System and methods employing mock thresholds to generate actual reading thresholds in flash memory devices

• US 8,751,726 B2
• Filed: 09/17/2008
• Issued: 06/10/2014
• 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
• Primary Examiner(s): BRADLEY, MATTHEW A
• Application Number:
  US12/596,438
• Publication Number:
  US 20100146191A1
• Time in Patent Office: 2,092 Days
• Field of Search: 711/103, 711/154
• US Class Current: 711/103
• 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 reading at least one page within an erase sector of a flash memory device, the method comprising computing at least one mock reading threshold; using the at least one mock reading threshold to perform at least one mock read operation of at least a portion of at least one page within the erase sector, thereby to generate a plurality of logical values; defining a set of reading thresholds based at least partly on the plurality of logical values; and reading at least one page in the erase sector using the set of reading thresholds.

Claims

1. A method for reading at least one page within an erase sector of a flash memory device, the method comprising:

computing at least one mock reading threshold;

wherein the mock threshold that is far from an adequate conventional reading threshold useful for reconstructing a logical value from a physical value residing in a flash memory cell;

using said at least one mock reading threshold to perform at least one mock read operation of at least a portion of at least one page within said erase sector, thereby to generate a plurality of logical values;

defining a set of reading thresholds based at least partly on said plurality of logical values; and

reading at least one page in said erase sector using said set of reading thresholds.

2. A method according to claim 1 wherein said defining of a set of reading thresholds based at least partly on said plurality of logical values includes estimating a mean and standard deviation of at least one physical level over at least a portion of at least one page belonging to the erase sector of the flash memory device based on said at least one mock reading operation.

3. A method according to claim 2 wherein said estimating is performed for at least a first physical level residing above a second physical level and below a third physical level and wherein estimating the mean and standard deviations of said first physical level comprises interpolating between previously found means and standard deviations of said second and third levels.

5. A method according to claim 1 wherein said defining of a set of reading thresholds based at least partly on said plurality of logical values includes identifying at least one non-populous region of a predetermined size, within a distribution of physical values of a population of cells belonging to a page within the erase sector.

6. A method according to claim 5 wherein at least one reading threshold in said set of reading thresholds is determined to be a central value within said non-populous region.

9. A method according to claim 1 wherein said defining comprises seeking a local maximum in a threshold voltage distribution and determining at least one reading threshold based at least partly on said local maximum.

10. A method according to claim 1 wherein said defining comprises seeking a local minimum in a threshold voltage distribution and determining at least one reading threshold based at least partly on said local minimum.

11. A method according to claim 1 wherein said defining comprises:

identification of a threshold voltage distribution of a highest program level in a probability distribution of physical values within said erase sector; and

preventing misidentification of the threshold voltage distribution of the highest program level by comparing the number of cells whose threshold voltage is larger than the candidate maximizing voltage, to a threshold.

12. A method according to claim 1 and also comprising guaranteeing a similar number of cells in each program level.

13. A method according to claim 1 wherein, each time said computing, using, defining and reading are performed, said reading thresholds as most recently modified are stored in a data base in association with characterization information, said method also comprising, for at least one page in said erase sector:

accessing said data base and reviewing at least some of said characterization information to determine whether said database includes reading thresholds suitable for said page;

if said database does include suitable thresholds, using said suitable thresholds to read said page; and

if said database does not include suitable thresholds, performing said computing, using, defining and reading steps for said page.

14. A method according to claim 13 wherein said characterization information includes:

identification of row neighborhood, number of program/erase cycles which rows in said row neighborhood underwent, and retention period which rows in said row neighborhood endured.

15. A method according to claim 12 wherein said guaranteeing comprises scrambling of bits stored in said erase sector.

19. A method according to claim 1 wherein said computing comprises translating the mock reading thresholds along a physical value axis of a probability distribution of physical values within said erase sector, according to at least one characteristic of a highest program level within said probability distribution.

4. A method for rapidly assessing functionality of erase sectors in a flash memory, the method comprising:

estimating a mean and standard deviation of at least one physical level over at least a portion of at least one page belonging to an erase sector of a flash memory device;

wherein said estimating is performed for at least a first physical level residing above a second physical level and below a third physical level and wherein estimating the mean and standard deviations of said first physical level comprises interpolating between previously found means and standard deviations of said second and third levels.

7. A method for assessing effects of retention and cycling on individual erase sectors in a flash memory which has undergone retention and cycling, the method comprising:

receiving a first centerpoint of a rightmost lobe of a distribution of physical values within the cells of a new flash memory device and a second centerpoint of a rightmost lobe of a distribution of physical values within the cells of an entirely degraded flash memory device;

generating a histogram representing distribution of physical values within the cells of an individual erase sector, over a range extending at least between said second and first centerpoints; and

computing at least one statistic of a rightmost lobe of the histogram.

8. A method according to claim 7 wherein said at least one statistic comprises at least one of a mean and a standard deviation.

16. A system for reading at least one page within an erase sector of a flash memory device, the system comprising:

mock threshold computation apparatus operative to compute at least one mock reading threshold;

wherein the mock threshold that is far from an adequate conventional reading threshold useful for reconstructing a logical value from a physical value residing in a flash memory cell;

a reading threshold set generator operative to define a set of reading thresholds based at least partly on said plurality of logical values; and

reading apparatus using said at least one mock reading threshold to perform at least one mock read operation of at least a portion of at least one page within said erase sector, thereby to generate a plurality of logical values and subsequently using said set of reading thresholds based at least partly on said plurality of logical values to read at least one page in said erase sector using said set of reading thresholds.

17. A system for rapidly assessing functionality of erase sectors in a flash memory, the system comprising:

flash memory physical level distribution statistic estimation apparatus estimating a mean and standard deviation of at least one physical level over at least a portion of at least one page belonging to an erase sector of a flash memory device and including interpolation apparatus, wherein said apparatus receives and operates upon at least a first physical level residing above a second physical level and below a third physical level by using said interpolation apparatus to interpolate between previously found means and standard deviations of said second and third levels.

18. A system for assessing effects of retention and cycling on individual erase sectors in a flash memory which has undergone retention and cycling, the system comprising:

a histogram generator operative to generate a histogram representing distribution of physical values within the cells of an individual erase sector, over a range extending at least between a first centerpoint of a rightmost lobe of a distribution of physical values within the cells of a new flash memory device and a second centerpoint of a rightmost lobe of a distribution of physical values within the cells of an entirely degraded flash memory device; and

a rightmost lobe analyzer operative to compute at least one statistic of a rightmost lobe of the histogram.