Method of achieving wear leveling in flash memory using relative grades

• US 7,224,604 B2
• Filed: 11/29/2005
• Issued: 05/29/2007
• Est. Priority Date: 03/14/2005
• Status: Active Grant

• Current Assignee: Western Digital Israel Limited (Western Digital Corporation)
• Original Assignee: SanDisk IL Ltd. (Western Digital Corporation)
• Inventors: Lasser, Menahem
• Primary Examiner(s): NGUYEN, VAN THU T
• Application Number:
  US11/288,161
• Publication Number:
  US 20060203546A1
• Time in Patent Office: 546 Days
• Field of Search: 365/185.09, 711/103, 711/156
• US Class Current: 365/185.09
• 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 2216/18 Flash erasure of all the cells in an array, sector or block simultaneously

Abstract

For each block of a memory, a number is calculated that is a function of how many times the block has been erased and of how many times at least one other block has been erased. The numbers are stored in the memory device that includes the memory. The numbers are updated as needed when blocks are erased. Blocks are selected to be erased in accordance with their numbers. Preferably, each block'"'"'s function is the block'"'"'s relative grade.

Claims

1. A method of managing a memory, of a memory device, that includes a plurality of blocks, the method comprising the steps of, for each block:

(a) calculating a number that is a function of how many times said each block has been erased and of how many times at least one other block has been erased; and

(b) storing said number in the memory device;

wherein, for at least two of the blocks, said at least one other block of a first of said at least two blocks is different from said at least one other block of a second of said at least two blocks.

2. The method of claim 1, further comprising the step of, for each block:

(c) updating said number whenever said each block is erased.

3. The method of claim 1, further comprising the step of, for each block:

(c) updating said number whenever one of said at least one other block is erased.

4. The method of claim 1, further comprising the step of:

(c) selecting at least one block to be erased at least in part in accordance with said numbers.

5. The method of claim 1, wherein said storing is effected by steps including, for each of at least one of the blocks, storing said number in said each block.

6. The method of claim 1, wherein said storing is effected by steps including, for each of at least one of the blocks, storing said number outside of said each block.

7. The method of claim 1, wherein, for each block, said function is a relative grade of said each block.

8. The method of claim 7, wherein, for each of at least one of the blocks, said relative grade depends on a difference between an erase count of said each block and an erase count of one other block.

9. The method of claim 7, wherein, for each of at least one of the blocks, said relative grade depends on a difference between an erase count of said each block and an average erase count of at least two other blocks.

10. A memory device comprising:

(a) a memory that includes a plurality of blocks; and

(b) a controller for managing said memory by steps including;

for each said block;

(i) calculating a number that is a function of how many times said each block has been erased and of how many times at least one other said block has been erased, and

(ii) storing said number in the memory device;

wherein, for at least two of the blocks, said at least one other block of a first of said at least two blocks is different from said at least one other block of a second of said at least two blocks.

11. The memory device of claim 10, wherein said memory is a flash memory.

12. A computer-readable storage medium having computer-readable code embodied on said computer-readable storage medium, the computer-readable code for managing a memory, of a memory device, that includes a plurality of blocks, the computer-readable code comprising:

(a) program code for, for each block, calculating a number that is a function of how many times said each block has been erased and of how many times at least one other said block has been erased; and

(b) program code for, for each block, storing said number in the memory device;

wherein, for at least two of the blocks, said at least one other block of a first of said at least two blocks is different from said at least one other block of a second of said at least two blocks.

13. A method of managing a memory, of a memory device, that includes a plurality of blocks, the method comprising the steps of, for each block:

(a) calculating a number that is a function of how many times said each block has been erased and of how many times at least one other block has been erased; and

(b) storing said number in the memory device;

wherein, for each block, said function is a relative grade of said each block; and

wherein, for each of at least one of the blocks, said relative grade depends on a difference between an erase count of said each block and an erase count of a neighbor of said each block.

