You've Come a Long Way, USB

Electronic News, July 23, 2001 by Steven Larky, Steve Kolokowsky, MARK McCoy

THE UNIVERSAL SERIAL Bus (USB) has been around for a while. USB is an open interface standard that has been included on PC motherboards since late 1997. In 1998, the iMac was introduced, featuring only two I/O interfaces--USE and Ethernet. The first versions of the USB spec (1.0 and 1.1) supported maximum transfer rates of 1Mbyte/sec. with a raw bit rate of l2Mbits/sec. Windows '98 provided the first OS support for USE with built-in drivers for a range of devices, including mice, keyboards and speakers. In April 2000, the USE Implementers Forum (Microsoft, Intel, Philips, HP, Compaq, NEC, and Lucent) released version 2.0 of the USB specification. This release added a new high-speed transfer rate. High-speed USB is 480Mbits/sec., 40 times faster than the fastest rate offered in USB 1.1.

USB supports two types of transfers for moving large blocks of data: bulk for moving data that cannot tolerate errors and isochronous (iso) for moving data that cannot tolerate delay. Bulk transfers ensure the data is transferred reliably with no lost or corrupted data. However, the amount of data transferred in a given period of time is not guaranteed. Bulk data is "fill data" that only uses the bus when there is no other traffic. Isochronous transfers are scheduled to occur at a defined rate that has been negotiated between the host PC and the device. However the data may arrive corrupted or not be sent from time to time.

Selecting between iso and bulk is fairly straightforward. If the integrity of the data is most important, then bulk transfers should be used. If it is more important for the data to be on time than correct, iso transfers should be used. At first glance, it seems illogical to select iso as iso provides corrupted data on time instead of correct data later. But there are several instances in which data cannot be late. A simple example is an audio stream during a telephone call. The audio must keep up with real-time in order to make the phone usable. Losing a few snippets of conversation is preferred over the listener falling further and further behind the speaker. Another example is video, where it is preferable to miss a frame of the image and maintain the real-time flow of the image.

USB bandwidth is divided into 1,000 frames per second. Isochronous transfers in USB 1.1 were limited to a single packet per device endpoint per frame. The largest packet was 1,023 bytes, yielding an aggregate data transfer rate of 1Mbyte/sec. USE 2.0 not only increased the data rate to 480Mbits/sec., it also subdivided each of the frames into 8 microframes to reduce the need for buffering in the device. USB 2.0 also increased the number of 1,024 byte packets per microframe to 3 for each device. Thus, the achievable iso data rate in USB 2.0 jumps from 1Mbyte/sec. to 24Mbytes/sec. (1,000 frames/sec. x 8 microframes/frame x 3 packets/microframe x 1,024 bytes/packet).

The most obvious multimedia application that is opened up with the introduction of USE 2.0 high-speed iso is for inexpensive video cameras. USB video cameras have been available for years, but the limited bandwidth of full-speed USB has resulted in disappointing resolutions and frame rates. A full-speed USB video camera only has the bandwidth to transmit uncompressed CIF (352x288) video at 5 frames/sec. This device would require more than 90 percent of the available bandwidth that must be shared by all USB devices in the system. At high speed, the same camera could operate at 30 frames/sec. and use just 10 percent of the available bandwidth. As a real-world example, currently available digital video cameras transmit at a constant data rate of 4.5Mbytes/sec. for video and audio combined. This is well within the capabilities of high-speed iso.

Windows support of USB is based on a layered architecture that minimizes and can even eliminate the need for custom device drivers. At the lowest level is the USB driver (USBD). Layered above the USBD are class drivers, drivers that are generic to a range of devices with similar characteristics. For iso support, the class driver is the Stream Class Driver (stream.sys). Audio and video drivers are layered above stream.sys. Windows '98 includes support for audio devices, and Windows 2000 improves upon this support by reducing the possibility of audio data being delayed by other operating system tasks. Windows '98 SE and Windows 2000 add some support for digital video. USE 2.0 high-speed devices can use the streaming driver to access up to 24Mbytes/sec. of bandwidth.

PCI hosts that support USB 2.0 appeared in retail shops in April. Microcontrollers that support the new 2.0 standard are sampling now. The Cypress EZ-USB FX2, for example, is an 8-bit microcontroller that is USB 2.0 compliant and supports iso transfers at a sustained rate of up to 24Mbytes/sec. A flexible external interface allows it to be easily connected to a multimedia data source such as a video camera ASIC. Firmware and driver examples included in the developer's kit can help to bootstrap the iso development and have transfers up and running on the first day of development.