NAND flash memory is ideal for industrial applications. This is because, unlike hard disks, they are insensitive to shocks and vibrations. However, the technology also has disadvantages, such as the limited write and read cycles of NAND memories. Nevertheless, they are suitable for applications that require a long service life. Provided that the memories are evaluated according to requirements: This is because there are different NAND memory technologies.

SLC NAND memories, for example, have the longest service life - a basic rule that still applies. SLC NAND memory (single level cell) allows by far the most write and erase cycles and therefore has the longest service life.

SLC and MLC memory in comparison

The SLC memories from Cactus Technologies sold by Syslogic have A-grade flash cells (NAND) from Toshiba with 43- or 32-nanometer technology. These are the largest NAND shrinks available. They allow 100,000 and 80,000 write and erase cycles respectively - meaning they do not need to be replaced for a long time. The only disadvantages of SLC memories are the higher price compared to MLC (Multi Level Cell) memories and the limited storage capacities.

SLC memories are suitable for all applications where a long service life is important and which do not exceed the required storage capacity of 16 GB. It is also worth investing in SLC memory for applications with high thermal loads, as it functions much more reliably under extreme temperatures than MLC memory, for example.

MLC memories were long frowned upon in the industry. However, there are now memory manufacturers who use clever firmware to extend the longevity and reliability of MLC flash memories. Nevertheless, MLC NAND achieves up to 30 times fewer read and write accesses than SLC memory. Accordingly, MLC memory is only recommended for applications where either longevity is not a key factor or where very large storage capacities are required. The durability of MLC memories can be extended by using large memory capacities. This distributes the read and write accesses across more NAND cells.

pSLC memory - the alternative

One innovation in industrial flash memory is pSLC technology (Pseudo Single Level Cell). To explain this, it is important to know how a NAND flash memory works (see Figure 1). NAND cells work by means of a transistor channel (source-drain) and two gates, a control gate and a floating gate. The floating gate is insulated from the control gate and transistor channel by an oxide layer. If electrons are pushed through the oxide layer into the floating gate by a storage voltage (tunnel effect), they remain there permanently, even without voltage.

Figure 1: How a NAND flash memory works: Depending on where the electrons are located, the state 0 or 1 is read out.

© Syslogic

To read the memory cell, a read voltage is applied to the transistor and the current flowing between the source and drain is measured. If the floating gate is charged - i.e. there are many electrons in the floating gate - the state is 0, because there is no current flowing between the source and drain.

The electrons can be released again using a quenching voltage. If there are few electrons in the floating gate, the state is 1 because current is flowing between the source and drain.

All NAND cells wear out over time and the oxide layer degrades. The thicker the oxide layer, the slower this process progresses. Accordingly, NAND memories with large shrinks are more durable than those with small ones. The time during which information remains error-free once it has been stored is called retention. NAND cells with a thick oxide layer therefore achieve high retention.

SLC memories only have two charge states, almost no electrons or very many electrons in the floating gate. In MLC memories, four different charge states per cell are stored at different voltage levels, which corresponds to 2 bits.

Functionality of SLC and MLC NAND

Figure 2: Cactus Technologies offers a broad portfolio of industrial flash memory, such as SLC, pSLC and MLC memory in all common form factors such as SD and microSD Card, Compact Flash, CFast, 2.5-inch disk or m.2.

© Syslogic

SLC NAND with signs of wear can be read for much longer than MLC NAND with signs of wear. With only two charge states, the assignment is easy, even if the charge state is no longer as clear as with a new cell. With four charge states (MLC-NAND), the assignment is much more difficult in comparison. Even small signs of wear and tear are enough and the charge states can no longer be assigned. Accordingly, SLC memories allow a much higher number of write and read cycles per flash cell than MLC memories. In addition, SLC memories are significantly faster than MLC memories due to the clear voltage differences.

It is possible to use MLC NAND but only store 1 bit on it. This approach is known as 'Fast Page Mode', some manufacturers refer to it as 'MLC+' or 'Turbo Mode'. The main advantage of fast page mode is the speed: read and write speeds are increased at the expense of storage capacity. All MLC NAND memories can be operated in fast page mode without the need for firmware adjustments.

The disadvantage is that the service life (endurance) of the individual flash cell (NAND) is only marginally longer than that of conventionally used MLC memories. Although only two charge states are stored in fast page mode, the difference in voltage levels is just as small as with four charge states. Accordingly, the disadvantages of MLC NAND mentioned above come into play: susceptibility to errors due to difficult assignment of the voltage levels, limited read/write cycles and the resulting limited service life.

What pSLC can do

With pSLC technology, sometimes also called 'SLC Light', the MLC NAND is also written with just one bit. At the same time, however, the voltage differences between the two charge states are increased. The advantages of SLC memory come into play due to the significant voltage differences. The charge states can be assigned more easily than in fast page mode or with conventionally used MLC NAND, which allows more write and read cycles. At the same time, the significant voltage differences ensure reduced susceptibility to data errors. Accordingly, the data security and longevity of pSLC memories are significantly better than those of conventional MLC memories or MLC memories in fast page mode. However, in order to increase the voltage differences between the charge states, the memory manufacturer must adapt the firmware. In contrast to Fast Page Mode, pSLC technology requires special MLC NAND that support pSLC technology. With pSLC technology, the service life of MLC NAND can be significantly increased. With pSLC, only half of the physical storage capacity is available. A 32 GB pSLC memory is therefore physically a 64 GB MLC memory. However, the service life is not only doubled, but increased six-fold due to the greater voltage differences between the charge states. This makes pSLC memory a worthwhile investment for many applications.

In terms of service life, however, pSLC cannot hold a candle to classic SLC memories, as pSLC is based on MLC NAND technology. Accordingly, real SLC NAND achieves 5 times more write and read cycles than pSLC NAND. SLC technology therefore remains the undisputed leader in terms of service life and is best suited for industrial applications. Especially when very high storage capacities are required or when write and erase cycles are limited, industrial companies turn to pSLC or MLC memory. For small and medium capacities, it pays to invest in genuine SLC industrial memory. Even if flash memory is exposed to extreme thermal loads, it is worth investing in SLC memory, as there are special SLC NANDs that are manufactured for the extended temperature range between -40 and +90 °C.

Author: Patrik Hellmüller is responsible for Public Relations at Syslogic.