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 has become increasingly widespread. Although the single-chip microcomputer itself has certain anti-interference ability, the control system composed of the single-chip microcomputer as the core still has the problem of electromagnetic interference in the application. In order to prevent external EMI to the system and ensure the safe and reliable operation of the single-chip control system, corresponding EMS measures must be taken.
In the working environment of the single-chip microcomputer system, there are often many strong current equipment, especially the start of the motor and the pull-in of the relay will cause strong interference to the single-chip microcomputer. If you use an oscilloscope, you can see that there is obvious glitch interference on the power voltage waveform. In addition, limited by conditions, sometimes there must be a long distance between the various parts of the single-chip control system, the data and control lines use longer wires and there is no good shielding measure, which will make electromagnetic interference more likely to be mixed into the system.
In short, the EMI of the single-chip microcomputer system always enters by means of radiation, power circuit, etc. There are three main ways. The first is the input way, which causes the analog signal to be distorted and the digital signal to produce errors. If the system is based on the problematic signal The result of the operation processing will inevitably be wrong. The second is the output path. Interference will be superimposed on each output signal, causing confusion in the output signal, and the true processing result of the system cannot be expressed. The third is the internal bus interference of the single-chip microcomputer. The interference makes the internal digital signals on the control, address, and data buses disorder, causing MCU errors, program runaways, and even crashes.
2 Main research directions of EMS technologyIn view of the causes and ways of interference in the single-chip system, the main research direction of EMS technology focuses 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. Corresponding to these three couplings, electrostatic shielding, magnetic field shielding and electromagnetic shielding can be adopted. The research direction of shielding technology is mainly the shielding effectiveness of various materials such as metal, magnetic, composite materials, such as the shielding effectiveness of various structures such as multi-layer, single-layer, and pores, the shielding effectiveness of shields of various shapes and shields The design 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 relays, isolation transformers or photoelectric isolation devices for isolation. Its characteristic is that the ground system of the two parts of the circuit can be separated, cutting off the possibility of coupling through the ground system.
Filtering is a technique used to cut off noise propagation in the frequency domain. The research direction of filtering technology is to use filter components such as capacitors and inductors to filter out part of the spectrum signals that are not needed, and only retain the required signals. For example, for the power supply filter, only the power supply frequency of 50Hz is reserved to filter out all other high and low frequency electromagnetic noises.
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 neutral wire and various ground wires in the system. The main consideration is how to correctly arrange the digital ground analog ground, the design of the ground body, the impedance of the ground wire at various frequencies, and so on.
The hardware anti-jamming measures create a basically clean working environment for the microcontroller system, but it does not guarantee that there is absolutely no clutter. Therefore, the anti-jamming measures of the programming software should be added. Software anti-jamming technology is when the system is interfered, a specific program in the microcontroller system takes effect, so that the system resets normally, such as a watchdog or when the input signal is interfered, it is filtered through specific programming techniques to remove the fake and save the truth. Programming auxiliary methods such as digital filters. The programming design of this technology is diversified and flexible, which can save a lot of hardware cost and make debugging and operation more convenient.
3 Specific applications of EMS technology3.1 Hardware EMS technology application
(1) Good grounding method
The operating frequency of the single-chip control system is low, and the interference frequency that affects it is mostly below 1MHz. Therefore, it is advisable to use a point of independent grounding, but it should be noted that the length of the ground wire should not exceed 1/20 of the wavelength. One-point grounding has two methods, one-point grounding in series and one-point grounding in parallel. When using one-point grounding in series, 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 level is low The priority is arranged in the place closest to the power source. The relative parallel connection of one point grounding will make the voltage drop of each branch current on the wire not affect each other, will not cause interference, and the effect will be better.
(2) Optical isolation
Using photoelectric isolation devices on the input and output channels for information transmission, which can electrically isolate the single-chip system from various sensors, switches, relays and other mechanisms, just like a PLC, and interfere with most external devices Will be blocked out. However, useful digital signals can be transmitted without problems by photoelectric coupling, while analog signals can be transmitted using linear optocouplers to ensure quality.
(3) Hardware filtering
Connecting some RC low-pass filters when low-frequency signal transmission of the single-chip system is needed can greatly weaken the effect of various high-frequency interference signals. When the single-chip system has high requirements for the power environment, a power filter can be used, and only the power frequency of 50Hz is reserved to filter out all other high and low frequency electromagnetic noises.
(4) Shield
Shielding can play a very good role in the interference caused by various electromagnetic induction. The core system of the single-chip microcomputer is surrounded by a metal shell, and then the metal shell or metal gate can be grounded to guide the electromagnetic interference into 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 single-chip system surrounded by a metal shield, a shielded line should be used, and the shielding layer and the shell should be connected to the system reference point at the same point. Systems with different reference ground points should be shielded separately, and they cannot be co-located in a metal shielding shell.
3.2 Software EMS technology application
(1) Digital filter. The software method is adopted to suppress the noise superimposed on the analog input signal to read the really useful information. The following are several commonly used filtering methods a. Program judgment filtering b. Median filtering c. Arithmetic average filtering d. De-extreme average filtering e. Weighted average filtering f. Moving average filtering
(2) Software interception technology. When the program is disturbed and "runs away", measures are taken to return the program to its normal track. Common anti-jamming techniques include: software interception techniques (software traps, etc.) often use a. NOP instructions to use b. Unused Interrupt area trap c. Unused EPROM space trap d. Program area trap
(3) Program operation monitoring system (watchdog) When the program flies into an endless loop, redundant instructions and software traps are powerless, and the system will be completely paralyzed. To this end, a running monitoring system (watchdog) should be set up in the program, which should have the following characteristics: a. It can work independently and basically does not rely on 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 and reset the system in time.
3.3 Other EMS technology applications
(1) In the long-line transmission of the single-chip microcomputer system, the use of twisted-pair shielded wire as the transmission line can effectively suppress the common mode noise field and electromagnetic field interference. But it should be noted that impedance matching must be performed on the transmission line to avoid reflection and signal distortion.
(2) Attention should be paid to level matching when designing the peripheral circuit of the microcontroller system. For example, TTL "1" level is 2.4~5V, "0" level is 0~0.4V; and CMOS input "1" level is 4.99~5V, and "0" level is 0~0.01V. Therefore, when a CMOS device accepts a TTL output, a level shifter or a pull-up resistor must be added to its input. Otherwise, the CMOS device will be in an uncertain state.
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