Determining read thresholds based upon read error direction statistics

• US 9,368,225 B1
• Filed: 11/21/2012
• Issued: 06/14/2016
• Est. Priority Date: 11/21/2012
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Avago Technologies General IP PTE Limited (Broadcom, Inc.)
• Inventors: Pinkovich, Evgeni, Weingarten, Hanan
• Primary Examiner(s): Le, Toan
• Application Number:
  US13/683,479
• Time in Patent Office: 1,301 Days
• Field of Search: 365/185.03, 365/185.11, 365/185.25, 365/185.21
• US Class Current: 1/1
• CPC Class Codes: G06F 11/1048 using arrangements adapted for a specific error detection or correction feature
• 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 2211/5634 Reference cells
• CPC Class Codes: G11C 7/065 Differential amplifiers of latching type

Abstract

There is provided a method for setting read thresholds to be used for reading multiple bits per cell flash memory cells, the method may include reading, by a read circuit, the flash memory cells using a set of current read thresholds to provide current read results; finding, by an error evaluation circuit, current read errors direction statistics associated with the current read results; determining multiple read threshold changes based upon the current read error direction statistics, without determining a contribution of each current read threshold to the current read error direction statistics; and altering multiple current read thresholds, by the multiple read threshold updates, to provide a set of next read thresholds.

Claims

1. A method for setting read thresholds to be used for reading multiple bits per cell flash memory cells, the method comprises:

reading, by a read circuit, the flash memory cells using a set of current read thresholds to provide current read results;

finding, by an error evaluation circuit, current read errors direction statistics associated with the current read results;

determining multiple read threshold changes based upon the current read error direction statistics, without determining a contribution of each current read threshold to the current read error direction statistics; and

altering multiple current read thresholds, by the multiple read threshold updates, to provide a set of next read thresholds;

wherein at least one of the following is true;

(i) the method further comprises receiving or calculating old read errors direction statistics associated with multiple previously obtained read results, and wherein the determining is further responsive to the old read error direction statistics;

(ii) each of the multiple read threshold changes does not exceed a smallest change amount allowable by the read circuit;

(iii) the determining of the multiple read threshold changes comprises determining, for each current read threshold, a read threshold change;

(iv) the determining of the multiple read threshold changes is further responsive to at least one physical characteristic of the flash memory cells;

(v) the determining of the multiple read threshold changes comprises finding a maximal margin estimator to the set of next read thresholds;

(vi) the determining of the multiple read threshold changes comprises applying a Kalman filter on the current read error direction statistics and on old read errors direction statistics associated with multiple previously obtained read results;

(vii) the method further comprises finding a set of optimal read thresholds; and

(viii) the determining the multiple read threshold changes is responsive to optimal and sub-optimal read thresholds.

2. The method according to claim 1, comprising receiving or calculating the old read errors direction statistics associated with the multiple previously obtained read results, and wherein the determining is further responsive to the old read error direction statistics.

3. The method according to claim 1, wherein each of the multiple read threshold changes does not exceed the smallest change amount allowable by the read circuit.

4. The method according to claim 1, wherein the determining of the multiple read threshold changes comprises determining, for each current read threshold the read threshold change.

5. The method according to claim 4, comprises determining a read threshold change associated with a current read threshold in response to a value of the current read threshold and in response to the current read errors direction statistics, wherein the current read errors direction statistics comprise a number of current read errors along a first direction and a number of current read errors along an second direction opposite to the first direction.

6. The method according to claim 4, wherein the read threshold change is a linear combination of the value of the current read threshold, the number of current read errors along the first direction and the number of current read errors along the second direction.

7. The method according to claim 1, wherein the determining of the multiple read threshold changes is further responsive to the at least one physical characteristic of the flash memory cells.

8. The method according to claim 1, wherein the determining of the multiple read threshold changes comprises finding the maximal margin estimator to the set of next read thresholds.

9. The method according to claim 8, wherein the finding of the maximal margin estimator comprises finding a minimum of an square absolute value of a difference between (i) a first vector of a set of optimal read thresholds, and (ii) a second vector that equals a multiplication of a read threshold and error matrix by a set of multiple read threshold changes;

wherein the read threshold and error matrix comprises multiple rows, each row comprises a set of read thresholds applied during a read attempt of the group of flash memory cells and read error direction statistics associated with the read attempt.

