USB设备中间层详解：状态与功能架构

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USB (Universal Serial Bus) 是一种广泛应用于现代电子设备中的标准化接口，它允许高速、简单地连接和通信。本资料PDF主要关注USB 2.0版本，特别是第9章——USB设备框架，这一部分深入探讨了设备的三个核心层：总线接口、数据路由和特定功能。在USB设备架构中，最底层是总线接口，它负责传输和接收数据包，确保数据在设备与主机之间高效流动。总线接口是所有通信的基础，是数据传输的主通道。中间层，即数据路由层，负责处理总线接口与设备上不同端点之间的数据传输。端点是USB设备上的数据接收器或发送器，它们可以作为数据的流入或流出点。这些端点根据设备的功能和需求进行配置，如鼠标、键盘或ISDN接口等。章节9.1详述了USB设备的不同状态，这些状态对USB系统和主机来说是可见的。可见设备状态包括以下几种： 1. 挂起（Suspended）：当设备在低功耗模式下，不需与主机交互时处于此状态。 2. 配置（Configured）：设备完成初始化后，准备好接收或提供服务的状态，此时已设置了至少一个端点配置。 3. 待命（Powered）：设备已连接并供电，但尚未配置，可能处于可被发现和配置的模式。 4. 就绪（Initialized）：设备已经配置，并且所有端点都可用，能够响应主机的请求。 5. 默认配置（Default Configured）：设备默认的、预设的工作模式，通常只有一个配置可供使用。 6. 备用配置（Alternate Configurations）：除了默认配置外，设备可能有其他配置供用户选择。此外，还有一些内部状态，对设备本身管理和维护操作至关重要，但对外部是不可见的。了解这些状态对于正确管理和优化USB设备的行为至关重要，因为设备必须在不同的状态下正确响应主机的命令，以确保高效和稳定的通信。本资料PDF深入介绍了USB设备的工作原理，特别是其数据流管理机制和状态控制，这对于开发和维护兼容USB标准的硬件和软件都具有很高的参考价值。通过理解这些概念，工程师可以设计出更加高效和用户友好的USB设备。

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Universal Serial Bus Specification Revision 2.0

245

settings for a specific interface. The default setting when a device is initially configured is alternate setting

zero.

In support of adaptive device drivers that are capable of managing a related group of USB devices, the

device and interface descriptors contain Class, SubClass, and Protocol fields. These fields are used to

identify the function(s) provided by a USB device and the protocols used to communicate with the

function(s) on the device. A class code is assigned to a group of related devices that has been characterized

as a part of a USB Class Specification. A class of devices may be further subdivided into subclasses, and,

within a class or subclass, a protocol code may define how the Host Software communicates with the

device.

Note: The assignment of class, subclass, and protocol codes must be coordinated but is beyond the scope of

this specification.

9.2.4 Data Transfer

Data may be transferred between a USB device endpoint and the host in one of four ways. Refer to

Chapter 5 for the definition of the four types of transfers. An endpoint number may be used for different

types of data transfers in different alternate settings. However, once an alternate setting is selected

(including the default setting of an interface), a USB device endpoint uses only one data transfer method

until a different alternate setting is selected.

9.2.5 Power Management

Power management on USB devices involves the issues described in the following sections.

9.2.5.1 Power Budgeting

USB bus power is a limited resource. During device enumeration, a host evaluates a device’s power

requirements. If the power requirements of a particular configuration exceed the power available to the

device, Host Software shall not select that configuration.

USB devices shall limit the power they consume from V

BUS to one unit load or less until configured.

Suspended devices, whether configured or not, shall limit their bus power consumption as defined in

Chapter 7. Depending on the power capabilities of the port to which the device is attached, a USB device

may be able to draw up to five unit loads from V

BUS after configuration.

9.2.5.2 Remote Wakeup

Remote wakeup allows a suspended USB device to signal a host that may also be suspended. This notifies

the host that it should resume from its suspended mode, if necessary, and service the external event that

triggered the suspended USB device to signal the host. A USB device reports its ability to support remote

wakeup in a configuration descriptor. If a device supports remote wakeup, it must also allow the capability

to be enabled and disabled using the standard USB requests.

Remote wakeup is accomplished using electrical signaling described in Section 7.1.7.7.

9.2.6 Request Processing

With the exception of SetAddress() requests (see Section 9.4.6), a device may begin processing of a request

as soon as the device returns the ACK following the Setup. The device is expected to “complete”

processing of the request before it allows the Status stage to complete successfully. Some requests initiate

operations that take many milliseconds to complete. For requests such as this, the device class is required to

define a method other than Status stage completion to indicate that the operation has completed. For

example, a reset on a hub port takes at least 10 ms to complete. The SetPortFeature(PORT_RESET) (see

Chapter 11) request “completes” when the reset on the port is initiated. Completion of the reset operation is

Universal Serial Bus Specification Revision 2.0

246

signaled when the port’s status change is set to indicate that the port is now enabled. This technique

prevents the host from having to constantly poll for a completion when it is known that the request will take

a relatively long period of time.

9.2.6.1 Request Processing Timing

All devices are expected to handle requests in a timely manner. USB sets an upper limit of 5 seconds as the

upper limit for any command to be processed. This limit is not applicable in all instances. The limitations

are described in the following sections. It should be noted that the limitations given below are intended to

encompass a wide range of implementations. If all devices in a USB system used the maximum allotted

time for request processing, the user experience would suffer. For this reason, implementations should

strive to complete requests in times that are as short as possible.

9.2.6.2 Reset/Resume Recovery Time

After a port is reset or resumed, the USB System Software is expected to provide a “recovery” interval of

10 ms before the device attached to the port is expected to respond to data transfers. The device may ignore

any data transfers during the recovery interval.

After the end of the recovery interval (measured from the end of the reset or the end of the EOP at the end

of the resume signaling), the device must accept data transfers at any time.

9.2.6.3 Set Address Processing

After the reset/resume recovery interval, if a device receives a SetAddress() request, the device must be able

to complete processing of the request and be able to successfully complete the Status stage of the request

within 50 ms. In the case of the SetAddress() request, the Status stage successfully completes when the

device sends the zero-length Status packet or when the device sees the ACK in response to the Status stage

data packet.

After successful completion of the Status stage, the device is allowed a SetAddress() recovery interval of

2 ms. At the end of this interval, the device must be able to accept Setup packets addressed to the new

address. Also, at the end of the recovery interval, the device must not respond to tokens sent to the old

address (unless, of course, the old and new address is the same).

9.2.6.4 Standard Device Requests

For standard device requests that require no Data stage, a device must be able to complete the request and

be able to successfully complete the Status stage of the request within 50 ms of receipt of the request. This

limitation applies to requests to the device, interface, or endpoint.

For standard device requests that require data stage transfer to the host, the device must be able to return the

first data packet to the host within 500 ms of receipt of the request. For subsequent data packets, if any, the

device must be able to return them within 500 ms of successful completion of the transmission of the

previous packet. The device must then be able to successfully complete the status stage within 50 ms after

returning the last data packet.

For standard device requests that require a data stage transfer to the device, the 5-second limit applies. This

means that the device must be capable of accepting all data packets from the host and successfully

completing the Status stage if the host provides the data at the maximum rate at which the device can accept

it. Delays between packets introduced by the host add to the time allowed for the device to complete the

request.

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