Several types of memory devices, such as Flash memories, use arrays of analog memory cells for storing data. Each analog memory cell holds a certain level of a given physical quantity such as an electrical charge or voltage, which represents the data stored in the cell. The levels of this physical quantity are also referred to as analog values. In Flash memory, for example, each memory cell holds a certain amount of electrical charge. The range of possible analog values is typically divided into regions, each region corresponding to a programming state or programming level that represents one or more data bit values. Data is typically read from Flash memory cells by comparing the analog values of the cells to one or more read thresholds.
Various techniques for adapting read thresholds are known in the art. For example, U.S. Pat. No. 8,369,141, whose disclosure is incorporated herein by reference, describes a method for operating a memory that includes a plurality of analog memory cells. The method includes storing data in the memory by writing first storage values to the cells. Second storage values are read from the cells, and a Cumulative Distribution Function (CDF) of the second storage values is estimated. The estimated CDF is processed so as to compute one or more thresholds. A memory access operation is performed on the cells using the one or more thresholds.
U.S. Pat. No. 8,000,135, whose disclosure is incorporated herein by reference, describes a method for data storage including storing data in a group of analog memory cells by writing into the memory cells in the group respective storage values, which program each of the analog memory cells to a respective programming state selected from a predefined set of programming states, including at least first and second programming states, which are applied respectively to first and second subsets of the memory cells, whereby the storage values held in the memory cells in the first and second subsets are distributed in accordance with respective first and second distributions. A first median of the first distribution is estimated, and a read threshold, which differentiates between the first and second programming states, is calculated based on the estimated first median. The data is retrieved from the analog memory cells in the group by reading the storage values using the calculated read threshold.
U.S. Pat. No. 8,259,506, whose disclosure is incorporated herein by reference, describes a method for data storage including storing multiple sets each including one or more read thresholds for use in reading data from a plurality of analog memory cells. The memory cells are arranged in multiple erasure blocks each including multiple pages, and each set of read thresholds is associated with a respective page in one of the erasure blocks. A first page, which belongs to a given erasure block and is not associated with any of the stored sets of read thresholds, is read by retrieving a stored set of read thresholds that is associated with a second page in the given erasure block, adapting the retrieved set of read thresholds to match the first page, and reading the first page using the adapted set of read thresholds.
U.S. Pat. No. 8,694,854, whose disclosure is incorporated herein by reference, describes a method for data storage including storing data in analog memory cells by programming the memory cells with respective analog input values. After storing the data, respective analog output values are read from the memory cells using multiple read thresholds, which define multiple ranges of the analog output values. Respective numbers of read errors in the data, corresponding to the analog output values falling in the ranges, are assessed. The stored data is recovered based on respective numbers of the read errors assessed in the ranges.
U.S. Pat. No. 8,773,904, whose disclosure is incorporated herein by reference, describes a method including storing data in a group of analog memory cells. The memory cells in the group are read using first read thresholds to produce first readout results, and re-read using second read thresholds to produce second readout results. Third read thresholds, which include at least one of the first read thresholds and at least one of the second read thresholds, are defined. Readout performance of the first, second and third read thresholds is evaluated based on the first and second readout results. The first, second or third read thresholds are selected based on the evaluated readout performance, and data recovery is performed using the selected read thresholds.