By Fred Rastgar
Newsflash: Optimum product design needs USB connectivity and the development tools that eliminate implementation headaches.
The universal serial bus (USB) connector made its debut back in 1995 when a group of seven companies defined it as a means to overcome certain inadequacies the industry saw in PC I/O connections. Since then, it has undergone a huge transformation to add speed and performance. Today, an estimated one billion USB-enabled devices are installed worldwide.
In the last three years, two new USB specifications were released. The first, USB 2.0m added 480-Mbit/s capability and new transactions to maximize bus bandwidth utilization. It also provided new hub structures to ensure compatibility with existing devices. The second, USB On-The-Go (OTG), creates a virtual peer-to-peer capability for USB enabling a new dual-role device that can operate as a peripheral or host using the same mini-AB connector.
Electrically, a USB device consists of four wires: Two provide power and two provide a bidirectional differential signaling pair. Cables are polarized using different non-reversible connectors at either end, although one end may be made captive to reduce costs. Hubs can be used to expand the number of available connections to a host. Devices can be attached and configured automatically.
USB vs. PC I/O
One USB connector can support up to 127 USB devices, and each device can have multiple endpoints (often registers or buffers) that can be addressed from the host for read or write. A special endpoint (endpoint-0) is used to dynamically configure the device from the host.
This simplicity equates to very-low-cost implementation with minimal chipset and processing overheads required by the device. The USB protocol has the intelligence to tell the host what type of device they are and what they can do. This "plug and play" feature, when compared to previous serial port devices, provides a user-friendly experience.
Standard PC I/O connectors have several drawbacks. First, they lack hot plugs and cannot provide automatic configuration. Most have limited system resources such as I/O addresses and non-sharable interruption request lines. Standard connector types are limited, yet there are vastly different cable types. It is also more costly to add new peripheral connections.
The USB communication protocol provides solutions to these and other drawbacks of traditional PC I/O connectivity (see Table). However, in simpler devices such as computer mouse and keyboard connections, the use of USB connectors may be overkill in terms of functionality and performance.
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The USB OTG specification is expected to have a significant impact on the development of new products since devices that normally would be considered peripherals would now have host roles as well. The host has traditionally been in the PC and was a very stable, well-controlled stack/IP. But with the development of OTG hosts, the requirement now exists for more verification that the OTG device can work well with all its targeted peripherals.
The most likely candidates for OTG include products such as digital cameras, cell phones, and PDAs. The need to connect to a PC would be eliminated. For example, a digital camera could transfer pictures directly to a printer or a PDA could virtually become, in effect, a miniature PC.
Development headaches
The USB is designed to provide communication services between a host and attached USB physical devices. However, the simple view an end user sees of attaching one or more USB devices to a host is, in fact, more complicated to implement than is indicated by simply plugging it in. The designer must handle more complexity.
With its long list of features and benefits, USB connectivity still presents significant challenges to product designers. For example, designers must modify the configuration requirements of their product to ensure compatibility and adequate performance when sharing a hub with other traffic. There are also a multitude of possible problems that could arise associated with power, cabling, conflicts, updates, and software compatibility.
When it comes to developing a product with USB protocol, arguably the most essential tool to assist the developer is a protocol analyzer (see Figure). These tools cross the boundaries of hardware and software to enable developers to fully observe the behavior of the system during the early development stages.
 The protocol analyzer provides developers with a vital tool for adapting USB to a wide range of products. |
A protocol analyzer is often used in addition to an oscilloscope and volt/current meter but usually replaces the traditional logic analyzer for analysis of problems on a shared bus. Although a logic analyzer can show raw bits the same as a protocol analyzer, the user is typically left to chop the bitstream up and manually identify and decode the packets. A protocol analyzer takes this information a level further to enable the user to "see" the "stuff bit" in the non-return-to-zero inverted encoding format and evolves it into a more readable form.
To fully use the available high-speed bandwidth, a device must support a broader transaction capability. A protocol analyzer can ensure correct implementation of new states and correctly handle new error conditions. An analyzer with two channels makes the development of a hub infinitely easier, since it enables monitoring of the upstream and downstream traffic simultaneously. It can then reconstruct split transactions that may be spaced over several frames with significant intervening traffic.
When selecting a protocol analyzer in designing USB-capable products, there are several key features to keep in mind. An analyzer with a high-impedance probe does not distort or enhance the electrical signals. Triggering capability is essential for set-specific combinations of conditions to capture a specific problem.
An intuitive user interface can interpolate the protocol trace in terms that are consistent with the protocol specification. Bits and bytes are translated into meaningful representations without having to constantly refer to the specification. Other features would include dual channels, OTG support, traffic generation, and support of all necessary speeds.
A good protocol analyzer will not only show if the system is working, but it will also highlight any problems experienced on the bus. The analyzer will also explain why the error or warning has occurred and what the spec-compliant reading should have been.
With the proper tools, developers can more easily adapt USB to a wide range of products. The future for USB will likely include its adoption by the wireless product world. Although there is no official activity to date, many people believe the efforts in 802.15.3a (ultra-wideband) will lead to a USB 2.0-compliant wireless interface. For now, however, no protocol has been defined for 802.15.3a.
The versatility and flexibility of USB has boosted its popularity such that more than 100,000 products worldwide have been developed with USB connectivity. As development tools make it easier to incorporate USB into additional products, the simplicity of its "plug and play" nature will make it the connector of choice for end users.
FRED RASTGAR is senior product marketing manager at CATC (Computer Access Technology Corp.), 2403 Walsh Ave., Santa Clara, CA 95051. Tel: (408) 727-6600; email: frastgar@catc.com.
