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  • what matters do we need to pay attention in SCM control panel design?
    Date :2017-2-20


    The most basic process of designing a circuit board can be divided into three steps: the design of the circuit schematic, the generation of the network table, the printed circuit board design. Whether it is on-board device layout or alignment, and so have a specific request.
    For example, input and output traces should be avoided as far as possible in order to avoid interference. Two signal lines parallel to the line is necessary to add ground isolation, two adjacent layers of wiring to be perpendicular to each other, parallel to produce parasitic coupling. Power and ground should be divided into two layers perpendicular to each other. Line width, the digital circuit PCB can be used to do a wide ground circuit, that constitutes a ground network (analog circuit can not be used), with a large area of ​​copper.
    The following article on the microcontroller control board design need to pay attention to the principles and some details of the problem were described.
    First, the component layout
    In the layout of the components, should be related to the components as far as possible close to some, for example, the clock generator, crystal, CPU clock input are easy to produce noise, they should be placed close to the time. For those devices that are prone to noise, small current circuits, high current circuit switching circuits, etc., should be kept away from the logic control circuit and memory circuit (ROM, RAM) of the microcontroller. If possible, these circuits can be made separately Board, which is conducive to anti-interference, improve the reliability of circuit work.
    Second, decoupling capacitors
    Try to install decoupling capacitors next to key components such as ROM, RAM and other chips. In fact, printed circuit board alignment, pin wiring and wiring may contain a large inductance effect. Large inductors may cause severe switching noise spikes on the Vcc traces. The only way to prevent the noise spikes on the Vcc traces is to place a 0.1 μF electron decoupling capacitor between VCC and the power ground. If a circuit-mount element is used on the board, it can be mounted directly on the Vcc pin with a chip capacitor directly. It is best to use a ceramic capacitor because this capacitor has a low electrostatic loss (ESL) and high-frequency impedance, and this capacitor temperature and time on the stability of the media is also very good. Try not to use tantalum capacitors because of its high impedance at high frequencies.
    In the installation of decoupling capacitors need to pay attention to the following:
    1. In the printed circuit board power input side of the connection of about 100uF electrolytic capacitor, if the volume allows, then the capacitance is larger.
    2. In principle, each IC chip next to the need to place a 0.01uF ceramic capacitors, if the circuit board is too small and no less than 10, can be placed around every 10 a 1 ~ 10 tantalum capacitors.
    3. For decoupling capacitors with weak resistance to interference, large current variations at shutdown, and storage elements such as RAM and ROM, a power supply line (Vcc) and ground must be connected.
    4. The lead of the capacitor should not be too long, especially the high frequency bypass capacitor can not lead.
    Third, ground line design
    In the single-chip control system, there are many types of ground, systematically, shielded, logical, analog ground, ground wiring is reasonable, will determine the anti-jamming capability of the circuit board. In the design of ground and grounding point, you should consider the following questions:
    1. Logically and analog ground to separate wiring, can not be combined, their respective ground lines were connected with the corresponding power ground. In the design, the analog ground should be as thick as possible, and try to increase the lead-side grounding area. In general, for the input and output of the analog signal, and between the microcontroller circuit is best isolated by optocoupler.
    2. In the design of logic circuit printed circuit version, the ground should form a closed-loop form, improve the circuit's anti-jamming capability.
    3. Ground should be as thick as possible. If the ground is very fine, then the ground resistance will be larger, resulting in changes in the ground potential with the current changes, resulting in signal level instability, resulting in decreased anti-interference ability of the circuit. In the case of wiring space to ensure that the main ground to ensure that the width of at least 2 ~ 3mm above the component pin on the ground wire should be about 1.5mm.
    4. Pay attention to the choice of ground. When the signal frequency on the circuit board is less than 1MHz, the influence of the electromagnetic induction between the wiring and the element is very small, and the circulation caused by the grounding circuit has a great influence on the interference. Therefore, it is necessary to use a little grounding so that it does not form a loop. When the circuit board signal frequency is higher than 10MHz, because the wiring of the inductive effect is obvious, ground impedance becomes very large, then the formation of ground circuit loop is no longer a major problem. So should be used to ground more, try to reduce the ground impedance.
    Four, the other
    1. Power cord in addition to the layout according to the size of the current as much as possible to increase the width of the cable, the wiring should also make the power line, ground line alignment and data line alignment side of the line in the wiring work at the end, The ground floor will be the bottom of the circuit board is not covered with the place covered, these methods will help enhance the anti-interference ability of the circuit.
    2. The width of the data line should be as wide as possible to reduce the impedance. The width of the data line is at least not less than 0.3mm (12mil), if the use of 0.46 ~ 0.5mm (18mil ~ 20mil) is more ideal.
    3. Since a vias on the board will bring about 10pF of capacitance, this will introduce too much interference for high-frequency circuits, so the number of vias should be reduced as much as possible when wiring. Again, too many vias can cause the mechanical strength of the circuit board to decrease.



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