Supporting USB - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 26

USB 2.0 ports that support the bus speeds used by mass-storage devices: 12
megabits/sec and 480 megabits/sec. Windows and other operating systems
support USB's mass-storage class. A USB host interface is a good solution if
you need a host controller to communicate with other devices or if you want
to use off-the-shelf USB storage devices.
This chapter introduces the USB interface and USB's mass storage class.
The Interface in Brief
The Universal Serial Bus is an interface and protocol that enable a single
host computer to communicate with a variety of peripheral devices. USB is
appropriate for just about any kind of mass-storage device, including hard
drives, CD and DVD drives, and flash drives.
The USB specifications are available from the USB Implementers Forum
(USB-IF) ( www.usb.org) . The USB-IF is the non-profit corporation
founded by companies involved with developing the USB specification. The
USB-IF also sponsors a developers Web forum, provides software and hard-
ware to help in developing and testing products, and develops compliance
tests for devices, hosts, and related hardware.
The USB 2.0 specification is the main document that defines the interface.
Hosts and Devices
Every USB communication is between a host and a device. The host is in
charge of the bus. Devices communicate only when requested to do so by
the host. The only exception is the remote-wakeup feature, which enables a
device in the low-power Suspend state to request communications with the
host.
A USB host is a computer that contains USB host-controller hardware, a
root hub with one or more USB ports, and program code to manage com-
munications and events on the bus. The host-controller hardware formats
data for transmitting on the bus and converts received data to a format that
host software can understand. The host controller also performs functions
related to managing communications on the bus. The root hub has one or
more connectors for attaching devices. The root hub, in combination with
the host controller, detects newly attached and removed devices, carries out