1.Layout of electronic components
When laying out electronic components, relevant components should be put close to each other, such as, clock generator, crystal oscillator, input end of CPU clock, which easily generate noise, so they should be placed closer to each other. For components which easily produce noise, small current circuits, large current circuits, switching circuit, etc., they should be away from the logic control circuit with single chip as well as ROM and RAM. If possible, these circuits can be made into circuit boards, which will be beneficial to resisting interference and improve reliability of the circuits.
Decoupling capacitance should be installed near the key components, such as, ROM, RAM, etc.. Inductive effects may occur in the wiring, pins connecting and routing of the printed circuit board. Once the inductive effects are too strong, heavy switching noise spike may be produced in the Vcc routing. The only way to prevent heavy switching noise spike is to install an electronic decoupling capacitance of 0.1uF between the VCC and power floor. If the surface assembling components are used on the PCB, chip capacitance can be installed next to the components and fixed well at the Vcc pins. It is best to use ceramic chip, because this kind of capacitance has low ESL and high-frequency resistance. Besides, this kind of capacitance and temperature enjoys good stability. Tantalum capacitance should not be used because it has a high level of resistance under high-frequency circumstances.
Several points need to be paid special attention when installing decoupling capacitance:
·On the power input ends of the printed circuit board, an electrolytic capacitor of 100 uF should be installed. If the volume allows, it would be better if the capacitor is larger.
·In principle, a ceramic capacitor of 0.01uF should be installed near every integrated circuit chip. If the gap between circuit boards are too large to place the ceramic capacitor, a tantalum capacitor of 1~10 can be placed around every 10 chips.
·For components whose anti-interference ability is weak and current change is large when cutting off, and storage components, such as RAM, ROM, decoupling capacitance can be installed between the Vcc and floor wires.
·The lead of the capacitance should never be too long, especially for the high-frequency bypass, there should be no leads.
3.Ground wires design
In the single chip controller system, there are many kinds of ground wires, such as the systematic ground wires, screen ground wires, logic ground wires, simulating ground wires, etc.. Whether ground wires can be laid out reasonably determines the anti-interference ability of the circuit board. Therefore, the following factors should be taken into consideration when designing ground wires and earth connection point.
·Logic ground wires and simulating ground wires should be separated and never be combined, whose ground wires should be connected to the corresponding power lines. Simulating wires should be thickened and the ground contact area of the leading out terminal should also be enlarged. Generally speaking, simulating signal, be it from input or output, should be separated by optocoupler with single chip circuit.
·When designing printed circuit boards of logical circuits, ground wires should form a closed ring to improve the anti-interference ability.
·Ground wires should be thickened as much as possible. If the ground wires are too thin, electric resistance from the ground wires will be too large, causing changes in electrical potential due to changes of currency. Thus, the electrical level of the signal will not be stable, leading to a reduction in the anti-interference ability. When routing room allows, the length of the ground wires should be at least over 2~3mm and the ground wires on the pins of components should be around 1.5mm.
·More attention should be paid to the location of earth connection point. When the signal frequency on the circuit board is lower than 1MHz, electromagnetic induction between routing and components will be very small. On the contrary, the circular current ground circuit forms will have larger influence on the interference, so a little floor connection is necessary so that there won’t be loopback. When the signal frequency on the circuit board is higher than 10MHz, inductive effect of the routing will be obvious, so that the resistance from ground wires will be huge. At this time, it is no longer a problem for the ground circuit to form circular current. Therefore, more connections to the floors are needed so as to reduce the ground wires’ resistance.
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