The role of the capacitor next to the MCU crystal and the analysis of the oscillation circuit

火辣西米秀

The role of the capacitor next to the MCU crystal and the analysis of the oscillation circuit [Copy link]

Most MCU enthusiasts do not understand why a 22pF capacitor is needed on both sides of the MCU crystal, because this capacitor can be omitted in some cases. I have referred to many books, but there is little explanation. What is often mentioned is the stabilization effect and load capacitance, etc., but there is no in-depth theoretical analysis.
The problem is that many enthusiasts do not care about these two capacitors. They think that it is enough to follow the reference design. I am the same. It was not until a mobile phone project had a problem with this capacitor and lost several million that I began to really consider the role of this capacitor.
In fact, the real name of the MCU oscillation circuit is "three-point capacitor oscillation circuit", please refer to the picture on the webpage.

Y1 is a crystal, equivalent to the inductor in the three-point circuit. C1 and C2 are capacitors. 5404 and R1 realize an NPN transistor. You can refer to the three-point capacitor oscillation circuit in the high-frequency book. Next, let's analyze this circuit.
5404 must have a resistor, otherwise it is in the saturation cutoff area, not the amplification area. R1 is equivalent to the bias of the transistor, which puts 5404 in the amplification area. Then 5404 is an inverter, which realizes the role of the NPN transistor. The NPN transistor is also an inverter when connected in the common emitter method.
Next, I will use a popular method to explain the working principle of this three-point oscillation circuit. You can also read the book directly.
As we all know, the condition for a sinusoidal oscillation circuit to oscillate is that the system amplification factor is greater than 1. This is easy to achieve, and the phase satisfies 360°. Next, we will mainly explain this phase problem:
5404 is an inverter, which means that it realizes 180° phase shift, so C1, C2 and Y1 are needed to realize 180° phase shift. Coincidentally, when C1, C2 and Y1 form resonance, 180° phase shift can be realized. The simplest way to achieve this is to use the ground as a reference. When resonating, the current passing through C1 and C2 is the same. The ground is between C1 and C2, so the voltage is just opposite, realizing 180° phase shift.
When C1 increases, the amplitude of C2 increases, and when C2 decreases, the amplitude also increases.
Sometimes C1 and C2 can oscillate without welding. This does not mean that there is no C1 and C2, but it is caused by the distributed capacitance of the chip pins, because C1 and C2 do not need to be very large, so this is very important. Next, we will analyze the influence of these two capacitors on the oscillation stability.
Because the voltage feedback of 7404 depends on C2, if C2 is too large, the feedback voltage is too low, which is also unstable. If C2 is too small, the feedback voltage is too high, the stored energy is too little, and it is easy to be disturbed by the outside world, and it will also radiate and affect the outside world. The effect of C1 is just the opposite to that of C2. Because when we lay out the board, assuming that the double-sided board is relatively thick, the influence of distributed capacitance is not very large. If it is a high-density multi-layer board, it is necessary to consider distributed capacitance, especially for oscillation circuits such as VCO, which should be considered.
For some projects used in industrial control, it is recommended not to use the crystal method for oscillation. Secondly, directly connect an active crystal oscillator.
Many times, people will use 32.768K clock crystals to make clocks, instead of using the crystal frequency division of the microcontroller to make clocks. Many people don’t understand this reason. In fact, this is related to the stability of the crystal. The higher the frequency of the crystal, the harder it is to make a high Q value, and the frequency stability is not high. The stability of the 32.768K crystal is good in all aspects, forming an industrial standard, which is easier to make high.