NAND Flash memory has evolved into one of the fastest-growing enabling technologies today for the growing consumer electronics industry. As the cost-per-bit of NAND flash memory has declined, and storage capacities have increased, this technology has become a cost-effective, high-performance storage solution for a wide range of consumer products and applications, including digital still and video cameras, MP3 players, PDAs, cellular phones, set-top boxes, USB drives, and many others. As a result of growth in new and traditional NAND applications, industry analyst firm Web-Feet Research projects the market for NAND flash will increase from $2.1 billion in 2002 to $18.2 billion in 2008. (Web-Feet Research Quarterly NOR and NAND Forecasts, October 2003.)
However, in spite of such robust projections and continued widespread industry use in consumer electronics devices, there are still many misconceptions in the marketplace about the ease of use, effectiveness and performance of NAND Flash memory, especially in comparison to NOR Flash. It is not always widely understood that, for high-density, high-performance applications, NAND performance criteria exceeds that of NOR in two important aspects required for storing information.
This article dispels some of the misconceptions and inaccuracies surrounding NAND Flash memory. More specifically, this piece closely examines the issues of speed, reliability, ease of integration, storage capacity, and long-term performance of NAND Flash memory.
Misconception #1: NAND Flash is slower than NOR Flash.
The Truth: The performance characteristics of NAND Flash, as compared to NOR Flash, are faster write (or program) speed, quicker erase speed, and medium read speed. As a result, these attributes make NAND Flash memory an ideal storage solution for low-cost, high-density, high-speed, program/erase applications.
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 Toshiba NOR and NAND Flash, for a 64KB erasable unit of memory, NAND outperforms NOR by a wide margin, at approximately 17 milliseconds for NAND, and approximately 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.
Misconception #2: NAND Flash is unreliable.
The Truth: This statement is simply not true. Akin to a hard disk drive's acceptance with few concerns about bad sectors, NAND Flash works in a similar way in that the controller or related software maps around bad memory areas and error correction code (ECC) is used to rectify bit errors. Virtually all commercially available controllers or related software for NAND Flash have built-in ECC to automatically fix bit errors.
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 commonly used Hamming codes, ECC and Reed Solomon in hard drives and CD-ROMs.
Misconception #3: NAND Flash is difficult to integrate into a system.
The Truth: This also is an erroneous assumption. NAND Flash possesses indirect, or I/O-like, access. As a result, it must be accessed through a command sequence, instead of through the direct application of an address to the address lines. In addition, NAND Flash has internal command, address, and data registers. Presently, a vast selection of NAND controllers and software drivers are available which make system integration fairly straightforward.
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.
Misconception #4: MLC NOR is close to matching NAND capacities.
The Truth: MLC NOR cannot touch NAND in terms of basic capacity. The maximum available density currently available in Multi-Level Cell (MLC) NOR Flash is 256Mb. The highest density for MLC NAND Flash at the present time is 4Gb, and the highest density for Single-Level Cell (SLC) NAND Flash is 2Gb.
Misconception #5: MLC NAND cannot withstand extended use.
The Truth: MLC NAND has a different rating for the number of read/write cycles in comparison to SLC NAND. Currently, Toshiba SLC NAND is rated at approximately 100,000 cycles, and the rating for Toshiba MLC NAND is approximately 10,000 cycles. While there are applications for which SLC is better suited, the 10,000 cycles of MLC NAND is more than sufficient for a wide range of consumer applications, from storing documents to digital photos. For example, if a 256MB MLC card can typically store around 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. This total is so far beyond the average number of photos taken by a typical user that the difference in endurance is not significant for this specific application.
For those unfamiliar with the technology, MLC NAND allows each memory cell to store two bits of information, compared to one bit per cell for SLC NAND. This results in a larger capacity and lower bit cost. While SLC NAND may be more appropriate for some applications where high performance is required, the difference will not affect many common consumer applications, including most digital cameras. MLC NAND provides a very competitive level of performance and makes high-density NAND cards more affordable, accounting for its growing popularity among consumers.
Misconception #6: MLC NAND does not have the performance, nor the endurance, to reliably store digital photos.
The Truth: As mentioned previously, Toshiba MLC NAND is rated for approximately 10,000 read/write cycles, a level lower than that of SLC NAND, but more than sufficient to meet the needs of the vast majority of consumers. A significant portion of the NAND Flash memory cards available today are based on MLC NAND, and the continuing rapid growth of this market is an indication that the performance is meeting consumers' needs.
The aforementioned digital camera example is poignant evidence of MLC NAND's ability to adequately store and view large numbers of digital photos. Thus, with respect to digital cameras, the use of MLC NAND is a non-issue.
Misconception #7: MLC NAND cannot handle streaming video.
The Truth: The performance of Toshiba MLC NAND is sufficient to support the transfer rate of 6 to 8 megabits per second, required to store MPEG2 compressed video on a memory card. This works out to approximately 1MB/second. Toshiba MLC NAND can transfer and write at approximately 1.7MB/sec.
Misconception #8: SLC NAND is a generation ahead of MLC NAND.
The Truth: For Toshiba, SLC development leads MLC by only one month. 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. The real issue is market acceptance, not actual time to market, for the next generation of technology. Currently, Toshiba MLC development is timed to match market acceptance rates, with 512MB and 1GB cards widely available today to meet industry demand.
Misconception #9: The additional circuitry required for MLC NAND uses a significant amount of real estate.
The Truth: The circuitry required for MLC NAND is relatively minimal. A 4Gb MLC NAND chip provides approximately 1.95 times greater density than a 2Gb SLC NAND chip. In reality, the more important question to the user is: "What density is available in a chip today?" Right now, the highest density MLC NAND Flash in production is 4Gb, whereas the highest density SLC NAND in mass production is 2Gb, using stacked die. The market demand for higher removable storage densities makes the lower-cost, higher-density MLC NAND card more attractive to users and continues to enable the emergence of new applications.
The minimum specified capacity of 2Gb SLC NAND is 271, 417, 344 bytes, compared to 529, 317, 888 bytes for 4Gb MLC NAND. This equates to a factor of approximately 1.95 times greater density.
Misconception #10: NAND Flash is a slow storage technology.
The Truth: As previously mentioned, this assertion is false. 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 random access of a 2 kB file, a typical hard disk drive may take approximately 10ms to retrieve a file. NAND Flash would take about 0.13 ms to retrieve the same file. For a comparable write function with the 2 kB file, NAND could be as much as 20 times faster. Because it is a solid state memory with no moving parts, NAND features a significantly shorter random access time compared to a mechanical hard disk drive.
In conclusion, NAND Flash memory is a dynamic and viable technology for data storage in a variety of consumer electronics applications. More importantly, NAND Flash exhibits a number of speed, reliability, ease of integration, storage capacity, and long-term performance advantages over NOR. While NOR will still be used in some products, NAND Flash memory represents the wave of the future, and will continue to be incorporated into a variety of designs.
Brian T. Kumagai is the business development manager for NAND flash memory products in the memory business unit of Toshiba America Electronic Components. Since joining the company in 1998, Mr. Kumagai has played a critical role in many strategic initiatives for the company's memory portfolio. He has more than a decade of product management and engineering experience in the electronics components arena, and previously held positions with Toshiba, Hughes Aircraft, and Intel Corp. Mr. Kumagai holds a B.S. in industrial technology and electronics from California State University, San Jose, and a MBA from California State University, Fullerton.