RETENTION-DRIFT-HISTORY-BASED NON-VOLATILE MEMORY READ THRESHOLD OPTIMIZATION

• US 20140229131A1
• Filed: 01/31/2014
• Published: 08/14/2014
• Est. Priority Date: 05/04/2012
• Status: Active Grant

• Current Assignee: Seagate Technology LLC (Seagate Technology Holdings Plc)
• Original Assignee: LSI Corporation (Broadcom, Inc.)
• Inventors: COHEN, Earl T., ZHONG, Hao
• Granted Patent: US 9,645,177 B2
• Time in Patent Office: Days
• US Class Current: 702/64
• CPC Class Codes: G01R 19/0084 measuring voltage only all subgroups of G01R19/00 take precedence
• CPC Class Codes: G11C 16/0483 comprising cells having several storage transistors connected in series
• CPC Class Codes: G11C 16/26 Sensing or reading circuits; Data output circuits
• CPC Class Codes: G11C 16/28 using differential sensing or reference cells, e.g. dummy cells
• CPC Class Codes: G11C 16/3418 Disturbance prevention or evaluation; Refreshing of disturbed memory data
• CPC Class Codes: G11C 16/3427 Circuits or methods to prevent or reduce disturbance of the state of a memory cell when neighbouring cells are read or written
• CPC Class Codes: G11C 16/349 Arrangements for evaluating degradation, retention or wearout, e.g. by counting erase cycles
• CPC Class Codes: G11C 16/3495 Circuits or methods to detect or delay wearout of nonvolatile EPROM or EEPROM memory devices, e.g. by counting numbers of erase or reprogram cycles, by using multiple memory areas serially or cyclically
• CPC Class Codes: G11C 29/021 in voltage or current generators
• CPC Class Codes: G11C 29/028 with adaption or trimming of parameters
• CPC Class Codes: G11C 7/14 Dummy cell management; Sense reference voltage generators

Abstract

An SSD controller dynamically adjusts read thresholds in an NVM to reduce errors due to device threshold voltage distribution shifts, thus improving performance, reliability, and/or cost of a storage sub-system, such as an SSD. A retention drift clock uses one or more reference pages (or ECC units or blocks) on one or more NVM die as read threshold over time/temperature references, and uses a function of those values as a measure of drift (over time/temperature). At some initial time, the one or more reference pages are programmed and an initial read threshold is measured for each of the one or more reference pages. In some embodiments, read threshold values are averaged among one or more of: all references pages on the same die; and all reference pages in the same one or more die in an I/O device.

Claims

1. A method comprising:

recording a sampled value of a voltage drift reference timestamp associated with programming a particular one of a plurality of groups of pages of a non-volatile memory; and

determining a voltage threshold, of a reading of at least some of the particular group of pages programmed, based at least in part on the recorded sampled value of the voltage drift reference timestamp and a sampled value of the voltage drift reference timestamp associated with the reading.

2. The method of claim 1, wherein the sampled values of the voltage drift reference timestamp are according to a voltage threshold of one or more reference locations of the non-volatile memory.

3. The method of claim 1, further comprising:

subsequent to the recorded sampled value of the voltage drift reference timestamp differing by less than a specified difference with respect to the sampled value of the voltage drift reference timestamp as of a time of the reading, using a default voltage threshold as the voltage threshold of the reading.

4. The method of claim 1, further comprising:

periodically sampling a reference location of the non-volatile memory to determine a current voltage threshold of the reference location; and

wherein the periodically sampling determines, at least in part, the sampled values of the voltage drift reference timestamp.

5. The method of claim 4, wherein the reference location is a page of the non-volatile memory.

6. The method of claim 4, wherein the reference location is one of the pages of the particular group of pages.

7. The method of claim 4, wherein the reference location is in a same block of the non-volatile memory as at least one of the pages of the particular group of pages.

8. The method of claim 4, wherein at least some of the sampled values of the voltage drift reference timestamp respectively comprise a respective generation number and a respective threshold portion, wherein the generation number is according to a master generation number, and the threshold portion is according to the current voltage threshold of the reference location as of the associated periodically sampling.

9. The method of claim 8, further comprising:

subsequent to the current voltage threshold of the reference location exceeding a bound, updating the reference location, and incrementing the master generation number.

10. The method of claim 1, wherein the recorded sampled value of the voltage drift reference timestamp is of a time of the programming and the sampled value of the voltage drift reverence timestamp associated with the reading is of a time of the reading.

