Service Guide

LTE Standard Module Series

EG91_Series_Hardware_Design

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A common mode choke L1 is recommended to be added in series between the module and your 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. 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.



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 pay

attention to the selection of the device. Typically, the stray capacitance should be less than 2 pF.



Keep the ESD protection devices as close to the USB connector 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 9600 bps, 19200 bps, 38400 bps, 57600 bps, 115200 bps,

230400 bps, 460800 bps and 921600 bps baud rates, and the default is 115200 bps. It supports RTS

and CTS hardware flow control, and is used for AT command communication and data transmission.

 The debug UART interface supports 115200 bps baud rate. It is used for Linux console and log

output.

The following tables show the pin definition of the two UART interfaces.

Table 11: Pin Definition of Main UART Interfaces

Pin Name Pin No. I/O Description Comment

RI 39 DO Ring indication

1.8 V power domain

If unused, keep it open.

DCD 38 DO Data carrier detect

CTS 36 DO Clear to send