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Which USB is Right for Your Application?: Part 2


(You can read Part 1 by clicking here.)

The Universal Serial Bus (USB) peripheral interface is ubiquitous across all personal computing platforms, as well as many industrial and infrastructure platforms. At the same time, however, the correct version for a given application, i.e., USB 1.0, USB 1.1, USB 2.0, USB On-the-Go (OTG), or WirelessUSB (WUSB), can lead to confusion.

The release of USB 1.1, combined with the native operating system support offered by Microsoft, enabled the rapid adoption of USB hosts in the PC. Additionally, it drove the conversion of many peripheral devices from legacy interfaces such as serial (RS-232), PS-2 (mice and keyboards), and parallel ports (Centronix and IEEE-1284 for printers) to this common interface standard.

The release of USB 2.0 enabled high-speed connections. An even greater explosion in the number of available USB peripherals greatly enhanced the end-user experience. Part 1 summarized the evolution of the USB standard. Part 2 addresses common applications and determine which flavor of USB is best for a given application.

USB 1.1, USB 2.0, USB OTG, or WUSB?
Briefly, let's review the different USB specifications to help frame the balance of the article. The terms USB 1.1, USB 2.0, USB OTG and WUSB are all commonly used today. In many cases, however, they have created confusion among engineers and end-users. USB 2.0 is the official version superseding USB 1.1, As a superset of USB 1.1 it offers three speeds for data transfers:

  • low-speed (LS) at 1.5 Mbps
  • full-speed (FS) at 12 Mbps
  • high-speed (HS) at 480 Mbps

The first two are defined exactly the same as they were in USB 1.1. USB OTG is an addendum to the USB 2.0 specification that defines a new class of devices. This extends the functionality of a peripheral product to include limited host capabilities. An OTG device can use any of the speeds available in USB 2.0. Certified WUSB is the latest extension. It defines a wireless interface that combines the speed and security of wired USB technology with the ease-of-use for wireless technology. Certified WUSB supports robust, high-speed wireless connectivity by utilizing the common WiMedia MB-OFDM ultra-wideband (UWB) radio platform as developed by the WiMedia Alliance. It supports speeds of up to 480 Mbps at 3m, and up to 110 Mbps at 10m. Each version has its strengths and weaknesses. A brief overview of cost vs. power vs. throughput for each implementation helps set a foundation for defining which specification is best for selected applications.

Low- and High-Speed USBs
The best strengths of low-speed USB are low cost and low power. At 1.5 Mbps, the transceiver's specifications are very limited, but down in cost and power. An obvious weakness is data throughput. With a bit-rate of 1.5 Mbps, actual data throughput is limited to less than 1 Mbps due to the overhead of the USB specification.

On the other end of the standard is high-speed USB for data throughput. However, increased data throughput drives up costs and significantly increases power consumption. The cost of the actual USB controller is higher than full-speed or low-speed USBs. The cost for board implementation is higher because signal quality becomes a greater technological challenge at 480 Mbps, versus 12 Mbps or lower.

Bridging the gap between low-speed and high-speed is full-speed USB, which is in the middle on all three vectors.

Since USB OTG can be implemented at any of the three speed nodes the same discussion above applies when comparing different speed versions of USB OTG to each other. It is interesting to compare OTG to standard-wired USB. One of the original intentions of the USB specification was to allow for low-cost peripheral implementations by putting much of the "brains" in the host, not in the peripheral.

This dictated the host-centric bus nature of USB, but allowed for extremely cost-effective peripheral implementations. The cost of the "brains" would be absorbed once per personal computer, instead of once per every device that would be connected to the bus. OTG changes this dynamic by implementing limited host functionality into a standard USB peripheral, which drives up the cost no matter how limited the additional functionality.

It does this not just at the silicon USB controller level where host functionality must be integrated, but across the entire product where additional memory and processing power will need to be implemented to accommodate host functionality. One benefit of USB OTG is the ability to share data between USB devices in the absence of a computer. Specific examples and case studies will be cited later on when we discuss applications.

The biggest advantage of Certified WUSB over wired USB is ease of use. No longer are cables required to transfer data to/from your computer. Also, now you can power many peripheral devices directly off of the USB cable. Without the cable, a device needs to provide its own power, either a standard AC power adapter or battery.

