USB 3.0 system includes a USB host, USB device, and USB interconnection.
- The host initiates most of the activities on the USB bus. It sends data to the devices or requests data from the devices.
- USB devices respond to the host. Except for the Endpoint Ready (ERDY) signal, they cannot initiate any data transfer.
01
USB Host
The USB host consists of a host controller and a root hub. The root hub provides additional connection ports for the host controller. The host can be connected to one or multiple devices.
The USB system has only one host. The USB host initiates and schedules all transactions on the USB bus and acts as the bus manager. It consists of the SuperSpeed communication layer on the host side, which includes the host controller and the root hub, as well as relevant system software, device drivers, or application software.
The USB host supports all USB speeds and is backward compatible, capable of handling both newer USB 3.0 SuperSpeed traffic and existing USB 2.0 High-Speed (HS), Full-Speed (FS), and Low-Speed (LS) traffic simultaneously. It detects when devices are connected and assigns a unique address to each device, which, along with routing information, is used to forward data packets to the target device.
The USB host also provides power to all connected devices.
02
USB Device
USB devices receive requests from the host and respond to them. They are peripheral devices on the bus and cannot initiate any data transfers except for the ERDY signal. They consist of the SuperSpeed communication layer on the device side, which includes the device controller and device/function. USB devices support SuperSpeed and HS operations, one at a time, and can also operate in FS and LS modes if desired.
03
USB Hub
A USB hub provides additional connection points/ports for the host. The USB system allows up to five hubs to be linked and cascades up to 127 devices, including the root hub. USB hubs forward data packets to specific devices/hubs using routing strings in the headers. They should be self-powered and always provide power to all devices connected to their downstream ports when the host is powered on. However, support for charging applications is only available when the hub is in a powered-off state.
USB Dual Role Devices (DRDs) can function as either a host or a device. They support protocols that allow them to switch roles and determine their connection status when the USB bus is powered off. They also support protocols used in USB 2.0 for similar purposes.
04
USB Signal
A USB 3.0 cable has three USB data paths: a pair of twisted wire signal pairs (half-duplex) for USB 2.0, and two twisted wire signal pairs (Tx and Rx) for USB 3.0. There is also a power pair. The cable can reach a length of up to 3 meters and is color-coded blue. It supports SuperSpeed operations as well as backward compatibility with LS/FS/HS.
In USB 3.0 terminology, the upstream port faces the host, while the downstream port is away from the host. When the host receives data, it is referred to as IN, and when it sends data, it is referred to as OUT.
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