Things to Consider When Designing a USB Board

  In the USB2.0 design, the USB differential D+/D- works in the high-speed mode of 480Mbps, and the system clock can work at 12 MHz, 48 MHz and 60 MHz. It belongs to the high-speed design part in the hardware design. There are many aspects that need special attention. Because USB cables tend to form monopole antennas, RF current must be prevented from coupling into the cable.

D+/D-: High-speed differential signal, which is easily interfered by external noise and affects the transmission quality of the signal.

Power supply signal VBUS: The power supply ripple on the power supply signal pin will cause great interference to the data transmission signal, so it must be filtered. And the ground signal should also be filtered to reduce interference. Also pay attention to voltage sag for high current loads.

D+/D- equal-length control: When working in high-speed mode or USB3.0 rate, it is necessary to ensure D+/D- equal-length wiring. D+ and D– traces should not differ by more than 50 mils (1.25 mm) in length to avoid signal skew and to prevent AC voltage effects

D+ and D– traces should be less than 3 inches (75 mm) long. It is recommended that the length of the two traces be no greater than 1 inch

In routing, the right angle should be bent twice at 45° or rounded (instead of a right angle of 90°)

Impedance control: D+/D- differential wiring, impedance control 90Ω±10%

No Stub (no fulcrum): When adding test points or some devices, try to ensure that the differential signal has no fulcrum, as shown below:

Complete reference plane: D+/D- impedance implementation requires a complete reference plane, no split reference planes can occur. If separate reference planes must not appear, jumper capacitors need to be connected, as shown below:

Now the chip integration is very high, and the peripheral design of USB_Host is relatively simple, as shown below:

Current-limiting device: RT9702, USB2.0 standard load requires a maximum current of 500mA from VBUS, and it will be turned off if it exceeds 500mA

Electrostatic protection: SRV05 prevents static electricity from damaging the connector signal, place it close to the USB interface

Magnetic beads L1/L2: Eliminate high-frequency noise on power signals and enhance anti-jitter performance. The resistance value of the magnetic beads is between 47ohm and 1000ohm (at 100MHz signal frequency), placed close to the connector

R157/R158: Matching resistors to prevent signal overshoot, the resistance is generally between 10Ω-33Ω, placed close to the USB controller

CP1: The large capacitor ensures that the VBUS voltage drops too much at the moment of plugging and unplugging or when a large amount of data is transmitted, and it is placed close to the USB interface.