USBinaNutshell - CSDN文库

usb

5星 · 超过95%的资源需积分: 35 22 浏览量更新于2023-03-03 评论 1 收藏 175KB PDF 举报

身份认证购VIP最低享 7 折!

领优惠券(最高得80元）

资源详情

资源评论

资源推荐

USB in a Nutshell.

Making Sense of the USB Standard.

USB in a Nutshell Page 2 www.beyondlogic.org

Starting out new with USB can be quite daunting. With the USB 2.0 specification at 650 pages one could easily be

put off just by the sheer size of the standard. This is only the beginning of a long list of associated standards for

USB. There are USB Class Standards such as the HID Class Specification which details the common operation of

devices (keyboards, mice etc) falling under the HID (Human Interface Devices) Class - only another 97 pages. If

you are designing a USB Host, then you have three Host Controller Interface Standards to choose from. None of

these are detailed in the USB 2.0 Spec.

The good news is you don’t even need to bother reading the entire USB standard. Some chapters were churned

out by marketing, others aimed at the lower link layer normally taken care off by your USB controller IC and a

couple aimed at host and hub developers. Lets take a little journey through the various chapters of the USB 2.0

specification and briefly introduce the key points.

Chapter Name Description Pages

1 Introduction Includes the motivation and scope for USB. The most important piece

of information in this chapter is to make reference to the Universal

Serial Bus Device Class Specifications. No need reading this chapter.

2 Terms and

Abbreviations

This chapter is self-explanatory and a necessary evil to any standard. 8

3 Background Specifies the goals of USB which are Plug’n’Play and simplicity to the

end user (not developer). Introduces Low, Full and High Speed ranges

with a feature list straight from marketing. No need reading this chapter

either.

4 Architectural

Overview

This is where you can start reading. This chapter provides a basic

overview of a USB system including topology, data rates, data flow

types, basic electrical specs etc.

5USB Data Flow

Model

This chapter starts to talk about how data flows on a Universal Serial

Bus. It introduces terms such as endpoints and pipes then spends

most of the chapter on each of the data flow types (Control, Interrupt,

Isochronous and Bulk). While it’s important to know each transfer type

and its properties it is a little heavy on for a first reader.

6 Mechanical

This chapter details the USB’s two standard connectors. The important

information here is that a type A connector is oriented facing

downstream and a type B connector upstream. Therefore it should be

impossible to plug a cable into two upstream ports. All detachable

cables must be full/high speed, while any low speed cable must be

hardwired to the appliance. Other than a quick look at the connectors,

you can skip this chapter unless you intend to manufacture USB

connectors and/or cables. PCB designers can find standard footprints

in this chapter.

7 Electrical This chapter looks at low level electrical signalling including line

impedance, rise/fall times, driver/receiver specifications and bit level

encoding, bit stuffing etc. The more important parts of this chapter are

the device speed identification by using a resistor to bias either data

line and bus powered devices vs self powered devices. Unless you are

designing USB transceivers at a silicon level you can flip through this

chapter. Good USB device datasheets will detail what value bus

termination resistors you will need for bus impedance matching.

8 Protocol Layer Now we start to get into the protocol layers. This chapter describes the

USB packets at a byte level including the sync, pid, address, endpoint,

CRC fields. Once this has been grasped it moves on to the next

protocol layer, USB packets. Most developers still don’t see these

lower protocol layers as their USB device IC’s take care of this.

However an understanding of the status reporting and handshaking is

worthwhile.

9USB Device

Frame Work

This is the most frequently used chapter in the entire specification and

the only one I ever bothered printing and binding. This details the bus

enumeration and request codes (set address, get descriptor etc) which

make up the most common protocol layer USB programmers and

designers will ever see. This chapter is a must read in detail.

USB in a Nutshell Page 3 www.beyondlogic.org

10 USB Host

Hardware and

Software

This chapter covers issues relating to the host. This includes frame and

microframe generation, host controller requirements, software

mechanisms and the universal serial bus driver model. Unless you are

designing Hosts, you can skip this chapter.

11 Hub Specification

Details the workings of USB hubs including hub configuration, split

transactions, standard descriptors for hub class etc. Unless you are

designing Hubs, you can skip this chapter.

143

So now we can begin to read the parts of the standard relevant to our needs. If you develop drivers (Software)

for USB peripherals then you may only need to read chapters,

• 4 - Architectural Overview

• 5 - USB Data Flow Model

• 9 - USB Device Frame Work, and

• 10 - USB Host Hardware and Software.

Peripheral hardware (Electronics) designers on the other hand may only need to read chapters,

• 4 - Architectural Overview

• 5 - USB Data Flow Model

• 6 - Mechanical, and

• 7 - Electrical.

USB in a NutShell for Peripheral Designers

Now lets face it, (1) most of us are here to develop USB peripherals and (2) it's common to read a standard

and still have no idea how to implement a device. So in the next 7 chapters we focus on the relevant parts

needed to develop a USB device. This allows you to grab a grasp of USB and its issues allowing you to further

research the issues specific to your application.

