Application of STWD100xP system hardware anti-electromagnetic interference technology

1 Generation of EMC in Embedded Systems

The magnitude of electromagnetic interference generated by an embedded system and its ability to resist electromagnetic interference are closely related to the function of the system itself. Different embedded systems generate different electromagnetic interference and have different abilities to resist electromagnetic interference.

1.1 High-order harmonic interference

According to the Fourier series, any periodic signal S(t) is not a single waveform, but is composed of DC component, fundamental wave and harmonics. The harmonic frequency is very rich. The periodic signal S(t) can be expressed by formula (1):

DC component, when n=1, it indicates the fundamental wave of the signal, n=2, 3, 4, ..., it indicates the 2nd, 3rd, 4th, ... harmonics of the signal, where the fundamental wave is the main component of the signal. In many cases, the characteristics of the signal show fundamental wave characteristics. For example, in the power supply system, the AC signal is S(t)=380sin 2π50t, which means that the mains signal has a fundamental wave of n=1. The harmonic frequency of the signal increases continuously near the fundamental wave, and the amplitude decays rapidly. Theoretically, there is only a little impulse signal in the spectrum of sine and cosine signals. The harmonics generated by the sine signal itself are small, while other electromagnetic interferences are mixed in the sine signal, causing large harmonics in the system. The system should make use of the fundamental wave of the signal, and suppress and filter out the harmonics of the signal. The sine signal itself has poor suppression ability for external interference signals, which is prone to EMC , seriously interfering with the system operation and affecting the system stability.

1.2 The main frequency of the embedded system is too high, causing interference

With the development of electronic technology, CPU processing speed is getting faster and faster, data throughput is getting larger and larger, and the main frequency is getting higher and higher. The main frequency of the 8-bit 51 single-chip microcomputer system is 12MHz, the main frequency of the 16-bit Lingyang single-chip microcomputer system is 50MHz, and the main frequency of the Altera Cyclone series FPGA chip system can reach 200MHz. The main frequency of the embedded system with Inter PAX255, PAX270, and PAX310 as the core can reach 820MHz. If the CPU main frequency is too high, the signal change rate will be too fast, and places with large signal amplitudes are prone to interference. According to the Fourier Series and Euler Formula, the frequency spectrum of the square wave signal is the Sine signal. The Sine signal is a continuous signal with large harmonics, which can easily cause unstable system operation. 2 Anti-EMC technology for embedded system hardware

There are many factors that make embedded systems unstable and unreliable. Interference can come from the system itself or from other systems outside the system. Software processing can play a certain role in system interference, but it is difficult to support the situation of software without hardware, and the problem of system anti-interference is always difficult to solve. In order to increase system stability and reliability and reduce system CPU expenses in programmable systems, the following methods are often used:

2.1 Suppressing power supply harmonic interference

In programmable electronic systems, solving the power supply interference problem can solve more than half of the interference of the entire system. Many interferences come from the power supply. Excessive power supply interference will cause system instability. Pure sinusoidal signals will not have interference . The key sinusoidal signal is in a very noisy environment, and then the sinusoidal signal will have additional interference. For example: if the power cord is too long, parasitic interference will be generated on the power cord. You can add a small magnetic bead or magnetic ring filter to the power cord. If the requirements are not high, you can add a 100Ω resistor to attenuate the interference signal so that the interference signal has no effect on the system. For switching power supplies, the main reason is that the oscillator oscillation frequency is too high, causing harmonics, which not only affects the power supply but also the entire system. Switching power supplies are more efficient and are used in many places, but linear power supplies are still used in systems with higher requirements.

2.2 Select the CPU with the lowest frequency that can meet the system requirements

If the CPU main frequency is too high, the system clock will speed up, the frequency of the square wave generated by the system clock will increase, and the harmonics caused by the square wave will increase accordingly. From the Fourier Series and Euler Formula, it can be seen that the harmonic amplitude of a single square wave is much larger than that of a sine wave, and the harmonic frequency is much more complex. Therefore, the higher the system main frequency, the more complex the harmonics generated, which affects the stability of the system. However, with the development of electronic technology, the CPU main frequency is getting higher and higher, the electronic system is becoming more and more complex, the functions are constantly increasing, and the integration is constantly improving. But for the CPU, no matter where it is used, the system main frequency is sufficient.

