PCB Technical - How to design the single-chip control board PCB?

The most basic process of designing a circuit board can be divided into three steps: circuit schematic design, netlist generation, and printed circuit board design. No matter it is the device layout on the board or the wiring, etc., there are specific requirements.

For example, the input and output wiring should be avoided as much as possible to avoid interference. The parallel routing of the two signal lines must be separated by the ground wire, and the wiring of two adjacent layers should be perpendicular to each other as much as possible. Parasitic coupling is likely to occur in parallel. The power and ground wires should be separated into two layers as far as possible to be perpendicular to each other. In terms of line width, a wide ground wire can be used as a loop for the digital circuit PCB, that is, a ground network (analog circuit cannot be used in this way), and a large area of copper is used.

The following article explains the principles and some details that need to be paid attention to in the PCB design of the microcontroller control board.

1. Component layout

In terms of the layout of components, the components related to each other 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. Decoupling capacitor

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 directly against the components and fixed 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 during turn-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. Ground wire design

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 ground wires and grounding points, the following issues should be considered:

▪ The logic ground and the 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 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 2-3mm, and the ground wire on the component pin should be about 1.5mm.

▪ 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 effect, and the circulating current formed by the grounding circuit has a greater influence on interference, it is necessary to use a point of grounding so that it does not form a loop. When the signal frequency on the circuit board is higher than 10MHz, due to the obvious inductance effect of the PCB layout design, the ground line impedance becomes very large, and the circulating current formed by the ground circuit is no longer a major problem. Therefore, multi-point grounding should be used to reduce the ground impedance as much as possible.

4. other

·In addition to the layout of the power line, the width of the trace should be as thick as possible according to the size of the current. In the PCB layout design, the routing direction of the power line and the ground line should be consistent with that of the data line. Work in the PCB layout design. In the end, use ground wires to cover the bottom of the circuit board where there are no traces. These methods all help to enhance the anti-interference ability of the circuit.

The width of the data line should be as wide as possible to reduce impedance. The width of the data line is at least not less than 0.3mm (12mil), and it is more ideal if it is 0.46~0.5mm (18mil~20mil).

▪ Because a via on the circuit board will bring about 10pF capacitance effect, this will introduce too much interference for high frequency circuits, so when designing the PCB layout, the number of vias should be reduced as much as possible. Furthermore, too many vias will also reduce the mechanical strength of the circuit board.