[b]1 Introduction[/b] Currently, microcontroller systems play a crucial role in spaceborne instruments. These instruments often operate in complex space environments with numerous interference sources. For example, high-energy charged particles primarily affect microcontroller systems through single-event effects, causing single-event upsets, which can lead to program corruption and system malfunction. Therefore, it is essential to fully consider anti-interference design in system design to improve system reliability. For microcontroller systems, there are two types of interference: one originates from the external environment and other electrical equipment, affecting the normal operation of the microcontroller system through conduction and radiation; the other originates from within the system, determined by its structure, manufacturing process, and the interference generated by internal components during operation, transmitted through address lines, power lines, signal lines, and distributed capacitance, affecting the system's operating state. Anti-interference measures for microcontroller systems mainly focus on fault avoidance and fault tolerance design in both hardware and software aspects to improve system reliability. [b]2 Causes of Interference[/b] 2.1 Interference Sources Interference sources refer to components, devices, or signals that generate interference. The interference generated includes: (1) Electromagnetic interference, such as relay switch starting, electrostatic discharge, power grid voltage fluctuations, etc., which may cause transient surge voltages of varying degrees, which may cause IC and semiconductor device PN junction burnout, oxide layer breakdown, etc. (2) Human interference, such as mechanical vibration, relay contact jitter, electromagnetic coupling caused by component installation and circuit board wiring, poor connector contact, cold solder joint, amplifier self-excitation, power supply ripple, etc. (3) Environmental interference, such as noise and ambient temperature and humidity, as well as changes in sunspots, space particle radiation, etc. 2.2 Interference transmission path The impact of interference on the microcontroller system is mainly transmitted through three paths, including: (1) Input system. Under normal circumstances, the detection object of the spaceborne instrument is often a weak physical signal, which is composed of an amplified operational amplifier circuit and a high-precision A/D conversion circuit. If interference is introduced, it will cause the input analog signal to be distorted and the digital signal to be wrong, thereby increasing the error of the acquired data. (2) Output system. Once interfered with, the output signals will be disordered and cannot reflect the true output of the microcontroller system normally. However, the output circuit of a microcontroller generally has a high level and is not easily interfered with. It is important to pay attention to its interference effect on other circuits. (3) CPU system. This interference is mainly caused by the internal clock and noise of the CPU. It can cause the digital signals on the microcontroller system bus to be disordered, the CPU to get the wrong address signal, the program to run away or dead loop, resulting in output error, and this error will be continuously transmitted, causing the system to fail. 2.3 Sensitive devices In the microcontroller system of spaceborne instruments, some devices such as A/D, D/A converters, weak signal amplifiers, etc. are usually used, which are easily interfered with and are also important reasons for interference. [b]3 Common anti-interference technology for microcontrollers[/b] Anti-interference is to take corresponding methods to eliminate the interference source, suppress the propagation path, reduce the sensitivity of the circuit or components to noise interference, and enable the microcontroller system to operate normally and stably. 3.1 Suppressing the interference effect of interference source Take corresponding measures to suppress the interference effect for different interference sources. (1) Add a freewheeling diode to the relay coil to eliminate the back electromotive force interference generated when the coil is disconnected. (2) Connect a spark suppression circuit in parallel across the relay contacts. It is usually an RC series circuit. The resistor is usually selected from several K to tens of K, and the capacitor is selected from 0.01uF to reduce the impact of electric sparks. (3) Keep the interference source (motor, relay) as far away from the sensitive device (microcontroller) as possible. 3.2 Hardware interference suppression technology Hardware anti-interference has the characteristics of high efficiency. As long as the relevant parameters are reasonably arranged and selected, appropriate hardware anti-interference measures can suppress most of the interference in the system. 3.2.1 Power supply anti-interference design Microcontroller systems are very sensitive to power supply noise. Power supply switching, instantaneous short circuit and interference pulses from the power grid will cause microcontroller malfunction. The main measures are to add a filter circuit and voltage regulator to the power supply to reduce interference. 3.2.2 Shielding interference technology In the space environment, due to the radiation of charged particles in space, devices with low anti-radiation ability will flip or lock, and in severe cases, the devices will fail. Aluminum or tantalum shields can be used for shielding reinforcement. In spaceborne instruments, shielded wires are used for the leads of highly sensitive weak signal detection probes to avoid interference from external signals. Components that may generate electromagnetic pulses, such as high-voltage power supplies, and highly sensitive components, such as preamplifiers, are shielded with metal covers to reduce interference. 