Application of EMC Technology in Single Chip Microcomputer System

EMC electromagnetic compatibility includes EMI (interference) and EMS (susceptibility), that is, electromagnetic interference and electromagnetic anti-interference. With the development of intelligent technology, the application of single-chip microcomputers is becoming more and more extensive. Although the single-chip microcomputer itself has a certain anti-interference ability, the control system composed of single-chip microcomputer as the core still has the problem of electromagnetic interference in the application. In order to prevent the external EMI of the system and ensure the safe and reliable operation of the single-chip microcomputer control system, corresponding EMS measures must be taken.

1 Analysis of the causes of EMI

In the working environment of the single-chip microcomputer system, there are often many strong electrical devices, especially the motor start-up and the relay pull-in will cause strong interference to the single-chip microcomputer. If you use an oscilloscope, you can see obvious burr interference on the power supply voltage waveform. In addition, due to conditions, sometimes the various parts of the single-chip microcomputer control system must be far apart, and the data and control lines use longer wires without good shielding measures, which makes it easier for electromagnetic interference to mix into the system.

In short, EMI always enters the MCU system in the form of radiation, power supply circuit, etc. There are three main ways. The first is the input path, which causes the analog signal to be distorted and the digital signal to produce errors. If the system performs operations based on the problematic signal, the result will inevitably be wrong. The second is the output path. The interference will be superimposed on each output signal, causing the output signal to be chaotic and unable to express the real processing result of the system. The third is the interference of the internal bus of the MCU. The interference makes the internal digital signals on the control, address, and data buses chaotic, causing the MCU to make mistakes, the program to run away, and even crash.

2 Main research directions of EMS technology

In view of the causes and ways of interference in single-chip microcomputer systems, the main research directions of EMS technology focus on hardware shielding, isolation, filtering, grounding and software programming.

Shielding is mainly used to cut off the propagation path of electromagnetic noise formed by electrostatic coupling, inductive coupling or alternating electromagnetic field coupling. Electrostatic shielding, magnetic field shielding and electromagnetic shielding can be used to correspond to these three types of coupling. The research direction of shielding technology is mainly the shielding effectiveness of various materials such as metal, magnetic, composite materials, such as multi-layer, single-layer, pore and other structures, the shielding effectiveness of shielding bodies of various shapes, the design of shielding bodies, and the relationship between shielding and grounding.

Isolation is used to cut off the propagation path of electromagnetic noise in the form of conduction. The research direction of isolation technology mainly uses DC/AC relays, isolation transformers or photoelectric isolation devices for isolation. Its characteristic is that it can separate the ground wire systems of two circuits and cut off the possibility of coupling through the ground wire system.

Filtering is a technology used to cut off noise propagation in the frequency domain. The research direction of filtering technology is to use filter devices such as capacitors and inductors to filter out unnecessary part of the spectrum signal and only retain the required signal. For example, for the power filter, only the power frequency of 50Hz is retained and all other high and low frequency electromagnetic noises are filtered out.

Grounding is a common path for useful signals and electromagnetic noise. The research direction of grounding technology is the grounding methods of safety ground, signal ground, power neutral line and various ground lines in the system. The main considerations are how to correctly arrange digital and analog ground, the design of the grounding body, the impedance of the ground line at various frequencies, etc.

Hardware anti-interference measures create a basically clean working environment for the MCU system, but they cannot guarantee that there is absolutely no noise, so programming software anti-interference measures should be added. Software anti-interference technology is that when the system is interfered with, a specific program inside the MCU system plays a role, so that the system resets and operates normally, such as a watchdog, or when the input signal is interfered with, a specific programming technique is used to filter out the false and retain the true, such as a digital filter. This technology has a variety of flexible programming designs, can save a lot of hardware costs, and is more convenient for debugging operations.

