MCU minimum system composition and power supply/reset/oscillation circuit analysis

    The minimum system of the single-chip microcomputer is mainly composed of power supply, reset, oscillation circuit and expansion part. The minimum system schematic is shown in the figure.

   Power Module

    For a complete electronic design, the first 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 basis for the smooth operation of the system. Although the 51 single-chip microcomputer has been used for the earliest time and has the widest application range, in actual use, a typical problem is that compared with other series of single-chip microcomputers, the 51 single-chip microcomputer is more susceptible to interference and the program will run away. An important means to overcome this phenomenon is to configure a stable and reliable power supply module for the single-chip microcomputer system.

    Power module circuit diagram

    The power supply module in this minimum system can be powered by the USB port of the computer or by an external stable 5V power supply module. A power indicator LED is connected to the power circuit. R11 in the figure is the current limiting resistor of the LED. S1 is the power switch.

    Reset Circuit

    The setting and resetting of a microcontroller are both to initialize the circuit to a certain state. Generally speaking, the function of a microcontroller reset circuit is to initialize a state machine to an empty state, for example. Inside the microcontroller, when it is reset, the microcontroller loads some registers and storage devices with a value preset by the manufacturer.

    The principle of the microcontroller reset circuit is to connect an external resistor and capacitor to the reset pin RST of the microcontroller to achieve power-on reset. The reset is valid when the reset level lasts 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 time constant of the RC circuit.

    The reset circuit consists of two parts: button reset and power-on reset.

    (1) Power-on reset: The STC89 series single chip is reset at a high level. Usually, a capacitor is connected to VCC on the reset pin RST, and then a resistor is connected to GND. This forms an RC charge and discharge loop to ensure that the RST pin has a high level for a sufficient time to reset the microcontroller when it is powered on, and then returns to a low level to enter normal working state. The typical values ​​of this resistor and capacitor are 10K and 10uF.

    (2) Push-button reset: Push-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 pulled to a high level. Moreover, due to the charging of the capacitor, the high level will be maintained for a period of time to reset the microcontroller.

    Oscillator Circuit

    There are crystal oscillators in the microcontroller system. The crystal oscillator plays a very important role in the microcontroller system. The full name of the crystal oscillator is crystal oscillator. It combines with the internal circuit of the microcontroller to generate the clock frequency required by the microcontroller. The higher the clock frequency provided by the microcontroller crystal oscillator, the faster the microcontroller runs. The execution of all instructions connected to the chip is based on the clock frequency provided by the microcontroller crystal oscillator.

    Under normal working conditions, the absolute accuracy of ordinary crystal oscillator frequency can reach 50 parts per million. Advanced ones have higher accuracy. Some crystal oscillators can also adjust the frequency within a certain range by applying an external voltage, which is called a voltage-controlled oscillator (VCO). Crystal oscillators use a crystal that can convert electrical energy and mechanical energy to each other in a resonant state to provide stable and accurate single-frequency oscillation.

    The function of the microcontroller crystal oscillator is to provide a basic clock signal for the system. Usually a system shares a crystal oscillator to facilitate synchronization of various parts. Some communication systems use different crystal oscillators for baseband and radio frequency, and synchronize them by electronically adjusting the frequency.

    Crystal oscillators are usually 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 an 11.0592MHz crystal oscillator as the oscillation source. Since the microcontroller has an internal oscillation circuit, it only needs to connect a crystal oscillator and two capacitors externally. The capacitance is generally between 15pF and 50pF.