二、架构、系统、协议和供电

1.USB系统结构

2.USB拓扑结构

USB总线基于分层的星状拓扑结构
以Hub为中心，连接周围设备
总线上最多可连接127个设备
Hub串联数量最多5个
线缆长度最长5米

3.USB拓扑结构示例

4.USB逻辑部件

5.USB设备供电方式

1）Two possible power configurations

Self-powered: power provided from external power-supply
Bus-powered: power provided from VBUS (5v)

2）对总线供电设备，才区分低功耗和高功耗USB设备之分

For bus-powered device, two options are possible:

Low-power devices :maximum power consumption is 100mA
High-power devices :maximum power consumption is 100mA during bus enumeration and 500mA after configuration

3）During device enumeration, the device indicates to host its power configuration (self-powered/bus-powered) and its power consumption in the device configuration descriptor (see slide after)

6.USB配置描述符（以Joystick为例）

Configuration Descriptor

0x09,

/* 描述符的长度： 9字节 */

USB_CONFIGURATION_DESCRIPTOR_TYPE,

/* 描述符的类型: 0x02 配置描述符(Configuration) */

JOYSTICK_SIZ_CONFIG_DESC, 0x00,

/* 完整的描述符包括接口描述符、端点描述符和类描述符的长度 */

0x01,

/* 配置所支持的接口数目： 1*/

0x01,

/* 用SetConofiguration()选择此配置，所指定的配置号： 1*/

0x00,

/* 用于描述此配置的字符描述符的索引号： 0 */

0xE0,

/* 供电配置： B7(1 保留), B6(自供电), B5(远程唤醒), B4-B0(0 保留) */

0x32,

/* 最大功耗，以2mA为单位计算： 0x32表示 50×2 ＝100mA */

7.USB挂起模式

USB device should enter in USB Suspend mode when the bus is idle for more than 3 ms 该【USB挂起模式】是针对USB设备来说的
In suspend mode, when the device is bus powered , the current drawn from VBUS power shouldn’t exceed 2.5mA
USB host prevents device from entering in suspend mode by periodically issuing Start of Frame (SOF) or Keep Alive for LS

For High-speed, SOF is sent every micro-frame 125us +/- 65ns
For Full-speed, SOF is sent every frame 1ms +/- 500ns
For Low-speed, Keep Alive (End of Packet) is sent every 1ms in absence of low-speed data

Exist from Suspend mode can be USB设备要退出【挂起模式】可以由Host唤醒，或者自己唤醒

Initiated from host by issuing the resume signaling
Initiated from device by issuing the remote wakeup signaling

8.USB 连接器

1）标准接口/Mini接口

2）Micro接口

9.USB总线信号

USB使用的是差分传输模式，两个数据线D+和D-

差分信号1： D+ > VOH (2.8V) and D- < VOL(0.3V)
差分信号0： D- > VOH and D+ < VOL
复位信号（Reset）： D+ and D- < VOL for >= 10ms
IDLE状态： J 状态
恢复信号（Resume）： K 状态
SOP：从IDLE状态切换到K状态
EOP：持续2位时间的SE0信号，后跟随1位时间的J状态
SYNC： 3个重复的KJ状态切换，后跟随2位时间的K状态

10.USB设备的插入检测和速度检测

Full/high Speed: Pull-up on D+

Low Speed: Pull-up on D-

The 1.5K pull-up allows the host to detect the device attachment and its supported speed
High-speed device is detected first as full-speed device then high-speed capability is detected through bus handshakemechanism called “chirp sequence”