USB On-The-Go – USB technology for PDAs and mobile phones

The recent explosion in the number of digital electronics devices on the market has fuelled the need for an industry standard method of interconnecting such devices and peripherals such as printers. The dozens of connectivity methods used by mobile product manufacturers – including proprietary docks, dongles, slots, connectors and the various different memory card technologies – bears witness to this need.

The market penetration of USB – by early 2001 there were over 1.1 billion USB-enabled PCs, peripherals and consumer electronics devices in the worldwide customer base – makes it a natural candidate for mobile point-to-point connectivity. However, its master-slave protocol – which relies on the intelligence of the host – is a major disadvantage of USB. Implementation of the entire USB host on portable electronics devices is simply not feasible. Not only will a standard USB host significantly cripple the battery life of a low-powered device, the connectors are also too large for the form factor of these miniature devices.

To address the problem, several mobile phone, PDA and mobile product manufacturers have collaborated to define a new supplement to USB 2.0 specification, called USB On-The-Go (OTG). Finalised in December 2001, the new standard defines new mini connectors, negotiation rules and power requirements used in both host and peripheral. The specification also calls for a new type of device called dual-role peripheral, capable of acting as either a host or peripheral, depending on how users connect the cable to its unique mini-AB receptacle. When connected to the mini-A plug, the dual-role device behaves as a host; when the mini-B plug is connected instead, the device acts as a peripheral. The OTG peripheral can also determine the power consumed the by host during a connection in order to preserve battery power.

To encourage adoption, and to help consumers identify peripherals able to connect to their dual-role devices, the USB Promoter Group requires that all OTG dual-role devices come with a Targeted Peripheral List of supported hardware. Eventually, it is planned for USB OTG dual-role devices to be able to support a large class of peripherals via a universal driver.

On the face of it, Bluetooth – the wireless technology designed from ground up to allow interconnection of consumer electronics devices – might appear to be a competitor to USB OTG. In fact, the two technologies are capable of co-existence on many portable devices and are seen as being complimentary, USB OTG allowing connection to the huge installed base of USB-capable devices.

Knoppix on a USB Key for booting and trouble shooting.