Electromagnetic compatibility design of single chip microcomputer system

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

① It does not interfere with other systems;
② It is not sensitive to the emission of other systems;
③ It does not interfere with the system itself.

If the interference cannot be completely eliminated, it must be minimized. Interference is either directly (through conductors, common impedance coupling, etc.) or indirectly (through crosstalk or radiation coupling). Electromagnetic interference is generated through conductors and through radiation. Many electromagnetic emission sources, such as light, 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 receive unwanted signals. In high-speed microcontroller systems, clock circuits are usually the largest source of broadband noise. These circuits can generate harmonic distortion up to 300 MHz and should be removed from the system. In addition, in microcontroller systems, the reset lines, interrupt lines, and control lines are the most susceptible.

1 Interference coupling mode

(1) Conducted EMI
One of the most obvious but often overlooked paths that can cause noise in a circuit is through a conductor. A wire passing through a noisy environment can pick up noise and send it to other circuits to cause interference. Designers must avoid wires picking up noise and use decoupling methods to remove noise before it 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, the power line must be decoupled before it enters 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, and the ground currents from the two circuits flow through the common ground impedance. The ground potential of circuit 1 is modulated by ground current 2, and the noise signal or DC offset is coupled from circuit 2 to circuit 1 through the common ground impedance.

(3) Radiated coupling
Coupling through radiation is generally called crosstalk. Crosstalk occurs when current flows through a conductor and generates an electromagnetic field, which induces transient currents in adjacent conductors.

(4) Radiated
emission 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 unintentional voltage drop, which raises all ground connections in the circuit above the system ground potential. In terms of electric field size, CM radiation is a more serious problem than DM radiation. 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 low power supply voltage affects sensitivity.

② Frequency. High frequencies produce more emissions, and periodic signals produce more emissions. In high-frequency single-chip microcomputer systems, current spikes are generated when devices are switched; in analog systems, current spikes are generated when 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, multi-point and mixed. When the frequency is below 1 MHz, the single-point grounding method can be used, but it is not suitable for high frequencies; in high-frequency applications, multi-point grounding is best used. Mixed grounding is a method of using single-point grounding for low frequencies and multi-point grounding for high frequencies. Ground layout is the key, and the ground loops of high-frequency digital circuits and low-level analog circuits must never be mixed.

④ PCB design. Proper printed circuit board (PCB) wiring is crucial to preventing EMI.

⑤ Power supply decoupling. When devices are switched, transient currents are generated on the power supply line, which must be attenuated and filtered out. Transient currents from high di/dt sources cause ground and trace "emission" voltages, and high di/dt generates a large range of high-frequency currents, which excite components and cables to radiate. The change of current and inductance flowing through the wire will cause voltage drop. Reducing the inductance or the change of current over time can minimize the voltage drop.

3 Electromagnetic compatibility design of printed circuit board (PCB)

PCB is the support of circuit elements and devices in the single-chip system, and it provides electrical connections between circuit elements 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 system. Practice has proved that even if the circuit schematic is designed correctly, improper design of printed circuit board will also have an adverse effect on the reliability of the single-chip system. For example, if two thin parallel lines of the printed board are close to each other, the delay of the signal waveform will be formed, and reflected noise will be formed at the terminal 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 meeting the requirements of anti-interference design.

3.1 General principles of PCB design

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

(1) Layout of special components
First, the size of the PCB should be considered: when the PCB size is too large, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases; when it is too small, the heat dissipation is poor and the adjacent lines are easily interfered with. After determining the PCB size, determine the location of the special components. Finally, layout all the components of the circuit according to the functional units of the circuit.

The following principles should be followed when determining the location of special components:
① Shorten the connection between high-frequency components as much as possible, and try to reduce their distributed parameters and mutual electromagnetic interference. Components that are susceptible to interference should not be too close to each other, and the input and output components should be kept as far away 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 discharge causing accidental short circuits. Components with high voltage should be placed as far as possible in places that are not easily touched by hands during debugging.

③ Components weighing more than 15 g should be fixed with a bracket and then soldered. Those large and heavy components that generate a lot of heat should not be installed on the printed circuit board, but should be installed on the chassis bottom plate of the whole machine, and heat dissipation should be considered. Thermistors should be kept away from heating elements.

④ For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, and micro switches, the structural requirements of the entire machine should be considered. If the adjustment is inside the machine, it should be placed in a convenient place on the printed board; if the adjustment is outside the machine, its position should be consistent with the position of the adjustment knob on the chassis panel.
⑤ Leave space for the positioning holes on the printed board and the fixed bracket.

(2) General component layout
When laying out all the components of the circuit according to the functional units of the circuit, the following principles should be followed:

① Arrange the positions of each functional circuit unit according to the flow of the circuit so that the layout is convenient for signal flow and the signal can maintain the same direction as much as possible.

② Take the core component of each functional circuit as the center and lay it out around it. Components should be arranged evenly, neatly, and compactly on the PCB, and the leads and connections between components should be minimized and shortened as much as possible.

③ For circuits working at high frequencies, the distribution parameters between components should be considered. In general, components should be arranged in parallel as much as possible in the circuit. This is not only beautiful, but also easy to assemble and solder, and 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 best shape of the circuit board is a rectangle. The aspect ratio is 3:2 or 4:3. When the surface size of the circuit board is greater than 200 mm × 150 mm, the mechanical strength of the circuit board should be considered. [page]

(3) Wiring
The wiring principles are as follows:

① The wires used for the input and output terminals should be avoided to be adjacent and parallel. It is best to add a ground wire between the wires to avoid feedback coupling.

