USB is a new generation interface standard jointly launched by the world-famous computer and communication companies, known as Universal Serial Bus (Universal Serial Bus), 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 ultra high speed, which respectively corresponds to
USB1.0, USB1.1, USB2.0, and USB3.0, and USB3.0 is divided into GEN1,GEN2 and so on. The latest generation is USB4, with a transmission speed of 40Gbit/s, a three-stage voltage of 5V/12V/20V, and a maximum power supply of 100W. The new Type C interface allows forward and reverses blind insertion.
USB is widely used and has the following advantages
- Suitable for 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-swap of USB devices and configure them automatically.
- PC interface line is very short, and USB devices do not need users to set up ports, so no matter the user’s use of convenience, or from the occupation of resources, USB is very good;
- When connected to a USB device, the full-speed USB port 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.
- USB interface chips are cheap, which greatly promotes the development and application of USB devices.
Before the appearance of USB, the typical computer interface has a parallel port, serial port, mouse port, keyboard port, monitor port, and all kinds of card interfaces, etc., with these interfaces corresponding to a variety of different cables, in terms of transmission speed, these interfaces are low-speed problems. On the technical side, this design is prone to I/O conflicts, insufficient interrupts, and the need to design a new interface card for every new peripheral. Today’s computer external equipment is in pursuit of high speed and high versatility. USB interface to meet this requirement, and with its fast speed, easy to use, low-cost advantages, quickly received the support of many PC manufacturers and semiconductor manufacturers, peripherals to USB have become an inevitable trend.
USB system usage classification and system description
USB is designed for desktop computers, not mobile environments. The software system supports various host controllers to ensure the expansion of USB in the future . USB is a cable bus that supports data transfer between a host computer and various plug-and-play peripherals. A standard protocol predetermined by the host enables various devices to share USB bandwidth, and the bus allows peripherals to be added, set, used, and removed while other devices and the host are running.
Description of USB system:
A USB system is mainly defined as three parts: USB interconnect, USB device, and USB host.
USB interconnection refers to the operation of connecting and communicating between USB devices and hosts, including the following aspects:
- (1) Bus topology: various connection modes between USB devices and hosts;
- (2) Internal hierarchy: according to performance superposition, USB tasks are assigned to each hierarchy of the system;
- (3) Data flow mode: describes the flow mode of data in the system from the producer to the user through USB;
- (4) USB scheduling: USB provides a shared connection. Available connections are scheduled to support synchronous data transfer and to avoid priority checking overhead.
USB devices are shown below
- (1) Network hub, which provides more connection points to USB;
- (2) Functional devices: provide specific functions for the system, such as ISDN connection, digital joystick, or loudspeaker.
USB devices provide standard USB ports based on:
- (1) The application of USB protocol;
- (2) Feedback on standard USB operations, such as setting and reset;
- (3) Descriptive information of standard performance;
USB bus transmission mode
The USB2.0 bus is a round-robin bus. The host control port initializes all data transmission. Each bus performs an action and transmits up to three packets. According to the rules established before transmission, at the beginning of each transmission, the host controller sends a USB packet describing the type, direction, USB device address, and a terminal number of the transmission operation. This packet is usually called a Token packet. The USB device retrieves its own data from the proper location of the decoded packet. Data is transferred either from host to device or from device to host. At the beginning of transmission, the direction of data transmission is indicated by a flag packet, and then the sender begins to send packets containing information or to indicate that there is no data transmission. The receiver also sends a handshake packet to indicate whether the transmission was successful. The transfer of USB data between sender and receiver can be regarded as a channel between host and device ports. There are two types of channels: flows and messages. Unlike message data, stream data does not have the structure defined by USB, and channels are dependent on data bandwidth, type of delivery service, and port characteristics such as direction and buffer size. USB has a special channel, the default control channel, which is a message channel that exists when the device is started and provides an entry point for setting up the device, querying the status of the device, and entering control information.