Human Interface or Input Devices
Examples of human interface or input device (HID) applications include mice, track balls, keyboards, joysticks and game controllers. Typically, these devices use the interrupt data transfer feature, and are polled by the host periodically to determine if they have data to provide to the active application. Usually mice are polled for data every 8 ms, and respond with 32 bits of data. A keyboard transmits 64 bits of data over the same interval. Even advanced joystick and game controllers (multiple buttons and force feedback) are polled only every 8 ms and transmit 6 bytes of data per request.

This drives a maximum data throughput rate of around 8 kbps (0.008 Mbps), well under the low-speed bit rate of 1.5 Mbps. Humans just can not type or click fast enough to generate higher data rates than what can be satisfied by low-speed USB. Hence, HID devices are very low-bandwidth consuming devices and will not eclipse the kbps range in the near future. Keeping these as low-speed devices also helps to keep cost down.

These devices are not candidates for implementing any type of host functionality. This is due to the increased cost to implement the host, and the necessary increased processing power, thereby eliminating them as candidates for full-speed OTG devices. They are good candidates as peripherals for an OTG-enabled device such as adding a portable keyboard to enter data into a Personal Digital Assistant (PDA) or mobile handset when away from your computer. The issue that needs to be investigated is whether or not this can be implemented such that the current requirements are less than the 8 mA specified in the USB OTG specification, or will a battery be required.

Wireless mice and keyboards have been around for many years. As a matter of fact, I'm using one now to type this article. Each was implemented using the proprietary wireless solution chosen by the manufacturer. Each of these implementations requires a wireless transceiver device is plugged into a standard USB port to enable a connection back to the PC. In many cases, you can not simply update just your mouse or keyboard, or add additional wireless HID devices to this system unless you purchase/add a new transceiver for the new device.

By using the standards-based implementation provided by Certified WUSB, you can overcome this issue. Any new Certified WUSB keyboard, mouse, or joystick can be added to the system just like a standard wired USB peripheral. In the near-term, an external Certified WUSB transceiver will be required to enable the connectivity. However, over the next few years, this Certified WUSB transceiver will be integrated into the computer eliminating the need for an external transceiver, further lowering the cost of implementing the peripheral. One drawback of any wireless solution HID device is that batteries are required. Wired solutions get all the power they need from the USB cable.

Mass Storage Devices
Mass Storage Class (MSC) devices include external hard disk drives (HDD), DVD-RW, CD-RW, flash card readers, ZIP drives, magneto-optical (MO) drives and USB flash drives. The choice of USB speeds is to ensure the USB is NOT the bottleneck. For example, Figure 1 shows the speed required for data transfer, assuming there is no bottleneck. Then we'll see which USB speed is required for the target applications. If we assumed ideal bandwidth usage (no overhead), then we can analyze the time it would take to move 1 GB of data between the drive and the PC.


Figure 1.2: Ideal time required for 1-GB data transfers

(Click to enlarge image)


Obviously, the ideal bandwidth is unrealistic. Therefore, the times listed in Figure 1 are faster than what can be achieved in real applications. In this example, USB high-speed is the only adequate choice for this interface. The question then becomes, "Would any of the non-standard USB implementations, Certified WUSB or OTG, be a good choice for these applications?"

Inherently, USB flash drives are very small and are easily powered off the wired USB connection. So these are not great candidates for wireless applications. Adding a battery increases cost and does not buy the end-user any more functionality. Having a flash drive capable of initiating an OTG session may make sense, but probably adds more cost to these devices than the added functionality can justify.

For "stationary" drives that always stay connected to a computer, a wired USB connection is a better solution. Most likely, the user will have a wall power connection due to the power requirements of the drive. If the device is intended to be "mobile" or portable, then either OTG or WUSB may be better choices. Usually mobile drives are not a standalone drives, but rather are some form of consumer entertainment devices such as an MP3 player, Digital Still Camera (DSC), or mobile phone/PDA with an HDD for expanded storage.

Wireless connectivity for these devices is the ideal solution. The end-user walks up to their desk and places the device next to the PC to synchronize or "sync" files with the PC, without having to find the right cable since many devices use non-standard USB connectors. OTG is another option " especially for letting cameras talk to printers, or letting phones/PDAs talk to each other.

Digital Still Cameras
DSCs, no matter the resolution (VGA up to greater than eight megapixel), all have some very similar characteristics. Currently, all of these cameras store digital still pictures on storage cards, such as compact flash, SmartMedia, and MediaStick or onto embedded hard disk drives. All DSCs are battery-operated and, therefore, typically do not draw power over the USB connection unless it is to charge the battery. Additionally, these devices are used away from the computer so they only connect to the PC for final data image editing, printing, storing or forwarding files.

