A Preliminary Study on Bus Technology

0Introduction

If there is a single-plank bridge that can only accommodate one person, and people at both ends want to cross the bridge, in order to avoid crowding and congestion, we have to take effective measures. For example, we can stipulate that the person at one end can cross the bridge at a certain time, and the person at the other end can wait for the arrival of his time period to cross the bridge. At the same time, we can also stipulate that when there are many people, they must cross the bridge in the order of first come first served or age. In this inadvertently, we have experienced the most primitive idea of ​​modern electronic information data transmission through the bus according to the time-sharing system.

With the development of modern network information, especially in terms of cost and space, bus transmission replacing point-to-point transmission is the current hot spot. Its emergence will provide the greatest convenience and the most effective technical solution for information transmission. If a microprocessor and its components and peripherals are connected and communicated using point-to-point lines, all the connections will be complicated and even difficult to implement.

At present, a series of activities related to our daily life all involve the application of bus technology, such as surfing the Internet, calling relatives and friends, using USB flash drives to store information, etc. Although the popular buses take different forms, their main principled ideas are nothing more than time division system, frequency division system, phase division system and code division system. As the saying goes, "When the enemy comes, the general will stop him; when the water comes, the earth will flood him." Faced with a wide variety of buses, we can only master and flexibly use the various bus technologies that are currently or will be used, only by starting from the basic principles and understanding its essence from the bottom of our hearts, and not being confused by its diverse appearance.

1 Definition and classification of bus

1.1 Definition

The English word "bus" is "BUS", which is the same as "public car" in Chinese. This is a very vivid example. For example, the route of a public bus is fixed, and anyone can take the bus to any stop on the route. If we compare ourselves to electronic signals, this is the real reason why it is called "BUS" instead of "CAR" in English. Of course, from a professional point of view, a bus is a structural form that describes the transmission line of electronic signals. It is a collection of signal lines and a public channel for transmitting information between subsystems [1]. The bus can enable information transmission, exchange, sharing and logical control between various components in the entire system. For example, in a computer system, it is a public channel for the CPU, memory, input and output devices to transmit information. The various components of the host are connected through the host, and the external devices are connected to the bus through the corresponding interface circuits.

1.2 Classification

There are many ways to classify buses, such as external and internal buses, system buses and non-system buses, etc. The following are some of the most commonly used classification methods [2].

1.2.1 By function

The most common way to divide the data bus is by function, which can be divided into the address bus, data bus and control bus. In some systems, the data bus and address bus can be shared under the control of the address latch, that is, multiplexed.

The address bus is used to transmit addresses. In the design process, the most common method is to select the storage address of the external memory from the CPU address bus. The number of bits of the address bus often determines the size of the memory storage space. For example, if the address bus is 16 bits, the maximum storage space is 216 (64KB).

The data bus is used to transmit data information. It can be divided into unidirectional transmission and bidirectional transmission data bus. The bidirectional transmission data bus usually adopts a bidirectional three-state bus. The number of bits of the data bus is usually consistent with the word length of the microprocessor. For example, the word length of the Intel 8086 microprocessor is 16 bits, and its data bus width is also 16 bits. In actual work, the data transmitted on the data bus is not necessarily the complete data.

The control bus is used to transmit control signals and timing signals. For example, when the microprocessor operates the external memory, it must first send read/write signals, chip select signals, and read interrupt response signals through the control bus. The control bus is generally bidirectional, and its transmission direction is determined by the specific control signal, and its number of bits is also determined according to the actual control needs of the system.

1.2.2 By transmission method

According to the data transmission method, buses can be divided into serial buses and parallel buses. In principle, parallel transmission is better than serial transmission, but its cost will increase. In layman's terms, the parallel transmission path is like a multi-lane highway, while serial transmission only allows one car to pass through a single-lane highway. Currently, common serial buses include SPI, I2C, USB, IEEE1394, RS232, CAN, etc.; while parallel buses are relatively less in variety, and common ones include IEEE1284, ISA, PCI, etc.

1.2.3 Classification by clock signal

According to whether the clock signal is independent, it can be divided into synchronous bus and asynchronous bus. The clock signal of synchronous bus is independent of data, that is, a separate line is used as the clock signal line; while the clock signal of asynchronous bus is extracted from data, and the edge of data signal is usually used as clock synchronization signal.

2 Basic principles of bus transmission

According to the above definition of bus, the basic function of bus is to transmit signals. In order to effectively and timely transmit the information of each subsystem, to prevent the signals from interfering with each other and to avoid overcrowding in physical space, the best way is to use multiplexing technology [3]. In other words, the basic principle of bus transmission is multiplexing technology. Multiplexing refers to a mechanism in which multiple users share a common channel. Currently, the most common ones are time division multiplexing, frequency division multiplexing and code division multiplexing.