② The minimum width of the printed circuit board wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them. When the copper foil thickness is 0.5 mm and the width is 1 to 15 mm, the temperature rise will not exceed 3°C when a current of 2 A passes through. Therefore, a wire width of 1.5 mm can meet the requirements. For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually selected. Of course, as long as it is allowed, wide wires should be used as much as possible, especially power lines and ground lines. The minimum spacing between 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 process allows, the spacing can be less than 0.1-0.2 mm.

③ The bends of printed conductors are generally arc-shaped, while right angles or included angles will affect the electrical performance in high-frequency circuits. In addition, try to avoid using large-area copper foil, otherwise, when heated for a long time, the copper foil is prone to expansion and falling off. When a large area of ​​copper foil must be used, it is best to use a grid shape, which is conducive to removing the volatile gas generated by the heat of the adhesive between the copper foil and the substrate.

(4) Solder
pads The center hole of the solder pad should be slightly larger than the diameter of the device lead. A solder pad that is too large is prone to cold soldering. The outer diameter D of the solder 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 solder pad can be (d+1.0) mm.

3.2 PCB and circuit anti-interference measures

The anti-interference design of printed circuit boards is closely related to the specific circuit. Here we only explain several common measures for PCB anti-interference design.

(1) Power line design
According to the current of the printed circuit board, try to increase 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 line consistent with the direction of data transmission, which helps to enhance the anti-noise ability.

(2) Ground line design
In the design of the single-chip microcomputer system, grounding is an important method to control interference. If grounding and shielding can be used correctly, most interference problems can be solved. The ground line structure in the single-chip microcomputer system generally includes system ground, chassis ground (shielded ground), digital ground (logic ground) and analog ground. The following points should be noted in the ground line design:

① Correctly choose single-point grounding and multi-point grounding. In low-frequency circuits, the operating frequency of the signal is less than 1 MHz, and its wiring and the inductance between devices have little effect, while the loop current formed by the grounding circuit has a greater impact on interference, so a single-point grounding method should be adopted. When the signal operating frequency is greater than 10 MHz, the ground line impedance becomes very large. At this time, the ground line impedance should be reduced as much as possible, and multi-point grounding should be adopted nearby. When the operating frequency is between 1 and 10 MHz, if one-point grounding is used, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise a 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. They should be connected to the ground wire of the power supply end respectively. The ground of the low-frequency circuit should be connected to the ground at a single point as much as possible. When the actual wiring is difficult, it can be connected in series and then connected in parallel; the high-frequency circuit should be connected to the ground at multiple points in series, and the ground wire should be short and thick. Use a large area of ​​grid-shaped 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 uses a very thin line, the ground potential will change with the change of the current, resulting in unstable timing signal level of the electronic product 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 should be greater than 3 mm.

④ The ground wire forms a closed loop. When designing a grounding system for a printed circuit board consisting only of digital circuits, making the grounding line 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 grounding line, a large potential difference will be generated on the grounding line, causing the anti-noise capability to decrease; if the grounding line is formed into a loop, the potential difference will be reduced, improving the anti-noise capability of the electronic equipment.

(3) Decoupling capacitor configuration
One of the common practices in PCB design is to configure appropriate decoupling capacitors at various key locations of the printed circuit board. The general configuration principle of decoupling capacitors is:
① Connect a 10~100μF electrolytic capacitor 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 circuit board, a 1~10 pF tantalum capacitor can be arranged for every 4~8 chips.

③ For devices with weak noise immunity and large power supply changes when turned off, such as RAM and ROM memory devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip.

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

In addition, the following two points should be noted:

① When there are contactors, relays, buttons and other components on the printed circuit board, they will generate large spark discharges when operating them, and an RC circuit must be used to absorb the discharge current. Generally, R is 1~2 kΩ and C is 2.2~47μF.

② The input impedance of CMOS is very high and is easily inductive. Therefore, when using it, the unused end should be grounded or connected to the positive power supply.

(4) Oscillator
Almost all microcontrollers have an oscillator circuit coupled to an external crystal or ceramic resonator. On the PCB, the leads of the external capacitor, crystal or ceramic resonator should be as short as possible. RC oscillators are potentially sensitive to interference signals and can generate very short clock cycles, so it is best to choose crystals or ceramic resonators. In addition, the shell of the quartz crystal should be grounded.

(5) Lightning protection measures
For single-chip microcomputer systems used outdoors or for power lines and signal lines that are introduced from the outdoors to indoors, lightning protection issues for the system must be considered. Common lightning protection devices include: gas discharge tubes, TVS (Transient Voltage Suppression), etc. When the power supply voltage of a gas discharge tube is greater than a certain value, usually tens of V or hundreds of V, the gas breaks down and discharges, directing the strong impulse pulse on the power line to the ground. TVS can be seen as two Zener diodes connected 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 pass hundreds or even thousands of A of current transiently.

Conclusion

In order to improve the electromagnetic compatibility of single-chip microcomputer systems, 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 electromagnetic compatibility needs to be considered at all stages of the design, manufacture, installation and operation of single-chip microcomputer systems. Only in this way can the system be guaranteed to operate stably, reliably and safely in the long term.