Preliminary Study on Bus Technology Learning

Preliminary Study on Bus Technology Learning

Abstract: Describes the basic idea of ​​bus technology with vivid metaphors, points out the basic classification of bus and the basic principles of bus transmission, and the most basic knowledge that should be mastered in the learning process, which can throw a brick for beginners The role of introduction.

Keywords: bus technology, basic knowledge, study and discussion

Chinese graphic number: G714 Document identification code: A

0 Preface

If a single-wood bridge that can only be visited by one person, people at both ends want to cross the bridge, in order not to be crowded or blocked, then we have to take effective measures. For example, stipulate which side of the bridge a person crosses at a certain time, and the person at the other side waits for the time period when the other side crosses the bridge. At the same time, it can also stipulate that people should cross the bridge in the order of first-come-first-served or older. Inadvertently, we realized the most primitive idea of ​​the transmission of modern electronic information data through the bus in a time-division system.

The development of modern network information, especially in terms of cost and space, the replacement of point-to-point transmission by bus transmission is the hotspot of current development. Its emergence will provide the greatest convenience and the most effective technical solution for information transmission. If a microprocessor and its components and peripheral devices are connected by point-to-point lines for communication, all the connections will be complicated and even difficult to achieve.

At present, a series of activities related to our living habits all involve the application of bus technology, such as our Internet access, calling relatives and friends, and using U disk to store information. Although popular buses take different forms, their main principle ideas are nothing more than time division systems, frequency division systems, phase division systems, and code division systems. As the saying goes, "The soldiers will stop, the water will flood the soil." Facing a wide variety of buses, we can only learn from the basic principles and understand its essence from the bones, and not be fooled by its various forms of appearance, in order to master and use it flexibly. Various bus technologies that are currently or will be used.

1 Definition and classification of bus

1.1 Definition

The bus is called "BUS" in English, which is our Chinese "public bus". This is very vivid. For example, the route taken by the public bus is certain. Any one of us can take the public bus to any of the public bus routes. A site. If we compare ourselves to electronic signals, this is why the English language calls it "BUS" rather than "CAR". Of course, from a professional perspective, a bus is a structural form that describes an electronic signal transmission line, a collection of signal lines, and a common channel for transmitting information between subsystems [1]. Through the bus, the information between the components in the entire system can be transmitted, exchanged, shared, and logically controlled. For example, in a computer system, it is a common channel for the CPU, memory, input, and output devices to transfer information. Various components of the host are connected through the host, and external devices are connected to the bus through the corresponding interface circuit.

1.2 Classification

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

1.2.1 By function

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

The address bus is specifically used to transfer addresses. In the design process, the most common thing is to choose the storage address of the external memory from the CPU address bus. The number of bits on 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 transfer data information, and it has a one-way transmission and a two-way transmission data bus. The two-way transmission data bus usually uses a two-way three-state bus. The number of bits on the data bus usually corresponds to the word length of the microprocessing. For example, the Intel 8086 microprocessor has a word length of 16 bits, and its data bus width is also 16 bits. In actual work, what is transmitted on the data bus is not necessarily data in a complete sense.

The control bus is used to transfer 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, its transmission direction is determined by specific control signals, and its number of bits also depends on the actual control needs of the system.

1.2.2 According to transmission mode

According to the way of data transmission, the bus can be divided into serial bus and parallel bus. From the principle point of view, the parallel transmission method is actually better than the serial transmission method, but its cost will increase. In layman's terms, the path of parallel transmission is like a multi-lane highway, while serial transmission allows only one car to pass through a single-lane highway. At present, the common serial buses are SPI, I2C, USB, IEEE1394, RS232, CAN, etc .; and the parallel buses are relatively few in types, such as IEEE1284, ISA, PCI, etc.

