Microcomputer Electromagnetic Interference Technology

Research on Electromagnetic Interference Technology of SCM Application System

The single-chip microcomputer system is more and more widely used in industrial applications, and it is an important technical means in product development and production. However, due to the harsh electromagnetic environment in which the system is located, the single-chip microcomputer is often subject to various internal and external interferences when it is working. Work is adversely affected. In order to ensure the reliability and safety of the single-chip microcomputer system, it is necessary to understand the cause of the interference, which is an important issue to solve the interference.

1. MCU system interference categories and analysis

1.1 Elements of electromagnetic interference in single-chip microcomputer system

There are three factors in the electromagnetic interference problem of the single-chip microcomputer system, namely, the electromagnetic interference source, the coupling path, and the sensitive equipment.

Among them, the interference source refers to the components, equipment and signals that generate interference; the coupling path refers to the path and medium from the interference source to the sensitive equipment. The typical coupling way is the conduction through the wire and the radiation in the space; sensitive equipment refers to the disturbed object, such as single-chip microcomputer, amplifier, digital-to-analog converter, etc.

1.2 Classification of electromagnetic interference of single chip microcomputer

Electromagnetic interference in single-chip microcomputer systems is usually divided into several types, which are classified according to the propagation mode, the cause of noise, and the waveform characteristics. According to the propagation mode, it can be divided into series mode noise and common mode noise; 2) According to the cause, it can be divided into high frequency oscillation noise, discharge noise, surge noise; 3) Waveform characteristics can be divided into pulse voltage, continuous sine wave, pulse sequence etc.

The interference source of the single-chip microcomputer system is mainly electromagnetic energy interference. Interference sources are mainly divided into internal interference sources and external interference sources:

(1) The internal interference source mainly comes from the mutual interference between the printed circuit board and the circuit; mainly due to the unreasonable design of the printed circuit board inside the system, incorrect layout of components and grounding, etc., the microcontroller system cannot work normally .

(2) External interference sources are mainly electromagnetic waves and electromagnetic fields. Strong electromagnetic field interference signals will affect the work of the single-chip microcomputer system. Strong external interference signals mainly enter the internal system of the single-chip microcomputer through the power supply. Therefore, the anti-interference method of the power supply is a research focus of our electromagnetic anti-interference.

1.3 The influence of electromagnetic interference on the microcontroller

1) The data acquisition error of the single-chip microcomputer system increases, which makes the RAM data tampered and reduces the reliability of the data.

2) The single-chip microcomputer control system fails, and when the automatic control system is subjected to electromagnetic interference, misoperation and miscontrol and loss of control may occur, which reduces the effectiveness and reliability of the single-chip microcomputer control system.

3) The program runs abnormally, and the interference makes the PC value of the single-chip microcomputer reach the unused address space for meaningless operation, or the program runs in the normal address space, but the interference makes the program jump to the place where it should not go or run Into an endless loop.

2. Electromagnetic Interference Suppression Technology

2.1 Hardware anti-jamming technology

Hardware anti-interference technology is the preferred anti-interference measure in the application and design of single-chip microcomputer systems. It can effectively block the interference propagation path, suppress interference sources, and reasonably arrange and select relevant parameters. Hardware anti-interference measures can suppress most electromagnetic interference. Commonly used The hardware interference is as follows.

1) Printed circuit board design. Whether the design and layout of the printed circuit board are reasonable or not is very important to the reliability of the single-chip microcomputer system. Here is a key step in the generation, propagation and absorption of noise.

From the perspective of reducing radiation interference, multi-layer boards should be used as much as possible. The inner layer is used as the power layer and the ground layer respectively. Form a uniform ground plane for the signal, increase the distributed capacitance between the signal line and the ground plane, and suppress its internal force radiating to space. For multi-layer circuit boards, the ground planes in different areas must meet the 20 H rule at the edge (that is, the edge of the ground plane should be 20 H longer than the edge of the power layer or the signal line layer, and H is the ground plane and the signal layer. between heights).

Power lines, ground lines, and printed circuit board traces should maintain low impedance to high-frequency signals. In the case of high frequency, power lines, ground lines or printed circuit board traces will all become small antennas for receiving and transmitting interference. Reducing this kind of interference is to reduce the high-frequency impedance of the power line, ground wire, and printed circuit board wiring itself, and the arrangement should be appropriate, as short and straight as possible. When there are different functional circuits on the circuit board, the different types of circuits should be separated, and their grounding should also be separated; no signal lines can pass through the cracks on the ground surface.

An independent ground wire is used on the I/O interface to provide a clean place for the filter and shielding layer, and the filter should be as close as possible to the cable inlet and outlet. The ground wire of the high-speed clock should be as short as possible, do not change layers, and the corners should not be 90°, and keep as far away from the I/O port as possible; the heat sink installed on the chip should be connected to the signal ground; the driving circuit of the I/O interface should be close.

