Myths about NOR and NAND Flash

Myth 1: NAND Flash is slower than NOR.

The Reality:
The performance characteristics of NAND Flash are: fast write (or program) speed , fast erase speed and medium read speed. This makes NAND Flash ideal for low cost, high density, high speed, program/erase applications. Read More

Although NOR Flash offers a slight advantage in random read access times, NAND offers significantly faster program and erase times. For high performance data storage requirements, such as storing digital photos, downloading music and other advanced features popular in today’s cell phones, the write/erase speeds of NAND provide a distinct performance advantage. This high performance is also what has made NAND Flash cards so widely used in data storage applications such as digital cameras.

Comparing the time required to perform a typical program and erase sequence for NOR and NAND Flash, for a 64KB erasable unit of memory, NAND outperforms NOR by a wide margin, at 17 milliseconds for NAND, and 2.4 seconds for NOR. In a system application, this difference is large enough to be easily noticed by the user. For the read function, the NAND performance is sufficient to support the system requirement, without a noticeable delay for the user.

Today, many designers build upon the conventional cell phone memory architecture by increasing density of the NOR and PSRAM, and adding NAND Flash to obtain greater performance and capacity for data storage.

Myth 2: NAND is not reliable

The Reality:
Just as a hard disk drive is widely accepted with little concerns about bad sectors, NAND works in a similar way in that the controller maps around bad memory areas and error correction code (ECC) is used to correct bit errors. All controllers for NAND Flash have built-in ECC to automatically correct bit errors. Read More

The industry standard is to correct any bit error to a level comparable to that of hard disk drives, or 10-14, which means one bit uncorrectable error every 1014 bits (12.5 terabytes). System designers have long been aware of the benefits of using ECC to detect and correct errors Historically, memory subsystems have used Hamming codee, ECC and Reed Solomon are common in hard drives and CDROMs.

Myth 3: NAND Flash is hard to integrate into a system.

The Reality:
NAND Flash has an indirect or I/O-like access. Therefore, it must be accessed through a command sequence instead of through the direct application of an address to the address linesNAND Flash also has internal command, address and data registers. Today, a wide selection of NAND controllers and software drivers are available, making integration into a system relatively simple. Read More

Although this interface may appear more cumbersome than the direct interface of NOR Flash, a notable advantage is the relative ease in upgrading to a higher density chip. Because of the indirect interface, the external pinout, or connection to the host, does not change with the density of the chip. This is similar to the hard disk drive interface in which different densities of hard disk drives could use the same cable interface.

Myth 4: MLC NOR is close to matching NAND capacities.

The Reality:
The maximum available density currently available in MLC NOR Flash is 256Mb. The highest available capacity for MLC NAND Flash is currently 2Gb, and the highest available capacity for SLC NAND Flash is 1Gb. Read More

A common method to increase the capacity of NOR Flash is to store multiple charge levels (typically four) enabling the storage of 2 bits in a memory cell, also known as Multi-level Cell (or MLC) NOR. However, by implementing MLC architecture, the effective speed is further reduced and write/erase endurance is also reduced.

Myth 5: MLC NAND won’t hold up under extended use.

The Reality:
MLC Flash has a different rating for the number of read/write cycles compared to SLC NAND Flash. Currently, SLC Flash is rated to have approximately 100,000 cycles and MLC Flash is rated to have approximately 10,000 cycles. However, if a 256MB MLC card can typically store 250 pictures from a 4-megapixel camera (a conservative estimate), its 10,000 read/write cycles, combined with wear-leveling algorithms in the controller, will enable the user to store and/or view approximately 2.5 million pictures within the expected useful life of the card. That number is so far beyond the average number of photos taken by the typical user that the difference in endurance is not significant for this application. Read More

For those not familiar with the technology, MLC NAND Flash allows each memory cell to store two bits of information, compared to one bit-per-cell for SLC NAND Flash, resulting in a larger capacity and lower bit cost. While SLC NAND may be more appropriate for some specific applications, the difference will not affect the many common consumer applications, including most digital camera users. MLC NAND provides a very competitive level of performance and makes high density NAND cards more affordable, resulting in its growing popularity among consumers.

Myth 6: MLC NAND does not have the performance or endurance to reliably store your digital photos.

The Reality:
MLC NAND is rated to have approximately 10,000 cycles, a level that is lower than SLC NAND, but more than sufficient to meet the needs of the vast majority of consumer users. A significant portion of the NAND Flash-based memory cards on the market today are made from MLC NAND, and the continuing rapid growth of this market can be considered an indication that the performance is meeting consumers’ needs. Read More

If a 256MB MLC card can typically store 250 pictures from a 4-megapixel camera (a conservative estimate), its 10,000 read/write cycles, combined with wear-leveling algorithms in the controller, will enable the user to store and/or view approximately 2.5 million pictures within the expected useful life of the card. That number is so far beyond the average number of photos taken by the typical user that the difference in endurance is not significant for this application.

Myth 7: MLC NAND does not have high enough performance for streaming video.

The Reality:
The performance of MLC NAND is sufficient to support the 6 to 8 Mbits/second, transfer rate needed to store MPEG2 compressed video on a memory card. This works out to approximately 1MB/second. MLC NAND can transfer and write approximately 1.7MB/second.

Myth 8: SLC NAND is a generation ahead of MLC NAND.

The Reality:
On Toshiba’s roadmap, SLC development leads MLC by only two to three months. Presently, for each new generation, SLC chips are designed with MLC requirements in mind, so there is little lag-time between the two types of NAND. Read More

The real issue is market acceptance, not actual time-to-market for the next generation. Currently, MLC development is well-timed to match market acceptance, with 512MB and 1GB cards widely available today to meet market demand.

Myth 9: The additional circuitry needed for MLC NAND takes up a significant amount of real estate.

The Reality:
The circuitry required for MLC NAND is relatively minimal. A 4Gb MLC NAND Flash chip provides approximately 1.95 times greater density than a 2Gb SLC NAND chip. We believe that the more important question to the user is “what density can you get in a chip today?” Presently, the highest density MLC NAND Flash in production is 4Gb, whereas the highest density SLC NAND in mass production is 2Gb. The market demand for ever-higher densities of removable storage makes the lower-cost, higher density MLC card attractive to users and continues to enable new applications to emerge. Read More

The rated storage capacity of 2Gb SLC NAND is 271MB, compared to 529MB for a 4Gb MLC NAND Flash chip, for a density increase of approximately 1.95 times greater.

Myth 10: NAND Flash is a slow storage technology.

The Reality:
NAND Flash offers excellent performance for data storage. As a point of comparison, it can offer significantly faster performance and reliability than a hard disk drive, depending on the number and size of files transferred. For a random access of a 2kB file, a typical hard disk drive might take approximately 10ms to retrieve a file, while NAND Flash would take about 0.13ms to retrieve a similar size file. For a comparable write function with the 2kB file, NAND could be as much as 20 times faster. Because it is a solid state memory with no moving parts, NAND flash features a significantly shorter random access time compared to a mechanical hard disk drive.

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Silicon Forest
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