The USB On-The-Go specification allows direct interconnectivity between computer peripherals and mobile devices using the popular USB standard.
The specification was released as a supplement to the USB 2.0 specification at the end of last year. It is available for public comment and can be downloaded at www.usb.org/developers/onthego.
A number of companies are listed on the specification as contributors include ACON, Cypress, Ericsson, Hewlett-Packard, inSilicon, Intel, MCCI, Microsoft, Motorola, NEC, Nokia, Onspec, OPTi, Palm, Philips, SoftConnex and Texas Instruments and TransDimension.
The additions to the existing USB specification are minimised to ensure interoperability with the widest range of the more than 900 million USB-based products in use today.
While USB, as currently defined, requires one side to be a host, and one side to be a peripheral, the USB On-The-Go specification defines a protocol that enables USB products to function in a dual role as either host or peripheral, enabling direct point-to-point connections.
With traditional USB, the host side is very complex while the peripheral side is very simple. However, as products that have traditionally been peripherals become more intelligent, adding in additional USB host functionality is rapidly becoming a necessity. Transdimension recently demonstrated operational prototypes. “With our track record of success in the embedded USB host controller market, we have tremendous visibility into USB usage in Post-PC appliances and mobile products,” said David Murray, vice president of marketing for TransDimension and one of the authors of the original USB specification.
TransDimension's OTG243 single-chip, dual-role controller is the first fully integrated solution. With an embedded OTG243 controller, any OTG-compliant device can be a self-sufficient USB host or a USB peripheral at the user's option.
The TransDimension architecture gains its performance advantage by locating the bulk list structure in controller memory. The bulk list consists of endpoint descriptors (ED), which identify the endpoint address to/from which data is to be transferred and the transfer descriptors (TD), which point to the data in memory.
Other architectures require working through the entire list of EDs and TDs, whether active or not, to send a bulk output transfer.
If there is no immediate handshake from the destination device, the entire process of accessing and computing EDs and TDs and reading a data packet from memory must be repeated. The OTG243 only lists active EDs and TDs for faster work through and the use of local memory reduces contention on the system bus and CPU. An internal buffer eliminates the need to repeatedly read data packets during connection retries.
For real-time (isochronous) data transfers, the OTG243 uses loose timing and mega buffering techniques to ensure quality of transmission in the event of bus errors or delays in operating system scheduling. Unlike bulk transfers, the USB specification guarantees bandwidth access for isochronous transfers but does not guarantee data delivery. When data is lost due to a bus error, the controller will continue to send an empty frame, causing clicks and pops in audio and visual artifacts in video or, in the worst case, killing the stream altogether. The OTG243 requires less frequent access to the system bus and can withstand interrupts during isochronous transfers without affecting data delivery.
Other OTG devices recently released include the ISP1392 from Philips Semiconductors and the SL811HR_OTG reference board from Cypress Semiconductor.
The ISP1362 is an OTG compliant USB 2.0 host and peripheral controller. These capabilities allow the chip to act as USB host, USB peripheral, or both functions simultaneously. The host and peripheral roles can be interchanged through the Host Negotiation Protocol (HNP).
The Cypress prototype board contains the SL811HS USB host/peripheral controller and an SL11R microcontroller. As well as components, the board comes with schematic diagrams, firmware, an assembler, and compiler.
Strategic alliance on USB 2.0
TransDimension and NEC Electronics have set up strategic alliance for the development and production of high-speed Universal Serial Bus (USB) 2.0 high-speed controllers for embedded applications. Under terms of the agreement, TransDimension will market worldwide the solutions jointly developed through the alliance.
TransDimension, provides a market-proven embedded USB 1.1 and 2.0 host stacks and a range of class drivers. NEC is a core member of the USB Implementers Forum, is a leading silicon vendor offering high-speed USB 2.0 host controllers in large volumes today, in addition to providing a hub controller and integrated drive electronics (IDE) bridge chip.
Published in Embedded Systems (Europe) April 2002