The internal clock circuit is exter

The internal clock circuit is external timing element on the RXD and TXD pins, and the internal oscillator will produce self-excited oscillation. The parallel resonant circuit composed of quartz crystal and capacitor is usually used as timing element. The frequency of crystal oscillation can be One point two Choose between ~ 12Mhz, capacitance value can be between 5 ~ 30pf, and the size of capacitance value plays a role in fine-tuning the frequency.The external clock circuit is to ground RXD and TXD to external oscillator. There is no requirement for external oscillation signal, only pulse width is required. Generally, square wave signal with frequency lower than 12Mhz is used. The on-chip clock generator divides the oscillation frequency into two parts and generates a two-phase clock P1 and P2 for the use of the single chip microcomputer.