What is USB, what are the advantages, device classification and transmission methods of USB?

USB is a new generation interface standard jointly launched by world-renowned computer and communication companies. The full name is Universal Serial Bus (Universal Serial Bus). It is a fast and flexible bus interface. It is a serial communication standard developed to solve the contradiction between the increasing number of PC peripherals and the limited motherboard slots and ports.

USB is generally divided into USB low speed, USB full speed, USB high speed and USB super high speed, which correspond to USB1.0, USB1.1, USB2.0 and USB3.0 respectively, and USB3.0 is divided into GEN1, GEN2 and so on. The latest generation is USB4, the transmission speed is 40Gbit/s, the three-stage voltage is 5V/12V/20V, the maximum power supply is 100W, and the new Type C interface allows positive and negative blind insertion.

USB is widely used and has the following advantages:
Applicable to a variety of peripherals, so that it does not need to prepare different interfaces and protocols for different peripherals;
Windows can automatically detect the hot-plugging of USB devices and configure them automatically;
The interface cable on the PC is very scarce, and the USB device does not require the user to set the port, so in terms of user convenience, or in terms of resource occupancy, USB is excellent;
When connecting a USB device, the full-speed USB interface can reach 12Mbit/s. Considering the status, control and error information, the maximum theoretical speed can still reach 9.6Mbit/s, which is unmatched by other serial interface protocols, and USB also supports low-speed transmission of 1.5Mbit/s.
The low price of USB interface chips also greatly promotes the development and application of USB devices.
Before the emergence of USB, typical computer interfaces included parallel port, serial port, mouse port, keyboard port, monitor port, and various card-type interfaces, etc. There were various cables corresponding to these interfaces. In terms of transmission speed, These interfaces all have the problem of low speed. In terms of technology, this design is prone to I/O conflicts, insufficient interrupts, and a new interface card must be designed for each new peripheral. Today’s computer peripherals are pursuing high speed and high versatility. The USB interface has adapted to this requirement, and has quickly gained strong support from many PC manufacturers and semiconductor manufacturers due to its advantages of high speed, convenient use and low cost. The transition of peripherals to USB has become an inevitable trend.

The classification of the use of the USB system and the description of the system
The original intention of the USB design was for desktop computers and not for use in mobile environments. The software system ensures future expansion of USB by providing support for various host controllers [3]. USB is a cable bus that supports data transfer between a host and a wide variety of plug-and-play peripherals. A standard protocol predetermined by the host enables various devices to share the USB bandwidth, and the bus allows peripherals to be added, set up, used, and removed while other devices and the host are running.
Description of the USB system:
A USB system is mainly defined as three parts: USB interconnection, USB device, and USB host.
The interconnection of USB refers to the operation of connection and communication between the USB device and the host, which mainly includes the following aspects:

(1) Topological structure of the bus: various connection methods between the USB device and the host;
(2) Internal hierarchical relationship: According to performance stacking, USB tasks are assigned to each level of the system;
(3) Data flow mode: describes the flow of data in the system from the producer to the consumer through USB;
(4) USB scheduling: USB provides a shared connection. The available connections are scheduled to support synchronous data transfer and to avoid the overhead of prioritization.
The USB devices are as follows:
(1) Network hub, which provides more connection points to USB;
(2) Functional devices: Provide specific functions for the system, such as ISDN connections, digital joysticks or speakers.

The main basis of the USB standard interface provided by the USB device:

(1) The use of the USB protocol;
(2) Feedback to standard USB operations, such as set and reset;
(3) Descriptive information of standard performance;
USB bus transmission mode:
The USB2.0 bus is a polling bus, and the host controls the port to initiate all data transfers. Each bus performs an action to transmit up to three packets. According to the principle established before transmission, at the beginning of each transmission, the host controller sends a USB data packet describing the type, direction, USB device address and terminal number of the transmission operation. This data packet is usually called a token packet. ). The USB device fetches its own data from the proper position of the decoded data packet. The data transfer direction is either from the host to the device or from the device to the host. At the beginning of transmission, the direction of data transmission is marked by a flag packet, and then the sender starts to send packets containing information or indicates that there is no data transmission. The receiving end should also send a handshake data packet accordingly to indicate whether the transmission is successful. The USB data transmission between the sender and the receiver can be regarded as a channel between the host and the port of the device. There are two types of channels: streams and messages. Unlike message data, stream data does not have the structure defined by USB, and channels are related to data bandwidth, type of transfer service, and port characteristics (such as direction and buffer size). There is a special channel in USB – the default control channel, which belongs to the message channel and exists when the device is started, thus providing an entry for device settings, query status and input control information.

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