In electronics, a multi-level cell (MLC) is a memory element capable of storing more than a single bit of information.
MLC NAND flash is a flash memory technology using multiple levels per cell to allow more bits to be stored using the same number of transistors. In single-level cell (SLC) NAND flash technology, each cell can exist in one of two states, storing one bit of information per cell. Most MLC NAND flash memory has four possible states per cell, so it can store two bits of information per cell. This reduces the amount of margin separating the states and results in the possibility of more errors. Multi-level cells which are designed for low error rates are sometimes called enterprise MLC (eMLC).
The primary benefit of MLC flash memory is its lower cost per unit of storage due to the higher data density, and memory-reading software can compensate for a larger bit error rate. The higher error rate necessitates an error correcting code (ECC) that can correct multiple bit errors; for example, the SandForce SF-2500 Flash Controller can correct up to 55 bits per 512-byte sector with an unrecoverable read error rate of less than one sector per 1017 bits read. The most commonly used algorithm is Bose-Chaudhuri-Hocquenghem (BCH code). Other drawbacks of MLC NAND are lower write speeds, lower number of program-erase cycles and higher power consumption compared to SLC flash memory.
A few memory devices go the other direction, and use two cells per bit, to give even lower bit error rates. The Intel 8087 uses two-bits-per-cell technology, and in 1980 was one of the first devices on the market to use multi-level ROM cells. Some solid-state disks use part of a MLC NAND die as if it were single-bit SLC NAND, giving higher write speeds.
Samsung has pioneered high-performance MLC technology with three bits per cell for eight total states. This is commonly referred to as Triple Level Cell (TLC) and was first seen in the 840 EVO Series SSDs. SanDisk X4 flash memory cards are based on four bits per cell technology, which uses 16 states. The negative aspects of MLC are amplified with TLC, but TLC benefits from higher storage density and lower cost.
Flash memory stores data in individual memory cells, which are made of floating-gate transistors. Traditionally, each cell had two possible states, so one bit of data was stored in each cell in so-called single-level cells, or SLC flash memory. SLC memory has the advantage of faster write speeds, lower power consumption and higher cell endurance. However, because SLC memory stores less data per cell than MLC memory, it costs more per megabyte of storage to manufacture. Due to faster transfer speeds and longer life, SLC flash technology is used in high-performance memory cards.
- Micron's MLC NAND Flash Webinar
- SandForce SF-2600/SF-2500 Product Info 2013-10-22
- A Tour of the Basics of Embedded NAND Flash Options EE Times 2013-08-27
- "Automotive EEPROMs use two cells per bit for ruggedness, reliability" by Graham Prophet 2008-10-02
- "Four-state cell called density key" article by J. Robert Lineback. "Electronics" magazine. 1982 June 30.
- Geoff Gasior. "Samsung's 840 EVO solid-state drive reviewed: TLC NAND with a shot of SLC cache". 2013.
- Allyn Malventano. "New Samsung 840 EVO employs TLC and pseudo-SLC TurboWrite cache". 2013.
- Samsung. "Samsung Solid State Drive: TurboWrite Technology White Paper". 2013.
- Samsung SSD 840 Series - 3BIT/MLC NAND Flash
- SanDisk Ships World’s First Flash Memory Cards with 64 Gigabit X4 (4-Bits-Per-Cell) NAND Flash Technology
- NAND Flash - The New Era of 4 bit per cell and Beyond EE Times 2009-05-05
- "Samsung SSD 840: Testing the Endurance of TLC NAND". AnandTech. 2012-11-16. Retrieved 2014-04-05.