USB枚举，USB Emulation，从字面意思看，就是去列举USB，而列举啥呢，其实就是USB的初始化。
简单来说，USB的枚举，对应的就是USB的Host和Device之间的对话，即Host根据Device所报告上来的参数，得知USB的device是啥类型的，具有啥功能，然后初始化相关参数。
接下来，就USB Device就可以正常工作了。
所以，可以简单的理解为，USB枚举，就是USB设备的初始化（init）。
USB枚举过程
1 获取USB设备的设备描述符 : 使用默认地址0
2 成功后复位设备
3 对设备进行地址设置 : 使设备从地址状态进入寻址状态
4 再次获取设备描述符 : 使用新的地址
5 获取设备配置符
6 根据设备配置符中的配置符总长度，获取其他配置符（接口，端点等）
7 设置配置 : 使设备从地址状态进入配置状态
http://www.lvr.com/usbcenum.htm
Enumeration Steps
The USB 2.0 specification defines six device states.
During enumeration, a device moves through the Powered, Default, Address, and Configured states.
(The other states are Attached and Suspend.)
In each state, the device has defined capabilities and behavior.
Typical USB 2.0 Sequence
The steps below are a typical sequence of events that occurs during enumeration of a USB 2.0 device under Windows.
Device firmware shouldn’t assume that enumeration requests and events will occur in a particular order.
To function successfully, a device must detect and respond to any control request or other bus event at any time.
1. The system has a new device.
A user attaches a device to a USB port, or the system powers up with a device attached.
The port may be on the root hub at the host or on a hub that connects downstream from the host.
The hub provides power to the port, and the device is in the Powered state.
The device can draw up to 100 mA from the bus.
2. The hub detects the device.
The hub monitors the voltages on the signal lines (D+ and D-) at each of its ports.
The hub has a pull-down resistor of 14.25k–24.8kW on each line.
A device has a pull-up resistor of 900–1575W on D+ for a full-speed device or D- for a low-speed device.
High-speed-capable devices attach at full speed.
On attaching to a port, the device’s pull-up brings its line high, enabling the hub to detect that a device is attached.
On detecting a device, the hub continues to provide power but doesn’t yet transmit USB traffic to the device.
Chapter 15 has more on how hubs detect devices.
3. The host learns of the new device.
Each hub uses its interrupt endpoint to report events at the hub.
The report indicates only whether the hub or a port (and if so, which port) has experienced an event.
On learning of an event, the host sends the hub a Get Port Status request to find out more.
Get Port Status and the other hub-class requests described are standard requests that all hubs support.
The information returned tells the host when a device is newly attached.
4. The hub detects whether a device is low or full speed.
Just before resetting the device, the hub determines whether the device is low or full speed
by examining the voltages on the two signal lines.
The hub detects the device’s speed by determining which line has a higher voltage when idle.
The hub sends the information to the host in response to the next Get Port Status request.
A USB 1.x hub may instead detect the device’s speed just after a bus reset.
USB 2.0 requires speed detection before the reset so the hub knows
whether to check for a high-speed-capable device during reset as described below.
5. The hub resets the device.
When a host learns of a new device, the host sends the hub a Set Port Feature request that asks the hub to reset the port.
The hub places the device’s USB data lines in the Reset condition for at least 10 ms.
Reset is a special condition where both D+ and D- are logic low.
(Normally, the lines have opposite logic states.)
The hub sends the reset only to the new device.
Other hubs and devices on the bus don’t see the reset.
6. The host learns if a full-speed device supports high speed.
Detecting whether a device supports high speed uses two special signal states.
In the Chirp J state, only the D+ line is driven and in the Chirp K state, only the D- line is driven.
During the reset, a device that supports high speed sends a Chirp K.
A high-speed-capable hub detects the Chirp K and responds with a series of alternating Chirp K and Chirp J.
On detecting the pattern KJKJKJ, the device removes its full-speed pull-up
and performs all further communications at high speed.
If the hub doesn’t respond to the device’s Chirp K, the device knows it must continue to communicate at full speed.
All high-speed devices must be capable of responding to control requests at full speed.
7. The hub establishes a signal path between the device and the bus.
The host verifies that the device has exited the reset state by sending a Get Port Status request.
