“As the single-chip microcomputer system is more and more widely used in consumer electronics, medical, industrial automation, intelligent instrumentation, aerospace and other fields, the single-chip microcomputer system is facing the increasingly serious threat of electromagnetic interference (EMI).
As the single-chip microcomputer system is more and more widely used in consumer electronics, medical, industrial automation, intelligent instrumentation, aerospace and other fields, the single-chip microcomputer system is facing the increasingly serious threat of electromagnetic interference (EMI). Electromagnetic compatibility (EMC) includes both the emission and sensitivity of the system. A single-chip system is electromagnetically compatible if it meets the following three conditions:
① No interference to other systems;
② Not sensitive to emissions from other systems;
③ No interference to the system itself.
If the interference can not be completely eliminated, but also to minimize the interference. The generation of interference is either direct (coupling via conductors, common impedance, etc.) or indirect (coupling via crosstalk or radiation). Electromagnetic interference occurs through conductors and through radiation. Many sources of electromagnetic emission, such as lighting, relays, DC motors, and fluorescent lamps, can cause interference; AC power lines, interconnecting cables, metal cables, and internal circuits of subsystems may also generate radiation. or received an undesired signal. In high-speed microcontroller systems, clock circuits are often the largest source of broadband noise. These circuits can generate harmonic distortions up to 300 MHz, which should be removed from the system. In addition, in the single chip system, the most easily affected are the reset line, the interrupt line and the control line.
1. Interference coupling method
(1) Conducted EMI
One of the most obvious and often overlooked paths that can cause noise in a circuit is through conductors. A wire running through a noisy environment can pick up noise and send it to other circuits to cause interference. Designers must avoid wire pickup and use decoupling to remove noise before it causes interference. The most common example is when noise enters a circuit through power lines. If the power supply itself or other circuits connected to the power supply are the source of interference, the power lines must be decoupled before they enter the circuit.
(2) Common impedance coupling
Common impedance coupling occurs when currents from two different circuits flow through a common impedance. The voltage drop across the impedance is determined by the two circuits from which the ground current flows through the common ground impedance. The ground potential of circuit 1 is modulated by ground current 2, and the noise signal or DC compensation is coupled from circuit 2 to circuit 1 via a common ground impedance.
(3) Radiation coupling
Radiated coupling is known as crosstalk. Crosstalk occurs when current flows through conductors, creating an electromagnetic field that induces transient currents in adjacent conductors.
(4) Radiated emissions
There are two basic types of radiated emissions: differential mode (DM) and common mode (CM). Common mode radiation or monopole antenna radiation is caused by an unintentional voltage drop that raises all ground connections in a circuit above the system ground potential. In terms of electric field magnitude, CM radiation is a more serious problem than DM radiation. In order to minimize CM radiation, the common mode current must be reduced to zero with a practical design.
2. Factors affecting EMC
① Voltage. Higher supply voltages mean larger voltage amplitudes and more emissions, while lower supply voltages affect sensitivity.
② Frequency. High frequencies produce more emissions, and periodic signals produce more emissions. In a high-frequency microcontroller system, a current spike is generated when the device switches; in an analog system, a current spike is generated when the load current changes.
③ Ground. In all EMC problems, the main problem is caused by improper grounding. There are three signal grounding methods: single-point, multi-point, and mixed. When the frequency is lower than 1 MHz, the single-point grounding method can be used, but it is not suitable for high frequency; in high-frequency applications, it is better to use multi-point grounding. Hybrid grounding is a single-point grounding method for low frequencies and multi-point grounding for high frequencies. The layout of the ground wire is the key, and the ground loops of high-frequency digital circuits and low-level analog circuits must not be mixed.
④ PCB design. Proper printed circuit board (PCB) routing is critical to preventing EMI.
⑤ Power decoupling. When devices switch, transient currents are generated on the power supply lines and must be attenuated and filtered. Transient currents from high di/dt sources cause ground and traces to “shoot” voltages, and high di/dt generates large-scale high frequency currents that excite components and radiate from cables. Changes in current flow and inductance through the wire cause a voltage drop, which can be minimized by reducing the inductance or the change in current over time.
