The minimum system of single-chip microcomputer is mainly composed of power supply, reset, oscillation circuit and extension parts. A minimal system schematic is shown in Figure 4.1. Figure 4.1 Minimum system circuit diagram Power supply module Figure 4.1.1 Circuit diagram of power module For a complete electronic design, the primary problem is to provide a power supply module for the entire system. The stability and reliability of the power supply module is the premise and foundation for the smooth operation of the system. Although the 51 single-chip microcomputer has the earliest use time and the widest application range, in the actual use process, a typical problem is that compared with other series of single-chip microcomputers, the 51 single-chip microcomputer is more prone to interference and the phenomenon of program running away, to overcome this phenomenon. An important means that appears is to configure a stable and reliable power supply module for the microcontroller system. The power supply of the power supply module in this minimum system can be supplied through the USB port of the computer, or it can be supplied by an external stable 5V power supply module. A power indicator LED is connected to the power supply circuit, and R11 in the figure is the current limiting resistor of the LED. S1 is the power switch. reset circuit Figure 4.1.2 Reset circuit diagram The setting and resetting of the single-chip microcomputer are all to initialize the circuit to a certain state. Generally speaking, the function of the single-chip reset circuit is to initialize a state machine to an empty state. Registers and storage devices are loaded with a value preset by the manufacturer. The principle of the reset circuit of the single-chip microcomputer is to connect an external resistor and capacitor to the reset pin RST of the single-chip microcomputer to realize a power-on reset. The reset is valid when the reset level persists for more than two machine cycles. The duration of the reset level must be greater than two machine cycles of the microcontroller. The specific value can be calculated by the RC circuit to calculate the time constant. The reset circuit consists of two parts: key reset and power-on reset. (1) Power-on reset: STC89 series single-chip and high-level reset, usually connect a capacitor to VCC on the reset pin RST, and then connect a resistor to GND, thus forming an RC charge and discharge loop to ensure that the microcontroller is on When the RST pin is powered on, there is enough time for a high level to reset, and then it returns to a low level to enter the normal working state. The typical values ​​of this resistor and capacitor are 10K and 10uF. (2) Button reset: Button reset is to connect a switch in parallel with the reset capacitor. When the switch is pressed, the capacitor is discharged and RST is also pulled to a high level, and due to the charging of the capacitor, it will maintain a high level for a period of time. Reset the microcontroller. Oscillation circuit Figure 4.1.3 Oscillation circuit diagram There are crystal oscillators in the single-chip microcomputer system. The crystal oscillator plays a very important role in the single-chip system. The whole process is called crystal oscillator. It combines the internal circuit of the single-chip microcomputer to generate the clock frequency required by the single-chip microcomputer. The higher the clock frequency provided by the single-chip crystal oscillator, the faster the single-chip microcomputer runs. Fast, the execution of all instructions connected to the single chip is based on the clock frequency provided by the crystal oscillator of the single chip. Under normal working conditions, the absolute accuracy of ordinary crystal frequency can reach 50 parts per million. Advanced is more accurate. Some crystal oscillators can also adjust the frequency within a certain range by an applied voltage, which is called a voltage-controlled oscillator (VCO). The crystal oscillator uses a crystal that can convert electrical energy and mechanical energy to work in a resonance state to provide stable and precise single-frequency oscillation. The role of the single-chip crystal oscillator is to provide the basic clock signal for the system. Usually a system shares a crystal oscillator, which is convenient for all parts to keep synchronized. Some communication systems use different crystals for the fundamental frequency and radio frequency, and maintain synchronization by electronically adjusting the frequency. Crystal oscillators are often used in conjunction with phase-locked loop circuits to provide the clock frequency required by the system. If different subsystems require clock signals of different frequencies, they can be provided by different phase-locked loops connected to the same crystal oscillator. STC89C51 uses a 11.0592MHz crystal oscillator as the oscillation source. Since the microcontroller has an oscillation circuit inside, only one crystal oscillator and two capacitors can be connected externally. The capacitance of the capacitor is generally between 15pF and 50pF. Xiaobian recommended reading: What are the parts of the minimum system of single-chip microcomputer, and what is the function of each part? Debugging method of the minimum system of single-chip microcomputer The concept of the minimum system of single-chip microcomputer_How to draw the minimum system of single-chip microcomputer
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