nRF91 Series as a Zephyr-compatible modem
Overview
Starting with nRF Connect SDK v2.6.0, the Serial LTE Modem (SLM) 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 via AT commands.
This is made possible by SLM’s support of CMUX and PPP and Zephyr’s cellular modem driver.
The nRF52840 SoC present in the nRF9160 DK can even be made to be the controlling chip with the appropriate configuration (described below). This is only possible on the nRF9160 DK, not on the other nRF91 Series DKs.
Note
As of nRF Connect SDK v2.6.1, Zephyr’s GNSS driver does not support the nRF91 Series running SLM. This functionality is part of planned future work.
Configuration
Both SLM 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 Configuration files section for information on how to compile with additional configuration files and a description of some of the mentioned Kconfig fragments.
Note
The maximum recommended baud rate is 460 800. At higher baud rates (921 600, 1 000 000), it is possible for bytes to come in faster than the chip is able to handle, which causes the buffer space to run out if it goes on for too long. UART RX is not disabled in that case, which results in data loss and communication failures.
At a baud rate of 460 800, the maximum throughput is slightly below that of the nRF91 Series modem when using LTE-M.
nRF91 Series SiP running SLM
The following configuration files must be included:
overlay-cmux.conf- To enable CMUX.overlay-ppp.conf- To enable PPP.overlay-zephyr-modem.conf- To tailor SLM to how Zephyr’s cellular modem driver works. This enables the CONFIG_SLM_START_SLEEP Kconfig option, which makes the nRF91 Series SiP start only when the power pin is toggled.
In addition, if the controlling chip is an external MCU, the following files must also be included:
overlay-zephyr-modem-external-mcu.conf- To define the power pin. Make sure to update the defined pin so that it matches your setup.overlay-external-mcu.overlay- To configure which UART SLM will use. The actual configuration of the UART is defined in the*_ns.overlayoverlay file matching your board in theboardsdirectory. Make sure to update the UART configuration (pins, baud rate) so that it matches your setup. You can do this by modifying the properties of the node of the UART to be used, by editing eitheroverlay-external-mcu.overlayor the overlay file matching your board in theboardsdirectory.
Or, if the controlling chip is the nRF52840 of the nRF9160 DK, the following files must also be included:
overlay-zephyr-modem-nrf9160dk-nrf52840.conf- To define the power pin.overlay-zephyr-modem-nrf9160dk-nrf52840.overlay- To configure the UART to be routed between the nRF9160 and the nRF52840 of the DK.
Finally, if you want more verbose logging that includes the AT commands and responses, you can enable debug logging by uncommenting CONFIG_SLM_LOG_LEVEL_DBG=y in the 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:
prj.conf- This file enables various Zephyr APIs, most of which are needed for proper functioning of the application.boards/nrf9160dk_nrf52840.conf- This file tailors the configuration of the modem subsystem and driver to the SLM. 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 removingCONFIG_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:
boards/nrf9160dk_nrf9160_ns.overlay- The UART configuration and power pin can be customized according to your setup.
If the controlling chip is the nRF52840 of the nRF9160 DK:
boards/nrf9160dk_nrf52840.overlay- The UART and power pin are configured to be routed to the nRF9160.
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
When built with the Zephyr-compatible modem configuration, SLM will put the nRF91 Series SiP to deep sleep when powered on. Zephyr’s cellular modem driver running on the controlling chip will take care of waking up the nRF91 Series SiP, so it is advised to first flash SLM to the nRF91 Series SiP.
However, before flashing the SLM 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 SLM 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.
Or if the controlling chip is the nRF52840 of the nRF9160 DK:
Make sure that the PROG/DEBUG SW10 switch on the DK is set to NRF91 when flashing SLM, and to NRF52 when flashing the Zephyr application. The switch also affects which chip is reset when the Reset button is pressed.
No wiring is needed as the routing between the pins happens internally.
To observe the operation sequence, you can view the application logs coming from both chips.
By default, SLM 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.
If SLM is running on the nRF9160 DK, the PROG/DEBUG SW10 switch needs to be set to NRF91 to be able to receive the RTT logs.
However, for convenience you may want to redirect SLM’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 Additional configuration section for information on how to do this.
The logs output via UART can be seen by connecting to the appropriate UART with a serial communication program.
Under Linux, if the controlling chip is the nRF52840 of the nRF9160 DK, the device file of its UART 0 will typically be /dev/ttyACM1.
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:
If power management is enabled (the
CONFIG_PM_DEVICEKconfig option is set toy): when the application powers on the modem (by callingpm_device_action_run(<dev>, PM_DEVICE_ACTION_RESUME)as the sample does), the cellular modem driver will toggle the modem’s power pin to wake it up.If power management is not enabled, the cellular modem driver will automatically proceed and expect SLM to already be started and in a pristine state. In this case, SLM should be compiled with the CONFIG_SLM_START_SLEEP Kconfig option set to
n, and CONFIG_SLM_POWER_PIN can be left undefined.The cellular modem driver will start sending AT commands to SLM. 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=1command). This will result in SLM’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 Cellular modem sample does this.