What are the principles and details of MCU control PCB board design?

Last Update Time: 2019-08-08 10:52:18

The most basic process of designing a circuit board can be divided into three major steps: the design of the circuit schematic, the generation of a network table, and the design of a printed circuit board. Whether it is on the board layout or routing, etc. have specific requirements.

 

For example, input and output traces should be avoided as much as possible to avoid interference. Parallel routing of the two signal lines must be separated by grounding, and the wiring of two adjacent layers should be perpendicular to each other as much as possible, and parasitic coupling is easy to occur in parallel. The power supply and ground should be placed as perpendicular to each other on two layers. In terms of line width, a digital circuit PCB can be used as a circuit with a wide ground line, that is, a ground network (the analog circuit cannot be used in this way), and a large area is used for copper plating.

 

The following article explains the principles and some details of the PCB design of the microcontroller control board.

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1. Decoupling capacitor

Try to install decoupling capacitors on critical components such as ROM, RAM, etc. In fact, printed circuit board traces, pin connections, and wiring can all have large inductive effects. Large inductors can cause severe switching noise spikes on the Vcc trace. The only way to prevent switching noise spikes on the Vcc trace is to place a 0.1uF electronic decoupling capacitor between VCC and the power ground. If a surface mount component is used on the board, a chip capacitor can be used to directly abut the component and be fixed on the Vcc pin. It is best to use a ceramic capacitor because of its low electrostatic loss (ESL) and high frequency impedance, and the stability of the capacitor temperature and time is also good. Try not to use tantalum capacitorsbecause of its high impedance at high frequencies.

 

Pay attention to the following points when placing the decoupling capacitor:

Connect an electrolytic capacitor of about 100uF at the power input end of the printed circuit board. If the volume allows, the larger the capacitance, the better.

For components with weak anti-interference ability, large current change during shutdown, and memory components such as RAM and ROM, a decoupling capacitor should be connected between the power supply line (Vcc) and the ground.

In principle, a 0.01uF ceramic capacitor is required next to each integrated circuit chip. If the space of the circuit board is too small to be placed, a 1-10 tantalum capacitor can be placed around every 10 chips.

The lead of the capacitor should not be too long, especially the high-frequency bypass capacitor can not be leaded.

2. Component layout

In terms of component layout, the related components should be placed as close as possible. For example, clock generators, crystal oscillators, and CPU clock inputs are prone to noise, and should be placed close to them when placed. For those devices that are prone to noise, small current circuits, high-current circuit switching circuits, etc., try to keep them away from the logic control circuit and memory circuit (ROM, RAM) of the microcontroller, and if possible, make these circuits into circuits. The board is good for anti-interference and improves the reliability of the circuit operation.

 

3. Ground design

In the MCU control system, there are many types of ground wires, such as system ground, shield ground, logic ground, analog ground, etc. Whether the ground wire is laid out properly will determine the anti-interference ability of the circuit board. When designing ground and ground points, you should consider the following questions:

 

The logic ground and the analog ground are to be separately routed, and they cannot be used together, and their respective ground lines are respectively connected to the corresponding power ground lines. In the design, the analog ground wire should be as thick as possible, and the grounding area of the lead end should be increased as much as possible. In general, the analog signal for input and output is preferably isolated from the microcontroller circuit by an optocoupler.

 

The ground wire should be as thick as possible. If the ground wire is very thin, the ground wire resistance will be large, causing the ground potential to change with the change of the current, resulting in unstable signal level, resulting in a decrease in the anti-interference ability of the circuit. In the case where the wiring space allows,