printed circuit board wiring

(1) Wiring priority

Key signal line priority: analog small signal, high-speed signal, clock signal and synchronization signal and other key signals priority wiring

Density first principle: Start wiring from the most complex connections on the single plate . Start wiring from the most densely wired area of the single plate.

Points to note:

A. Try to provide a special wiring layer for key signals such as clock signals, high-frequency signals and sensitive signals, and ensure the minimum loop area. If necessary, manual priority wiring, shielding and increasing safety spacing should be adopted. Ensure signal quality.

b. EMC environment between the power layer and the ground is poor, so signals sensitive to interference should be avoided.

c. The network with impedance control requirements should be wired as far as possible according to the line length and line width requirements.

(2) Four specific routing methods

1, clock wiring:

The clock line is one of the biggest factors affecting EMC. Make fewer holes in the clock line, avoid walking with other signal lines as far as possible, and stay away from general signal lines to avoid interference with signal lines. At the same time, the power supply on the PCB board should be avoided to prevent interference between the power supply and the clock.

If there is a special clock chip on the board, it can not go under the line, should be laid under the copper, if necessary, can also be special to its land. For many chip reference crystal oscillator, these crystal oscillator should not be under the line, to lay copper isolation.

2. Right Angle routing:

Right-angle cabling is generally required to avoid the situation in PCB wiring, and has almost become one of the standards to measure the quality of wiring, so how much impact will right-angle cabling have on signal transmission? In principle, right-angle routing will cause the line width of the transmission line to change, resulting in impedance discontinuity. In fact, not only right Angle routing, ton Angle, acute Angle routing may cause impedance changes.

The influence of right-angle routing on signal is mainly reflected in three aspects:

First, the corner can be equivalent to the capacitive load on the transmission line, slowing down the rise time;

Second, impedance discontinuity will cause signal reflection;

Third, EMI produced by the right Angle tip.

3. Differential routing:

Differential Signal is used more and more widely in the design of high-speed circuits, because the most important signals in circuits always use differential structure. Definition: In plain English, it means that the driver sends two equivalent, inverting signals, and the receiver determines whether the logical state is "0" or "1" by comparing the difference between the two voltages. The pair carrying the differential signal is called differential routing.

Compared with ordinary single-ended signal routing, differential signal has the most obvious advantages in the following three aspects:

a. Strong anti-interference ability, because the coupling between the two differential wires is very good, when there is noise interference from the outside, it is almost coupled to the two lines at the same time, and the receiver only cares about the difference between the two signals, so the common mode noise from the outside can be completely cancelled out.

b.It can effectively inhibit EMI. Similarly, because the polarity of two signals is opposite, the electromagnetic fields radiated by them can cancel each other out. The closer the coupling, the less electromagnetic energy released to the outside world.

c.Precise timing positioning. Since the switching change of differential signal is located at the intersection of two signals, unlike ordinary single-ended signal which depends on the high and low threshold voltage, it is less affected by technology and temperature, which can reduce the error in timing and is more suitable for the circuit with low amplitude signal. LVDS (low voltage differential signaling), which is popular at present, refers to this small amplitude differential signaling technology.

For PCB engineers, the most important thing is to ensure that the advantages of differential routing can be fully utilized in the actual routing. Perhaps as long as the contact with PCB Layout people will understand the general requirements of differential routing, that is, "equal length, equal distance".

The equal length is to ensure that the two differential signals maintain opposite polarity at all times and reduce the common-mode component. Equidistance is mainly to ensure that the difference impedance is consistent and reduce reflection. "As close as possible" is sometimes a requirement for differential routing.

4. Snake line:

Serpentine line is a kind of Layout which is often used in layout. Its main purpose is to adjust the delay and meet the requirements of system timing design. The first thing designers need to realize is that snake-like wires can destroy signal quality and change transmission delay, and should be avoided when wiring. However, in actual design, in order to ensure sufficient holding time of signals, or to reduce the time offset between the same group of signals, it is often necessary to deliberately wind.

Points to note:

Pairs of differential signal lines, generally parallel lines, as little as possible through the hole, must be punched, should be two lines together, in order to achieve impedance matching.

A group of buses with the same attributes should be routed side by side as far as possible to achieve equal length. The hole leading from the patch pad is as far away from the pad as possible.

(3) Common wiring rules

1. Control rules for the direction of routing:

That is, the direction of the adjacent layers is orthogonal. Avoid running different signal lines in the same direction on adjacent floors to reduce unnecessary inter-floor interference; When this situation is difficult to avoid due to board structure restrictions (such as some backplanes), especially when the signal rate is high, it is necessary to consider the ground plane to isolate each wiring layer and the signal line to isolate each signal line.

2. Open-loop inspection rules for routing:

In general, an Dangling Line on one end was not allowed, mainly in order to avoid the "antenna effect" and reduce unnecessary interference radiation and acceptance, which might bring unpredictable results.

3. Check rules for impedance matching:

The wiring width of the same network should be consistent, the change of line width will cause the characteristic impedance of the line is not uniform, when the transmission speed is high will produce reflection, in the design should try to avoid this situation. In some conditions, such as plug-in lead wire, BGA package lead wire similar structure, may not be able to avoid the change of line width, should try to reduce the effective length of the inconsistent part in the middle.

4. Control rules of cable length:

That is, the short line rule, in the PCB design should try to make the wiring length as short as possible, in order to reduce the interference problem caused by too long line, especially some important signal lines, such as the clock line, must put its oscillator close to the device. In the case of driving multiple devices, the network topology should be determined according to the specific situation.