Product Info

LTE Standard Module Series

EC25 Series Hardware Design

EC25_Series_Hardware_Design 53 / 134

USB_DP

USB_DM

GND

USB_DP

USB_DM

GND

Close to Module

Test Points

ESD Array

NM_0R

Minimize these stubs

Module

MCU

USB_VBUS

VDD

Figure 19: Reference Circuit of USB Application

A common mode choke L1 is recommended to be added in series between the module and customer’s

MCU in order 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 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 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.

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 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. It is

important to 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 lines, so

please pay attention to the selection of the device. Typically, the stray capacitance should be less

than 2 pF.

⚫ Keep the ESD protection devices to the USB connector as close as possible.

3.11. UART Interfaces

The module provides two UART interfaces: the main UART interface and the debug UART interface. The

following shows their features.

⚫ The main UART interface supports 4800 bps, 9600 bps, 19200 bps, 38400 bps, 57600 bps, 115200