TLC memory writes slower than MLC memory because it stores three bits of data per cell, rather than the one bit stored by SLC and the two bits stored by MLC. Writing three bits of data to a single cell requires more complex programming than writing a single or two bits of data, resulting in slower write speeds and shorter endurance levels. Additionally, the number of program and erase cycles that TLC memory can endure is significantly lower than SLC and MLC memory, further reducing its overall write performance.
More complex programming is required because eash cell can hold three bits of data (with TLC memory) and because of this trait, when new data is added to the cell, the original data must be erased, remembered, and then re-written back to the same cell, in addition to the new bit of data being added to the three layer cell. The speed at which a memory block gets erased depends on the type of memory being used. Generally, Single Level Cell (SLC) memory gets erased the quickest, while Multi Level Cell (MLC) and Triple Level Cell (TLC) memory take longer due to their increased layers. As a side note, the size of the memory block, as well as the type of controller being used, can also affect the speed of erase operations.
The five steps when writing to memory space in TLC memory are as follows:
- Erase – The existing data in the memory cell must be erased before new data can be written.
- Program – The new data is programmed into the memory cell.
- Verify – The new data is verified to ensure it was written correctly.
- Refresh – The memory cell is refreshed to prevent data corruption due to charge leakage.
- Read – The data is read from the memory cell to ensure it was written correctly.
If additional data is to be written to another bit of the same cell in a TLC memory, the existing bit must be erased first and then added back in. The overhead in processing slows down the overall speed of the device and directly affects the performance.
To get a bit more information about SLC memory, please visit our original post about this back from 2006.