Difficult Problems:
When designing a 51-chip system PCB, why is the crystal oscillator required to be next to the microcontroller?
The reasons are as follows:
The crystal oscillator generates electrical vibration at a fixed frequency by electric excitation, and the vibration generates current feedback to the circuit. After the circuit receives the feedback, the signal is amplified, and the amplified electrical signal is used to excite the mechanical vibration of the crystal oscillator. The crystal oscillator generates vibration again. The current is fed back to the circuit, and so on. When the excitation electric signal in the circuit and the nominal frequency of the crystal oscillator are the same, the circuit can output a strong, frequency-stable sine wave. The shaping circuit then converts the sine wave into a square wave for use in the digital circuit.
The problem is that the crystal's output capability is limited, it only outputs electrical energy in milliwatts. Inside the IC (integrated circuit), this signal is amplified by a few hundred times or even thousands of times by an amplifier to be used normally.
The crystal oscillator and the IC are usually connected by copper traces. This trace can be regarded as a length of wire or a number of wires. When the wire is cut, the current will be generated. The longer the wire, the stronger the current generated. In reality, magnetic lines of force are not common, and electromagnetic waves are everywhere, such as: radio broadcasts, television tower launches, cell phone communications, and so on. The connection between the crystal and the IC becomes the receiving antenna. The longer it is, the stronger the received signal is, and the stronger the generated electrical energy is until the received electrical signal strength exceeds or approaches the signal strength generated by the crystal. The output of the amplifying circuit in the IC will no longer be a square wave of a fixed frequency, but a messy signal, which causes the digital circuit to fail to work synchronously and error.
Therefore, when drawing a PCB (circuit board), the closer the crystal oscillator is to its amplifier circuit (IC pin), the better. 
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