The type of storage media a camera has is not important. Rather, the issue is the amount of storage it has. Lower-capacity cameras fill up faster with data, so need to be refreshed more often. This frequent refresh rate drives the need to "clear" the camera quickly so the user can continue using the camera. High-capacity cameras follow the same usage pattern as the external storage discussed earlier.

In either case, the issue is making sure the USB connection is NOT the bottleneck when moving images from the camera to the computer. As with external storage, the larger the data pipe, the better! Therefore, high-speed is clearly the best choice. For some low-end cameras where cost is critical and capacity is low, full-speed may still be used to meet the cost goals.

DSCs are an ideal end-equipment for OTG technology. The ability to walk up to a printer in a store and print out the photos in your camera, without connecting to a PC, is an ideal application. One issue is whether or not the chosen cameras and printers on each other's targeted peripheral lists? An OTG-enabled DSC could attempt to serve the "printer" class of USB devices, but this could be an issue if a printer requires special drivers to do high-quality photo printing. Conversely, having the printer act as host to the camera might enable the printer to support mass storage peripherals, and access the camera with a PC. Embedding flash media readers in many photo printers have positively impacted the market for OTG-enabled cameras or photo printers.

Given the portable nature of DSCs, when downloading images to a PC just to clear out the in-camera storage, Certified WUSB seems like an ideal option. The only potential drawback is the cost to implement WUSB in DSC market segments that are cost-sensitive. However, eliminating the cost of shipping a cable and potentially the USB connecter from the camera itself, may offset the added cost of implementing WUSB. Obviously, eliminating the connector from a DSC can not happen until WUSB is as widely adopted as wired USB is today.

Portable Media (Audio or Video) Players
Like with DSCs, the real issue with portable audio or video players is the type of storage used and the capacity. There are two categories: HDD-based; and flash-based. The same considerations addressed with DSCs apply to these devices when discussing the "sync" user application. High-speed is the only truly adequate solution for all but the very low-end of these products. The only real difference between DSCs and portable media players (PMP) is the potential application for the PMP to stream isochronous, real-time data to a PC for playback, or in an OTG environment directly to a speaker/headset system.

You'll soon see that for all but the very high-end, next-generation audio formats, full-speed is more than adequate for streaming audio applications. However, when streaming real-time video the bandwidth jumps up into the 30 Mbps range and again drives the need for a high-speed connection. Much like DSCs, these devices are prime targets for both OTG and WUSB applications. In this case, target peripherals are speakers or headsets rather than printers.

Mobile Phones / PDAs
These devices have similar usage patterns as with both DSCs and PMPs. Again, how much storage capacity is required? Capacity for these devices is growing exponentially as more and more functionality is added such as an MP3 player, a mega-pixel DSC, or increased contact database size, etc. As capacity increases, the "sync" connection needs to be high-speed. As with both DSCs and PMPs, mobile phones and PDAs are prime targets of OTG. They are some of the first commercially available devices that implement this functionality and have all of the prime characteristics for WUSB implementation. Most likely, they will be the front runners with WUSB functionality added natively rather than via an external device wireless adapter.

Printers
As with mass storage devices, the first thing to consider with a printing device is, "Where will the data throughput bottleneck occur?" Laser-type printers and inkjet/bubble jet printers have different data throughput requirements. Other printer options, such as including data compression, also can affect bandwidth requirements. However, these features affect cost, which is critical for consumer devices.

Inkjet/bubble jet printers are most commonly used by consumers. Accordingly, they are the most cost-conscious. To minimize cost, compression features are most often off-loaded to the PC for processing. This in turn drives the need for higher data throughput, especially for sending color documents. A high-speed USB interface enables the highest page-per-minute rate on the printer. For inkjet printers without compression, the ideal bandwidth can approach 300 Mbps.

This can drop into the range of 10 Mbps for printers with data compression engines, but at the expense of adding cost to the printer. So, even for ink-jet printers with compression, consumers will overstress a full-speed USB connection. Thus, a high-speed USB connection is a far better choice. With cost being so important to this market segment, many printers will be shipped as full-speed devices with lower page-per-minute specs, but still satisfy the consumer's price needs.

