USB 7 great military rules that circuit boards must read!

USB It is a fast, dual, synchronous transmission, inexpensive, and easy-to-use hot swappable serial communication. Due to its advantages such as fast data transmission, convenient interface, and support for hot plugging, USB devices have been widely used. Currently, the above are available in the market. Most products use USB 2.0 as the interface, but many hardware novices encounter a lot of trouble with USB in applications, usually encountering various problems with the USB interface after PCB installation.
For example, if communication is unstable or unable to communicate, checking the schematic and soldering is not a problem, perhaps it is necessary to suspect that the PCB design is unreasonable at this time. Painting meets the requirements. USB 2.0 data transmission requires PCB to play an extremely important role in the performance and reliability of the product.

USB The protocol is defined by two differential power lines (D, D -). If an analog signal is input and the USB device is stable, the differential power lines must be arranged strictly according to the rules of differential signals. Based on the author's years of experience in USB design, product design, and debugging:
1. When arranging components, try to minimize the distance between differential lines (√ in a reasonable way, x in an unreasonable way);
2. Prioritize drawing differential lines and try not to have more than two pairs of via holes on a pair of differential lines (via holes can increase the parasitic inductance of the line, thereby affecting the signal integrity of the line). Symmetrical placement is required (√ in a reasonable way, x in an unreasonable way);
3. Symmetrical parallel routing ensures tight coupling between two wires and prevents 90 ° lines, arcs, or 45 ° lines, which are all good routing methods (√ in a reasonable way, x in an unreasonable way);
4. The placement of differential series resistors, test points, and pull-up and pull-down resistors (√ in a reasonable way, x in an unreasonable way);

5. Due to factors such as pin distribution, via holes, and wiring space, differential line lengths are prone to mismatch. Once the line lengths do not match, timing will shift, introducing common mode interference and reducing signal quality. Therefore, the corresponding differences are compensated for the mismatch situation, so that the line length matches, and the length difference is generally controlled within 5 minutes. The compensation principle is to compensate for the difference in length;
6. To reduce crosstalk, if space allows, the distance between other signal networks and the ground to the differential line should be at least 20mil (20mil is empirical). If the distance between the ground and the differential line is too close, it will affect the impedance of the differential line;
7. The USB output current is 500mA, and attention should be paid to the VBUS and GND line spacing. If using 1Oz copper, the line width should be greater than 200mil. Of course, the wider the line spacing, the better the integrity of the power supply.