3.2.3 Isolation and Anti-interference Technology Isolation can cut off external interference channels, thereby achieving the purpose of isolating on-site interference. On the other hand, it can separate two signal lines to minimize crosstalk. Commonly used isolation methods include opto-isolation, transformer isolation, and relay isolation. a. Opto-isolation is achieved using optocouplers. The electrical signals at the input and output ends of an optocoupler are indirectly coupled through light, thus possessing high electrical isolation and interference suppression capabilities. In addition, it also provides good safety protection because it has a high withstand voltage between the input and output circuits. b. Pulse transformers can achieve digital signal isolation. Pulse transformers have few turns, and the primary and secondary windings are wound on both sides of a ferrite core, with a distributed capacitance of only a few pF, so they can be used as isolation devices for pulse signals. c. There is no electrical connection between the relay coil and contacts. Therefore, the relay coil can be used to receive electrical signals, and the contacts can be used to send and output signals, avoiding direct contact between weak and strong signals and achieving interference isolation. 3.2.4 Printed Circuit Board (PCB) Anti-interference Design With the rapid development of technology, PCB density is getting higher and higher, and the quality of PCB design has a great impact on the microcontroller system. a. When designing, try to choose a multi-layer PCB board, with one layer as the ground layer and one layer as the power layer. This choice can form a good decoupling circuit, and adding ground shielding can prevent low potential difference and coupling between components. b. The crystal oscillator should be as close as possible to the microcontroller pins, and the leads should be as short as possible. Use ground lines to isolate the clock area, and ground and fix the crystal oscillator shell. c. Power lines and ground lines should be as thick as possible, which can reduce voltage drop and reduce coupling noise. d. The ground lines of TTL and CMOS devices should be in a radial mesh pattern, avoiding loops. The interface between the two devices should consider level matching. e. When routing printed circuit boards (PCBs), separate weak signal circuits from circuits prone to noise pollution. Separate signal lines from high-voltage control lines and power lines, maintaining a certain distance between them. Distinguish between AC lines, DC regulated power lines, digital signal lines, analog signal lines, digital ground, and analog ground. Larger spacing and shorter lines result in lower noise. Signal lines should be kept as far away as possible from high-voltage lines. If conditions limit the distance, use capacitors or other methods to suppress electromagnetic noise. Isolate digital and analog areas with ground lines, separate digital ground from analog ground, and connect them to power ground at a single point. AC ground and signal ground should not be shared. f. Avoid 90° bends in routing to reduce high-frequency noise emissions. Minimize loop area to reduce induced noise. g. Use perpendicular, oblique, or curved routing between component and solder surfaces to avoid parallel runs and reduce parasitic coupling. Avoid excessively long parallel segments between adjacent conductors. h. Microcontrollers and high-power devices should be grounded separately, and high-power devices should be placed at the edge of the circuit board as much as possible. i. Signal lines with excessively high frequency signals should have terminating matching resistors. To improve interference suppression capability, detector connection cables can use metal mesh shielded wire to suppress electrostatic induction and twisted pair shielded wire to suppress electromagnetic induction. j. High-frequency circuits should be grounded at multiple points nearby to avoid coupling between ground lines, while low-frequency circuits should be grounded at a single point to reduce ground loops caused by ground lines. 3.2.5 EFT Anti-interference Technology When the sinusoidal signal of the oscillation circuit is subjected to external interference, some glitches will be superimposed on its waveform. When shaped by a Schmitt trigger circuit, these glitches will become trigger signals that interfere with the normal clock signal. Alternating between Schmitt trigger circuits and RC filters can eliminate these glitches; this is the EFT anti-interference technology. 3.2.6 Reducing the Interference of Sensitive Components a. Select appropriate components according to circuit parameters, and try to use components with high integration, low temperature drift, good anti-interference performance, and low power consumption. b. Unused I/O ports of the microcontroller should not be left floating; they should be grounded or powered. c. For other ICs, ground or power supply the idle terminals without changing the system logic. d. Minimize the use of microcontroller crystal oscillators and select low-speed digital circuits, provided the speed requirement is met. e. Connect a 0.01uF to 0.1uF decoupling capacitor in parallel between VCC and ground for each IC on the circuit board to act as a filter. Pay attention to capacitor wiring; the connection should be close to the power supply and as short and thick as possible. Otherwise, it will increase the equivalent series resistance of the capacitor, affecting the filtering effect. f. Use power monitoring and watchdog circuits for the microcontroller. g. The standard decoupling circuit for a microcontroller consists of a 100uF capacitor in parallel with a 0.1uF high-frequency capacitor. These two capacitors should be as close as possible to the V/V point to reduce loop effects. h. The microcontroller's data bus, address bus, and control bus are the only channels for information exchange between the microcontroller and the outside world. To improve bus reliability, configure bus drivers and pull-up resistors. 3.3 Software Interference Suppression Technology Hardware anti-interference measures often cannot completely eliminate interference, and microcontroller systems are still susceptible to damage. Software anti-interference technology can further reduce the impact of various interferences. 