Jan Axelson, USB Complete

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At the start of each code example, a sidehead indicates the programming lan-

guage:

The .NET code is compatible with the .NET Framework Version 2.0 and later.

Example applications are available for free download from www.Lvr.com.

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VB Visual Basic .NET Microsoft

VC# Visual C# .NET Microsoft

PBP PICBASIC PRO microEngineering Labs, Inc.

C18 MPLAB C compiler for

PIC18 CPUs

Microchip Technology Inc.

xxi

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This book uses the abbreviations and symbols below to express quantities and

units:

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ppico

-12

nnano

-9

micro

-6

m milli

-3

kkilo

Kkilo

(1024)

Mmega

or 2

depending on context

Ggiga

or 2

depending on context

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ampere

Ffarad

Ω

ohm

Vvolt

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ssecond

Hz Hertz (cycles per second)

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Binary values have a trailing subscript “b”. Example: 10100011

. An

exception is when it’s clear from the context that the values are binary.

Example: Set bits 6..5 to 01.

Hexadecimal values have a trailing “h”. Example: A3h.

All other values are decimal. Example: 163.

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in. inch

ft foot

m meter

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bbit

Bbyte

bps bits per second

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USB is much too big a topic to write about without help. I have many people to

thank.

My technical reviewers provided feedback that helped make the book as com-

plete and accurate as possible. With that said, every error in this book is mine

and mine alone. A big thanks to Paul E. Berg, Greg Burk, Robert Dunstan,

John Garney, Bill Jacobus, Kosta Koeman, and Matt Leptich.

Others I want to thank for their support are Phyllis Brown of J. Gordon Elec-

tronic Design, Michael DeVault of DeVaSys Embedded Systems, Traci Donnell

of the USB-IF, David Flowers of Microchip Technology, Inc., Laurent Guin-

nard of Ellisys, Tim Harvey of CWAV, Inc., Blake Henry of Bitwise Systems,

John Hyde of usb-by-example.com, Rahman Ismail and Jeff Ravencraft of Intel

Corporation, Dr. Bob Miller of Trace Systems, Inc., and Jeff Schmoyer of

microEngineering Labs, Inc.

For their help with the previous editions this edition builds on, thanks to

Joshua Buergel, Gary Crowell, Fred Dart, Wendy Dee, Lucio DiJasio, Keith

Dingwall, Dave Dowler, Mike Fahrion, David Goll, John M. Goodman, Lane

Hauck, David James, Christer Johansson, Geert Knapen, Alan Lowne, Jon

Lueker, Brad Markisohn, Rich Moran, Bob Nathan, Walter Oney, Amar Rajan,

Marc Reinig, Rawin Rojvanit, Glenn M. Roberts, Robert Severson, Craig R.

Smith, and Dave Wright.

I hope you find the book useful and welcome your comments at

jan@Lvr.com.

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At over two billion new installed units per year, USB is the most successful per-

sonal-computer interface ever. Every recent PC has USB ports that can connect

to keyboards, mice, game controllers, scanners, cameras, printers, drives, and

more. USB is reliable, fast, versatile, power-conserving, inexpensive, and sup-

ported by major operating systems. USB 3.0’s new SuperSpeed bus means USB

is likely to continue to dominate as the interface of choice for an ever-expand-

ing selection of peripherals.

This chapter introduces USB, including its advantages and limits, some history

about the interface and recent enhancements to it, and a look at what’s involved

in designing and programming a device with a USB interface.

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USB is a likely solution any time you want to use a computer to communicate

with an external device. Internal devices, such as fingerprint readers, can use

USB as well. The interface is suitable for mass-produced, consumer devices as

well as specialized, small-volume products and one-of-a-kind projects.

Chapter 1

To be successful, an interface has to please two audiences: the users who want to

use the devices and the developers who design the hardware and write the code

that communicates with the devices. USB has features to please both groups.

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From the user’s perspective, the benefits of USB are ease of use, fast and reliable

data transfers, low cost, and power conservation. Table 1-1 compares USB with

other interfaces.

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Ease of use was a major design goal for USB, and the result is an interface that’s

a pleasure to use for many reasons:

One interface for many devices. USB is versatile enough for just about any

standard PC peripheral function. Instead of having a different connector and

cable type for each peripheral function, one interface serves many.

Automatic configuration. When a user connects a USB device to a PC, the

operating system detects the device and loads the appropriate software driver.

The first time the device connects, the operating system may prompt the user to

insert a disc with driver software, but other than that, installation is automatic.

Users don’t need to reboot before using the device.

Easy to connect. A typical PC has multiple USB ports, and hubs make it easy

to add ports without opening up the PC.

Convenient cables. USB connectors are small and compact compared to con-

nectors used by other interfaces such as RS-232. To ensure reliable operation,

the USB specification defines electrical requirements for cables. A cable seg-

ment can be as long as 5 m depending on bus speed. With hubs, again depend-

ing on bus speed, a device can be as far as 30 m from its host PC.

Wireless options. USB originated as a wired interface, but technologies are

now available for wireless communications with USB devices.

Hot pluggable. Users can connect and disconnect a USB device whenever they

want, whether or not the system and device are powered, without damaging the

PC or device. The operating system detects when a device is attached and read-

ies it for use.

