89c51 microcontroller minimum system, detailed function explanation of 89c51 minimum system schematic diagram

　　The minimum system of a single-chip microcomputer is composed of some components necessary to form a single-chip microcomputer system. In addition to the single-chip microcomputer, it also needs to include a power supply circuit, a clock circuit, and a reset circuit. Microcontroller minimum system circuit schematic diagram.

     Detailed function explanation of 89c51 minimum system schematic diagram

　　1: Power circuit

　　The power circuit is the power supply circuit of the microcontroller, usually 3.3V or 5V. The specific amount depends on the operating voltage of various types of microcontrollers. It is usually 5V, here refers to the normal situation.

　　2: Clock circuit

　　The clock circuit is a crystal oscillator circuit. Generally, a 12Mhz crystal oscillator is selected to facilitate the use of timer and counter functions. There is a high-gain inverting amplifier in AT 89C51, which is the main unit that constitutes the internal oscillator. XTAL2 and pin XTAL1 are the output and input terminals of the amplifier respectively.

　　The off-chip quartz crystal or ceramic resonator and amplifier together form a self-excited oscillator. The bypass capacitors Cl and C2 and the external quartz crystal (or ceramic resonator) are connected to the amplifier with feedback function to form a parallel feedback oscillation circuit.

　　Even if there are no very strict requirements for external bypass capacitors Cl and C2, the size of the capacitor will slightly affect the stability of the oscillator frequency, the amplitude of the oscillation frequency, the ease of starting the oscillation procedure, temperature stability, etc. . If a quartz crystal is used, the capacitor is usually 30pF±10pF, and if a ceramic resonator is used, the capacitor is usually 40pF±10F.

　　3: Reset circuit

　　Whether it is when the microcontroller is first connected to the power supply or when a fault occurs during operation, it is necessary to

　　To reset. The reset circuit is used to restore the state of each circuit inside the microcontroller to a certain initial value and start working from this state. The reset condition of the microcontroller: its RST pin must maintain a high level for two (or more) machine cycles. The reset form of the microcontroller: power-on reset, button reset.

　　In the power-on reset circuit, capacitor charging is used to achieve reset. At the moment when the power is turned on, the potential on the RST pin is high level (Vcc). After the power is turned on, the capacitor is quickly charged. As charging proceeds, the potential on the RST pin will gradually decrease to low level. . As long as the time when the high level appears on the RST pin is greater than two machine cycles, a normal reset can be achieved.

　　In the key reset circuit, when the key is not pressed, the circuit is the same as the power-on reset circuit. For example, when the RESET button is pressed while the microcontroller is running, the charged capacitor will quickly discharge through the 200Ω resistor circuit, causing the potential on the RST pin to quickly change to a high level. This high level will remain until the button is pressed. Release, thereby meeting the conditions for microcontroller reset to achieve button reset.