USB EXPLAINED: AN INTRODUCTION TO USB AND ITS FUTURE
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USB HISTORY
The Universal Serial Bus (USB) was introduced to the world of PC computing to solve the growing problem of how
to connect peripherals to a PC. The world of desktop computers needed a way, not only to physically connect
devices, but also allow them to be identified and categorized to enable a simple ease of use and method of discovery.
Prior to USB, many people overloaded the use of their serial and parallel ports to support more than one peripheral
on that port.
Stephen’s story:
“I had a printer, two dongles, and a scanner on a single parallel port. It was so complex that booting with the scanner
selected meant that the dongle was not recognized. An A/B switch was added to make all of the components work
together. This meant booting the computer with the switch set correctly in order to access that particular device and its
accompanying software. Reboots were necessary to run the different sets of software and their respective peripherals. It
was very frustrating and not very user friendly.”
In 1996, the 1.0 USB specification was released to solve this type of problem, not only with serial and parallel ports,
but with keyboards, mice, and many other low/full-speed devices. USB 1.0 offered a single bus at either 12 Mb/s
bandwidth (full speed) or 1.5 Mb/s (low speed) shared between up to 127 peripherals, all with different needs. It is
superseded by the USB 1.1 specification released in September 1998.
USB is a host-centric tree topology where all devices are hooked to the host via a set of hubs. Each device supports
up to 32 endpoints: 16 in and 16 out. Each endpoint is defined as either: control, bulk, interrupt, or isochronous.
The basic types of transactions are as follows:
■■ Control. Used to configure and enumerate a peripheral, allowing the host software to interrogate the
peripheral to find out what it is and how it can use it. Every peripheral needs to support this traffic type.
■■ Bulk. Reliable data transport that guarantees sequential reception of data. Retransmits are used to ensure
the data is received correctly. Depending on the current bus utilization, the throughput with bulk transfers
will vary. Typical uses of bulk transport are Disk-On-Keys and printers.
■■ Interrupt. Meant for low bandwidth transfers with periodic transport needs. The host pre-schedules an
interval at which the peripheral is queried. Unlike bulk, this transport type is guaranteed allocation on the bus
for transport. The typical use is keyboards, mice, track-balls, and game controllers.
■■ Isochronous. Guaranteed bandwidth, but not guaranteed reception. This method is typically associated
with video cameras and audio speakers. It is used to insure that the data always has a path. During
enumeration and configuration, the host determines if a configuration setting has enough bandwidth to talk
with a certain peripheral. If sufficient bandwidth is not available, it rejects the configuration selection in favor
of less bandwidth consumption.
All peripherals support a single control endpoint and any number of bulk, interrupt, and isochronous endpoints to
fit their application up to the maximum of 32 endpoints.
The USB Implementers Forum saw that adoption was good, but with the competition of FireWire, many high-speed
peripherals were bypassing USB, and therefore, USB needed to grow with the increased demand for bandwidth.
The USB 2.0 specification released in April of 2000, added the key feature of high-speed devices, cranking up the
USB bandwidth to 480Mb/s or 40 times full-speed USB bus speed. The USB 2.0 specification maintained backward
compatibility with all USB 1.1 compliant host and peripherals as interoperability was essential. Disk-On-Keys were
considered a “killer app” for USB 2.0.


Besides providing a universal serial bus on which all peripherals talk, the USB Implementers Forum also adopted a
class approach to device drivers. No longer do vendors of peripherals need to provide both a host and a peripheral
driver, they can provide their functionality as part of a particular class of devices. Then the host will provide a set of
class drivers for use with a set of peripherals.
One of the classes is the human interface device (HID) class. It is well adopted by providers of mice, keyboards,
joysticks, and trackballs. Most of these devices do not ship with a host driver as they just use the functionality
provided in the host for the HID class.
In addition to the HID class, there is mass storage class (MSC), printer, video, audio, communications, hub, and
device firmware upgrade (DFU). Where the needs are unique, the vendor can provide a vendor-specific driver for
use with a particular host operating system. Each class is as complex or as simple as needed to accomplish the
basic set of peripherals. For instance, the communications class is made up of a set of communications paradigms
including modem and Ethernet subclasses. Each subclass represents a set of peripherals that have specific needs,
and are subsets of the entire communications class.
USB support for embedded designs, facilitating the implementation of both peripheral and host functionality, is
available from a number of manufacturers, including Mentor Graphics.