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introduction

As the single-chip microcomputer system is more and more widely used in consumer electronics, medical treatment, industrial automation, intelligent instrumentation, aerospace and other fields, the single-chip microcomputer system is facing an increasingly serious threat of electromagnetic interference (EMI). Electromagnetic compatibility (EMC) includes the emission and sensitivity of the system. If a single-chip microcomputer system meets the following three conditions, the system is EMC:

① No interference to other systems;

② Insensitive to the emission of other systems;

③ There is no interference to the system itself.

If interference cannot be completely eliminated, it should also be minimized. The generation of interference is either direct (through conductor, common impedance coupling, etc.) or indirect (through crosstalk or radiation coupling). Electromagnetic interference is generated through conductors and radiation. Many electromagnetic emission sources, such as lighting, relays, DC motors and fluorescent lamps, can cause interference; AC power lines, interconnection cables, metal cables and internal circuits of subsystems may also generate radiation or receive unwanted signals. In high-speed single-chip microcomputer systems, clock circuits are usually the largest source of broadband noise. These circuits can produce harmonic distortion as high as 300 MHz, which should be removed from the system. In addition, the reset line, interrupt line and control line are the most easily affected in the single chip microcomputer system.

1. Coupling mode of interference

(1) Conductive EMI

One of the most obvious and often overlooked paths that can cause noise in a circuit is through a conductor. A wire passing through a noisy environment can pick up the noise and send it to other circuits to cause interference. The designer must avoid the noise of wire picking up and remove the noise by decoupling before the noise causes interference. The most common example is noise entering the circuit through the power line. If the power supply itself or other circuits connected to the power supply are interference sources, they must be decoupled before the power line enters the circuit.

(2) Common impedance coupling

Common impedance coupling occurs when current from two different circuits flows through a common impedance. The voltage drop on the impedance is determined by two circuits, and the ground current from the two circuits 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 through common ground impedance.

(3) Radiative coupling

Radiated coupling is commonly known as crosstalk. Crosstalk occurs when a current flows through a conductor, generating an electromagnetic field that induces transient currents in adjacent conductors.

(4) Radiation emission

There are two basic types of radiative emission: differential mode (DM) and common mode (CM). Common mode radiation or monopole antenna radiation is caused by unintentional voltage drop, which raises all connections in the circuit above the system ground potential. In terms of the size of the electric field, CM radiation is a more serious problem than DM radiation. In order to minimize cm radiation, a practical design must be used to reduce the common mode current to zero.

2 factors affecting EMC

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

② Frequency. High frequencies produce more emissions, and periodic signals produce more emissions. In the high-frequency single-chip microcomputer system, when the device is switched, the current peak signal is generated; In the analog system, the current spike signal is generated when the load current changes.

③ Grounding. Among all EMC problems, the main problem is caused by improper grounding. There are three signal grounding methods: single point, multipoint and hybrid. When the frequency is lower than 1 MHz, the single point grounding method can be adopted, 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 for low frequency and multi-point grounding for high frequency. The ground wire layout is the key. The ground circuit of high-frequency digital circuit and low-level analog circuit must not be mixed.

④ PCB design. Proper printed circuit board (PCB) wiring is essential to prevent EMI.

⑤ Power decoupling. When the device is switched, transient current will be generated on the power line, which must be attenuated and filtered out. The transient current from the high di/dt source leads to the “emission” voltage of the ground and trace. The high di/dt generates a wide range of high-frequency current, which excites the radiation of components and cables. 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 with time.

3. EMC design of printed circuit board (PCB)

PCB is the support of circuit components and devices in single chip microcomputer system. It 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 impact 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 also have an adverse impact on the reliability of single chip microcomputer system. For example, if the two thin parallel lines of the printed board are close together, 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 PCB, we should pay attention to the correct method, abide by the general principles of PCB design, and meet the requirements of anti-interference design.

3.1 general principles of PCB design

In order to obtain the best performance of electronic circuits, the layout of components and wires is very important. In order to design PCB with good quality and low cost, the following general principles should be followed.

(1) Layout of special components

First of all, we should consider the size of PCB: when the size of PCB is too large, the printing line is long, the impedance increases, the anti noise ability decreases, and the cost also increases; If it is too small, the heat dissipation is poor, and the adjacent lines are easy to be disturbed. After determining the size of PCB, determine the position of special components. Finally, according to the functional units of the circuit, all components of the circuit are laid out.

The following principles shall be observed when determining the position of special components:

① Try to shorten the wiring between high-frequency components 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 as far away as possible.

② There may be a high potential difference between some components or wires, so the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be arranged in places that are not easy to touch during commissioning.

③ Components weighing more than 15 g should be fixed with brackets and then welded. Those large, heavy and heat generating components should not be installed on the printed board, but on the chassis bottom plate of the whole machine, and heat dissipation should be considered. The thermal element should be far away from the heating element.

④ For the layout of potentiometers, adjustable inductance coils, variable capacitors, microswitches and other adjustable components, the structural requirements of the whole machine should be considered. If it is adjusted in the machine, it should be placed on the printing board where it is convenient to adjust; If it is adjusted outside the machine, its position should be consistent with the position of the adjustment knob on the chassis panel.

⑤ Reserve the position occupied by the positioning hole of the printed board and the fixed support.

(2) General component layout

According to the functional units of the circuit, the layout of all components of the circuit shall comply with the following principles:

① Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal flow, and keep the signal in the same direction as much as possible.

