LTE Module Series
EC21 Hardware Design
EC21_Hardware_Design Confidential / Released 42 / 94
The USB interface is recommended to be reserved for firmware upgrade in your design. The following
figure shows a reference circuit of USB interface.
USB_DP
USB_DM
GND
USB_DP
USB_DM
GND
R1
R2
Close to Module
R3
R4
Test Points
ESD Array
NM_0R
NM_0R
0R
0R
Minimize these stubs
Module
MCU
USB_VBUS
VDD
Figure 19: Reference Circuit of USB Application
In order to ensure the integrity of USB data line signal, components R1, R2, R3 and R4 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.
In order to ensure the USB interface design corresponding with the USB 2.0 specification, please comply
with the following principles:
It is important to route the USB signal traces as differential pairs with total grounding. The impedance
of USB differential trace is 90 ohm.
Do not route signal traces under crystals, oscillators, magnetic devices or RF signal traces. It is
important to route the USB differential traces in inner-layer with ground shielding on not 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.
1. EC21 module can only be used as a slave device.
2.
“*” means under development.
NOTES
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