Methods for writing non-volatile memories for increased endurance

• US 7,245,556 B1
• Filed: 12/28/2005
• Issued: 07/17/2007
• Est. Priority Date: 12/28/2005
• Status: Expired due to Fees

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Corporation (Western Digital Corporation)
• Inventors: Gongwer, Geoffrey S., Pinto, Yosi, Honen, Oren
• Primary Examiner(s): Nguyen; Tuan T.
• Application Number:
  US11/321,217
• Publication Number:
  US 20070147168A1
• Time in Patent Office: 566 Days
• Field of Search: 365/236, 365/218
• US Class Current: 365/236
• CPC Class Codes: G06F 12/0246 in block erasable memory, e.g. flash memory
• CPC Class Codes: G06F 2212/1036 Life time enhancement
• CPC Class Codes: G06F 2212/7211 Wear leveling
• 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 8/10 Decoders

Abstract

A memory system that incorporates methods of amplifying the lifetime of a counter made up of memory elements, such as EEPROM cells, having finite endurance. A relatively small memory made up of a number of individually accessible write segments, where, depending on the embodiment, each write segment is made up of a single memory cell or a small number of cells (e.g., a byte). A count is encoded so that it is distributed across a number of fields, each associated with one of the write segments, such that as the count is incremented only a single field (or, in the single bit embodiments, occasionally more than one field) is changed and that these changes are evenly distributed across the fields. The changed field is then written to the corresponding segment, while the other write segments are unchanged. Consequently, the number of rewrites to a given write segment is decreased, and the lifetime correspondingly increased, by a factor corresponding to the number of write segments used.

Claims

1. In a memory system including a plurality N of individually accessible sections of multiple erasable and re-programmable non-volatile memory cells, a method of storing a numerical value that comprises:

encoding the numerical value as a binary value composed of N fields, such that for each incrementing the numerical value changes only a single one of said fields and the changes to the N fields are substantially uniformly distributed among the N fields as the numerical value is incremented; and

storing the change to said single one of the N fields in a corresponding one of the N sections as the numerical value is incremented while leaving the other ones of the N fields unchanged.

2. The method of claim 1, wherein the changes to the N fields are cyclically distributed among the N fields as the numerical value is incremented.

3. The method of claim 1, wherein each of the individually accessible sections contains a byte of storage capability.

4. The method of claim 1, wherein said storing comprises storing the fields in binary form in the memory cells of the sections.

5. The method of claim 1, wherein each of the fields are of the same number of bits.

6. The method of claim 1, wherein the numerical value is a M-bit value, where M is a multiple of N.

7. The method of claim 1, wherein the memory system includes a controller portion and a memory portion and said individually accessible sections are formed on the controller.

8. The method of claim 7, wherein the numerical value corresponds to an event on the memory portion.

9. The method of claim 1, wherein the memory system includes a controller portion and a memory portion and said individually accessible sections are formed as part of a state machine on the memory portion.

10. The method of claim 1, wherein said memory cells of the individually accessible sections are EEPROM memory cells.

11. In a memory system having a plurality of N individually accessible, rewritable non-volatile segments, a method comprising:

incrementing the value of a register;

encoding the register value into a plurality of N binary fields, each of the fields corresponding to a respective one of the individually accessible, rewritable segments, where the encoding is such that as the register value is incremented the number of fields that change for each count is minimized and the changes to the fields are substantially uniformly distributed; and

rewriting the incremented register value in the segments in which the respective field value has changed.

12. The method of claim 11, wherein the encoding uses a balanced Gray code.

13. The method of claim 11, wherein each of the individually accessible, rewritable segments is comprised of a plurality of memory cells.

14. The method of claim 13, wherein the changes to the N fields are cyclically distributed among the N fields as the register value is incremented.

15. The method of claim 13, wherein each of the individually accessible, rewritable segments contains a byte of storage capability.

16. The method of claim 11, wherein N is greater than or equal to three and each of the individually accessible, rewritable segments is comprised of a single memory cell.

17. The method of claim 16, wherein the encoding uses a balanced Gray code.

18. The method of claim 11, wherein each of the individually accessible, rewritable segments are of the same number of bits.

19. The method of claim 11, wherein the register value is a M-bit value, where M is a multiple of N.

20. The method of claim 11, wherein the memory system includes a controller portion and a memory portion and said individually accessible, rewritable segments are formed on the controller.

21. The method of claim 20, wherein the register value corresponds to an event on the memory portion.

22. The method of claim 11, wherein the memory system includes a controller portion and a memory portion and said individually accessible, rewritable segments are formed as part of a state machine on the memory portion.

23. The method of claim 11, wherein said individually accessible, rewritable segments are formed of EEPROM memory cells.