Over the last few years, the number and variety of portable electronic devices have exploded. Cell phones, digital cameras, PDAs, and MP3 players are increasing in capability and in the amount of data they can store and process. These new, improved devices give users convenience and capabilities they could only dream about a few years ago. But protocol, system, and device designers must address new issues and problems that accompany these powerful, new devices. Their unique characteristics require different considerations than more conventional, wall-powered systems.
Naturally, as portable devices gain power and store more data, it becomes increasingly desirable for them to share information among one another and with nonportable devices. But handheld devices present problems in data entry and display that don't exist with other powerful electronic devices, like PCs. Portable devices are small, so they typically don't have a very useful general-purpose human interface. Usually, they have small display screens and a limited number of buttons.
This creates a serious problem in offering data entry and retrieval capability to match the device's power and storage capacity. For instance, it would be easy to build a cell phone that could store 10,000 phone numbers—but practically impossible to enter that many names and numbers by hand via the phone's tiny keypad.
Contributing to the problem is the possible size of data to be transferred. A digital camera can easily store hundreds of megabytes of data, which would take a long time to transfer over a slow connection. Various types of data transfers also might be necessary. Users may want to connect their cell phones to PCs to download stored messages; to other phones to share phone numbers; to cameras to upload photographs; or to PDAs to access a wireless network.
However, portable devices like cell phones are too small to have dedicated connectors for each of these diverse tasks. Further, the cost of so many connectors would be prohibitive in a cost-sensitive portable device. Also, many users of portable devices aren't technically inclined, so any means of transferring data has to be simple and intuitive. Required is an inexpensive, ubiquitous, user-friendly, high-speed means of connecting a diverse array of devices and transferring data among them.
USB To The Rescue? One candidate for such a connection is the Universal Serial Bus (USB). It's already an established mode of connecting PCs to peripherals, ranging from simple items like mice and keyboards to more complicated devices like printers and MP3 players. Nearly every PC sold over the last three years has USB capability, creating an installed base of over 1 billion PCs and peripherals. USB also is inexpensive, reliable, and easy to use. It delivers high performance, with data transfer speeds ranging from 1.5 to 480 Mbits/s.
But standard USB requires a host—typically a PC. Data can only be moved between two peripherals by passing through the host. So why not simply put host capability into a port-able device? Unfortunately, there are some problems associated with this solution.
USB is a master-slave protocol, designed for use with one host and many potential peripherals. As such, the intelligence is largely centered on the host side of the connection. The amount of intelligence required to implement the full USB specification on a host could burden simple, dedicated-purpose portable devices. Also, USB uses directed cables, with one type of connector for plugging into a host and another type for the peripheral end.
A portable device, though, may sometimes need to act as a host (a digital camera sending pictures to a printer), but as a peripheral at other times (a digital camera uploading pictures to a desktop PC). Even the relatively small USB connector can be too large for the tiny portable devices of today and tomorrow. Using separate connectors for host and peripheral operation is out of the question.
Finally, USB assumes that the host has plenty of power to supply to connected devices, which may take their power entirely from the bus. This could place an unmanageable strain on small portable devices with very limited stores of power. Because USB has so many advantages, there must be a way to get around its shortcomings and make it practical for use in portable devices.
USB OTG: The solution to the problem has arrived in a new supplement to the USB specification called "USB On-The-Go." Released on Dec. 18, 2001, the On-The-Go (OTG) supplement addresses the needs of portable devices. OTG defines smaller connectors that can be used in both host and peripheral roles; protocols for devices to negotiate the host or peripheral role; lower power requirements; enhanced power conservation modes; and restrictions designed to reduce the burden of software overhead.
Even with the new features, OTG maintains full compatibility with the existing USB specification. One major mechanical enhancement is the addition of new types of connectors. Mini-A and Mini-B plugs and receptacles have been added to the original USB A and B connectors, along with a "Mini-AB" receptacle. These plugs and receptacles are considerably smaller than their original full-size counterparts (Fig. 1).
The Mini-AB receptacle is used on dual-role devices. An ID pin, not present in standard USB plugs, is shorted to ground in the Mini-A plug and unconnected in the Mini-B plug. With this ID pin, a dual-role device identifies which type of plug is connected, determining the device's default role.