Device and method for resolving an LM flag issue

• US 9,159,437 B2
• Filed: 06/21/2013
• Issued: 10/13/2015
• Est. Priority Date: 06/11/2013
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Enterprise IP LLC (Western Digital Corporation)
• Inventors: Jeon, Seungjune, Kwong, Charles, Zhu, Jiangli
• Primary Examiner(s): Yang, Han
• Assistant Examiner(s): Chen, Xiaochun L
• Application Number:
  US13/924,313
• Publication Number:
  US 20140365836A1
• Time in Patent Office: 844 Days
• Field of Search: 714/100, 714/E11.001, 714/721
• US Class Current: 1/1
• 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
• CPC Class Codes: G11C 29/021 in voltage or current generators
• CPC Class Codes: G11C 29/028 with adaption or trimming of parameters

Abstract

The reliability with which data can be read from a storage medium, such as flash memory storage medium, is enhanced by updating an upper limit of a reading threshold voltage window for a respective portion of the storage medium. For each memory cell in the respective portion of the storage medium, a memory controller is configured to perform a plurality of sensing operations and obtain results from the plurality of sensing operations, where the plurality of sensing operations includes sensing operations using a predefined range of offsets from a previously established reading threshold voltage. The memory controller is further configured to determine the updated upper limit of the reading threshold voltage window based on the-results from the plurality of sensing operations, and store the updated upper limit of the reading threshold voltage window for the respective portion of the storage medium.

Claims

1. A method performed by a memory controller of updating an upper limit of a reading threshold voltage window for a respective portion of a storage medium, the method comprising:

during read patrol operations, adjusting one or more reading threshold voltages associated with the respective portion of the storage medium;

after adjusting the one or more reading threshold voltages, detecting a predefined lower-middle (LM) flag issue; and

in response to detecting the LM flag issue, performing a sequence of operations comprising;

for each memory cell of a plurality of memory cells in the respective portion of the storage medium;

performing a plurality of sensing operations, wherein the plurality of sensing operations includes sensing operations using a predefined range of offsets from a previously established reading threshold voltage; and

obtaining results from the plurality of sensing operations;

determining an updated upper limit of the reading threshold voltage window based on the results from the plurality of sensing operations performed on the plurality of memory cells in the respective portion of the storage medium; and

storing the updated upper limit of the reading threshold voltage window for the respective portion of the storage medium.

2. The method of claim 1, further comprising, for each memory cell of the plurality of memory cells in the respective portion of the storage medium:

comparing the results from the plurality of sensing operations to expected results for the plurality of sensing operations; and

determining a number of sensing operations in error.

3. The method of claim 2, wherein comparing the results comprises:

performing a matrix multiplication operation of a 1×

2M vector comprising the results from the plurality of sensing operations by a 2M×

(3M+1) expected results matrix comprising the expected results for the plurality of sensing operations, wherein M is an integer number of sensing operations; and

determining a hamming distance, for the respective memory cell, based on a result of the matrix multiplication operation.

4. The method of claim 3, wherein:

the matrix multiplication operation comprises an XOR and summation operation for each column of the expected results matrix;

determining the number of sensing operations in error comprises the hamming distance for the respective memory cell;

generating a histogram of the hamming distances determined for the plurality of memory cells in the respective portion of the storage medium; and

determining the updated upper limit of the reading threshold voltage window comprises determining the updated upper limit of the reading threshold voltage window in accordance with the generated histogram of hamming distances.

5. The method of claim 2, further comprising:

for each memory cell of the plurality of memory cells in the respective portion of the storage medium, determining a hamming distance based on the number of sensing operations in error;

generating a histogram of the hamming distances determined for the plurality of memory cells in the respective portion of the storage medium; and

wherein determining the updated upper limit of the reading threshold voltage window comprises determining the updated upper limit of the reading threshold voltage window in accordance with the generated histogram of hamming distances.

