DRA722_17 Datasheet, PDF (360/408 Pages) Texas Instruments – Infotainment Applications Processor

DRA722, DRA724, DRA725, DRA726
SPRS956B – MARCH 2016 – REVISED JANUARY 2017
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USB PHY
D+
Connector
D-
VBUS
GND
D+
D-
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Cable
Connector
SPRS906_PCB_USB20_06
Figure 8-26. USB PHY Connector and Cable Connector
8.5.2.2.2.8 DP/DM Vias
When a via must be used, increase the clearance size around it to minimize its capacitance. Each via
introduces discontinuities in the signal’s transmission line and increases the chance of picking up
interference from the other layers of the board. Be careful when designing test points on twisted pair lines;
through-hole pins are not recommended.
8.5.2.2.2.9 Image Planes
An image plane is a layer of copper (voltage plane or ground plane), physically adjacent to a signal routing
plane. Use of image planes provides a low impedance, shortest possible return path for RF currents. For a
USB board, the best image plane is the ground plane because it can be used for both analog and digital
circuits.
• Do not route traces so they cross from one plane to the other. This can cause a broken RF return path
resulting in an EMI radiating loop as shown in Figure 8-27. This is important for higher frequency or
repetitive signals. Therefore, on a multi-layer board, it is best to run all clock signals on the signal
plane above a solid ground plane.
• Avoid crossing the image power or ground plane boundaries with high-speed clock signal traces
immediately above or below the separated planes. This also holds true for the twisted pair signals (DP,
DM). Any unused area of the top and bottom signal layers of the PCB can be filled with copper that is
connected to the ground plane through vias.
360 Applications, Implementation, and Layout
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