APPARATUS AND METHODS FOR GENERATING ROW-SPECIFIC READING THRESHOLDS IN FLASH MEMORY

• US 20100131809A1
• Filed: 09/17/2008
• Published: 05/27/2010
• Est. Priority Date: 12/05/2007
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: DensBits Technologies Ltd. (Broadcom, Inc.)
• Inventors: Katz, Michael
• Granted Patent: US 8,453,022 B2
• Time in Patent Office: Days
• US Class Current: 714/719
• CPC Class Codes: G06F 12/0246 in block erasable memory, e.g. flash memory
• CPC Class Codes: G06F 2212/1032 Reliability improvement, data loss prevention, degraded operation etc
• CPC Class Codes: G11C 16/349 Arrangements for evaluating degradation, retention or wearout, e.g. by counting erase cycles

Abstract

A method for generating a set of at least one row-specific reading threshold for reading at least portions of pages of data within an erase sector of a flash memory device, the method comprising predetermining at least one initial reading threshold; performing the following steps for at least one current logical page: generating bit error characterizing information regarding at least one corresponding bit error within at least one cell representing at least a logical portion of at least one successfully reconstructed previous logical page; and computing at least one row-specific reading threshold based on said bit error characterizing information and on a previous threshold initially comprising said initial threshold and subsequently comprising a row-specific reading threshold computed for a successfully reconstructed previous logical page; and reading at least a portion of said current logical page using said at least one row-specific reading threshold.

Claims

1. A method for generating a set of at least one row-specific reading threshold for reading at least portions of pages of data within an erase sector of a flash memory device, the method comprising:

predetermining at least one initial reading threshold;

performing the following steps for at least one current logical page;

generating bit error characterizing information regarding at least one corresponding bit error within at least one cell representing at least a logical portion of at least one successfully reconstructed previous logical page; and

computing at least one row-specific reading threshold based on said bit error characterizing information and on a previous threshold initially comprising said initial threshold and subsequently comprising a row-specific reading threshold computed for a successfully reconstructed previous logical page; and

reading at least a portion of said current logical page using said at least one row-specific reading threshold.

2. A method according to claim 1 wherein said bit error characterizing information comprises identification of a reading threshold associated with the bit error.

3. A method according to claim 1 wherein said bit error characterizing information comprises a direction of the bit error.

4. A method according to claim 1 wherein a first number of threshold errors whose direction, with respect to said reading threshold, is from left to right is compared to a second number of threshold errors whose direction, with respect to said reading threshold, is from right to left, wherein the row-specific reading threshold depends upon at least one of the sign and magnitude of the difference between said first and second numbers of threshold errors.

5. A method according to claim 1 and also comprising:

selecting a subset of rows within said erase sector; and

identifying a set of logical pages residing within said subset of rows; and

performing said generating, computing and reading for said set of logical pages and for less than all pages in said erase sector.

6. A method according to claim 5 and wherein said generating, computing and reading are performed only for said set of logical pages.

7. A method according to claim 1 wherein said reading thresholds are corrected only if the number of bit errors per page is in a process of change of at least a predetermined magnitude.

8. A method according to claim 7 wherein said reading thresholds are corrected only if the difference between the number of bit errors encountered during reconstruction of said current page and the number of bit errors occurring during reconstruction of at least one previous page is larger than a predetermined number.

9. A method according to claim 1 wherein said generating is performed for at least one CSB page residing in a row corresponding to an MSB page also residing in said row, based at least partly on values read from said MSB page.

10. A method according to claim 1 wherein said generating is performed for an LSB page residing in a row corresponding to an MSB and at least one CSB page also residing in said row, based at least partly on values read from at least one of said MSB and CSB pages.

11. A method according to claim 9 wherein said generating performed for said CSB page is based at least partly on values read on-the-fly from said MSB page.

12. A method according to claim 10 wherein said generating performed for said LSB page is based at least partly on values read on-the-fly from at least one of said MSB and CSB pages.

13. A method according to claim 9 wherein said generating performed for said CSB page is based at least partly on stored values previously read from said MSB page.

14. A method according to claim 10 wherein said generating performed for said LSB page is based at least partly on stored values previously read from at least one of said MSB and CSB pages.

15. A method for using flash memory to store data, the method comprising:

writing at least one page of data to said flash memory;

reading said at least one page of data from said flash memory using a set of reading thresholds, including generating bit error characterizing information regarding at least one corresponding bit error within at least one cell representing at least a logical portion of at least one successfully reconstructed previous logical page; and

subsequently, using said flash memory so as to take into account said bit error characterizing information.

16. A method according to claim 15 wherein said bit error characterizing information comprises identification of a reading threshold which is associated with the bit error.

17. A method according to claim 15 wherein said bit error characterizing information comprises a direction of the bit error.

18. A method according to claim 15 wherein said using comprises reading at least one subsequent page of data so as to take into account said bit error characterizing information.

19. A method according to claim 15 wherein said using comprises detecting portions of said flash memory which suffer from read-disturb phenomenon based on said bit error characterizing information.

20. A method according to claim 19 wherein said detecting comprises detecting an overly large number of bit errors whose source is a reading threshold which is closest, within said set of reading thresholds, to zero voltage, and whose direction is from left to right.

21. A system for generating a set of at least one row-specific reading threshold for reading at least portions of pages of data within an erase sector of a flash memory device, the system comprising:

apparatus for predetermining at least one initial reading threshold;

a bit error analyzer operative, for at least one current logical page, to generate bit error characterizing information regarding at least one corresponding bit error within at least one cell representing at least a logical portion of at least one successfully reconstructed previous logical page; and

a bit error-based threshold generator operative to compute at least one row-specific reading threshold based on said bit error characterizing information and on a previous threshold initially comprising said initial threshold and subsequently comprising a row-specific reading threshold computed for a successfully reconstructed previous logical page; and

a flash memory cell reader operative to read at least a portion of said current logical page using said at least one row-specific reading threshold.

22. A system for using flash memory to store data, the system comprising:

apparatus for writing in flash memory operative to write at least one page of data to the flash memory;

a bit error characterizing reader operative to read said at least one page of data from said flash memory using a set of reading thresholds, including generating bit error characterizing information regarding at least one corresponding bit error within at least one cell representing at least a logical portion of at least one successfully reconstructed previous logical page; and

a bit error-based controller operative to control said flash memory so as to take into account said bit error characterizing information.