How to deal with some theoretical conflicts in actual wiring
Basically, it is right to isolate the modulo / digital ground. It should be noted that the signal trace should not cross the moat, and the return current path of the power supply and signal should not be too large.
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The crystal oscillator is an analog positive feedback oscillation circuit. To have a stable oscillation signal, the specifications of loop gain and phase must be met. The oscillation specifications of this analog signal are easily susceptible to interference. Even if ground guard traces are added, the interference may not be completely isolated. And too far away, the noise on the ground plane will also affect the positive feedback oscillation circuit. Therefore, the distance between the crystal and the chip must be close.
There are indeed many conflicts between high-speed wiring and EMI requirements. However, the basic principle is that some electrical characteristics of the signal cannot meet the specifications due to the resistance and capacitance or ferrite bead added by EMI. Therefore, it is best to use the technique of arranging traces and PCB stackup to solve or reduce EMI problems, such as high-speed signals going through the inner layer. Finally, use resistance or capacitor or ferrite bead to reduce the damage to the signal.
Why are the differential pairs routed close and parallel?
The routing of differential pairs should be properly close and parallel. The so-called proper approach is because this distance will affect the value of differential impedance, which is an important parameter for designing differential pairs. The need for parallelism is also due to the consistency of the differential impedance. If the two wires are far away or near, the differential impedance will be inconsistent, which will affect signal integrity and timing delay.