What is the working principle of the crystal oscillator in the microcontroller?

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What is the working principle of the crystal oscillator in the microcontroller? [Copy link]

The crystal oscillator is an indispensable component in the microcontroller. As long as the CPU is used, there must be a crystal oscillator. So how does the crystal oscillator work?

What is a crystal oscillator?

Crystal oscillator generally refers to crystal oscillator. Crystal oscillator refers to a thin slice cut from a quartz crystal at a certain azimuth angle, referred to as a chip.

A quartz crystal resonator, referred to as a crystal oscillator, is an elliptical object as shown below.

The crystal element that adds IC to the package to form an oscillation circuit is called a crystal oscillator. Its products are generally packaged in metal shells, but there are also glass shells, ceramics or plastic packages.

Working Principle of Crystal Oscillator

A quartz crystal oscillator is a resonant device made using the piezoelectric effect of a quartz crystal. Its basic structure is as follows: a thin slice is cut from a quartz crystal at a certain azimuth angle, a silver layer is applied on its two corresponding surfaces as electrodes, a lead is welded on each electrode and connected to the pin, and then a package shell is added to form a quartz crystal resonator, referred to as quartz crystal or crystal, crystal oscillator. Its products are generally packaged in metal shells, but also in glass shells, ceramics or plastics.

If an electric field is applied to the two electrodes of a quartz crystal, the chip will produce mechanical deformation. Conversely, if mechanical pressure is applied to both sides of the chip, an electric field will be generated in the corresponding direction of the chip. This physical phenomenon is called the piezoelectric effect.

If an alternating voltage is applied to the two poles of the chip, the chip will produce mechanical vibrations, and the mechanical vibrations of the chip will generate an alternating electric field.

Under normal circumstances, the amplitude of the mechanical vibration of the chip and the amplitude of the alternating electric field are very small, but when the frequency of the applied alternating voltage is a certain value, the amplitude increases significantly, much larger than the amplitude at other frequencies. This phenomenon is called piezoelectric resonance, which is very similar to the resonance phenomenon of the LC circuit. Its resonant frequency is related to the cutting method, geometry, size, etc. of the chip.

When the crystal is not vibrating, it can be regarded as a flat plate capacitor called electrostatic capacitance C. Its size is related to the geometric size of the chip and the electrode area, and is generally about a few picofarads to tens of picofarads. When the crystal oscillates, the inertia of mechanical vibration can be equivalent to inductance L.

Generally, the value of L is from tens of millihenries to hundreds of millihenries. The elasticity of the chip can be equivalent to the capacitance C, which is very small, generally only 0.0002 to 0.1 pF. The loss caused by friction when the chip vibrates is equivalent to R, which has a value of about 100 ohms.

Since the equivalent inductance of the chip is large, while C and R are small, the quality factor Q of the loop is very large, which can reach 1000 to 10000. In addition, the resonant frequency of the chip itself is basically only related to the cutting method, geometry, and size of the chip, and can be made accurately, so the oscillation circuit composed of quartz resonators can obtain high frequency stability.

Computers have a timing circuit. Although the word "clock" is generally used to refer to these devices, they are not actually clocks in the usual sense. It might be more appropriate to call them timers.

A computer timer is usually a precision machined quartz crystal that oscillates within its tension limits at a frequency that depends on how the crystal itself is cut and the amount of tension it is subjected to. There are two registers associated with each quartz crystal, a counter and a holding register.

Each oscillation of the quartz crystal decrements the counter by 1. When the counter reaches 0, an interrupt is generated and the counter is reloaded from the holding register. This method makes it possible to program a timer to interrupt 60 times per second (or at any other desired frequency). Each interrupt is called a clock tick.

Electrically, a crystal oscillator can be equivalent to a two-terminal network consisting of a capacitor and a resistor in parallel and a capacitor in series. In electrical engineering, this network has two resonant points, which are divided into series resonance and parallel resonance according to the frequency.

Due to the characteristics of the crystal itself, the distance between these two frequencies is quite close. In this extremely narrow frequency range, the crystal oscillator is equivalent to an inductor, so as long as a suitable capacitor is connected in parallel at both ends of the crystal oscillator, it will form a parallel resonant circuit.

This parallel resonant circuit can be added to a negative feedback circuit to form a sinusoidal oscillation circuit. Since the frequency range of the crystal oscillator being equivalent to an inductor is very narrow, the frequency of the oscillator will not change much even if the parameters of other components change greatly.

The crystal oscillator has an important parameter, which is the load capacitance value. By selecting a parallel capacitor equal to the load capacitance value, the nominal resonant frequency of the crystal oscillator can be obtained.

In a general crystal oscillator circuit, a crystal is connected to both ends of an inverting amplifier, and then two capacitors are connected to both ends of the crystal. The other end of each capacitor is connected to the ground. The capacitance of these two capacitors in series should be equal to the load capacitance. Please note that the pins of general ICs have equivalent input capacitance, which cannot be ignored.

The load capacitance of a general crystal oscillator is 15 picobars or 12.5 picobars. If the equivalent input capacitance of the component pin is considered, two 22 picobars capacitors to form the crystal oscillator circuit are a better choice.

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Blkhumor

Very good explanation, the general crystal capacitance is 22pf to 47pf. For specific applications, you should refer to the application circuit section of the manual