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TransDimension Implements New OTG for USBBy Michael Foley
The OTG243 controller allows devices to be USB hosts or USB peripherals at the user's discretion. OTG is a newly released supplement to the USB 2.0 specification and specifically addresses interface standards for dual-role products that must operate on low-power portable devices and use miniaturized cables and connectors.
"The OTG243 architecture is a major technological achievement that starts with the premise that there are no PC resources, such as a PCI (peripheral control interface) bus, multi-megabyte memory and Windows operating system to rely upon," said David Murray, vice president of marketing at TransDimension.
The major challenge of an embedded USB controller is to guarantee large bulk and isochronous (streaming data) transfers under conditions of bandwidth and power constraints. The OTG specification does not guarantee throughput, Murray noted, and bulk transfer performance can vary significantly in competitive products.
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.
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