Doc. No: Unex-QSG-21-003
21/30
A printed version of this document is an uncontrolled copy
© 2023 Unex Technology Corporation – Company Confidential
routing example for two pairs of USB data buses is shown in
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The impedance value should be as close as possible to 90 Ohms differential.
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The differential pair signals should be all referenced to ground.
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Differential pair route in parallel and in equal length.
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The amount of vias and corners used for the USB signal layout should be
minimized; this is to prevent the occurrence of reflection and impedance changes.
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Each pair of USB data lines is required to be parallel to each other with the same
trace length, and not parallel with other signals to minimize crosstalk.
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Separate the signal traces into similar groups and route similar signal traces
together. In addition, it is recommended to have differential pairs routed together on
the system board.
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For the USB traces, do not route them under oscillators, crystals, clock
synthesizers, magnetic devices or IC’s which could be using duplicate clocks.
Figure 8: USB data line routing example
9.6.
Serial Port
The serial port is typically a secondary interface between the SOM-352 mPCIe module
and OEM hardware. The levels for SOM-352 UART is 3.3V TTL logic level.
Depending on the design of serial port on the OEM hardware, a level translator circuit
might be needed to make the system operate properly (e.g., 5V to 3.3V or 1.8V to 3.3V).
The only configuration that does not need level translation is the 3.3V UART.
