Reset circuit design of the smallest microcontroller system

I explained it yesterday, which is the minimum system. The minimum system of the microcontroller includes: reset circuit, crystal oscillator circuit, power supply circuit, and download circuit. Start today by explaining each circuit and combining it with the actual design to demonstrate the actual circuit.

1. What is

The purpose is to generate a reset signal on the reset pin of the microcontroller, which puts the microcontroller in a reset state and causes the microcontroller program to be executed from the beginning, thereby avoiding the microcontroller program from running away.

2. What is the function of the microcontroller reset circuit?

The main function of the microcontroller reset is to load the value of the special function register as the default value. Because the microcontroller is disturbed by external interference during the operation and the data in the register is confused, it cannot continue to execute the program normally (called a crash) or the instructions of the microcontroller do not execute according to the program. If it points to an unknown location (commonly known as running away), then the microcontroller needs to be reset to make the program start running again.

3. What are the ways to reset a microcontroller?

There are generally three ways to reset a microcontroller: power-on reset, watchdog reset, manual reset, etc.

Power-on reset: When the microcontroller is powered on, it sends a reset signal (high level or low level for a certain period of time) to the reset pin to reset the microcontroller. After stabilization, the microcontroller starts executing the program.

Watchdog reset: Watchdog reset is different from power-on reset. Power-on reset causes the microcontroller to execute the program from the beginning; while during watchdog reset, when a certain program block is out of control, the program counter is cleared to zero. The program is executed from the beginning. In order to make the microcontroller system run normally, the dog must be fed regularly in the program.

Manual reset: Connect a button to the reset pin of the microcontroller, and manually press the button to reset the microcontroller.

4. Reset level of microcontroller

Different microcontrollers have different reset levels. Some microcontrollers reset at a high level, and some microcontrollers reset at a low level. Let's look at the reset circuit.

Microcontroller power-on high-level reset circuit:

Power-on high level reset circuit

At the moment of power-on, since the voltage across the capacitor cannot mutate, the potential at both ends of the capacitor is VCC (at this time the charging current is maximum and the capacitor is equivalent to a short circuit), that is, RST is high level, and as VCC passes through the RC circuit, the capacitor is Charging, when the voltage across the capacitor reaches VCC, the capacitor is equivalent to an open circuit (at this time, the capacitor charging current is 0, that is, DC isolation), the level of RST is low level, and the microcontroller operates normally.

Microcontroller power-on low-level reset circuit:

Power-on low level reset circuit

At the moment of power-on, since the voltage across the capacitor cannot mutate, the potential at both ends of the capacitor is GND, that is, RST is low level. As VCC charges the capacitor through the RC circuit, when the voltage across the capacitor reaches VCC, RST The level is high level and the microcontroller is working normally.

Manual reset circuit:

Manual flat reset circuit

This circuit is also a power-on high level reset circuit. When the microcontroller is working normally, if button S1 is pressed, the reset pin RST gets a high level, thereby realizing reset.