How clock signal is generated in microcontroller?

Whether it is a single-chip microcomputer or a microprocessor, their core is a large-scale sequential logic circuit, and the power driving the sequential logic circuit is an accurate and stable clock source-don’t underestimate the attributive “accurate and stable”, in fact The reason why human technology can develop so stably and at a high speed is that it is inseparable from an accurate and stable clock source. Today qihe smt pick and place machine sharing how clock signal is generated in microcontroller.

For example, the crystal oscillator used by the microcontroller is a relatively accurate clock source.Before the crystal oscillator, the oscillation source is generally LC oscillation circuit and other circuits, but they will be affected by the outside world (vibration, temperature, etc.), and their error may reach the kHz level. Its accuracy is also difficult to guarantee under long-time working conditions. The development of radio and digital circuits forced the birth of an accurate and stable clock source, so there was a crystal oscillator.

When a quartz crystal is cut in a certain way and a voltage is applied to it, it will start to vibrate under the action of an external electric field. This property is called “piezoelectricity”. This gives the quartz crystal the ability to resonate with external oscillations. Crystals cut in the same way have the same oscillation frequency, which makes it possible to mass-produce quartz crystals.

The inside of a practical quartz crystal oscillator typically looks like this: a precision-cut crystal is placed in the middle, sandwiched between two chemically treated metal oxides, and packaged in a strong metal case. This is why the circuit diagram symbol of the crystal oscillator is a crystal sandwiched between two electrodes. The application circuit of a typical quartz crystal oscillator is shown in the figure below:

This circuit is called a Pierce oscillator and uses an inverter to complete the circuit. The two capacitors below generally have the same capacitance value, and their values can slightly affect the oscillation frequency of the entire circuit. Their role is to form a bandpass filter with the quartz crystal, forming a 180-degree phase shift at the resonant frequency of the quartz crystal. This circuit is most commonly found in the clock section of microcontrollers.
For quartz crystals, the finer they are cut, the higher their resonant frequency – but it is not unlimited. If the cut is too fine, the quartz crystal will be easily broken. And this limit frequency is about 20MHz. How are those crystals above 20MHz made?

In fact, for crystals above 20MHz, they use harmonics in the crystal’s oscillator frequency. We can enhance the odd harmonics of the crystal and take advantage of it. For example, if I want to use a 50MHz crystal, I can make a 10MHz crystal and make its structure enhance the 5 times harmonic (50MHz). For such a crystal, if a pure 50MHz is required, it is necessary to use an external resonant circuit to select the harmonic of this 50MHz.

For something like a clock source, we never mind its high precision, the higher the better. But even an oscillator with a long history like a crystal oscillator is enough for us to study it for a while!

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