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nRF91_as_Zephyr_modem.rst

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nRF91 Series as a Zephyr-compatible modem

Overview

The |SM| application can be used to turn an nRF91 Series SiP into a standalone modem that can be used through Zephyr's cellular modem driver. This means that the controlling chip can run a Zephyr application that seamlessly uses Zephyr's IP stack instead of offloaded sockets through AT commands.

This is made possible by |SM|'s support of CMUX and PPP and Zephyr's cellular modem driver.

Note

It is not yet possible to make use of the nRF91 Series' GNSS functionality when Zephyr's cellular modem driver controls the SiP. Also, the driver's support of power saving features is quite limited, only allowing complete power off of the SiP.

Configuration

Both |SM| and the Zephyr application running on the controlling chip must be compiled with the appropriate configuration to make them work together.

For that, Kconfig fragments and devicetree overlays must be added to the compilation command.

See the :ref:`sm_config_files` section for information on how to compile with additional configuration files and a description of some of the mentioned Kconfig fragments.

nRF91 Series SiP running |SM|

The following configuration files must be included:

In addition, if the controlling chip is an external MCU, the following configurations must also be included:

  • :file:`overlay-external-mcu.overlay` - To configure UART (pins, baud rate), DTR and RI pins that |SM| will use. Make sure to update the UART configuration (pins, baud rate) so that it matches your setup.

Finally, if you want more verbose logging that includes the AT commands and responses, you can enable debug logging by uncommenting CONFIG_SM_LOG_LEVEL_DBG=y in the :file:`prj.conf` configuration file.

Controlling chip running Zephyr

Configuration files found in `Zephyr’s Cellular modem`_ sample are a good starting point.

Specifically, regardless of what the controlling chip is, the Kconfig options found in the following files are needed:

  • :file:`prj.conf` - This file enables various Zephyr APIs, most of which are needed for proper functioning of the application.
  • :file:`boards/nrf91601dk_nrf9160_ns.conf` - This file tailors the configuration of the modem subsystem and driver to the |SM|. It makes the application's logs be output on UART 0 and also enables the debug logs of the cellular modem driver. If you do not want the debug logs output by the driver, you may turn them off by removing CONFIG_MODEM_LOG_LEVEL_DBG=y.

In addition, depending on what the controlling chip is, the following devicetree overlay files are also needed. They define the modem along with the UART it is connected to and its power pin.

If the controlling chip is an external MCU:

Developing the Zephyr application

To get started developing the Zephyr application running on the controlling chip, look at the code of `Zephyr’s Cellular modem`_ sample to see how the modem is managed and used. You can even compile, flash and run the sample to verify proper operation of the modem.

Flashing and running

However, before flashing the |SM| built with the Zephyr-compatible modem configuration, make sure that the nRF91 Series modem has been set to the desired system mode. For this, you will need a regular |SM| running in the nRF91 Series SiP to be able to run AT commands manually. To set the modem to the desired system mode, issue an AT%XSYSTEMMODE command followed by an AT+CFUN=0 command so that the modem saves the system mode to NVM. For example, to enable only LTE-M, issue the following command: AT%XSYSTEMMODE=1,0,0,0 You need to do this because the modem's system mode is not automatically set at any point, so the one already configured will be used.

Additionally, if the controlling chip is an external MCU, make sure that the UART and the power pin are wired according to how they are configured in both the external MCU and the nRF91 Series SiP.

To observe the operation sequence, you can view the application logs coming from both chips.

By default, |SM| will output its logs through RTT, and the Zephyr application running on the controlling chip through its UART 0. The RTT logs can be seen with an RTT client such as JLinkRTTViewer. However, for convenience you may want to redirect |SM|'s logs to the SiP's UART 0 so that you do not need to reconnect the RTT client every time the board is reset. See the :ref:`sm_logging` section for information on how to do this.

The logs output through UART can be seen by connecting to the appropriate UART with a serial communication program.

After both applications have been flashed to their respective chips and you are connected to receive logs, you can reset the controlling chip. When the Zephyr application starts up, the following happens:

  • The cellular modem driver will start sending AT commands to |SM|. It will enable the network status notifications, gather some information from the modem, enable CMUX, and set the modem to normal mode (with an AT+CFUN=1 command). This will result in |SM|'s PPP starting automatically when the network registration is complete.
  • From this point onwards, once the Zephyr application has brought up the driver's network interface, it will be able to send and receive IP traffic through it. The `Zephyr’s Cellular modem`_ sample does this.