Chapter
1
About USB
This chapter presents a quick introduction to USB. The first section in this chapter
introduces the basic concepts of the USB specification Revision 2.0. The second section
explores the data flow model. The third section gives details about the device operation.
Lastly, the fourth section describes USB device logical organization.
The full protocol is described extensively in the USB Specification Revision 2.0 at
http://www.usb.org .
1-1 INTRODUCTION
The Universal Serial Bus (USB) is an industry standard maintained by the USB Implementers
Forum (USB-IF) for serial bus communication. The USB specification contains all the
information about the protocol such as the electrical signaling, the physical dimension of
the connector, the protocol layer, and other important aspects. USB provides several
benefits compared to other communication interfaces such as ease of use, low cost, low
power consumption and, fast and reliable data transfer.
1-1-1 BUS TOPOLOGY
USB can connect a series of devices using a tiered star topology. The key elements in USB
topology are the host , hubs , and devices , as illustrated in Figure 1-1. Each node in the
illustration represents a USB hub or a USB device. At the top level of the graph is the root
hub, which is part of the host. There is only one host in the system. The specification allows
up to seven tiers and a maximum of five non-root hubs in any path between the host and a
device. Each tier must contain at least one hub except for the last tier where only devices
are present. Each USB device in the system has a unique address assigned by the host
through a process called enumeration (see section 1-4-3 on page 25 for more details on
enumeration).
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