METHOD FOR DECODING DATA IN NON-VOLATILE STORAGE USING RELIABILITY METRICS BASED ON MULTIPLE READS

• US 20080250300A1
• Filed: 03/29/2007
• Published: 10/09/2008
• Est. Priority Date: 03/29/2007
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: Sandisk Technologies Incorporated (Western Digital Corporation)
• Inventors: Chin, Henry, Zhao, Dengtao, Mokhlesi, Nima
• Granted Patent: US 7,904,793 B2
• Time in Patent Office: Days
• US Class Current: 714/780
• CPC Class Codes: G06F 12/00 Accessing, addressing or allocating within memory systems or architectures digital input from, or digital output to record carriers, e.g. to disk storage units, G06F3/06
• CPC Class Codes: G11C 11/5642 Sensing or reading circuits; Data output circuits
• CPC Class Codes: G11C 16/04 using variable threshold transistors, e.g. FAMOS
• CPC Class Codes: G11C 16/28 using differential sensing or reference cells, e.g. dummy cells
• CPC Class Codes: G11C 29/00 Checking stores for correct operation ; Subsequent repair; Testing stores during standby or offline operation
• CPC Class Codes: G11C 29/02 Detection or location of defective auxiliary circuits, e.g. defective refresh counters
• CPC Class Codes: G11C 29/028 with adaption or trimming of parameters
• CPC Class Codes: H03M 13/11 using multiple parity bits
• CPC Class Codes: H03M 13/37 Decoding methods or techniques, not specific to the particular type of coding provided for in groups H03M13/03 - H03M13/35

Abstract

Data stored in non-volatile storage is decoded using iterative probabilistic decoding and multiple read operations to achieve greater reliability. An error correcting code such as a low density parity check code may be used. In one approach, initial reliability metrics, such as logarithmic likelihood ratios, are used in decoding read data of a set of non-volatile storage element. The decoding attempts to converge by adjusting the reliability metrics for bits in code words which represent the sensed state. If convergence does not occur, e.g., within a set time period, the state of the non-volatile storage element is sensed again, current values of the reliability metrics in the decoder are adjusted, and the decoding again attempts to converge. In another approach, the initial reliability metrics are based on multiple reads. Tables which store the reliability metrics and adjustments based on the sensed states can be prepared before decoding occurs.

Claims

1. A method for decoding data in non-volatile storage, comprising:

performing multiple sense operations on at least one non-volatile storage element; and

determining a programming state of the at least one non-volatile storage element using iterative probabilistic decoding, the iterative probabilistic decoding uses reliability metrics which are based on the multiple sense operations.

2. The method of claim 1, wherein the reliability metrics comprise logarithmic likelihood ratios.

4. The method of claim 1, further comprising:

for each of the multiple sense operations, providing bits which represent a sensed programming state of the at least one non-volatile storage element, the reliability metrics are provided for each of the bits.

5. The method of claim 4, further comprising:

reading a table which cross references sensed programming states and bits to reliability metrics.

6. The method of claim 1, wherein the iterative probabilistic decoding attempts to satisfy parity checks of an error correction code.

9. The method of claim 1, wherein the multiple sense operations include a first sense operation and at least one subsequent sense operation, the iterative probabilistic decoding iterates initially using first reliability metrics which are based on the first sense operation but not the at least one subsequent sense operation, the first reliability metrics are adjusted to obtain adjusted reliability metrics as the probabilistic decoding iterates, the method further comprising:

adjusting the adjusted reliability metrics, as the probabilistic decoding iterates further, based on the at least one subsequent sense operation.

10. The method of claim 9, wherein the adjusted reliability metrics are adjusted to indicate a higher reliability when a sensed programming state of the at least one subsequent sense operation is consistent with a sensed programming state of the first sense operation.

12. The method of claim 9, wherein the multiple sense operations include a first sense operation and at least one subsequent sense operation, and the iterative probabilistic decoding iterates initially using first reliability metrics which are based on the first sense operation but not the at least one subsequent sense operation, the method further comprising:

restarting the iterative decoding process using probability metrics which are based on the first sense operation and the at least one subsequent sense operation.

