Jan Axelson, USB Complete

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Chapter 1

intention of communicating with line printers. USB makes no such assump-

tions and is suitable for just about any peripheral type.

For communicating with common peripherals such as printers, keyboards, and

drives, USB classes specify device requirements and protocols. Developers can

program a device to conform to a class specification instead of having to rein-

vent everything from the ground up.

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This book focuses on Windows programming for PCs, but other computers

and operating systems also have USB support, including Linux and Apple

Computer’s Macintosh. Some real-time kernels also support USB.

At the most basic level, an operating system that supports USB must do three

things:

• Detect when devices are attached to and removed from the system.

• Communicate with newly attached devices to find out how to exchange

data with them.

• Provide a mechanism that enables software drivers to pass communications

between the USB hardware and applications that want to access USB

peripherals.

At a higher level, operating-system support may also mean the inclusion of class

drivers that enable applications to access specific types of devices. If the operat-

ing system doesn’t include a driver appropriate for a specific device, the device

vendor must provide the driver.

Microsoft continues to improve and add to the class drivers included with Win-

dows. Supported device types include human interface devices (keyboards,

mice, game controllers), speakers and other audio devices, modems, drives,

still-image and video cameras, scanners, printers, and smart-card readers. Filter

drivers can support device-specific features and abilities within a class. Applica-

tions use Application Programming Interface (API) functions or other software

components to access devices via their drivers.

Devices that have vendor-specific functions can sometimes use a supported

class such as the communications-device or human-interface device class. Other

options for vendor-specific functions include Microsoft’s WinUSB driver and

generic drivers from other sources. Some chip companies provide drivers that

developers can use with the company’s chips.

USB Basics

Writers of USB device drivers for Windows can use Microsoft’s Windows

Driver Foundation (WDF) model. The WDF provides a framework that sim-

plifies the task of writing drivers.

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On the device side, the hardware must include a controller chip that manages

USB communications. The device is responsible for responding to requests that

identify and configure the device and for reading and writing other data on the

bus. Some controllers perform some functions entirely in hardware.

Many USB controllers are based on popular microcontroller architectures such

as Intel Corporation’s 8051 or Microchip Technology’s PIC® with added hard-

ware support for USB communications. Other controllers don’t contain a CPU

but instead provide a serial or parallel interface to an external microcontroller. If

you’re already familiar with a chip architecture that has a USB-capable variant,

you don’t need to learn a new architecture. Most chip companies provide exam-

ple code to help you get started.

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The USB Implementers Forum, Inc., or USB-IF (www.usb.org), is the

non-profit corporation founded by the companies that developed the USB

specification.

The USB-IF’s mission is to support the advancement and adoption of USB

technology. To that end, the USB-IF offers information, tools, and testing sup-

port. The information includes the specification documents, white papers,

FAQs, and a Web forum. The tools include software and hardware to help in

developing and testing products. The support for testing includes compliance

tests to verify proper operation and compliance workshops where developers

can have their products tested and certified to display a USB logo.

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All of USB’s advantages mean that it’s a good candidate for many devices. But a

single interface can’t handle every task.

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Limits of USB include distance constraints, no support for peer-to-peer com-

munications or broadcasting, and lack of support in older hardware and operat-

ing systems.

Chapter 1

Distance. USB was designed as a desktop-expansion bus where devices are rela-

tively close at hand. Other interfaces, including RS-232, RS-485, IEEE-1394b,

and Ethernet, allow much longer cables. To extend the distance between a

device and its host computer, an option is to use USB to connect to a nearby

device that functions as a bridge to a long-distance interface to the end circuits.

Peer-to-Peer Communications. Every USB communication is between a host

computer and a device (except for one option introduced with USB 3.0). The

host is a PC or other computer with host-controller hardware. The device con-

tains device-controller hardware. Hosts can’t talk to each other directly, and

devices can’t talk to each other directly. Other interfaces, such as IEEE-1394,

allow direct device-to-device communication.

USB provides a partial solution with the USB On-The-Go option. An

On-The-Go device can function as both a device and a limited-capability host

that communicates with other devices.

Two USB hosts can communicate with each other via a bridge cable that con-

tains two USB devices with a shared buffer. USB 3.0 defines a new host-to-host

cable for SuperSpeed. With driver support, this cable can support host-to-host

communications.

Broadcasting. USB doesn’t support sending data simultaneously to multiple

devices (except for USB 3.0 timestamp packets). The host must send the data

to each device individually. If you need broadcasting ability, use IEEE-1394 or

Ethernet.

Legacy Hardware. Older “legacy” computers and peripherals don’t have USB

ports. The issue of supporting legacy equipment has faded, however, as older

systems are retired.

