Doc. No: Unex-QSG-21-003
16/23
A printed version of this document is an uncontrolled copy
© 2021 Unex Technology Corporation – Company Confidential
9.3.
Brown Out
•
If the 5V power supply drops below 3.5V, the SOM-301(v2)/SOM-351 will enter
standby mode.
•
A SOM in standby mode will stay in this mode until reset.
•
Once the power supply returns to normal, the SOM-301(v2)/SOM-351 can reboot
into normal operation with the rising edge of PERST# or EX_RSTn signal.
9.4.
Grounding
•
Ensure good GND connection between the ground of the module and the ground of
the system board.
•
Grounding of the external components (e.g., capacitors) should be connected to
the same reference ground of the module and not just on the top layer, use more
than one via whenever possible to ensure good GND connection.
9.5.
USB Data Lines
The SOM-301(v2)/SOM-351 mPCIe module includes a Universal Serial Bus (USB)
transceiver, which operates at USB high-speed (480 Mbits/s). It is compliant with the USB
2.0 specification and can be used for control and data transfers as well as for diagnostic
monitoring and firmware update.
The USB port is typically the main interface between the SOM-301(v2)/SOM-351
mPCIe module and OEM hardware. Since the USB_D+ and USB_D- signals have a clock
rate of 240 MHz, the signal traces must be routed carefully. Minimize trace lengths, number
of vias, and capacitive loading.
The layout guidelines for the USB data lines (mPCIe pin 36, 38) is listed below. And a
routing example for two pairs of USB data buses is shown in
•
The impedance value should be as close as possible to 90 Ohms differential.
•
The differential pair signals should be all referenced to ground.
•
Differential pair route in parallel and in equal length.
•
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.
•
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.
•
Separate the signal traces into similar groups and route similar signal traces
