USB - USB在消费领域的应用

Switching in USB Consumer Applications

通用串行总线（USB）已成为满足终端设备之间日益增长的快速数据传输需求的主流接口--例如，在个人电脑和便携式设备（如手机、数码相机和个人媒体播放器）之间下载和上传数据。

The universal serial bus (USB) has become a dominant interface to fulfill the ever increasing needs for rapid data transfer between end devices—for example, downloading and uploading data between PCs and portable devices such as cell phones, digital cameras, and personal media players.

CMOS 开关可用于 USB 系统中数据线的连接和布线。通过选择合适的开关，设计人员可以增强现有设计而不是开发新设计，从而大大缩短设计周期。在本文中，我们将介绍 USB，然后探讨开关在提高便携式媒体播放器、手机和无线笔式驱动器等应用的性能方面所起的关键作用。我们还展示了开关的关键参数对整个系统设计的影响，并讨论了基本的设计挑战，如满足带宽要求与最大限度减少信号反射之间的权衡。此外，我们还建议如何通过精心的电路板布局最大限度地扩大眼图的开口。

CMOS switches can be used for connecting and routing data lines in USB systems. By selecting suitable switches, designers can significantly shorten design cycles by enhancing existing designs rather than developing new ones. In this article, we describe the USB, then go on to explore the crucial role of switches in improving performance in applications such as portable media players, cell phones, and wireless pen drives. We also show how key parameters of the switches affect the overall system design and discuss basic design challenges, such as the trade-offs between meeting bandwidth requirements and minimizing signal reflections. Additionally, we suggest how to maximize the opening in eye diagrams by careful board layout.

What Is USB and Why Has It Become So Popular?

USB 已成为全球最流行的 PC 与外设通信标准。键盘、打印机、数据存储设备和手机等众多外围设备都可以通过 USB 标准连接到个人电脑。以前使用串行端口和并行端口的设备正在向 USB 迁移，同时硬盘驱动器和数码相机等设备的设计者通常会选择 USB 而不是其他标准，如火线或串行端口通信。与手机、MP3 播放器和游戏机的连接也是最近的一项发展。

USB has become the most popular standard for PC-to-peripheral communication in the world. Keyboards, printers, data-storage devices, and mobile phones are among the many peripherals that can be connected to a PC, employing the USB standard. Devices that previously used serial ports and parallel ports are migrating to USB, while designers of devices such as hard drives and digital cameras are often choosing USB over other standards, such as FireWire or serial-port communication. Connectivity to mobile phones, MP3 players, and game consoles is another recent development.

USB 的主要魅力在于即插即用。将设备插入个人电脑，由个人电脑识别；然后，在首次安装适当的软件后，设备将始终被主机识别--这是一种用户友好的握手方式。

USB’s main attraction is the ability to plug and play. The device is plugged into the PC, recognized by the PC; then, after the first installation of appropriate software, the device will always be recognized by the host PC—a user-friendly handshake.

USB 实施者论坛公司是一个由计算机和电子行业领先公司赞助的行业标准制定机构，负责制定 USB 标准。设备设计可以获得 USB 认证并在产品上使用 USB 标志，但必须通过非常严格的软件和硬件测试。这确保了所有通过 USB 认证的设备，无论是个人电脑还是外围设备，都能从软件和硬件两个角度正确地进行互联。该标准确保所有经过认证的软件程序、连接器、电缆、信号驱动程序和接收器都符合标准，从而保证了互联性（图 1a）。

The USB Implementers Forum, Inc., an industry-standard-generating body sponsored by leading companies from the computer and electronics industry, lays down the standards for USB. Device designs can receive USB certification—and use the USB symbol on a product, but only after passing very strict software and hardware tests. This ensures that all USB-certified devices, whether PC or peripheral, will function correctly when interconnected, from the standpoints of both software and hardware. The standard ensures that all certified software routines, connectors, cables, signal drivers, and receivers comply, ensuring interconnnectability (Figure 1a).

Figure 1a. USB devices: a port expander, a pen drive, and a webcam.

