FX3 Programmers Manual, Doc. # 001-64707 Rev. *C 13
2. Introduction to USB
The universal serial bus (USB) has gained wide acceptance as the connection method of choice for
PC peripherals. Equally successful in the Windows and Macintosh worlds, USB has delivered on its
promises of easy attachment, an end to configuration hassles, and true plug-and-play operation. The
USB is the most successful PC peripheral interconnect ever. In 2006 alone, over 2 billion USB
devices were shipped and there are over 6 billion USB products in the installed base today.
2.1 USB 2.0 System Basics
A USB system is an asynchronous serial communication 'host-centric' design, consisting of a single
host and a myriad of devices and downstream hubs connected in a tiered-star topology. The USB
2.0 Specification supports the low-speed, full-speed, and high-speed data rates. It employs a
half-duplex two-wire signaling featuring unidirectional data flow with negotiated directional bus
transitions.
2.1.1 Host, Devices, and Hubs
The USB system has one master: the host computer. Devices implement specific functions and
transfer data to and from the host (for example: mouse, keyboard, and thumb drives). The host owns
the bus and is responsible for detecting a device as well as initiating and managing transfers
between various devices. Hubs are devices that have one upstream port and multiple down stream
ports and connect multiple devices to the host creating a tiered topology. Associated with a host is
the host controller that manages the communication between the host and various devices. Every
host controller has a root hub associated with it. A maximum of 127 devices may be connected to a
host controller with not more than seven tiers (including root hubs). Because the host is always the
bus master, the USB direction OUT refers to the direction from the host to the device, and IN refers
to the device to host direction.
2.1.2 Signaling Rates
USB 2.0 supports following signaling rates:
■ A low-speed rate of 1.5 Mbit/s is defined by USB 1.0.
■ A full-speed rate of 12 Mbit/s is the basic USB data rate defined by USB 1.1. All USB hubs
support full speed.
■ A high-speed (USB 2.0) rate of 480 Mbit/s introduced in 2001. All high-speed devices are
capable of falling back to full-speed operation if necessary; they are backward compatible.
2.1.3 Layers of Communication Flow
A layered communication model view is adopted to describe the USB system because of its
complexity and generic nature. The components that make up the layers are presented here.
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