2.1 Time Division Multiple Access (TDMA)

Time division multiplexing is to divide the channel into multiple time periods according to time. Signals from different sources will require responses in different time periods, and the transmission times of each other's signals will not overlap on the time coordinate axis.

2.2 Frequency Division Multiplexing (FDMA)

Frequency division multiplexing is to divide the available frequency band of the channel into several non-overlapping frequency bands, and the spectrum of each signal after frequency modulation occupies one of the frequency bands, so as to achieve the transmission of multiple signals of different frequencies in the same channel. When the receiving end receives the signal, it will use appropriate bandpass filters and frequency demodulators to restore the original signal.

2.3 Code Division Multiple Access (CDMA)

Code division multiplexing means that each transmitted signal will have its own specific identification code or address code. The receiving end will distinguish the transmission information on the public channel based on different identification codes or address codes. The transmission information will only be received when the identification codes or address codes are completely consistent.

3 Bus communication protocol

For learning about the bus, understanding its communication protocol is the most critical step in the whole process. All materials introducing bus technology will spend a lot of space to describe its protocol, especially the seven-layer definition of ISO/OSI. In fact, to understand the protocol of a bus, the most important thing is to understand the characteristics and meaning of each bit of the bus frame data. The transmission and reception of valid data between each node of the bus is achieved through the judgment and confirmation of each node on the frame data bit or segment.

Figure 1

Figure 1 shows a data frame of one byte of data transmitted on a common I2C bus. The bus is a two-wire serial bus consisting of a data line SDA and a clock SCL. The circuit module connected to the bus can act as both a transmitter (host) and a receiver (slave). In addition to the control code (including the slave identification code and access address code) and the data code, the frame data also includes a start signal, an end signal, and a response signal [4].

Start signal: When SCL is at a high level, SDA jumps from a high level to a low level and starts transmitting data.

Control code: used to select the operation target and object, that is, to connect the circuit to be controlled and determine the type and object of control. During the reading period, that is, when the SCL clock line is at the clock pulse high level, the data bit on SDA will not jump.

Data code: It is the specific useful data (such as contrast, brightness, etc.) and information sent by the host to the slave. During reading, the data bit on SDA will not jump.

Acknowledge signal: After receiving 8 bits of data, the receiver sends a specific low level to the sender. The read/write direction is exactly opposite to other data bits, that is, the slave writes the low level and the host reads the low level.

End signal: When SCL is at a high level, SDA jumps from a low level to a high level to indicate the end of data frame transmission.

Of course, the definition of data bits or segments of different buses is definitely different, but based on the same principle, you can quickly understand the characteristics and features of its protocol. Although the size of its information frame is different, a specific data bit or data segment is similar to the I2C bus mentioned in this article, and its meaning and function will be defined according to the requirements of its protocol.

4 Main technical indicators

The main technical indicators for evaluating a bus are the bus bandwidth (i.e., transmission rate), data bit width (bit width), operating frequency, and reliability and stability of transmitted data.

4.1 Bandwidth (transmission rate), bit width and operating frequency

The bandwidth of the bus refers to the amount of data transmitted on the bus per unit time, that is, the maximum data transmission rate of MB per note. The bit width of the bus refers to the number of bits of binary data that the bus can transmit simultaneously, or the number of bits of the data bus, that is, the concept of bus width such as 32 bits and 64 bits; the wider the bit width of the bus, the greater the data transmission rate, and the wider the bandwidth of the bus. The operating clock frequency of the bus is in MHz, which is related to the transmission medium, the amplitude of the signal and the transmission distance. Under the same hardware conditions, the frequency when we use differential signal transmission is often much higher than that of unilateral signal, because the amplitude of the differential signal is only half of that of the unilateral signal.

The bandwidth, bit width and operating frequency of the bus are closely related. The relationship between them is:

4.2 Reliability of Transmitted Data

Reliability is the most critical parameter for evaluating the bus. Without reliability, the transmitted data is all wrong information, and the bus loses its practical significance. In order to improve the reliability of the bus, the following measures are usually adopted:

The transmitter monitors the bus before sending the data frame. The data frame can be sent to the bus only when it detects that the bus is in an idle state. This avoids data conflicts between different nodes.

The twisted pair differential signal is used to transmit data to reduce the voltage rise and fall of a single line and reduce the high-order harmonics generated by the signal edge.

It is appropriate to make the edge of the data have a certain slope.

Add matching resistors and capacitors to reduce the transmission of signals on the bus and balance the distributed capacitance on the bus.

Use appropriate network topology and shielding technology to reduce interference from other signals.

Another way is to improve the reliability of data transmission through software measures such as digital filtering, data verification and error correction.

5 Conclusion

Learning is a gradual process, and the learning and understanding of bus technology is also a process of continuous updating as the technology continues to develop. Confucius said, "If you want to do your work well, you must first sharpen your tools." Only by starting from the most basic principles and laying a solid foundation can you integrate and apply them in future learning, continue to make progress, and further deepen the knowledge points and broaden your knowledge.