對(duì)同時(shí)兼容usb2.0和usb3.0的設(shè)備來說，從物理信號(hào)和軟件邏輯上來看，都是完全分開的。從物理信號(hào)上看，usb2.0用D+,D-，而usb3.0用sstx+,sstx-,ssrx+,ssrx-。從軟件邏輯上看，usb2.0和usb3.0分屬不同的總線。
usb3.0設(shè)備插入檢測(cè)時(shí)，并不需要用到D+,D-信號(hào)。
參考下面的內(nèi)容：
https://electronics.stackexchange.com/questions/297031/usb-3-x-ss-enumeration
https://electronics.stackexchange.com/questions/107669/usb3-with-fewer-wires
https://www.totalphase.com/support/articles/200349256-USB-Background#s1.1.3.4
可以看到usb插拔檢測(cè)的大致流程：
- USB plug is connected; SS Tx and Rx pairs are connected to HOST, D+ D- are connected as well, but initially do nothing.
- The SS host Tx (transmitter) periodically sends short common-mode pulses of positive polarity, and watches for line response. If the line is connected and therefore terminated with 2x45 Ohms, the RC of this is different from open line. The transmitter identifies this, and proceeds to Polling.LFPS signaling. See section 6.11 of USB 3.0/3.1 specs. The process is called "Rx Detect".
- The attached device tries to do the same with its Tx, but only upon seeing VBUS. If termination is detected, it starts its own Polling.LFPS signaling. These two processes are independent and asynchronous, see section 6.9.2
- Whoever side starts LFPS first, it waits for the link partner to start its own LFPS. When the first partner detects valid LFPS bursts, it ends LFPS and moves to link training sequencing. The partner follows.
- If one partner does not see the polling from other side in 360 ms, it moves into "compliance pattern". (actually, only device side does this automatically, the host needs to be specially enabled for this).
- If the LFPS handshake was successful, both partners proceed into link training, sending TSQ, then TS1 and then TS2 "ordered sets", and then the link moves into active "U0" state, exchanging "link management" packets and determining link capabilities.
- Only then the protocol layer kicks in and SS enumeration starts.
- Only if both link partners failed to detect Rx termination, the SS device proceeds with USB 2.0 connection pulling up D+ line with 1.5k resistor. Then the link proceeds with USB 2.0 protocol.
另外，usb3.0 tx,rx前面的ss應(yīng)該是指super speed。各個(gè)usb版本的速率也是不同的：
usb1.0 low speed 1.5Mbps
usb1.1 full speed 12Mbps
usb2.0 high speed 480Mbps
usb3.0 super speed 5Gbps