No user settings. USB devices don’t have user-selectable settings such as port

addresses and interrupt-request (IRQ) lines, so users have no jumpers to set or

configuration utilities to run.

USB Basics

Table 1-1: USB is more flexible than other interfaces, which often target a

specific use.

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USB 3.0 dual simplex

serial

127 (per bus) 9 (typical)

(up to 49

with 5 hubs)

5 G Mass storage,

video

USB 2.0 half duplex

serial

127 (per bus) 16 (98 ft.

with 5 hubs)

1.5M, 12M,

480M

Keyboard,

mouse, drive,

speakers, printer,

camera

eSATA serial 2 (port

multiplier

supports 16)

63GDrives

Ethernet serial 1024 1600 10G General network

communications

IEEE-1394b

(FireWire 800)

serial 64 300 3.2G Video, mass

storage

IEEE-488

(GPIB)

parallel 15 60 8M Instrumentation

synchronous

serial

40 18 3.4M Microcontroller

communications

Microwire synchronous

serial

8 10 2M Microcontroller

communications

MIDI serial current

loop

2 (more with

flow-through

mode)

50 31.5k Music, show

control

Parallel Printer

Port

parallel 2 (8 with

daisy-chain

support)

10–30 8M Printers,

scanners, disk

drives

RS-232

(EIA/TIA-232)

asynchronous

serial

2 50–100 20k (115k

with some

hardware)

Modem, mouse,

instrumentation

RS-485

(TIA/EIA-485)

asynchronous

serial

32 unit loads

(some chips

allow up to

256 devices)

4000 10M Data acquisition

and control

systems

SPI synchronous

serial

8 10 2.1M Microcontroller

communications

Chapter 1

No power supply required (sometimes). The USB interface includes

power-supply and ground lines that provide a nominal +5V from the PC or a

hub. A device that requires up to 500 mA (USB 2.0) or 900 mA (USB 3.0) can

draw all of its power from the bus instead of using a dedicated power supply. In

contrast, devices that use other interfaces may have to provide a power supply

inside the device or an external supply.

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USB supports four bus speeds: SuperSpeed at 5 Gbps, high speed at 480 Mbps,

full speed at 12 Mbps, and low speed at 1.5 Mbps. SuperSpeed requires a USB

3.0 host controller in the host PC. USB 2.0 host controllers support low, full,

and high speeds.

The bus speeds describe the rate that information travels on the bus. In addi-

tion to application data, the bus must carry status, control, and error-checking

information. Plus, multiple devices can share a bus. Thus, the data throughput

for an individual device’s data is less than the bus speed. The USB protocols

support data transfers at around 400 MB/s for SuperSpeed, 53 MB/s for high

speed, 1.2 MB/s for full speed, and 800 B/s for low speed. Hardware and soft-

ware limitations can result in lower real-world rates, however.

The USB 1.0 specification defined low and full speeds. Full speed was intended

for most peripherals that had been using RS-232 (serial) and parallel ports.

Full-speed data-transfer rates are comparable to the speeds of these earlier inter-

faces. Mice tend to use low speed because the less stringent cable requirements

allow flexible cables. Low-speed devices may have lower manufacturing cost due

in part to cheaper cables. High speed became an option with the release of USB

2.0, and USB 3.0 defined SuperSpeed.

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USB’s reliability is due to both the hardware and the protocols. The hardware

specifications for USB drivers, receivers, and cables ensure an electrically quiet

interface that eliminates most noise that could cause data errors. The USB pro-

tocols enable detecting errors in received data and notifying the sender so it can

retransmit. Hardware performs the detecting, notifying, and retransmitting

without software or user support.

USB Basics

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Because the host computer provides most of the intelligence to control the

interface, components for USB devices are inexpensive. A device with a USB

interface is likely to cost the same or less than an equivalent device with a differ-

ent interface.

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Power-saving circuits and protocols reduce a device’s power consumption while

keeping the device ready to communicate when needed. Reducing power con-

sumption saves money, helps the environment, and for battery-powered

devices, allows a longer time between recharges.

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Many of the user advantages described above also make things easier for devel-

opers. For example, USB’s cable standards and error checking mean that devel-

opers don’t have to worry about specifying cable characteristics or providing

error checking in software.

Other advantages help the hardware designers who select components and

design the circuits in devices and the programmers who write firmware embed-

ded in the devices and software to communicate with devices.

The benefits result from the flexibility built into the USB protocol, the support

in the controller chips and operating system, and the support available from the

USB Implementers Forum.

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USB’s four transfer types and four speeds make the interface feasible for many

types of peripherals. USB has transfer types suited for exchanging large and

small blocks of data, with and without time constraints. For data that can’t tol-

erate delays, USB can guarantee bandwidth. These abilities are especially wel-

come under Windows where accessing peripherals in real time is often a

challenge. Although the operating system, device drivers, and application soft-

ware can introduce unavoidable delays, USB makes it as easy as possible to

achieve transfers that are close to real time even on desktop systems.

Unlike other interfaces, USB doesn’t assign specific functions to signal lines or

make other assumptions about how the system will use the interface. For exam-

ple, the status and control lines on the PC’s parallel port were defined with the