The USB 1.1 standard was complex enough before High Speed was thrown into USB 2.0. In order to help

understand the fundamental principals behind USB, we omit many areas specific to High Speed USB 2.0

devices. Once a grasp of USB 1.1 is obtained, these additional 2.0 details should be easy to pick up.

Introducing the Universal Serial Bus

USB version 1.1 supported two speeds, a full speed mode of 12Mbits/s and a low speed mode of 1.5Mbits/s.

The 1.5Mbits/s mode is slower and less susceptible to EMI, thus reducing the cost of ferrite beads and quality

components. For example, crystals can be replaced by cheaper resonators. USB 2.0 which is still yet to see

day light on mainstream desktop computers has upped the stakes to 480Mbits/s. The 480Mbits/s is known as

High Speed mode and was a tack on to compete with the Firewire Serial Bus.

USB Speeds

• High Speed - 480Mbits/s

• Full Speed - 12Mbits/s

• Low Speed - 1.5Mbits/s

The Universal Serial Bus is host controlled. There can only be one host per bus. The specification in itself,

does not support any form of multimaster arrangement. However the On-The-Go specification which is a tack

on standard to USB 2.0 has introduced a Host Negotiation Protocol which allows two devices negotiate for the

role of host. This is aimed at and limited to single point to point connections such as a mobile phone and

personal organiser and not multiple hub, multiple device desktop configurations. The USB host is responsible

for undertaking all transactions and scheduling bandwidth. Data can be sent by various transaction methods

using a token-based protocol.

In my view the bus topology of USB is somewhat limiting. One of the original intentions of USB was to reduce

the amount of cabling at the back of your PC. Apple people will say the idea came from the Apple Desktop Bus,

where both the keyboard, mouse and some other peripherals could be connected together (daisy chained)

using the one cable.

However USB uses a tiered star topology, simular to that of 10BaseT Ethernet. This imposes the use of a hub

somewhere, which adds to greater expense, more boxes on your desktop and more cables. However it is not

as bad as it may seem. Many devices have USB hubs integrated into them. For example, your keyboard may

contain a hub which is connected to your computer. Your mouse and other devices such as your digital camera

USB in a Nutshell Page 4 www.beyondlogic.org

can be plugged easily into the back of your keyboard. Monitors are just another peripheral on a long list which

commonly have in-built hubs.

This tiered star topology, rather than simply daisy chaining devices together has some benefits. Firstly power to

each device can be monitored and even switched off if an overcurrent condition occurs without disrupting other

USB devices. Both high, full and low speed devices can be supported, with the hub filtering out high speed and

full speed transactions so lower speed devices do not receive them.

Up to 127 devices can be connected to any one USB bus at any one given time. Need more devices? - Simply

add another port/host. While earlier USB hosts had two ports, most manufacturers have seen this as limiting

and are starting to introduce 4 and 5 port host cards with an internal port for hard disks etc. The early hosts had

one USB controller and thus both ports shared the same available USB bandwidth. As bandwidth requirements

grew, we are starting to see multi-port cards with two or more controllers allowing individual channels.

The USB host controllers have their own specifications. With USB 1.1, there were two Host Controller Interface

Specifications, UHCI (Universal Host Controller Interface) developed by Intel which puts more of the burden on

software (Microsoft) and allowing for cheaper hardware and the OHCI (Open Host Controller Interface)

developed by Compaq, Microsoft and National Semiconductor which places more of the burden on

hardware(Intel) and makes for simpler software. Typical hardware / software engineer relationship. . .

With the introduction of USB 2.0 a new Host Controller Interface Specification was needed to describe the

Contributors include Intel, Compaq, NEC, Lucent and Microsoft so it would hopefully seem they have pooled

together to provide us one interface standard and thus only one new driver to implement in our operating

systems. Its about time.

USB as its name would suggest is a serial bus. It uses 4 shielded wires of which two are power (+5v & GND).

The remaining two are twisted pair differential data signals. It uses a NRZI (Non Return to Zero Invert)

encoding scheme to send data with a sync field to synchronise the host and receiver clocks.

USB supports plug’n’plug with dynamically loadable and unloadable drivers. The user simply plugs the device

into the bus. The host will detect this addition, interrogate the newly inserted device and load the appropriate

driver all in the time it takes the hourglass to blink on your screen provided a driver is installed for your device.

The end user needs not worry about terminations, terms such as IRQs and port addresses, or rebooting the

computer. Once the user is finished, they can simply lug the cable out, the host will detect its absence and

automatically unload the driver.

The loading of the appropriate driver is done using a PID/VID (Product ID/Vendor ID) combination. The VID is

supplied by the USB Implementor's forum at a cost and this is seen as another sticking point for USB. The

latest info on fees can be found on the USB Implementor’s Website

Other standards organisations provide an extra VID for non-commercial activities such as teaching, research or

fiddling (The Hobbyist). The USB Implementors forum has yet to provide this service. In these cases you may

wish to use one assigned to your development system's manufacturer. For example most chip manufacturers

will have a VID/PID combination you can use for your chips which is known not to exist as a commercial

device. Other chip manufacturers can even sell you a PID to use with their VID for your commercial device.

