Systems for reverse reading in non-volatile memory with compensation for coupling
Shifts in the apparent charge stored by a charge storage region such as a floating gate in a non-volatile memory cell can occur because of electrical field coupling based on charge stored in adjacent (or other) charge storage regions. Although not exclusively, the effects are most pronounced in situations where adjacent memory cells are programmed after a selected memory cell. To account for the shift in apparent charge, one or more compensations are applied when reading storage elements of a selected word line based on the charge stored by storage elements of other word lines. Efficient compensation techniques are provided by reverse reading blocks (or portions thereof) of memory cells. By reading in the opposite direction of programming, the information needed to apply (or select the results of) an appropriate compensation when reading a selected cell is determined during the actual read operation for the adjacent word line rather than dedicating a read operation to determine the information.
1. A non-volatile memory system, comprising:
a first set of select gates;
a second set of select gates;
a plurality of word lines including a first word line adjacent said first set of select gates and a last word line adjacent said second set of select gates;
a plurality of non-volatile storage elements coupled to said plurality of word lines; and
managing circuitry in communication with said plurality of non-volatile storage elements, said managing circuitry programs said storage elements beginning with said first word line and ending with said last word line by changing a threshold voltage of selected ones of said storage elements in accordance with a target memory state, said managing circuitry reads said non-volatile storage elements beginning with said last word line and ending with said first word line, said managing circuitry applies one or more compensations when reading each word line except said last word line based on reading a word line adjacent to said each word line in a direction of said second set of select gates, said managing circuitry temporarily stores data from said plurality of non-volatile storage elements in a memory beginning with data from non-volatile storage elements coupled to said last word line and ending with data from non-volatile storage elements coupled to said first word line; and
an output, said managing circuitry provides data from said non-volatile storage elements coupled to said plurality of word lines to said output in a sequence beginning with data from non-volatile storage elements coupled to said first word line and ending with data from non-volatile storage elements coupled to said last word line.
2. The non-volatile memory system of claim 1 , wherein said managing circuitry applies one or more compensations when reading each word line except said last word line by:
performing a plurality of subreads and selecting a result of one of said subreads for each storage element of said each word line based on reading a word line adjacent to said each word line in a direction of said second set of select gates.
3. The non-volatile memory system of claim 2 , wherein said managing circuitry performing a plurality of subreads for each word line except said last word line includes:
performing a first subread by applying a first set of read reference voltages to said each word line and sensing a conduction of said storage elements of said each word line; and
performing a second subread by applying a second set of read reference voltages to said each word line and sensing a conduction of said storage elements of said each word line.
4. The non-volatile memory system of claim 3 , wherein said managing circuitry selecting a result of one of said subreads includes:
selecting a result of said first subread for each storage element of said each word line having an adjacent memory cell on said adjacent word line that was determined to be in a first physical state during said reading said word line adjacent to said each word line; and
selecting a result of said second subread for each storage element of said each word line having an adjacent memory cell of said adjacent word line that was determined to be in a second physical state during said reading said word line adjacent to said each word line.
5. The non-volatile memory system of claim 2 , wherein said managing circuitry performing a plurality of subreads for each word line except said last word line includes:
performing a first subread by applying a set of read reference voltages to said each word line, applying a first voltage to said word line adjacent said each word line, and sensing a conduction of said storage elements of said each word line while applying said set of read reference voltages and said first voltage; and
performing a second subread by applying said set of read reference voltages to said each word line, applying a second voltage to said word line adjacent said each word line, and sensing a conduction of said storage elements of said each word line while applying said set of read reference voltages and said second voltage.
6. The non-volatile memory system of claim 5 , wherein said managing circuitry selecting a result of one of said subreads includes:
selecting a result of said first subread for each storage element of said each word line having an adjacent memory cell of said adjacent word line that was determined to be in a first physical state during said reading said word line adjacent to said each word line; and
selecting a result of said second subread for each storage element of said each word line having an adjacent memory cell of said adjacent word line that was determined to be in a second physical state during said reading said word line adjacent to said each word line.
7. The non-volatile memory system of claim 1 , wherein:
said non-volatile storage elements coupled to said plurality of word lines are formed on one or more memory chips; and
said managing circuitry temporarily storing data from said plurality of non-volatile storage elements includes buffering said data at said one or more memory chips.
8. The non-volatile memory system of claim 1 , wherein:
said non-volatile storage elements coupled to said plurality of word lines are formed on one or more memory chips;
at least a portion of said managing circuitry is on a different chip; and
said managing circuitry temporarily storing data from said plurality of non-volatile storage elements includes buffering said data at a memory on said different chip.
9. The non-volatile memory system of claim 1 , further comprising:
a random access memory;
said managing circuitry temporarily storing data from said plurality of non-volatile storage elements includes buffering said data in said random access memory.
10. The non-volatile memory system of claim 1 , wherein said managing circuitry applies one or more compensations when reading each word line except said last word line by:
performing a single subread while applying at least one bit line based compensation for each storage element coupled to said each word line based on reading said word line adjacent to said each word line.
11. The non-volatile memory system of claim 10 , wherein said managing circuitry applying at least one bit line based compensation for each storage element includes:
controlling at least one of an integration time, a pre-charge voltage, and a breakpoint voltage used by a sense module for reading said each storage element.
12. The non-volatile memory system of claim 1 , further comprising:
one or more data latches associated with a bit line of said each storage element;
wherein said managing circuitry, after reading storage elements of each word line and before reading storage elements of another word line, stores data values for each storage element of said each word line in said one or more data latches associated with said each storage element;
wherein said managing circuitry, after storing said data values for each storage element of said each word line, buffers said data values in a different memory.
13. The non-volatile memory system of claim 12 , wherein:
said managing circuitry storing data values for each storage element of said each word line includes, for each word line except said last word line, overwriting one or more data values from a previously read storage element of an adjacent word line in said one or more data latches; and
said managing circuitry buffering said data values includes, for each word line except said last word line, storing said data values with data values from previously read storage elements of an adjacent word line.
14. The non-volatile memory system of claim 1 , wherein said managing circuitry programs said non-volatile storage elements coupled to said plurality of word lines using full sequence programming.
15. The non-volatile memory system of claim 1 , wherein said managing circuitry programs said non-volatile storage elements coupled to said plurality of word lines using upper page/lower page programming.
16. The non-volatile memory system of claim 1 , wherein said managing circuitry programs said non-volatile storage elements coupled to said plurality of word lines using last first mode programming.
17. The non-volatile memory system of claim 1 , wherein said non-volatile storage elements are multi-state non-volatile storage elements.
18. The non-volatile memory system of claim 1 , wherein said managing circuitry includes at least one of a controller and a state machine.
19. The non-volatile memory system of claim 1 , wherein said plurality of non-volatile storage elements are part of a NAND flash memory system.