FreeRTOS tutorial for new CDH members

We developed this project as a way to get people up-to-speed with using the FreeRTOS middleware.

FreeRTOS is an real-time operating system for microcontrollers designed to schedule tasks and introduce multitasking.

If you encounter any difficulties, please contact Junqi on AACT Slack channel or via email [email protected]

Getting started

Things you need

• Visual Studio 2019 (community version should be free)
• a Window system

Step 0

• Download Visual Studio 2019
• When installing, choose the Desktop Development with C++ package , otherwise the project won't open
• The directory we will be using is FreeRTOSv10.0.1\FreeRTOS\Demo\WIN32-MSVC. We will be modifying the main.c file.
• Use this FreeRTOS documentation for more information on the specific API.
• The most common API are:

Task Creation	TaskControl
xTaskCreate()	vTaskDelay()
xTaskDelete()	vTaskDelayUntil()
	vTaskSuspend()
	vTaskResume()

Step 1

• Navigate to the directory FreeRTOSv10.0.1\FreeRTOS\Demo\WIN32-MSVC. Inside, you will find WIN32.vcxproj file. If you have Visual Studio installed, double clicking the file will open the project in Visual Studio.
• Open main.c and navigate to line 173
• On line 173, you will find three xTashHandle, this is used to reference FreeRTOS tasks we will create later. Right now, they will be set to NULL

Step 2

• Head to line 158, This is where we will create our tasks. Follow the documentation and example codes to implement the API call.
• Create the tasks with the following parameters

Task	pritority	stack depth
Task1	3	100
Task2	4	100
Task3	5	100

• *Note: task with higher, i.e. larger number, pritority will take precedence when scheduled at the same time. You will see this later in the result.

Step 3

• Head to line 170, this is where we will define the tasks itself.
• The internal structure of a task is similar to our main funtion, where the the setup code is place in front of a infinity loop (while (1) or for(;;)).
• The schedular tracks the infinity loop and will handle the tasks switching; the program does not get trap in one loop forever.
• Use vTaskDelay() to set the delay interval.
• Use printf() and xTaskGetTickCount() to display both task number and the current time ticks.

Step 4

• vTaskStartScheduler() on line 166 is used to initlize and start the schedular. Nothing needs to be done here.

Step 5

• On the top of the VS IDE, choose build the project and run the windows debug.
• A window should pop up and once the program is running. Read through the log and indentify that the period matches the delay.
• *Note from earlier: when tasks are running at the same time, tasks with higher priority get to operate first, this is why we see tasks3 finished before tasks2 during the same time tick.

If you have trouble with the program, Or is finished the all steps

Head to the solution branch and take a look at the code I wrote.

Final thoughts

FreeRTOS helps us achieve multitasking ability by abstracting many internal mechanisms. We will mainly be developing our flight software within the tasks definition. I hope you learned the basic structure of FreeRTOS and feel less intimitated by it. Let me or Jay know if you have any other questions!