In the PCB routing rules, there is a principle of “key signal lines first”, that is, key signals such as power, analog signals, high-speed signals, clock signals, differential signals and synchronization signals are routed first. Next, let’s take a closer look at the routing requirements of these key signals.
Analog signal routing requirements
The main feature of analog signals is poor anti-interference, and the protection of analog signals is mainly considered during routing.
The processing of analog signals is mainly reflected in the following points:
1. To increase its anti-interference ability, the routing should be as short as possible.
2. Some analog signals can give up the impedance control requirements, and the routing can be appropriately thickened.
3. Limit the routing area, try to complete the routing in the analog area, and stay away from digital signals.
High-speed signal routing requirements
1. Multi-layer routing
High-speed signal routing circuits are often highly integrated and have a high routing density. The use of multi-layer boards is both necessary for routing and an effective means to reduce interference. Reasonable selection of the number of layers can significantly reduce the size of the printed board, make full use of the middle layer to set up the shield, better achieve nearby grounding, effectively reduce parasitic inductance, effectively shorten the transmission length of the signal, and significantly reduce cross-interference between signals.
2. The fewer bends in the lead, the better
The fewer bends in the lead between the pins of high-speed circuit devices, the better. It is best to use full straight lines for the leads of high-speed signal wiring circuit wiring. If a turn is required, a 45° bend line or arc turn can be used. This requirement is only used to improve the adhesion strength of the steel foil in low-frequency circuits, but in high-speed circuits, meeting this requirement can reduce the external emission and mutual coupling of high-speed signals, and reduce signal radiation and reflection.
3. The shorter the lead, the better
The shorter the lead between the pins of high-speed signal wiring circuit devices, the better. The longer the lead, the greater the distributed inductance and distributed capacitance, which will have a lot of impact on the passage of high-frequency signals in the system. At the same time, it will also change the characteristic impedance of the circuit, causing reflection and oscillation in the system.
4. The less inter-layer alternation of leads, the better
The less inter-layer alternation of leads between pins of high-speed circuit devices, the better. The so-called “the less inter-layer alternation of leads, the better” means that the fewer vias used in the component connection process, the better. According to measurements, a via can bring about 0.5pf of distributed capacitance, resulting in a significant increase in circuit delay. Reducing the number of vias can significantly increase the speed.
5. Pay attention to parallel cross interference
High-speed signal routing should pay attention to the “cross interference” introduced by the close parallel routing of signal lines. If parallel distribution cannot be avoided, a large area of ”ground” can be arranged on the reverse side of the parallel signal lines to greatly reduce interference.
6. Avoid branches and stumps
High-speed signal routing should try to avoid branches or stumps. Stumps have a great impact on impedance and can cause signal reflection and overshoot, so we usually avoid stumps and branches when designing. Using daisy chain wiring will reduce the impact on the signal.
7. Signal lines should be routed in the inner layer as much as possible
High-frequency signal lines on the surface layer are prone to generate large electromagnetic radiation and are also easily interfered by external electromagnetic radiation or factors. Routing high-frequency signal lines between the power supply and the ground line will reduce the radiation generated by the power supply and the bottom layer absorbing electromagnetic waves.
Requirements for clock signal routing
In digital circuit design, the clock signal is a signal that oscillates between high and low states and determines the performance of the circuit. The clock circuit plays an important role in the midpoint of the digital circuit and is also the main source of electromagnetic radiation. The clock processing method is also something that needs to be paid special attention to during PCB routing. At the beginning, the clock tree is sorted out and the relationship between various clocks is clarified, so that the routing can be handled better. In addition, the clock signal is often a difficult point in EMC design, and projects that need to pass EMC test indicators should pay special attention to it.
In addition to the conventional impedance control and equal length requirements, the clock line also needs to pay attention to the following issues:
1. Try to choose the preferred routing layer for the clock signal.
2. Try not to cross the partition, let alone route along the partition area.
3. Pay attention to the spacing between the clock signal and other signals, which should be at least 3W.
4. For designs with EMC requirements, try to choose inner-layer wiring when the line is long.
5. Pay attention to the termination matching of the clock signal.
6. Do not use a daisy chain structure to transmit the clock signal, but a star structure, that is, all clock loads are directly connected to the clock power driver.
7. All wires connected to the input/output of the crystal oscillator should be as short as possible to reduce noise interference and the influence of distributed capacitance on the crystal oscillator.
8. The ground wire of the crystal oscillator capacitor should be connected to the device using a wire as wide and short as possible; the digital ground pin closest to the crystal oscillator should have as few vias as possible.
9. In digital circuits, the clock signal is usually a signal with fast edge changes and large external crosstalk. Therefore, in the design, the clock line should be surrounded by a ground wire and more ground wires should be used to reduce distributed capacitance, thereby reducing crosstalk; for high-frequency signal clocks, try to use a low-voltage clock signal and ground wrapping method, and pay attention to the integrity of the ground wrapping holes.
Differential signal wiring requirements
Differential signals, also known as differential signals, use two completely identical signals with opposite polarities to transmit one data path, and rely on the difference in the level of the two signals for judgment. In order to ensure that the two signals are completely consistent, they must be kept parallel during wiring, and the line width and line spacing must remain unchanged.
On the circuit board, the differential traces must be two lines of equal length, equal width, close proximity, and on the same layer.
1. Equal length: Equal length means that the lengths of the two lines should be as equal as possible, in order to ensure that the two differential signals always maintain opposite polarities. Reduce common mode components.
2. Equal width and equal distance: Equal width means that the trace widths of the two signals need to be consistent, and equal distance means that the spacing between the two lines should remain unchanged and parallel.
Reminder: Try to provide dedicated wiring layers for key signals such as clock signals, high-frequency signals, and sensitive signals, and ensure their minimum loop area. Use shielding and increased safety spacing to ensure signal quality.