A bit in the returned data indicates whether the device is still in the reset state.
If necessary, the host repeats the request until the device has exited the reset state.
When the hub removes the reset, the device is in the Default state.
The device’s USB registers are in their reset states, and the device is ready to respond to control transfers at endpoint zero.
The device communicates with the host using the default address of 00h.
8. The host sends a Get Descriptor request to learn the maximum packet size of the default pipe.
The host sends the request to device address 00h, endpoint zero.
Because the host enumerates only one device at a time,
only one device will respond to communications addressed to device address 00h even if several devices attach at once.
The eighth byte of the device descriptor contains the maximum packet size supported by endpoint zero.
A Windows host requests 64 bytes but after receiving just one packet (whether or not it has 64 bytes),
the host begins the Status stage of the transfer.
On completing the Status stage, Windows requests the hub to reset the device as in step 5 above.
The USB 2.0 specification doesn’t require a reset here.
The reset is a precaution that ensures that the device will be in a known state when the reset ends.
9. The host assigns an address.
When the reset is complete, the host controller assigns a unique address to the device by sending a Set Address request.
The device completes the Status stage of the request using the default address and then implements the new address.
The device is now in the Address state.
All communications from this point on use the new address.
The address is valid until the device is detached, a hub resets the port, or the system reboots.
On the next enumeration, the host may assign a different address to the device.
10. The host learns about the device’s abilities.
The host sends a Get Descriptor request to the new address to read the device descriptor.
This time the host retrieves the entire descriptor.
The descriptor contains the maximum packet size for endpoint zero,
the number of configurations the device supports, and other basic information about the device.
The host continues to learn about the device by requesting the one or more configuration descriptors specified in the device descriptor.
A request for a configuration descriptor is actually a request for the configuration descriptor
followed by all of its subordinate descriptors up to the number of bytes requested.
A Windows host begins by requesting just the configuration descriptor’s nine bytes.
Included in these bytes is the total length of the configuration descriptor and its subordinate descriptors.
Windows then requests the configuration descriptor again, this time requesting the number of bytes in the retrieved total length.
The device responds by sending the configuration descriptor followed by all of the configuration’s subordinate descriptors,
including interface descriptor(s), with each interface descriptor followed by any endpoint descriptors for the interface.
Some configurations also have class- or vendor-specific descriptors. This chapter has more on what the descriptors contain.
11. The host assigns and loads a device driver (except for composite devices).
After learning about a device from its descriptors, the host looks for the best match in a driver to manage communications with the device.
Windows hosts use INF files to identify the best match.
The INF file may be a system file for a USB class or a vendor-provided file that contains the device’s Vendor ID and Product ID.
Chapter 9 has more about selecting a driver.
For devices that have been enumerated previously, Windows may use stored information instead of searching the INF files.
After the operating system assigns and loads the driver, the driver may request the device to resend descriptors or send other class-specific descriptors.
An exception to this sequence is composite devices, which can have different drivers assigned to multiple interfaces in a configuration.
The host can assign these drivers only after enabling the interfaces, so the host must first configure the device as described below.
12. The host’s device driver selects a configuration.
After learning about a device from the descriptors, the device driver requests a configuration
by sending a Set Configuration request with the desired configuration number.
Many devices support only one configuration.
If a device supports multiple configurations, the driver can decide
which configuration to request based on information the driver has about how the device will be used,
or the driver can ask the user what to do or just select the first configuration.
(Many drivers only select the first configuration.)
On receiving the request, the device implements the requested configuration.
The device is now in the Configured state and the device’s interface(s) are enabled.
For composite devices, the host can now assign drivers.
As with other devices, the host uses the information retrieved from the device
to find a driver for each active interface in the configuration.
The device is then ready for use.
Hubs are also USB devices, and the host enumerates a newly attached hub in the same way as other devices.
If the hub has devices attached, the host enumerates these after the hub informs the host of their presence.
Attached state.
If the hub isn’t providing power to a device’s VBUS line, the device is in the Attached state.
The absence of power may occur if the hub has detected an over-current condition or
if the host requests the hub to remove power from the port.
With no power on VBUS, the host and device can’t communicate,
so from their perspective, the situation is the same as when the device isn’t attached.
Suspend State.