14. The method of claim 1, further comprising:

obtaining voltage threshold profiles of the at least one non-volatile storage element for each of a plurality of programming states based on a plurality of sense operations performed on a set of non-volatile storage elements;

providing a set of reliability metrics based on the voltage threshold profiles; and

accessing the reliability metrics which are used in the iterative probabilistic decoding from the set of reliability metrics.

15. The method of claim 14, wherein the providing the set of reliability metrics comprises:

determining a conditional probability function f1(u|X) based on the plurality of sense operations, where u indicates a sensed threshold voltage and X represents a programmed state of the plurality of programming states; and

determining a probability function f2(v) based on the plurality of sense operations, for each programming state, where v indicates a deviation of a sensed threshold voltage from a threshold voltage free of read noise.

16. The method of claim 15, wherein the providing the set of reliability metrics further comprises:

determining logarithmic likelihood ratios based on the conditional probability function f1(u|X) and the probability function f2(v).

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17. A method for decoding data in non-volatile storage, comprising:

performing a first sense operation on at least one non-volatile storage element;

providing a first code word based on the first sense operation;

performing a decoding process for the first code word using a first set of reliability metrics which is based on the first code word; and

if the decoding process does not meet a first condition, performing a second sense operation on the at least one non-volatile storage element and adjusting the decoding process based on the second sense operation.

21. The method of claim 17, wherein the first condition comprises the decoding process converging within a given time period, converging within a given number of iterations, or satisfying a given number of parity checks.

24. The method of claim 17, wherein the first code word comprises bits which indicate whether a voltage threshold of the at least one non-volatile storage element is above or below each of a plurality of different voltage threshold levels, as indicated by the first sense operation.

25. The method of claim 17, wherein, if the decoding process does not meet a second condition, the method further comprises:

performing a third sense operation on the at least one non-volatile storage element; and

adjusting the decoding process based on the third sense operation.

26. The method of claim 17, wherein the first set of reliability metrics comprises logarithmic likelihood ratios, the first code word comprises multiple bits, and the logarithmic likelihood ratios are provided for each bit.

31. The method of claim 17, wherein the decoding process is adjusted based on:

(a) reliability metrics which are based on the second sense operation and (b) reliability metrics in the adjusted decoding process.

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33. A method for decoding data in non-volatile storage, comprising:

performing a first sense operation on at least one non-volatile storage element;

providing first data based the first sense operation which represents a sensed programming state of the at least one non-volatile storage element;

starting an iterative decoding process for the first data using probability metrics which are based on the first data;

performing a second sense operation on the at least one non-volatile storage element;

providing second data based on the second sense operation which represents a sensed programming state of the at least one non-volatile storage element; and

continuing the iterative decoding process by adjusting most recently used values of the probability metrics based on the second data.

34. The method of claim 33, wherein the second sense operation is started after starting the iterative decoding process.

37. The method of claim 33, wherein the probability metrics comprise logarithmic likelihood ratios for bits in the first data.

40. The method of claim 33, wherein:

the first data comprises multiple bits in respective bit positions; and

for each bit position, the probability metrics represent a probability that the associated bit is not errored.

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41. A method for decoding data in non-volatile storage, comprising:

performing a first sense operation on at least one non-volatile storage element;

providing first data based the first sense operation which represents a sensed programming state of the at least one non-volatile storage element;

starting an iterative decoding process for the first data using probability metrics which are based on the first data;

providing a metric which indicates a progress of the iterative decoding process; and

if the metric indicates the progress is unsatisfactory, providing second data based on a second sense operation which represents a sensed programming state of the at least one non-volatile storage element, and restarting the iterative decoding process using probability metrics which are based on the first and second data.

44. The method of claim 41, wherein the metric is based on whether or not the iterative decoding process approaches convergence within a given time period, whether or not the iterative decoding process approaches convergence within a given number of iterations, or a number of satisfied parity checks in the iterative decoding process.

