The design of the printed circuit board is very important for the anti-interference of the single-chip system. It is necessary to design the printed circuit board and wiring in accordance with the three principles of controlling the noise source as much as possible, minimizing the propagation and coupling of noise, and minimizing the absorption of noise.
The printed circuit board should be reasonably partitioned. The single-chip system can be divided into three areas, namely the analog area (fear of interference), digital area (that is, interference and interference) and power drive area (interference source).
The single-point connection of the printed circuit board and the single-point grounding principle supply power. The power and ground wires in the three areas are led out in three ways. Noise components and non-noise components should be far away.
Design tips
1. Coil the clock oscillating circuit and special high-speed logic circuit to make the surrounding electric field close to zero.
2. I / O drive device and power amplifier should be as close as possible to the edge of the board and the connector.
3. High-speed devices are not necessary for low-speed devices, and high-speed devices are only used in key places.
4. Use the clock with the lowest frequency that meets the system requirements. The clock generator should be as close as possible to the device that uses the clock. The quartz crystal oscillator shell should be grounded, the clock line should be as short as possible, and do not lead everywhere. The area of ​​the ground should be increased under the quartz oscillator and under the noise sensitive device, and no other signal lines should be used. The clock line perpendicular to the I / O line has less interference than the parallel I / O line, and the clock line should be far away from the I / O line.
5. Use a 45-degree line instead of a 90-degree fold line to reduce the emission of high-frequency signals. For single and double panels, the power and ground cables should be as thick as possible. The vias of the signal line should be as few as possible.
6. The 4-layer board has 20dB lower noise than the double panel. The 6-layer board is 10dB lower than the 4-layer board. When economically allowed, use multi-layer boards as much as possible.
7. The key line should be as short and thick as possible, and add protective grounds on both sides. If the sensitive signal and the noise field signal are led out through a flat cable. It should be led out by ground wire-signal-ground wire.
8. Do not form a loop in any signal line. If unavoidable, the loop should be as small as possible.
9. For A / D type devices, the digital part and the analog part would rather go around and not cross. Noise-sensitive lines should not be parallel to high-speed lines and high-current lines
10. SCM and other IC circuits, if there are multiple power supplies and grounds, add a decoupling capacitor at each end. A decoupling capacitor should be added to each IC, and a monolithic or ceramic capacitor with good high-frequency signals should be selected as the decoupling capacitor. When the decoupling capacitor is soldered to the board, the pins should be as short as possible. The use of large-capacity tantalum capacitors or polyester capacitors instead of electrolytic capacitors as energy storage capacitors for circuit charging is because electrolytic capacitors have a large distributed inductance and are ineffective at high frequencies. If electrolytic capacitors are used, they should be paired with decoupling capacitors with good high-frequency characteristics.
11. I / O ports that are not used by the microcontroller should be defined as outputs. The signal from the high noise area should be filtered. A discharge diode should be placed outside the relay coil. You can use a resistor in series to soften the transition edge of the I / O line or provide some damping.
12. When necessary, high-frequency choke devices made of copper wire wound ferrite can be added to the power line and ground line to block the conduction of high-frequency noise. Weak signal leads, high-frequency, high-power leads should be shielded. The lead wire and ground wire should be twisted.
13. The printed circuit board is too large or the frequency of the signal line is too high, so that the delay time on the line is greater than or equal to the signal rise time. The line should be treated as a transmission line, and the terminal matching resistance should be added.
14. Try not to use the IC socket, solder the IC directly to the board, the IC seat has a large distributed capacitance
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