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 proprietary AT commands.

Requirements

The application supports the following development kit:

Hardware platforms

PCA

Board name

Build target

nRF9160 DK

PCA10090

nrf9160dk_nrf9160

nrf9160dk_nrf9160ns

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.

Overview

The nRF9160 SiP integrates both a full LTE modem and an application MCU, which enables 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 that are specific to the serial LTE modem application are described in the AT commands documentation. In addition to these proprietary AT commands, the application 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 carriage return and 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 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.

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_NATIVE_TLS

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_GPIO_WAKEUP - Support of GPIO wakeup

This option enables using GPIO to wake up nRF9160 from deep sleep mode. Select this option if you enable CONFIG_SLM_START_SLEEP to put nRF9160 into deep sleep mode after startup.

This option is selected by default.

CONFIG_SLM_START_SLEEP - Enter sleep on startup

This option makes nRF9160 enter deep sleep after startup.

This option is not selected by default. It requires CONFIG_SLM_GPIO_WAKEUP to be selected.

CONFIG_SLM_INTERFACE_PIN - Interface GPIO to wake up or exit idle mode

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 when CONFIG_SLM_CONNECT_UART_2 is selected.

Note that when CONFIG_SLM_CONNECT_UART_0 is selected, Button 1 can be used to exit idle mode, but not to wake up from sleep mode.

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 carriage return and line feed.

CONFIG_SLM_TCP_FILTER_SIZE - Size of IPv4 address allowlist

This option specifies the number of IPv4 addresses that you can add to an allowlist for TCP connections. If the list is set, only connections from the specified addresses are allowed.

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_TCP_CONN_TIME - Connection timeout in seconds for TCP server

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

CONFIG_SLM_DATAMODE_HWFC - UART HWFC for data mode

This option specifies whether UART hardware flow control is required for data mode. By default, UART hardware flow control is required.

CONFIG_SLM_DATAMODE_TERMINATOR - Pattern string to terminate data mode

This option specifies a pattern string to terminate data mode. The default pattern string is “+++”.

CONFIG_SLM_DATAMODE_SILENCE - Silence time to exit data mode

This option specifies the time (in seconds) of UART silence before and after the pattern string that is used to exit data mode. The default value is 1 second.

CONFIG_SLM_GPS - GPS support in SLM

This option enables additional AT commands for using GPS service.

CONFIG_SLM_SUPL_SERVER - SUPL server

This option specifies the SUPL server to use for retrieving SUPL A-GPS data.

CONFIG_SLM_SUPL_PORT - SUPL server port

This option specifies the port to use for the specified SUPL server.

CONFIG_SLM_FTPC - FTP client support in SLM

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

CONFIG_SLM_FTP_SERVER_PORT - FTP service port on remote host

This option specifies the port to use when connecting to an FTP server.

CONFIG_SLM_FTP_USER_ANONYMOUS - FTP client anonymous login user

This option specifies the user name to use for anonymous login on an FTP server.

CONFIG_SLM_FTP_PASSWORD_ANONYMOUS - FTP client anonymous login password

This option specifies the password to use for anonymous login on an FTP server.

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.

Additional configuration

Check and configure the following library options that are used by the sample:

To save power, console and logging output over UART_0 is disabled in this application. This information is logged to RTT instead. See Connecting via 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 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_nrf9160ns 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 a sample 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.

Dependencies

This application uses the following nRF Connect SDK libraries:

It uses the following sdk-nrfxlib libraries:

In addition, it uses the following samples: