Rewritable multibit non-volatile memory with soft decode optimization
A non-volatile memory system including multi-level storage optimized for ramp sensing and soft decoding is provided. Sensing is performed at a higher bit resolution than an original user data encoding to improve the accuracy of reading state information from non-volatile storage elements. Higher resolution state information is used for decoding the original user data to improve read performance through improved error handling. Ramp sensing is utilized to determine state information by applying a continuous input scanning sense voltage that spans a range of read compare points. Full sequence programming is enabled as is interleaved coding of the user data over all of the data bit sets associated with the storage elements.
1. A method of reading non-volatile storage, comprising:
selecting a word line associated with a read operation, the word line is in communication with a group of non-volatile storage elements that are programmable to a plurality of physical states;
applying to the word line a continuous scanning voltage signal that spans a range of read reference levels for the plurality of physical states;
determining a threshold voltage of each non-volatile storage element based on time-domain sensing while applying the continuous scanning voltage signal;
determining state information for each non-volatile storage element based on the threshold voltage from the time-domain sensing, wherein the state information exceeds a number of encoded bits in each non-volatile storage element;
transferring from the non-volatile memory the state information for each storage element; and
performing an initial decode to determine data for the plurality of storage elements based on the state information for each non-volatile storage element.
2. The method of claim 1 , wherein:
the initial decode is a hard bit decode that uses a number of bits in the state information for each storage element that is equal to the number of encoded bits.
3. The method of claim 2 , further comprising:
after the initial decode, performing one or more soft bit decodes based on the state information, the one or more soft bit decodes use a number of bits in the state information for each storage element that exceeds the number of encoded bits.
4. The method of claim 1 , wherein:
the state information includes an index for each non-volatile storage element, the index indicating a sensed threshold voltage of the non-volatile storage element.
5. The method of claim 4 , wherein:
applying to the word line the continuous scanning voltage signal includes applying the continuous scanning voltage by a digital to analog converter for the word line; and
determining the threshold voltage of each non-volatile storage element includes determining a control setting of the digital to analog converter when the non-volatile storage element begins to conduct under application of the continuous scanning voltage.
6. The method of claim 5 , wherein the index is based on the control setting of the digital to analog converter.
7. The method of claim 1 , wherein:
the state information includes a plurality of hard bits and one or more soft bits for each non-volatile storage element;
transferring the state information comprises transferring all of the hard bits for each non-volatile storage element from the memory together with the one or more soft bits for each non-volatile storage element; and
the method further comprises receiving all of the hard bits and the all of the soft bits associated with the word line in a single sequence from the non-volatile memory.
8. The method of claim 7 , further comprising:
compressing the soft bits associated with the word line prior to transferring the soft bits.
9. The method of claim 7 , wherein a number of the plurality of hard bits is equal to the number of encoded bits.
10. A method of reading non-volatile storage, comprising:
selecting by a controller in communication with a non-volatile memory a word line corresponding to a read request, the word line is in communication with a group of non-volatile storage elements that are programmable to a plurality of physical states;
applying to the word line a scanning sense voltage signal that spans a range of read reference levels for the plurality of physical states;
determining a threshold voltage of each non-volatile storage element based on time-domain sensing while applying the scanning sense voltage signal;
determining a plurality of hard bits and one or more soft bits for each non-volatile storage element based on a sensed threshold voltage from the scanning sense voltage signal;
transferring from the non-volatile memory to the controller the plurality of hard bits and the one or more soft bits for each storage element; and
performing an initial decode to determine data for the plurality of storage elements based on the plurality of hard bits and the one or more soft bits for each non-volatile storage element.