MCU control board design principles and some details

1. Component layout

In terms of the layout of components, related components should be placed as close as possible. For example, clock generators, crystal oscillators, and CPU clock input terminals are prone to noise, so they should be placed closer together. For those devices that are prone to noise, small current circuits, large current circuit switch circuits, etc., they should be kept as far away from the logic control circuits and storage circuits (ROM, RAM) of the microcontroller as possible. If possible, these circuits can be made into circuit boards separately, which is conducive to anti-interference and improves the reliability of circuit operation.

2. Decoupling capacitors

Try to install decoupling capacitors next to key components, such as ROM, RAM and other chips. In fact, PCB board traces, pin connections and wiring 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 the Vcc trace is to place a 0.1uF electronic decoupling capacitor between VCC and the power ground. If surface mount components are used on the PCB board, chip capacitors can be used directly close to the components and fixed on the Vcc pin. It is best to use ceramic capacitors because this type of capacitor has lower 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.

The following points should be noted when placing decoupling capacitors:

(1) Connect an electrolytic capacitor of about 100uF across the power input terminal of the PCB board. If the volume allows, a larger capacitance is better.

(2) In principle, a 0.01uF ceramic capacitor needs to be placed next to each integrated circuit chip. If the gap in the circuit board is too small to accommodate it, a 1~10uF tantalum capacitor can be placed for every 10 chips.

(3) For components with weak anti-interference ability, 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.

(4) The leads of the capacitor should not be too long, especially high-frequency bypass capacitors should not have leads.

3. Ground wire design

In the single-chip control system, there are many types of ground wires, including system ground, shield ground, logic ground, analog ground, etc. Whether the ground wire layout is reasonable will determine the anti-interference ability of the circuit board. When designing the ground wire and grounding point, the following issues should be considered:

(1) The logic ground and analog ground should be wired separately and cannot be used together. Their respective ground wires should be connected to the corresponding power ground wire. When designing, the analog ground wire should be as thick as possible, and the grounding area of ​​the lead-out end should be as large as possible. Generally speaking, for the input and output analog signals, it is best to isolate them from the microcontroller circuit through an optical coupler.

(2) When designing the printed circuit board of a logic circuit, its ground wire should form a closed loop to improve the circuit's anti-interference ability.

(3) The ground wire should be as thick as possible. If the ground wire is very thin, the ground wire resistance will be large, causing the ground potential to change with the current, resulting in unstable signal level and reduced anti-interference ability of the circuit. If the wiring space allows, the width of the main ground wire should be at least 2~3mm, and the ground wire on the component pin should be around 1.5mm.

(4) Pay attention to the selection of grounding points. When the signal frequency on the circuit board is lower than 1MHz, the electromagnetic induction effect between the wiring and the components is small, while the loop current formed by the grounding circuit has a greater impact on interference, so one-point grounding should be used to prevent it from forming a loop. When the signal frequency on the circuit board is higher than 10MHz, the inductance effect of the wiring is obvious, and the ground impedance becomes very large. At this time, the loop current formed by the grounding circuit is no longer a major problem. Therefore, multi-point grounding should be used to minimize the ground impedance.

4. Others

(1) In addition to increasing the width of the power line as much as possible according to the current size, the routing direction of the power line and the ground line should be consistent with the routing direction of the data line during wiring. At the end of the wiring work, the ground line is used to cover the bottom layer of the circuit board where there is no routing. These methods are helpful to enhance the circuit's anti-interference ability.

(2) The width of the data line should be as wide as possible to reduce impedance. The width of the data line should be at least 0.3mm (12mil), and 0.46~0.5mm (18mil~20mil) is more ideal.

(3) Since a via in a circuit board will bring about a capacitance effect of about 10pF, which will introduce too much interference to high-frequency circuits, the number of vias should be reduced as much as possible during wiring. In addition, too many vias will also reduce the mechanical strength of the circuit board.