The basic process of designing circuit boards can be divided into three major steps: the design of circuit schematic diagram, the generation of network tables and the design of printed circuit boards. There are specific requirements, whether on the board, the layout of the device or the line.
In the layout of components, the related elements should try to put a little closer, for example, clock generator, crystal oscillator, CPU clock input is easy to produce noise, you should put them in the place closer. For those devices, easy to produce noise in small current circuit, high current circuit switching circuit, logic control circuit and memory circuit should be kept away from the microcontroller (ROM, RAM), if possible, these can be also made circuit board, which is beneficial to the anti interference, improve the reliability of the circuit.
Try to install decoupling capacitors beside key components such as ROM and RAM chips. In fact, printed circuit board routing, pins, wiring and wiring may contain greater inductance effect. Large inductance may cause severe switching noise spikes on the Vcc line. The only way to prevent switching noise spikes on the Vcc line is to place an 0.1uF decoupling capacitor between the VCC and the power source. If a surface mount element is used on the circuit board, the chip capacitor can be directly attached to the element and fixed on the Vcc pin. It is best to use ceramic capacitors, this is because the electrostatic capacitance has low loss and high frequency impedance, the stability of medium temperature and time on the capacitance is also very good. Try not to use tantalum capacitors because of their high impedance at high frequencies.
The following points should be noted when placing decoupling capacitors:
At the power input end of the printed circuit board, the electrolytic capacitor is connected about 100uF, and the capacitance is better if the volume is allowed.
Ceramic capacitor chip integrated circuit principle, each side needs to put a 0.01uF, if the gap is too small and the circuit board is placed, each of the 10 chip tantalum capacitor can be placed around a 1~10.
For components with weak anti-interference ability, large current variation during shutdown, and RAM, ROM and other storage devices, a decoupling capacitor should be inserted between the power line (Vcc) and ground wire.
The lead of the capacitor should not be too long, especially the high frequency bypass capacitor can not be led.
In the single chip microcomputer control system, there are many kinds of ground wire, which are systematic, shielded, logical, analog and so on. Whether the ground wire is reasonable or not will determine the anti-interference ability of the circuit board. The following questions should be considered when designing ground and ground points:
Logically and physically, they should be routed separately, not in combination, with their respective ground wires connected to the corresponding power ground. In the design, the simulated ground wire should be as bold as possible, and the ground area of the extraction terminal will be increased as much as possible. Generally speaking, for analog signals of input and output, it is best to isolate them through optocoupler through the circuit of single chip microcomputer.
When designing a printed circuit board for logic circuits, the ground wire shall form a closed loop form to improve the anti-interference capability of the circuit.
The ground shall be as thick as possible. If the ground wire is very thin, then the earth wire resistance will be larger, resulting in the grounding potential changes with the current, resulting in instability of the signal level, resulting in the ability of anti-interference of the circuit. In the case of routing space, make sure the width of the main ground is at least above 2~3mm, and the grounding wire on the component pin should be around 1.5mm.
Pay attention to the location of the site. When the circuit board signal frequency is lower than 1MHz, due to electromagnetic induction effect between wiring and components is very small, and the larger effect of grounding circuit formed by circulation on the interference, so the grounding point, so it does not form a loop. When the signal frequency is higher than 10MHz on the circuit board, because the inductance effect of the wiring is obvious, the earth wire impedance becomes very big. At this point, the circulation of the grounding circuit is no longer the main problem. Therefore, multipoint earthing should be adopted and ground impedance can be decreased as much as possible.
In addition to the power line layout according to the current size to go bold line width, the wiring should also make the power line and ground line direction and data lines and the way side in wiring work finally, with ground bottom circuit board did not take place on line, these methods helps to enhance the anti-interference ability of the circuit.
Since an over circuit of the circuit board will bring about 10pF capacitive effect, this will lead to too much interference for the high frequency circuit, so the number of vias should be reduced as much as possible during wiring. In addition, excessive vias can also cause a reduction in the mechanical strength of the circuit board.