USB System Architecture

USB Peripheral Connection

As stated in the previous chapters, USB provides a single type of connector for attaching peripherals to a system. USB 2.0 also supports three different speeds of USB devices:

•low-speed devices — 1.5Mb (megabits)/second

•full-speed devices — 12Mb/second

•high-speed devices — 480Mb/second

All USB devices attach via a USB hub that provides one or more ports. Figure 2- 27 on page 68 illustrates a variety of devices attached to USB ports provided by the system. Hub ports may support only fulland low-speed or may support all three speeds as illustrated in Figure 2-27. A device’s speed is detected when it is attached to the hub port. (Refer to Chapter 5, entitled "LS/FS Signaling Environment," on page 93, and Chapter 11, entitled "The High-Speed Signaling Environment," on page 217 for details).

Some devices such as keyboards and mice typically operate at low speed, while other devices such as digital telephones must operate at either full or high speed. However, several connection issues can exist depending on the device speed and the hub port capability as listed below:

•full-speed hub ports (1.x hubs) — support for LS and FS devices only

•high-speed hub ports (2.0 hubs) — support for LS, FS, and HS devices

Due to related EMI differences at the different transmission rates, the cables used for low-speed versus full-/high-speed devices are subject to different electrical characteristics. See Chapter 3 for details regarding the electrical characteristics of the cables.

Full-Speed Hubs

Hubs based on the 1.0 and 1.1 versions of the specification can support only lowand full-speed devices. These hubs block all full-speed traffic from reaching low-speed devices attached to its ports. low-speed transactions targeting these devices will always be preceded by a preamble packet that serves as a command to l.x hubs to enable their low-speed ports. This ensures that lowspeed devices see only the low-speed transactions.

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