11. The method of claim 2, wherein at least some of the voltage drift reference timestamps respectively comprise a determined set of one or more optimal read threshold voltages of a voltage drift reference comprising the reference locations.

12. The method of claim 11, wherein the non-volatile memory comprises SLC memory and the determined set comprises a single optimal read threshold voltage of the one or more optimal read threshold voltages.

13. The method of claim 11, wherein the non-volatile memory comprises MLC memory and the determined set comprises a plurality of optimal read threshold voltages of the one or more optimal read threshold voltages.

14. The method of claim 11, further comprising:

associating at least some of the voltage drift reference timestamps with respective representations of time.

15. The method of claim 14, wherein the respective representations of time are according to an offset counter to a system event.

16. The method of claim 14, wherein the respective representations of time are according to a value derived from an actual time-clock.

17. An apparatus comprising:

means for recording a sampled value of a voltage drift reference timestamp associated with programming a particular one of a plurality of groups of pages of a non-volatile memory; and

means for determining a voltage threshold, of a reading of at least some of the particular group of pages programmed, based at least in part on the recorded sampled value of the voltage drift reference timestamp and a sampled value of the voltage drift reference timestamp associated with the reading.

18. The apparatus of claim 17, wherein the sampled values of the voltage drift reference timestamp are according to a voltage threshold of a reference location of the non-volatile memory.

19. The apparatus of claim 17, further comprising:

means for, subsequent to the recorded sampled value of the voltage drift reference timestamp differing by less than a specified difference with respect to the sampled value of the voltage drift reference timestamp as of the time of the reading, using a default voltage threshold as the voltage threshold of the reading.

20. The apparatus of claim 17, further comprising:

means for periodically sampling a reference location of the non-volatile memory to determine a current voltage threshold of the reference location; and

wherein the periodically sampling determines, at least in part, the sampled values of the voltage drift reference timestamp.

21. The apparatus of claim 20, wherein the reference location is a page of the non-volatile memory.

22. The apparatus of claim 20, wherein the reference location is one of the pages of the particular group of pages.

23. The apparatus of claim 20, wherein the reference location is in a same block of the non-volatile memory as at least one of the pages of the particular group of pages.

24. The apparatus of claim 20, wherein at least some of the sampled values of the voltage drift reference timestamp respectively comprise a respective generation number and a respective threshold portion, wherein the generation number is according to a master generation number, and the threshold portion is according to the current voltage threshold of the reference location as of the associated periodically sampling.

25. The apparatus of claim 24, further comprising:

means for, subsequent to the current voltage threshold of the reference location exceeding a bound, updating the reference location, and incrementing the master generation number.

26. A non-transitory tangible computer readable medium having a set of instructions stored therein that when executed by a processing element cause the processing element to perform operations comprising:

recording a sampled value of a voltage drift reference timestamp associated with programming a particular one of a plurality of groups of pages of a non-volatile memory; and

determining a voltage threshold, of a reading of at least some of the particular group of pages programmed, based at least in part on the recorded sampled value of the voltage drift reference timestamp and a sampled value of the voltage drift reference timestamp associated with the reading.

27. The computer readable medium of claim 26, wherein the sampled values of the voltage drift reference timestamp are according to a voltage threshold of a reference location of the non-volatile memory.

28. The computer readable medium of claim 26, the operations further comprising:

subsequent to the recorded sampled value of the voltage drift reference timestamp differing by less than a specified difference with respect to the sampled value of the voltage drift reference timestamp as of the time of the reading, using a default voltage threshold as the voltage threshold of the reading.

29. The computer readable medium of claim 26, the operations further comprising:

periodically sampling a reference location of the non-volatile memory to determine a current voltage threshold of the reference location; and

wherein the periodically sampling determines, at least in part, the sampled values of the voltage drift reference timestamp.

30. The computer readable medium of claim 29, wherein the reference location is a page of the non-volatile memory.

31. The computer readable medium of claim 29, wherein the reference location is one of the pages of the particular group of pages.

32. The computer readable medium of claim 29, wherein the reference location is in a same block of the non-volatile memory as at least one of the pages of the particular group of pages.

33. The computer readable medium of claim 29, wherein at least some of the sampled values of the voltage drift reference timestamp respectively comprise a respective generation number and a respective threshold portion, wherein the generation number is according to a master generation number, and the threshold portion is according to the current voltage threshold of the reference location as of the associated periodically sampling.

34. The computer readable medium of claim 33, the operations further comprising:

subsequent to the current voltage threshold of the reference location exceeding a bound, updating the reference location, and incrementing the master generation number.