The clock circuit is an oscillating circuit that moves accurately like a clock. Any work is in accordance with the sequence of time. Then the circuit that generates this time is the clock circuit. The clock circuit is generally composed of a crystal oscillator, a crystal oscillator, a control chip and a matching capacitor, such as Figure 1 shows.
Figure 1 Clock circuit
There are the following precautions for clock circuit PCB design:
1. The crystal circuit layout needs to be prioritized. The overall layout is compact. The layout should be on the same layer as the chip and placed as close as possible to avoid punching holes. The crystal trace should be as short as possible, away from interference sources, and as far away as possible from the edge of the board;
2. If the crystal circuit is placed on a different layer from the chip during the layout process, it should be as close to the chip as possible to shorten the wiring, and the crystal wiring needs to be covered to avoid interference;
3. The traces of the crystal and the clock signal need to be ground-covered throughout the process. At least one GND via should be added every 200-300mil for the ground-covered line, and the ground reference plane of the adjacent layer must be complete, as shown in Figure 2;
4. The current layer of the crystal can be surrounded by GND routing to form a ground ring, and GND via holes are placed in the ground ring to connect to the adjacent GND plane layer to isolate noise, as shown in Figure 3.
Figure 2 Crystal layout and wiring
Figure 3 crystal oscillator layout and wiring
4. Clock traces Xin and Xout and the projection area under the crystal prohibit any traces to avoid noise coupling into the clock circuit;
5. The adjacent layer below the crystal must ensure a complete reference plane to avoid cross-segmentation, which helps to isolate noise and maintain crystal output, as shown in Figure 4 below.
Figure 4 The second layer is a complete reference plane
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