A device enters the Suspend state after detecting no bus activity, including SOF markers, for at least 3 ms.
In the Suspend state, the device should limit its use of bus power.
Both configured and unconfigured devices must support this state.
Chapter 16 has more about the Suspend state.
SuperSpeed Differences
Enumerating SuperSpeed devices has some differences compared to USB 2.0:
- On detecting a downstream SuperSpeed termination at a port, a hub initializes and trains the port’s link. Enumeration then proceeds at SuperSpeed with no need for further speed detecting.
- The host isn’t required to reset the port after learning of a new device.
- The bus-current limits are 150 mA before configuration and 900 mA after configuration.
- The host sends a Set Isochronous Delay request to inform the device of the bus delay for isochronous packets.
- The host sends a Set SEL request to inform the device of the system exit latency (the amount of time required to transition out of a low-power state).
- Protocols for entering and exiting the Suspend state differ.
- For hubs, the host sends a Set Hub Depth request to set the hub-depth value.
http://www.kernelsword.com/article.asp?id=104
1、集线器检测新设备
主机集线器监视着每个端口的信号电压，当有新设备接入时便可觉察。（集线器端口的两根信号线的每一根都有15kΩ的下拉电阻，而每一个设备在D+都有一个1.5kΩ的上拉电阻。当用USB线将PC和设备接通后，设备的上拉电阻使信号线的电位升高，因此被主机集线器检测到。）
2、主机知道了新设备连接
每个集线器用中断传输来报告在集线器上的事件。当主机知道了这个事件，它给集线器发送一个Get_Status请求来了解更多的消息。返回的消息告诉主机一个设备是什么时候连接的。
3、集线器重新设置这个新设备
当主机知道有一个新的设备时，主机给集线器发送一个Set_Feature请求，请求集线器来重新设置端口。集线器使得设备的USB数据线处于重启（RESET）状态至少10ms。
4、集线器在设备和主机之间建立一个信号通路
主机发送一个Get_Status请求来验证设备是否激起重启状态。返回的数据有一位表示设备仍然处于重启状态。当集线器释放了重启状态，设备就处于默认状态了，即设备已经准备好通过Endpoint 0 的默认流程响应控制传输。即设备现在使用默认地址0x0与主机通信。
5、集线器检测设备速度
集线器通过测定那根信号线（D+或D-）在空闲时有更高的电压来检测设备是低速设备还是全速设备。（全速和高速设备D+有上拉电阻，低速设备D-有上拉电阻）。
6、获取最大数据包长度
向address0发送USB协议规定的Get_Device_Descriptor命令，以取得却缺省控制管道所支持的最大数据包长度，并在有限的时间内等待USB设备的响应，主机一次只能列举一个USB设备，所以同一时刻只能有一个USB设备使用缺省地址0。
7、主机分配一个新的地址给设备
主机通过发送一个Set_Address请求来分配一个唯一的地址给设备。设备读取这个请求，返回一个确认，并保存新的地址。从此开始所有通信都使用这个新地址。
8、主机向新地址重新发送Get_Device_Descriptor命令，此次读取其设备描述符的全部字段，以了解该设备的总体信息，如VID，PID。
9、主机向设备循环发送Get_Device_Configuration命令，要求USB设备回答，以读取全部配置信息。
10、主机发送Get_Device_String命令，获得字符集描述（unicode），比如产商、产品描述、型号等等。
11、此时主机将会弹出窗口，展示发现新设备的信息，产商、产品描述、型号等。
12、根据Device_Descriptor和Device_Configuration应答，PC判断是否能够提供USB的Driver，一般能提供几大类的设备，如游戏操作杆、存储、打印机、扫描仪等，操作就在后台运行。但是有的不可以，所以在此时将会弹出对话框，索要USB的Driver。
13、加载了USB设备驱动以后，主机发送Set_Configuration（x）命令请求为该设备选择一个合适的配置(x代表非0的配置值)。如果配置成功，USB设备进入“配置”状态，并可以和客户软件进行数据传输。此时，常规的USB完成了其必须进行的配置和连接工作。查看注册表，能够发现相应的项目已经添加完毕，至此设备应当可以开始使用。不过，USB协议还提供了一些用户可选的协议，设备如果不应答，也不会出错，但是会影响到系统的功能。