Single-Chip Microcomputer Anti-Interference Design
In electronic system design, to avoid detours and save time, the requirements for anti-interference should be fully considered and met, avoiding taking remedial measures for interference after the design is completed. The basic elements that form interference are three:
(1) Interference source: This refers to components, devices, or signals that produce interference. Described in mathematical terms: places with large du/dt and di/dt are interference sources. For example: lightning, relays, thyristors, motors, high-frequency clocks, etc., can all become interference sources.
(2) Propagation path: This refers to the path or medium through which interference travels from the interference source to sensitive devices. Typical interference propagation paths include conduction through wires and radiation through space.
(3) Sensitive devices: These are objects that are easily affected by interference. For example: A/D, D/A converters, microcontrollers, digital ICs, weak signal amplifiers, etc.
The basic principle of anti-interference design is: 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. Suppressing the Interference Source
Suppressing the interference source involves minimizing the du/dt and di/dt of the interference source as much as possible. This is the most prioritized and important principle in anti-interference design, often yielding twice the result with half the effort. Reducing the du/dt of the interference source is mainly achieved by paralleling a capacitor across the interference source. Reducing the di/dt of the interference source is achieved by series-connecting an inductor or resistor in the interference source circuit, as well as adding a freewheeling diode.
Common measures to suppress the interference source include:
(1) Adding a freewheeling diode to the relay coil to eliminate the counter-electromotive force interference generated when the coil is disconnected. Adding only a freewheeling diode may delay the disconnection time of the relay; adding a voltage stabilizing diode allows the relay to operate more times within a unit time.
(2) Connecting a spark suppression circuit (usually an RC series circuit, with resistance generally selected from several K to dozens of K, and capacitance selected at 0.01uF) across the relay contact points to reduce the effect of sparks.
(3) Adding a filter circuit to the motor. Note that the leads of the capacitor and inductor should be as short as possible.
(4) Each IC on the circuit board should be paralleled with a 0.01μF～0.1μF high-frequency capacitor to reduce the influence of the IC on the power supply. Pay attention to the layout of the high-frequency capacitor, the connection line should be close to the power end and as thick and short as possible; otherwise, it is equivalent to increasing the equivalent series resistance of the capacitor, which will affect the filtering effect.
(5) Avoid 90-degree wiring during wiring to reduce high-frequency noise emission.
(6) Paralleling an RC suppression circuit across the thyristor to reduce the noise generated by the thyristor (this noise may severely damage the thyristor if serious).
According to the propagation path of interference, it can be divided into two types: conducted interference and radiated interference.
Conducted interference refers to interference that propagates to sensitive devices through wires. Since the frequency band of high-frequency interference noise is different from that of useful signals, the propagation of high-frequency interference noise can be cut off by adding a filter to the wire, and sometimes an isolation optocoupler can also be added to solve the problem. Power noise has the greatest harm and should be particularly noted. Radiated interference refers to interference that propagates to sensitive devices through spatial radiation. The general solution method is to increase the distance between the interference source and the sensitive device, isolate them with ground lines, and add shielding covers to sensitive devices.
Common measures to cut off the interference propagation path include:
(1) Fully consider the impact of the power supply on the microcontroller. If the power supply is well designed, half of the entire circuit's anti-interference problem is solved. Many microcontrollers are very sensitive to power noise, so filters or voltage regulators should be added to the microcontroller power supply to reduce the interference of power noise on the microcontroller. For example, a π-type filter circuit composed of magnetic beads and capacitors can be used, and when conditions do not require high standards, a 100Ω resistor can also replace the magnetic bead.
(2) If the I/O port of the microcontroller is used to control noise-generating devices such as motors, isolation (adding a π-type filter circuit) should be added between the I/O port and the noise source.
(3) Pay attention to crystal oscillator wiring. The crystal oscillator should be as close as possible to the microcontroller pin, and the clock area should be isolated with ground lines, the crystal oscillator shell should be grounded and fixed. This measure can solve many difficult problems.
(4) Reasonably partition the circuit board, such as strong/weak signals, digital/analog signals. Try to keep the interference source (such as motors, relays) away from sensitive components (such as microcontrollers).
(5) Use ground lines to isolate the digital area from the analog area, the digital ground and analog ground should be separated, and finally connected to the power ground at one point. The wiring of A/D, D/A chips follows this principle, and manufacturers have already considered this requirement when allocating the pin arrangement of A/D, D/A chips.
(6) The ground lines of the microcontroller and high-power devices should be separately grounded to reduce mutual interference. High-power devices should be placed as far as possible on the edge of the circuit board.
(7) Using anti-interference components such as magnetic beads, magnetic rings, power filters, and shielding covers at key locations like the microcontroller I/O port, power lines, and circuit board connecting lines can significantly enhance the anti-interference performance of the circuit.
3. Enhancing the Anti-Interference Performance of Sensitive Devices
Enhancing the anti-interference performance of sensitive devices means considering how to minimize the pickup of interference noise from the side of the sensitive devices, as well as methods to quickly recover from abnormal states. Common measures to enhance the anti-interference performance of sensitive devices include:
(1) Minimize the area of loop circuits during wiring to reduce induced noise.
(2) Make power lines and ground lines as thick as possible during wiring. In addition to reducing voltage drop, it is more important to reduce coupled noise.
(3) Do not leave unused I/O ports of the microcontroller floating; they should be grounded or connected to the power supply. Other idle ends of ICs should be grounded or connected to the power supply without changing the system logic.
(4) Use power monitoring and watchdog circuits for the microcontroller, such as IMP809, IMP706, IMP813, X25043, X25045, etc., to greatly enhance the overall anti-interference performance of the circuit.
(5) Under the premise that speed meets the requirements, try to reduce the oscillation frequency of the microcontroller and select low-speed digital circuits.
(6) IC components should be directly soldered onto the circuit board as much as possible, minimizing the use of IC sockets.
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