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Comparativa e specifiche sulle Compact Flash Industriali

SLC vs. MLC: An Analysis of Flash Memory


4. Multi-Level Cell (MLC) Flash

As the name suggests, there are multiple values that an MLC cell can represent. The values can be interpreted as four distinct states: 00, 01, 10, or 11.

Table 3: MLC Levels
Value State
00 Fully Programmed
01 Partially Programmed
10 Partially Erased
11 Fully Erased

These four states yield two bits of information. As seen in table 3, the value of the two bits range from fully programmed to fully erased.
Figure 4: Voltage Reference for MLC
As seen in figure 2, a Flash cell’s ability to store charge is why MLC technology works. Since the delta between each level has decreased, the sensitivity between each level increased. Thus, more rigidly controlled programming is needed to manipulate a more precise amount of charge stored on the floating gate. In order for a Flash cell to be considered MLC technology, the cell must exhibit two characteristics:

  1. Precise charge placement
  2. Precise charge sensing

Thus, MLC Flash works the same way as SLC Flash. The threshold voltage Vt, is used to manipulate the state of the Flash. Once again, the amount of charge on the floating gate is what determines the threshold voltage. As seen in figure 4, current MLC technology uses two bits, or 4 levels. However, it is possible to hold more bits. Equation 1 is a generic equation to follow to determine how many states are needed for the desired bits.

Equation 1       States = 2N

N is equal to the number of desired bits per cell. For example, for a cell to hold three bits, you need eight states equal to: 000, 001, 010, 011, 100, 101, 110, 111.

MLC Flash is used in consumer applications that do not require long term reliability such as consumer grade USB Flash drives, portable media players, and Compact Flash cards.