Wiring requirements for PCB key signal lines

PCB wiring is a very important part of Design de PCB. Wiring is a relatively complex job. It requires exquisite design and precise skills. It is a huge test for designers.

In PCB wiring rules, the layout of the key signal line is very important. That is, the key signal lines such as power supply, dictation signal, high-speed signal, clock signal, differential signal, and synchronization signal. Understand the wiring requirements of the key signal line.

Sims signal wiring requirements

The main feature of analog signals is poor anti-interference. When wiring, the protection of analog signals is mainly considered.

Simulation signal processing is mainly reflected in the following points:

1. In order to increase its anti-interference capability, the wiring should be as short as possible.
2. Some analog signals can abandon impedance control requirements, and the wiring can be appropriately thickened.
3. Limited wiring area, try to complete wiring in the simulation area as much as possible, and stay away from digital signals.

High-speed signal wiring requirements

1. Multi -layer wiring
High-speed signal wiring circuits are often highly integrated and the wiring density is large. The use of multi-layer boards is both necessary for wiring and an effective means to reduce interference. Reasonable selection of the number of layers can greatly reduce the size of the printing plate, can make full use of the middle layer to set the shield, can better achieve near ground, can effectively reduce parasitic inductance, can effectively shorten the transmission length of the signal, greatly reduce the signal to greatly reduce signal cross-interference in the middle.

2. The less the lead bend, the better.
The less lead between the tubes of the high-speed circuit device, the better. The leader of high-speed signal wiring circuit wiring is best to adopt a full line, which needs to be turned. You can use a 45 ° folding line or an arc to turn. One requirement can reduce the transmission of high-speed signals and coupling between them, and reduce the radiation and reflection of the signal.

3. The shorter the lead, the better
The shorter the lead between the high-speed signal wiring circuit device, the better. The longer the lead, the greater the distributed inductance and distributed capacitance value brought about. It has a great impact on the transmission of the high frequency signal of the system. At the same time, it will also change circuit characteristics, which will cause the system to reflect and oscillate.

4. The less alternation between the lead layers, the better
The less the lead layer between the tubes of the high speed circuit device, the better. The so-called “less interlayer lead, the better,” refers to the fewer holes used in the connection process of the component, the better. It is reported that overholes can bring about 0.5PF distributed capacitors, which causes the delay of the circuit to increase significantly, and reducing the number of overholes can significantly increase the speed.

5. Pay attention to parallel cross-sections

High-speed signal wiring should pay attention to the “cross interference” introduced by the close parallel wiring of the signal line. If parallel distribution cannot be avoided, a large area of “ground” can be arranged on the reverse side of the parallel signal line to significantly reduce interference.

6. Avoid branches and stumps
High-speed signal wiring should try to avoid branches or stumps. Stump resistance has a great impact, which can cause the signal to reflect and overlap, so we usually avoid tree stumps and branches when designing. Chrysanthemum chain is used to reduce signal impact.

7. Signaling line as much as possible in the inner layer
High frequency signal lines are prone to large electromagnetic radiation at the surface layer, and they are also vulnerable to external electromagnetic radiation or factors. The high frequency signal line is between the power supply and the ground line. The radiation generated by the absorption of electromagnetic waves through the power supply and the underlying layer will be greatly reduced.

Clock signal wiring requirements

In digital circuit design, the clock signal is a signal oscillating between high and low state, determining the performance of the circuit. The clock circuit has an important position in the digital circuit, and at the same time, it is the main source of electromagnetic radiation. The processing method of the clock also needs to pay special attention to PCB wiring. From the beginning, clarify the clock tree and clarify the relationship between the clocks, and it can be handled better when wiring. And the clock signal is often the difficulty of EMC design, and it is necessary to pay attention to the items that need to pass the EMC test index.

In addition to conventional impedance control and equivalent requirements, the clock line also needs to pay attention to the following issues:

1. Choose the best wiring layer as much as possible.
2. Try not to split the clock signal as much as possible, let alone wiring along the segmentation area.
3. Pay attention to the spacing of the clock signal and other signals, at least 3W.
4. Design with EMC requirements. Try to choose the inner layer wiring as much as possible.
5. Pay attention to the end matching of the clock signal.
6. Do not use the chrysanthemum chain structure to transmit the clock signal, but the star structure should be used. That is, all clock loads connect directly to the clock power drive.
7. The wires at all input/output terminals of the connected wafer are as short as possible to reduce the effects of noise interference and distributed capacitors on the wafer.
8. The crystal capacitance ground wire should be connected to the device as wide and short as possible; the closest digital pins of the crystal oscillator should be reduced as much as possible.
9. In digital circuits, the usual clock signal is a signal with fast edge changes, which are large. Portanto, in the design, the clock line should be surrounded by ground lines and hit more ground wires to reduce distributed capacitors, thereby reducing string disturbances; the high frequency signal clock is used to use a low voltage control clock signal. Integrity of the hole.

Differential signal wiring requirements

Differential signals are also called differential signals. With two exactly the same, opposite signal transmission all the way, relying on two signal level differences to make a judgment. In order to ensure that the two signals are completely consistent, it is necessary to keep parallel when wiring, and the line width and line spacing remain unchanged.

How to wire PCB key signals

On the circuit board, the differential wiring must be two lines equal, wide, close, and close, and at the same level.
1. Equivalent: The length of the two lines means that the length of the two lines should be as long as possible, in order to ensure that the two differential signals always maintain the opposite polarity at all times. Reduce co-mode weight.
2. Equal width means that the width of the two signals needs to be consistent, and distance means that the distance between the two lines remains unchanged and keeps parallel.
Reminder: Try to provide a special wiring layer for key signals such as clock signals, high frequency signals, and sensitive signals, and ensure its smallest circuit area. Methods such as shielding and increasing the safety distance to ensure signal quality.

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