Matters needing attention in the design of the clock on the circuit board are mainly considered from three aspects.
1. PCB layout
1) the clock crystal and related circuits should be arranged in the center of the PCB with a good stratum, not near the I/O interface. The clock generation circuit can not be made into daughter card or daughter board, but must be made on a separate clock board or carrier board. As shown in the figure below, it’s best not to route some floors below the green box.
2) Only the devices related to the clock circuit should be laid in the PCB clock circuit area, so as to avoid laying other circuits. Do not lay other signal lines near or under the crystal: use the ground plane under the clock generating circuit and crystal. If other signals pass through this plane, it violates the function of the image plane. If signals pass through this ground plane, there will be a small ground loop which will affect the continuity of the ground plane. These ground loops will cause problems at high frequencies.
3) For the clock crystal and clock circuit, shielding measures can be adopted for shielding treatment.
4) If the clock shell is made of metal, copper must be laid under the crystal during PCB design, and ensure that this part has a good electrical connection with the complete ground plane (through porous grounding).
5) Benefits of laying the ground under the clock crystal: the circuit inside the crystal oscillator will generate RF current. If the crystal is packaged with a metal shell, the DC power supply pin is the support of DC voltage reference and RF current loop reference inside the crystal, and the transient current generated by RF radiation in the shell is released through the ground plane. In a word, the metal shell is a single-ended antenna, and sometimes two or more layers of the nearest image layer and ground plane layer are sufficient for the radiation coupling of RF current to the ground. The lower floor of the crystal is also good for heat dissipation.
6) The clock circuit and the lower ground of the crystal will provide a mapping plane, which can reduce the common-mode current generated by the related crystals and clock circuits, thus reducing the radio frequency radiation. The ground plane also has an absorption effect on the differential-mode radio frequency current. This plane must be connected to the complete ground plane through multiple points, and it is required to pass through multiple vias, which can provide low impedance. To enhance the effect of this ground plane, the clock generation circuit should be close to this ground plane.
7)SMT packaged crystals will have more radio frequency energy radiation than those with metal shell: because surface-mounted crystals are mostly plastic packages, the radio frequency current inside the crystals will radiate into space and be coupled to other devices.
2. Special circumstances and consequences
1) common clock routing
It is better to connect the fast rising edge signal and the clock signal in radial topology than to connect the network in series with a single common driving source, and each trace should be wired by termination measures according to its characteristic impedance.
2) Clock transmission line shall be layered with PCB.
Clock routing principle: arrange a complete image plane layer next to the clock routing layer, reduce the routing length and control the impedance. Wrong cross-layer wiring and impedance mismatch of can lead to:
The use of line vias and jumps leads to the incompleteness of the image loop.
Surge voltage on the image plane due to the change of the voltage on the signal pin of the device with the change of the signal.
If the 3W principle is not considered, different clock signals will cause crosstalk.
3. PCB wiring
1) The clock line must run in the inner layer of multilayer PCB. And be sure to take the stripline; If you want to walk in the outer layer, you can only take the microstrip line.
2) Walking in the inner layer can guarantee a complete image plane. It can provide a low impedance RF transmission path and generate magnetic flux to offset the magnetic flux of their source transmission lines. The closer the distance between the source and the return path is, the better demagnetization will be. Due to the enhanced degaussing ability, each complete planar image layer of high-density PCB can provide 6-8dB suppression.
3) The advantages of clock cloth multilayer board: one or more layers can be specially used for complete power supply and ground plane, which can be designed into a good decoupling system, reducing the area of ground loop, differential mode radiation, EMI, impedance level of signal and power supply return path, keeping the consistency of impedance of the whole route, and reducing crosstalk between adjacent routes.
