MCS-51 has an inverting amplifier inside. Xtal1 and xtal2 are the input and output terminals of the inverting amplifier respectively. After external timing feedback elements are connected, an oscillator is formed to generate a clock and send it to each component inside the single chip microcomputer. AT89C51 is a CMOS 8-bit microprocessor. Its clock circuit is different from NMOS single chip microcomputer in structure.

There is a controllable negative feedback inverting amplifier inside the CMOS single chip microcomputer (such as AT89C51), and the oscillator is composed of an external crystal oscillator (or ceramic resonator) and a capacitor. The following figure is the clock circuit block diagram of the CMOS single chip microcomputer. The operation of the oscillator is controlled by the / PD end. The software sets “1” PD (i.e. special function register PCON. 1) to make / PD = 0, the oscillator stops working, and the whole single chip microcomputer stops working, so as to achieve the purpose of power saving. Clear “0” PD to make the oscillator work and generate clock, and the single chip microcomputer will operate normally. Sys in the figure is a crystal oscillator or ceramic resonator. The clock frequency generated by the oscillator is mainly determined by sys parameters (the frequency indicated on the crystal oscillator). Capacitors C1 and C2 have two functions: one is to make the oscillator vibrate, and the other is to fine tune the frequency f of the oscillator (C1 and C2 are large and f becomes small), with a typical value of 30pf.



Figure clock circuit diagram of CMOS single chip microcomputer

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