Printed circuit board

PCB design of single-chip control board design principles

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 network tables, and the design of the printed circuit board. Whether the layout of the devices on the board or alignment, etc. have specific requirements.
For example, the input and output alignments should be parallel to avoid interference. Two signal lines parallel alignment necessary is to add ground isolation, two adjacent layers of wiring to try to perpendicular to each other, parallel easy to produce parasitic coupling. Power and ground should be divided as far as possible in two layers perpendicular to each other. Line width, the digital circuit PCB available wide ground line to do a circuit, that is, to form a ground network (analog circuits can not be used in this way), with a large area of copper laying.
The following article on the microcontroller control board design needs to pay attention to the principles and some of the details of the issue is explained.
1. Component layout
In the layout of components, the components should be placed as close as possible to each other, for example, the clock generator, crystal, CPU clock input are prone to noise, when placed close to them. For those easy to generate noise devices, small current circuits, high current circuit switching circuits, etc., should be as far away as possible from the logic control circuit and storage circuit of the microcontroller (ROM, RAM), if possible, these circuits can be made into a separate circuit board, which is conducive to anti-interference, improve the reliability of the circuit work.
2. Decoupling capacitors
Try to install decoupling capacitors next to key components, such as ROM, RAM and other chips. In fact, the printed circuit board alignment, pin connection and wiring may contain a large inductive effect. Large inductors can cause severe switching noise spikes in the Vcc path. The only way to prevent switching noise spikes on the Vcc path is to place a 0.1uF electronic decoupling capacitor between VCC and power ground. If a surface mount component is used on the board, a chip capacitor can be used directly adjacent to the component and secured on the Vcc pin. It is best to use porcelain chip capacitors, this is because this capacitor has a low electrostatic loss (ESL) and high frequency impedance, in addition to the temperature and time of this capacitor dielectric stability is also very good. Try not to use tantalum capacitors because of its high impedance at high frequencies.
The following points need to be noted when placing decoupling capacitors.
-Electrolytic capacitors of about 100uF are connected across the power input of the printed circuit board, and a larger capacity is better if the size allows.
-In principle, each IC chip needs to be placed next to a 0.01uF porcelain chip capacitor, if the gap of the board is too small to place, you can place a tantalum capacitor of 1~10 for every 10 chips or so.
– For components with weak anti-interference ability, large current changes when turned off and storage components such as RAM and ROM, decoupling capacitors should be accessed between the power line (Vcc) and the ground line.
-The lead of the capacitor should not be too long, especially the high frequency bypass capacitor should not be with lead.
3. Ground design
In the microcontroller control system, there are many types of ground, including system ground, shield ground, logic ground, analog ground, etc. Whether the ground layout is reasonable, will determine the board’s ability to resist interference. When designing the ground and grounding points, the following issues should be considered.
-Logical ground and analog ground to be wired separately, can not be combined, will be their respective ground lines are connected to the corresponding power ground. In the design, the analog ground should be as thick as possible, and try to increase the grounding area of the lead. Generally speaking, for the input and output analog signals, it is best to isolate them from the microcontroller circuit through an optocoupler.
-In the design of the printed circuit version of the logic circuit, the ground should form a closed-loop form to improve the circuit’s anti-interference ability.
-The ground line should be as thick as possible. If the ground line is very thin, the ground resistance will be larger, causing the ground potential to vary with the current, resulting in an unstable signal level and leading to a decrease in the circuit’s immunity to interference. In case the wiring space allows, make sure that the width of the main ground line is at least 2~3mm, and the ground line on the component pins should be about 1.5mm.
– Pay attention to the choice of grounding point. When the signal frequency on the board is lower than 1MHz, the influence of electromagnetic induction between the wiring and components is small, while the grounding circuit forms a loop current on the interference, so a point of grounding should be used so that it does not form a loop. When the signal frequency on the board is higher than 10MHz, the ground impedance becomes large due to the obvious inductive effect of the wiring, and the loop current formed by the grounding circuit is no longer a major problem at this time. Therefore, multiple points of grounding should be used to minimize the ground impedance.
4. Other
-Power line arrangement in addition to the size of the current as much as possible to thicken the width of the line, in the wiring should also make the power line, ground line alignment direction and data line alignment body in line with the wiring work at the end, with the ground line will be the bottom of the circuit board without the line of the place paved, these methods help to enhance the circuit’s anti-interference ability.
-The width of the data lines should be as wide as possible to reduce impedance. Data line width of at least not less than 0.3mm (12mil), if the use of 0.46 ~ 0.5mm (18mil ~ 20mil) is more ideal.
-As a result of the circuit board will bring about 10pF of capacitive effect, which for high-frequency circuits, will introduce too much interference, so when wiring, the number of vias should be reduced as much as possible. In addition, too many vias will also cause the mechanical strength of the board is reduced.


您的电子邮箱地址不会被公开。 必填项已用 * 标注