Identification and operation of sub-prime blocks in nonvolatile memory
In a block-erasable nonvolatile memory array, blocks are categorized as bad blocks, prime blocks, and sub-prime blocks. Sub-prime blocks are identified from their proximity to bad blocks or from testing. Sub-prime blocks are configured for limited operation (e.g. only storing non-critical data, or data copied elsewhere, or using some additional or enhanced redundancy scheme).
1. A method of operating blocks of a memory array comprising:
categorizing the blocks into a plurality of categories including prime blocks, sub-prime blocks, and bad blocks;
wherein the categorizing includes identifying a bad block from test results obtained from the bad block, and subsequently identifying sub-prime blocks based on their proximity to the bad block;
subsequently selecting individual blocks in different planes for parallel operation in metablocks such that each block in a metablock is selected from the same category;
configuring metablocks formed from prime blocks as prime metablocks that are operated in a first manner; and
configuring metablocks formed from sub-prime blocks as sub-prime metablocks that are operated in a second manner that is different from the first manner.
2. The method of claim 1 wherein the categorizing includes:
testing each of the blocks to obtain test results;
subsequently, comparing test results for a block with first criteria to determine if the block is a bad block; and
comparing the test results for the block with second criteria to determine if the block is a sub-prime block.
3. The method of claim 2 further comprising: categorizing blocks that are not determined to be bad blocks, and are not determined to be sub-prime blocks, as good blocks.
4. The method of claim 1 wherein the categorizing includes identifying bad blocks from testing, and identifying sub-prime blocks from proximity to bad blocks and from testing of sub-prime blocks.
5. The method of claim 1 wherein the metablocks formed from sub-prime blocks are limited to operation as MLC blocks and the metablocks formed from prime blocks are configured for operation as either SLC or MLC blocks.
6. The method of claim 1 wherein the metablocks formed from sub-prime blocks are limited to store certain types of data and the metablocks formed from prime blocks are used to store any data sent to the memory array.
7. The method of claim 1 wherein operation in the second manner includes assigning a write-erase cycle count to a sub-prime block that is greater than a number of write-erase cycles actually undergone by the sub-prime block.
8. A method of operating erase blocks of a memory array comprising:
individually testing a plurality of erase blocks to determine a number of memory cells with threshold voltages that are between target threshold voltage ranges associated with logic states for each of the plurality of erase blocks;
categorizing the plurality of erase blocks into a plurality of categories based on the number, the plurality of categories including prime erase blocks, and sub-prime erase blocks including categorizing erase blocks that are in close proximity to bad blocks as sub-prime blocks;
operating prime erase blocks in a first manner;
operating sub-prime erase blocks in a second manner that is different from the first manner; and
wherein operation in the second manner is restricted to storing only data that is recoverable from other blocks in the memory array.
9. The method of claim 8 wherein the first manner includes operation in both MLC and SLC modes, and the second manner restricts sub-prime erase blocks to use in SLC mode.
10. The method of claim 8 wherein operation in the second manner is restricted to blocks used for internal copying of data.
11. The method of claim 8 wherein the data is recoverable from another copy of the data that is stored in another block in the memory array.
12. The method of claim 8 wherein the data is recoverable by performing an exclusive OR operation (XOR) on data stored in another block in the memory array.
13. The method of claim 8 categorizing the plurality of erase blocks into a plurality of categories includes categorizing erase blocks that are in close proximity to bad blocks as sub-prime blocks.
14. A method of operating erase blocks of a memory array comprising:
individually testing a plurality of erase blocks to obtain block-specific test information;
storing the block-specific test information;
subsequently, categorizing the plurality of erase blocks into a plurality of categories based on the block-specific test information, the plurality of categories including prime erase blocks, and sub-prime erase blocks;
identifying bad blocks and categorizing erase blocks that are in close proximity to bad blocks as sub-prime blocks;
configuring prime erase blocks for general use in storing user data; and
configuring sub-prime erase blocks for limited use.
15. The method of claim 14 wherein the testing includes determining a number of memory cells with threshold voltages that are between target threshold voltage ranges associated with logic states for each of the plurality of erase blocks.
16. The method of claim 14 wherein prime metablocks are configurable as either Single Level Cell (SLC) blocks or Multi Level Cell (MLC) blocks, and sub-prime erase blocks are configured exclusively as SLC blocks.
17. The method of claim 14 wherein prime metablocks are configurable as either Single Level Cell (SLC) blocks or Multi Level Cell (MLC) blocks, and sub-prime erase blocks are configured exclusively as SLC blocks.
18. The method of claim 14 wherein the sub-prime erase blocks are configured for use that is limited to storing data that is already stored in another block in the memory array, or can be reproduced from data stored in another block in the memory array.