SCM system printed circuit board design skills
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The design of the printed circuit board is very important for the anti-interference of the single-chip system. Design printed circuit boards and wiring in accordance with the three principles of controlling noise sources as much as possible, minimizing noise propagation and coupling, and minimizing noise absorption.
The printed circuit board should be properly partitioned. The single-chip system can usually be divided into three zones, namely the analog zone (fear of interference), the digital zone (ie, interference and interference) and the power drive zone (interference source).
The printed circuit board is powered by a single point power supply and a single point grounding principle. The power and ground lines of the three areas are led out in three ways. The noise component is farther away from the non-noise component.
Design skills
1. Coil the clock oscillating circuit and the special high-speed logic circuit to ground so that the surrounding electric field is close to zero.
2. The I/O driver device and power amplifier are as close as possible to the side of the board, close to the connector.
3. Can use low-speed devices without high-speed devices, high-speed devices are only used in key places.
4. Use a clock that meets the minimum frequency required by the system. The clock generator is as close as possible to the device that uses the clock. The quartz crystal oscillator case should be grounded, and the clock line should be as short as possible, and should not be cited everywhere. Below the quartz oscillator, the area below the noise-sensitive device is to be enlarged and the other signal lines should not be taken. The clock line is perpendicular to the I/O line and has less interference than the parallel I/O line. The clock line is far from the I/O line.
5. Use a 45-degree line and do not use a 90-degree fold line to reduce the emission of high-frequency signals. Single panel, double panel, power cord and ground wire should be as thick as possible. The number of vias in the signal line should be as small as possible.
6. 4 layer board is 20dB lower than double panel noise. The 6-layer board is 10 dB lower than the 4-layer board. Try to use multi-layer boards when the economy allows.
7. The key lines should be as short as possible and as thick as possible, with protective ground on both sides. If the sensitive signal and the noise field band signal are taken out through a flat cable. It should be extracted by the ground-signal-ground line.
8. Do not form a loop on any signal line. If it is unavoidable, the loop should be as small as possible.
9. For A/D type devices, the digital part and the analog part are better to go around and not to cross. Noise sensitive lines should not be parallel to high speed lines and high current lines
10. Single-chip microcomputer and other IC circuits. If there are multiple power supplies and grounds, add a decoupling capacitor to each end. Add a decoupling capacitor to each IC, and choose a monolithic or ceramic capacitor with a high-frequency signal as the decoupling capacitor. When the decoupling capacitor is soldered to the board, the pins should be as short as possible. A large-capacity tantalum capacitor or a polyester capacitor is used instead of an electrolytic capacitor as a storage capacitor for charging a circuit because the electrolytic capacitor has a large distributed inductance and is ineffective for high frequency. If electrolytic capacitors are used, they must be paired with decoupling capacitors with good high frequency characteristics.
11. The I/O port that is not used by the MCU should be defined as an output. Signals from the high noise region are filtered. A discharge diode should be placed outside the relay coil. A string of resistors can be used to soften the edge of the I/O line or provide some damping.
12. When needed, a high-frequency choke device made of copper wire wound ferrite can be added to the power line and the ground line to block the transmission of high-frequency noise. Weak signal lead-out lines, high-frequency, high-power lead-out cables should be shielded. The lead wire and the ground wire are to be twisted.
13. The printed circuit board is too large or the signal line frequency is too high, so that the delay time on the line is greater than or equal to the signal rise time. The line is processed according to the transmission line, and the terminal matching resistor is added.
14. Try not to use the IC socket, solder the IC directly to the board, and the IC holder has a large distributed capacitance.