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What this article shares to you is about how to solve the problem of single-chip microcomputer EMC, the editor thinks it is very practical, so I share it with you to learn. I hope you can get something after reading this article.

The editor quietly tells you how to solve the problem of electromagnetic compatibility from the PCB design of single-chip microcomputer to software processing.

1. Factors affecting EMC

1. Voltage: the higher the power supply voltage, the greater the voltage amplitude, the more emission, and the lower power supply voltage affects the sensitivity.

2. Frequency: high frequency produces more emission, periodic signal produces more transmission. In the high-frequency single-chip microcomputer system, the current spike signal is generated when the device is switched on, and in the simulation system, the current spike signal is generated when the load current changes.

3. Grounding: in all EMC problems, the main problem is caused by inappropriate grounding. There are three signal grounding methods: single point, multipoint, and hybrid. When the frequency is lower than 1MHz, the single-point grounding method can be used, but it is not suitable for high frequency; in high-frequency applications, it is best to use multi-point grounding. Hybrid grounding is a method of single-point grounding in low frequency and multi-point grounding in high frequency. Ground wire layout is the key, and the grounding circuits of high-frequency digital circuits and low-level analog circuits should not be mixed.

4. PCB design: proper wiring of printed circuit boards (PCB) is very important to prevent EMI.

5. Power supply coupling: when the device is switched, transient currents will be generated on the power line, and these transient currents must be attenuated and filtered. Transient currents from high di/dt sources cause ground and stitch "emission" voltages, and high di/dt generates a wide range of high frequency currents that excite components and cable radiation. The change of current and inductance flowing through the wire will lead to voltage drop, which can be minimized by reducing the change of inductance or current over time.

Second, the hardware processing method of interference measures.

1. Electromagnetic compatibility design of printed circuit board (PCB).

PCB is the support of circuit components and devices in single-chip microcomputer system, which provides the electrical connection between circuit components and devices. With the rapid development of electronic technology, the density of PCB is getting higher and higher. The quality of PCB design has a great influence on the electromagnetic compatibility of single-chip microcomputer system. Practice has proved that even if the circuit schematic design is correct and the printed circuit board design is improper, it will have an adverse impact on the reliability of single-chip microcomputer system. For example, if the two thin parallel lines of the printed circuit board are very close, the delay of the signal waveform will be formed, and the reflected noise will be formed at the end of the transmission line. Therefore, when designing printed circuit boards, we should pay attention to adopt correct methods, abide by the general principles of PCB design, and should meet the design requirements of anti-interference. In order to achieve the best performance of electronic circuits, the layout of components and wires are very important.

2. Electromagnetic compatibility design of input / output

In the single-chip microcomputer system, the input / output is also the transmission wire of the interference source, and the pick-up source of receiving radio frequency interference signal, we should generally take effective measures when designing: ① uses the necessary common mode / differential mode suppression circuit, but also take certain filtering and anti-electromagnetic shielding measures to reduce the advance of interference. ② takes various isolation measures (such as photoelectric isolation or magnetoelectric isolation) as far as possible, so as to block the propagation of interference.

3. Design of reset circuit of single chip microcomputer.

In the single-chip microcomputer system, the watchdog system plays a very important role in the operation of the whole single-chip microcomputer. Because all the interference sources can not be isolated or removed, once entering the normal operation of the CPU interference program, then the reset system combined with software processing measures will become an effective error correction and defense barrier. There are two commonly used reset systems:

① external reset system. The external watchdog circuit can be designed or built with a special watchdog chip. However, they each have their own advantages and disadvantages. Most of the dedicated watchdog chips can not respond to the low-frequency "dog-feeding" signal, while the high-frequency "dog-feeding" signal can respond. it causes it to produce a reset action under the low-frequency "dog-feeding" signal but not under the high-frequency "dog-feed" signal, so that if the program system falls into an endless loop, and the loop happens to have a "dog-feed" signal. Then the reset circuit will not be able to achieve its due function. However, we can design a system with bandpass "dog feeding" circuit and other reset circuits is a very effective external monitoring system.

② now more and more single-chip computers have their own on-chip reset system, so that users can easily use its internal reset timer, but there are some types of single-chip microcomputer whose reset instructions are too simple, so there will also be "dog feeding" instructions like the dead cycle mentioned above, making it lose its monitoring function. There are some single-chip microcomputer on-chip reset instructions to do better, generally they "feed the dog" signal into a fixed format of multiple instructions to be executed in order, if there is a certain error, the "feed dog" operation is invalid, this greatly improves the reliability of the reset circuit.

4. Oscillator

Most single-chip microcontrollers have an oscillator circuit coupled to an external crystal or ceramic resonator. On PCB, the shorter the lead of a capacitor, crystal or ceramic resonator, the better. RC oscillator has latent sensitivity to interference signal, it can produce a very short clock cycle, so it is best to choose crystal or ceramic resonator. In addition, the quartz crystal shell should be grounded.

5. Lightning protection measures

The lightning protection of the single-chip microcomputer system used outdoors or the indoor power lines and signal lines introduced from outdoor crowding should be considered. Commonly used lightning protection devices are: gas discharge tube, TVS and so on. The gas discharge tube is that when the voltage of the power supply is greater than a certain value, it is usually tens of volts or hundreds of volts, and the gas breakdown discharge leads the strong impulse pulse on the power line into the earth. TVS can be regarded as two Zener diodes in parallel and in opposite directions, which turn on when the voltage at both ends is higher than a certain value. Its characteristic is that it can transiently pass through hundreds of thousands of A current.

III. Software processing methods for interference measures

The interference signal produced by the electromagnetic interference source can not be completely eliminated in some specific cases (for example, in some cases with bad electromagnetic environment), and will eventually enter the core unit of CPU processing, so it will often be interfered in some large-scale integrated circuits, resulting in not working properly or in the wrong state. In particular, devices such as RAM, which use bistability for storage, often flip under strong interference, making the original storage "0" into "1", or "1" into "0"; the timing and data of some serial transmission will be changed due to interference; more serious will destroy some important data parameters; the consequences are often very serious. In this case, the quality of the software design directly affects the anti-interference ability of the whole system.

1. The program will roughly meet the following situations due to electromagnetic interference:

The ① program runs away. This situation is the most common interference result, generally speaking, there is a good reset system or software frame measurement system, which will not have much impact on the whole operating system.

② endless loop or abnormal program code running. Of course, this kind of endless loop and abnormal program code is not intentionally written by the designer. We know that the instructions of the program are made up of bytes, some are single-byte instructions and some are multi-byte instructions. When the interference occurs, the PC pointer changes, so that the original program code is reorganized to produce unpredictable executable program code. So, this kind of error is fatal. It is possible to modify important data parameters and may produce a series of error states such as unpredictable control output.

2. Measures for storing important parameters

In general, we can use error detection and correction to effectively reduce or avoid this situation. According to the principle of error detection and correction, the main idea is that when the data is written, the check code of a certain number of digits according to the written data is saved together with the corresponding data; when it is read out, the check code is also read out and judged. If there is an error, it will be automatically corrected, the correct data will be sent out, and the corrected data will be written back to cover the original error data at the same time; if there are two-bit errors, an interrupt report will be generated and notify CPU to handle the exception. All these actions are completed automatically by software design, and have the characteristics of real-time and automatic completion. Through this design, the anti-interference ability of the system can be greatly improved, thus the reliability of the system can be improved.

The above are several major methods to solve the problem of single-chip microcomputer EMC, the editor believes that there are some knowledge points that we may see or use in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.

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