Anti-jamming design, the basic elements are inseparable from these three

There are three basic elements that form interference:

(1) Interference source refers to the component, device or signal that generates interference. It is described in mathematical language as follows: du/dt, where the di/dt is large is the source of interference. Such as: lightning, relays, thyristors, motors, high-frequency clocks, etc. may become sources of interference.

(2) Propagation path refers to the path or medium that interferes with the propagation from the interference source to the sensitive device. Typical interference propagation paths are conduction through the wires and radiation from the space.

(3) Sensitive devices refer to objects that are easily disturbed. Such as: A / D, D / A converter, microcontroller, digital IC, weak signal and so on.

The basic principle of anti-interference design is to suppress the interference source, cut off the interference propagation path, and improve the anti-interference performance of sensitive devices. (similar to the prevention of infectious diseases)

1. Suppress interference sources

To suppress the interference source is to reduce the du/dt, di/dt of the interference source as much as possible. This is the most important and most important principle in anti-jamming design, and it often has a multiplier effect. Reducing the du/dt of the interference source is mainly achieved by connecting capacitors across the interference source. Reducing the di/dt of the interferer is achieved by connecting the inductor or resistor in series with the source loop and adding a freewheeling diode.

Common measures to suppress interference sources are as follows:

(1) The relay coil adds a freewheeling diode to eliminate the interference generated when the coil is disconnected. Adding a freewheeling diode will delay the turn-off time of the relay. After the Zener diode is added, the relay can move more times per unit time.

(2) Connect the spark suppression circuit at both ends of the relay contact (usually RC, the resistance is generally selected from a few K to tens of K, and the capacitor is selected as 0.01uF) to reduce the spark effect.

(3) Add a filter circuit to the motor, pay attention to the capacitor and inductor leads as short as possible.

(4) Each IC on the board should be connected with a high frequency capacitor of 0.01μF ~ 0.1μF to reduce the impact of the IC on the power supply. Pay attention to the wiring of high-frequency capacitors. The wiring should be close to the power supply terminal and be as short and as short as possible. Otherwise, it will increase the equivalent series resistance of the capacitor, which will affect the filtering effect.

(5) Avoid 90 degree fold line when wiring, reduce high frequency noise.

(6) The RC suppression circuit is connected to both ends of the thyristor to reduce the noise generated by the thyristor (this thyristor may break down when the noise is severe).

According to the propagation path of interference, it can be divided into two types: conducted interference and radiated interference.

Conducted interference is the interference that propagates through a wire to a sensitive device. The high-frequency interference noise and the frequency band of the useful signal are different, and the propagation of the high-frequency interference noise can be cut off by adding a filter to the wire, and sometimes the isolation optocoupler can be added. Power supply noise is the most harmful, so pay special attention to handling. Radiation interference refers to interference that propagates through a spatial radiation to a sensitive device. The general solution is to increase the distance between the interferer and the sensitive device, isolate them with ground and add a shield to the sensitive device.

2. Common measures to cut off the interference propagation path are as follows

(1) Fully consider the impact of the power supply on the microcontroller. The power supply is well done, and the anti-interference of the entire circuit is solved. Many single-chip microcomputers are very sensitive to power supply noise. It is necessary to add a filter circuit or a voltage regulator to the power supply of the single-chip microcomputer to reduce the interference of the power supply noise on the single-chip microcomputer. For example, a magnetic bead and a capacitor can be used to form a π-shaped filter circuit. Of course, a 100Ω resistor can be used instead of the magnetic bead when the condition is not high.

(2) If the I/O port of the MCU is used to control noise devices such as motors, isolation should be added between the I/O port and the noise source (increasing the π-shaped filter circuit). Control noise devices such as motors, and add isolation between the I/O port and the noise source (increasing the π-shaped filter circuit).

(3) Pay attention to the crystal wiring. The crystal oscillator and the MCU pins are placed as close as possible, and the clock region is isolated by the ground wire. The crystal oscillator case is grounded and fixed. This measure can solve many difficult problems.

(4) Reasonable division of the circuit board, such as strong and weak signals, digital and analog signals. Keep interference sources (such as motors, relays) away from sensitive components (such as microcontrollers) as much as possible.

(5) Separate the digital zone from the analog zone with a ground wire, separate it from the analog ground, and finally connect it to the power ground at one point. A/D and D/A chip wiring are also based on this principle. This requirement has been considered when the manufacturer assigns A/D and D/A chip pinouts.

(6) The single chip and the large ground wire should be grounded separately to reduce mutual interference. Place high-power devices on the edge of the board as much as possible.

(7) Anti-interference components such as magnetic beads, magnetic rings, power supply filters, and shields can be used in key areas such as I/O ports, power lines, and circuit board connectors to significantly improve the anti-jamming performance of the circuit.

3. Improve the anti-interference performance of sensitive devices

Improving the anti-interference performance of sensitive devices refers to the method of minimizing the pickup of interference noise and recovering from an abnormal state as soon as possible from the sensitive device side.

Common measures to improve the anti-jamming performance of sensitive devices are as follows:

(1) Minimize the loop loop area during wiring to reduce induced noise.

(2) When wiring, the power and ground wires should be as thick as possible. In addition to reducing the voltage drop, it is more important to reduce the coupling noise.

(3) For the idle I/O port of the MCU, do not hang it, ground it or connect it to the power supply. The idle ends of other ICs are grounded or connected to the power supply without changing the system logic.

(4) The power supply monitoring and watchdog circuit for the single-chip microcomputer, such as IMP809, IMP706, IMP813, X25043, X25045, etc., can greatly improve the anti-interference performance of the whole circuit.

(5) Under the premise that the speed can meet the requirements, try to reduce the crystal oscillator of the single-chip microcomputer and select the low-speed digital circuit.

(6) The IC device is soldered directly on the circuit board as much as possible, and the IC holder is used less.

Let's talk about the experience in this area.

Software:

1. Always clear the unused code space to “0”, because this is equivalent to NOP, which can be returned when the program runs away;

2, add a few NOP before the jump instruction, the purpose is the same as 1;

3. In the absence of hardware WatchDog, software can be used to simulate WatchDog to monitor the running of the program;

4. When dealing with the adjustment or setting of external device parameters, in order to prevent the external device from being disturbed due to interference, the parameter can be resent periodically, so that the external device can be restored as soon as possible;

5, anti-interference in communication, you can add data check digits, you can take 3 to take 2 or 5 to take 3 strategy;

6. When there is a communication line, such as I^2C, three-wire system, etc., in practice, we find that the normal state of the Data line, CLK line, and INH line is set high, and the anti-interference effect is better than low.

Hardware:

1. The line of the ground wire and power cord is definitely important!

2, the decoupling of the line;

3. Separation of numbers and models;

4. Each digital component must have 104 capacitors between the ground and the power supply;

5, in the application of relays, especially at high current, anti-relay contact spark interference on the circuit, can be between the relay coil and a 104 and diode, indirect contact 472 capacitor at the contact and the normally open end, the effect is good!

6. In order to prevent crosstalk of I/O port, the I/O port can be isolated by diode isolation, gate isolation, photocouple isolation, electromagnetic isolation, etc.

7, of course, the anti-interference of the multi-layer board is certainly better, but the cost is several times higher.

8. Choosing a device with strong anti-interference ability is more effective than any method. This should be the most important.

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