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Easy to understand USB PCB wiring

2022-01-03

Easy to understand USB PCB wiring

Most comprehensive USB PCB wiring experience

USB protocol defines that two differential signal lines (D +, D -) transmit digital signals. If the USB device works stably, the differential signal lines must be arranged and wired in strict accordance with the rules of differential signals. According to our years of experience in USB related product design and debugging:
1. During PCB component layout, try to make the differential line as short as possible, and shorten the routing distance of the differential line (√) as a reasonable way, × (in an unreasonable manner);
2. Give priority to drawing differential lines. Try not to exceed two pairs of vias on a pair of differential lines (vias will increase the parasitic inductance of the line, thus affecting the signal integrity of the line), and place them symmetrically (√ is a reasonable way, × (in an unreasonable manner);
3. Parallel routing for PCB can ensure tight coupling between two wires and avoid 90 ° routing. Arc or 45 ° is a better routing method (√ is a reasonable method, × (in an unreasonable manner);
4. The placement of differential series resistance capacitance, test points and pull-down resistance (√) is a reasonable way, × (in an unreasonable manner);
5. Due to pin distribution, vias, routing space and other factors, the differential line length is easy to mismatch. Once the line length does not match, the timing will shift, and common mode interference will be introduced to reduce the signal quality. Therefore, it is necessary to compensate the mismatch of the differential pair to match the line length. The length difference is usually controlled within 5MIL. The compensation principle is where the length difference occurs and where the length difference is compensated;
6. In order to reduce crosstalk, if space permits, the distance between other signal networks and the ground from the differential line shall be at least 20MIL (20MIL is an empirical value). Too close distance between the ground and the differential line will affect the impedance of the differential line;
7. The output current of USB is 500mA. Pay attention to the linewidth of VBUS and GND. If 1oz copper foil is used, the linewidth greater than 20MIL can meet the current carrying requirements. Of course, the wider the linewidth, the better the integrity of the power supply.
The line width and line spacing of the differential line signal of the ordinary USB device can be consistent with the line width and line spacing of the whole board signal. However, when the working speed of USB device is 480 Mbits / s, it is not enough to only do the above points. We also need to control the impedance of the differential signal. Controlling the impedance of the differential signal line is very important to the integrity of the high-speed digital signal, because the differential impedance affects the eye diagram, signal bandwidth, signal jitter and interference voltage on the signal line. The differential line impedance is generally controlled at 90 (± 10%) ohms (refer to the instructions in the chip manual for specific values). The differential line impedance is inversely proportional to the line widths W1, W2 and T1, inversely proportional to the dielectric constant ER1, directly proportional to the line spacing S1 and directly proportional to the distance H1 of the reference layer
   The reference stack of PCB four-layer board, in which the middle two layers are the reference layer, and the reference layer is usually GND or power, and the reference layer corresponding to the difference line must be complete and cannot be divided, otherwise the impedance of the difference line will be discontinuous. If the four layer board is designed by lamination as shown in Figure 2, the line width of 4.5mil and the line spacing of 5.5mil are usually used to design the TDOA line, which can meet the differential impedance of 90 Ω. However, the 4.5mil linewidth and 5.5mil line spacing are only our theoretical design values. Finally, the circuit board factory will adjust the linewidth line spacing and the distance to the reference layer according to the required impedance value and combined with the actual production situation and plate.
USB is a fast, bidirectional, synchronous transmission, cheap and convenient hot plug serial interface. Due to the advantages of fast data transmission, convenient interface and support for hot plug, USB devices are widely used. At present, the market is dominated by USB2 0 is the interface of most products, but many hardware novices encounter many problems in USB application. Often after PCB assembly, USB interface has various problems, communication is unstable or unable to communicate. There is no problem in checking schematic diagram and welding. Maybe it is necessary to suspect that PCB design is unreasonable at this time. Drawing meets USB2 0 data transmission requirements PCB plays a very important role in product performance and reliability. The above is the best wiring mode summarized by our company according to many years of experience, which is easy to understand.