2) Select components. Components are the foundation of the system and also an important link in the control system. Only by selecting components reasonably can the reliability and stability of the entire system be improved. Therefore, components with high integration, strong anti-interference ability, and low power consumption should be selected. electronic devices.

3) Grounding technology. Signal grounds are usually divided into single-point grounding, multi-point grounding, and hybrid grounding. When the signal frequency is less than 1MHz, use single-point grounding; when the operating frequency is 1 MHz to 10 MHz, when using single-point grounding, the length of the ground wire must not exceed 1/20 of the wavelength, otherwise multi-point grounding is used; the signal operating frequency is greater than 10 MHz, multi-point grounding should be used in order to reduce ground impedance. The grounding point of the multi-level circuit should be selected at the input end of the low-level circuit, so that this end is closest to the reference position, the grounding of the input stage is shortened, and the possibility of electromagnetic interference is reduced.

4) Isolation technology. Through isolation, external interference can be cut off, and at the same time play the role of restraining drift and safety protection. Generally divided into physical isolation and optical isolation. Physical isolation is generally aimed at the input signal of the front-end of the single-chip microcomputer, which is divided into two parts in industrial measurement. (1) The display and control part is called the secondary instrument; (2) the sensor is called the primary instrument. Sometimes the distance between the primary instrument and the secondary instrument is far away, and the signal is easily disturbed during the transmission process, and its signal line is much larger than the power line; photoelectric isolation is to isolate the electrical signals of the two circuit systems, and transmit the signal through the optocoupler. It can not only ensure the transmission of noisy signals from one circuit to another, but also ensure the correct transmission of signals. In DC and low-frequency systems, optoelectronic coupling is mostly used for isolation. Because light is used as a medium for indirect coupling, it has high electrical isolation and interference suppression capabilities.

2.2 Software anti-jamming technology

Although the hardware anti-jamming technology is adopted, it is difficult to ensure that the single-chip microcomputer system will not be disturbed due to the complexity of the reasons for the interference and the large randomness, so the software anti-jamming technology is used to supplement it.

1) Instruction redundancy technology. When the MCU is strongly disturbed, the value of the program counter PC will change, the program will deviate from the normal running track, there will be random flying, the value of the operation will change, and the operand will be wrongly put into the operation code. Instruction redundancy is similar to software traps, but there are differences. Software traps are used in unused areas of program memory, while instruction redundancy is usually in programs. The practice is to insert some NOP instructions after normal instructions or put valid bytes Rewriting, the use of instruction redundancy technology can not only make the runaway program regular, but also help to eliminate random interference and improve reliability.

2) Software trap technology. Software trap refers to a series of instructions that can restore the runaway program to normal operation or return to the initial state. Usually, the NOP empty instruction is used as a software trap. When the program is out of control, as long as the PC points to these units, after performing several empty operations in succession, The program will execute the following normal program or be forced to jump to a specified location and automatically return to normal. When the program is working normally, the software trap will not affect the normal operation of the system.

3) Digital filtering technology. Digital filtering is realized by program, and it can be shared by multiple channels without adding hardware equipment. It has high reliability and high stability, and can filter low-frequency signals. The analog RC filter is limited by the capacitance and the frequency cannot be too low, and the flexibility is good, and different filtering methods can be changed by changing the program.

4) Watchdog. Sometimes the single-chip microcomputer will execute the program out of order when it accepts electromagnetic interference. If the program enters an endless loop, the single-chip microcomputer will crash. The way to solve this problem is to add a watchdog to the system.

Add the following program in each program segment:

Among them, YS is the delay subroutine, WATCHDOG is the watchdog subroutine, the counting value (counting time) of the counter should be greater than the cycle time of the main program once, otherwise the system will always reset and cannot run normally. During normal operation, every time the program runs LCALL WATCHDOG, a pulse is sent from P2.7 to clear the counter, and its Qn output terminal is always at low level, which will not reset the system. Once the system is disturbed by electromagnetic interference, the program enters an endless loop. If the program cannot be executed normally, the pulse cannot be sent from P2.7, which will make the counter full and Qn be covered with "1" and send it to the RESET pin of 8051 through the 47uF capacitor to reset the system and execute the main program again.

Summarize

In the design of single-chip application system, as long as the hardware composition of the system is carefully analyzed, the components are carefully selected, the interference sources and sensitive components are confirmed, and the reasonable layout of them can improve the stability of the system. Practice has proved that only by taking measures from two aspects of hardware and software, with hardware anti-interference technology as the main and software anti-interference technology as a supplement, the combination of the two can effectively eliminate the influence of interference signals and improve system stability.

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