Xtal1 and xtal2 refer to the two pins commonly used to connect "crystal resonator" on 8051 series single chip microcomputer. In principle, these two pins are connected with an inverter in MCU. The inverter and the external "crystal oscillator" form a pierce oscillator. Because the components of the oscillator integrated in the device can't be simpler. Just one inverter and one resistor are very suitable for the design and manufacturing process of various digital IC.
When analyzing the working principle of this pierce oscillator, we may as well describe it as the following ideal circuit form:
The knowledge of analog electronics tells us that when we expect to get an oscillating circuit whose output signal frequency is the picture, the circuit must satisfy two conditions in the pictur
The loop phase shift of the picture.
The closed loop gain is 1.
In the above circuit diagram of Pierce oscillator, it is not difficult to find that the inverter U1 provides 180 for any frequency component °， That is, the amount of phase shift of the picture. At the same time, the inverter can be regarded as a buffer between the input and output, so by adjusting the output characteristics of the inverter, it is easier to get 1 loop gain.
Some people will ask here, the phase shift is only in the picture, and even one of the above two conditions is not met, how can it start to vibrate? The crux of the problem lies in the other components in the circuit.
First of all, for a "crystal oscillator" in the circuit, quartz crystal itself has piezoelectric effect. After proper treatment of quartz crystal, a kind of piezoelectric resonance device can be obtained, which is the common quartz crystal resonator (hereinafter referred to as QCR). By analyzing the physical characteristics of QCR, it can be found that the piezoelectric resonance process of QCR can be expressed almost perfectly by the following ideal circuit model.