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47. A method for operating non-volatile storage, comprising:

performing a plurality of sense operations on a set of non-volatile storage elements;

providing a set of reliability metrics based on the sense operations; and

storing the set of reliability metrics for use by an iterative probabilistic decoding process in determining a programming state of at least one non-volatile storage element in the set of non-volatile storage elements based on at least first and second subsequent sense operations.

48. The method of claim 47, further comprising:

obtaining voltage threshold profiles of the at least one non-volatile storage element for each of the plurality of programming states based on the plurality of sense operations; and

providing the set of reliability metrics based on the voltage threshold profiles.

52. The method of claim 47, wherein the plurality of sense operations are performed on the set of non-volatile storage elements at different ages of the set of non-volatile storage elements.

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53. A non-volatile storage system, comprising:

a set of non-volatile storage elements; and

one or more control circuits in communication with the set of non-volatile storage elements, the one or more control circuits perform multiple sense operations on at least one non-volatile storage element of the set and determine a programming state of the at least one non-volatile storage element using iterative probabilistic decoding, the iterative probabilistic decoding uses reliability metrics which are based on the multiple sense operations.

54. The non-volatile storage system of claim 53, wherein the reliability metrics comprise logarithmic likelihood ratios.

55. The non-volatile storage system of claim 53, wherein for each of the multiple sense operations, the one or more control circuits provide bits which represent a sensed programming state of the at least one non-volatile storage element, the reliability metrics are provided for each of the bits.

56. The non-volatile storage system of claim 55, wherein the one or more control circuits read a table which cross references sensed programming states and bits to reliability metrics.

57. The non-volatile storage system of claim 53, wherein the iterative probabilistic decoding attempts to satisfy parity checks of an error correction code.

58. The non-volatile storage system of claim 53, wherein the multiple sense operations include a first sense operation and at least one subsequent sense operation, the iterative probabilistic decoding iterates initially using first reliability metrics which are based on the first sense operation but not the at least one subsequent sense operation, the first reliability metrics are adjusted to obtain adjusted reliability metrics as the probabilistic decoding iterates, and the one or more control circuits adjust the adjusted reliability metrics, as the probabilistic decoding iterates further, based on the at least one subsequent sense operation.

59. The non-volatile storage system of claim 58, wherein the one or more control circuits adjusts the adjusted reliability metrics to indicate a higher reliability when a sensed programming state of the at least one subsequent sense operation is consistent with a sensed programming state of the first sense operation.

60. The non-volatile storage system of claim 58, wherein the multiple sense operations include a first sense operation and at least one subsequent sense operation, and the iterative probabilistic decoding iterates initially using first reliability metrics which are based on the first sense operation but not the at least one subsequent sense operation, and the one or more control circuits restarts the iterative decoding process using probability metrics which are based on the first sense operation and the at least one subsequent sense operation.

61. The non-volatile storage system of claim 53, wherein the one or more control circuits obtain voltage threshold profiles of the at least one non-volatile storage element for each of a plurality of programming states based on a plurality of sense operations performed on a set of non-volatile storage elements, provide a set of reliability metrics based on the voltage threshold profiles, and access the reliability metrics which are used in the iterative probabilistic decoding from the set of reliability metrics.

62. The non-volatile storage system of claim 61, wherein the one or more control circuits provide the set of reliability metrics by determining a conditional probability function f1(u|X) based on the plurality of sense operations, where u indicates a sensed threshold voltage and X represents a programmed state of the plurality of programming states, and determining a probability function f2(v) based on the plurality of sense operations, for each programming state, where v indicates a deviation of a sensed threshold voltage from a threshold voltage free of read noise.

63. The non-volatile storage system of claim 62, wherein the one or more control circuits provide the set of reliability metrics by determining logarithmic likelihood ratios based on the conditional probability function f1(u|X) and the probability function f2(v).

