Application description

The Serial LTE Modem (SLM) application demonstrates how to use the nRF9160 as a stand-alone LTE modem that can be controlled by AT commands.

The application accepts both the modem-specific AT commands documented in the nRF91 AT Commands Reference Guide and proprietary AT commands documented in SLM-specific AT commands.

Overview

The nRF9160 SiP integrates both a full LTE modem and an application MCU, enabling you to run your LTE application directly on the nRF9160.

However, you might want to run your application on a different chip and use the nRF9160 only as a modem. For this use case, the serial LTE modem application provides an interface for controlling the LTE modem through AT commands.

The proprietary AT commands specific to the serial LTE modem application are described in the SLM-specific AT commands documentation. In addition to these, the application also supports the nRF91 AT commands described in the AT Commands Reference Guide.

Communicating with the modem

The nRF9160 DK running the serial LTE modem application serves as the host. As a client, you can use either a PC or an external MCU.

Connecting with a PC

To connect to the nRF9160 DK with a PC, make sure that CONFIG_SLM_CONNECT_UART_0 is defined in the application. It is defined in the default configuration.

Use LTE Link Monitor to connect to the nRF9160 DK. See How to connect with LTE Link Monitor for instructions. You can then use this connection to send or receive AT commands over UART, and to see the log output of the nRF9160 DK.

Alternatively, you can use a terminal emulator like PuTTY to establish a terminal connection to the nRF9160 DK. See How to connect with PuTTY for instructions.

Note

The default AT command terminator is a carriage return followed by a line feed (\r\n). LTE Link Monitor supports this format. When connecting with another terminal emulator, make sure that the configured AT command terminator corresponds to the line terminator of your terminal. You can change the termination mode in the application configuration.

Connecting with an external MCU

If you run your user application on an external MCU (for example, an nRF52 Series DK), you can control the modem on the nRF9160 directly from the application. See the nRF52 client for serial LTE modem application repository for a sample implementation of such an application.

To connect with an external MCU, you must set the configuration option CONFIG_UART_2_NRF_HW_ASYNC_TIMER and CONFIG_SLM_CONNECT_UART_2 in the serial LTE modem application configuration.

The following table shows how to connect an nRF52 Series DK to the nRF9160 DK to be able to communicate through UART:

nRF52 Series DK

nRF9160 DK

UART TX P0.6

UART RX P0.11

UART RX P0.8

UART TX P0.10

UART CTS P0.7

UART RTS P0.12

UART RTS P0.5

UART CTS P0.13

GPIO OUT P0.27

GPIO IN P0.31

UART instance in use:

  • nRF52840 or nRF52832 (UART0)

  • nRF9160 (UART2)

UART configuration:

  • Hardware flow control: enabled

  • Baud rate: 115200

  • Parity bit: no

  • Operation mode: IRQ

Note

The GPIO output level on the nRF9160 side must be 3 V. You can set the VDD voltage with the VDD IO switch (SW9). See the VDD supply rail section in the nRF9160 DK User Guide for more information.

Requirements

The application supports the following development kit:

Hardware platforms

PCA

Board name

Build target

nRF9160 DK

PCA10090

nrf9160dk_nrf9160

nrf9160dk_nrf9160_ns

The sample is configured to compile and run as a non-secure application on nRF91’s Cortex-M33. Therefore, it automatically includes the Secure Partition Manager that prepares the required peripherals to be available for the application.

You can also configure it to use TF-M instead of Secure Partition Manager.

Configuration

See Configuring your application for information about how to permanently or temporarily change the configuration.

Configuration options

Check and configure the following configuration options for the sample:

CONFIG_SLM_CUSTOMIZED - Flag for customized functionality

This flag can be used to enable customized functionality. To add your own custom logic, enclose the code by #if defined(CONFIG_SLM_CUSTOMIZED) and enable this flag.

CONFIG_SLM_NATIVE_TLS - Use Zephyr mbedTLS

This option enables using Zephyr’s mbedTLS. It requires additional configuration. See Native TLS sockets for more information.

CONFIG_SLM_EXTERNAL_XTAL - Use external XTAL for UARTE

This option configures the application to use an external XTAL for UARTE. See the nRF9160 Product Specification (section 6.19 UARTE) for more information.

CONFIG_SLM_CONNECT_UART_0 - UART 0

This option selects UART 0 for the UART connection. Select this option if you want to test the application with a PC.

CONFIG_SLM_CONNECT_UART_2 - UART 2

This option selects UART 2 for the UART connection. Select this option if you want to test the application with an external CPU.

CONFIG_SLM_START_SLEEP - Enter sleep on startup

This option makes nRF9160 enter deep sleep after startup. It is not selected by default.

CONFIG_SLM_INTERFACE_PIN - Interface GPIO to wake up from sleep or exit idle

This option specifies which interface GPIO to use for exiting sleep or idle mode. By default, P0.6 (Button 1 on the nRF9160 DK) is used when CONFIG_SLM_CONNECT_UART_0 is selected, and P0.31 is used when CONFIG_SLM_CONNECT_UART_2 is selected.

CONFIG_SLM_SOCKET_RX_MAX - Maximum RX buffer size for receiving socket data

This option specifies the maximum buffer size for receiving data through the socket interface. By default, this size is set to NET_IPV4_MTU (576), which is defined in Zephyr. The maximum value is 708, which is the maximum segment size (MSS) defined for the modem.

