How to determine whether the crystal oscillator in the circuit is overdriven

　　How to judge whether the crystal oscillator in the circuit is overdriven?

　　Resistor RS is often used to prevent the crystal oscillator from being overdriven. Overdriving the crystal oscillator will gradually wear out and reduce the contact plating of the crystal oscillator, which will cause the frequency to rise. You can use an oscilloscope to detect the OSC output pin. If a very clear sine wave is detected and the upper and lower limits of the sine wave meet the clock input requirements, the crystal oscillator is not overdriven. On the contrary, if the peaks and troughs of the sine wave are flattened and the waveform becomes square, the crystal oscillator is overdriven. At this time, resistor RS is needed to prevent the crystal oscillator from being overdriven. The easiest way to judge the value of resistor RS is to connect a 5k or 10k trimming resistor in series, starting from 0 and slowly increasing it until the sine wave is no longer flattened. This method can find the closest resistor RS value.

　　How to choose capacitors in the crystal oscillator circuit?

　　(1) C1, C21, because each crystal oscillator has its own characteristics, it is best to select external components according to the values ​​provided by the manufacturer.
　　(2) Within the permissible range, the lower the C1 and C2 values, the better. A larger C value is beneficial to the stability of the oscillator, but it will increase the start-up time.
　　(3) The C2 value should be greater than the C1 value, so that the crystal oscillator can start up faster when powered on.