3. Electromagnetic compatibility design of printed circuit board (PCB)
The PCB is the support for circuit components and devices in the single-chip microcomputer system, and it provides electrical connections 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 the single-chip microcomputer system. Practice has proved that even if the circuit schematic design is correct, the printed circuit board is not properly designed, it will also have an adverse effect on the reliability of the single-chip microcomputer system. For example, if two thin parallel lines on a printed board are close together, a delay in the signal waveform will be created, resulting in reflected noise at the end of the transmission line. Therefore, when designing a printed circuit board, attention should be paid to adopting the correct method, complying with the general principles of PCB design, and should meet the requirements of anti-interference design.
3.1 General principles of PCB design
To obtain the best performance of electronic circuits, the layout of components and wiring is very important. In order to design a good quality and low cost PCB, the following general principles should be followed.
(1) Special component layout
First of all, the size of the PCB should be considered: when the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will also increase; if it is too small, the heat dissipation will be poor, and the adjacent lines will be easily interfered. After determining the size of the PCB, determine the location of special components. Finally, according to the functional units of the circuit, all components of the circuit are laid out.
Observe the following principles when locating special components:
① Shorten the connection between high-frequency components as much as possible, and try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference should not be too close to each other, and input and output components should be kept as far apart as possible.
② There may be a high potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be arranged as far as possible in places that are not easily accessible by hand during debugging.
③ Components weighing more than 15 g should be fixed with brackets and then welded. Those components that are large, heavy and generate a lot of heat should not be installed on the printed board, but should be installed on the chassis bottom plate of the whole machine, and the heat dissipation problem should be considered. Thermal elements should be kept away from heating elements.
④ For the layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, and micro switches, the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on the printed board where it is convenient to adjust; if it is adjusted outside the machine, its position should be adapted to the position of the adjustment knob on the chassis panel.
⑤ Reserve the position occupied by the positioning holes of the printed board and the fixing bracket.
(2) General component layout
According to the functional unit of the circuit, when laying out all the components of the circuit, the following principles should be followed:
① Arrange the positions of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signals keep the same direction as possible.
② Take the core components of each functional circuit as the center, and make a layout around it. Components should be evenly, neatly and compactly arranged on the PCB, minimizing and shortening the leads and connections between components.
③ For circuits operating at high frequencies, the distribution parameters between components should be considered. In general, the components should be arranged in parallel as far as possible, so that it is not only beautiful, but also easy to install and weld, and is easy to mass produce.
④ Components located at the edge of the circuit board are generally not less than 2 mm away from the edge of the circuit board. The optimal shape of the circuit board is a rectangle. The aspect ratio is 3:2 or 4:3. When the size of the circuit board surface is larger than 200 mm × 150 mm, the mechanical strength of the circuit board should be considered.
The principles of wiring are as follows:
① The wires used at the input and output terminals should avoid being adjacent and parallel as much as possible, and it is better to add a ground wire between the wires to avoid feedback coupling.
② The minimum width of the printed board wires is mainly determined by the adhesion strength between the wires and the insulating substrate and the value of the current flowing through them. When the thickness of the copper foil is 0.5 mm and the width is 1-15 mm, the temperature rise will not be higher than 3 ℃ through the current of 2 A. Therefore, a wire width of 1.5 mm suffices. For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually chosen. Of course, whenever possible, use as wide a wire as possible, especially power and ground wires. The minimum spacing of wires is mainly determined by the worst-case wire-to-wire insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, as long as the process allows, the spacing can be less than 0.1 to 0.2 mm.
③ The corners of printed wires are generally arc-shaped, and right angles or included angles will affect electrical performance in high-frequency circuits. In addition, try to avoid the use of large-area copper foil, otherwise, the copper foil will easily expand and fall off when heated for a long time. When a large area of copper foil must be used, it is best to use a grid shape, which is beneficial to eliminate the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.
The pad center hole is slightly larger than the device lead diameter. If the pad is too large, it is easy to form a virtual solder. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the lead hole diameter. For high-density digital circuits, the minimum diameter of the pad is desirable (d+1.0) mm.
3.2 PCB and circuit anti-interference measures
The anti-jamming design of the printed circuit board is closely related to the specific circuit. Here, only a few common measures for the anti-jamming design of the PCB are explained.
