EMC electromagnetic compatibility includes EMI (interference) and EMS (suscepTIbility), which is electromagnetic interference and electromagnetic interference. With the development of intelligent technology, the application of single-chip microcomputers has become increasingly widespread. Although the MCU itself has a certain anti-interference ability, the control system with the MCU as the core has the problem of electromagnetic interference in the application. In order to prevent external EMI to the system and ensure that the MCU control system operates safely and reliably, corresponding EMS measures must be taken.
1 Analysis of the causes of EMI
In the working environment of the single-chip system, there are often many strong electrical equipment, especially the motor starting and the relay's pull-in will have strong interference to the single-chip microcomputer. If you use the oscilloscope, you can see that there is obvious burr interference on the power supply voltage waveform. In addition, subject to conditions, sometimes there is a long distance between the various parts of the MCU control system. The data and control lines use long wires and there is no good shielding measures, which makes electromagnetic interference easier to mix into the system.
In short, the EMI of the MCU system always enters in the form of radiation, power circuit, etc. There are three main ways. The first is the input path, which makes the analog signal appear distorted, the digital signal generates an error, and the system performs according to the problematic signal. The result of the arithmetic processing 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 confusing and the actual processing result of the system cannot be expressed. The third is the internal bus interference of the MCU. The interference makes the internal digital signals on the control, address and data bus confusing, causing the MCU to go wrong, the program runs away, and even crashes.
2 The main research direction of EMS technology
In view of the causes and ways of interference in the MCU system, 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 electromagnetic noise propagation path formed by electrostatic coupling, inductive coupling or alternating electromagnetic field coupling. Electrostatic shielding, magnetic field shielding and electromagnetic shielding can be adopted corresponding to the three couplings respectively. The research direction of shielding technology is mainly the shielding effectiveness of various materials such as metal, magnetic, composite materials, such as shielding effectiveness of various structures such as multilayer, single layer, and pores, shielding effectiveness of shielding bodies of various shapes, and shielding bodies. The design and the relationship between shielding and grounding.
Isolation is the propagation path for cutting off electromagnetic noise in the form of conduction. The research direction of isolation technology is mainly isolated by using straight AC relays, isolation transformers or opto-isolated devices. It is characterized by dividing the grounding system of the two-part circuit and cutting off the possibility of coupling through the grounding system.
Filtering is a technique used to cut off noise propagation in the frequency domain. The research direction of the filtering technology is to filter a part of the spectrum signal that is not needed by using a filter element such as a capacitor inductor, and only retain the required signal. For example, for the power supply filter, only the power frequency of the power frequency of 50 Hz is reserved, and all other high and low frequency electromagnetic noises are filtered out.
Grounding is a common path that provides useful signals and electromagnetic noise. The research direction of grounding technology is the grounding method of safety ground, signal ground, power supply neutral line and various ground wires in the system. The main consideration is how to correctly arrange the digital ground analog ground, the design of the grounding body, the impedance of the ground wire at various frequencies, and so on.
The hardware anti-interference measures create a basic clean working environment for the single-chip system, but there is no guarantee that there will be absolutely no clutter, so the anti-interference measures of the programming software should be added. The software anti-jamming technology is a special program in the MCU system after the system is disturbed, so that the system resets normally, such as a watchdog or a specific programming technique to filter the input signal when it is disturbed. Programming aids such as digital filters. This technology programming design is flexible and flexible, which can save a lot of hardware costs and make debugging more convenient.
3 Specific application of EMS technology
3.1 Hardware EMS Technology Application
(1) Good grounding method
The operating frequency of the MCU control system is relatively low, and the interference frequency acting on it is mostly below 1MHz. Therefore, it is advisable to use a single independent grounding, but it should be noted that the grounding length should not exceed 1/20 of the wavelength. One-point grounding method has two methods: one-point grounding in series and one-point grounding in parallel. When using one-point grounding in series, in order to prevent interference, the grounding line between each branch should be as short as possible, and the wire diameter should be thick enough, especially the level is low. The priority is placed near the power source. The relative parallel grounding will make the voltage drop generated by each branch current on the wire not affect each other, and will not form interference, and the effect is better.
(2) Photoelectric isolation
The use of opto-isolated devices for information transmission on the input and output channels can electrically isolate the MCU system from various sensors, switches, relays, etc., just like PLCs, which interfere with most external devices. Both will be blocked. The useful various types of digital signals are transmitted by means of optocoupler without problems, and the analog signals can be transmitted using linear optocouplers to ensure quality.
(3) hardware filtering
When some low-frequency signal transmission of the single-chip microcomputer system is required, some RC low-pass filters are connected, which can greatly weaken the effects of various high-frequency interference signals. When the single-chip system has high requirements on the power supply environment, the power supply filter can be used to retain only the power frequency of the power frequency of 50 Hz, and filter out all other high and low frequency electromagnetic noise.
(4) Shielding
Shielding can play a good role in the interference caused by various electromagnetic inductions. The core system of the single-chip microcomputer is surrounded by a metal shell, and then the metal casing or the metal gate is grounded to introduce electromagnetic interference into the earth, thereby removing interference. The grounding point of the shielded enclosure should be connected to the grounding point of the system signal reference ground. If the signal line is drawn from the single-chip microcomputer system surrounded by the metal shield, the shielded wire should be used, and the shielding layer and the outer casing should be connected to the system reference point at the same point. Special systems with different reference grounding points should be shielded separately and should not be co-located in a metal shield.
3.2 Software EMS Technology Application
(1) Digital filter. Software is used to suppress noise superimposed on the analog input signal to read really useful information. The following are several common filtering methods a. Program judgment filter b. Median filter c. Arithmetic average filter d. Go to extreme value average filter e. Weighted average filter f. Moving average filter
(2) Software interception technology. In the case that the program is subject to interference "running", take measures to return the program to the normal track. Common anti-jamming technologies are: software interception technology (software traps, etc.) often use a.NOP instruction to use b. unused Interrupt zone trap c. Unused EPROM space trap d. Program area trap
(3) Program operation monitoring system (watchdog) When the program fly into an infinite loop, redundant instructions and software traps are powerless, and the system will be completely paralyzed. A running watch system (watchdog) should be set up in this program, which should have the following characteristics: a. It can work independently and basically does not depend on the MCU. b. The MCU interacts with the system for a fixed period of time, indicating that it is normal. c. When the MCU enters an infinite loop, it can detect and reset the system in time.
3.3 Other EMS technology applications
(1) In the long-distance transmission of the single-chip system, the use of twisted-pair shielded wires as the transmission line can effectively suppress the common mode noise field and electromagnetic field interference. However, it should be noted that the transmission line must be impedance matched to avoid reflection and distortion of the signal.
(2) Pay attention to level matching when designing the peripheral circuit of the single chip system. For example, the TTL "1" level is 2.4~5 volts, the "0" level is 0~0.4 volts; 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 a TTL output, a level shifter or pull-up resistor is applied to its input. Otherwise, the CMOS device is in an indeterminate state.
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