for said cells;

b) obtaining a linear dependency function which refers to at least one of the cells and at least one cell coupled thereto;

c) computing an estimated charge level based on the linear dependency function; and

d) determining program levels for said cells respectively such that the aggregated standard variation between a corresponding estimated charge level and the detected charge level of each cell is minimized.

14. A method for determining program levels useful for reading cells of a flash memory, said method comprising:

a) obtaining charge level initial threshold values;

b) detecting charge levels for said cells using said initial threshold values;

c) determining a program level for each of said cells based on the detected charge levels, including, for each of at least one individual of said cells, performing the following operations iteratively, at least once;

i) computing new threshold values for the measurement of charge level of said individual cell based on the previous detected charge level of said individual cell and on previous determined program levels of said individual cell and of at least one cell adjacent thereto;

ii) detecting charge levels for said individual cell based on said new threshold values; and

iii) determining a new program level for said individual cell based on the last detected charge levels of said individual cell.

15. The method according to claim 14, wherein said computing comprises:

For each individual one of said cells;

a) obtaining the last determined program levels of at least one other cell;

b) obtaining the corresponding joint conditional probability densities; and

c) computing new threshold values for the cell according to maximum likelihood criteria.

16. The method according to claim 14, wherein a charge level of at least one of said cells depends at least on the cell'"'"'s programmed value and the programmed value of one additional cell and the corresponding joint conditional probability densities have the following form:

17. The method according to claim 14, wherein a charge level of at least one of said cells depends on the cell'"'"'s programmed value and the programmed value of one additional cell and the corresponding joint conditional probability density functions have the following form:

18. The method according to claim 14 wherein if said computing new threshold values results in at least one threshold value that was already used in a previous iteration for said individual cell then skipping the operation of detecting charge levels for said individual cell based on the threshold value that was already used.

19. A method for determining program levels useful for reading cells of a flash memory, said method comprising:

a) obtaining charge level initial threshold values;

b) detecting charge levels for said cells using said initial threshold values;

c) determining voltage regions for each of said cells based on the detected charge levels,

d) for each of at least one individual of said cells, performing the following operations iteratively, at least once;

i) computing new threshold values for the measurement of charge levels of said individual cell based on the detected voltage regions of said individual cell and at least one cell adjacent thereto;

ii) detecting charge levels for said individual cell based on said new threshold values; and

iii) determining a new voltage region for at least said individual cell based on the last determined charge level of said individual cell and the determined voltage regions of at least one other cell, and

e) determining the program levels based on the last determined voltage regions.

20. The method according to claim 19, wherein said computing comprises:

for each individual one of said cells;

a) obtaining last determined voltage regions;

b) obtaining the corresponding region based joint conditional probability densities; and

c) computing new threshold values for the cell according to maximum likelihood criteria.

21. The method according to claim 19, wherein said computing further comprises, for each individual one of said cells, choosing the threshold values for the measurement of charge levels such that the aggregated entropy of the determined program levels of said cells is minimized given the last detected charge levels.

22. The method according to claim 19, wherein said computing further comprises, for all of said cells simultaneously, choosing the threshold values for the measurement of charge levels such that the aggregated entropy of the determined program levels of said cells is minimized given the last detected charge levels.

23. A method of reading cells of a flash memory, said method comprising:

sorting each of a plurality of cells into several categories depending on whether the cells'"'"' charge levels are above or below predetermined charge level boundary points, wherein said boundary points are independent of the charge levels in said cells; and

determining each cell'"'"'s program level as a function of at least the category to which it and at least one neighboring cell thereof, belong.

24. A method according to claim 23, wherein said determining further comprising:

obtaining joint conditional probability densities for a plurality of combinations of program levels of said cells; and

determining program levels for said cells respectively such that an aggregated joint probability value of said joint conditional probability densities is maximized.

