1、 Circuit board layout design steps
Generally speaking, the basic process of layout design can be divided into three steps.
(1) Design of circuit schematic diagram
The design of circuit schematic diagram is mainly based on PROTEL099′ s Advanced Schematic system to draw a circuit schematic diagram. In this process, we should make full use of all kinds of schematic drawing tools and editing functions provided by PROTEL99 to achieve our goal, that is, to get a correct and beautiful circuit schematic diagram.
(2) generate a network table
Network table is a bridge between circuit schematic design (SCH) and printed circuit board design (PCB), and it is the soul of PCB automation. The network table can be obtained from circuit schematic diagram or extracted from printed circuit board.
(3) layout design of printed circuit board
The design of printed circuit board (PCB) is mainly aimed at another important part of PROTEL99. In this process, we use the powerful functions provided by PROTEL99 to realize the layout design of PCB and complete difficult tasks.
Two. Draw a simple circuit diagram
2.1 schematic design process
The design of the schematic diagram can be completed according to the following process.
(1) After designing the drawing size Protel 99/ Schematic, first of all, we should conceive the part drawing and design the drawing size. The size of the drawing depends on the scale and complexity of the circuit diagram. Setting an appropriate drawing size is the first step in designing the schematic diagram.
(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) According to the needs of the circuit diagram, users of rotating parts take out the parts from the parts library and place them on the drawings, and define and set the serial number and package of the placed parts.
(4) Wiring with schematic diagram Use various tools provided by Protel 99/Schematic to connect the elements on the drawing with electrically significant wires and symbols to form a complete schematic diagram.
(5) Adjust the circuit. Further adjust and modify the preliminarily drawn circuit diagram to make the schematic diagram more beautiful.
(6) Report output: Various reports are generated by various report tools provided by Protel 99/Schematic, the most important of which is the network table, which is used to prepare for the subsequent circuit board design.
(7) File saving and printout The last step is file saving and printout.
The layout design principle of single-chip microcomputer should follow the following principles:
(1) As for the layout of components, the related components should be placed as close as possible. For example, the clock generator, crystal oscillator and the clock input of CPU are prone to noise, so they should be placed closer together. For those devices, low-current circuits, high-current circuits and switching circuits that are prone to noise, try to keep them away from the logic control circuits and memory circuits (ROM and RAM) of the single chip microcomputer. If possible, these circuits can be made into circuit boards, which is conducive to anti-interference and improves the reliability of circuit work.
(2) Try to install decoupling capacitors beside key components, such as ROM, RAM and other chips. In fact, the wiring, pin connection and wiring of printed circuit boards may all contain large inductance effect. The large inductance of may cause serious switching noise spikes on Vcc traces. The only way to prevent the switching noise spike on Vcc wiring is to place an electronic decoupling capacitor of 0.1uF between VCC and power supply ground. If a surface mount component is used on the circuit board, a chip capacitor can be directly attached to the component and fixed on the Vcc pin. It is best to use ceramic capacitor, because it has low electrostatic loss (ESL) and high-frequency impedance, and its dielectric stability at temperature and time is also very good. Try not to use tantalum capacitor, because its impedance is high at high frequency. Pay attention to the following points when placing decoupling capacitors:
Connect an electrolytic capacitor of about 100uF across the power input end of the printed circuit board. If the volume allows, it is better to have a larger capacitance.
In principle, a ceramic capacitor of 0.01uF should be placed next to each IC chip. If the gap of the circuit board is too small to fit, a tantalum capacitor of 1 ~ 10 can be placed every 10 chips or so.
For components with weak anti-interference ability and large current variation when turned off, and RAM, ROM and other storage components, decoupling capacitors should be connected between the power line (Vcc) and the ground.
The lead of the capacitor should not be too long, especially the high-frequency bypass capacitor should not have a lead.
(3) In the single chip microcomputer control system, there are many kinds of ground wires, such as systematic, shielded, logical, analog, etc. Whether the ground wires are properly laid out will determine the anti-interference ability of the circuit board. When designing the ground wire and grounding point, the following issues should be considered:
The logic ground and analog ground should be wired separately, and cannot be used together. Their respective ground wires should be connected to the corresponding power supply ground wires. In design, the analog ground wire should be as bold as possible, and the grounding area of the lead-out terminal should be increased as much as possible. Generally speaking, the input and output analog signals should be isolated from the single chip microcomputer circuit by optocoupler.
When designing the printed circuit board of logic circuit, its ground wire should form a closed loop to improve the anti-interference ability of the circuit.
The ground wire should be as thick as possible. If the ground wire is thin, the resistance of the ground wire will be large, resulting in the change of the ground potential with the change of current, resulting in the instability of the signal level and the decline of the anti-interference ability of the circuit. When the wiring space allows, the width of the main ground wire should be at least 2 ~ 3 mm, and the ground wire on the component pins should be about 1.5 mm.
Layout design should pay attention to the choice of grounding point. When the signal frequency on the circuit board is lower than 1MHz, because the electromagnetic induction between wiring and components has little influence, and the circulating current formed by the grounding circuit has great influence on the interference, a point grounding should be adopted to prevent it from forming a loop. When the signal frequency on the circuit board is higher than 10MHz, due to the obvious inductance effect of the wiring, the impedance of the ground wire becomes very large, and the circulating current formed by the grounding circuit is no longer the main problem. Therefore, multi-point grounding should be adopted to minimize the impedance of ground wire.
The layout of the power line should not only be as bold as possible according to the current, but also make the routing direction of the power line and the ground line consistent with the routing square of the data line when wiring. At the end of the wiring work, the ground line should be used to cover the place where there is no routing on the bottom of the circuit board. These methods are all helpful to enhance the anti-interference ability of the circuit.
The width of the data line should be as wide as possible to reduce the impedance. The width of the data line should be at least 0.3mm(12mil), and it is more ideal if it is 0.46 ~ 0.5 mm (18 mil ~ 20 mil).
As a via hole in the circuit board will bring about 10pF capacitance effect, which will introduce too much interference to the high-frequency circuit, so the number of vias should be reduced as much as possible when wiring. Furthermore, too many vias will also reduce the mechanical strength of the circuit board.