Laser printers target a somewhat higher-end market, so are not quite as cost-sensitive. This allows compression to be included to offload some of the processing. Typically, these printers have a much higher page-per-minute target than the inkjet segment. Thus, the high bandwidth of high-speed USB is needed to achieve these goals.

Most likely, implementing Certified WUSB in the cost-conscious inkjet/bubble jet printer market will be cost-prohibitive. The laser segment, on the other hand, may be more willing to absorb these costs. Traditionally, they have a stronger need for easy connections between a printer and notebook PC for small office/home office applications.

Adding OTG functionality to either type of printer enables direct printing from a portable consumer device such as DSCs or PDAs, especially in higher-end printers that are beginning to include small digital displays to view photos/documents and perform edits. Although not beneficial while connected to the PC, it can be of great use for dual-role OTG devices.

PC Cameras or Webcams
Webcams typically fall into two camps: those that use compression and those that send raw data streams. Both types use isochronous data streams. The first typically provides 30 frames per second (fps) of uncompressed video data at a resolution of 640x480 megapixels. The second is more likely to run at 24 fps at 1024x768 resolution, supporting RGB-24 (millions of colors, 24-bit).

Additionally, most of these devices are bus-powered. Like the inkjet market, they are average sellers and price-sensitive. Table 1 and Figure 2 show bandwidth requirements for uncompressed data for selected webcam scenarios.


Table 1.2: Ideal time required for 1GB data transfers

(Click to enlarge image)



Figure 2.2: WebCam bandwidth requirements

(Click to enlarge image)


Figure 2 shows that the uncompressed webcam, implementing high-speed USB is an absolute must! Compressed data in the camera still usually requires 30 Mbps or more of throughput. Again, a solid argument for a high-speed USB connection.

USB OTG does not make much sense for webcam applications where the camera functions as the host. Being a peripheral to an OTG device is not that feasible, either because webcams require an OTG host to supply over 8mA, or we do not want the added cost of external power or batteries.

Again, if we consider Certified WUSB, the issue of power and cost appears to make this an unattractive option. The average selling price of a webcam is approximately $60-$100. Economically, it may not be feasible to include the added cost of WUSB silicon at $5-$10 or higher. Additionally, WUSB requires the camera manufacturer to include an AC power converter, eliminating one of the benefits of WUSB as well as increasing the price.

Speakers/Headsets
When you consider the isochronous bandwidth requirements for speakers, the first thing to review is what audio format do they support: Dolby digital (AC-3), DTS Digital Surround, THX, or next generation uncompressed sound? Dolby Digital is a 5.1-channel surround sound format, and is the standard for DVD-Video. DTS Digital Surround is a 5.1-channel surround sound format, similar to but competing with Dolby Digital. THX Surround EX, also known as Dolby Digital EX, refers to the Dolby Digital version of the new "6.1"-channel surround sound format that extends the 5.1-channel surround sound format with one (or two) additional speaker(s) located in the back of the audience.

There also are competing formats to Dolby Digital that use uncompressed data. Top-end speakers operate on 24-bit, 96-kHz per channel data streams with no compression. Table 2 summarizes the bandwidth requirements for these formats.



(Click to enlarge image)


The only format listed that exceeds the capabilities of full-speed is the next generation uncompressed audio.

Speakers do not seem to be targeted as full OTG devices. They do seem to make sense as a target peripheral OTG-enabled video or music players such as MP3 or portable DVD players. When it comes to WUSB, the focus will be on higher-end speakers that can support the added bill of material (BOM). These types of speakers are high-end gaming speaker systems for PCs, and home entertainment component speakers and subwoofers. Generic PC speakers (low-cost speakers normally packaged with PCs or with low-end application-specific products) will not take advantage of the technology due to an increased price and the consumer not wanting to pay extra for a sub-average wireless speaker.

Even more so than speakers, bandwidth requirements for headsets are easily satisfied by full-speed. Generally, these headsets only need 128 kbps of bandwidth to drive two streams of high-quality audio. Like speakers, OTG headsets do not seem to make sense. But as target peripherals they are an ideal solution. High-end headsets (>$199 average retail price) can use WUSB technology to connect both to the PC for applications such as VoIP, and connecting to PDAs, mobile phones, etc.

The Future: what is next for USB?
To really assess this "need for speed," we need to understand what is driving it. There are two overreaching trends that appear in many of the above end-equipment discussions: richer content and increased storage capacity. It's like the chicken and the egg: is richer content driving a demand for more capacity, or is the increase in storage capacity increase the end-user desire for richer content?

