USB包(packet)由SOP,SYNC,Packet内容和EOP组成.
SOP信号-------------瞬态信号
协议中的描述：7.1.7.4.1
The start of a packet (SOP) is signaled by the originating port by driving the D+ and D- lines from the Idle stateto the opposite logic level (K state).
This switch in levels represents the first bit of the SYNC field
. Hubs mustlimit the change in the width of the first bit of SOP when it is retransmitted to less than ± 5 ns. Distortion can beminimized by matching the nominal data delay through the hub with the output enable delay of the hub
也就是idle状态到相反的K状态。电平上的切换代表SYNC 字段的第一个bit
低速设备SOP信号：总线从IDLE状态（J状态：差分0）切到K状态（差分1），即可完成低速SOP信号的发送。
全速设备SOP信号：总线从IDLE状态（J状态：差分1）切到K状态（差分0），即可完成全速SOP信号的发送。
3.2 EOP信号-----------
低速设备EOP信号：总线持续2位时间的SE0（单端0）状态，后跟随1位时间的J状态（差分0），即可完成低速EOP信号的发送。
全速设备EOP信号：总线持续2位时间的SE0（单端0）状态，后跟随1位时间的J状态（差分1），即可完成全速EOP信号的发送。
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高速IDLE \SOP\ EOP信号定义，spec 7.1.7.4.2 High-speed Signaling
The high-speed Idle state is when both lines are nominally at GND.-----高速idle定义，d+和d-都拉低
The source of the packet signals the Start of Packet (SOP) in high-speed mode by driving the D+ and D- lines from the high-speed Idle state to the K state. This K is the first symbol of the SYNC pattern (NRZI sequenceKJKJKJKJ KJKJKJKJ KJKJKJKJ KJKJKJKK) as described in Section 7.1.10.----------高速SOP 定义，IDLE--->K状态。
The high-speed End of Packet (EOP) begins with a transition from the last symbol before the EOP to theopposite symbol. This opposite symbol is the first symbol in the EOP pattern (NRZ 01111111 with bit stuffing disabled) as described in Section 7.1.13.2.-------eop第一个符号是前面的crc前面的最后一个符号取反的符号。
Upon completion of the EOP pattern, the driver ceases to inject current into the D+ or D- lines, and the lines return to the high-speed Idle state.
The high-speed SOF EOP is aspecial case. This SOF EOP is 40 symbols without a transition (rather than 8 for a non-SOF packet)
.--------高速sof eop是个特殊情况，40个符号没有符号变化
The fact that the first symbol in the EOP pattern forces a transition simplifies the process of determining precisely which is the last bit in the packet prior to the EOP delimiter. -----EOP的第一个符号是为了简化操作，保证精确分开和packet直接的精确界限划分开
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The high-Speed mode, on the other hand, has been modified to use an 8-bit NRZ signal of "01111111," without stuffed bit insertion. The first symbol indicating an EOP is the reversal value of the last symbol immediately before the EOP. When the end of the EOP pattern is reached, the driver stops the flow of current to the D+ or D- line, and switches the line to high-Speed idle state.
另一方面，高速模式已被修改为使用“01111111”的 8 位 NRZ 信号，而无需插入填充位。指示 EOP 的第一个符号是紧接在 EOP 之前的最后一个符号的反转值。当到达 EOP 模式结束时，驱动器停止电流流向 D+ 或 D- 线，并将线切换到高速空闲状态。
---------------非sof包的eop 格式
The format of theSOFpacket is shown in Figure12. ---------sof packet 格式
sof的eop是5B=40bit
The EOP for a high-Speed SOF (Start Of Frame) is a special case, and has a length of 40 bits, not 8 bits. This is used to detect the disconnection of a device. When a device is disconnected, the 45 ohm resistance further from the hub's downstream port is lost. However, this is not sufficient to detect the disconnection of a device, since the driver does not send a current to the D+ or the D- line while in the high-Speed idle state. Simply put, the disconnection of a device can only actually be detected while there is a transaction on the bus. The 40-bit long EOP is used to maintain the output level until the signal returns to the transmitting end after being reflected at the end of the cable. When the output level exceeds approx. 600 mV, the device is considered to be disconnected.
高速SOF（Start Of Frame）的EOP是一个特例，它的长度为40位，而不是8位。这是用来检测一个设备的断开连接的。当一个设备断开时，离集线器下游端口更远的45欧姆电阻就会丢失。然而，这并不足以检测设备的断开，因为在高速空闲状态下，驱动器不会向D+或D-线发送电流。简单地说，只有在总线上有交易的时候才能真正检测到设备的断开。40位长的EOP用于维持输出电平，直到信号在电缆末端反射后返回到发送端。当输出电平超过约600 mV时，设备被认为是断开了。