If you need to connect a legacy peripheral to a USB port, a solution is an intel-

ligent adapter that converts between USB and the older interface. Several

sources have adapters for use with peripherals with RS-232, RS-485, and paral-

lel ports. An adapter is useful only for devices that use protocols supported by

the adapter’s device driver. For example, most parallel-port adapters support

communications only with printers, not with other parallel-port peripherals.

RS-232 adapters work with most RS-232 devices.

If you want to use a USB device with a computer that doesn’t support USB, a

solution is to add USB capabilities to the computer. To do so, you need to add

USB host-controller hardware and use an operating system that supports USB.

The hardware is available on expansion cards that plug into a PCI slot or on a

USB Basics

replacement motherboard. For Windows systems, the edition must be Win-

dows 98 or later.

If upgrading the PC to support USB isn’t feasible, you might think an adapter

would be available to translate a peripheral’s USB interface to the PC’s RS-232,

parallel, or other interface. An adapter is rarely an option when the computer

has the legacy interface because an adapter that contains host-controller hard-

ware and code is too expensive to design and manufacture for its limited mar-

ket.

Even on new systems, users may occasionally run applications on older operat-

ing systems such as DOS. Without a driver, the operating system can’t access a

USB device. Although it’s possible to write a USB driver for DOS, few device

vendors provide one. An exception is mice and keyboards, which the system

BIOS typically supports to ensure that the devices are usable any time, includ-

ing from within DOS and from the BIOS screens that you can view on

boot-up.

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For developers, challenges to USB are the complexity of the protocols, operat-

ing-system support for some applications, and for small-scale developers, the

need to obtain a Vendor ID.

Protocol Complexity. A USB device is an intelligent device that must respond

to requests and other events on the bus. Controller chips vary in how much

firmware support they require to perform USB communications. In most cases,

to program a USB device, you need to be familiar with the USB protocols for

exchanging data on the bus. On the host-computer side, device drivers insulate

application programmers from having to know many of the low-level details

about the protocols and hardware interface. Device-driver writers need to be

familiar with USB protocols.

In contrast, some older interfaces can connect to very simple circuits that soft-

ware addresses directly. For example, the PC’s original parallel printer port is a

series of digital inputs and outputs. You can connect to input and output cir-

cuits such as relays, switches, and analog-to-digital converters with no com-

puter intelligence required on the device side. With a driver to enable port

access, applications can monitor and control the individual bits on the ports.

USB is a shared bus with defined protocols, and the operating system prevents

applications from directly accessing the hardware. To access a USB device,

applications must communicate with a class or device driver that in turn com-

Chapter 1

municates with the lower-level USB drivers that manage communications on

the bus. Devices must support protocols that enable the PC to detect, identify,

and communicate with the device.

Evolving Support in the Operating System. The class drivers included with

Windows enable applications to communicate with many devices. Often, you

can design a device to use one of the provided drivers. If not, you may be able to

use or adapt a driver provided by a chip company or other source. If you need

to provide your own driver, a third-party driver toolkit can help in developing

the driver.

Fees. The USB-IF’s website provides the USB specifications, related docu-

ments, software for compliance testing, and much more at no charge. Anyone

can develop USB software without paying a licensing fee.

Every USB device contains a Vendor ID and a Product ID that identify the

device to the operating system. At this writing, the USB-IF charges a $2000

administrative fee for the rights to a Vendor ID. The owner of the Vendor ID

assigns Product IDs. Joining the USB-IF (at $4000/year) gets you a Vendor ID

along with other benefits such as admittance to compliance workshops.

Devices that don’t undergo compliance testing and don’t display the USB-IF

logo have lower-cost options. Some chip companies, including Future Technol-

ogy Devices International Limited (FTDI) and Microchip Technology, will

assign a range of Product IDs to a customer for use in products with the com-

pany's Vendor ID, typically at no charge. Chips that perform all of their USB

communications in hardware can use a Vendor ID and Product ID embedded

in the hardware. An example is FTDI's USB device controllers.

Companies that sell products that implement a USB specification must sign an

adopters agreement. The agreement grants a royalty-free, non-exclusive patent

license to implement the specification. You must submit a signed agreement

within the later of one year after first sale of a product or one year after the spec-

ification’s release. See the agreements (at www.usb.org) for the legal specifics.

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For some devices, the choice is between USB and Ethernet. Ethernet’s advan-

tages include the ability to use very long cables, support for broadcasting, and

familiar Internet protocols. Ethernet hardware is more complex and expensive

than typical USB device hardware, however. USB is also more versatile, with

four transfer types and defined classes for different device functions.

USB Basics

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Another interface option for some devices is IEEE-1394. Apple Computer’s

implementation of the interface is called Firewire. Advantages to IEEE-1394

are support for peer-to-peer communications and broadcasting and more bus

power available to devices (up to 1.5A at 30V).