64. A non-volatile storage system, comprising:

a set of non-volatile storage elements; and

one or more control circuits in communication with the set of non-volatile storage elements, the one or more control circuits perform a first sense operation on at least one non-volatile storage element of the set, provide a first code word based on the first sense operation, perform a decoding process for the first code word using a first set of reliability metrics which is based on the first code word, and if the decoding process does not meet a first condition, perform a second sense operation on the at least one non-volatile storage element and adjust the decoding process based on the second sense operation.

65. The non-volatile storage system of claim 64, wherein the first condition comprises the decoding process converging within a given time period, converging within a given number of iterations, or satisfying a given number of parity checks.

66. The non-volatile storage system of claim 64, wherein the first code word comprises bits which indicate whether a voltage threshold of the at least one non-volatile storage element is above or below each of a plurality of different voltage threshold levels, as indicated by the first sense operation.

67. The non-volatile storage system of claim 64, wherein if the decoding process does not meet a second condition, the one or more control circuits perform a third sense operation on the at least one non-volatile storage element and adjust the decoding process based on the third sense operation.

68. The non-volatile storage system of claim 64, wherein the first set of reliability metrics comprises logarithmic likelihood ratios, the first code word comprises multiple bits, and the logarithmic likelihood ratios are provided for each bit.

69. The non-volatile storage system of claim 64, wherein the decoding process is adjusted based on:

(a) reliability metrics which are based on the second sense operation and (b) reliability metrics in the adjusted decoding process.

70. A non-volatile storage system, comprising:

a set of non-volatile storage elements; and

one or more control circuits in communication with the set of non-volatile storage elements, the one or more control circuits perform a first sense operation on at least one non-volatile storage element of the set, provide first data based the first sense operation which represents a sensed programming state of the at least one non-volatile storage element, start an iterative decoding process for the first data using probability metrics which are based on the first data, perform a second sense operation on the at least one non-volatile storage element, provide second data based on the second sense operation which represents a sensed programming state of the at least one non-volatile storage element, and continue the iterative decoding process by adjusting most recently used values of the probability metrics based on the second data.

71. The non-volatile storage system of claim 70, wherein the second sense operation is started after starting the iterative decoding process.

72. The non-volatile storage system of claim 70, wherein the first data comprises multiple bits in respective bit positions;

and

for each bit position, the probability metrics represent a probability that the associated bit is not errored.

73. A non-volatile storage system, comprising:

a set of non-volatile storage elements; and

one or more control circuits in communication with the set of non-volatile storage elements, the one or more control circuits perform a first sense operation on at least one non-volatile storage element, provide first data based the first sense operation which represents a sensed programming state of the at least one non-volatile storage element, start an iterative decoding process for the first data using probability metrics which are based on the first data, provide a metric which indicates a progress of the iterative decoding process, and if the metric indicates the progress is unsatisfactory, provide second data based on a second sense operation which represents a sensed programming state of the at least one non-volatile storage element, and restart the iterative decoding process using probability metrics which are based on the first and second data.

74. The non-volatile storage system of claim 93, wherein the metric is based on whether or not the iterative decoding process approaches convergence within a given time period, whether or not the iterative decoding process approaches convergence within a given number of iterations, or a number of satisfied parity checks in the iterative decoding process.

75. A non-volatile storage system, comprising:

a set of non-volatile storage elements; and

one or more control circuits in communication with the set of non-volatile storage elements, the one or more control circuits perform a plurality of sense operations on a set of non-volatile storage elements, provide a set of reliability metrics based on the sense operations, and store the set of reliability metrics for use by an iterative probabilistic decoding process in determining a programming state of at least one non-volatile storage element in the set of non-volatile storage elements based on at least first and second subsequent sense operations.

76. The non-volatile storage system of claim 75, wherein the one or more control circuits obtain voltage threshold profiles of the at least one non-volatile storage element for each of the plurality of programming states based on the plurality of sense operations, and provide the set of reliability metrics based on the voltage threshold profiles.

77. The non-volatile storage system of claim 75, wherein the plurality of sense operations are performed on the set of non-volatile storage elements at different ages of the set of non-volatile storage elements.