Another more notable feature of USB, is its transfer modes. USB supports Control, Interrupt, Bulk and

Isochronous transfers. While we will look at the other transfer modes later, Isochronous allows a device to

reserve a defined about of bandwidth with guaranteed latency. This is ideal in Audio or Video applications

where congestion may cause loss of data or frames to drop. Each transfer mode provides the designer trade-

offs in areas such as error detection and recovery, guaranteed latency and bandwidth.

Connectors

All devices have an upstream connection to the host and all hosts have a downstream connection to the

device. Upstream and downstream connectors are not mechanically interchangeable, thus eliminating illegal

loopback connections at hubs such as a downstream port connected to a downstream port. There are

commonly two types of connectors, called type A and type B which are shown below.

USB in a Nutshell Page 5 www.beyondlogic.org

Figure 1 : USB Connectors

Type A plugs always face upstream. Type A sockets will typically find themselves on hosts and hubs. For

example type A sockets are common on computer main boards and hubs. Type B plugs are always connected

downstream and consequently type B sockets are found on devices.

It is interesting to find type A to type A cables wired straight through and an array of USB gender changers in

some computer stores. This is in contradiction of the USB specification. The only type A plug to type A plug

devices are bridges which are used to connect two computers together. Other prohibited cables are USB

extensions which has a plug on one end (either type A or type B) and a socket on the other. These cables

violate the cable length requirements of USB.

USB 2.0 included errata which introduces mini-USB B connectors. The details on these connectors can be

found in Mini-B Connector Engineering Change Notice. The reasoning behind the mini connectors came from

the range of miniature electronic devices such as mobile phones and organisers. The current type B connector

is too large to be easily integrated into these devices.

Just recently released has been the On-The-Go specification which adds peer-to-peer functionality to USB.

This introduces USB hosts into mobile phone and electronic organisers, and thus has included a specification

for mini-A plugs, mini-A receptacles, and mini-AB receptacles. I guess we should be inundated with mini USB

cables soon and a range of mini to standard converter cables.

Pin Number Cable Colour Function

1RedV

BUS

(5 volts)

2WhiteD-

3 Green D+

4 Black Ground

Table 1 : USB Pin Functions

Standard internal wire colours are used in USB cables, making it easier to identify wires from manufacturer to

manufacturer. The standard specifies various electrical parameters for the cables. It is interesting to read the

detail the original USB 1.0 spec included. You would understand it specifying electrical attributes, but

paragraph 6.3.1.2 suggested the recommended colour for overmolds on USB cables should be frost white -

how boring! USB 1.1 and USB 2.0 was relaxed to recommend Black, Grey or Natural.

PCB designers will want to reference chapter 6 for standard foot prints and pinouts.

Electrical

Unless you are designing the silicon for a USB device/transceiver or USB host/hub, there is not all that much

you need to know about the electrical specifications in chapter 7. We briefly address the essential points here.

As we have discussed, USB uses a differential transmission pair for data. This is encoded using NRZI and is

bit stuffed to ensure adequate transitions in the data stream. On low and full speed devices, a differential ‘1’ is

transmitted by pulling D+ over 2.8V with a 15K ohm resistor pulled to ground and D- under 0.3V with a 1.5K

ohm resistor pulled to 3.6V. A differential ‘0’ on the other hand is a D- greater than 2.8V and a D+ less than

0.3V with the same appropriate pull down/up resistors.

The receiver defines a differential ‘1’ as D+ 200mV greater than D- and a differential ‘0’ as D+ 200mV less than

D-. The polarity of the signal is inverted depending on the speed of the bus. Therefore the terms ‘J’ and ‘K’

states are used in signifying the logic levels. In low speed a ‘J’ state is a differential 0. In high speed a ‘J’ state

is a differential 1.

USB transceivers will have both differential and single ended outputs. Certain bus states are indicated by

single ended signals on D+, D- or both. For example a single ended zero or SE0 can be used to signify a

1234

Receptacle Type A

Receptical Type B

剩余29页未读，继续阅读

zzia_cy

2015-06-14

很好的入门指导

yangchun301

粉丝: 62
资源: 358

会员权益专享

USB in a Nutshell

评论12

会员权益专享

最新资源

USB in a Nutshell

评论12

USB in a Nutshell(USB概述)

usb-in-a-nutshell 英文版（加书签）

USB in a Nutshell-英文

python in a nutshell azw3

linux in a nutshell

java in a nutshell 7th edition pdf

java in a nutshell

c# 10 in a nutshell

xml in a nutshell

c# 5.0 in a nutshell

c# 6.0 in a nutshell

c# 5.0 in a nutshell.pdf网盘

最近6年的C#参考书写在此处。要求序号、作者、书名、出版社、出版时间。

c#从入门到精通第九版

c#方向比较著名的文献

c#从入门到精通第六版

.net相关电子书推荐

c#开发一个图书管理系统

推荐10本.net程序员读的书

NEMU shell命令

会员权益专享

最新资源