This option impacts the total RAM usage.

CONFIG_SLM_CR_TERMINATION - CR termination

This option configures the application to accept AT commands ending with a carriage return.

CONFIG_SLM_LF_TERMINATION - LF termination

This option configures the application to accept AT commands ending with a line feed.

CONFIG_SLM_CR_LF_TERMINATION - CR+LF termination

This option configures the application to accept AT commands ending with a carriage return followed by a line feed.

CONFIG_SLM_TCP_POLL_TIME - Poll timeout in seconds for TCP connection

This option specifies the poll timeout for the TCP connection, in seconds.

CONFIG_SLM_SMS - SMS support in SLM

This option enables additional AT commands for using the SMS service.

CONFIG_SLM_GNSS - GNSS support in SLM

This option enables additional AT commands for using GNSS service.

CONFIG_SLM_AGPS - nRF Cloud A-GPS support in SLM

This option enables additional AT commands for using the nRF Cloud A-GPS service. It is not selected by default.

CONFIG_SLM_PGPS - nRF Cloud P-GPS support in SLM

This option enables additional AT commands for using the nRF Cloud P-GPS service. It is not selected by default.

CONFIG_SLM_CELL_POS - nRF Cloud cellular positioning support in SLM

This option enables additional AT commands for using the nRF Cloud Cellular Positioning service. It is not selected by default.

CONFIG_SLM_FTPC - FTP client support in SLM

This option enables additional AT commands for using the FTP client service.

CONFIG_SLM_MQTTC - MQTT client support in SLM

This option enables additional AT commands for using the MQTT client service.

CONFIG_SLM_HTTPC - HTTP client support in SLM

This option enables additional AT commands for using the HTTP client service.

CONFIG_SLM_TWI - TWI support in SLM

This option enables additional AT commands for using the TWI service.

Additional configuration

To save power, console and logging output over UART_0 is disabled in this application. This information is logged to RTT instead. See Connecting using RTT for instructions on how to view this information.

To switch to UART output, change the following options in the prj.conf file:

# Segger RTT
CONFIG_USE_SEGGER_RTT=n
CONFIG_RTT_CONSOLE=n
CONFIG_UART_CONSOLE=y
CONFIG_LOG_BACKEND_RTT=n
CONFIG_LOG_BACKEND_UART=y

Configuration files

The sample provides predefined configuration files for both the parent image and the child image. You can find the configuration files in the applications/nrf9160/serial_lte_modem directory.

The following files are provided:

  • prj.conf - This configuration file contains the standard configuration for the serial LTE modem application.

  • overlay-native_tls.conf - This configuration file contains additional configuration options that are required to use Native TLS sockets. You can include it by adding -DOVERLAY_CONFIG=overlay-native_tls.conf to your build command. See Providing CMake options.

  • child_secure_partition_manager.conf - This configuration file contains the project-specific configuration for the Secure Partition Manager child image.

Native TLS sockets

By default, the secure socket (TLS/DTLS) is offloaded onto the modem. However, if you require customized TLS/DTLS features that are not supported by the modem firmware, you can use a native TLS socket instead. The serial LTE modem application will then handle all secure sockets used in TCP/IP, TCP/IP proxy, and MQTT.

If native TLS is enabled, the Credential storage management %CMNG command is overridden to map the security tag from the serial LTE modem application to the modem. You must use the overridden AT%CMNG command to provision the credentials to the modem. Note that the serial LTE modem application supports security tags in the range of 0 - 214748364.

The configuration options that are required to enable the native TLS socket are defined in the overlay-native_tls.conf file.

Note

The following limitations exist for native TLS sockets:

  • PSK, PSK identity, and Public Key are currently not supported.

  • DTLS server is currently not supported.

  • AT%CMNG=1 is not supported.

  • FTP client and HTTP client do currently not support native TLS.

Building and running

This sample can be found under applications/nrf9160/serial_lte_modem in the nRF Connect SDK folder structure.

The sample is built as a non-secure firmware image for the nrf9160dk_nrf9160_ns build target. Because of this, it automatically includes the Secure Partition Manager. You can also configure it to use TF-M instead of SPM.

See Building and programming an application for information about how to build and program the application.

Testing

The testing instructions focus on testing the application with a PC client. If you have an nRF52 Series DK running a client application, you can also use this DK for testing the different scenarios.

After programming the sample to your development kit, test it by performing the following steps:

  1. Connect the kit to the computer using a USB cable. The kit is assigned a COM port (Windows) or ttyACM device (Linux), which is visible in the Device Manager.

  2. Connect to the kit with LTE Link Monitor. If you want to use a different terminal emulator, see Connecting with a PC.

  3. Reset the kit.

  4. Observe that the nRF9160 DK sends a Ready\r\n message on UART.

  5. Enter AT+CFUN=1 to turn on the modem and connect to the network.

  6. Enter AT+CFUN? and observe that the connection indicators in the LTE Link Monitor side panel turn green. This indicates that the modem is connected to the network.

  7. Send AT commands and observe the responses from the nRF9160 DK. See Testing scenarios for typical test cases.