1.4 Multi-Level Cell and Single-Level Cell NAND Flash

The two most common types of NAND Flash technology are the Single-Level Cell (SLC) and the Multi-Level Cell (MLC).

In SLC NAND Flash, each cell exists in one of two states, storing one bit of information per cell. This technology is used in most high-performance media card and wireless processor applications.

MLC NAND Flash was developed more recently to achieve a higher bit density, creating a Flash chip with greater memory capacity for a given die size. MLC NAND Flash uses multiple levels per cell to accommodate more bits using the same number of transistors. In most cases, an MLC NAND Flash has four possible states per cell, and thus it can store two bits of information per cell. The primary benefit of MLC Flash is its lower cost per unit of storage due to its higher data density. However, since MLC technology reduces the amount of margin separating the states, there is a greater risk of errors. The higher error rate of MLC NAND results in a lower write speed, a smaller number of program-erase cycles and higher power consumption compared to SLC Flash memory.

Compared to SLC, MLC architecture requires more ECC bits to compensate for the resulting higher error rate. Thus, the algorithm and related software are more complex.

To reduce software complexity, Microchip SAM9X60 and SAM9X7 devices feature a built-in PMECC module that supports MLC ECC requirements.