Compared to USB, where a host computer manages the interface, IEEE-1394

devices have more responsibilities and thus tend to be more complex and

expensive to implement. SuperSpeed USB exceeds IEEE-1394b’s bus speed of

3.2 Gbps. While every new PC has USB ports, IEEE-1394 ports are less com-

mon and thus may require adding ports on expansion cards. For some devices,

such as drives, either interface works well, and some devices support both inter-

faces.

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The main reason why new interfaces don’t appear very often is that existing

interfaces have the advantage of all of the peripherals that users don’t want to

scrap. By choosing compatibility with the existing Centronics parallel interface

and RS-232 serial-port interface, the developers of the original IBM PC sped

up the design process and enabled users to connect to printers and modems

already on the market. These interfaces proved serviceable for close to two

decades. But as computers became more powerful and the number and kinds of

peripherals increased, the older interfaces became a bottleneck of slow commu-

nications with limited options for expansion.

A break with tradition makes sense when the desire for enhancements is greater

than the inconvenience and expense of change. This is the situation that

prompted the development of USB.

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The Universal Serial Bus Specification Revision 1.0 was released in January 1996.

USB capability first became available on PCs with the release of Windows 95’s

OEM Service Release 2, available only to vendors installing Windows 95 on

PCs they sold. The USB support in these versions was limited and buggy, and

there weren’t many USB peripherals available, so use of USB was limited in this

era.

The situation improved with the release of Windows 98 in June 1998. By this

time, many more vendors had USB peripherals available, and USB began to

Chapter 1

take hold as a popular interface. Windows 98 Second Edition (SE) fixed bugs

and further enhanced the USB support. The original edition of Windows 98 is

called Windows 98 Gold to distinguish it from Windows 98 SE.

This book concentrates on PCs running Windows XP and later. Much of the

information also applies to Windows 98, Windows 2000, and Windows Me.

Windows NT never supported USB except via third-party software. However,

all of the editions mentioned above except Windows 95/98/Me are considered

NT-based Windows editions because they build on the Windows NT kernel.

In this book, the term PC encompasses all of the various computers that share

the common ancestor of the original IBM PC. A host computer is any com-

puter that can communicate with USB devices.

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The Universal Serial Bus Specification Revision 1.1 (September 1998) added one

new transfer type (interrupt OUT). In this book, USB 1.x refers to USB 1.0

and 1.1.

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As USB gained in popularity and PCs became more powerful, demand grew for

a faster bus speed. Investigation showed that a bus speed 40× faster than full

speed could remain backwards-compatible with the low- and full-speed inter-

faces. April 2000 saw the release of the Universal Serial Bus Specification Revision

2.0, which added high speed at 480 Mbps. High speed made USB more attrac-

tive for peripherals such as printers, disk drives, and video cameras. Windows

added support for USB 2.0 in Windows XP SP2. The USB 2.0 specification

replaced USB 1.1.

Except for hubs, a USB 2.0 device can support low speed, full speed, or high

speed, and a high-speed-capable device can support full speed when connected

to a USB 1.x bus. A USB 2.0 hub must support all three USB 2.0 speeds. The

ability to communicate at any speed increases the complexity of the hubs but

conserves bus bandwidth and eliminates a need to use different hubs for differ-

ent speeds.

USB 2.0 is backwards compatible with USB 1.x. In other words, USB 2.0

devices can use the same connectors and cables as 1.x devices, and a USB 2.0

device works when connected to a PC that supports USB 1.x or USB 2.0,

except for a few devices that function only at high speed and thus require USB

2.0 support.

USB Basics

When USB 2.0 devices first became available, there was confusion among users

about whether all USB 2.0 devices supported high speed. To reduce confusion,

the USB-IF released naming and packaging recommendations that emphasize

speed and compatibility rather than USB version numbers. A product that sup-

ports high speed should be labeled “Hi-Speed USB,” and messages on the pack-

aging might include Fully compatible with Original USB and Compatible with

the USB 2.0 Specification. For products that support low or full speed only, the

recommended messages on packaging are Compatible with the USB 2.0 Specifi-

cation and Works with USB and Hi-Speed USB systems, peripherals and cables.

The recommendations advise avoiding references to low or full speed on con-

sumer packaging.

To use high speed, a high-speed-capable device must connect to a USB 2.0 or

USB 3.0 host computer with only USB 2.0 or USB 3.0 hubs between the host

and device. USB 2.0 and USB 3.0 hosts and hubs can also communicate with

USB 1.x devices.

The USB-IF releases revisions and additions to the USB specification via Engi-

neering Change Notices (ECNs). Table 1-2 lists ECNs to the USB 2.0 specifi-

cation.

