In an era when lower data rates and smaller data volumes (or sizes) were required by users, a single storage unit may have been sufficient to accommodate all of a user's data needs. But, in recent years, reducing data reading/writing periods and/or securing larger-capacity storage spaces have become important, for instance, in order to process larges quantities of multimedia data and/or real-time data. In particular, the physical limitations of storage materials may make it difficult to achieve high frequency operations and/or larger capacities for single storage units. To overcome such limitations, multi-channel memory systems have been proposed, which include a plurality of similar and/or different memory devices coupled to one another.
Today, various kinds of memory devices may be used as storage units, e.g., hard disk drives (HDDs) configured to store and read data by rotating aluminum disks coated with magnetic materials, optical disks, such as CD-ROMs or digital versatile disks (DVDs) configured to store information such as voice, images, or characters, and/or nonvolatile memories, such as flash memories.
Some such memory devices may frequently encounter errors or malfunctions while reading data, for example, due to the physical limitations thereof. For example, in hard disk drives, closer track spacing, the use of weaker signals to avoid interference, and/or increased rotation speeds may be used to meet ever-increasing storage demands. However, as the limits of such technologies are pushed, errors may occur more frequently. Furthermore, errors and/or failures in the hard disks may be caused by particles floating therein, electrostatic discharge (ESD), temperature, and/or humidity related effects while reading data therefrom.
Flash memories are nonvolatile devices that may retain data even without a power supply. Although not as fast as dynamic memories that may be used as main memories in personal computers, flash memory devices may offer advantages over hard disks in reading rate and/or resistance to external impact. As such, flash memories may be employed in mobile or portable devices that are operated by batteries. Another advantage of flash memory may be durability.
Flash memory may be used as nonvolatile storage units for computing systems, and may be capable of electrically erasing and rewriting data. In contrast to electrically erasable and programmable read-only memories (EEPROMs), flash memories may erase and/or write data in units of blocks and/or sectors. Due to lower costs than EEPROMs, flash memory may be used in applications that require large-capacity, nonvolatile, solid-state storage units. Typically, flash memory may be used in digital music players, digital cameras, and/or mobile phones. Moreover, flash memory may be used in universal serial bus (USB) drives for storing and transferring data between computing systems.
In a flash memory device, data may be retained in a memory cell array formed of floating gate transistors called memory cells, each of which stores bit information. For a single-level-cell (SLC) flash memory device, data stored in a unit memory cell may be sensed based on threshold voltage distributions corresponding to respective data states ‘1’ and ‘0’. For example, when a reference voltage is applied to a control gate of the memory cell, the data (‘1’ or ‘0’) stored in the cell may be determined based on current flow through the memory cell. However, since the actual threshold voltage distributions of the memory cells may not be within designed voltage ranges, errors may result from data readings. This phenomenon may become more serious, for example, due to charge loss or leakage, time lapse, temperature elevation, capacitive coupling by programming adjacent memory cells, reading adjacent memory cells, cell defects, and so forth.