1. In the layout of components on a PCB, related components should be placed as closely as possible. For instance, the clock generator, crystal oscillator, and CPU clock input are particularly susceptible to noise, so they should be situated near one another. Additionally, devices prone to noise, such as low-current circuits, high-current switching circuits, etc., should be kept away from the logic control circuits and storage circuits (ROM, RAM) of the microcontroller whenever possible. If feasible, these noisy circuits can be isolated onto separate boards, which helps enhance anti-interference measures and improve circuit reliability.

2. Decoupling capacitors should be installed adjacent to key components like ROM, RAM, and other chips. Printed circuit board traces and pin connections can exhibit significant inductance effects, which may lead to severe switching noise spikes on the Vcc trace. The most effective method to mitigate these noise spikes is to place a 0.1µF decoupling capacitor between Vcc and ground. If surface-mount components are utilized, chip capacitors can be placed directly next to the components and soldered onto the Vcc pin. Ceramic capacitors are preferable due to their low equivalent series inductance (ESL) and high-frequency performance, as well as their excellent dielectric stability in terms of temperature and time. It’s advisable to avoid tantalum capacitors, as their impedance tends to be higher at elevated frequencies.


Pay attention to the following points when placing decoupling capacitors:

1. Connect a 100µF electrolytic capacitor across the power input of the printed circuit board. If space allows, a capacitor with a larger capacitance is preferable.

2. In principle, a 0.01µF ceramic capacitor should be positioned close to each integrated circuit chip. If the circuit board’s spacing is too limited, you can use a 1-10µF tantalum capacitor for every 10 chips.

3. For components that exhibit weak interference resistance and experience significant current changes when powered down—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 capacitors should not be excessively long, especially for high-frequency bypass capacitors, which should ideally have no leads.

**3. Ground Wire Design**

5. In a microcontroller system, various ground types exist, including system ground, shield ground, logic ground, and analog ground. The effectiveness of ground wire layout directly influences the circuit board’s ability to resist interference. When designing ground wires and grounding points, consider the following:

6. Logic ground and analog ground should be kept separate and not combined. Their respective ground wires should connect to the corresponding power ground lines. Ideally, the analog ground wire should be as thick as possible, and the grounding area of the terminal should be maximized. Generally, isolating input and output analog signals from the microcontroller circuit using optocouplers is advisable.

7. In the design of the printed circuit board for logic circuits, the ground wire should form a closed-loop structure to enhance the circuit’s interference resistance.

8. The ground wire should be as thick as feasible. A thin ground wire increases resistance, causing fluctuations in ground potential with current changes, leading to unstable signal levels and reduced interference immunity. If space permits, ensure that the main ground wire width is at least 2-3mm, while component pin ground wires should be approximately 1.5mm.

9. Be mindful when selecting grounding points. If the signal frequency on the circuit board is below 1MHz, electromagnetic induction effects between traces and components are minimal. Therefore, using a single grounding point to avoid loops is advisable. Conversely, for signal frequencies above 10MHz, the inductive effects of PCB layout design increase ground line impedance significantly, making circulating current less of a concern. In this case, employing multi-point grounding is beneficial to minimize ground impedance.

**4. Additional Considerations**

10. In addition to power line layout, trace width should be maximized according to current size. The routing direction of power and ground lines should align with that of the data lines. In the final PCB layout, utilize ground wires to cover areas of the circuit board without traces. These practices enhance the circuit’s interference resistance.

11. The width of data lines should be as broad as possible to minimize impedance. Data line width should be no less than 0.3mm (12mil), with 0.46-0.5mm (18mil-20mil) being optimal.

12. Since a via hole in the circuit board can introduce about 10pF of capacitance, potentially causing excessive interference in high-frequency circuits, it’s essential to minimize the number of vias in PCB layout design. Additionally, an excessive number of vias can compromise the mechanical strength of the circuit board.