3 Specific applications of EMS technology

3.1 Hardware EMS Technology Application

(1) Good grounding method

The operating frequency of the single-chip control system is low, and the interference frequency acting on it is mostly below 1MHz, so it is advisable to use a single-point independent grounding, but it should be noted that the length of the ground wire should not exceed 1/20 of the wavelength. There are two types of single-point grounding: series single-point grounding and parallel single-point grounding. When using series single-point grounding, in order to prevent interference, the ground wire between each branch should be as short as possible, and the wire diameter should be thick enough. Especially the low-level one should be arranged at the place closest to the power supply. The parallel single-point grounding will make the voltage drop generated by the current of each branch on the wire not affect each other, will not cause interference, and the effect is better.

(2) Photoelectric Isolation

Using optoelectronic isolation devices on input and output channels to transmit information can electrically isolate the microcontroller system from various sensors, switches, relays and other mechanisms, just like PLC, blocking most external device interference. Useful digital signals can be transmitted without problems using optoelectronic coupling, and analog signals can be transmitted using linear optocouplers to ensure quality.

(3) Hardware filtering

When the microcontroller system needs to transmit low-frequency signals, some RC low-pass filters can be connected to greatly weaken the effects of various high-frequency interference signals. When the microcontroller system has high requirements for the power supply environment, a power supply filter can be used to only retain the power supply frequency of 50Hz and filter out all other high and low frequency electromagnetic noise.

(4) Shielding

Shielding can play a very good role in the interference caused by various electromagnetic induction. Surrounding the core system of the microcontroller with a metal shell, and then grounding the metal shell or metal gate can lead the electromagnetic interference to the earth, thereby removing the interference. The grounding point of the shielding shell should be connected to the system signal reference ground point. If there is a signal line drawn from the microcontroller system surrounded by the metal shield, a shielded line should be used, and its shielding layer and shell should be connected to the system reference point at the same point. Special systems with different reference grounding points should be shielded separately, and they cannot be placed in the same metal shielding shell.

3.2 Software EMS Technology Application

(1) Digital filter. Use software to suppress the noise superimposed on the analog input signal in order to read truly useful information. The following are some commonly used filtering methods: a. Program judgment filter b. Median filter c. Arithmetic mean filter d. De-extreme average filter e. Weighted average filter f. Sliding average filter

(2) Software interception technology. When a program is disturbed and "runs away", measures are taken to get the program back on track. Common anti-interference technologies include: Software interception technology (software traps, etc.) is often used a. NOP instruction use b. Unused interrupt area trap c. Unused EPROM space trap d. Program area trap

(3) Program operation monitoring system (watchdog) When the program jumps into an infinite loop, redundant instructions and software traps are powerless and the system will be completely paralyzed. For this reason, a program should have an operation monitoring system (watchdog), which should have the following characteristics: a. It can work independently and basically does not rely on the MCU. b. The MCU interacts with the system once within a fixed period of time, indicating that it is currently normal. c. When the MCU enters an infinite loop, it can detect it in time and reset the system.

3.3 Other EMS technology applications

(1) In the long-distance transmission of the single-chip microcomputer system, the use of twisted-pair shielded cables as transmission lines can effectively suppress common-mode noise and electromagnetic interference. However, it should be noted that impedance matching of the transmission lines is necessary to avoid reflection and signal distortion.

(2) When designing the peripheral circuits of a single-chip microcomputer system, attention should be paid to level matching. For example, the TTL "1" level is 2.4~5 volts, and the "0" level is 0~0.4 volts; while the CMOS input "1" level is 4.99~5 volts, and the "0" level is 0~0.01 volts. Therefore, when a CMOS device accepts TTL output, a level converter or pull-up resistor must be added to its input end, otherwise the CMOS device will be in an uncertain state.

(3) When expanding the microcontroller, its driving capacity should not be exceeded, otherwise the entire system will not work properly. If overload driving is required, a bus driver should be added, such as 74LS244, 74LS245, etc.

(4) Unused input terminals of CMOS circuits are not allowed to float. Otherwise, the logic level will be abnormal and the circuit will be susceptible to external interference and malfunction. When designing, the redundant input terminals can be connected to the positive power supply or ground according to the actual situation.