Calibrating non-volatile memory read thresholds
View Patent ↗In a read scan operation, a first read level window is scanned for a first candidate read level that activates the fewest number of memory cells in relation to other candidate read levels within that window. A second read level window for a second candidate read level is then configured based on a correlation between at least one of the two adjacent memory states and one or more other adjacent memory states associated with the second read level window. The second read level window is scanned for a second candidate read level that activates the fewest number of memory cells, or results in the fewest bit errors, in relation to other candidate read levels within the second read level window. Next, a read operation is configured to use the first candidate read level and the second candidate read level.
1. A method of performing a read scan operation on a memory having a plurality of memory cells, the method comprising:
scanning a first read level window for a first candidate read level that activates a fewest number of memory cells in relation to other candidate read levels within the first read level window, the first read level window configured to test read levels between two adjacent memory states;
configuring a second read level window for a second candidate read level based on a correlation between at least one of the two adjacent memory states and one or more other adjacent memory states associated with the second read level window;
scanning the second read level window for a second candidate read level that activates the fewest number of memory cells in relation to other candidate read levels within the second read level window; and
configuring a read operation to use the first candidate read level and the second candidate read level.
2. The method of claim 1 , wherein configuring the second read level window comprises:
determining a correlation factor by searching a correlation data structure based on an identifier for one of the two adjacent memory states;
applying the correlation factor to the second read level window such that the correlation affects candidate read levels of the second read level window.
3. The method of claim 2 , wherein applying the correlation factor comprises multiplying a candidate read level of the second read level window by the correlation factor such that the candidate read level accounts for the correlation factor.
4. The method of claim 2 , wherein applying the correlation factor comprises changing a predefined order for the candidate read levels of the second read level window such that candidate read levels that account for the correlation are used in the scanning before other candidate read levels.
5. The method of claim 1 , wherein the configured second read level window comprises fewer candidate read levels than the first read level window.
6. The method of claim 1 , wherein the correlation comprises a positive correlation between a first memory state and a second memory state.
7. The method of claim 1 , wherein the correlation comprises a negative correlation between a first memory state and a second memory state.
8. The method of claim 1 , wherein scanning the first read level window comprises:
testing the first candidate read level and the second candidate read level in a predefined order; and
designating the first candidate read level as the first read level in response to the second candidate read level activating more memory cells than the first candidate read level.
9. The method of claim 1 , wherein scanning the second read level window further comprises iteratively testing candidate read levels in a predefined order and wherein configuring the second read level window comprises changing the predefined order based on the correlation.
10. The method of claim 9 , wherein changing the predefined order comprises skipping iteratively testing a first candidate read level in response to the second candidate read level activating the fewest number of memory cells in relation to other candidate read levels within the second read level window, the second candidate read level determined based on the correlation.
11. The method of claim 1 , further comprising determine the correlation based on a first memory state from the two adjacent memory states and a second memory state from the other adjacent memory states associated with the second read level window in response to the first memory state and the second memory state having a stable correlation.
12. An apparatus, comprising:
a three-dimensional memory array of memory cells; and
a read scan circuit configured to:
iteratively sense a set of memory cells using a first set of candidate read levels until a candidate read level activates a fewest number of memory cells in relation to other candidate read levels within the first set of candidate read levels;
determine a first read level for a first memory state based on the candidate read level that activates the fewest number of memory cells;
retrieve a correlation between the first memory state and a second memory state;
determine a second read level for the second memory state using the correlation; and
set the first read level and the second read level for subsequent read operations.
13. The apparatus of claim 12 , wherein the read scan circuit determines the second read level by multiplying a current read level for the second memory state by a correlation factor representative of the correlation.
14. The apparatus of claim 12 , wherein the read scan circuit determines the second read level by:
choosing a second set of candidate read levels based on the correlation, wherein the second set of candidate read levels is smaller than the first set of candidate read levels; and
determining the second read level for the second memory state by iteratively testing the second set of candidate read levels until the candidate read level activates a fewest number of memory cells in relation to the other candidate read levels within the second set of candidate read levels.
15. The apparatus of claim 12 , further comprising:
a volatile memory coupled to the read scan circuit, the volatile memory comprising a correlation data structure configured to store correlation factors for each memory state of the set of memory cells;
wherein the correlation factor represents the correlation and multiplying the candidate read level by the correlation factor modifies the candidate read level to account for the correlation.
16. The apparatus of claim 12 , wherein the correlation between the first memory state and the second memory state comprises a width correlation configured to represent a relationship between a widening of cell threshold voltage distribution in the first memory state in relation to the widening of cell threshold voltage distribution in the second memory state.
17. The apparatus of claim 12 , wherein the correlation between the first memory state and the second memory state comprises a shift correlation configured to represent a relationship between a shifting of cell threshold voltage distribution in the first memory state in relation to the shifting of cell threshold voltage distribution in the second memory state.
18. A system, comprising:
a non-volatile memory array comprising a plurality of memory dies; and
a storage controller comprising:
a read/write circuit configured to write data to memory cells of the plurality of memory dies;
a read scan circuit configured to implement a read scan operation for a storage block of a plurality of storage blocks of the non-volatile memory array, the read scan operation configured to test candidate read levels of a set of candidate read levels based on a correlation between two memory states for memory cells of the storage block;
an error correction code decoder configured to determine an estimated bit error rate for the data read during the read scan operation; and
a calibration circuit configured to calibrate memory cells based on read levels determined by the read scan circuit.
19. The system of claim 18 , wherein the read scan circuit is configured to:
determine a first read level for reading data programmed to a first set of memory cells of the storage block, the first set of memory cells having one of the two memory states;
determine the correlation between the two memory states;
determine a second read level for reading data programmed to a second set of memory cells of the storage block, the second set of memory cells having one of two other memory states, the second read level determined based on the correlation between at least one of the two memory states and at least one of the two other memory states;
set the first read level as a current read level for reading data from the first set of memory cells; and
set the second read level as a current read level for reading data from the second set of memory cells.
20. The system of claim 18 , wherein the correlation comprises one or more of a width correlation and a shift correlation.