1. Definition
Universal Serial Bus (USB) is a serial bus standard for connecting computer systems to external devices. It is also a technical specification for input/output interfaces, widely used in personal computers and mobile devices, and has expanded to other related fields such as photographic equipment, digital televisions (set-top boxes), and game consoles. The latest generation is USB 3.1 , with a transmission speed of 10 Gbit/s, three-stage voltage (5V/12V/20V), and a maximum power supply of 100W. The new Type-C connector is reversible. Figure 1 shows several commonly used USB interfaces. This type of USB interface is widely used in MID (Multi-Instrument Device) products.
Figure 1 Common USB Interfaces
There are two different definitions for commonly used USB, as shown in Table 1. Select the appropriate type of USB as needed.
Table 1 USB Pin Definitions
2. USB Interface Layout and Routing Requirements
1) The USB port is placed close to the edge of the board and extends a certain distance beyond the edge for easy plugging and unplugging;
2) ESD and common-mode inductors should be placed near the USB interface in the following order: ESD - common-mode inductor - resistor and capacitor. Also, pay attention to the distance between the ESD and USB, leaving a 1.5mm gap to account for post-soldering, as shown in Figure 2.
Figure 2 USB layout
3) USB cables should be differential, with an impedance of 90 ohms and grounding. The total length should ideally not exceed 1800mil. The length should be equalized, similar to the HDMI cable routing.
4) In order to suppress electromagnetic radiation, the differential lines of USB are preferentially routed on the inner layer and the traces are guaranteed to have a complete reference plane. Otherwise, if the traces cross the split, it will cause impedance discontinuity and increase the impact of external noise on the differential lines.
5) When routing USB differential cables, minimize the number of vias for layer changes. Vias can cause discontinuities in line impedance. Add a pair of return ground vias at each layer change location for signal return, as shown in Figure 3.
Figure 3 shows the addition of GND holes at the differential drilling layer change location.
⑥ If the positioning posts on both sides of the USB are connected to the protective ground, ensure that the distance from GND is 2MM when splitting, and drill more holes in the protective ground area to ensure full connection, and use a ferrite bead to bridge it with GND, as shown in Figure 4.
Figure 4 Isolation of USB protective ground
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