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The Universal Serial Bus 3.0 Specification Revision 1.0 was released in November

2008, with the first USB 3.0 device-controller hardware expected to follow

about a year later. Windows will likely support USB 3.0 sometime after the

release of Windows 7, the successor to Windows Vista.

USB 3.0 defines a new dual-bus architecture with two physical buses that oper-

ate in parallel. USB 3.0 provides a pair of wires for USB 2.0 traffic and addi-

tional wires to support the new SuperSpeed bus at 5 Gbps. SuperSpeed offers a

more than 10× increase over USB 2.0’s high speed. Plus, unlike USB 2.0,

SuperSpeed has a pair of wires for each direction and can transfer data in both

directions at the same time. USB 3.0 also increases the amount of bus current

devices can draw and defines protocols for more aggressive power saving and

more efficient transfers.

USB 3.0 is backwards compatible with USB 2.0. USB 3.0 hosts and hubs sup-

port all four speeds. USB 2.0 cables fit USB 3.0 receptacles.

USB 3.0 supplements, but doesn’t replace, USB 2.0. Low, full, and high-speed

devices continue to comply with USB 2.0 and can’t take advantage of USB 3.0’s

features such as higher bus-current limits and larger data packets.

Chapter 1

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As USB became the interface of choice for all kinds of peripherals, developers

began to ask for a way for USB peripherals to access other USB devices. For

example, a user might want to attach a printer to a camera or a keyboard to a

PDA. The On-The-Go (OTG) Supplement to the USB 2.0 Specification defines a

limited-capability host function that devices can implement to enable commu-

nicating with USB peripherals.

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Developers who want to design devices with wireless interfaces have several

choices. The Wireless USB Promoter Group’s Wireless Universal Serial Bus Spec-

ification defines the Certified Wireless USB (WUSB) interface for communi-

cating at up to 480 Mbps. Cypress Semiconductor's WirelessUSB enables

Table 1-2: Engineering change notices (ECNs) correct, add to, and clarify the

USB 2.0 specification.

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Mini-B Connector 10/2000

Errata 12/2000

Pull-up/Pull-Down Resistors (loosened tolerances) 05/2002

Interface Association Descriptor 05/2003

Rounded Chamfer for the Mini-B Plug (recommendation) 10/2003

Unicode UTF-16LE for String Descriptors 02/2005

Inter-Chip USB Supplement (chip-to-chip interconnects without external cables) 03/2006

USB On-The-Go V1.3 (defines devices that can also function as hosts) 12/2006

Micro-USB connector 04/2007

Link Power Management (optional power saving capabilities) 07/2007

Hi-Speed Interchip Electrical Specification (chip-to-chip interconnects without

external cables)

09/2007

Suspend Current Limit Changes 04/2008

USB 2.0 Phase-locked SOFs 12/2008

5V Short Circuit Withstand Requirement Change 12/2008

Device Capacitance 12/2008

Material Change 12/2008

MicroUSB Micro-B ID Pin Resistance and Tolerance stack-up between D+ and D- 12/2008

USB Basics

implementing wireless devices that function as low-speed USB devices. Another

option is to use an adapter that converts between USB and a wireless interface

such as Zigbee, Bluetooth, or WiFi.

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USB communications require a host computer with USB support, one or more

devices with USB ports, and hubs, connectors, and cables as needed to connect

the devices to the host computer.

The host computer is a PC or other computer that contains a USB host con-

troller and a root hub. The host controller formats data for transmitting on the

bus and translates received data to a format that operating-system components

understand. The host controller also helps manage communications on the bus.

The root hub has one or more connectors for attaching devices. The root hub

and host controller together detect attached and removed devices, carry out

requests from the host controller, and pass data between devices and the host

controller. In addition to the root hub, a bus may have one or more external

hubs.

Each device has hardware and firmware as needed to communicate with the

host computer. The USB specifications define the cables and connectors that

connect devices to their hubs.

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The topology, or arrangement of connections, on the bus is a tiered star (Figure

1-1). At the center of each star is a hub, and each connection to the hub is a

point on the star. The root hub is in the host. An external hub has one

upstream (host-side) connector for communicating with the host and one or

more downstream (device-side) connectors or internal connections to embed-

ded devices. A typical hub has two, four, or seven ports. When multiple hubs

connect in series, you can think of the series as a tier, one above the next.

The tiered star describes only the physical connections. In programming, all

that matters is the logical connection. Host applications and device firmware

don’t need to know or care whether the communication passes through one hub

or five.

Up to five external hubs can connect in series with a limit of 127 peripherals

and hubs including the root hub. However, bandwidth and scheduling limits

can prevent a single host controller from communicating with this many