Memory system and method of operating memory system using soft read voltages

A method of operating a memory system, the method comprising:

• applying a plurality of hard read voltages to a word line;
• sequentially applying at least one pair of soft read voltages corresponding to each hard read voltage among the plurality of hard read voltages to the word line;
• and
• correcting a readout error of data programmed in nonvolatile memory cells connected to the word line on the basis of reliable data, the reliable data being read from the nonvolatile memory cells using each of the hard read voltages and the corresponding at least one pair of soft read voltages,
• wherein each pair of the at least one pair of soft read voltages has differently set voltage differences from the corresponding hard read voltage on the basis of a degree of asymmetry of adjacent threshold voltage distributions having different bit values of hard data identified by the corresponding hard read voltage, and
• wherein the differences from the corresponding hard read voltage for each pair of the at least one pair of soft read voltages correspond to results of counting the number of arbitrary bit values read using a voltage level in a threshold voltage distribution corresponding to an erase state or a threshold voltage distribution having the highest voltage level.

A memory system comprising:

• a flash memory comprising a plurality of memory cells arranged in a plurality of word lines;
• and
• a memory controller for controlling a read operation of the flash memory, the memory controller being configured to provide a normal read command to the flash memory to read normal read data from the plurality of non-volatile memory cells, and to provide a soft decision command to the flash memory when an error is detected in the normal read data,
• wherein the flash memory is configured to apply a first hard read voltage, a first soft read voltage, and a second soft read voltage paired with the first soft read voltage to a selected word line in response to the soft decision command to read corresponding memory cells, wherein a voltage level difference between the first soft read voltage and the first hard read voltage is indicated by a first voltage value and a voltage level difference between the second soft read voltage and the first hard read voltage is indicated by a second voltage value, the second voltage value being different from the first voltage value.