6. The method of claim 5, further comprising determining a most frequent non-zero hamming distance based on the histogram;

and

wherein determining an updated upper limit of the reading threshold voltage window comprises determining the updated upper limit of the reading threshold voltage window in accordance with the most frequent non-zero hamming distance.

7. The method of claim 1, further comprising:

generating a histogram based on the results from the plurality of sensing operations for each of the plurality of memory cells in the respective portion of the storage medium;

determining a number of sensing operations in error based on the histogram; and

wherein determining the updated upper limit of the reading threshold voltage window comprises determining the updated upper limit of the reading threshold voltage window in accordance with the number of sensing operations in error.

8. The method of claim 1, wherein the read patrol operations are performed after a predefined number of PE cycles have been performed on the respective portion of the storage medium.

9. The method of claim 1, wherein the read patrol operations are performed in accordance with a predefined schedule.

10. The method of claim 1, wherein detecting the predefined LM flag issue comprises detecting the predefined LM flag issue in a last programmed word line in an open block of the respective portion of the storage medium.

11. The method of claim 1, further comprising:

determining that a codeword cannot be decoded;

after determining that the codeword cannot be decoded, detecting the predefined LM flag issue; and

wherein performing the plurality of sensing operations for each memory cell of the plurality of memory cells in the respective portion of the storage medium comprises performing the plurality of sensing operations in response to detecting the LM flag issue.

12. The method of claim 1, wherein the upper limit of the reading threshold voltage window is stored in a memory of the memory controller.

13. The method of claim 1, wherein the reading threshold voltage window is a first reading threshold voltage window;

and

the method further comprising updating an upper limit for one or more reading threshold voltage windows distinct from the first reading threshold voltage window.

14. A memory controller for updating an upper limit of a reading threshold voltage window for a respective portion of a storage medium, the memory controller comprising:

a storage medium interface for coupling the memory controller to the storage medium; and

one or more modules, including a management module that includes one or more processors and memory storing one or more programs configured for execution by the one or more processors, the one or more modules coupled to the storage medium interface and configured to;

during read patrol operations, adjust one or more reading threshold voltages associated with the respective portion of the storage medium;

after adjusting the one or more reading threshold voltages, detect a predefined lower-middle (LM) flag issue; and

respond to detecting the LM flag issue by performing a sequence of operations comprising;

for each memory cell of a plurality of memory cells in the respective portion of the storage medium;

perform a plurality of sensing operations, wherein the plurality of sensing operations includes sensing operations using a predefined range of offsets from a previously established reading threshold voltage; and

obtain results from the plurality of sensing operations;

determine an updated upper limit of the reading threshold voltage window based on the results from the plurality of sensing operations performed on the plurality of memory cells in the respective portion of the storage medium; and

store the updated upper limit of the reading threshold voltage window for the respective portion of the storage medium.

15. The memory controller of claim 14, wherein the one or more modules are further configured to:

for each memory cell of the plurality of memory cells in the respective portion of the storage medium;

compare the results from the plurality of sensing operations to expected results for the plurality of sensing operations; and

determine a number of sensing operations in error.

16. The memory controller of claim 15, wherein comparing the results comprises:

performing a matrix multiplication operation of a 1×

2M vector comprising the results from the plurality of sensing operations by a 2M×

(3M+1) expected results matrix comprising the expected results for the plurality of sensing operations, wherein M is an integer number of sensing operations; and

determining a hamming distance, for the respective memory cell, based on a result of the matrix multiplication operation.

17. The memory controller of claim 16, wherein:

the matrix multiplication operation comprises an XOR and summation operation for each column of the expected results matrix;

determining the number of sensing operations in error comprises the hamming distance for the respective memory cell;

the one or more modules are further configured to generate a histogram of the hamming distances determined for the plurality of memory cells in the respective portion of the storage medium; and

determining the updated upper limit of the reading threshold voltage window comprises determining the updated upper limit of the reading threshold voltage window in accordance with the generated histogram of hamming distances.

