Maintaining an average erase count in a non-volatile storage system

• US 20040177212A1
• Filed: 10/28/2002
• Published: 09/09/2004
• Est. Priority Date: 10/28/2002
• Status: Active Grant

• Current Assignee: Innovative Memory Systems, Inc. (Wi-LAN Inc.)
• Original Assignee: SanDisk Corporation (Western Digital Corporation)
• Inventors: Qawami, Bahman, Sabet-Sharghi, Farshid, Chang, Robert C.
• Granted Patent: US 7,035,967 B2
• Time in Patent Office: Days
• US Class Current: 711/103
• CPC Class Codes: G06F 12/0246 in block erasable memory, e.g. flash memory
• 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

Abstract

Methods and apparatus for maintaining an average erase count in a system memory of a non-volatile memory system are disclosed. According to one aspect of the present invention, a method for determining an average number of times each block of a number of blocks within a non-volatile memory of a memory system has been erased includes obtaining an erase count for each block that indicates a number of times each block has been erased. Once all the erase counts have been obtained, the erase counts are summed, and an average erase count that indicates the average number of times each block of the number of blocks has been erased is created by substantially dividing the sum by the number of blocks.

Claims

1. A data structure, the data structure being arranged in a system memory associated with a non-volatile memory system, the non-volatile memory system including a non-volatile memory which includes a plurality of blocks, the data structure comprising:

a first indicator, the first indicator being arranged to provide an indication of a number of times each usable block of the plurality of blocks has been erased; and

a second indicator, the second indicator being arranged to indicate a total number of blocks included in the plurality of blocks.

2. The data structure of claim 1 further including:

a header, wherein the header is arranged to contain the first indicator and the second indicator.

3. The data structure of claim 1 wherein the first indicator is an average erase count, the average erase count being arranged to indicate an average number of times each block in the plurality of blocks has been erased.

4. The data structure of claim 1 wherein the non-volatile memory is a NAND flash memory.

5. A non-volatile memory system comprising:

a non-volatile memory, the non-volatile memory including a plurality of blocks;

a system memory; and

means for indicating in the system memory an average number of times each block included in the plurality of blocks has been erased.

6. The non-volatile memory system of claim 5 wherein the means for indicating in the system memory the average number of times each block included in the plurality of blocks has been erased include means for indicating in a data structure the average number of times each block included in the plurality of blocks has been erased.

7. The non-volatile memory system of claim 6 wherein the data structure is an erase count block.

8. The non-volatile memory system of claim 5 wherein the non-volatile memory is a NAND flash memory.

9. A method of determining an average number of times each block of a number of blocks within a non-volatile memory of a memory system has been erased, the method comprising:

obtaining an erase count for each block of the number of blocks, the erase count for each block of the number of blocks being arranged to indicate a number of times each block of the number of blocks has been erased, wherein the number of blocks is a total number of usable blocks within the non-volatile memory;

creating a sum of the erase counts for the number of blocks; and

creating the average erase count, the average erase count being arranged to indicate the average number of times each block of the number of blocks has been erased, wherein creating the average erase count includes substantially dividing the sum by the number of blocks.

10. The method of claim 9 wherein the erase count for a first block of the number of blocks is obtained from the first block.

11. The method of claim 10 wherein the erase count for the first block is obtained from a redundant area of the first block.

12. The method of claim 9 wherein the erase count for a first block of the number of blocks is obtained from an erase count block in the memory system, the erase count block being arranged to contain the erase counts for each block of the number of blocks.

13. The method of claim 9 wherein each block of the number of blocks is a usable block.

14. The method of claim 9 wherein the non-volatile memory is a NAND flash memory.

15. A method for substantially preparing an erased block for use from a non-volatile memory of a non-volatile memory system, the non-volatile memory including a plurality of blocks, wherein the plurality of blocks includes the erased block, the method comprising:

obtaining the erased block;

obtaining an indicator that characterizes a number of times each block in the plurality of blocks has been erased; and

storing the indicator that characterizes a number of times each block in the plurality of blocks has been erased in the erased block as an indication of a number of times the erased block has been erased.

16. The method of claim 15 further including:

determining when an indicator of a number of times the erased block has been erased is available, wherein obtaining the indicator that characterizes a number of times each block in the plurality of blocks has been erased includes obtaining the indicator that characterizes the number of times each block in the plurality of blocks has been erased when it is determined that the indicator of the number of times the erased block has been erased is not available, and storing the indicator that characterizes the number of times each block in the plurality of blocks has been erased includes storing the indicator that characterizes the number of times each block in the plurality of blocks has been erased when it is determined that the indicator of the number of times the erased block has been erased is not available.

17. The method of claim 16 further including:

storing the indicator of the number of times the erased block has been erased in the erased block when it is determined that the indicator of the number of times the erased block has been erased is available, wherein when the indicator of the number of times the erased block has been erased is stored in the erased block, the indicator that characterizes the number of times each block in the plurality of blocks has been erased is not stored in the erased block.

18. The method of claim 17 wherein the indicator that characterizes the number of times each block in the plurality of blocks has been erased is an average erase count which substantially indicates an average number of times each block in the plurality of blocks has been erased.

