How to use an oscilloscope to measure whether the crystal oscillator is oscillating

　　When I was testing the circuit board, I found that one circuit board could not be programmed. So I checked it:

　　1. There is no problem with power supply and ground

　　2. Use an oscilloscope to test whether the crystal oscillator is oscillating. I found a strange problem. The 24MHz quasi-sine wave appears at the XOUT end, but not at the XIN end? What is the reason? I have not found the reason.

　　So I had to change the main chip. I didn't know how to install QFP128 in the past, but now it is very easy to disassemble and assemble. Thanks to my friends for their guidance (I am a little satisfied, because I didn't dare to install the most common patches in the past).

　　After changing the chip, although the program can be burned, I measured the crystal oscillator again, but there is still no XIN and XOUT. It can be confirmed that the chip has been working normally, so why can't the crystal oscillator be measured? The circuit is correct, and I can only check whether there is a problem with the oscilloscope.

　　I used a probe to do self-calibration on the oscilloscope, and the 5V 1KHz square wave was normal. So where is the problem? Is it that the crystal oscillator cannot be measured? No way, I have measured it before.

　　I found that the probe was in the X1 position, so I tried changing it to the X0 position, and suddenly I found that it was there, and the oscillation waveform was there. It was strange, why was there no XIN at X1, but there was at X0?

　　Starting from the probe, I found from the Tektronix page that it is related to the capacitance of the probe.

　　The characteristic parameters of an oscilloscope probe are:

　　correspond:

　　P2200 x 1X/1X 200MHz/6MHz 1MO/1MO 16pF/95pF 300V/150V

　　Typical input C is very important here. It is 95pF at 1X. Such a capacitance affects the oscillation of the crystal. The matching capacitance of the crystal is 30pF. Therefore, it is normal that the waveform cannot be measured by XIN, and it will not affect the chip operation. If you want to measure whether it is oscillating, use a probe with smaller capacitance, such as the 10X gear. Of course, the quality of the crystal itself is also very important.

　　Similarly, from the above analysis, we can see that in terms of accuracy, strictly speaking, the waveform measured by the oscilloscope is distorted, not the actual value. Of course, for the crystal oscillator, whether or not to add capacitors, and how much capacitor to add, it affects the shape quality of the waveform, not the frequency. So the system will work normally.