Sponsored Article

Multi-Level Cell NAND Flash – The Consumer Choice

Sponsored by Toshiba America Electronic Components, Inc.

By Richard A. Quinnell

February, 2005

NAND Flash memory has arisen as the memory technology of choice for many non-volatile storage applications, from music players to solid-state disk drives. Multi-level cell (MLC) Flash technology offers nearly twice the storage density of traditional single-level cell (SLC) Flash. With performance characteristics that match many consumer applications, MLC can lower the cost per bit of non-volatile solid-state storage.

MLC NAND Flash memory uses an architecture that goes beyond traditional binary logic. Rather than simply being on or off, storage transistors in this technology are able to enter one of four stable states allowing them to encode data to achieve a storage density of two bits per memory cell. Factoring in the control circuitry, MLC NAND Flash device designs achieve a storage density increase of approximately 1.95 times that of SLC Flash, effectively doubling the capacity of Flash memory.

ADDITIONAL INFO For more information on NAND Flash Memory visit: www.mlcnand.toshiba.com To learn more about Toshiba's Multi-Level Cell NAND Flash visit: Ask the Flash Expert SeeToshiba’s second NAND Flash Memory series article: “Fast Future for Flash”

The use of MLC technology can lower the cost of memory storage systems. In the case of memory cards, for example, the final retail price of an MLC Flash card can be as much as one third less than SLC cards of the same capacity. This gives MLC Flash the right mix of cost and performance needed to satisfy the most demanding consumer applications.

Developers evaluating the performance of Flash memory typically consider three parameters: read speed, write speed, and endurance. In consumer applications, read speed is important in the playback of stored audio and video as well as in the retrieval of data files from mass storage. Write speed is a significant factor in the recording of audio and video signals. Endurance, the number of write/erase cycles a Flash memory cell can tolerate, affects the useable lifetime of a device in its application. MLC Flash technology meets or exceeds the requirements of consumer applications in all three categories.

Real-Time Video Performance

One of the most demanding applications from the standpoint of read and write performance is the recording and playback of video and audio signals, such as in a digital camera or digital video recorder. The exact performance level needed depends on the video quality desired. A high-definition (HD) video signal recording using MPEG-2 compression, the current standard for consumer video recording, requires a storage bandwidth of approximately 2.5 megabytes1 per second (MB/s2) or 20 megabits3 per second (Mb/s4). Other compression standards, such as H.264 and Windows Media® 9, have lower bandwidth needs.

MLC Flash memory is a perfect match for such video applications. Made with a 90nm process and using large-block architecture, MLC Flash systems achieve a read speed of up to 108Mb/sec—fast enough for 4x or faster playback speeds. Its write performance of up to 20Mb/s, allows real-time MPEG-2 video recording at HD quality. SLC Flash systems have slightly higher read performance of up to 128Mb/s and typical write performance of up to 45Mb/s, but the performance differences do not add significant value in video recording or playback.

Audio recording and playback in popular consumer applications, such as portable MP3 players, also fall well within the read and write performance of MLC Flash. Even at full CD quality stereo, audio recording and playback only require approximately 0.124 Mb/s storage bandwidth. As with video recording and playback, higher speed SLC Flash offers no advantage while the lower cost of MLC Flash is a significant asset.

High-speed Data Storage

Data storage applications, such as solid-state disk drives, do not have the real-time performance requirements of audio and video. Even so, the quality of the user experience does depend on the

"MLC NAND FLASH GIVES THE RIGHT MIX OF COST AND PERFORMANCE NEEDED TO SATISFY THE MOST DEMANDING CONSUMER APPLICATIONS"

memory access speed. Consumers want rapid access to their files, which can be quite large. To test the time required to download typical size Word, Excel and PowerPoint files, Toshiba America Electronic Components, Inc. (TAEC) simulated files stored on servers, calculating the average, median, and maximum file sizes to determine the typical representation of documents in an enterprise environment. As Table 1 shows, word processing documents were as large as 68MB and presentation files were as large as 399MB—although these types of files are typically much smaller (159 kilobytes⁵ and1.18MB, respectively).

Because solid-state disk drives are typically used to hold finished documents for transfer to another computer for file sharing or presentation display, read speed is the more important performance parameter. In the case of a word processing document, the speed of MLC memory allows the opening of a maximum-sized document in approximately 5 seconds. Whereas a maximum-sized presentation, comprising hundreds of slides, would take less than 30 seconds to load, files of a more typical size take less than a tenth of a second to load – virtually instantaneous from a user perspective.

Endurance plays a significant role in disk drive applications because users frequently replace the files. Endurance is also an important factor in many other digital storage applications, such as audio players and digital camera cards. The concern in such applications is that frequently-used cells at the beginning of memory can “wear out” with repeated use, rendering the entire memory useless even though most of the cells still function.

Long Utilization Lifetime

Early failure concerns were realistic in the early history of Flash memory, but with wear-leveling algorithms many of these fears have been alleviated. Wear-leveling algorithms balance the utilization of physical memory while retaining a consistent logical memory map for users, preventing early wear-out of individual memory locations. With such algorithms in place, extreme endurance offers no practical advantage.

The endurance of MLC Flash memory is a minimum of 10,000 write/erase cycles per cell, compared to around 100,000 cycles for SLC Flash. Even so, the endurance of MLC Flash is more than a match for consumer applications. Conservatively speaking, a 4 M-pixel camera with a 256 MB6 memory card, for instance, holds about 250 photographs at typical resolution. With wear-leveling algorithms in place, an endurance of 10,000 cycles corresponds to storage of 2.5 million photos over the card’s lifetime. That corresponds to taking 1,000 photographs a day for nearly 7 years, far beyond the needs of even the most prolific photographer.

The performance characteristics of MLC Flash memory, then, meet the needs of the most demanding consumer applications while offering an attractive cost advantage over SLC memory. Further, performance improvements from Toshiba are on the horizon for MLC. Increasing block size, for instance, will boost write performance and movement to a 70 nm fabrication process will increase both read and write speeds.

Toshiba is the world leader for MLC Flash technology. The company invented NAND Flash in 1986 and has maintained its' technology lead ever since.

For more information on NAND Flash visit: www.mlcnand.toshiba.com.

To learn more about Toshiba's Multi-Level Cell NAND Flash go to Ask the Flash Experts.

Look for Toshiba’s second NAND Flash Memory series: “Fast Future for Flash” coming in the March 31, 2005 issue of EDN.

ABOUT THE EDITOR: Richard A. Quinnell was an embedded systems designer for 15 years before turning to technology journalism. He was a staff writer for EDN for 10 years and is now a contributing editor and freelance writer for a number of high-tech publications.

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