Non-volatile memory device and method for reading cells
A non-volatile device and method of operating the device including changing a read reference level for reading a group of memory cells as a function of changes in a threshold voltage distribution of a different group of memory cells. The changing step may include determining a history read reference level of a history cell associated with a group of memory cells of a non-volatile memory cell array and comparing sensed logical state distributions with stored logical state distributions.
1. A method for selecting a read reference level associated with a given logical state for a set of non-volatile memory (“NVM”) cells, said method comprising:
deriving an initial read reference level from a history cell associated with the set of NVM cells; and
comparing logical state distribution of the set of NVM cells sensed using the initial read reference level against a stored logical state distribution of the set of NVM cells.
2. The method according to claim 1 , wherein deriving an initial read reference level comprises determining a reference level for the history cell.
3. The method according to claim 2 , wherein the initial read reference level associated with a given logical state for a set of NVM cells is at a level not equal to the reference level of the history cell from which it is derived.
4. The method according to claim 3 , wherein a difference between the initial read reference level associated with a given logical state for a set of NVM cells and the reference level for the history cell from which it is derived is based on a predetermined margin to be maintained.
5. The method according to claim 2 , wherein the initial read reference level associated with a given logical state for a set of NVM cells is at a level substantially equal to the read reference level of the history cell from which it is derived.
6. The method according to claim 1 further comprising adjusting the initial read reference level based on results of the comparison of logical state distributions.
7. The method according to claim 2 , wherein the initial read reference level is adjusted upward if the number of NVM cells sensed at a given logical state is higher than a stored value.
8. The method according to claim 2 , wherein the initial read reference level is adjusted downward if the number of cells sensed at a given logical state is lower than a stored value.
9. An non-volatile memory (“NVM”) device comprising:
an NVM array including a set of non-volatile memory (“NVM”) cells; and
control logic adapted derive an initial read reference level from a history cell associated with the set of NVM cells; and
to compare logical state distribution of the set of NVM cells sensed using the initial read reference level against a stored logical state distribution of the set of NVM cells.
10. The device according to claim 9 , wherein the control logic is adapted to derive an initial read reference level by determining a reference level for the history cell.
11. The device according to claim 10 , wherein the initial read reference level associated with a given logical state for a set of NVM cells is at a level not equal to the reference level of the history cell from which it is derived.
12. The device according to claim 11 , wherein a difference between the initial read reference level associated with a given logical state for a set of NVM cells and the reference level for the history cell from which it is derived is based on a predetermined margin to be maintained.
13. The device according to claim 10 , wherein the initial read reference level associated with a given logical state for a set of NVM cells is at a level substantially equal to the reference level of the history cell from which it is derived.
14. The device according to claim 9 , wherein said control logic is further adapted to adjust the initial read reference level based on results of the comparison of logical state distributions.
15. The device according to claim 10 , wherein the initial read reference level is adjusted upward if the number of NVM cells sensed at a given logical state is higher than a stored value.
16. The device according to claim 10 , wherein the initial read reference level is adjusted downward if the number of cells sensed at a given logical state is lower than a stored value.