TLC flash (triple level cell flash)

TLC flash (triple level cell flash) is a type of solid-state NAND flash memory that stores three bits of data per cell of flash media.

TLC flash (triple level cell flash) is a type of solid-state NAND flash memory that stores three bits of data per cell of flash media. TLC flash is less expensive than single-level cell (SLC) and multi-level cell (MLC) solid-state flash memory, which makes it appealing for consumer devices that use solid-state storage.

Each of the three bits of data in a TLC flash cell is either programmed (0) or erased (1), which means the cell has a total of eight different states (000, 001, 010, 011, 100, 101, 110, 111).

MLC flash doubles the amount of storage on a single die because each MLC cell contains two bits of data compared to the one bit of data in an SLC cell. TLC flash adds one more bit of data per cell, which means TLC adds 50% more storage than MLC flash.

A TLC flash die contains the same capacity as an MCL flash die, but the die is smaller, which accounts for the cost savings. An array of 16 billion SLC cells holds 16 GB of data. An array with 16 billion MLC cells has a capacity of 32 GB, hence the dramatic savings.

But to increase the capacity to 64 GB you would have to double the number of bits in an MLC cell (from two bits to four bits), which is not what TLC flash is. To use TLC flash and get three bits of data per cell, manufacturers will achieve the same capacity as the MLC die (32 GB) but reduce the die size from 16 billion cells to 10.667 billion cells, which accounts for the cost savings of using TLC flash over MLC flash.

The drawbacks to using TLC flash are performance, reliability, and longevity.

TLC flash will deliver less performance than SLC flash and MLC flash because each time a cell is read its voltage level, or state, must be checked. A TLC flash cell has eight different voltage levels, so it will take longer to read each cell's voltage level.

The TLC flash media will also have a higher bit error rate (BER) than SLC and MLC flash because it has more voltage levels to check and more opportunities to misread the cell's state. SLC flash has only two states (0 or 1), MLC flash has four states (00, 01, 10, 11), and TLC flash has eight states.

TLC flash will also have lower write endurance than both SLC and MLC flash. Generally, the more bits of data the cell has, the fewer write cycles it will support. SLC memory cells can withstand up to 100,000 write cycles before failing. A 2-bit MLC memory cell can typically withstand up to 10,000 write cycles before failing. A TLC memory cell can sustain about 1,000 write cycles before failing, which is why thus far it has been limited to consumer-grade applications.

This was first published in November 2013

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