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Detailed description of the role of 22pf or 30pf capacitors in microcontroller crystal oscillator circuits

A senior who just learned about single-chip microcomputers told me that a 22pf or 30pf capacitor should be used in the crystal oscillator circuit of the single-chip microcomputer. I listened to the advice and everything was ok after soldering the circuit. I never thought about why. I knew the reasons but didn't know why. It was really sad. I seem to have been in a bad state recently and it is difficult to explain why. A few days ago, I followed the teacher to other laboratories to attend classes. In fact, I just listened to the teacher and the master talking about all kinds of embedded systems to the students in other laboratories. In addition, the master talked to me alone for a long time that day. I thank them from the bottom of my heart. They made me know how to reflect and adjust. I have a new understanding of what kind of learning attitude I should have and how to put it into action. This is much better than simply giving me some knowledge.

Speaking of this little knowledge point, I have such an experience, and I would like to share it with you. I once helped a girl make something. In fact, it was a super simple temperature acquisition system of ATMEGAL16 microcontroller. Although my welding skills are average, I am helping a girl and I am relatively attentive, so there should be no problem. However, the fact is not satisfactory. The smallest system welded out did not work. I checked the circuit several times with a multimeter but could not find the error. Then I suspected that the microcontroller was locked. I changed several microcontrollers but it still did not work. I still thought that I was a good student among my classmates. It was a bit ridiculous. In the end, I found that when I was short of original parts, I confusedly soldered two 0.1uf capacitors in the crystal oscillator circuit, causing the crystal oscillator to not oscillate, so the whole circuit did not work. It worked immediately after replacing it with a 22pf capacitor. I should have figured it out immediately, but I ignored it, which was really wrong. The teacher was right. We really lack the spirit of getting to the bottom of things.

Let's take a look at why 22pf or 30pf capacitors are used in crystal oscillator circuits instead of others.

In fact, the real name of the oscillation circuit of the microcontroller and some other ICs is "three-point capacitor oscillation circuit", as shown below

Y1 is a crystal, which is equivalent to the inductor in the three-point circuit. C1 and C2 are capacitors. The 5404 NOT gate and R1 realize an NPN transistor. Let's analyze this circuit next.

5404 must have a resistor, otherwise it is in the saturation cutoff area instead of 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 common emitter.

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. The phase satisfies 360 degrees, and a very small oscillation with the same oscillation frequency as the crystal oscillator is amplified. Next, we will mainly explain this phase problem:

Because 5404 is an inverter, that is to say, it realizes 180° phase shift, then 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. You can solve equations and treat Y1 as an inductor. You can also use the characteristics of capacitors and inductors, such as the capacitor voltage lags behind the current by 90°, and the inductor voltage leads the current by 90°. Both are possible. When C1 increases, the amplitude of C2 increases, and when C2 decreases, the amplitude also increases. Sometimes C1 and C2 can oscillate without soldering. This is not because there is no C1 and C2, but because of the distributed capacitance of the chip pins. Because C1 and C2 do not need to be very large, this is very important. Next, analyze the impact of these two capacitors on the oscillation stability.

Because the voltage feedback of 5404 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 a double-sided board, which is relatively thick, the influence of distributed capacitance is not very large. If it is a high-density multi-layer board, distributed capacitance needs to be considered.

For some projects used in industrial control, it is recommended not to use the passive crystal oscillator method to start the oscillation, but to directly connect the active crystal oscillator. This is mainly because the passive crystal oscillator needs to start the oscillation, and the industrial control project requires good stability, so the active crystal oscillator will be used directly. The higher the frequency of the crystal oscillator, the lower the stability, so when the speed requirement is not high, a lower frequency crystal oscillator will be used.