USB 基于串行主从架构。一般来说，PC 是主设备，称为主机（图 1b）；它控制各项作业。被称为外设的从设备告诉主机其带宽需求，然后开始数据交换。正常 USB 事件的完整序列包括以下步骤：

1. 外设插入主机（启动 USB 事件）

2. 握手（识别外设、分配带宽）

3. 批量数据传输（如传输至打印机），或外设轮询（鼠标）

4. 主机禁用外设

5. 外设断开连接

USB is based on a serial master-slave architecture. In general, the PC is the master, known as the host (Figure 1b); it controls the transaction. The slave, known as the peripheral, tells the host its bandwidth requirement, and then a data transaction is initiated. A complete sequence of normal USB events includes these steps: 1. Peripheral plugged into host (initiating USB event) 2. Handshaking (peripheral identified, bandwidth allocated) 3. Bulk data transfer (e.g., to printer), or peripheral polled (mouse) 4. Peripheral disabled by host 5. Peripheral disconnected

Figure 1b. Typical host- and peripheral USB devices.

USB 系统中使用的硬件通过双线（加地线）差分双向系统传输数据。数据线 D+ 和 D- 传输数据，如图 2 所示。数据只能单向传输，因此在一种情况下，主机传输数据的同时外设接收数据，然后外设传输数据的同时主机接收数据。USB 标准还包括一条 5 V 电源线。一般用于为下游设备供电，使 USB 笔式驱动器、网络摄像头和键盘等低功耗设备不再需要电池。

The hardware used in a USB system transmits data using a 2-wire (plus ground) differential bidirectional system. The data lines, D+ and D–, transmit the data as shown in Figure 2. Data can only be transmitted in one direction, so in one instance the host transmits while the peripheral receives, and then the peripheral transmits while the host receives. The USB standard also includes a 5-V power line. Generally used to power downstream devices, it obviates the need for batteries in low-power devices such as USB pen drives, webcams, and keyboards.

Figure 2. USB interconnections.

How Do USB 1.1 and USB 2.0 Compare?

USB 标准规定了三种数据传输速率： 低速（1.5 Mbps）、全速（12 Mbps）和高速（480 Mbps）。USB 1.1 设备的信号电平为 ±3.3-V，可以低速和全速运行。USB 2.0 设备的信号电平为 ±400-mV，可以低速、全速和高速运行。

The USB standard specifies three data rates: Low-Speed (1.5 Mbps), Full-Speed (12 Mbps), and Hi-Speed (480 Mbps). USB 1.1 devices have ±3.3-V signal levels and can operate at low- and full speeds. USB 2.0 devices have ±400-mV signal levels and can operate at low-, full-, and high speeds.

Table I. Comparison of USB 1.1 and USB 2.0

What Is USB On-The-Go (USB OTG)?

许多消费产品，如作为 USB 外围设备连接到电脑的手机和数码相机，也可以连接到其他 USB 设备。在这种情况下，个人电脑无法充当主机，因此需要由其中一个外设来承担责任。USB OTG 定义了一种双重角色的设备，它既可以充当主机，也可以充当外设，而且可以使用相同的连接器连接电脑和其他便携设备。

Many consumer products—such as cell phones and digital cameras, that connect to the PC as a USB peripheral—can also be connected to other USB devices. Since, in these circumstances, the PC cannot be the host, one of the peripherals needs to take on the responsibility. USB OTG defines a dual-role device, which can act as either a host or a peripheral—and can use the same connector for both PCs and other portable devices.

通过实现数字设备之间的基本功能，USB OTG 使这些 PC 外围设备的功能更强大，因此对消费者和企业用户来说更有价值。USB OTG 设备当然可以连接个人电脑，因为它们符合 USB 2.0 规范。此外，它们还具有有限的主机功能，可以连接到一系列目标明确的其他 USB 外围设备。当两个双重角色设备通过电缆连接在一起时，电缆会设置默认主机和默认外设。如果应用要求转换角色，主机协商协议（HNP）会提供一个握手来执行该功能，这种转换对用户来说是完全不可见的。