Quectel EG91 Series Hardware Design page 46

The USB interface is recommended to be reserved for firmware upgrade in customers' design. The
following figure shows areference circuit of USB interface.
A common mode choke L1 is recommended to be added in series between the module and
customer'sMCU in order to suppress EMI spurious transmission. Meanwhile, the 0Ω resistors (R3 and R4)
should beadded in series between the module and the test points so as to facilitate debugging, and the
resistors are
not mounted by default. In order to ensure the integrity of USB data line signal, L1/R3/R4
componentsmust be placed close to the module, and also these resistors should be placed close to each
other. Theextra stubs of trace must be as short as possible.
The following principles should be complied with when design the USB interface, so as to meet USB 2.0
specification.
 It is important to route the USB signal traces as differential pairs with total grounding. The impedance
of USB differential trace is 90Ω.
 Do not route signal traces under crystals, oscillators, magnetic devices and RF signal traces. It is
important to route the USB differential traces in inner-layer with ground shielding onnot only upper
and lower layers but also right and left sides.
 Pay attention to the influence of junction capacitance of ESD protection components on USB data
lines. Typically, the capacitance value should be less than 2pF.
 Keep the ESD protection components to the USB connector as close as possible.
NOTES
1. EG91 module can only be used as a slave device.
2. "*" means under development.
EG91_Hardware_Design
Figure 19: Reference Circuit of USB Interface
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LTE Module Series
EG91 Hardware Design