Hardware anti-interference in MCU System Design

 

How can we improve anti-interference capability and electromagnetic compatibility when developing electronic products with processors?
1. The following systems should pay special attention to anti-electromagnetic interference:
1. A system with extremely high clock frequency and extremely fast bus cycle.
2. The system contains high-power and high-current drive circuits, such as relays that generate sparks and large-current switches.
3. Systems with weak analog signal circuits and high-precision A/D conversion circuits.
2. Take the following measures to increase the system's anti-electromagnetic interference capability:
1. Select a low-frequency microcontroller:
Using a microcontroller with a low external clock frequency can effectively reduce noise and improve the system's anti-interference capability. For square waves and sine waves of the same frequency, the high-frequency components in the square waves are much more than those in the sine waves. Although the amplitude of the high-frequency component of the square wave is smaller than that of the fundamental wave, the higher the frequency, the more easily it is emitted as a noise source, the most influential High-frequency noise produced by the microcontroller is about three times the clock frequency.
2. Reduce distortion in Signal Transmission
The microcontroller is mainly manufactured using high-speed CMOS technology. The static input current at the signal input end is about 1mA, the input capacitor is about 10 PF, the input impedance is quite high, and the output end of the high-speed CMOS circuit has a considerable carrying capacity, that is, a considerable output value, when the output end of a door is directed to the input end with a very high input impedance through a long line, the reflection problem is very serious, which will cause signal distortion and increase system noise. When Tpd> Tr, it becomes a transmission line problem. signal reflection and impedance matching must be considered.
The delay time of the signal on the printed board is related to the characteristic impedance of the lead, that is, the dielectric constant of the printed circuit board. It can be roughly considered that the signal transmission speed in the printed board lead is between 1/3 and 1/2 of the speed of light. The Tr (Standard delay time) of commonly used logical Telephone components in a Microcontroller System is between 3 and 18ns.
On the printed circuit board, the signal passes through a 25cm-W resistor and a-long lead, and the online delay time is roughly 4 ~ Between 20 NS. That is to say, the shorter the lead on the print line, the better. The maximum length of the signal should not exceed 25 cm. In addition, the number of passing holes should be as small as possible, preferably no more than 2.
When the signal rising time is faster than the signal delay time, it must be processed by fast electronics. In this case, the impedance matching of transmission lines should be taken into account. For signal transmission between integrated blocks on a printed circuit board, avoid Td> Trd, the larger the printed circuit board, the faster the system is.
A rule used to draw conclusions about the design of printed circuit boards:
The delay time of the signal transmitted on the Printed Board should not be greater than the nominal delay time of the device used.
3. reduce cross interference between signal lines:
A step signal with a rising time of Tr is transmitted to the B end through the lead AB. The delay time of the signal on the AB line is Td. At point D, due to the forward transmission of the signal at point A, the signal reflection after point B and the delay of the AB line will be sensed after the Td time a page pulse signal with a width of Tr will be generated. At point C, because of the signal transmission and reflection on AB, a positive pulse signal with a width of twice the delay time of the signal on the AB line is generated. This is the cross interference between signals. The interference signal intensity is related to the di/at of the cpoint signal and the distance between lines. When the two signal lines are not very long, what we see on AB is the superposition of two pulses.
High Input Impedance, high noise, and high noise tolerance of micro-control manufactured by CMOS process. The digital circuit is a superposition of 100 ~ 200mv noise does not affect its work. If the AB line in the figure is a simulated signal, this interference becomes intolerable. For example, if a printed circuit board is a four-layer board with a large area or a dual-panel, and the opposite side of the signal line is a large area, the cross interference between the signals will become smaller. The reason is that the characteristic impedance of the signal line is reduced in a large area, and the signal reflection at the D end is greatly reduced. The characteristic impedance is inversely proportional to the square of the dielectric constant from the signal line to the ground, and proportional to the natural logarithm of the dielectric thickness. If the AB Line is a analog signal, to avoid the interference of the digital circuit signal line CD to the AB line, there must be a large area of ground below the AB line, the distance between the AB Line and the CD line is greater than the distance between the AB Line and the ground ~ 3 times. It can be locally shielded, and the left and right sides of a lead with a ground wire are laid.
4. Reduce Noise from Power Supply
While providing energy to the system, the power supply also adds noise to the power supply. The reset line, disconnection, and other control lines of the Controller in the circuit are most vulnerable to external noise interference. The strong interference on the electric network enters the circuit through the power supply. Even if the battery supply system, the battery itself also has high-frequency noise. Analog signals in analog circuits cannot withstand interference from power sources.
5. Pay attention to the high-frequency characteristics of the printing board and components.
In high-frequency situations, distribution inductance and capacitance of leads, passing holes, resistors, capacitors, and connectors on printed circuit boards cannot be ignored. The distribution inductance of the capacitor cannot be ignored, and the distribution capacitance of the inductor cannot be ignored. Resistance generates reflection on high-frequency signals. The distribution capacitance of the lead will take effect. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, the antenna effect will be generated, and the noise will be emitted through the lead.
The pass-through of the printed circuit board causes the capacitance of 0.6pf.
The packaging materials of an integrated circuit are introduced in 2 ~ 6pf capacitor.
The connector on a circuit board has a distribution inductance of 520nH. A 24-pin integrated circuit housing with dual-row direct connections introduces 4 ~ Distribution inductance of 18nH.
These small distribution parameters are negligible for the microcontroller systems at a low frequency in this line, and special attention must be paid to high-speed systems.
6. Reasonably arrange Components
The position of components arranged on the printed circuit board should fully consider the problem of electromagnetic interference resistance. One principle is that the lead wires between components should be as short as possible. In terms of layout, the analog signal, high-speed digital circuit, and noise sources (such as relays and large current switches) should be reasonably separated, minimum signal coupling between each other.
7. Handle the ground line
On printed circuit boards, power cords and ground wires are the most important. The main method to overcome electromagnetic interference is grounding.
For the dual panel, the ground line layout is particularly exquisite. By using the single point grounding method, the power supply and ground are connected from both ends of the power supply to the printed circuit board. The power supply is one point, one point. On the printed circuit board, there must be multiple return ground wires, which will be aggregated to the point of the Back-to-power supply. This is called single point grounding. The opening of analog, digital, and high-power devices means that the cables are separated and finally collected to the ground point. When connected to a signal other than a printed circuit board, a shielded cable is usually used. For high-frequency and digital signals, both ends of the shielded cable are grounded. Shielded Cables for low-frequency analog signals. It is better to ground one end.
Circuits that are very sensitive to noise and interference, or circuits with extremely high-frequency noise, should be shielded with metal masks.
8. Use the decoupling capacitor properly.
A good high-frequency Decoupling capacitor can remove high-frequency components up to 1 GHz. The high-frequency characteristics of ceramic chip capacitor or multilayer ceramic capacitor are better. When designing a printed circuit board, a decoupling capacitor must be added between the power supply and ground of each integrated circuit. Decoupling capacitor has two functions: one is the energy storage capacitor of the integrated circuit, which provides and absorbs the charge and discharge energy when the integrated circuit is opened and closed; the other is to bypass the high-frequency noise of the device. In digital circuits, a typical decoupling capacitor of 0.1uf has a 5nH distribution inductance, and its parallel resonance frequency is about 7 MHz, that is to say, there is a good decoupling effect for noise below 10 MHz, and there is almost no effect on noise above 40 MHz.
1 uf, 10uf capacitor, parallel resonance frequency above 20 MHz, the effect of high-frequency noise removal is better. It is often advantageous for a 1uf or 10uf High-Frequency Capacitor where the power supply enters the Printed Board, even for a battery-powered system.
Each 10-piece integrated circuit requires a charge-discharge capacitor, or storage capacitor. The capacitance size can be set to 10 uf. It is best not to use electrolytic capacitors. electrolytic capacitors are flushed by Two-layer swap films. The structure of the flushed is expressed as inductance at high frequencies. It is best to use a bold capacitor or a polycarbonate capacitor.
The decoupling capacitor value is not strictly selected and can be calculated according to C = 1/f; that is, 0.1 uf for 10 MHz, and 0.1 ~ Both 0.01uf and 0.01uf.
Iii. Experience in noise reduction and electromagnetic interference reduction.
High-speed chips are not needed when low-speed chips are used in key areas.
One resistor can be used to reduce the upper and lower hop speed of the control circuit.
Try to provide some form of damping for relays and so on.
Use the minimum frequency clock that meets the system requirements.
The clock generator is as close as possible to the device that uses the clock. The case of the Z crystal oscillator must be grounded.
Use a ground wire to circle the clock area. The clock line should be as short as possible.
The I/O drive circuit is as close as possible to the edge of the printing board, so that it can leave the printing board as soon as possible. Filter the signal entering the Printed Board, and filter the signal from the high noise area. At the same time, use the method of serial terminal resistance to reduce signal reflection.
The useless MCD end must be connected to a high or ground, or defined as the output end. The end of the power source must be connected to the integrated circuit. Do not leave it blank.
Do not enter the input end of the idle door circuit. The unneeded Op Amplifier is grounded at the front end and the negative end is connected to the output end. (10) Use a 45-line printing board instead of a 90-line wiring to reduce the transmission and coupling of high-frequency signals.
The printed board is partitioned by frequency and current switch characteristics, and the noise components and non-noise components are farther away.
Single-panel and dual-Panel use single-point power supply and single-point grounding, power cord, ground line as crude as possible, if economical, it can withstand multiple layers to reduce the power supply, local volume inductance.
Clock, bus, and chip selection signals should be kept away from the I/O Lines and connectors.
The analog voltage input line and reference voltage end should be as far away as possible from the digital circuit signal line, especially the clock.
For Class A/D devices, the digital part and the analog part should be unified and should not be crossed.
The clock line perpendicular to the I/O line has less interference than the parallel I/O line, and the clock element pin is far away from the I/O cable.
The element pins should be as short as possible, and the decoupling capacitor pins should be as short as possible.
The key lines should be as rough as possible, and the protected area should be added on both sides. High-speed lines must be short and straight.
The line sensitive to noise should not be parallel to the large current or high-speed switch line.
Do not strip the cables below the Z crystal and under the noise-sensitive devices.
Weak signal circuit, do not form a current loop around the low frequency circuit.
Do not form a loop for any signal. If it is inevitable, make the loop area as small as possible.
One decoupling capacitor for each integrated circuit. A small high-frequency bypass capacitor must be added to the edge of each electrolytic capacitor.
Use a large capacity of Ta capacitor or juku capacitor instead of electrolytic capacitor as the circuit charge-discharge and energy storage capacitor. When using a tubular capacitor, the housing must be grounded.