About the crystal section:
1. The two cables from the crystal to the MCU should not be too thin and should be as short and straight as possible. The smaller the area between the two cables and the two load capacitors, the better, it is best to use a wider strip separately to bring it to the MCU oscillating place, instead of connecting it to a large area copper foil;
2. The back of the crystal should be the whole copper foil. Do not take other lines or take other lines above the front of the crystal;
3. Some MCUs work with unsuitable crystals. If the amplitude is too high and the top cut distortion is generated, a strong base wave and strong harmonic radiation will be generated, in this case, we need to build a resistance of several dozen to several hundred euro pairs at the MCU end on the xout to reduce the amplitude peak value to 1/2 ~ Of The VCC ~ 2/3 is recommended;High-speed line, usually the SDRAM, And the vclk of digital video signals, it is best to have a copper layer on the back of the line, there is no condition, at least a wide ground line "escort ", the ability to package the location is more ideal. Sometimes it is necessary to rely on the serial resistance at the MCU end to eliminate overhead (overhead has a great impact on the radiation). Do not concatenate too much to avoid delay; in addition, there should be no parallel lines on the back, and the front should also be; in addition, USB should be followed by a differential line; Low-speed long lines cannot be ignored. RC should be used as much as possible to suppress high-frequency components. Close to the external interface, strings of magnetic pigs, power sources, and small common-mode inductance. All in all, excessive radiation is caused by a large dv/dt and a large loop area. We can find a way to suppress the voltage transient, reduce the signal overshoot, and reduce the loop area of the signal and current backflow. There is also the small antenna effect. Finally, there is no way to block it, and the cost is very expensive. |
