The USB protocol defines two differential signal lines (D+, D -) for transmitting digital signals. If USB devices need to work stably, differential signal lines must be strictly laid and wired according to differential signal rules. Based on the author’s years of experience in USB related product design and debugging, the following points can be summarized:
1. During component layout, it is important to keep the length of the differential line as short as possible in order to reduce the routing distance of the line.
2. When drawing the layout, prioritize the placement of the differential lines and try to avoid having more than two pairs of vias on one pair of differential lines. The presence of vias can increase the parasitic inductance of the line, affecting the signal integrity. Additionally, ensure that the vias are placed symmetrically.
3. Symmetrical parallel wiring helps in ensuring tight coupling of two wires and avoiding 90° wiring. It is recommended to use arc or 45° wiring for better results.
4. Pay attention to the arrangement of differential series connection resistance, capacitance, test points, and resistances.
5. Consider factors such as pin distribution, vias, and routing space which can lead to mismatched lengths of differential lines. Mismatched line lengths can shift timing and introduce common mode interference, reducing signal quality. It is essential to compensate for the mismatch of differential pairs to ensure that the line lengths match, with the length difference controlled within 5mil.
6. To reduce crosstalk, maintain a distance of at least 20mil between other signal networks and the ground differential line. If the ground and differential line are too close, the impedance of the differential line will be affected.
7. For USB, where the output current is 500mA, pay attention to the line width of VBUS and GND. Using 1oz copper foil, a line width greater than 20mil can meet current carrying requirements. A wider line width will enhance the integrity of the power supply.
The line width and spacing of the differential line signal in an ordinary USB device should be consistent with the entire board. However, for USB devices operating at 480 Mbits/s, controlling the impedance of the differential signal is crucial. Differential impedance affects the integrity of high-speed digital signals, impacting the eye diagram, signal bandwidth, signal jitter, and interference voltage. The differential line impedance is typically controlled at 90Ω (±10%), with parameters such as line width, dielectric constant, line spacing, and distance to the reference layer influencing impedance. Maintaining a complete and undivided reference layer is essential to ensure consistent impedance in the differential lines.
The above wiring guidelines are specifically for USB2.0 devices. Adhering to principles such as shortest length, tight coupling, equal length, and consistent impedance of the differential line, as well as focusing on the current carrying capacity of the USB power line, will ensure proper operation of USB devices. Mastering these principles is essential for successful USB device operation.
USB is a cheap and convenient hot swappable serial interface for fast, bidirectional, and synchronous transmission. Due to its advantages such as fast data transfer, convenient interfaces, and hot swappable support, USB devices are widely used. At present, most products on the market use USB 2.0 as an interface, but many hardware novices encounter many problems in USB applications. A common occurrence is that after PCB assembly, various problems occur with the USB interface, such as unstable communication or inability to communicate. Check if the schematic and welding are correct. Perhaps it is necessary to question whether the PCB design is reasonable at this time. Drawing a PCB that meets the data transmission requirements of USB2.0 plays an important role in product performance and reliability.