Overview of Methods for Improving Hardware Reliability of Single Chip Design

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1, circuit design

45% of the factors affecting the reliability of the MCU measurement and control system come from system design. In order to ensure the reliability of the measurement and control system, the worst case design should be made when designing the circuit.

The characteristics of various electronic components cannot be a constant value, and there is always a range of variation above and below their labeled values. At the same time, the supply voltage also has a range of fluctuations. The worst design (referring to the worst case of the working environment) is to consider the tolerances of all components and take the most unfavorable value. Accounting for each specified characteristic of the circuit. If this set of parameter values ​​is guaranteed to work properly, all other component values ​​within the tolerance range will enable the circuit to operate reliably. When designing the application system circuit, it is necessary to take corresponding measures according to the failure rate characteristics of the component and its place of use:

At the component level, parts that are prone to short circuits are replicated in series; those that are prone to open circuits are replicated in parallel and alarm and protection devices are placed in these sections.

2, component selection

(1) Model and tolerance After determining the component parameters, the type of the component is also determined, which depends mainly on the tolerance range allowed by the circuit. For capacitors, if used in a normal temperature environment, the general electrolytic capacitor can meet the requirements. For circuit systems with high capacitance tolerance requirements, electrolytic capacitors should not be used.

(2) The failure rate of derating components is doubled with the operating voltage. Therefore, the capacity of the system power supply is greater than the maximum value of the load, and the rated operating conditions of the components are various, such as current and voltage frequency, power, mechanical strength and ambient temperature. The derating is to reduce these parameters. In circuit design, the first consideration is to reduce its efficiency. When using a capacitor, reduce its operating voltage. The voltage used is generally less than 60% of the rated voltage. When selecting diodes and thyristors, the operating current should be lower than the rated current. For transistors, voltage regulators, etc., the power dissipation during operation should be considered.

The derating of integrated circuits is also considered in terms of electrical parameters and environmental factors. To reduce power consumption electrically, the CMOS chip and linear integrated circuit should reduce the power supply voltage or reduce the lower load if the output requirements are met. The TTL circuit has strict requirements on the power supply voltage. At this time, attention should be paid to their load carrying capacity. The temperature range of the civilian components is narrow. If it is used in industrial control, it should be derated in the overall design.

3, structural design

Structural reliability design is the final stage of hardware reliability design. The structure design should first pay attention to the installation method of components and equipment; secondly, the environmental conditions of the control system, such as ventilation, dehumidification, dustproof, etc.

4, noise suppression

Noise on the analog circuit will affect the accuracy of the system, causing malfunctions on the digital circuit. Therefore, in engineering design, suppression measures must be taken. Interference signals can be divided into two categories: serial mode interference and common mode interference. There are also a lot of mature circuits and special works for these two types of interference. The following commonly used methods are introduced as follows:

4.1 Measures against cross-mode interference

(1) Optical isolation, using optical coupling devices on the input and output channels for information transmission, so as to avoid the upper level interference to the next level.

(2) Hardware filter circuit, commonly used RC low-channel filter is connected to some low-frequency signal transmission circuits (such as thermocouple input lines, etc.), which can greatly weaken various types of high-frequency interference signals (such as various types of "burr" interference).

(3) Overvoltage protection circuit, a certain overvoltage protection circuit should be used on the input and output channels to prevent the introduction of high voltage damage to the microcontroller system.

(4) The anti-jamming power supply is adopted, and the power supply line of the microcomputer system is the main intrusion path of interference. The following measures are usually used:

1 The power supply line of the single-chip microcomputer system and the power-consuming equipment that generates interference are separately supplied with power;

2 connected to the grid through a low-pass filter and an isolation transformer;

3 The rectifier capacitor is connected to the filter capacitor. As shown in Figure 1, a high-quality voltage regulator circuit is selected.

4.2 Anti-common mode interference measures

Common mode interference is usually for balanced input signals. There are several ways to resist common mode interference:

(1) Balanced symmetrical input. When designing a signal source, it is usually balanced and symmetrical as various types of signals.

(2) Select a high quality differential amplifier.

(3) Have a good grounding system.

(4) The system grounding point should be properly connected; the high-power component ground wire and the low-power signal ground wire in the system should also be separately routed or thickened, the digital ground must be separated from the analog ground, and finally connected only at one point. If the digital ground in the system is not divided into the analog ground, the digital signal current forms interference (ground potential change) in the ground of the analog system, which distorts the analog signal. This is especially important for beginners.

(5) Shielding, surrounded by a metal casing or metal enamel, or a metal casing or metal 匣 grounding, can play a shielding role. For various disturbances caused by electromagnetic induction, special attention is paid to the grounding point of the shielded enclosure, which must be connected to the reference point of the signal.

Several of the above are very necessary hardware anti-jamming measures, which create a basically "clean" working environment for the microcontroller system.

references

1 Guangjun. Practical interface technology. Xi'an University of Electronic Science and Technology Press, 1997

2 Mao Nan, Sun Hao. Electronic circuit anti-interference practical technology. Beijing: National Defense Industry Press, 1996