Single chip microcomputer scheme: how does single chip microcomputer play a role in circuit protection?

Correct circuit protection methods can greatly improve the reliability of products or designs. Unfortunately, not all circuit protection takes the form of hardware such as fuses and transient voltage suppression. Some forms of circuit protection may come from software, which may cause confusion. In this paper, we will discuss how single chip microcomputer plays a role in circuit protection.

Circuit protection in single chip microcomputer (MCU)

1、 Basic MCU protection

When designing the circuit protection circuit for single chip microcomputer, we first consider the basic knowledge. This means that any digital pins (such as connectors) connected to conductors and exposed to the outside world need to be properly protected with zener diodes and current limiting resistors. If the analog input is also connected to the external conductor, zener diode and current limiting measures shall also be provided.

Due to its CMOS characteristics, single chip microcomputer design is also sensitive to noisy power rails (noisy power lines will lead to incorrect operation of logic gates). Therefore, it is important to use the correct type of decoupling capacitor. High capacity capacitors, such as polarization capacitors, can be used to deal with sudden power requirements of peripherals and high-power devices. On the other hand, small non-polar capacitors can be used to reduce the switching noise on the power rail.

It is usually necessary to place small non-polar capacitors with low equivalent series resistance (ESR) value on each pair of power pins of single chip microcomputer. For example, pic18f45k22 has two pairs of input power supplies (VDD and VSS), and each pair of input power supplies should be directly connected to a low value ceramic capacitor (e.g. 10nf).

2、 Type of circuit protection

So far, the above protection methods are usually passive and can only deal with surge. What about other forms of circuit protection?

Although voltage and current surges will damage the single chip microcomputer, unreliable power supply and power interruption will undoubtedly cause greater damage (only in the physical sense). Generally, when operating at reduced voltage, the circuit is rarely damaged, but the behavior of circuits involving digital logic may be unexpected.

From an outsider's point of view, MCU based circuits operating at reduced voltage can still drive the display and run the firmware (so it seems to work normally), but data from external sources (such as I2C devices) may become unreliable because they will not operate at the correct voltage level. This can lead to false reliability. If the circuit is under control For the whole equipment, only half of the circuit is working, otherwise there is a risk of serious injury.

So, how to protect the circuit from power reduction?

3、 Power down detection in single chip microcomputer

Although overvoltage and current can usually be reduced by using external discrete components (such as fuses and diodes), it is more difficult to reduce power (called power failure).

One way is to design a special The circuit can operate at low voltage level and actively monitor the power rail. The problem with this method is that it may increase the design price and the complexity of * * * final PCB. However, there are usually many resources that may reduce the cost of designers. That is the MCU itself. MCU based designs usually have spare peripherals, pins, memory and processing time. This means that designers can perform active circuit health monitoring without including other components in the design. Some microcontrollers contain power down peripherals, which may cause a reset and set a flag to indicate to the firmware that a power down has been detected.

So, how to realize the basic circuit monitoring system with single chip microcomputer?

1. Single chip microcomputer input voltage

The following is the basic layout of circuit protection based on single chip microcomputer, which only needs two hardware. Standby analog input and internal bandgap reference. When the bandgap reference voltage source provides a reference voltage, it is necessary to use an analog input to measure the external VDD voltage. Since ADC usually works, the importance of bandgap reference cannot be underestimated. The ADC will produce a binary output whose value is proportional to the ADC and the two reference voltages of the ADC. If the ADC reference is a power rail, the ADC reading of the VDD power rail will always be * * * value. The use of bandgap reference provides a voltage reference for ADC, which is independent of the power supply voltage, so it allows the single chip microcomputer to measure the power supply voltage.

2. Power down detection circuit

In addition, we can use power down detection circuit, but this is only useful when the power supply is enough to run MCU rather than external hardware. Unfortunately, this method can not protect the MCU from power failure. In case of power failure, special equipment is required Power down detection circuit or single chip microcomputer with its own power down detection system. For many designers, the use of additional external circuits is usually too expensive in terms of product cost, number of components and PCB area. Therefore, * * * uses a single chip microcomputer with built-in power down detection peripherals.

Now, the single chip microcomputer is equipped with. Once the power failure is detected, it will reset the MCU and set a special Special CPU flag to notify the user of power down reset. Since MCU performs reset operation, it must be reset Special procedures must be carried out first Special handler that checks the CPU flag (including power down flag). If this flag is set, specific measures can be taken, such as turning off peripherals and displaying warning lights.

Wuxi Sijie Microelectronics Co., Ltd. (hereinafter referred to as "Wuxi Sijie") was established in 2017 and is located in Wuxi, Jiangsu, the national integrated circuit and Internet of things industrial base. Wuxi Sijie has been selected as a private science and technology enterprise in Jiangsu Province, and "Gazelle enterprise" has been selected and put into storage. It has successfully passed the certification of intellectual property standard implementation system in early 2021. Registered capital of the company: RMB 10 million only. Main business scope: the development and sales of integrated circuits and supporting products, and the provision of a full set of technical consulting and technical services in the above related fields