The importance of power supply voltage monitoring in the microcontroller

The power supply is arguably the simplest device in the system of the designed system, but it is often placed at the end of the overall design. In order to ensure the reliable operation of the single chip microcomputer, the requirements for the power supply system are getting higher and higher. In the single-chip microcomputer system, for example, high-frequency and high-precision A/D data conversion generally requires that the voltage deviation does not exceed 5%. If the supply voltage of the chip is 5V, the voltage deviation cannot exceed 0.25V. Otherwise, the operation of the MCU may be unstable, and the accuracy of the input/output relationship may be degraded (such as A/D conversion accuracy), which may cause errors or confusion in the internal program of the system, and even affect the life of the chip and cause chip damage. Therefore, real-time monitoring of the voltage is required in the circuit through the voltage monitoring circuit, so that the power supply can provide a qualified and stable voltage for the chip. TL77xxA series chip is designed for the voltage monitoring of single-chip system (Figure 1, pin definition: REF reference voltage; CT delay; GND ground; VCC power supply; SENSE monitoring voltage; RESET reset output; chip select). It mainly has the following advantages: power-on automatic reset output, wide voltage monitoring range, built-in precision voltage source, self-contained temperature compensation, adjustable reset output time, and complementary reset output. Therefore, the TL77xxA series chips are widely used in single-chip systems. The following is a brief introduction to the principle of TL77xxA, and will illustrate its application in practice.

â—† Principle of TL77xxA series

l. Principle of TL77xxA series:

The hardware schematic of TL77xxA series integrated circuit is shown in Figure 2. The working voltage waveform is shown in Figure 4. The voltage is input from the SENSE pin to determine if the voltage is under voltage.

When the VCC pin voltage rises to 3.6V after power-on, the RESET output will go low. When the SENSE pin level is higher than VIT+, the internal timing function of the chip will generate a certain delay (the delay time is determined by the external capacitor CT, td=1.3X104XCT (unit: CT farad, td seconds)), RESET is low level RESET high level during the time. After the delay time expires, the RESET and RESET signals are turned off (RESET is high, RESET is low), and the power-on reset is completed. When the system suddenly runs out of power during normal operation, once the SENSE pin voltage is lower than VIT-, the RESET and RESET outputs will be low and high respectively and held until VCC is 2V. When VCC is below 2V, the RESET and RESET output signals are unknown. (For TL7705A, VIT+=V IT-=4.6V.)

Note: The REF pin must be connected to an external capacitor to eliminate the effects on the chip when the voltage changes rapidly. The typical value of the capacitor is 0.1 uf.

2. TL77xxA series specific model

TL77xxA series chips play an important role in voltage monitoring with its own characteristics, and the cost is low, the functions are many, the hardware lines are simple, and the TL77xxA series products can satisfy a variety of power supply systems. Below we will illustrate the application of TL77xxA in practical systems in several forms commonly used in microcontrollers.

Application of TL7705A in Voltage Monitoring in Single Chip Microcomputer System

1. General Application of TL7705A in AT89C52 Microcontroller

In the AT89C52 system of the input water level meter we studied, we use the TL7705A to monitor the voltage of the single-chip system. As shown in Figure 6, the TL7705A acts as a voltage monitoring chip for the AT89C52 system to improve system operational reliability. The connection between system chips is shown in Figure 6. Ct is an external capacitor whose value affects the RESET and RESET output signal hold time td. In order to ensure reliable reset of the AT89C52 in this system (at least two complete clock cycles T is required for AT89C52 reset, the system assumes T=100ns), and the CT is taken as 100pf, so that the AT89C52's demand for reset signals can be satisfied.

System power-on: After the VCC pin voltage rises to 3.6V, the RESET output will change from low level to high level, and the AT89C52 chip will be reset. The RESET pin voltage high state will remain low for 1.3μs. At this point the system is completed - a power-on reset.

Manual reset: After the system is powered on, once the button RST is pressed, the system outputs RESET and RESET immediately become low level and high level. When the button is restored, the RESET and RESET outputs will go high and low. System completed - one manual reset.

System power-down: When the SENSE pin voltage is lower than V IT-, the RESET output will go high until VCC is below 2V. At this point, the system completes a power-down reset.

Therefore, after installing a TL7705A, the system runs much more stably. The impact of the microcontroller power supply on the system will be very low.

2. Application of TL77xxA series chips in multi-voltage monitoring system

As shown in Figure 7, the system uses a TL7705A and two TL7712A to monitor the +12V, +SV, and -12V power supply voltages commonly used in the single-chip system. Once any one or several voltages of the system are powered down or the power supply is abnormal, SYSTEM RESET will change from high level to low level to inform the system and handle it accordingly (for example, switching from main power to standby power supply, etc.) ), providing a reliable guarantee for the stable and safe operation of the system.

summary

In the application of single-chip microcomputer, we put forward higher requirements for the safety and stability of the system under high-frequency operation. Some of the original technologies have not met the requirements. The power-reset circuit (Figure 8), which was originally used in single-chip systems, cannot meet the requirements because its reset is unreliable. Therefore, some advanced technologies need to be used in large numbers in practice. Our input water level gauge uses the TL7705A as the voltage monitoring chip of the AT89C52 system, which greatly improves the system operation reliability compared with the original power reset circuit. Therefore, the various problems that appear in the practical application of the single-chip microcomputer have become the difficult point that we have to solve as soon as possible. In today's increasingly mature chip technology, integrated chips are increasingly demanding power supplies. Therefore, we must not only solve the problem from the power supply and the system hardware circuit itself, but also start from the power supply voltage monitoring of the single chip microcomputer to ensure the normal and safe operation of the system. Therefore, the power supply voltage monitoring chip will be more and more valued and applied.

With the increasing application of single-chip microcomputers, and the working environment is relatively harsh, the reliability of the system operation will be higher and higher, so the stability requirements of its power supply are becoming more and more urgent. I believe this article will be in this regard. The power supply design has a certain reference value.