As MCU systems are gaining wider use in areas like consumer electronics, medical, industry automation, intelligent instruments and metering, aviation and aerospace, they also face an ever-growing threat caused by EMI (Electromagnetic interference). EMC (Electromagnetic compatibility) is a design against this problem and it covers two aspects of emission and sensitivity. An MCU system is said to be electromagnetically compatible if:

 

1. It does not cause interference with other systems;

2. It is not susceptible to emissions of other systems;

3. It does not cause interference with itself.

 

EMI must be reduced to the least, if not completely none. EMI can be radiated (through the air) or conducted (through common impedance or a physical medium). Many electromagnetic sources like light, relay, DC motors and fluorescent lamps can cause interference. AC power cords, connection cables, metal cables and internal circuits of the subsystem can cause radiation or receive unwanted signals. In high-speed MCU systems, clock circuits are usually the biggest source of bandwidth noise. They can produce as high as 300MHz of harmonic distortion, which must be eliminated in the system. The reset wires, interrupt wires and control wires are most vulnerable to interference.

 

a) Conductive EMI

One of the most apparent and yet easily ignored causes of noise is through conductors. A wire passing through noisy environment can pick up the noise and bring it to other circuitries to cause interference. Designers must prevent the wire from picking up noise and remove the noise through decoupling before interference occurs. A common example of this is the noise brought to a circuit by the power line. If the power supply itself or other circuit connecting to the power is the source of interference, the power line must be decoupled before entering into the circuit.

 

b) Common impedance coupling

Common impedance coupling occurs when two currents from different circuits pass through the same impedance. The voltage drop caused by impedance depends on the two circuits. The ground currents of the two circuits (say, 1 and 2) flows through the common ground impedance, the ground potential of circuit 1 is modulated by current 2, and the noise signal or DC compensator is coupled from circuit 2 to circuit 1 through the common ground impedance.

 

c) Radiated coupling

Coupling generated by radiation is referred to as “crosstalk”. Crosstalk happens when current passes through the conductor and generates electromagnetic field, which then induces transient current to the conductors nearby.

 

d) Radiated emission

There are two modes of radiated emission: Differential Mode (DM) and Common Mode (CM). CM radiation or monopole antenna radiation is generated by unintended voltage drop, which causes all the ground connection of the circuit rising beyond the system ground level. CM radiation is more serious than DM radiation in terms of the field size. To keep CM radiation as low as possible, proper design must be employed to drop the common mode current to zero.

 

a) Voltage. The higher the voltage, the greater the amplitude, and more emissions will be generated. But low voltage will influence the sensitivity.

 

b) Frequency. Higher frequency will cause more emissions, so are periodic signals. In high frequency MCU systems, turning on and off a device can produce spike signals. Load current changes in analog systems can also cause spike signals.

 

c) Grounding. Most EMC issues are caused by inappropriate grounding. There are three grounding methods: single point, multipoint and hybrid. If the frequency is lower than 1MHz, single point grounding can be used; while in high frequency applications, multipoint grounding is preferred. Hybrid grounding is a method that uses single point for low frequency and multipoint for high frequency. Layout is the key in grounding. The ground loop of the high frequency digital circuit and the low voltage analog circuit must not mix.

 

d)  PCB design. Proper PCB design is crucial to preventing EMI.

 

e)  Power source decoupling. Turning on and off a device can cause transient current on the power wires, which must be filtered or attenuated. Transient current from high di/dt source can cause the ground and stitch to “transmit” high voltage, and the high di/dt produces extensive high frequency current that cause components and wires to radiate. Current change in the wires and inductance can result in voltage drop. Reducing inductance or current change with time can minimize the voltage drop.

PCB is the holder for components and parts in an MCU system and provides electric connection for them. With the development of the electronic technologies, PCBs are becoming more compact. The design of PCB has a significant effect on EMC. A poor PCB layout can reduce the system reliability even if the schematic circuit is correct. For example, if two thin parallel wires are arranged too close on a PCB, signal waveform delay more occur and causes reflection noise at the end of the wires. Therefore, PCB design must conform to the general rules and take EMC into consideration.

