An overview of the USB 3.0 architecture, covering the USB Host, USB Device(s) and USB Interconnect, as well as the related receptacles, plugs and cables.
A USB 3.0 system has a USB Host, USB Device(s) and USB Interconnect. The Host initiates most of the activity on the bus. It sends data to the Device or requests data from the Device.
The Device responds to the Host. It cannot initiate any data transfers, apart from an Endpoint Ready (ERDY) signal.
The USB Host has a Host Controller and a Root Hub (Tier1 Hub), which provides additional connection ports to the host controller. The Host can be connected to one or more devices.
A USB system has only one Host. The USB Host initiates and schedules all the transactions on the USB bus, and acts as the bus master. It is comprised of the host-end SuperSpeed communication layers: a Host controller and a Root Hub; and related system software, device drivers, or application software.
The USB Host supports all USB speeds for backward compatibility, and can concurrently handle newer USB 3.0 SuperSpeed traffic and existing USB 2.0 Hi-Speed (HS), Full-Speed (FS) and Low-Speed (LS) traffic. It detects when devices are connected, and gives each a unique address, which is used, with the routing information, to forward packets to the target device.
The USB Host also provides power for all the devices attached to it.
A USB Device receives requests from the Host and responds to them, is a slave on the bus, and cannot initiate any data transfer expect ERDY. It is comprised of the device-end SuperSpeed communication layers: Device controller, and Device/Function. The USB Device supports SuperSpeed and HS operation, one at a time, and optionally, FS and LS modes.
A USB Hub provides additional connection points/ports to the host. The USB system allows up to five Hubs to be chained, and cascades up to 127 devices, including the root hub. A USB Hub uses a Routing String in the header to forward packets to the particular device/hub. It should be self-powered, and should always offer power to all devices connected to its downstream ports, when the host is powered on; but only if the Hub supports charging applications when the Host is off.
A USB Dual Role Device (DRD) acts either as a Host or a Device, and supports protocols that enable it to swap roles and to find its attachment status in the absence of power on the USB bus. It also supports protocols used in USB 2.0 to achieve similar ends.
USB 3.0 Receptacles are backward compatible with USB 2.0 connector plugs.
USB 3.0 cables and plugs are not compatible with USB 2.0 receptacles.
A USB 3.0 cable has three USB data paths: one twisted signal pair for USB 2.0 (uni-directional); and two twisted signal pairs for USB 3.0 (Tx and Rx). There is also a power pair. The cable, of up to 3 meters in length and colour-coded blue, supports SuperSpeed operation, as well as LS/FS/HS for backward compatibility.
In USB 3.0 terminology, an upstream port faces the Host while a downstream port faces away from the Host. Data transfers IN to the Host, and OUT, away from the Host.
If you are new to designing with USB, or looking for tips on implementing USB 3.0 IP, attend Synopsys’ “USB 3.0 University.” Topics in this instructional video series range from fuller descriptions of USB 3.0 Device Classes, USB 3.0 Architecture, USB 3.0 Topology, USB 3.0 Data Bursting, USB 3.0 Bulk Streaming, USB 3.0 Transaction Generation, USB 3.0 End Points, the USB 3.0 Initialisation process, and the general USB 3.0 SuperSpeed architecture. Click on the links below.