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Why the TLC NAND FLASH no good?
2017/2/15 14:54:54

So what is TLC?
TLC is a flash memory cell storage unit, its English is TLC = Trinary-Level Cell, that is, 3bit / cell. Prior to the invention of TLC, solid-state hard disk most of the use of SLC and MLC, namely Single-Level Cell single-layer unit and Multi-Level Cell multi-layer unit. As early as last year, Samsung 840evo solid state hard drive on the use of the TLC chip, and later found the existence of the old data read speed decline, Samsung insisted that the problem from the flash management algorithm error, not TLC flash memory particles.
At present, most of the solid-state hard drive using MLC particles, very few high-end products using SLC particles, because the price of some of the entry-level solid-state hard drive using TLC particles. From the cost of procurement point of view, TLC price relative to the MLC is much cheaper, let alone SLC.
Why is TLC bad?
In Xiaobian before the test, especially the solid-state hard drive read and write speed, the use of TLC chip Samsung 840evo and the use of MLC Kingston in reading speed almost all over 180MB / s, from the simple speed test on both The performance is not worse. But in fact the working principle determines the TLC fatal Mishap.
Access principle SLC architecture is 0 and 1 two charge value, that is, each cell can only access 1bit data, somewhat similar to the switch circuit, although simple but very stable. The MLC architecture can store four charge values (00, 01, 10, 11) at a time, so it has a good storage density. TLC can store 8 point values (000, 001, 010, 011, 100, 101, 110, and 111), so the density is higher, of course, lower cost.
The life problem is because of its working principle:
When you want to write data, you need to apply voltage to the control gate, and the source and drain voltages are 0V. The voltage forms an electric field so that electrons can enter the floating gate from the N channel through the silicon dioxide insulator. This process is often also a "tunneling". To delete the memory cell, it is necessary to apply a voltage to the P-type semiconductor and keep the voltage at the control gate at zero. After the electric field is formed, the electrons can pass through the silicon dioxide layer. This is why NAND flash has to delete the original data before rewriting the new data: you must first release the original electrons before you can re-enter the electronics. The problem is that the thickness of the insulating layer of silicon dioxide is only 10 nanometers in thickness, it is not never wear the King Kong's footer. After every "tunneling", it will wear out. When the silica is finally broken, the atomic bond breaks.
The SLC can tolerate more changes in the voltage state because it has only two voltage states. But in TLC flash memory, there are eight voltage states in the memory cell, so the hard drive's fault tolerance is very limited. That's where the problem is, the SLC flash has a larger voltage space, so it can tolerate a high peak voltage change until the entire block stops working because the erase data is too slow. This is why SLC flash memory has a higher number of erase cycles. The number of times a user can delete or write data is longer. And TLC voltage tolerance of the lowest, so TLC flash memory erase the number of natural minimum. So most of the current number of TLC write only about 1000 times. Durability (life) is the fatal weakness of TLC.
Of course, life can be achieved by controlling the chip checksum and intelligent wear balance algorithm so that the number of writes per storage unit can be evenly distributed, but this is not good enough for TLC. Still need to use 840evo example, foreign people have to be a durability test, programming / erase cycle (P / E) the number of times is about 1064 times, that is 250GB of life data write in 270TB or so.

Samsung 840 series uses its own 21nm process ToggleDDR2.0TLC flash particles
128GB iPhone 6 Even if you use three years, can you write data to 130TB? This may be the main reason why boldly daring to use TLC. However, it is said that Apple has recently considered the replacement of TLC flash memory thing, of course, everything is not the final message.

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