10. The method according to claim 8, wherein the finding of the maximal margin estimator comprises finding minimal and maximal sets of multiple read threshold changes that are associated with read error numbers that are amendable by the error evaluation circuit.

11. The method according to claim 8, comprising detecting erroneous flash memory cells that are flash memory cells that are associated with read errors that can not be corrected by the error evaluation circuit, and associating with read results of the erroneous flash memory cells a lower weight than a weight associated with flash memory cells that differ from the erroneous flash memory cells.

12. The method according to claim 8, comprising determining forbidden combinations of values of next read thresholds and wherein the determining of the multiple read threshold changes comprises preventing the set of the next read threshold to comprise any forbidden combination of values.

13. The method according to claim 1, wherein the determining of the multiple read threshold changes comprises applying the Kalman filter on the current read error direction statistics and on the old read errors direction statistics associated with multiple previously obtained read results.

14. The method according to claim 1, comprising finding the set of optimal read thresholds.

15. The method according to claim 14, comprising finding an optimal read threshold that is located between a pair of voltage read threshold lobes by ignoring voltage read threshold voltages that are associated with less than a predetermined number of flash memory cells.

16. The method according to claim 14, comprising finding an optimal read threshold that is located between a pair of voltage read threshold lobes by finding extremum read threshold values that once applied will result in a maximal number of errors correctable by the error evaluation circuit and calculating the optimal read threshold in response to the extremum read thresholds.

17. The method according to claim 1 comprising determining the multiple read threshold changes in response to the optimal and sub-optimal read thresholds.

18. A non-transitory computer readable medium that stores instructions to be executed by a computer for:

reading, by a read circuit, flash memory cells using a set of current read thresholds to provide current read results;

finding, by an error evaluation circuit, current read errors direction statistics associated with the current read results;

determining multiple read threshold changes, by a read threshold circuit, based upon the current read error direction statistics, without determining a contribution of each current read threshold to the current read error direction statistics; and

altering multiple current read thresholds by the multiple read threshold updates, to provide a set of next read thresholds;

wherein at least one of the following is true;

(i) the non-transitory computer readable medium further stores instructions for receiving or calculating old read errors direction statistics associated with multiple previously obtained read results, and wherein the determining is further responsive to the old read error direction statistics;

(ii) each of the multiple read threshold changes does not exceed a smallest change amount allowable by the read circuit;

(iii) the determining of the multiple read threshold changes comprises determining, for each current read threshold a read threshold change;

(iv) the determining of the multiple read threshold changes is further responsive to at least one physical characteristic of the flash memory cells;

(v) the determining of the multiple read threshold changes comprises finding a maximal margin estimator to the set of next read thresholds;

(vi) the determining of the multiple read threshold changes comprises applying a Kalman filter on the current read error direction statistics and on old read errors direction statistics associated with multiple previously obtained read results;

(vii) the non-transitory computer readable medium further stores instructions for finding a set of optimal read thresholds; and

(viii) the non-transitory computer readable medium further stores instructions for determining the multiple read threshold changes in response to optimal and sub-optimal read thresholds.

19. The non-transitory computer readable medium according to claim 18, that stores instructions for receiving or calculating the old read errors direction statistics associated with the multiple previously obtained read results, and wherein the determining is further responsive to the old read error direction statistics.

20. The non-transitory computer readable medium according to claim 18, wherein each of the multiple read threshold changes does not exceed the smallest change amount allowable by the read circuit.

21. The non-transitory computer readable medium according to claim 18, wherein the determining of the multiple read threshold changes comprises determining, for each current read threshold, the read threshold change.

22. The non-transitory computer readable medium according to claim 21, comprises determining a read threshold change associated with a current read threshold in response to a value of the current read threshold and in response to the current read errors direction statistics, wherein the current read errors direction statistics comprise a number of current read errors along a first direction and a number of current read errors along an second direction opposite to the first direction.

23. The non-transitory computer readable medium according to claim 21, wherein the read threshold change is a linear combination of the value of the current read threshold, the number of current read errors along the first direction and the number of current read errors along the second direction.