18. The memory controller of claim 15, wherein the one or more modules are further configured to:

for each memory cell of the plurality of memory cells in the respective portion of the storage medium, determine a hamming distance based on the number of sensing operations in error;

generate a histogram of the hamming distances determined for the plurality of memory cells in the respective portion of the storage medium; and

wherein determining the updated upper limit of the reading threshold voltage window comprises determining the updated upper limit of the reading threshold voltage window in accordance with the generated histogram of hamming distances.

19. The memory controller of claim 18, wherein the one or more modules are further configured to determine a most frequent non-zero hamming distance based on the histogram;

and

wherein determining an updated upper limit of the reading threshold voltage window comprises determining the updated upper limit of the reading threshold voltage window in accordance with the most frequent non-zero hamming distance.

20. The memory controller of claim 14, wherein the one or more modules are further configured to:

generate a histogram based on the results from the plurality of sensing operations for each of the plurality of memory cells in the respective portion of the storage medium;

determine a number of sensing operations in error based on the histogram; and

wherein determining the updated upper limit of the reading threshold voltage window comprises determining the updated upper limit of the reading threshold voltage window in accordance with the number of sensing operations in error.

21. The memory controller of claim 14, wherein the read patrol operations are performed after a predefined number of PE cycles have been performed on the respective portion of the storage medium.

22. The memory controller of claim 14, wherein the read patrol operations are performed in accordance with a predefined schedule.

23. The memory controller of claim 14, wherein detecting the predefined LM flag issue comprises detecting the predefined LM flag issue in a last programmed word line in an open block of the respective portion of the storage medium.

24. The memory controller of claim 14, wherein the one or more modules are further configured to:

determine that a codeword cannot be decoded;

after determining that the codeword cannot be decoded, detect the predefined LM flag issue; and

wherein performing the plurality of sensing operations for each memory cell of the plurality of memory cells in the respective portion of the storage medium comprises performing the plurality of sensing operations in response to detecting the LM flag issue.

25. The memory controller of claim 14, wherein the upper limit of the reading threshold voltage window is stored in the memory of the management module.

26. The memory controller of claim 14, wherein the reading threshold voltage window is a first reading threshold voltage window;

and

the one or more modules are further configured to update an upper limit for one or more reading threshold voltage windows distinct from the first reading threshold voltage window.

27. A memory controller for updating an upper limit of a reading threshold voltage window for a respective portion of a storage medium, the memory controller comprising:

a storage medium interface for coupling the memory controller to the storage medium; and

means for adjusting one or more reading threshold voltages associated with the respective portion of the storage medium during read patrol operations;

means for detecting a predefined lower-middle (LM) flag issue after adjusting the one or more reading threshold voltages; and

means for performing a sequence of operations in response to detecting the LM flag issue, the sequence of operations comprising;

performing a plurality of sensing operations, for each memory cell of a plurality of memory cells in the respective portion of the storage medium, and obtaining results from the plurality of sensing operations, wherein the plurality of sensing operations includes sensing operations using a predefined range of offsets from a previously established reading threshold voltage; and

determining an updated upper limit of the reading threshold voltage window based on the results from the plurality of sensing operations performed on the plurality of memory cells in the respective portion of the storage medium, and

storing the updated upper limit of the reading threshold voltage window for the respective portion of the storage medium.

28. The memory controller of claim 27, wherein the sequence of operations further comprises:

for each memory cell of the plurality of memory cells in the respective portion of the storage medium,

comparing the results from the plurality of sensing operations to expected results for the plurality of sensing operations;

determining a number of sensing operations in error; and

determining a hamming distance based on the number of sensing operations in error;

wherein determining the updated upper limit of the reading threshold voltage window comprises determining the updated upper limit of the reading threshold voltage window in accordance with the hamming distances determined for the plurality of memory cells in the respective portion of the storage medium.

29. The memory controller of claim 27, further comprising:

means for determining that a codeword cannot be decoded;

wherein the means for detecting the LM flag issue detects the LM flag issue in accordance with a determination that the codeword cannot be decoded.