The clock signal of the single-chip microcomputer is generally obtained in two forms of circuit: internal vibration mode and external vibration mode. This system uses the internal oscillation mode and the external crystal oscillator(abbreviated as crystal oscillator) or the ceramic resonator is connected to pins XTAL1 and XTAL2 of AT89C51 to form the internal oscillation mode. Since the internal microcontroller has a high gain inverting amplifier, the external crystal forms a self-excited oscillator whichgenerates oscillator and oscillation clock pulses.