FN990
Family Hardware Design Guide
1VV0301752 Rev. 3 Page 49 of 92 2022-10-07
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Table 32: USB Interface Signals
Note: Consider placing a low-capacitance ESD protection component
to protect FN990 Family against ESD strikes.
6.3.1.3.1. USB Layout Guidelines
If third-party components are required for signal improvement, place them
closer to the USB connector.
There are relatively fast edge rates, so must be routed away from sensitive
circuits and signals (RF, audio and XO).
Maintain good isolation between the USB connector and RF antennas (especially
2.4 GHz).
Route the RF signals operating at a 2.4 GHz frequency with the highest isolation
possible from USB_SS_TX/RX traces.
USB SS Tx AC coupling capacitors are better placed close to source or the
ESD/connector side to keep good SI of main route on PCB.
Route differential pairs in the inner layers with a solid GND reference to have
good impedance control and to minimize discontinuities.
Keep isolation between the Tx pair, Rx pair, and DP/DM to avoid crosstalk.
The SS-USB Tx and Rx differential pair maximum length is recommended to be
less than 136 mm.
For USB 2.0 signal, maximum trace length should be less than 234 mm.
Type of guidance Guideline
Requirement
USB 3.1 Gen 2 USB 2.0
General Data rate 10 Gbps 480Mbps
Insertion loss at 5 GHz (dB) -7 dB N/A
Impedance 85 ohms differential
Bus length 136 mm 234 mm
Length matching Intra pair match < 0.7mm < 2mm
Spacing To all other signals > 4 x line width > 3 x line width
Tx lane to Rx lane > 4 x line width N/A
Component AC capacitance 220 nF N/A
Table 33: USB Routing Constraints
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