Noteworthy MCU control board design principles

The principles to be followed are as follows:

(1) In terms of the layout of 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 prone to noise. They should be brought closer. For those devices that are prone to noise, small current circuits, high current circuit switching circuits, etc., it should be kept away from the logic control circuit and storage circuit (ROM, RAM) of the single-chip microcomputer as far as possible. If possible, these circuits can be made into separate circuits board, which is conducive to anti-interference and improves the reliability of circuit work. lcd panel module

(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 inductive effects. A large inductance can cause severe switching noise spikes on the Vcc trace. The way to prevent switching noise spikes on the Vcc trace is to place a 0.1uF electronic decoupling capacitor between VCC and power ground. If surface-mount components are used on the board, chip capacitors can be used directly next to the components, fixed on the Vcc pin. Ceramic capacitors are used, because this capacitor has low electrostatic loss (ESL) and high frequency impedance, and the dielectric stability of this capacitor over temperature and time is also very good. Try not to use tantalum capacitors because of their higher impedance at high frequencies.

When placing decoupling capacitors, pay attention to the following points:
·Connect an electrolytic capacitor of about 100uF across the power input end of the printed circuit board. If the volume allows, it is better to have a larger capacitance.
·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 be placed, a 1-10 tantalum capacitor can be placed every 10 chips or so.
· For components with weak anti-interference ability and large current change 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 wire of the capacitor should not be too long, especially the high frequency bypass capacitor should not have a lead wire.

(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 ground wires and grounding points, the following issues should be considered:
·Logical 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, the input and output analog signals are isolated from the microcontroller circuit by 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 change of the current, resulting in unstable signal level, resulting in a decrease in the anti-interference ability of the circuit. 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 pins 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, since the electromagnetic induction between the wiring and the components has little influence, and the circulating current formed by the grounding circuit has a greater influence on the interference, it is necessary to use a little 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 wiring, the impedance of the ground wire 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 wire impedance as much as possible.
The layout of the power line should not only increase the width of the line as much as possible according to the size of the current, but also make the line direction of the power line and the ground line consistent with that of the data line. Cover the bottom layer of the circuit board where there are no traces, these methods will help to enhance the anti-interference ability of the circuit.
• The width of the data lines 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 0.46~0.5mm (18mil~20mil) is used.
·Since a via hole on the circuit board will bring about a 10pF capacitance effect, which will introduce too much interference for high-frequency circuits, so the number of via holes should be reduced as much as possible when wiring. Furthermore, too many vias can also reduce the mechanical strength of the circuit board.