PCB Tech - How to wire PCB key signal

In PCB wiring rules, there is a "key signal line priority" principle, that is, power supply, analog signal, high-speed PCB signal, clock signal, differential signal and synchronization signal and other key signals priority wiring. Next, we may wish to understand the wiring requirements of these key signals in detail.

Analog signal cabling requirements

The main characteristic of analog signal is poor anti-interference. The protection of analog signal is mainly considered when wiring.

Analog signal processing is mainly reflected in the following points:

1. In order to increase its anti-interference ability, the wiring should be as short as possible.

2. Part of the analog signal can abandon the impedance control requirements, wiring can be appropriately bold.

3. Limit the wiring area, try to complete the wiring in the analog area, away from the digital signal.

High-speed signal wiring requirements

1. Multi-layer cabling

High-speed signal wiring circuit is often high integration, wiring density, the use of multi-layer board is not only necessary for wiring, but also an effective means to reduce interference. Reasonable selection of layers can greatly reduce the size of the board, make full use of the intermediate layer to set the shield, realize the nearest grounding, effectively reduce the parasitic inductance, effectively shorten the transmission length of the signal, and greatly reduce the cross interference between signals.

2. Bend the lead as little as possible

The less lead bending between pins of high-speed circuit devices, the better. The fuses with full line of high-speed signal wiring circuit wiring, need to turn, 45 ° line or the circular arc transition are available, and the requirement in the low frequency circuit used to improve the fixation strength of steel foil, only in the high speed circuit, meet this requirement can reduce foreign high-speed signal transmission and mutual coupling, to reduce the radiation and reflection signals.

3. The shorter the lead, the better

The shorter the lead between pins of high speed signal wiring circuit components, the better. The longer the lead, the greater the value of distributed inductance and capacitance, the high frequency signal of the system through a lot of influence, but also change the characteristic impedance of the circuit, resulting in system reflection, oscillation and so on.

4. The less alternating between lead layers, the better

The less alternating lead layers between pins of high-speed circuit devices, the better. The so-called "as few layers of lead alternating as possible" refers to the connection of components with as few holes as possible. It has been measured that a single hole can bring about 0.5 pF of distributed capacitance, resulting in a significant increase in circuit delay, reducing the number of holes can significantly improve the speed.

5. Pay attention to parallel cross interference

In high-speed signal wiring, attention should be paid 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 opposite side of parallel signal lines to greatly reduce interference.

6. Avoid branches and stumps

High-speed signal wiring should avoid branching or Stub formation as far as possible. Stumps have a large effect on impedance and can lead to signal reflection and overshoot, so we should generally avoid stumps and branches in our design. Daisy chain wiring will reduce the impact on the signal.

7. Route signal cables through the inner layer

High frequency signal lines walking on the surface are prone to produce large electromagnetic radiation, and are also prone to interference by external electromagnetic radiation or factors. If the high frequency signal cable is routed between the power supply and ground wire, the radiation generated by the power supply and the bottom layer will be reduced by absorbing electromagnetic waves.

Requirements for clock signal cabling

In digital circuit design, clock signal is a signal oscillating between high and low states, which determines the performance of the circuit. Clock circuit plays an important role in digital circuit, and it is also the main source of electromagnetic radiation. The processing method of the clock is also needed to pay special attention to the PCB wiring. By sorting out the clock tree from the beginning and understanding the relationship between the various clocks, wiring will be better handled. And the clock signal is often a difficult point in EMC design, especially for projects that need to pass EMC test indexes.

In addition to the conventional impedance control and equal length requirements of the clock line, the following issues need to be paid attention to:

1. Select the best routing layer for the clock signal.

2. The clock signal should not cross the partition as far as possible, let alone route along the partition area.

3. Ensure that the distance between the clock signal and other signals is at least 3W.

4. For design with EMC requirements, inner wiring should be selected as long as possible.

5. Pay attention to the matching of clock signals.

6. Do not use a Daisy chain structure to transmit clock signals. Use a star structure, where all clock loads are directly connected to the clock power driver.

7. All wires connecting the input/output terminals of crystal oscillators should be as short as possible to reduce noise interference and the influence of distributed capacitance on crystal oscillators.

8. Crystal capacitance grounding wire shall be connected to the device with as wide and short wire as possible; The digital ground pins close to the crystal oscillator should minimize the hole.

9. In the digital circuit, the clock signal is usually the signal with fast edge change and large external crosstalk. Therefore, in the design, the clock line should be surrounded by ground lines and more ground lines to reduce the distributed capacitance, so as to reduce crosstalk; For high frequency signal clock, it is necessary to use low voltage off time clock signal and ground encapsulation mode as far as possible, and pay attention to the integrity of ground encapsulation drilling.

Differential signal wiring requirements

Differential signals, some also known as differential signals, use two exactly the same, opposite polarity of the signal transmission one way of data, depending on the level difference between the two signals for judgment. In order to ensure that the two signals are completely consistent, it is necessary to keep parallel in wiring, and the line width and line spacing remain unchanged.

How to wire PCB key signal

On a circuit board, differential wiring must be two wires of equal length and width, close together and at the same level.

1. Equal length: Equal length 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 opposite polarity. Reduce the common-mode component.

2. Equal width equal distance: Equal width means that the line widths of two signals should be the same, and equal distance means that the spacing between two lines should remain unchanged and keep parallel.

Reminder: as far as possible for the clock signal, high frequency signal, sensitive signal and other key signals to provide special wiring layer, and ensure its loop area. Shielding and increasing safety distance are adopted to ensure signal quality.