Electronic Design - Basic knowledge of professional PCB design

Chengdu Zicheng Electronics has a professional hardware design team, and here are some basic knowledge of PCB design for everyone.

1. If the designed circuit system includes FPGA devices, the Quartus II software must be used to verify the pin assignments before drawing the schematic. (Some special pins in FPGA cannot be used as ordinary IO).

2. The 4-layer board from top to bottom is: signal plane layer, ground, power, signal plane layer; from top to bottom, the 6-layer board is: signal plane layer, ground, signal inner electric layer, signal inner electric layer, Power and signal plane layer. For boards with 6 layers or more (the advantage is: anti-interference radiation), the internal electric layer wiring is preferred, and the plane layer is not allowed to go. It is forbidden to route the wiring from the ground or power layer (reason: the power layer will be divided, causing parasitic effects).

3. Wiring of multi-power supply system: If the FPGA+DSP system is used as a 6-layer board, there will usually be at least 3.3V+1.2V+1.8V+5V. 3.3V is generally the main power supply, and the power layer is directly laid, and it is easy to route the global power network through vias; 5V may generally be the power input, and only a small area of copper is required. And make it as thick as possible (you ask me how thick it should be-as thick as it can be, the thicker the better); 1.2V and 1.8V are the core power supply (if you directly use the wire connection method, you will encounter a lot of problems when facing BGA devices. Great difficulty), try to separate 1.2V and 1.8V during layout, and arrange the components connected within 1.2V or 1.8V in a compact area, and use copper to connect. In short, because the power supply network is spread over the entire PCB board, if The method of routing will be very complicated and will go around very far, using the method of paving copper is a good choice!

4. The wiring between adjacent layers adopts a cross method: it can reduce the electromagnetic interference between parallel wires (for high school and high school), and it is convenient for wiring.

5. What is the isolation method for analog and digital isolation? Separate the devices used for analog signals from those used for digital signals during layout, and then cut across the AD chip across the board!

The analog signal is laid with an analog ground, and the analog ground/analog power supply and the digital power supply are connected at a single point through an inductor/magnetic bead.

6. PCB design based on PCB design software can also be regarded as a software development process. Software engineering pays most attention to the idea of "iterative development" to reduce the probability of PCB errors.

(1) Check the schematic diagram, pay special attention to the power and ground of the device (power and ground are the blood of the system, and there can be no negligence);

(2) PCB package drawing (confirm whether the pins in the schematic diagram are wrong);

(3) After confirming the PCB package size one by one, add a verification label and add it to the package library of this design;

(4) Import the netlist and adjust the signal sequence in the schematic while layout (OrCAD component automatic numbering function can no longer be used after layout);

(5) Manual wiring (check the power ground network while cloth, as I said before: the power network uses the copper method, so use less wiring);

In short, the guiding ideology in PCB design is to draw back and correct the schematic diagram of the package layout while drawing (considering the correctness of signal connection and the convenience of signal routing).

7. The crystal oscillator is as close as possible to the chip, and there is no wiring under the crystal oscillator, and the network copper skin is laid. Clocks used in many places are wired in a tree-shaped clock tree.

8. The arrangement of the signals on the connector has a great influence on the difficulty of wiring, so it is necessary to adjust the signals on the schematic diagram while wiring (but do not renumber the components).

9. Design of multi-board connector:

(1) Use flat cable connection: the upper and lower interfaces are the same;

(2) Straight socket: the upper and lower interfaces are mirrored and symmetrical

10. Design of module connection signal:

(1) If the two modules are placed on the same side of the PCB, then the serial number of the supervisor should connect to the small and connect to the large (mirror connection signal);

(2) If two modules are placed on different sides of the PCB, then the serial number of the control system should be connected to small and large.

Doing so will place the signal to cross as in the right picture above. Of course, the above method is not a rule, I always say that everything changes as needed (this can only be understood by yourself), but in many cases it is very useful to design in this way.