Soft information module
View Patent ↗A system and method for generating reliability information, such as “soft information,” from a flash memory device is disclosed. A plurality of memory cells are read by a data storage controller at a first read level to obtain a plurality of program values. On an error indicator being received in connection with reading the plurality of memory cells, the plurality of memory cells are read one or more times at one or more different read levels to categorize the plurality of memory cells into two or more cell program regions. A confidence value is then assigned to each memory cell based on a corresponding cell program region for the memory cell, the confidence value being representative of a likelihood that the memory cell is programmed to a corresponding program value read at the first read level.
1. A method for generating reliability information from a flash memory device, comprising:
reading a plurality of memory cells at a first read level to obtain a plurality of program values:
receiving an error indicator in connection with the reading of the plurality of memory cells;
reading, in response to receiving the error indicator, the plurality of memory cells at one or more different read levels to categorize the plurality of memory cells into two or more cell program regions; and
assigning a confidence value to each memory cell based on a corresponding cell program region for the memory cell, the confidence value being representative of a likelihood that the memory cell is programmed to a corresponding program value read at the first read level.
2. The method of claim 1 , wherein the two or more cell program regions are bounded by the first read level and one of the different read levels.
3. The method of claim 2 , further comprising:
calculating a respective confidence value for each of the cell program regions based on how many of the plurality of memory cells are in the cell program region compared to one or more other program regions; and
assigning the confidence value calculated for each cell program region to each respective memory cell in the cell program region.
4. The method of claim 2 , wherein reading the plurality of memory cells at one or more different read levels comprises reading the plurality of memory cells using a second read level below the first read level and a third read level above the first read level, and
wherein a first program region comprises one or more cells having a programmed value between the first read level and the second read level and a second program region comprises one or more cells having a programmed value between the first read level and the third read level.
5. The method of claim 4 , wherein reading the plurality of memory cells at one or more different read levels comprises reading the plurality of memory cells using a fourth read level below the second read level and a fifth read level above the sixth read level,
wherein a third program region comprises one or more cells having a programmed value between the fourth read level and the second read level and a fourth program region comprises one or more cells having a programmed value between the fifth read level and the third read level.
6. The method of claim 1 , wherein the first read level is based on a predetermined threshold for distinguishing between two potential programming values of a memory cell, and
wherein a respective memory cell is determined to be in a respective cell program region if the respective memory cell cannot be read at the first read level but is read at one of the different read levels associated with the respective cell program region.
7. The method of claim 1 , wherein the confidence value assigned to each memory cell is retrieved from a lookup table by indexing the lookup table by the corresponding cell program region.
8. The method of claim 1 , wherein the confidence value is a log-likelihood ratio.
9. The method of claim 1 , further comprising:
determining one or more cell program distributions for the plurality of program values,
wherein, for a distribution, a lower confidence value is assigned to a cell program region comprising program values at an edge of the distribution than a cell program region comprising program values closer to a mean of the distribution.
10. The method of claim 1 , further comprising:
applying an initial read level to the plurality of memory cells to obtain a least significant bit value for each of the plurality of memory cells; and
determining the first read level based on the least significant bit value.
11. A system, comprising:
one or more flash memory devices;
a controller operable to initiate one or more memory commands to the one or more flash memory devices, the controller further operable to:
initiate a first read of a plurality of memory cells of the one or more flash memory devices;
determine, based on the first read, a program level for one or more of the plurality of memory cells;
receive an error indicator associated with the first read;
initiate, in response to the error indicator, multiple reads of the one or more memory cells at different levels; and
determine a probability that the one or more memory cells are programmed to the program level based on the multiple reads.
12. The system of claim 11 , wherein the plurality of memory cells are read using a first read level, the controller further operable to:
categorize the one or more memory cells into two or more cell program regions based on the multiple reads; and
determine the probability based on the program level and a cell program region in which the one or more memory cells are categorized.
13. The system of claim 12 , the controller further operable to:
calculate, for each cell program region, a confidence value based on how many of the plurality of memory cells are in the cell program region compared to one or more other program regions, and assign the confidence value to each memory cell in the cell program region.
14. The system of claim 12 , wherein the controller being operable to initiate multiple reads comprises the controller being operable to:
initiate reading the one or more memory cells using a second read level below the first read level and a third read level above the first read level, and
wherein a first program region comprises one or more cells having a programmed value between the first read level and the second read level and a second program region comprises one or more cells having a programmed value between the first read level and the third read level.
15. The system of claim 12 , wherein the controller being operable to initiate multiple reads comprises the controller being operable to:
initiate reading the one or more memory cells using a fourth read level below the second read level and a fifth read level above the sixth read level,
wherein a third program region comprises one or more cells having a programmed value between the fourth read level and the second read level and a fourth program region comprises one or more cells having a programmed value between the fifth read level and the third read level.
16. The system of claim 11 , the controller further operable to:
apply an initial read level to the one or more memory cells to obtain a least significant bit value for each memory cell; and
determine a first read level for the first read based on the least significant bit value.
17. The system of claim 11 , wherein determining the probability comprises associating the one or more memory cells with a log-likelihood ratio.
18. The system of claim 17 , wherein the log-likelihood ratio associated with each memory cell is retrieved from a lookup table by indexing the lookup table by the corresponding cell program region.
19. A machine-readable medium having instructions stored thereon that, when executed by a processor, perform a method for providing memory device readiness to a memory controller, the method comprising:
initiating a first read of a plurality of memory cells of one or more flash memory devices;
determining, based on the first read, a program level for one or more of the plurality of memory cells;
initiating, in response to the error indicator, multiple reads of the one or more memory cells at different levels;
categorize the one or more memory cells into two or more cell groups based on the multiple reads; and
determining a probability that the one or more memory cells are programmed to the program level based on the program level and a cell group in which the one or more memory cells are categorized.