24. The non-transitory computer readable medium according to claim 18, wherein the determining of the multiple read threshold changes is further responsive to the at least one physical characteristic of the flash memory cells.

25. The non-transitory computer readable medium according to claim 18, wherein the determining of the multiple read threshold changes comprises finding the maximal margin estimator to the set of next read thresholds.

26. The non-transitory computer readable medium according to claim 25, wherein the finding of the maximal margin estimator comprises finding a minimum of an square absolute value of a difference between (i) a first vector of a set of optimal read thresholds, and (ii) a second vector that equals a multiplication of a read threshold and error matrix by a set of multiple read threshold changes;

wherein the read threshold and error matrix comprises multiple rows, each row comprises a set of read thresholds applied during a read attempt of the group of flash memory cells and read error direction statistics associated with the read attempt.

27. The non-transitory computer readable medium according to claim 25, wherein the finding of the maximal margin estimator comprises finding minimal and maximal sets of multiple read threshold changes that are associated with read error numbers that are amendable by the error evaluation circuit.

28. The non-transitory computer readable medium according to claim 25, that stores instructions for detecting erroneous flash memory cells that are flash memory cells that are associated with read errors that can not be corrected by the error evaluation circuit, and associating with read results of the erroneous flash memory cells a lower weight than a weight associated with flash memory cells that differ from the erroneous flash memory cells.

29. The non-transitory computer readable medium according to claim 25, that stores instructions for determining forbidden combinations of values of next read thresholds and wherein the determining of the multiple read threshold changes comprises preventing the set of the next read threshold to comprise any forbidden combination of values.

30. The non-transitory computer readable medium according to claim 18, wherein the determining of the multiple read threshold changes comprises applying the Kalman filter on the current read error direction statistics and on the old read errors direction statistics associated with the multiple previously obtained read results.

31. The non-transitory computer readable medium according to claim 18, that stores instructions for finding the set of optimal read thresholds.

32. The non-transitory computer readable medium according to claim 31, that stores instructions for finding an optimal read threshold that is located between a pair of voltage read threshold lobes by ignoring voltage read threshold voltages that are associated with less than a predetermined number of flash memory cells.

33. The non-transitory computer readable medium according to claim 31, that stores instructions for finding an optimal read threshold that is located between a pair of voltage read threshold lobes by finding extremum read threshold values that once applied will result in a maximal number of errors correctable by the error evaluation circuit and calculating the optimal read threshold in response to the extremum read thresholds.

34. The non-transitory computer readable medium according to claim 18, that stores instructions for determining the multiple read threshold changes in response to the optimal and sub-optimal read thresholds.

35. A flash memory controller, comprising:

a read circuit that is arranged to read flash memory cells using a set of current read thresholds to provide current read results;

an error evaluation circuit that is arranged to find current read errors direction statistics associated with the current read results;

a read threshold calculator that is arranged to determine multiple read threshold changes based upon the current read error direction statistics, without determining a contribution of each current read threshold to the current read error direction statistics; and

wherein the flash memory controller is arranged to alter multiple current read thresholds, by the multiple read threshold updates, to provide a set of next read thresholds;

wherein at least one of the following is true;

(i) the flash memory controller is further configured to receive or calculate old read errors direction statistics associated with multiple previously obtained read results, and wherein the read threshold calculator is configured to determine the multiple read threshold changes further in response to the old read error direction statistics;

(ii) each of the multiple read threshold changes does not exceed a smallest change amount allowable by the read circuit;

(iii) the read threshold calculator is configured to determine the multiple read threshold changes by determining, for each current read threshold, a read threshold change;

(iv) the read threshold calculator is configured to determine the multiple read threshold changes by in further response to at least one physical characteristic of the flash memory cells;

(v) the read threshold calculator is configured to determine the multiple read threshold changes by finding a maximal margin estimator to the set of next read thresholds;

(vi) the read threshold calculator is configured to determine the multiple read threshold changes by applying a Kalman filter on the current read error direction statistics and on old read errors direction statistics associated with multiple previously obtained read results;

(vii) the flash memory controller is further configured to find a set of optimal read thresholds; and

(viii) the read threshold calculator is configured to determine the multiple read threshold changes in response to optimal and sub-optimal read thresholds.