The old partner load capacitance of the crystal oscillator is divided into two chip capacitors, C1 and C2. The function of the load capacitor is to reduce the interference caused by other clutter, thereby improving the stability of the circuit. When you select the load capacitance , you can calculate the formula according to the specific size of the capacitor (C1*C2)/(C1+C2)+6.24. This calculation formula is just a way to know the size of the capacitor, but it is best to follow the load capacitance value matched by the crystal manufacturer, which will reduce many other interferences.
Crystal oscillators have their own characteristics, or they must be combined to achieve the size of the capacitor. In both, the lower the C1 and C2 values, the better. Preferably, the C2 value is greater than the C1 value, and the larger C2 value is beneficial to the stability of the oscillator. The more commonly used value is between 15p-30p.
In the actual circuit, the oscillating waveform can also be observed by an oscilloscope to determine whether the oscillator is operating at its optimum state. When observing the oscillating waveform, the oscilloscope observes the OSCO pin (Oscillator output). It should select an oscilloscope probe with a bandwidth of 100MHz or more. This probe has high input impedance, small capacitive reactance, and relatively little influence on the oscillating waveform. (Because there is usually a capacitance of 10 to 20 pF on the probe, it is possible to reduce the capacitance at the OSCO pin to obtain a more realistic oscillation waveform when observing). A well-functioning oscillating waveform should be a beautiful sine wave with a peak-to-peak value greater than 70% of the supply voltage. If the peak-to-peak value is less than 70%, the external load capacitance on the OSCI and OSCO pins can be appropriately reduced. Conversely, if the peak-to-peak value is close to the power supply voltage and the oscillation waveform is distorted, the load capacitance can be appropriately increased.
Using an oscilloscope to detect the OSCI (Oscillator input) pin can easily cause the oscillator to stop. The reason is that some probes have a small impedance and cannot be directly tested. You can use the string capacitor method to test. For example, a commonly used 4MHz quartz crystal resonator, the external load capacitance recommended by the manufacturer is usually about 10 to 30pF. If the center value is 15pF, then C1 and C2 each take 30pF to obtain a series equivalent capacitance of 15pF. At the same time, considering the additional distributed capacitance of the circuit board, the chip pin capacitance, the crystal's own parasitic capacitance, etc. will affect the total capacitance value, so when actually configuring C1, C2, each can take about 20 ~ 15pF. And C1, C2 use ceramic chip capacitors is better.
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