The Protel design printed circuit board should consider the problem

The printed circuit board is the high density of various devices, the signal lines, power assembly. The printed circuit board design is good or bad is often a direct impact on the reliability of the system and the anti-interference ability. Printed circuit board design is never simple layout wire communication components, usually should consider the following aspects of the problem.

A ground design

(1) single-point ground with the choice of multi-point ground. In the low-frequency circuit, the operating frequency of the signal is less than 1MHz, the inductive effect of the wiring and components is smaller, while the circulation formed by the grounding circuit to the interference greater impact, and thus shielded wire point grounding. When the signal frequencies greater than 10MHz, wire impedance becomes very large, you should minimize ground impedance nearby multi-point grounding.

(2) separate digital and analog circuit. If the circuit board is both high-speed logic circuits and linear circuits, so they try to separate, while the ground between the two can not be mixed, should be connected to the power supply terminal ground as much as possible to increase the linear circuit ground area. Digital part and analog part of the A / D, D / A class of devices, prefer to bypass and do not cross.

(3) ground wire as bold. If the ground wire is very thin, the ground potential varies with the changes in the circuit, resulting in its signal level is unstable, the anti-noise performance deteriorates. Therefore, should the ground lines bold, via three times so that it allows current to the printed circuit board. If the possible grounding line width to more than 2mm for good. Printed line width and the current allowed to pass through a certain relationship, as shown in Figure 1, which shows a certain thickness of the copper foil conductor width allowed by the current curve and the general design, there must be three times the margin .

Second, the decoupling capacitor layout

Configured in various key parts of the printed circuit board decoupling capacitor should be regarded as a printed circuit board design, one of the most effective anti-jamming measures to improve the reliability of the whole system, the overall performance has very obvious effect. Circuit printed board designers should attract enough attention.

(1) In principle, each integrated circuit chip end of the power line placement should be a 0.01μF ceramic capacitors, printed circuit board small gap will not fit in every 4 to 8 chip power into the line-side placement a 1 ~ 10μF tantalum capacitor low noise. Because the high frequency impedance of the tantalum capacitor is very small, in the range of 500 kHz impedance is less than 1Ω, and small leakage current (0.5μA).

(2) for the capacity of the anti-noise is weak, the potential change in the shutdown device and a ROM, a RAM memory device in the chip's power supply line (Vcc) and ground (GND), direct access to the decoupling capacitor.

In the design of printed circuit boards, for terminations without some IC processing is also very important. The input of vacant vulnerable to interference. Usually, the gate circuit can be used the concentrated processing method shown in Figure 3.

(A) the unused leading ends of the resistor connected via a 2kΩ about +5 V power supply, so that it is at a high level.

(B) the leading end and using the unused input and pick up.

(C) if the same monolithic the unused NAND gates, may be grounded, and its output connected to another input NAND gate unused. If the door should connect the high level, the output high connect unused input terminal to the other door.

For the trigger, when the end of the set and reset terminal without, should be connected to high (+5 V), the CP end when not connected low. As shown in Figure 4.

Application Schmitt circuit when the introduction of a signal from the external circuit long before shaping, or plate in an RC integrator circuit, after also Schmidt circuit shaping. As shown in Figure 5.

Cloth when the power line, in addition to the current size, bold conductor width, but also to the power line ground towards the direction of the data transfer, in order to enhance its anti-noise function.