 

The components layout and wiring are crucial to the performance of the circuit. To design quality and low cost PCBs, the following rules must be complied with.

PCB size considerations: bigger PCB size means longer printed wires, and increased impedance, noise risks and cost, while too small a PCB size may result in poorer heat dissipation performance, and easier interference from the nearby wires. You should decide the PCB size first and then the positions of the special and other components.

 

Follow the following rules when positioning the special components:

a) Keep the wires between high frequency components as short as possible, and try to reduce the distributed parameters and mutual EMI. Keep distance between the interference sensitive components, and keep the input and output components as far as possible.

 

b) If high potential difference may exist between some components or wires, leave enough distance between them to avoid unexpected short circuit. And place components with high voltage in positions not easily touched at maintenance.

 

c) Fix components weighing more than 15g with support, and solder them. Install big, heavy and heating components at the bottom plate of the rack instead of the PCB, with heat dissipation considerations. Keep heat sensitive components far from the heating components.

 

d) The layout of adjustable components like potentiometer, adjustable inductor, variable capacitor and microswitch must suit the requirement of the whole system. If they are to be adjusted internally, place them at convenient positions of the PCB. If they are to be adjusted externally, place them at positions that correspond to the adjusting knob on the panel.

 

e) Leave space for the locating holes and the fastening brackets.

 

The placement of general components should follow the principles below:

a) Place each functional circuit unit in a way that is convenient for signal flow and allows the signal to flow in the same direction.

 

b) Place the units of each functional circuit around the key components. Arrange the components on the PCB evenly, tightly and neatly. Reduce the leading wires between components.

 

c) Consider the distributive parameter of components in high frequency circuits. It is common practice to arrange components in parallel. This has a better look and facilitates soldering and manufacturing.

 

d) Leave at least 2mm between the board edge and components near the edge. The ideal shape of a circuit board is rectangle, with a length width ratio of 3:2 or 4:3. If the board dimension exceeds 200 mm×150 mm, you must consider the mechanical force that the board bears.

 

The principle of wiring is as follows:

a) Do not parallel the input and output wires immediately. Use grounding wire between them to avoid feedback coupling.

 

b) The minimum wire width for a PCB is decided by the adhesive strength between the wire and the substrate as well as the current in the wire. If the copper foil is 0.5mm thick, 1 to 15mm wide, and the current is 2A, the temperature raise will be less than 3℃. So a wire with a width of 1.5mm will be suitable. For integrated circuit, especially digital circuit, the wire width of 0.2 to 0.3mm is generally used. Always use wider wire when possible, especially for power wire and grounding wire. The minimum distance between wires is decided by the insulation resistance between them and the breakdown voltage. For integrated circuit, especially digital circuit, keep the distance within 0.1 to 0.2mm if possible.

 

c) Use arc at the turning point(s) of the printed wire, because sharp angels can affect the electrical performance in high frequency circuits. In addition, try to avoid using large area of copper foil, otherwise, the copper foil may expand and fall off under long time exposure to heat. In case large area of copper foil cannot be avoided, try to use them in grid. This is beneficial for discharging the volatile air caused by heated adhesive between the copper foil and the baseboard.

 

d) The diameter of soldering pad hole should be larger than that of the wire. Pad being too big can cause dry joint. The outer diameter D should be generally not less than (d+1.2) mm, where d is the diameter of the wire. For high-density digital circuits, the pad diameter can be (d+1.0) mm.

 

The measures to avoid interference vary with different circuits. However, there are some common principles as shown in the following.

Depending on the current in the PCB, try to use thicker power wire to reduce resistance. Meanwhile, keep the power wire, ground wire in the same direction as the data flow. This helps reduce interference.