3.3.1 Instruction Redundancy Technology Taking the MCS-51 as an example, the CPU fetches instructions by first fetching the opcode and then the operands. When the CPU is interfered with, it often executes some operands as opcodes, causing program chaos. To quickly bring a "runaway" program back on track, single-byte instructions should be used more frequently, and two or more single-byte NOP instructions should be inserted at critical points, usually after double-byte and three-byte instructions. Additionally, two NOP instructions should be inserted before instructions that play a decisive role in program flow (such as RET, RETI, LCALL, SJMP, etc.) to ensure the program quickly returns to normal, or valid instructions can be rewritten to ensure correct instruction execution. This is instruction redundancy. 3.3.2 Software Trap Technology When a program "runs away" to a non-program area, instruction redundancy cannot solve the problem. In this case, a software trap can be set to interrupt the runaway program and redirect it to a designated location for error handling. For example, in MCS-51, assuming the entry address of the error handling routine is ERROR, the following instructions can be set in appropriate places: NOP, LJMP, ERROR. Software traps are mainly arranged in unused interrupt areas, unused program space, non-program space, program execution area, and interrupt service routine area. 3.3.3 Software Watchdog Technology When an out-of-control program enters an "infinite loop," redundant instructions and software traps are powerless, and a "watchdog" technology is usually used. Watchdog technology is divided into hardware and software types; this section mainly introduces software watchdogs. For example, MCS-51 has two timers, T0 and T1, which can be used to monitor the main program. Timer T0 monitors timer T1, timer T1 monitors the main program, and the main program monitors timer T0. The "watchdog" determines that the program has encountered an error based on the absence of operation within a specified time interval during program execution. This ring-structured software "watchdog" has good anti-interference performance. 3.3.4 Software Filtering Techniques Interference can cause instantaneous sampling errors or misreading of the input signal to the microcontroller. To eliminate the influence of interference, software filtering methods can be used. Commonly used software filtering methods include: a. Median Averaging Filtering: Multiple N samples are taken of the important signal. The maximum and minimum values ​​are removed, and the average of the remaining N-2 A/D conversion values ​​is taken. This method can eliminate sampling value deviations caused by occasional pulse interference. b. Program Judgment Filtering: Based on experience, the maximum deviation value ΔY between two samples is determined. If the difference between the two sampled signals is greater than ΔY, it indicates interference and should be removed. The previous sampled value is compared with the current sampled value. If it is less than or equal to ΔY, it indicates no interference, and the sampled value is valid. This method can filter out random interference and errors caused by sensor instability. c. Recursive Averaging Filtering: N consecutive sampled values ​​are considered as a queue with a fixed length. Each new sampled value is placed at the end of the queue, and the original head data is removed. The N data in the queue are then arithmetically averaged to obtain a new filtering result. This method has a good suppression effect on periodic interference. 3.3.5 Output Port Anti-interference Technology When peripheral devices operate, they often generate electromagnetic pulses, which affect the output signal. To reduce interference to the output channel, the method of periodically adding output port refresh instructions in the program can be used. The output port should be in a specified RAM unit in the program, and the I/O port is refreshed according to the contents of these RAM units when the program runs. Alternatively, the control command can be written repeatedly, with the repetition period as short as possible. This way, the correct signal arrives before the output device can respond to an interference, which can prevent malfunctions. [b]4 Conclusion[/b] Anti-interference technology is an important part of the design of microcontroller systems. Reasonable use of software and hardware anti-interference technology can minimize the generation of interference and restore the system to normal operation, thus ensuring stable operation of the system. In previous spaceborne instrument designs, anti-interference measures combining the above-mentioned methods have been adopted according to the actual situation of the system. Practice has proven that the above anti-interference methods are effective. Aerospace engineering requires highly reliable and high-quality products. Therefore, only by taking corresponding measures for different situations and minimizing the impact of interference can we ensure the long-term stable, reliable and safe operation of the instrument. [b]References[/b] [1] Wang Xingzhi, Wang Lei, et al. Anti-interference technology for single-chip microcomputer application systems [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 2001. [2] He Limin. Design of single-chip microcomputer application systems [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 1998.11. [3] Wang Liying, Jin Xiaoli. Anti-interference technology for microcomputer systems [J]. Modern Electronics Technology, 2006, (5): 108-110. [4] Zhang Jun, Peng Xuange. Hardware anti-interference technology for embedded systems [J]. Microcomputer Information, 2006, 5 (2): 16-17. The innovation of this article is that, based on the causes of interference and combined with the actual application of the spaceborne single-chip microcomputer system, it systematically and comprehensively introduces various anti-interference measures from both software and hardware aspects. On the hardware side, the paper introduces anti-interference technologies such as power supply, shielding, isolation, printed circuit boards, and EFT, as well as how to reduce interference from sensitive components. On the software side, it introduces anti-interference technologies such as instruction redundancy, software traps, software watchdog timers, software filtering, and output ports. Edited by: He Shiping