The role of two capacitors on the microcontroller crystal oscillator

These two capacitors are called the load capacitors of the crystal oscillator, which are connected to the two legs of the crystal oscillator and the capacitance to the ground, respectively. They are usually in the tens of pico hairs. They will affect the resonant frequency and output amplitude of the crystal oscillator. Generally, when you order a crystal oscillator, the supplier will ask you what the load capacitor is.

The load capacitance of the crystal oscillator = [(Cd*Cg)/(Cd+Cg)]+Cic+△C, where Cd and Cg are the capacitances connected to the two legs of the crystal oscillator and to the ground respectively, and the empirical value of Cic (internal capacitance of the integrated circuit) + △C (capacitance on the PCB) is 3 to 5pf.

The crystal pins of various logic chips can be equivalent to a capacitor three-point oscillator. The inside of the crystal pin is usually an inverter, or an odd number of inverters connected in series. A resistor is connected between the crystal output pin XO and the crystal input pin XI. For CMOS chips, the resistor is usually between several megahertz and tens of megahertz. Many chip pins already include this resistor inside, so there is no need to connect it outside the pin. This resistor is to make the inverter in a linear state at the beginning of oscillation. The inverter is like an amplifier with a large gain, which is easy to start. The quartz crystal is also connected between the input and output of the crystal pin, which is equivalent to a parallel resonant circuit. The oscillation frequency should be the parallel resonant frequency of the quartz crystal. The two capacitors next to the crystal are grounded, which are actually the voltage-dividing capacitors of the three-point capacitor circuit, and the grounding point is the voltage-dividing point. Taking the grounding point, i.e., the voltage-dividing point, as the reference point, the input and output of the oscillation pin are inverted, but from the perspective of the parallel resonant circuit, i.e., the two ends of the quartz crystal, a positive feedback is formed to ensure that the circuit continues to oscillate. When the chip is designed, these two capacitors have been formed. Generally, the two capacities are equal. The capacity varies according to the process and layout, but it is relatively small after all, and may not be suitable for a wide frequency range. When connected externally, it is about a few PF to tens of PF, depending on the frequency and the characteristics of the quartz crystal. It should be noted that the value of the two capacitors in series is parallel to the resonant circuit, which will affect the oscillation frequency. When the two capacitances are equal, the feedback coefficient is 0.5, which generally meets the oscillation conditions. However, if it is difficult to start oscillation or the oscillation is unstable, the capacitance of the input terminal to the ground can be reduced, and the value of the output terminal can be increased to increase the feedback amount.