PCB Layout Technology

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Board design steps

(1) Design of circuit schematic

The circuit schematic design is mainly based on PROTEL099's schematic design system (Advanced Schematic) to draw a circuit schematic. In this process, we should make full use of the various schematic drawing tools and various editing functions provided by PROTEL99 to achieve our goal, which is to obtain a precise and accurate circuit schematic.

(2) Generate a network table

The netlist is the bridge between the schematic design (SCH) and the printed circuit board design (PCB). It is the soul of the board automatically and can be obtained from the schematic or extracted from the printed circuit board.

(3) Printed circuit board design

The design of the printed circuit board mainly involves using PROTEL99's powerful functions to layout the board and complete the difficult work.

Drawing a simple circuit diagram

The design of the schematic can be done as follows:

(1) Design drawing size

After opening Protel 99/Schematic, plan the part drawing and design the drawing size. The size of the drawing is based on the size and complexity of the circuit diagram. Setting the appropriate drawing size is the first step in designing the schematic.

(2) Set Protel 99/Schematic design environment

Set Protel 99/Schematic design environment, including setting grid size and type, cursor type, etc. Most parameters can also use system default values.

(3) Rotating parts

According to the needs of the circuit diagram, take out the parts from the parts library, place them on the drawing, and define and set the part number and part package of the placed parts.

(4) Schematic wiring

Use the various tools provided by Protel 99/Schematic to connect the components on the drawing with electrically conductive wires and symbols to form a complete schematic.

(5) Adjusting the circuit

Further, adjust and modify the preliminary drawing of the circuit diagram to make the schematic diagram more aesthetically pleasing.

(6) Report output

Generate various reports using the reporting tools provided by Protel 99/Schematic. The most important report is the network table, which prepares for subsequent board design through the network table.

(7) File saving and printout

Save the final design as a file and print it out for future reference.

PCB Layout Technology

The following principles must be followed in the design of the MCU control board:

(1) Place related components as close as possible in terms of component layout. For example, clock generators, crystal oscillators, and CPU clock inputs are prone to noise and should be placed closer 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. If possible, make these circuits into circuits that are good for anti-interference and improve the reliability of the circuit operation.

(2) Install decoupling capacitors on critical components such as ROM and RAM. Printed circuit board traces, pin connections and wiring can all have large inductance effects that 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 the VCC and the power ground. Ceramic capacitors are ideal for this purpose because of their low electrostatic loss (ESL) and high frequency impedance. Avoid using tantalum capacitors because of their high impedance at high frequencies. 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. In principle, a 0.01uF ceramic capacitor needs to be placed next to each integrated circuit chip. If the space of the circuit board is too small, a 1~10 tantalum capacitor can be placed around every 10 chips.

(3) In the MCU control system, there are many types of ground wires, such as system ground, shield ground, logic ground, analog ground, etc. Proper grounding and ground points will determine the anti-interference ability of the circuit board. When designing ground and ground points, consider the following questions: separate the logic and the analog ground, and their respective ground lines are respectively connected to the corresponding power ground lines. At the time of 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.

When designing the printed circuit board for the logic circuit, it is important to consider the ground line to improve the circuit's anti-interference ability. The ground wire should be thick to prevent ground potential changes and signal instability. If possible, the main ground wire should be at least 2 to 3 mm wide, while the ground wire on component leads should be about 1.5 mm.

When choosing a grounding point, consider the signal frequency on the circuit board. If it is lower than 1MHz, use a grounding point to prevent the formation of a loop. For frequencies higher than 10 MHz, use multi-point grounding to minimize ground impedance.

In addition to the width of the current line, the power line should be thick, and the wiring direction of the power line and ground line should be consistent with the data line. After wiring, cover the bottom layer of the board with the ground line to enhance the circuit's immunity to interference.

The data line should be wide enough to reduce impedance, at least 0.3 mm (12 mils) and preferably 0.46 to 0.5 mm (18 mils to 20 mils) wide. Try to minimize the number of vias during routing, as too many vias can introduce interference, and decrease the mechanical strength of the board.

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