FreeRTOS

FreeRTOS is an open source real time operating system that can be used on many microcontrollers including RA6M3 that I used for a project. It can meet the hard-real time requirements where unabling to run a specific task before a deadline is a system failure. Users can control an interval of the task so that it executes periodically.

Task

FreeRTOS provides API functions for the Task. The task is implemented in C function which acts like the C main() with different prototype and additional constraints.

The prototype is defined in include/projdefs.h

typedef void (*TaskFunction_t)(void*);

Function definition should looks like

void deserializeEthernetFrame(void* param)
{
    for (;;)
    {
        /* work */
    }
    vTaskDelete(NULL);
}

One of the constraints is that a task must be terminated by using vTaskDelete(NULL) instead of return;. It is necessary because there is no automatic deallocation of kernel resource like general operating systems where a separation between user space and kernel space exists.

Create

There are many API functions for the creation of the task. They provides options for static memory allocation, resource privilege and affinity on SMP.

vTaskCreate() function is the most simple one.

BaseType_t xTaskCreate(
    TaskFunction_t pvTaskCode,
    const char* const pcName,
    configSTACK_DEPTH_TYPE usStackDepth,
    void* pvParameters,
    UBaseType_t uxPriority,
    TaskHandle_t* pxCreatedTask);
  • pvTaskCode a function pointer to be executed.
  • pcName a descriptive string for a task.
  • usStackDepth is multiplicated by the word size to decide the size of the stack. If the word size is 4 bytes and usStackDepth is 128, stack size is 512 bytes.
  • pvParameters is argument of the pvTaskCode function.
  • uxPriority defines the task’s priority. FreeRTOS uses priority based round robin scheduler.
  • pxCreatedTask is an address to store a handle or NULL.

Tick Interval

A task runs for a time slice that is equal to the interval of two consecutive ticks. The interval is compile-time constant and configTICK_RATE_HZ macro contains its value. If the value is 1000, tasks will run for 1 millisecond and be preempted by the scheduler.

Sleep

vTaskDelay() function places the calling task into the Blocked state for a fixed number of tick interrupts. Users can adjust the interval larger than the tick interval by using this function.

void vTaskDelay(TickType_t xTicksToDelay);

Example

void deserializeEthernetFrame(void* param)
{
    for (;;)
    {
        /* work */
        vTaskDelay(pdMS_TO_TICKS(10));
    }
    vTaskDelete(NULL);
}

pdMS_TO_TICKS is function macro which convert milliseconds to ticks. This task will be placed to Blocked state for 10 ms after work has been done.

Reference

Mastering the FreeRTOS Real Time Kernel v1.1.0