Memory system and method of controlling nonvolatile memory

摘要

According to an embodiment, a controller specifies a first voltage range that has a first distribution quantity, a second voltage range that is adjacent to a lower voltage side of the first voltage range, and a third voltage range that is adjacent to a higher voltage side of the first voltage range. The first distribution quantity is a minimum value of the memory cells. The controller determines a read voltage by using the first voltage range, a first representative voltage value in the first voltage range, the first distribution quantity, a second distribution quantity corresponding to the second voltage range, and a third distribution quantity corresponding to the third voltage range.

主权项

1. A memory system comprising:
a nonvolatile memory that includes a plurality of memory cells, each of the memory cells having a storage capacity of N bits in accordance with 2 to the power of N threshold voltage distributions, where N is a natural number that is equal to or greater than 1; and a controller configured to
read data from the plurality of memory cells by using four voltage values,associate three voltage ranges designated by the four voltage values with a number of the memory cells using the four voltage values,specify, from the three voltage ranges, a first voltage range, a second voltage range, and a third voltage range, the first voltage range having a first distributed number of memory cells, the first distributed number of memory cells being a minimum value of the memory cells, the second voltage range being adjacent to a lower voltage side of the first voltage range, the third voltage range being adjacent to a higher voltage side of the first voltage range,determine a read voltage by using the first voltage range, a first representative voltage value in the first voltage range, the first distributed number of memory cells, a second distributed number of memory cells, and a third distributed number of memory cells, the second distributed number of memory cells being a number of memory cells corresponding to the second voltage range, the third distributed number of memory cells being a number of memory cells corresponding to the third voltage range, andread the data from the nonvolatile memory by using the determined read voltage.