 

Grounding is an important measure to control interference in MCU systems. Proper grounding plus shielding can solve most interference problems. In MCU system, there are system grounding, case grounding (shield grounding), digital grounding (logic grounding) and analog grounding. Pay attention to the following in the layout of ground wire:

 

a) Use single ground and multipoint ground for appropriate situations. In low frequency circuits, the working frequency of signal is less than 1MHz, and EMI caused by wiring and components is insignificant, while thecirculation caused by the grounding circuit can cause more interference. In this situation, single point grounding should be adopted. If the working frequency is higher than 10MHz, the grounding impedance becomes very significant. In this situation, the adjacent multipoint grounding should be adopted to reduce the impedance. With the working frequency range 1 to 10MHz, if the ground wire length is less than 1/20 of the wavelength, single point grounding can be adopted, otherwise, multipoint grounding should be used.

 

b) Separate digital grounding and analog grounding. When a PCB has both high-speed logic circuit and linear circuit, use separate grounding for them. For low frequency circuit, single point parallel grounding is always desirable. In case of wiring difficulty, use partial serial and then parallel to ground; for high frequency circuit, it is desirable to use multipoint serial grounding, and the ground wire should be short and thick. Try to use large area grid foil near the high frequency components, increase the grounding area of the linear circuit.

 

c) The ground wire should be as thick as possible. A thin ground wire can cause the ground potential changes with the current, resulting in an unstable timing signal and reduced anti-interference performance. Thus, the ground wire should be as thick as possible to allow for three times of the allowed current in the PCB. If possible, use a ground wire thicker than 3mm.

 

d) Use closed loop for grounding. When a PCB circuitry is completely composed of digital circuits, using closed loop grounding can significantly reduce noise. The reason is: there are many components on the PCB, if many of them are high power consumption components, high potential difference can be generated in the ground wire due to thickness limit of the ground wire. This reduces the anti-interference capability. If closed loop is used for grounding, the potential difference can be reduced and thus improves the anti-interference capability.

 

In PCB design, it’s common to use some decoupling capacitors at key positions. The principles of setting decoupling capacitors are as follows:

a) Use an electrolytic capacitor (10~100μF) at the power source end. If possible, use one above 100μF.

 

b) In principle, each chip should be equipped with 0.01 μF ceramic capacitor. If the space on the PCB is not enough, use one 1 to10 μF Tantalum Capacitor for each 4 to 8 chips.

 

c) For components with low anti-interference capability and high voltage change when turning off, like the RAM, ROM, use a decoupling capacitor between the power wire and ground wire.

 

d) The lead of the capacitor cannot be too long; high frequency bypass circuit must not have lead.

 

In addition, note the following two points:

a) For PCBs with components like contactor, relay, button, etc, spark discharge could easily occur in their operation, use RC circuit to absorb the discharge. The typical R value is 1 to 2 kΩ, and C value 2.2 to 47μF.

 

b) CMOS has high input impedance and is sensitive to interference. When in use, connect the unused pin(s) to ground or the positive power supply.

 

Nearly all MCUs have an oscillator circuit that couples with external crystal or ceramic resonator. On the PCB a shorter lead for external capacitor and crystal or ceramic resonator is desirable. RC oscillator is potentially sensitive to interference, and can generate short clock period. So crystal or ceramic Resonator is more ideal for use. Note that the outer case of the quartz crystal should connect ground.

 

Lightning protection must be taken into consideration when an MCU system is used outdoors or when there are power wire, signal wire that go overhead from outdoor into indoor. Common lightning proof devices include: gas discharge pipe, TVS (Transient Voltage Suppression), etc. Gas discharge pipe is a device that triggers gas disruptive discharge when the power voltage exceeds a certain value (typically tens or hundreds of volts), and leads the strong pulses in the power wire into the ground. TVS can be thought as two parallel Zener diodes in different direction, which turns on when the voltage between the two ends exceeds a certain value. TVS can allow for several hundreds or even more amps of transient current to discharge.