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

• US 7,287,118 B2
• Filed: 02/25/2006
• Issued: 10/23/2007
• Est. Priority Date: 10/28/2002
• Status: Expired due to Term

• 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.
• Primary Examiner(s): Elmore; Stephen C.
• Assistant Examiner(s): KIM, DANIEL Y
• Application Number:
  US11/362,666
• Publication Number:
  US 20060149896A1
• Time in Patent Office: 605 Days
• Field of Search: 365/200, 365/230, 365/185.33, 365/185.02, 365/185.29, 365/230.03, 711/103, 711/154, 711/156, 711/102, 711/165
• 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 method of managing a non-volatile memory of a memory system according to an average number of times each block of a number of blocks within the non-volatile memory has been erased, the method comprising:

for a first block of the number of blocks, comparing an erase count stored in the first block, the erase count indicating a number of times that block has been erased, with an erase count value for the first block stored in a non-volatile portion of a system memory of the memory system;

responsive to the comparing step determining that the erase count stored in the first block is greater than the erase count value for the first block stored in the system memory, updating the erase count value for the first block stored in system memory to correspond to the erase count stored in the first block;

also responsive to the comparing step determining that the erase count stored in the first block is greater than the erase count value, obtaining an erase count-for each block of the number of blocks, wherein the number of blocks is a total number of usable blocks within the non-volatile memory;

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

creating an average erase count, from the sum of the obtained erase counts; and

storing an indicator of the average erase count in a non-volatile portion of the system memory of the memory system.

2. The method of claim 1 wherein the non-volatile memory is a NAND flash memory.

3. The method of claim 1, wherein the step of obtaining an erase count for each block of the number of blocks comprises:

for each block of the number of blocks, obtaining an erase count stored in that block;

and further comprising;

after the step of obtaining an erase count for each block of the number of blocks, updating the erase count value for each of the number of blocks stored in system memory to correspond to the obtained erase count for that block.

4. The method of claim 3, wherein the step of creating the average erase count comprises:

dividing the sum of the obtained erase counts by the number of blocks.

5. A non-volatile memory system comprising:

a non-volatile memory, the non-volatile memory including a number of blocks, each block including a redundant area for storing overhead data comprising an erase count for that block;

a system memory, associated with the non-volatile memory, and having a non-volatile portion including an erase count block; and

means for managing the erase count block by performing a sequence of operations comprising;

comparing an erase count stored in the redundant area of a first block of the number of blocks, with an erase count value associated with the first block that is stored in the erase count block, the erase count arranged to indicate a number of times the block has been erased;

updating the erase count value in the erase count block for the first block, to correspond to the erase count stored in the first block, responsive to the comparing means determining that the erase count stored in the first block is greater than the erase count value stored in the erase count block for the first block;

obtaining an erase count for each block of the number of blocks from the redundant area of each block, responsive to the comparing means determining that the erase count stored in the redundant area of the first block is less than the erase count value for that block stored in the erase count block;

creating an average erase count, indicating an average number of times each block of the number of blocks has been erased; and

storing a first indicator corresponding to the average erase count in the non-volatile portion of the system memory.

6. The non-volatile memory system of claim 5, wherein the operation of creating an average erase count comprises the operations of:

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

dividing the sum by the number of blocks.

7. The non-volatile memory system of claim 5, wherein the managing means comprises:

an erase count block management software submodule; and

a microprocessor for executing the erase count block management software submodule.

8. The non-volatile memory system of claim 7, wherein the erase count block management software submodule comprises:

code devices that obtain an erase count for each block of the plurality of blocks, the erase count for each block of the plurality of blocks being arranged to indicate a number of times each block of the plurality of blocks has been erased, wherein the plurality of blocks correspond to usable blocks within the non-volatile memory;

code devices that create a sum of the erase counts for the plurality of blocks;

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 plurality of blocks has been erased, wherein the code devices that create the average erase count include code devices that divide the sum by the number of blocks in the plurality of blocks; and

a memory area that stores the code devices.

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