2.3 Reduce signal attenuation, distortion and reflection during transmission

Modern integrated circuit manufacturing mainly uses CMOS technology, which has large input impedance, small output impedance, strong anti-interference ability, and is suitable for small signal amplification. The input current of CMOS integrated circuit is about 1 mA. The signal delay Td on the circuit board is related to the impedance of the circuit board lead. The greater the impedance, the greater the signal attenuation, the longer the signal delay, the greater the system heat, and the worse the system stability. In the process of high-speed system design, the PCB signal transmission delay Td should be considered to make the signal artificial delay Tr greater than the signal transmission delay.

2.4 Pay attention to PCB layout and high-frequency characteristics of components

The most important principle of PCB wiring is to keep the leads between components as short as possible, cross the signal lines as little as possible, and use as few vias as possible. The analog ground, digital ground, and high-frequency ground should be routed separately and then converged into one point. High-power devices, low-power devices, and devices with particularly large interference signals should be separated during layout. High-frequency components that are particularly sensitive to interference signals can be shielded with metal covers to avoid electromagnetic interference.

2.5 Use of decoupling capacitors

Generally, a decoupling ceramic capacitor should be placed next to the power supply pin of the system integrated circuit to the ground to filter out the parasitic interference generated by the power supply during transmission. The selection of decoupling capacitors is not strict, and generally it can be selected according to f=1/C. For the CPU control system, to filter out the parasitic interference of the 10-100 MHz power supply signal , the capacitor should be 0.01-0.1μF ceramic capacitor. In high-speed circuits with higher system requirements, a 1-10μF electrolytic capacitor can be used from the integrated circuit power supply to the ground to filter out the 1-10GHz harmonic interference of the power supply.

3 Anti-EMC technology for embedded system software

In order to enhance the stability and reliability of embedded systems, in addition to hardware anti-interference, embedded systems can also use certain anti-interference to make the system work more stable and reliable.

3.1 Information Redundancy Technology

To prevent interference, the system copies important data or files to multiple storage units. When data in one storage unit is damaged, the backup storage unit is immediately started to restore the data.

3.2 Time Redundancy Technology

In order to enhance system reliability, important areas are executed repeatedly and the results of the first and second executions are compared. If they are the same, they are considered correct. If they are different, there is a problem with the first or second execution or there is a problem with both executions, the program is executed a third and fourth time. If one of them is the same, it is considered correct.

3.3 Automatic detection and diagnosis technology

Automatic detection includes detection of embedded system RAM, ROM, flags, etc. When the computer is turned on, several units can be allocated in the RAM storage area to write different values ​​such as 55H, AAH, EEH, etc. to the RAM storage units. When executing the program, the values ​​of these storage units are read to see if they are equal to the values ​​placed at the start-up, so as to determine the correctness of the program execution. ROM is used to store program or table values, and the number of binary numbers 0 and 1 in the ROM area is used to detect the correctness of the system program execution. During the execution of the program, different data results in the RAM area have different effects on the flags. For example, in the PSW of the 51 single-chip microcomputer, when a carry occurs when two data are added, the 7th bit CY in the PSW is set by hardware. Users can query the 7th bit in the PSW to determine the correctness of the program execution.

3.4 Software Trap Technology

Software trap technology means that the program memory in the embedded system is not necessarily used up. For example, if the embedded system program counter PC value is wrong due to interference, the program will jump to unused storage space, causing system errors or program runaway. The software trap is to put empty instructions and unconditional jump instructions in these unused program memories. When the program runs away and runs to the empty program memory, the embedded system believes that the system is interfered with and the program execution is wrong, and the system automatically resets.

4 Conclusion

In the program-controlled fountain control system with AC contactor, relay, and motor control, the STWD100 hardware watchdog is added. The system operates normally after 50 consecutive switches; it works continuously for 12 hours without interruption and interference from motors and welding machines . The system has no abnormal conditions such as freezing and program running away. The operation is stable and reliable, which solves the interference caused by the operation of relays, AC contactors and other devices, causing CPU malfunction and freezing and other unstable phenomena. In practice, attention should be paid to mastering the time interval for feeding the dog, which is generally best at 1 to 2ms. Without occupying CPU resources, the STWD100 hardware watchdog can improve the system's working stability and reliability to 100%, and can be widely used in control systems with strong interference such as relays and contactors .