Circuit design principles of single chip microcomputer control board

Source: Date: 2020-11-25

In the circuit design process of the microcontroller control board, if you can follow the following principles, it will speed up the speed of our circuit design!
(1) In 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 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 microcontroller 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) 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 method 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. Ceramic capacitors are used, because this capacitor has low electrostatic loss (ESL) and high-frequency impedance. In addition, the dielectric stability of this capacitor over temperature and time is also very good. Try not to use tantalum capacitors, because their impedance is higher at high frequencies.
Circuit design needs to 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 be placed, you can place a 1-10 tantalum capacitor for every 10 chips.
For components with weak anti-interference ability and large current changes 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 of the capacitor should not be too long, especially the high frequency bypass capacitor can not take lead.
(3) In the single-chip microcomputer control system, there are many types of ground wires, such as system ground, shield ground, logic ground, analog ground, etc. The reasonable layout of ground wires will determine the anti-interference ability of the circuit board.
The following issues should be considered when designing the ground wire and ground point of the circuit
Logic 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 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, resulting in a decrease in the anti-interference ability of the circuit. If the wiring space permits, 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 the wiring and the components has little effect, and the circulation formed by the grounding circuit has a greater influence on the interference, it is necessary to use a point 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 ground line impedance becomes very large, and the circulating current formed by the grounding circuit is no longer a major problem. Therefore, multi-point grounding should be used to minimize the ground impedance.
In addition to making the layout of the power line as thick as possible according to the current, the wiring direction of the power line and the ground line should be consistent with the direction of the data line during the wiring. The bottom layer of the circuit board is covered with 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 0.46~0.5mm (18mil~20mil) is used.
Since a via on the circuit board will bring about a 10pF capacitance effect, this will introduce too much interference to the high-frequency circuit, so when wiring, 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.