14. A memory device comprising:

(a) a memory that includes a plurality of blocks; and

(b) a controller for managing said memory by steps including;

for each said block;

(i) calculating a number that is a function of how many times said each block has been erased and of how many times at least one other said block has been erased, and

(ii) storing said number in the memory device;

wherein, for each block, said function is a relative grade of said each block; and

wherein, for each of at least one of the blocks, said relative grade depends on a difference between an erase count of said each block and an erase count of a neighbor of said each block.

15. A computer-readable storage medium having computer-readable code embodied on said computer-readable storage medium, the computer-readable code for managing a memory, of a memory device, that includes a plurality of blocks, the computer-readable code comprising:

(a) program code for, for each block, calculating a number that is a function of how many times said each block has been erased and of how many times at least one other said block has been erased; and

(b) program code for, for each block, storing said number in the memory device;

wherein, for each block, said function is a relative grade of said each block; and

wherein, for each of at least one of the blocks, said relative grade depends on a difference between an erase count of said each block and an erase count of a neighbor of said each block.

16. A method of managing a memory, of a memory device, that includes a plurality of blocks, the method comprising the steps of, for each block:

(a) calculating a number that is a function of how many times said each block has been erased and of how many times at least one other block has been erased; and

(b) storing said number in the memory device;

wherein, for each block, said function is a relative grade of said each block; and

wherein, for each of at least one of the blocks, said relative grade depends on a difference between an erase count of said each block and an average erase count of at least two other blocks, one of said two other blocks being a neighbor of said each block.

17. A memory device comprising:

(a) a memory that includes a plurality of blocks; and

(b) a controller for managing said memory by steps including;

for each said block;

(i) calculating a number that is a function of how many times said each block has been erased and of how many times at least one other said block has been erased, and

(ii) storing said number in the memory device;

wherein, for each block, said function is a relative grade of said each block; and

wherein, for each of at least one of the blocks, said relative grade depends on a difference between an erase count of said each block and an average erase count of at least two other blocks, one of said two other blocks being a neighbor of said each block.

18. A computer-readable storage medium having computer-readable code embodied on said computer-readable storage medium, the computer-readable code for managing a memory, of a memory device, that includes a plurality of blocks, the computer-readable code comprising:

(a) program code for, for each block, calculating a number that is a function of how many times said each block has been erased and of how many times at least one other said block has been erased; and

(b) program code for, for each block, storing said number in the memory device;

wherein, for each block, said function is a relative grade of said each block; and

wherein, for each of at least one of the blocks, said relative grade depends on a difference between an erase count of said each block and an average erase count of at least two other blocks, one of said two other blocks being a neighbor of said each block.

19. A method of managing a memory, of a memory device, that includes a plurality of blocks, the method comprising the steps of, for each block:

(a) calculating a number that is a function of how many times said each block has been erased and of how many times at least one other block has been erased; and

(b) storing said number in the memory device;

wherein said calculating includes, for each block, truncating said function at a number of bits insufficient to store a maximum allowed number of erase cycles per block.

20. A memory device comprising:

(a) a memory that includes a plurality of blocks; and

(b) a controller for managing said memory by steps including;

for each said block;

(i) calculating a number that is a function of how many times said each block has been erased and of how many times at least one other said block has been erased, and

(ii) storing said number in the memory device;

wherein said calculating includes, for each block, truncating said function at a number of bits insufficient to store a maximum allowed number of erase cycles per block.

21. A computer-readable storage medium having computer-readable code embodied on said computer-readable storage medium, the computer-readable code for managing a memory, of a memory device, that includes a plurality of blocks, the computer-readable code comprising:

(a) program code for, for each block, calculating a number that is a function of how many times said each block has been erased and of how many times at least one other said block has been erased; and

(b) program code for, for each block, storing said number in the memory device;

wherein said calculating includes, for each block, truncating said function at a number of bits insufficient to store a maximum allowed number of erase cycles per block.