(1) Power cord design
According to the size of the printed circuit board current, try to thicken the width of the power line to reduce the loop resistance; at the same time, make the direction of the power line and the ground wire consistent with the direction of data transmission, which helps to enhance the anti-noise ability. kx6 electronic technology bar
(2) Ground wire design
In the single chip system design, grounding is an important method to control interference. The correct combination of grounding and shielding can solve most interference problems. The ground wire structure in the single-chip microcomputer system roughly includes system ground, chassis ground (shield ground), digital ground (logical ground) and analog ground. The following points should be paid attention to when designing the ground wire:
① Correctly select single-point grounding and multi-point grounding. In the low-frequency circuit, the working frequency of the signal is less than 1MHz, the inductance between its wiring and the device has little influence, and the circulating current formed by the grounding circuit has a greater influence on the interference, so a one-point grounding method should be used. When the operating frequency of the signal is greater than 10 MHz, the impedance of the ground wire becomes very large. At this time, the impedance of the ground wire should be reduced as much as possible, and the nearest multi-point grounding should be used. When the operating frequency is between 1 and 10MHz, if one-point grounding is used, the length of the grounding wire should not exceed 1/20 of the wavelength, otherwise the multi-point grounding method should be used.
② Separate the digital ground from the analog ground. There are both high-speed logic circuits and linear circuits on the circuit board. They should be separated as much as possible, and the ground wires of the two should not be mixed, and they should be connected to the ground wires of the power supply. The ground of the low-frequency circuit should be grounded in parallel at a single point as far as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel; the high-frequency circuit should be grounded in multiple points in series, and the ground wire should be short and thick. Use a grid-shaped large-area ground foil around the high-frequency components as much as possible, and try to increase the grounding area of the linear circuit.
③ The ground wire should be as thick as possible. If the ground wire is very thin, the ground potential will change with the change of the current, resulting in unstable timing signal level of electronic products and reduced anti-noise performance. Therefore, the ground wire should be as thick as possible so that it can pass three times the allowable current of the printed circuit board.If possible, the width of the ground wire
④ The ground wire forms a closed loop. When designing the ground wire system of a printed circuit board consisting of only digital circuits, making the ground wire a closed circuit can significantly improve the anti-noise capability. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are components that consume a lot of power, due to the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground wire, resulting in a decrease in noise immunity; If the ground wire forms a loop, the potential difference will be reduced and the anti-noise capability of the electronic equipment will be improved.
(3) Decoupling capacitor configuration
One of the common practices in PCB design is to configure appropriate decoupling capacitors in various key parts of the printed board. The general configuration principles of decoupling capacitors are:
① Connect an electrolytic capacitor of 10 to 100 μF across the power input terminal. If possible, it is better to connect more than 100μF.
② In principle, each integrated circuit chip should be arranged with a 0.01 pF ceramic capacitor. If there is not enough space on the printed board, a tantalum capacitor of 1 to 10 pF can be arranged every 4 to 8 chips.
③ For devices with weak anti-noise capability and large power changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip.
④ The lead wire of the capacitor should not be too long, especially the high frequency bypass capacitor should not have lead wire.
In addition, the following two points should be noted:
① When there are contactors, relays, buttons and other components on the printed board, large spark discharges will occur when operating them, and RC circuits must be used to absorb the discharge current. Generally, R is 1 to 2 kΩ, and C is 2.2 to 47 μF.
② The input impedance of CMOS is very high, and it is susceptible to induction, so when using it, the unused terminal should be grounded or connected to a positive power supply.
Almost all microcontrollers have an oscillator circuit coupled to an external crystal or ceramic resonator. On the PCB, the leads that require external capacitors, crystals or ceramic resonators should be as short as possible. RC oscillators are potentially sensitive to interfering signals and can generate very short clock cycles, so a crystal or ceramic resonator is the best choice. In addition, the case of the quartz crystal should be grounded.
(5) Lightning protection measures
The single-chip microcomputer system used outdoors or the power line and signal line introduced into the room from the outdoor overhead should consider the lightning protection of the system. Commonly used lightning protection devices are: gas discharge tube, TVS (Transient Voltage Suppression) and so on. The gas discharge tube is when the power supply voltage is greater than a certain value, usually tens or hundreds of V, the gas breakdown discharges, and the strong shock pulse on the power line is introduced into the ground. TVS can be regarded as two zener diodes in parallel and in opposite directions, which are turned on when the voltage at both ends is higher than a certain value. Its characteristic is that it can transiently pass currents of hundreds or even thousands of A.
In order to improve the electromagnetic compatibility of the single-chip microcomputer system, it is necessary not only to design the PCB board reasonably, but also to take corresponding measures in the circuit structure and software. Practice has shown that the electromagnetic compatibility needs to be considered in each stage of the design, manufacture, installation and operation of the single-chip microcomputer system. Only in this way can the long-term stable, reliable and safe operation of the system be ensured.