19. The method of claim 17 wherein storing the indicator of the number of times the erased block has been erased includes storing the indicator of the number of times the erased block has been erased in an overhead area of the erased block.

20. The method of claim 17 wherein storing the that characterizes the number of times each block in the plurality of blocks has been erased in the erased block includes storing the indicator that characterizes the number of times each block in the plurality of blocks has been erased in an overhead area of the erased block.

21. The method of claim 15 further including:

determining when an indicator of a number of times the erased block has been erased is available; and

storing the indicator of the number of times the erased block has been erased in the erased block when it is determined that the indicator of the number of times the erased block has been erased is available.

22. A non-volatile memory system comprising:

a non-volatile memory, the non-volatile memory including a number of blocks;

means for obtaining an erase count for each block of the number of blocks, the erase count for each block of the number of blocks being arranged to indicate a number of times each block of the number of blocks has been erased;

means for creating a sum of the erase counts for the number of blocks; and

means for creating the average erase count, the average erase count being arranged to indicate the average number of times each block of the number of blocks has been erased, wherein the means for creating the average erase count include means for substantially dividing the sum by the number of blocks.

23. The non-volatile memory system of claim 22 wherein the erase count for a first block of the number of blocks is obtained from one of the first block and an erase count block in the non-volatile memory system, the erase count block being arranged to contain the erase counts for each block of the number of blocks.

24. A non-volatile memory system comprising:

a non-volatile memory including a plurality of blocks;

means for obtaining an erased block from the plurality of blocks;

means for obtaining an indicator that characterizes a number of times each block in the plurality of blocks has been erased; and

means for storing the indicator that characterizes a number of times each block in the plurality of blocks has been erased in the erased block as an indication of a number of times the erased block has been erased.

25. The non-volatile memory system of claim 24 further including:

means for determining when an indicator of a number of times the erased block has been erased is available; and

means for storing the indicator of the number of times the erased block has been erased in the erased block when it is determined that the indicator of the number of times the erased block has been erased is available.

26. A memory system comprising

a non-volatile memory which includes a plurality of memory elements;

code devices that obtain an erase count for each memory element of the number of memory elements, the erase count for each memory element of the number of memory elements being arranged to indicate a number of times each memory element of the number of memory elements has been erased, wherein the number of memory elements is a total number of usable memory elements within the non-volatile memory;

code devices that create a sum of the erase counts for the number of memory elements;

code devices that create the average erase count, the average erase count being arranged to indicate the average number of times each memory element of the number of memory elements has been erased, wherein the code devices that create the average erase count include code devices that substantially divide the sum by the number of memory elements; and

a memory area that stores the code devices.

27. The memory system of claim 26 wherein the memory elements are blocks.

28. The memory system of claim 27 wherein the erase count for a first memory element of the number of memory elements is obtained from one of the first memory element and an erase count block arranged to contain the erase counts for each block of the number of blocks.

29. The memory system of claim 27 wherein each memory element of the number of memory elements is a usable block.

30. The memory system of claim 26 wherein the non-volatile memory is a NAND flash memory.

31. A memory system comprising:

a non-volatile memory, the non-volatile memory including a plurality of memory elements, wherein the plurality of memory elements includes the erased memory element;

code devices that obtain the erased memory element;

code devices that obtain an indicator that characterizes a number of times each memory element in the plurality of memory elements has been erased;

code devices that store the indicator that characterizes a number of times each memory element in the plurality of memory elements has been erased in the erased memory element as an indication of a number of times the erased memory element has been erased; and

a memory area that stores the code devices.

32. The memory system of claim 31 further including:

code devices that determine when an indicator of a number of times the erased memory element has been erased is available; and

code devices that store the indicator of the number of times the erased memory element has been erased in the erased block when it is determined that the indicator of the number of times the erased memory element has been erased is available.

33. The memory system of claim 31 wherein the plurality of memory elements is a plurality of blocks.

34. The memory system of claim 33 wherein the plurality of blocks are a plurality of usable blocks.

35. The memory system of claim 31 wherein the non-volatile memory is a NAND flash memory.

36. A method for maintaining an average erase count within a memory system, the average erase count being arranged to indicate an average number of times each block of a number of blocks of a non-volatile memory associated with the memory system is erased, the memory system further including a system memory, the method comprising:

summing erase counts associated with each block of the number of blocks, each erase count being arranged to indicate a number of times an associated block of the number of blocks has been erased, wherein summing the erase counts creates a sum;

dividing the sum by the number of blocks, wherein dividing the sum by the number of blocks creates the average erase count; and

storing the average erase count into the system memory.

37. The method of claim 36 further including:

initializing the average erase count in response to an initialization request within the memory system.

38. The method of claim 37 wherein initializing the average erase count includes setting the average erase count to a value of approximately zero.

39. The method of claim 36 further including:

updating the average erase count after the average erase count is stored into the system memory.

40. The method of claim 39 wherein updating the average erase count includes:

summing updated erase counts associated with each block of the number of blocks, each updated erase count being arranged to indicate the number of times an associated block of the number of blocks has been erased, wherein summing the updated erase counts creates an updated sum;

dividing the updated sum by the number of blocks, wherein dividing the updated sum by the number of blocks creates an updated average erase count; and

storing the average erase count in the system memory with the updated erase count.