1.2.3 According to the clock signal

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

2 Basic principles of bus transmission

According to the previous definition of the bus, the basic function of the bus is to transmit signals. In order to effectively and timely transmit the information of each subsystem, in order not to interfere with each other's signals and avoid overcrowding in the physical space, its most A good way is to use multiplexing technology [3], which means that the basic principle of bus transmission is multiplexing technology. The so-called multiplexing refers to a mechanism for multiple users to share a common channel. Currently, the most common ones are mainly time division multiplexing, frequency division multiplexing, and code division multiplexing.

2.1 Time Division Multiplexing (TDMA)

Time-division multiplexing divides the channel into multiple time periods according to time. Signals from different sources will require responses in different time periods. The transmission time of each other's signals will not overlap on the time 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 frequency modulated spectrum of each signal occupies one of the frequency bands, so as to realize the transmission of multiple channels of different frequency signals in the same channel . When the receiving end receives the signal, it will use an appropriate band-pass filter and frequency demodulator to restore the original signal.

2.3 Code Division Multiplexing (CDMA)

Code division multiplexing means that the transmitted signal will have its own specific identification code or address code. The receiving end will distinguish the transmission information on the common channel according to different identification codes or address codes. Only the identification code or address code The transmission information will be received only if they are completely consistent.

3 Bus communication protocol

For the study of the bus, understanding its communication protocol is the most critical step in the entire process. All the materials that introduce bus technology will spend a lot of space to describe its protocol, especially the seven-layer definition of ISO / OSI. In fact, to understand a bus protocol, the most important thing is to understand the characteristics and significance of each bit of the frame data of the bus. The transmission and reception of valid data between each node of the bus is through the judgment of each node on the frame data bit or segment And confidence to achieve.

figure 1

As shown in Figure 1 is a common I2C bus data frame of one byte of data transmission, the bus form is a two-wire serial bus composed of data line SDA and clock SCL, and the circuit module connected to the bus is Can be used as transmitter (master) and receiver (slave). In addition to control codes (including slave identification codes and access address codes) and data codes, frame data also includes a start signal, an end signal, and an answer signal [4].

Start signal: When SCL is high, SDA transitions from high to low, and data transfer begins.

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

Data code: It is the specific useful data (such as contrast, brightness, etc.) and information sent from the master to the slave. During reading, the data bits on SDA will not toggle.

Response signal: After receiving 8bit data, the receiver sends a specific low level to the sender. The direction of reading / writing is exactly opposite to other data bits, that is, the low level is written by the slave, and the low level is read by the master.

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

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

4 Main technical indicators

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

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

The bandwidth of the bus refers to the amount of data transferred on the bus per unit time, that is, the maximum data transfer rate of MB per banknote. The bit width of the bus refers to the number of bits of binary data that the bus can transmit at the same time, or the number of bits of the data bus, that is, the concept of bus widths such as 32-bit and 64-bit; the wider the bit width of the bus, the greater the data transmission rate. The wider the bus bandwidth. The working 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 of differential signal transmission is often much higher than that of the single-sided signal, because the amplitude of the differential signal is only half of the single-sided signal.

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

4.2 Reliability of transmitted data

Reliability is the most critical parameter to evaluate the bus. Without reliability, the transmitted data are all wrong information, and the actual meaning of the bus is lost. In order to improve the reliability of the bus, the measures usually adopted are:

Before sending data frames, the transmitter listens to the bus. Only when the bus is in the idle state can the data frame be transmitted to the bus, thus avoiding data conflicts between different nodes.

Use twisted pair differential signals to transmit data to reduce the voltage rise and fall of a single line and reduce the higher harmonics generated by the edges of the signal.

Appropriately let the data edge have a certain slope.

Increase the matching resistance and capacitance to reduce the signal transmission on the bus and balance the distributed capacitance on the bus.

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

There is also the use of digital filtering, data verification and error correction in software to improve the reliability of data transmission.

5 Conclusion

Learning is a gradual process, and learning and understanding of bus technology is also a process of continuous updating as its technology continues to develop. Zi said, "If you want to do good work, you must sharpen your weapon first." Only by starting from the most basic principles and laying a good foundation will you be able to integrate in the future study, and succeed in the future, further deepen the knowledge point and broaden the knowledge

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