Electronic Design - Printed circuit board (PCB) wiring points

1. Circuit board design steps

Generally speaking, the most basic process of designing a circuit board can be divided into three major steps.

(1). The design of the circuit schematic diagram: The design of the circuit schematic diagram is mainly based on the PROTEL099 schematic design system (Advanced Schematic) to draw a circuit schematic diagram. In this process, we must make full use of the various schematic drawing tools and various editing functions provided by PROTEL99 to achieve our goal, that is, to obtain a correct and exquisite schematic circuit diagram.

(2). Generate netlist: netlist is a bridge between circuit schematic design (SCH) and printed circuit board design (PCB). It is the soul of circuit board automation. The netlist can be obtained from the schematic circuit diagram or extracted from the printed circuit board.

(3). The design of the printed circuit board: The design of the printed circuit board is mainly for another important part of the PCB of PROTEL99. In this process, we use the powerful functions provided by PROTEL99 to realize the layout design of the circuit board., To complete difficult and other tasks.

2. Draw a simple circuit diagram

2.1 Schematic diagram 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, firstly conceive the part drawing and design the drawing size. The size of the drawing is determined according to the scale and complexity of the circuit diagram. Setting a suitable drawing size is the first step to design a good 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) Rotating parts according to the needs of the circuit diagram, the user removes the parts from the parts library and places them on the drawings, and defines and sets the serial numbers and parts packaging of the placed parts.

(4) Wiring with schematic diagram Using various tools provided by Protel 99/Schematic, connect the components on the drawing with wires and symbols with electrical meaning to form a complete schematic diagram.

(5) Adjust the circuit make further adjustments and modifications to the preliminarily drawn circuit diagram to make the schematic diagram more beautiful.

(6) Report output: Various reports are generated through various report tools provided by Protel 99/Schematic. The most important report is the network table. The network table is used to prepare for the subsequent circuit board design.

(7) File saving and print output The final step is file saving and print output.

The design principles of the single-chip control board need to follow the following principles:

(1) In the layout of PCB components, the related components should be placed as close as possible. For example, the clock generator, crystal oscillator, and the clock input of the CPU are all prone to noise, so they should be placed closer. For those devices that are prone to noise, low-current circuits, high-current circuit switching circuits, etc., keep them away from the logic control circuit and storage circuit (ROM, RAM) of the single-chip microcomputer as much as possible. If possible, these circuits can be made into circuits. Board, this is conducive to anti-interference and improve the reliability of circuit work.

(2) Try to install decoupling capacitors next to key components such as ROM, RAM and other chips. In fact, printed circuit board traces, pin connections and wiring, etc. may contain large inductance effects. Large inductance may cause severe switching noise spikes on the Vcc trace. The only way to prevent switching noise spikes on Vcc traces is to place a 0.1uF electronic decoupling capacitor between VCC and power ground. If surface mount components are used on the circuit board, chip capacitors can be used to directly tighten the components and fix them on the Vcc pin. It is best to use ceramic capacitors, because this type of capacitor has low electrostatic loss (ESL) and high frequency impedance, and the temperature and time of the dielectric stability of this type of capacitor are also very good. Try not to use tantalum capacitors, because their impedance is higher at high frequencies.

Pay attention to the following points when placing decoupling capacitors:

Connect a 100uF electrolytic capacitor across the power input end of the printed circuit board. If the volume permits, a larger capacitance is better.

In principle, a 0.01uF ceramic capacitor needs to be placed next to each integrated circuit chip. If the gap of the circuit board is too small to fit, you can place a 1-10 tantalum capacitor for every 10 chips.

For components with weak anti-interference ability and large current changes when turned off, and storage components such as RAM and ROM, a decoupling capacitor should be connected 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 cannot have lead.

(3) In the single-chip 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 properly laid out will determine the anti-interference ability of the circuit board.

When designing the ground wire and ground point of the PCB, the following issues should be considered:

Logic ground and analog ground should be wired separately and cannot be used together. Connect their respective ground wires to the corresponding power ground wires. When designing, the analog ground wire should be as thick as possible, and the grounding area of the terminal should be enlarged as much as possible. Generally speaking, it is best to isolate the input and output analog signals from the microcontroller circuit through optocouplers.

When designing the printed circuit board of the logic circuit, the ground wire should form a closed-loop form to improve the anti-interference ability of the circuit.

The PCB ground wire should be as thick as possible. If the ground wire is very thin, the resistance of the ground wire will be large, causing the ground potential to change with the current change, causing the signal level to be unstable, and the anti-interference ability of the circuit is reduced. If the wiring space allows, ensure that the width of the main ground wire is at least 2 to 3 mm, and the ground wire on the component pin should be about 1.5 mm.