25. A method according to claim 23, wherein said determining further comprising:

obtaining a linear dependency function which refers to at least one of the cells and at least one cell coupled thereto;

computing an estimated charge level based on the linear dependency function; and

determining program levels for said cells respectively such that the aggregated standard variation between a corresponding estimated charge level and detected charge level of each cell is minimized.

26. A method according to claim 23, wherein said determining further comprises:

determining a program level for each of said cells based on the detected category, including, for each of at least one individual of said cells, performing the following operations iteratively, at least once;

i) computing new charge level boundary points for the measurement of charge level of said individual cell based on the previous detected charge level of said individual cell and on previous determined program levels of said individual cell and of at least one of the neighboring cells;

ii) detecting charge levels for said individual cell based on said new charge level boundary points; and

iii) determining a new program level for said individual cell based on the last detected charge level of said individual cell.

27. A method according to claim 23, wherein said determining further comprises:

a) determining voltage regions for each of said cells based on the detected charge levels,

b) for each of at least one individual of said cells, performing the following operations iteratively, at least once;

i) computing new charge level boundary points for the measurement of charge levels of said individual cell based on the detected voltage regions of said individual cell and at least one cell adjacent thereto;

ii) detecting charge levels for said individual cell based on said charge level boundary points; and

iii) determining a new voltage region for at least said individual cell based on the last determined charge level of said individual cell and the determined voltage regions of at least one other cell, and

c) determining the program levels based on the last determined voltage regions.

28. A system for reading cells of a flash memory, said system comprising:

a micro-controller; and

an array of cells

wherein said micro-controller is operatively configured to;

compare a charge level of a first individual cell to a first plurality of charge level boundary points, thereby to determine a first program level of said first individual cell, and subsequently

compare a second charge level of a second individual cell to a second plurality of charge level boundary points,

wherein said first plurality of boundary points depends on at least one charge level of at least one neighbor of said first cell and said second plurality of boundary points depends on at least one charge level of at least one neighbor of said second cell such that said first and second pluralities of boundary points differ.

29. A system for determining program levels useful for reading cells of a flash memory, said system comprising:

a micro-controller; and

an array of cells

wherein said micro-controller is operatively configured to;

detect charge levels for said cells;

obtain joint conditional probability densities for a plurality of combinations of program levels of said cells; and

determine program levels for said cells respectively such that an aggregated joint probability value of said joint conditional probability densities is maximized.

30. A system for determining program levels useful for reading cells of a flash memory, said system comprising:

a micro-controller; and

an array of cells

wherein said micro-controller is operatively configured to;

a) detect charge levels for said cells;

b) obtain a linear dependency function which refers to at least one of the cells and at least one cell coupled thereto;

c) compute an estimated charge level based on the linear dependency function;

d) determine program levels for said cells respectively such that the aggregated standard variation between a corresponding estimated charge level and the detected charge level of each cell is minimized.

31. A system for determining program levels useful for reading cells of a flash memory, said system comprising:

a micro-controller; and

an array of cells

wherein said micro-controller is operatively configured to;

a) obtain charge level initial threshold values;

b) detect charge levels for said cells using said initial threshold values;

c) determine a program level for each of said cells based on the detected charge levels, including, for each of at least one individual of said cells, performing the following operations iteratively, at least once;

i) compute new threshold values for the measurement of charge level of said individual cell based on the previous detected charge level of said individual cell and on previous determined program levels of said individual cell and of at least one cell adjacent thereto;

ii) detect charge levels for said individual cell based on said new threshold values; and

iii) determine a new program level for said individual cell based on the last detected charge levels of said individual cell.

32. A system for determining program levels useful for reading cells of a flash memory, said system comprising:

a micro-controller; and

an array of cells

wherein said micro-controller is operatively configured to;

a) obtain charge level initial threshold values;

b) detect charge levels for said cells using said initial threshold values;

c) determine voltage regions for each of said cells based on the detected charge levels,

d) for each of at least one individual of said cells, performing the following operations iteratively, at least once;

i) compute new threshold values for the measurement of charge levels of said individual cell based on the detected voltage regions of said individual cell and at least one cell adjacent thereto;

ii) detect charge levels for said individual cell based on said new threshold values; and

iii) determine a new voltage region for at least said individual cell based on the last determined charge level of said individual cell and the determined voltage regions of at least one other cell, and

e) determine the program levels based on the last determined voltage regions.

