In this exercise, let’s use the LTE Link Controller library in nRF Connect SDK to establish the LTE connection. One advantage of using a high-level library instead of direct serial communication commands is that the library lets you asynchronously set up the LTE connection, which is what we will do in this exercise.

Exercise Steps

1. Open the base exercise code for this exercise in VS Code.

1.1 Clone the GitHub repository for this course.

Copy the link to the repository and use VS Code’s Command Palette to clone the repository.

1.2 In the nRF Connect for VS Code extension, select Open an existing application, and open the base code for this exercise, found in cell-fund/<version>/l2/l2_e2, of whichever version directory (<version>) you are using.

Due to changes in the nRF Connect SDK, make sure to use the version folder of the base code and exercise tabs that matches the SDK version you are running.

2. Enable the nRF Modem library.

Search for STEP 2 in prj.conf and add the following line:

CONFIG_NRF_MODEM_LIB=y

3. Enable and configure the LTE link controller library.

Search for STEP 3 in prj.conf and add the following lines

CONFIG_LTE_LINK_CONTROL enables the library and CONFIG_LTE_AUTO_INIT_AND_CONNECT disables the auto initialize and connect functionality.

CONFIG_LTE_LINK_CONTROL=y
CONFIG_LTE_AUTO_INIT_AND_CONNECT=n

4. Include the relevant header files.

Include the header file for the LTE link controller library.

In the main.c file, search for STEP 4 and add the following line

#include <modem/lte_lc.h>

5. Define the semaphore lte_connected.

Define a semaphore using K_SEM_DEFINE, with initial count 0 and max count 1, to be used when connecting.

Search for STEP 5 and add the following line

static K_SEM_DEFINE(lte_connected, 0, 1);

6. Define the function modem_configure().

Define the function modem_configure() to initialize the LTE connection.

Search for STEP 6 and add the following lines

int err = lte_lc_init_and_connect_async(lte_handler);
if (err) {
	LOG_ERR("Modem could not be configured, error: %d", err);
	return;
}

7. In the event handler for the link controller lte_handler().

In lte_handler(), in the event of changed registration status (LTE_LC_EVT_NEW_REG_STATUS), check if the modems registration status is connected (LTE_LC_NW_REG_REGISTERED_HOME or LTE_LC_NW_REG_REGISTERED_ROAMING), and if so, give the semaphore lte_connected.

Search for STEP 7.1 and add the following lines

case LTE_LC_EVT_NW_REG_STATUS:
	if ((evt->nw_reg_status != LTE_LC_NW_REG_REGISTERED_HOME) &&
	     (evt->nw_reg_status != LTE_LC_NW_REG_REGISTERED_ROAMING)) {
		break;
	}

	LOG_INF("Network registration status: %s",
		evt->nw_reg_status == LTE_LC_NW_REG_REGISTERED_HOME ?
		"Connected - home network" : "Connected - roaming");
	k_sem_give(&lte_connected);
	break;

7.2 Upon the event of changed RRC mode (LTE_LC_EVT_RRC_UPDATE), print the RRC mode.

Search for STEP 7.2 and add the following lines

case LTE_LC_EVT_RRC_UPDATE:
	LOG_INF("RRC mode: %s", evt->rrc_mode == LTE_LC_RRC_MODE_CONNECTED ? 
			"Connected" : "Idle");
	break;		

8. In main(), call modem_configure() to initiate the LTE connection.

Search for STEP 8 and add the following lines

err = modem_configure();
if (err) {
	LOG_ERR("Failed to configure the modem");
	return 0;
}

9. Take the semaphore lte_connected.

Take the semaphore using k_sem_take() with no timeout, which will only be given in the callback function once the modem’s registration status is connected.

Search for STEP 9 and add the following lines

k_sem_take(&lte_connected, K_FOREVER);

After taking the semaphore, the two next lines will only be run when the device is connected to the network at which point the semaphore is given.

10. Turn on the LED status LED

Upon successfully connecting to the network, turn on the status LED.

Search for STEP 10 and add the following line

dk_set_led_on(DK_LED2);

11. Build and flash the application.

11.1 Add a build configuration and select whichever board you are using:

• nRF9160 DK: nrf9160dk_nrf9160_ns
• Thingy:91: thingy91_nrf9160_ns

Make sure the Build after generating configuration option is enabled to trigger the build process after clicking on the Build Configuration button.

11.2 Flash the application to your device.

You should see the following log output on your console.

*** Booting nRF Connect SDK 2.3.0-3758bcbfa5cd ***
<inf> Lesson2_Exercise2: Initializing modem library
<inf> Lesson2_Exercise2: Connecting to LTE network
<inf> Lesson2_Exercise2: RRC mode: Connected
<inf> Lesson2_Exercise2: Network registration status: Connected - roaming
<inf> Lesson2_Exercise2: Connected to LTE network
<inf> Lesson2_Exercise2: RRC mode: Idle

In addition, the connection LED which is the LED that will indicate a connection (LED2 on the nRF91 DK or green LED on the Thingy:91) should be lit.

12. For further debugging purposes, let’s enable the AT Host library.

In the prj.conf file, search for STEP 12 and add the following lines to the prj.conf file

CONFIG_AT_HOST_LIBRARY=y
CONFIG_UART_INTERRUPT_DRIVEN=y

CONFIG_AT_HOST_LIBRARY is to enable the AT Host library and CONFIG_UART_INTERRUPT_DRIVEN enables interrupt support for UART so that the AT Host library can print asynchronously to the console.

The AT Host library automatically spawns a thread to print status AT commands.

13. Build and flash the application again.

You should notice intermittent unsolicited AT command notifications +CEREG and +CSCON being printed on the console among the log messages from the application.

*** Booting nRF Connect SDK 2.3.0-3758bcbfa5cd ***
<inf> Lesson2_Exercise2: Initializing modem library
<inf> Lesson2_Exercise2: Connecting to LTE network
+CEREG: 2,"76C1","030EB103",7
+CSCON: 1
<inf> Lesson2_Exercise2: RRC mode: Connected
+CEREG: 5,"76C1","030EB103",7,,,"11100000","11100000"
<inf> Lesson2_Exercise2: Network registration status: Connected - roaming
<inf> Lesson2_Exercise2: Connected to LTE network
+CSCON: 0
<inf> Lesson2_Exercise2: RRC mode: Idle

14. Enabling the AT Host library also allows us to send AT commands while the application is running.

Let’s read out the IMEI of our device by sending:

AT+CGSN=1

Observe the AT command response containing the 15-digit IMEI (censored here).

+CGSN: "3515161726xxxxx"
OK

The solution for this exercise can be found in l2/l2_e2_sol of whichever version directory you are using.