​Some principles summarized in PCB board design

We summarize some principles during printed circuit board designs:

Layout

1.  Layout refers to the reasonable layout of circuit components. What kind of placement is reasonable. A simple principle is modular and clear division. That is to say, people with a certain circuit foundation can see which printed circuit board is used to achieve what functions.

2. Specific design steps: First, generate the initial printed circuit board file based on the schematic, complete the pre layout of the printed circuit board, determine the relative layout area of the printed circuit board, and then tell the structure that the structure is based on the area we provide. Then, provide specific constraints based on the overall structure design.

3. Based on structural constraints, complete the drawing of board edges, positioning openings, and some prohibited areas, and then place the connectors.

4. Component placement principle: In general, the main control microcontroller (MCU) is placed in the center of the circuit board, and the interface circuit is placed close to the interface (such as network ports, USB, VGA, etc.), Most interfaces have electrostatic discharge protection and filtering functions. The principle followed is to protect before filtering.

5. Next is the power module. Usually, the main power module is placed at the power inlet (such as 5V of the system). Independent power modules (such as 2.5V provided by module circuits) can be placed in densely populated areas within the same power supply network according to actual conditions.

6. Some internal circuits are not connected to the connector. We usually follow a basic principle: high-speed and low-speed zoning, analog and digital zoning, interference source and sensitive receiver zoning.

7. Then, for individual circuit modules, design based on the current flow direction during circuit design.

The overall circuit layout is roughly like this, welcome to add and correct it.

Wiring

1. The most basic requirement for wiring is to ensure effective connectivity of all

networks. Connectivity is easy to achieve, but effectiveness is a vague concept. In fact, there are only two types of signals in the circuit: digital signals and analog signals. For digital circuits, it is to ensure sufficient noise tolerance, while for analog signals, it is to achieve zero loss as much as possible.

2. Before wiring, it is usually necessary to understand the entire printed circuit board laminate design, that is, to plan all wiring layers into: optimal wiring layer and suboptimal wiring layer...., The optimal wiring layer, which refers to the adjacent complete grounding layer, is generally used to lay important signals (including all signals in DDR, differential signals, analog signals, etc.). Other signals (I2C, UART, SPI, GPIO) pass through other layers and ensure that only the relevant signals of that circuit (such as DDR, network ports, etc.) are present Exist in important fields.

3. In high-speed signal wiring, reflection, crosstalk, electromagnetic compatibility, and other issues need to be considered, so impedance matching is generally required, such as single line 50R, differential line 100R, etc. The actual design should prevail (the principle is to ensure equal and continuous impedance). Cross talk mainly considers the 3W/2W principle, group grounding processing, etc.

4. The power supply and power circuit should first ensure sufficient load-bearing capacity, that is, the entire circuit of the power supply should be as thick and short as possible. From the perspective of electromagnetic compatibility, the echo is called a loop, forming a loop antenna and radiating outward, thereby minimizing the loop area as much as possible.

Grounding

1. Grounding and grounding design are very important in printed circuit board design, as grounding is an important reference plane. If there is a problem with the design of the grounding layer, other signals cannot be stable.

2. Usually, we can divide it into chassis grounding and system grounding. As the name suggests, chassis grounding is the grounding of the product's metal sheet connection, and system grounding is the reference plane for the entire circuit system.

3. The practical principle of general systems and cabinets is that the cabinet is divided into grounding and system, and then connected to high-voltage capacitors through magnetic beads or multi-point connections.

4. On the system: Functionally, it is divided into digital, analog, and power supply. (There has always been debate about the division of land. I come from here.)

Firstly, with a very reasonable layout, I believe that land can be divided. The meaning of the layout is very reasonable, that is, the digital area only has digital signals, the analog area only has analog signals, the power area only has power signals, and there is a complete grounding layer below. Because current and current are very similar, they both flow downwards and have a complete grounding layer below them. Therefore, based on the principle of shortest and lowest, they flow directly back down without escaping to other places.

However, in some cases, it is not ideal and there are some intersections in different areas. At this point, it is common to choose a single understanding point and use 0R resistors (magnetic beads are not recommended as they have filtering effects at high frequencies). The resistance is located in the area with the densest intersection and the smallest flow area.