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FreeRTOS Exercise Lab05

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FreeRTOS Lab 05

  • Demonstrate that the scheduler always selects the highest Ready state task to run
    by using the vTaskPrioritySet() API function to change the priority of two tasks relative to each other.
  • Two tasks are created at two different priorities.
    Neither task makes any API function calls that cause it to enter the Blocked state,
    • So both are in either Ready or Running state.
    • So the task with highest priority will always be the task selected by the scheduler to be in Running state
  • Expected Behavior
    1. Task 1 is created with the highest priority to be guaranteed to run first. Task 1 prints out a couple of strings before raising the priority of Task 2 to above its own priority.
    2. Task2 starts to run as it has the highest relative priority.
    3. Task 2 prints out a message before setting its own priority back to below that of Task 1.
    4. Task 1 is once again the highest priority task, so it starts to run and forcing Task 2 back into the Ready state.


Freertos changing tasks priorities.png


  • Functions to be Used
    1. vTaskPrioritySet(xTaskHandle pxTask, unsigned portBASE_TYPE uxNewPriority);
      To be used to change the priority of any task after the scheduler has been started.
      Available if INCLUDE_vTaskPrioritySet is set 1.
      Two parameters
      pxTask: Handle of the task whose priority is being modified. A task can change its own priority by passing NULL in place of a valid task handle.
      uxNewPriority: the priority to be set.
    2. unsigned portBASE_TYPE uxTaskPriorityGet(xTaskHandle pxTask);
      To be used to query the priority of a task
      Available if INCLUDE_vTaskPriorityGet is set 1
      pxTask: Handle of the task whose priority is being modified. A task can query its own priority by passing NULL in place of a valid task handle.
      Returned value: the priority currently assigned to the task being queried