In terms of framework, USB3.0 SuperSpeed is backward compatible with USB2.0, but there are still some major differences in the protocols between the two:
•USB2.0 transaction has three parts (token, data and handshake). USB3.0 SuperSpeed also has these three parts but the usage is different (the token packet is integrated in the header packet and In DPH, various types of handshake packets are in the form of TP packets); for OUT transactions, the token is merged into the data packet; for IN transactions, the token is replaced by the handshake packet.
•USB2.0 does not support bursting, and USB3.0 SuperSpeed supports continuous bursting;
•USB2.0 is a half-duplex (half-duplex) broadcast bus, and USB3.0 SuperSpeed is a dual-simplex (full-duplex) non-broadcast bus that supports simultaneous IN and OUT transactions;
•USB2.0 uses polling mode, USB3.0 overspeed uses asynchronous notification mode;
•USB2.0 does not support streaming capabilities, but USB3.0 SuperSpeed supports the Stream mode of bulk endpoints;
•USB2.0 does not have a mechanism to enter a low power consumption state during the isochronous transmission interval, while USB 3.0 overspeed allows it to automatically enter a low power consumption state during the isochronous transmission service interval (entering low power consumption during non-service periods); The SS host sends a PING packet to the target synchronization device before the service interval to allow synchronization transfers to begin before transitioning to the power-active state.
• USB2.0 devices cannot no-tify the host of the tolerable delay time before entering a low-power state (the maximum delay time for a device to no-tify the host of entering a low-power state). USB3.0 Overspeed provides Latency Tolerance messages;
•USB2.0 sends frame packets/small frame packets at a fixed 1ms/125us interval (USB 2.0 full speed and high speed mode). Under USB3.0 overspeed, the device can send an Interval Adjustment message to the host to adjust the interval from 125us to +/-13.333us;
•USB2.0 power management is always host-oriented (host initialized), and both ends of the USB3.0 SuperSpeed link support power management; therefore, whenever idle, exit, or communication are needed, each link can be entered independently Low power state.
•USB2.0 only performs error detection, recovery, and flow control at the end-to-end level for each transaction. USB3.0 overspeed performs end-to-end (packet retry) and link level (header packet retry) ) split these functions.
The USB3.0 super-speed full-duplex bus physical layer allows simultaneous bidirectional communication. The USB 3.0 SuperSpeed protocol allows multiple data packets (bursts) to be sent before a handshake packet is received. For OUT transmission, the information contained in the USB 2.0 token packet (device address and endpoint information) is merged into the packet header, so no additional token packet is required. For input transfer IN, the USB 3.0 SuperSpeed host sends a handshake packet (ACK) to the device to request data (and indicate whether the data is correct). The device can respond by returning data or a STALL handshake packet, or it can return a Not Ready (NRDY) handshake packet to delay transmission until the device is ready.
USB2.0 packets are broadcast. Each connected device parses the address, endpoint, and direction information of each packet to decide whether it should respond. USB3.0 SuperSpeed packets have routing information, and the HUB determines which device each packet should be delivered to. There is only one exception. The Isochronous Timestamp Packet (ITP) is broadcast to each device.
The USB2.0 query method has been replaced by asynchronous notification. USB 3.0 SuperSpeed transfers begin the transfer by issuing a request from the host, followed by a response from the device. If the device can accept the request, it receives data or sends data; if the endpoint is stopped, the device should respond with a STALL handshake packet; if the device cannot accept the request due to lack of cache space or no data, it should use an NRDY response to tell the host that it is not available yet. Process the request. When the device can accept the request, the device will actively send an endpoint ready (ERDY) asynchronous notification to the host and the host will reschedule the transmission transaction.
Unicast and restricted multicast packets, as well as asynchronous notifications, allow links that are not actively transmitting packets to enter a reduced power state. Upstream and downstream ports jointly determine that their links enter a low power state. The hub will be passed to the upstream port. By allowing link partners to independently control their link power states, the hub passes the highest link power state visible to any downstream port to the upstream port, quickly bringing the bus into the lowest allowed power state.
