The Cohabitation of NAND Flash and USB 3.0
As USB 3.0 becomes ubiquitous in mobile devices, can NAND flash handle the throughput?
Many factors influence data-transfer performance, including the type of storage used in a device. The storage options available to designers offer performance and price tradeoffs. NAND flash, a type of non-volatile memory, is used in mobile products to store phone numbers, apps, photos, videos and music. As USB 3.0 becomes ubiquitous in highperformance mobile devices, the question is whether the NAND flash in a device can effectively handle USB 3.0 throughput.
NAND Flash Formats
NAND flash is packaged in a variety of ways. You may be familiar with it as the secure data (SD) cards that slip into your digital camera, or a USB flash drive that you carry around to transfer files. Those USB flash drives (sometimes called “jump drives” or “thumb drives”) can offer blazing speed like the Lexar Triton’s 155 Mbps read/150 Mbps write, or slower speeds like the giveaways handed out at trade shows.
NAND flash appears in ultrabooks as solid state drives (SSDs). SSDs consume about one-third the power of hard disk drives (HDDs) because SSDs do not have any mechanical moving parts. SSDs are also about 3X faster than HDDs. However, SSDs cost about 3X more than HDDs, so ultrabook manufacturers typically keep SSDs to 128GB or less.
|Figure 1: Possible sources of latency|
NAND speed is limited only by the price that mobile device manufacturers are willing to pay. Every year, the price of NAND flash drops, so manufacturers are able to add in more and faster NAND memory for the same price.
When designing products that will ship two years from now, it’s key to consider the future cost and performance of NAND flash. NAND flash performance also affects USB performance, which can determine design decisions around which generation of USB to implement.
Factors Affecting USB Performance
Both hardware and software will affect the performance of the connection between a USB device (e.g., USB flash drive), and a host, (e.g., PC, tablet PC, game console, digital TV). Figure 1 shows possible sources of latency, with a USB 3.0 xHCI host on the left, and a USB 3.0 peripheral on the right. Minimizing latency in both the hardware and software, on the host and the device, will ensure optimal USB performance.
The operating system plays a part on the hardware as well as the software. If the software layers are loaded down with lots of applications or software, the system will slow down in the same way a PC does when too many windows are open. In addition, the speed of the application and the quality of the drivers will impact performance. A well-written driver uses memory correctly and uses parameter settings in USB to maximize throughput.
Hosts and devices with high-performance hardware that offer plenty of CPU cycles allow the USB to take advantage of fast throughput. First-in, first-out (FIFO) memories stored in local RAM must be adequately sized to receive and process incoming and outgoing data, and the bus on the hardware must be fast enough to move the data from the USB controller to the CPU. If the RAM allocated is either too slow or too small, the system will wait for the FIFO to empty (data to be transferred elsewhere) before moving more data through USB.
The hardware must be synchronized with the OS and driver to transfer data into the system RAM or NAND for use by the CPU. For example, if a video card and the USB 3.0 host are competing for bandwidth on the same hardware bus, USB throughput will decrease.
The USB 3.0 PHY must be built to transmit cleanly and, more importantly, to receive and clean up data after it travels through the USB cable. Keep in mind that the USB cable can also affect throughput: A bad cable, poorly constructed, can interfere with traffic travelling between the client and the host.
On the client side, the same hardware and software factors can affect throughput to and from the peripheral (e.g., smart phone or flash drive). In addition, if the peripheral includes a spinning USB hard drive or a USB printer inkjet head, mechanical limitations can affect speed as well.
This also means that the NAND memory (hardware) is one part of the system (possibly in both the host and the device) that can impact throughput. Slower NAND can slow overall throughput, while faster NAND, like that found in the Lexar Triton flash drive, can make good use of USB 3.0 bandwidth (as shown in the video demo here).
The Ideal System
USB throughput is not a function of just the controller and the PHY, but includes an entire system of software, buses, operating systems and cables. Even when the controller and PHY can achieve SuperSpeed USB 3.0 speeds of 400 megabytes per sec (MBps), or 4 Gbps, performance is proven on an ideal system of high-performing parts. In the system used in the video referenced above that demonstrates the performance of the DesignWare® USB 3.0 PHY and controller, RAM was used for storage instead of NAND. Since RAM is instantaneous, the demo wasn’t limited by NAND read/write speeds. This helped to idealize the system by eliminating a possible latency.
Even when most of a system can handle USB 3.0 speeds, NAND flash speed often remains a limitation. However, as the demand for fast access increases, so do NAND speeds. Apple’s most recent launch of the new MacBook Pro advertises read speeds of 500 MBps. So, it is possible to read data incredibly quickly off the NAND flash. This capability allows huge spreadsheets and large video files to appear on screen faster, and editing, saving and compiling to be done in a flash. The NAND flash in such systems is designed to deliver performance that can support USB 3.0 speeds.
What Will Your Market Bear?
A fundamental consideration for determining whether to implement faster NAND flash is the target market. In a given market, there can be one to three market leaders that make the fastest, fanciest “tier 1” products. These market leaders make the most money both by margin and by volume. The rest of the market players fight for the lower-margin business. Companies making “tier 2” products generate revenue by differentiating on one or two features. Companies making “tier 3” products typically make money based on volume. Many companies offer products that fit into different tiers. This year’s tier 2 product can be cost-reduced in the second year, and price-reduced to compete with tier 3 the third year.
If you are competing with the market leaders in tier 1 products, USB 3.0 is now a requirement. In the case of smartphones and tablets, TI’s OMAP 5 and Samsung Exynos 5 have already moved.
If you are competing with tier 2 products, you either target to move your product into tier 1, or hold a strong feature lead in tier 2 to command a price premium over your competitors. In both of these cases, adopting USB 3.0 is required—either to move into tier 1, or to give your customers the value that they demand. Staying with USB 2.0 while the competition adopts USB 3.0 will allow your competition to take over the feature lead, and therefore the price premium.
In tier 3 products, it is possible that USB 2.0 will be good enough for the next few years. However, it is just as likely that USB 3.0 will be required to maintain a foothold in the market. Tier 3 products without USB 3.0 may simply need to compromise on price even further.
The Future of Cohabitation
By 2014 or 2015 most, if not all, tier 1 and tier 2 products will support USB 3.0, so system design starts this year must include USB 3.0 to compete. Product architects should demand that NAND be upgraded to keep up with USB 3.0. Operating at over 100 Mbps, this new breed of NAND is much faster than today’s SD memory and 3x faster than effective USB 2.0 speeds. Tier 1 products will push the speed limits of NAND to over 400 MBps (4 Gbps) as consumers get used to, and require, USB 3.0 speeds.
The question that you need to answer for your product is: In which tier will it compete? If you want to win against market leaders’ tier 1 products by offering the highest performance, you need USB 3.0. It’s only after that decision is made that you need to find the NAND flash that meets your system’s required speed. As NAND flash technology gets faster and cheaper and USB 3.0 adoption becomes ubiquitous, USB will, in fact drive the broader use of fast NAND flash. Fast NAND will complement USB 3.0 and cohabitate in every electronic product in true harmony.
Eric Huang is currently senior product marketing manager for Semiconductor USB Digital IP at Synopsys, Inc. He has been working on USB since 1995, starting with the world’s first BIOS that supported USB keyboards and mice. He also served as chairman of the USB On-The-Go Working Group for the USB Implementers Forum from 2004-2006. Huang received an M.B.A. from Santa Clara University, an M.S. in engineering from University of California Irvine, and a B.S. in engineering from the University of Minnesota.