Non-volatile semiconductor storage system
In a memory cell array, memory cells enabled to store plural-bit data are arranged in matrix. The bit-line control circuit is connected to bit-lines to control the bit-lines. A word line control circuit applies a plural-bit data read voltage as a word line voltage to the word line. The plural-bit data read voltage is larger than an upper limit of one of plural threshold voltage distributions and smaller than a lower limit of another threshold voltage distribution. Furthermore, it applies a soft-value read voltage as a word line voltage to the word line. The soft-value read voltage is smaller than an upper limit of a threshold voltage distribution and larger than a lower limit thereof. The likelihood calculation circuit calculates likelihood of the plural-bit data stores in the memory cells based on the soft-value.
1. A non-volatile semiconductor storage system comprising:
a memory cell array comprising a plurality of memory cells enabled to store multi-value data, the memory cells being arranged along a plurality of bit-lines and a plurality of word-lines;
a bit-line control circuit connected to the bit-lines to control the bit-line;
a word-line control circuit controlling applying a plural-bit data read voltage as a word line voltage to the word line, the plural-bit data read voltage being larger than an upper limit of one of plural threshold voltage distributions representing the plural-bit data and smaller than a lower limit of another threshold voltage distribution with higher threshold voltages than the one of the plural threshold voltage distributions, and applying a soft-value read voltage as a word line voltage to the word line, the soft-value read voltage being smaller than an upper limit of each of the plurality of the threshold voltage distributions and larger than a lower limit thereof to generate a plurality of soft-values; and
a likelihood calculation circuit calculating a likelihood of plural-bit data stored in the memory cell based on the soft-value generated by setting the word-line voltage at the soft-value read voltage.
2. The non-volatile semiconductor storage system according to claim 1 , further comprising an error correction circuit executing data error correction for the plural-bit data based on the likelihood.
3. The non-volatile semiconductor storage system according to claim 2 , wherein the error correction circuit is enabled to execute error detection and error correction using the plural-bit data and redundant data while it reads the soft-value in parallel.
4. The non-volatile semiconductor storage system according to claim 2 , wherein the error correction circuit is enabled to execute error correction using the plural-bit data held in the plurality of memory cells and redundant data, and when this is judged to have ended up in failure, configured to start error correction based on the likelihood calculated using the plural-bit data and the redundant data as well as the soft-value.
5. The non-volatile semiconductor storage system according to claim 3 , wherein the redundant data is stored in the memory cell.
6. The non-volatile semiconductor storage system according to claim 1 , wherein when all errors has been corrected by the error correction based on the redundant data, the error correction circuit is configured to output the data after the error correction.
7. The non-volatile semiconductor storage system according to claim 1 , wherein when error correction based on the likelihood calculated using the soft-value is judged to have ended up in failure, the number of the soft-value read voltages is increased, and again the soft-values are obtained to calculate the likelihood by the likelihood calculation circuit.
8. The non-volatile semiconductor storage system according to claim 1 , wherein the likelihood calculation circuit is configured to generate a first soft-value first by setting the soft-value read voltage at a first voltage having an approximately intermediate value of each of the threshold value distributions, and subsequently, generate a second soft-value by setting the soft-value read voltage at a second voltage having an value between the first voltage and an upper limit or a lower limit of the threshold value distribution, and calculate the likelihood based on the first and second soft-values,
wherein the first and second soft-values are generated by lowering the first voltage or the second voltage in a step-like manner, and, based on a state of data read when the first voltage or the second voltage is set at a certain value, inverting data read when the first voltage or the second voltage is set at a value lower than the certain value by one step.
9. The non-volatile semiconductor storage system according to claim 8 , wherein in case data read when the first voltage or the second voltage is set at the certain value coincides with data read when the first voltage or the second voltage is set at a value lower than the certain value by one step, the data is inverted and output as the first soft-value or the second soft-value.
10. The non-volatile semiconductor storage system according to claim 8 , wherein when error correction based on the likelihood calculated using the soft-value is judged to have ended up in failure, the number of the second voltages is increased to newly obtain the soft-values and to calculate the likelihood by the likelihood calculation circuit.
11. The non-volatile semiconductor storage system according to claim 1 , wherein the soft-value read voltage varies with time.
12. The non-volatile semiconductor storage system according to claim 1 , further comprising a data refresh circuit executing rewriting of data in the memory cell to restrict change in threshold voltages of the memory cell with time.
13. The non-volatile semiconductor storage system according to claim 1 , further comprising a first likelihood calculation circuit calculating likelihood of the plural-bit data based on the plural-bit data and the redundant data, and a second likelihood calculation circuit calculating likelihood of the plural-bit data based on the soft-values.
14. The non-volatile semiconductor storage system according to claim 1 , further comprising an error correction circuit executing error correction based on either the likelihood calculated by the first likelihood calculation circuit, or the likelihood calculated by the second likelihood calculation circuit.
15. The non-volatile semiconductor storage system according to claim 1 , further comprising:
a flag cell selected simultaneously with a plurality of the memory cells selected each of the word-lines and storing flag data indicating a writing situation of the multi-value data to the memory cells; and
a flag data storage circuit storing the flag data to be written in the flag cell.
16. The non-volatile semiconductor storage system according to claim 15 , wherein the flag data is rewritten when the upper page compared to the first page has been written.