Creating an application

In this exercise, we will learn how to create a working “Hello World” application in the nRF Connect SDK from scratch. We are only doing this for educational purposes to expose you to all the elements of an nRF Connect SDK application.

Keep in mind that this is not the recommended way to create an application. As we explained in lesson 1, the recommended way is by using one of the samples that you can find in the SDK as a baseline.

Exercise steps

1. In the exercise folder for this lesson, create a folder and name it l3_e1.

2. Open this folder and create the files needed for a minimum working application.

• prj.conf
• CMakeLists.txt
• src/main.c (this means create a subdirectory called src and then create the empty file main.c)

3. Open CMakeLists.txt and add the minimum functions needed for the application to build (use any editor).

cmake_minimum_required(VERSION 3.20.0)
find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
project(hello_world)
target_sources(app PRIVATE src/main.c)

cmake_minimum_required ensures the build fails if the CMake version is too old.
find_package pulls in the Zephyr build systems, which creates a CMake target named app.
project sets the name of the project.
target_sources adds the source file to the project.

4. In main.c, include the header file for Zephyr Kernel and the printk module like this:

#include <zephyr/kernel.h>
#include <zephyr/sys/printk.h>

5. In main.c, define the main function to continuously print the statement “Hello World!” and then sleep for a small amount of time (to avoid flooding the console).

int main(void)
{
	while (1) {
		printk("Hello World!\n");
		k_msleep(1000);
	}
}

6. Let’s add this as an existing application in nRF Connect for VS Code.

Go to nRF Connect for VS Code and under the Welcome View, select Open an existing application and select the directory fund_less3_exer1. Observe the project showing up under the Applications View.

7. Just like in Lesson 1, select Add Build Configuration by hovering your mouse over the project name in the Applications View and selecting the icon that appears. Choose the correct board, then select Build Configuration.

8. Flash the application to your board.

9. To view the output of the application, you need to configure a terminal emulator on your machine. You can work in any terminal emulator you want.

Step 9.1 describes how to view the output using the built-in terminal emulator in the nRF Connect extension in VS Code, while step 9.2 describes how to view the output using PuTTY. You can choose either one.

9.1 To use the terminal emulator in VS code, first go to the “Connected Devices” tab and open the drop list.

Then open the drop list for that connected device.

Then click the “Connect To Serial Port In nRF Terminal” icon as shown below.

Then choose the serial terminal which has the following configuration:

• Baud rate: 115200 baud/sec
• 8 bits/character, no parity, 1 stop bit (8n1)
• No flow control: rts and cts = off

As shown below:

You should be able to observe the output as shown below

9.2 To use PuTTY, you can download it from here. PuTTY does not require any installation, it is a single executable that you just need to download. 

There are two important parameters when using the UART console:

• Speed (baud rate) – the default speed set in the devicetree of Nordic Semiconductor development kits is 115200 bps.
• Serial line (COM number) – unlike the speed, this differs from machine to machine. You can find the serial line listed along your board name in the Connected Devices View inside VS code as shown below:

Note the COM number assigned to your board. In the screenshot above, it’s COM4, but this value is likely to be different on your host machine.  

10. Open PuTTY, and follow the steps below:

1. Select Serial as the connection type.
2. Set the correct serial line as the COM number assigned to your device.
3. Set the speed to 115200.
4. Start the connection by pressing Open and a session will start.

11. Reset your board, and you should be able to observe the output, “Hello World!” printed every second. Depending on which terminal you are using, your output will look like one of the two below images.

*** Booting nRF Connect SDK v2.8.0-preview1-11645184a54d ***
*** Using Zephyr OS v3.7.99-adcffa835a8e ***
Hello World!
Hello World!
Hello World!

The solution for this exercise can be found in the GitHub repository, l3/l3_e1_sol of whichever version directory you are using.