FreeRTOS Simple Tasks
Keywords: Embedded systems, ARM, FreeRTOS, Tasks, STM32F4
Code Link: Tutorial Source Code Github – Keil
In this tutorial we will demonstrate how to create a FreeRTOS Task. Tasks are main building blocks of any Application that wants to use a real-time OS. A task is a piece of code that can be scheduled by OS scheduler and dedicated for specific functionality. In FreeRTOS tasks are the key component of OS Kernel that requires the following prerequisites to meet before a task is recognized by FreeRTOS Kernel and put forward to scheduler.
1. Task Function: Task function define what task is all about i.e implementation of Task functionality e.g. blinking LED, reading sensors data etc. The function implementation must follow void functionName (void *); function declaration and must never return. functionName can be any valid C-function name. Following is a sample Task function code definition.
/*
Sample Task funciton
*/
void myTaskFtn (void * pvParam) {
for (;;) {
/* Task implementation */
...
}
}
2. Task Name: Any Valid ACSII string of maximum length configMAX_TASK_NAME_LEN (including the NULL terminator) or simply NULL. configMAX_TASK_NAME_LEN is defined in FreeRTOSConfig.h header file. This is a descriptive name for the task. This is not used by FreeRTOS in any way. It is included purely as a debugging aid.
3. Stack Size: In freeRTOS, every task has its own stack that stores TCB (Task Control Block) and other stack related operations while the task is being executed. It also stores processor context before a context switch (switching to other task). Stack size must be sufficient to accommodate all local variables + processor context (registers etc.).
4. Task Priority: Tasks in freeRTOS are executed based on Priority. Task Priority can be any number between 0 and configMAX_PRIORITIES – 1. Higher the priority number, higher will be the task priority. Scheduler always choose highest priority task available in ready state. Once higher priority tasks let go of processor then other tasks are given chance to execute.
5. Task Handler: A task handler is a pointer of type xTaskHandle and is used to refer to Task outside Task Function. This handle is used to perform various operations on tasks like Task deletion, Changing Priority etc. from outside the Task Function body. If your application doesn’t refer to a task from outside Task Function body, then the Task Handler can be set to NULL.
FreeRTOS Tasks API:
The following API is used to create a FreeRTOS Task.
portBASE_TYPE xTaskCreate(
pdTASK_CODE pvTaskCode,
const portCHAR * const pcName,
unsigned portSHORT usStackDepth,
void *pvParameters,
unsigned portBASE_TYPE uxPriority,
xTaskHandle *pvCreatedTask
);
Table-1 summarizes xTaskCreate(...) function parameters and return value.
Don’t worry if you haven’t pick anything from the above table. We will explain it here via simple Task Example.
Tutorial Scenario:
In this tutorial we will create two FreeRTOS Tasks i.e. T1 and T2. Both tasks will be given equal priorities i.e. 1. Upon execution, each task prints its own execution message.
T1 Prints: Task-1 Running.
T2 Prints: Task-2 Running.
Note: As both Tasks are of equal priorities; so both will be given equal execution time as shown in Figure-1.
Note: Apart from above tasks an IDLE task is created in the background for various duties like Garbage collection i.e. reclaiming memory of deleted tasks etc. This task is crated by freeRTOS Kernel with fixed priority of 0 and runs only when there is no other task running.
Let’s get back to programming. Following are the steps.
Steps:
1. The first step is to create task functions – See Table-1. Task function must never return and must follow void functionName (void *) function declaration In our case, task function just prints its execution message forever after some dummy delay.
/*
Task Function for Task-1
*/
void vTask1(void * pvParams) {
volatile unsigned int i = 0;
for (;;) {
printf("Task-1 Running.\n");
/*Dummy Delay - Lazy work */
for (i =0; i < 50000; ++i);
}
}
/*
Task Function for Task-2
*/
void vTask2(void * pvParams) {
volatile unsigned int i = 0;
for (;;) {
printf("Task-2 Running.\n");
/*Dummy Delay - Lazy work */
for (i =0; i < 50000; ++i);
}
}
2. Next Step is to create two Tasks via xTaskCreate() APIs. We are not passing any parameters to task function neither we need any task handler so both are kept NULL – Refer to Table-2.
xTaskCreate (vTask1, "T1", 150, NULL, 1, NULL); xTaskCreate (vTask2, "T2", 150, NULL, 1, NULL);
3. The next step is to start FreeRTOS Scheduler so that the tasks we created be executed.
/*
Start the Scheduler
*/
vTaskStartScheduler();
4. Finally Compile source code (github link given at the start of the tutorial), connect STM32F4-Discovery board to PC and enter debug mode. Open printf serial window via View -> Serial Windows -> Debug (printf) Viewer. Run the Program, You will see Task execution messages as shown in bellow Video.
Note: We have routed printf messages to ST-Link debugger via ARM-ITM. There is a dedicated tutorial on how to redirect printf messages to debugger. Link to the tutorial is given bellow.
For complete source code refer to Github link given at the start of this tutorial.
Click the full screen button for more clear view.
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