Useful Information | Why do we need to connect a capacitor to ground on each side of the MCU crystal?

Latest update time：2021-11-02 19:22

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Many MCU developers do not understand why a capacitor to ground is required on each side of the MCU crystal, because this capacitor can sometimes be removed.

I have referred to many books, but found that there is very little explanation in the books. The most mentioned things are often: the capacitance to ground has a stabilizing effect or is equivalent to load capacitance, etc., and there is no in-depth theoretical analysis.

On the other hand, many enthusiasts simply ignore the two capacitors next to the crystal, thinking that they can just follow the reference design. But in fact, this is the oscillation circuit of the MCU, also known as the "three-point capacitor oscillation circuit", as shown in Figure 1.

Figure 1: MCU three-point capacitor oscillation circuit

Among them, Y1 is a crystal, which is equivalent to the inductor in the three-point type; C1 and C2 are capacitors, and 5404 and R1 realize an NPN transistor (you can refer to the three-point capacitor oscillation circuit in the high-frequency book).

Next, we will analyze this circuit for you:

First of all, 5404 must be connected with a resistor, otherwise it will be in the saturation cutoff area instead of the amplification area, because R1 is equivalent to the bias of the transistor, which can make 5404 in the amplification area and act as an inverter, thereby realizing the role of the NPN transistor, and the NPN transistor is also an inverter when connected in common emitter.

Secondly, I will use a popular method to explain to you the working principle of this three-point oscillation circuit.

As we all know, the oscillation condition of a sinusoidal oscillation circuit is: the system gain is greater than 1, which is relatively easy to achieve; but on the other hand, the phase must also satisfy 360°.

The problem lies in the phase: since 5404 is an inverter, it has already achieved a 180° phase shift, so it only needs C1, C2 and Y1 to achieve a 180° phase shift again.

Coincidentally, when C1, C2 and Y1 form resonance, a 180-degree phase shift can be achieved; the simplest way to achieve this is to use the ground as a reference. During resonance, since the currents passing through C1 and C2 are the same and the ground is between C1 and C2, the voltages are exactly opposite, thus achieving a 180-degree phase shift.

Furthermore, when C1 increases, the amplitude at C2 increases; when C2 decreases, the amplitude also increases. Sometimes, even without soldering C1 and C2, the oscillation can still occur, but this phenomenon is not caused by not soldering C1 and C2, but by the distributed capacitance of the chip pins, because the capacitance values of C1 and C2 do not need to be very large, which is very important.

So, what effect do these two capacitors have on oscillation stability?

Since the voltage feedback of 5404 depends on C2, if C2 is too large, the feedback voltage is too low, and the oscillation is not stable; if C2 is too small, the feedback voltage is too high, and the stored energy is too little, it is easy to be disturbed by the outside world, and it will also radiate and affect the outside world. The role of C1 is exactly the opposite of that of C2.

When laying out the board, assuming it is a double-sided board and relatively thick, the influence of distributed capacitance is not very large;

However, if it is a high-density multi-layer board, distributed capacitance needs to be considered, especially for oscillation circuits such as VCO, where distributed capacitance should be considered even more.

Therefore, for projects used in industrial control, it is recommended not to use crystal oscillators, but to directly connect an active crystal oscillator.

Many times people will use 32.768K clock crystals as clocks instead of using the crystal frequency division of a single-chip microcomputer. Many people probably don’t understand the reason for this. 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 higher Q value, and the frequency stability is also relatively poor. The 32.768K crystal has good performance in terms of stability and other aspects, and has formed an industrial standard, which is relatively easy to increase.

It is also worth mentioning that 32.768K is half of 16-bit data, and the highest 1-bit carry flag is reserved, which is also very convenient for internal digital calculation processing of timing counters.

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