The Inner Workings of Solid State Flash: SLC versus MLC

Because all solid state flash products are not created equal and flash storage is finding its way into more and more embedded computing applications, system designers should understand the critical tradeoffs between competing technologies when evaluating flash products. Most commonly, the endurance and reliability required in end-user applications help dictate the appropriate storage technology to use. Two well-known flash storage technologies, Single Level Cell (SLC) and Multi Level Cell (MLC), offer distinct advantages, depending on a user’s needs.

The Technical Distinction

The difference between MLC and SLC can be found in the voltage level treatment and control within the cells. SLC technology stores a single bit in two binary states in each cell (0 or 1), while MLC stores two bits in four binary states in each cell (00, 01, 10 or 11) (Figure 1). In either case, the binary states are determined by the differentiation in charge levels on the floating gate. Advances in MLC technology are turning out 3- and 4-bit chips with 8 and 16 binary states per cell, respectively.

The downside is that increasing the number of potential binary states within a cell reduces the delta between voltage thresholds, blurring the distinction between cell values, especially in the face of cell degradation over the life of the drive. This drives the need for tighter and more time consuming error correction algorithms and is the reason why SLC flash is faster than MLC.

Reliability

Wider design and operating margins in SLC flash lead to higher reliability, better endurance and longer life when compared to MLC flash. Drive manufacturers specify endurance as the number of block level write/erase cycles allowed before errors rise to unacceptable levels.