Product Info

LTE Standard Module Series
EC21_Series_Mini_PCIe_Hardware_Design
30
/ 75
USB_DP
USB_DM
GND
USB_DP
USB_DM
GND
L1
Close to Module
R3
R4
Test Points
NM_0R
NM_0R
Minimize these stubs
Module
MCU
ESD Array
Figure 6: Reference Circuit of USB Interface
A common mode choke L1 is recommended to be added in series between the module and customer’s
MCU to suppress EMI spurious transmission. Meanwhile, the 0 Ω resistors (R3 and R4) should be added
in series between the module and the test points to facilitate debugging, and the resistors are not
mounted by default. In order to ensure the integrity of USB data trace signal, L1 & R3 & R4 components
must be placed close to the module, and also these resistors should be placed close to each other. The
extra stubs of trace must be as short as possible.
To meet USB 2.0 specification, the following principles should be complied with when design the USB
interface.
 Route the USB signal traces as differential pairs with ground surrounded. The impedance of USB
differential trace is 90 Ω.
 Do not route signal traces under crystals, oscillators, magnetic devices and RF signal traces. Route
the USB differential traces in inner-layer of the PCB, and surround the traces with ground on that
layer and with ground planes above and below.
 Junction capacitance of the ESD protection device might cause influences on USB data traces, so
you should pay attention to the selection of the device. Typically, the capacitance value should be
less than 2 pF.
 Keep the ESD protection devices as close to the USB connector as possible.