An example of richer content is in the digital still camera market. In 1998, DSCs averaged 1-megapixel. In 2004 it was 5-megapixels. Today, it is approaching 10-megapixels. Currently, mobile phones are implementing 1.3-megapixel camera functions, and trending to the 5-megapixel range over the next few years. There is no reason to expect this trend to slow down.

Concurrently, DSCs are transitioning from optical zoom (a bulky and expensive electromechanical sub-system) to digital zoom. Digital zoom drives a need for higher resolution to enable the DSC user to zoom in on a digital image already captured while still maintaining the original high resolution image. Both trends require storing and uploading of larger image files. The average JPEG camera picture file has grown from 100 KB in 1998 to an average of 4 MB today. As an added comment, these devices have a high refresh rate. Users want/need to move the content off the device to free up space, for it to be refilled quickly.

Other examples of richer content include HDD-based portable media players " whether MP3 or video players. Much of the content is intended to be taken "on-the-go." Movies are transitioning from standard definition (requiring 5.7GB storage) to high-definition movies (25 GB storage). MP3 players with up to 40 GB of storage, and the ability to carry 10,000 MP3 songs, are common.

Storage capacity growth outpaced any technological evolution of the past few decades, even exceeding Moore's law. When it comes to storage capacity, we need to consider the trends in hard disk drives (HDD) and flash memory. The average HDD in 1996 was 3.5" in diameter, turned at 3,600 RPM, and held 540 MB of storage. The average HDD in 2004 was 2.5", turned at 9,600 RPM, and held 160 GB of storage.

HDD storage is also decreasing in size. Today there are products including a 5-GB, 1" HDD and 0.85" drives holding 4 GB. These new drives are aiming at small portable applications such as mobile phones and MP3 players. Using a trend line and factoring the information above, we can project that in 2008 the average (not maximum) hard disk drive will hold 2,750 GB, will be 2.1" in diameter, and will turn at 15,700 RPM, maintaining the current capacity annual growth rate of 104 percent (double Moore's law growth).

Flash memory first appeared in 1989 and had a capacity of 1 MB. By 2004, flash memories reached 1 GB. If we maintain that same trend line, in 2008 there will be a 6.4-GB Flash memory chip (following Moore's law more closely, with annual storage capacity growth rate of 58 percent).

Looking at Figure 3, we can analyze the speed that will be required to move 20 GB of data between the computer and the storage device, assuming 50 percent bandwidth efficiency in 10 seconds, 1 minute, and 5 minutes. Five minutes is a good guideline for the "threshold of pain" that many users will tolerate for instant "on-the-go" functionality, and most desire these updates to take a minute or less.

Very rarely will a user be transferring the entire contents of one of these devices. In this case, we can estimate that a transfer speed in the 5 Gbps range seems like a good target for any future USB speed enhancements. Figure 4 shows data transfer times for various data image sizes assuming 50 percent bandwidth efficiency.


Figure 3.2: Analysis of speed required to move 20 GB of data between the computer and the storage device (assuming 50 percent bandwidth efficiency in 10 seconds, 1 minute, and 5 minutes).

(Click to enlarge image)




Figure 4.2: Data transfer times for various data image sizes (assuming 50 percent bandwidth efficiency).

(Click to enlarge image)


Which USB is right for you?
Part I reviewed a brief history of PC peripherals, the need for a common interface, and the evolution of the USB specification. Then it looked at USB specifications to understand how it impacts the end-user experience. Lastly, it reviewed the history of the specification, and reviewed the different flavors of USB that are in common use today.

Part 2 took a look at common applications and discussed the pros and cons of using different versions of USB to determine which USB is best for each. Then, it peered into the future for some of these applications to determine that in 2009 and beyond there will be the need for an even fatter data throughput pipe.

About the Author
Dan Harmon has held the position of Product Line Marketing Manager for Catalog Interface Connectivity Products at Texas Instruments for the last six years. During his 20-year career at TI, Dan engineered night-vision FLIR systems, and was a camera-design engineer on CCD Imaging Products before becoming the CCD Product Marketing Engineer. He earned a BSEE from the University of Dayton and a MSEE from the University of Texas in Arlington. You can reach Dan at [email protected]

(Note: All Trademarks are held by their respective companies.)


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