Chapter 2: The Big Picture

Any high-speed capable devices attached to a 1.x hub must operate in fullspeed mode. Note that the minimum requirement is that a high-speed device must permit access to its descriptors at full-speed, but may not function beyond that basic capability.

High-Speed Hubs

Each USB high-speed capable port must support the attachment and operation of high-, fulland low-speed devices. The HS hub interface detects the speed of the device and makes the necessary adjustments to operate at the required speed.

High-Speed Devices

The hub repeats high-speed packets to all ports that have high-speed devices attached. The high-speed devices decode the high-speed packets to determine if they are being targeted by the host.

Lowand Full-Speed Devices

When a lowor full-speed device is attached to a high-speed hub port, the hub checks for high-speed split transactions that are targeted for one of the lowor full-speed devices attached to its ports. When this occurs the hub translates the high-speed split transaction into the required lowor full-speed transaction and delivers it to the target device.

Topology

USB employs a tiered star topology where hubs provide attachment points for USB devices. The host controller contains the root hub, which is the origin of all USB ports in the system. As illustrated in Figure 2-27, three tiers are created by the three hubs: the root hub, a 2.0 hub, and a 1.x hub. Note that devices of any speed may be connected to any of the hub ports regardless of their speed.

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USB System Architecture

Figure 2-27: USB’s Tiered Star Topology

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