33. A system for reading cells of a flash memory, said system comprising:

a micro-controller; and

an array of cells

wherein said micro-controller is operatively configured to;

a) sort each of a plurality of cells into several categories depending on whether the cells'"'"' charge levels are above or below predetermined charge level boundary points, wherein said boundary points are independent of the charge levels in said cells; and

b) determine each cell'"'"'s program level as a function of at least the category to which it and at least one neighboring cell thereof, belong.

34. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform a method for reading cells of a flash memory, said method comprising:

comparing a charge level of a first individual cell to a first plurality of charge level boundary points, thereby to determine a first program level of said first individual cell, and subsequently

comparing a second charge level of a second individual cell to a second plurality of charge level boundary points,

wherein said first plurality of boundary points depends on at least one charge level of at least one neighbor of said first cell and said second plurality of boundary points depends on at least one charge level of at least one neighbor of said second cell such that said first and second pluralities of boundary points differ.

35. A computer program product comprising a computer useable medium having computer readable program code embodied therein for reading cells of a flash memory, said computer program product comprising:

computer readable program code for causing the computer to compare a charge level of a first individual cell to a first plurality of charge level boundary points, thereby to determine a first program level of said first individual cell, and subsequently

computer readable program code for causing the computer to compare a second charge level of a second individual cell to a second plurality of charge level boundary points,

wherein said first plurality of boundary points depends on at least one charge level of at least one neighbors of said first cell and said second plurality of boundary points depends on at least one charge level of at least one neighbors of said second cell such that said first and second pluralities of boundary points differ.

36. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform a method for determining program levels useful for reading cells of a flash memory, said method comprising:

a) detecting charge levels for said cells;

b) obtaining joint conditional probability densities for a plurality of combinations of program levels of said cells; and

c) determining program levels for said cells respectively such that an aggregated joint probability value of said joint conditional probability densities is maximized.

37. A computer program product comprising a computer useable medium having computer readable program code embodied therein for determining program levels useful for reading cells of a flash memory, said computer program product comprising:

computer readable program code for causing the computer to detect charge levels for said cells;

computer readable program code for causing the computer to obtain joint conditional probability densities for a plurality of combinations of program levels of said cells; and

computer readable program code for causing the computer to determine program levels for said cells respectively such that an aggregated joint probability value of said joint conditional probability densities is maximized.

38. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform a method for determining program levels useful for reading cells of a flash memory, said method comprising:

a) detecting charge levels for said cells;

b) obtaining a linear dependency function which refers to at least one of the cells and at least one cell coupled thereto;

c) computing an estimated charge level based on the linear dependency function; and

d) determining program levels for said cells respectively such that the aggregated standard variation between a corresponding estimated charge level and the detected charge level of each cell is minimized.

39. A computer program product comprising a computer useable medium having computer readable program code embodied therein for determining program levels useful for reading cells of a flash memory, said computer program product comprising:

computer readable program code for causing the computer to detect charge levels for said cells;

computer readable program code for causing the computer to obtain a linear dependency function which refers to at least one of the cells and at least one cell coupled thereto;

computer readable program code for causing the computer to compute an estimated charge level based on the linear dependency function; and

computer readable program code for causing the computer to determine program levels for said cells respectively such that the aggregated standard variation between a corresponding estimated charge level and the detected charge level of each cell is minimized.

40. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform a method for determining program levels useful for reading cells of a flash memory, said method comprising:

a) obtaining charge level initial threshold values;

b) detecting charge levels for said cells using said initial threshold values; and

c) determining a program level for each of said cells based on the detected charge levels, including, for each of at least one individual of said cells, performing the following operations iteratively, at least once;

i) computing new threshold values for the measurement of charge level of said individual cell based on the previous detected charge level of said individual cell and on previous determined program levels of said individual cell and of at least one cell adjacent thereto;

ii) detecting charge levels for said individual cell based on said new threshold values; and

iii) determining a new program level for said individual cell based on the last detected charge levels of said individual cell.