② Take the core components of each functional circuit as the center and layout around it. Components and parts shall be evenly, neatly and compactly arranged on the PCB to minimize and shorten the leads and connections between components and parts.

③ For the circuit working at high frequency, the distribution parameters between components should be considered. In general, the components and parts should be arranged in parallel as far as possible, which is not only beautiful, but also easy to assemble and weld, and easy to mass produce.

④ Components and parts located at the edge of the circuit board are generally not less than 2 mm away from the edge of the circuit board. The best shape of the circuit board is rectangle. The aspect ratio is 3:2 or 4:3. The size of circuit board is greater than 200 mm × At 150 mm, the mechanical strength of the circuit board should be considered.

(3) Wiring

The principles of wiring are as follows:

① The conductors used at the input and output terminals should be avoided to be adjacent and parallel as much as possible, and it is best to add ground wires between lines to avoid feedback coupling.

② The minimum width of the printed board wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current flowing through them. When the thickness of 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, the wire width of 1.5 mm can meet the requirements. For integrated circuits, especially digital circuits, the wire width of 0.02 ~ 0.3 mm is usually selected. Of course, as long as it is allowed, use as wide a line as possible, especially the power line and ground wire. The minimum spacing of wires is mainly determined by the insulation resistance and breakdown voltage between wires in the worst case. For integrated circuits, especially digital circuits, as long as the technology allows, the spacing can be less than 0.1 ~ 0.2 mm.

③ The bend of printed wire is generally circular arc, and the right angle or included angle will affect the electrical performance in high-frequency circuit. In addition, try to avoid using large-area copper foil, otherwise, copper foil expansion and falling off will easily occur 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 conducive to eliminating the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.

(4) Pad

The central hole of the pad is slightly larger than the diameter of the device lead. If the pad is too large, it is easy to form false soldering. The outer diameter D of the pad is generally not less than (d 1.2) mm, where D is the diameter of the lead. For high-density digital circuits, the minimum diameter of the pad can be (d 1.0) mm.

3.2 anti interference measures for PCB and circuit

The anti-interference design of printed circuit board is closely related to the specific circuit. Here are just some common measures of PCB anti-interference design.

(1) Power cord design

According to the current of the printed circuit board, try to thicken the width of the power line and reduce the loop resistance; At the same time, make the direction of power line and ground line consistent with the direction of data transmission, which helps to enhance the anti noise ability.

(2) Ground wire design

In the design of single chip microcomputer system, grounding is an important method to control interference. Most interference problems can be solved if grounding and shielding can be correctly combined. The ground wire structure of single chip microcomputer system roughly includes system ground, shell ground (shielding ground), digital ground (logic ground) and analog ground. The following points should be paid attention to in the design of 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 1 MHz, its wiring and the inductance between devices have little influence, while the circulating current formed by the grounding circuit has a great influence on the interference, so one point grounding should be adopted. When the signal working frequency is greater than 10 MHz, the ground wire impedance becomes large. At this time, the ground wire impedance should be reduced as much as possible, and the nearest multi-point grounding should be adopted. When the working frequency is 1 ~ 10MHz, if one point grounding is adopted, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise the multi-point grounding method should be adopted.

② Digital and analog are separated. There are both high-speed logic circuits and linear circuits on the circuit board, so they should be separated as far as possible, and the ground wires of the two should not be mixed, and they should be connected with the ground wires of the power end respectively. The grounding of low-frequency circuit should adopt single point parallel grounding as far as possible. When the actual wiring is difficult, it can be partially connected in series and then connected in parallel; High frequency circuit should adopt multi-point series grounding, and the ground wire should be short and thick. Try to use a large area of grid foil around the high-frequency components, and try to increase the grounding area of the linear circuit.

③ The grounding wire should be thickened as much as possible. If the grounding wire uses a very thin line, the grounding potential will change with the change of current, causing the timing signal level of electronic products to be unstable and the anti noise performance to be reduced. Therefore, the grounding wire should be thickened as much as possible so that it can pass three times the allowable current of the printed circuit board. If possible, the width of the grounding wire should be greater than 3 mm.

④ The grounding wire forms a closed loop. When designing the ground wire system of printed circuit board composed of only digital circuits, making the ground wire into a closed circuit can significantly improve the anti noise ability. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are components that consume much power, due to the limitation of the thickness of the grounding wire, a large potential difference will be generated on the grounding wire, resulting in the decline of anti noise ability; If the ground wire forms a loop, the potential difference will be reduced and the anti noise ability of electronic equipment will be improved.

(3) Decoupling capacitor configuration

One of the conventional methods of PCB design is to configure appropriate decoupling capacitors in each key part of the printed board. The general configuration principle of decoupling capacitor is:

① The power input terminal is jumper 10 ~ 100 μ F electrolytic capacitor. If possible, connect 100 μ Above f is better.

② In principle, each integrated circuit chip should be equipped with a 0.01 pf ceramic chip capacitor. In case of insufficient gap in the printed board, a 1 ~ 10 pf tantalum capacitor can be arranged for every 4 ~ 8 chips.

③ For devices with weak noise resistance and large power change when turning off, such as RAM and ROM storage devices, decoupling capacitors should be directly connected between the power line and ground wire of the chip.

④ The capacitor lead should not be too long, especially the high-frequency bypass capacitor should not have a lead.

In addition, the following two points should be noted: