The main function of the clock circuit is to generate the clock control signal needed for its work. Based on the clock control signal of SCM, all the functional components of SCM can run in a certain order step by step. Therefore, the quality of the clock circuit directly affects the stability of the MCU system, and the clock frequency also directly affects the speed of the MCU.Internal oscillation mode and external oscillation mode are two common circuit forms to obtain the clock signal of microcontroller. The internal oscillation mode consists of an external ceramic oscillator or crystal resonator on pins XTAL1 and XTAL2. Because the MCU has a high-gain inverted amplifier inside it, when the crystal oscillator is connected to the outside, it forms a self-excited oscillator, which also generates oscillating pulses. The circuit of AT89C51 crystal oscillator is shown in Figure 3.2.