There are three main sources of electromagnetic disturbance to the industrial system: first, space field disturbance, which enters the system through electromagnetic wave radiation; the second is power disturbance, which directly damages the system; The third is the signal disturbance channel, which enters the system through the front and rear channels connected to the host. In order to ensure the accurate, reliable, stable and continuous operation of the instrument itself, it is necessary to adopt some inductive methods for the single-chip microcomputer application system, so that the system can be actively restored to normal operation after being disturbed, so as to ensure its long-term reliable and stable operation, in line with the electromagnetic compatibility specifications of the instrumentation.
1. Choose a low-frequency microprocessor
The selection of microprocessors with low external clock frequency can effectively reduce the noise and improve the anti-disturbance ability of the system, the same frequency of square wave and sine wave, the high frequency component in the square wave is more than that of the sine wave, the higher the frequency, the simpler the emission becomes the noise source. Therefore, under the premise of meeting the functional requirements of the system, the low-frequency microprocessor should be selected as much as possible.
2. Reasonable arrangement of wiring and components of printed circuit boards
The orientation of the arrangement should fully consider the problem of anti-electromagnetic disturbance, and the lead wires between the parts should be as short as possible. The delay time of the signal transmitted on the circuit board should not be greater than the nominal delay time of the device used. In terms of layout, the analog signal part, the high-speed digital signal part, and the noise source part (relay switch) are reasonably separated, so that the signal coupling between them is small.
The high-frequency choke ring is connected in series in the external interface line of the important and easily disturbed circuit board to press the high-frequency disturbance and also play a role in resisting the static electricity carried by the operator.
The optocoupler is used at the signal output and input end for optoresistive isolation, and the two ends of the optocoupler are separated from the power supply, which can effectively press the tip pulse and various noise disturbances. At the input end of the photocoupler, even if it is a high amplitude disturbance, it will be pressed because there is not enough energy to make the LED emit light.
Use the single-point grounding method. A 0.1μF decoupling capacitor is added to the power supply of each chip, which has two effects: on the one hand, it is the energy storage capacitor of the integrated circuit, which supplies and absorbs the charging and discharging energy of the integrated circuit when the integrated circuit opens and closes the door, and on the other hand, bypasses the high-frequency noise of the device. Add a 10μF de-high-frequency capacitor as soon as the power supply enters the board.
Try to use a 45-degree polyline instead of a 90-degree polyline to reduce the external emission and coupling of high-frequency signals. No signal forms a loop.
3. Choose shielding skills
The closed metal box made of copper or aluminum and other metal materials with outstanding conductive function can damage the electric field disturbance path and has the function of electrostatic shielding, and the shielding box can also damage the magnetic field disturbance path after grounding, and has the function of electromagnetic shielding.
4. Use hardware "watchdog"
The English name of the "watchdog" timer circuit is Watchdog Timer, or WDT for short. Its effect is to monitor the operation of the microcontroller, and once the "crash" is found, the normal operation of the reset signal rehabilitation program is announced. There are many types of WDT circuits, but the basic principle is the same. The WDT circuit is an independent timer, and the microcontroller will constantly announce the timing zero signal to WDT in normal operation, and clear the WDT timer, so that WDT can not generate a reset signal. If the single-chip microcomputer is dead, the WDT timer can not receive the zero signal, and the timer is full to generate an overflow signal to reset the single-chip microcomputer and the normal operation of the rehabilitation program.
However, the hardware watchdog is not flawless, and sometimes it crashes without resetting, so consider combining software anti-interference methods to deal with it.
5. Improve the reliability of software
In the process of system software development, in order to improve the reliability of the system, try to avoid mistakes and faults.
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