41. A computer program product comprising a computer useable medium having computer readable program code embodied therein for determining program levels useful for reading cells of a flash memory, said computer program product comprising:

computer readable program code for causing the computer to obtain charge level initial threshold values;

computer readable program code for causing the computer to detect charge levels for said cells using said initial threshold values; and

computer readable program code for causing the computer to determine a program level for each of said cells based on the detected charge levels, including, for each of at least one individual of said cells, performing the following operations iteratively, at least once;

i) compute new threshold values for the measurement of charge level of said individual cell based on the previous detected charge level of said individual cell and on previous determined program levels of said individual cell and of at least one cell adjacent thereto;

ii) detect charge levels for said individual cell based on said new threshold values; and

iii) determine a new program level for said individual cell based on the last detected charge levels of said individual cell.

42. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform a method for determining program levels useful for reading cells of a flash memory, said method comprising:

a) obtaining charge level initial threshold values;

b) detecting charge levels for said cells using said initial threshold values;

c) determining voltage regions for each of said cells based on the detected charge levels,

d) for each of at least one individual of said cells, performing the following operations iteratively, at least once;

i) computing new threshold values for the measurement of charge levels of said individual cell based on the detected voltage regions of said individual cell and at least one cell adjacent thereto;

ii) detecting charge levels for said individual cell based on said new threshold values; and

iii) determining a new voltage region for at least said individual cell based on the last determined charge level of said individual cell and the determined voltage regions of at least one other cell, and

e) determining the program levels based on the last determined voltage regions.

43. A computer program product comprising a computer useable medium having computer readable program code embodied therein for determining program levels useful for reading cells of a flash memory, said computer program product comprising:

computer readable program code for causing the computer to obtain charge level initial threshold values;

computer readable program code for causing the computer to detect charge levels for said cells using said initial threshold values;

computer readable program code for causing the computer to determine voltage regions for each of said cells based on the detected charge levels,

computer readable program code for causing the computer to perform iteratively, at least once, for each of at least one individual of said cells;

i) compute new threshold values for the measurement of charge levels of said individual cell based on the detected voltage regions of said individual cell and at least one cell adjacent thereto;

ii) detect charge levels for said individual cell based on said new threshold values; and

iii) determine a new voltage region for at least said individual cell based on the last determined charge level of said individual cell and the determined voltage regions of at least one other cell, and

computer readable program code for causing the computer to determine the program levels based on the last determined voltage regions.

44. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform a method of reading cells of a flash memory, said method comprising:

sorting each of a plurality of cells into several categories depending on whether the cells'"'"' charge levels are above or below predetermined charge level boundary points, wherein said boundary points are independent of the charge levels in said cells; and

determining each cell'"'"'s program level as a function of at least the category to which it and at least one neighboring cell thereof, belong.

45. A computer program product comprising a computer useable medium having computer readable program code embodied therein of reading cells of a flash memory, said computer program product comprising:

computer readable program code for causing the computer to sort each of a plurality of cells into several categories depending on whether the cells'"'"' charge levels are above or below predetermined charge level boundary points, wherein said boundary points are independent of the charge levels in said cells; and

computer readable program code for causing the computer to determine each cell'"'"'s program level as a function of at least the category to which it and at least one neighboring cell thereof, belong.

47. A method for determining program levels useful for reading cells of a flash memory, said method comprising:

a) detecting charge levels for said cells;

b) obtaining a set of coefficients α

j that minimize the aggregated standard variation (L₁−

{circumflex over (L)}₁);

c) iteratively, as long as a criterion is met;

computing an estimation of the charge level for each of said cells according to;

48. A method according to claim 47 wherein the criterion includes a limitation on the number of iterations.