Calculation of External Matching Capacitance of Crystal Load Capacitor and Experience of Crystal Oscillation Circuit Design (III)

Crystal load capacitor external matching capacitors CL1 and CL2 are calculated:

If the equivalent capacitance at both ends of the crystal oscillator is different from the nominal load capacitance of the crystal oscillator, the resonant frequency of the crystal oscillator output will deviate from the nominal operating frequency (also called the frequency offset), so it is reasonable to match the appropriate applied capacitor. It is important to make the equivalent capacitance across the crystal oscillator equal to or close to the load capacitance.

In order to maintain the load balance of the crystal, in practical applications, CG=CD is generally required.

According to the components of CG, you can get:

CG=Ci+CPCB+CL1=23.3pF

Crystal wiring requires the crystal to be as close as possible to the oscillating circuit, so the PCB is generally small, taking 0.2pF; Ci = 4.8pF. So the final calculation result is as follows: (The calculation process of CL2 is similar)

CL1=CL2=18.3pF≈18pF

Exceptions:

Nowadays, there are many internal capacitors added to the chip. Therefore, when designing, you only need to select the crystal according to the recommended load capacitance value of the chip datasheet. No additional capacitor is needed. However, because of the uncertainty of the actual design of the parasitic circuit, it is better to reserve the position of CL1/CL2.

The above calculations are based on the premise of CG=CD. There are some unexpected situations. For example, cypress's clock crystal with nvsram with RTC requires asymmetry on both sides, but fortunately, cypress gives detailed calculation process and selection reference.