Getting Started with Real-time Operating Systems

Video link: https://atollic.wistia.com/medias/gwgee6ev93

RTOS Introduction

microcontroller

• before , 4~16K RAM, 8MHz CPU
• now, 16~32K RAM

we can build really cool stuff.

Bare-metal scheduling technique

Round-Robin Scheduling

int main(void) {
    System_Init();
    while (1) {
        Task1();
        
        Task2();
        
        Task3();
    }
    
    return 1;
}

• pros
  • easy to implement
• cons
  • Adding new task will change the real-time attribute (need to tune again)
  • not the best for RTOS

Round-Robin scheduling improvement

With interrupts, we can change the current task.

Cooperative Scheduling

• Read system time

  • Run task1? yes: Task1, no: check next task
  • Run task2? yes: Task2, no: read system time

• procs

  • only need to check all task end with the deadline
  • can have interrupt
  • can have hard real-time

• cons

  • Can't preemptive

Task Concurrency

• Sequential Tasks
  • Task 1
  • Task 2
  • Task 3
  • Task 1
  • …
• Concurrent Tasks
  • Task 1
  • Task 3
  • Task 1
  • Task 2
  • Task 1
  • …

RTOS Characteristics

A Real-Time Operating System (RTOS) is an operating system designed to manage hardware resources of an embedded system with very precise timing and a high degree of reliability.

• Example 6 characteristics
  • Reliability
  • Predictability
  • Performance
  • Compactness
  • Scalability
  • Multi-Tasking

Bara-metal or RTOS

• 7 Reason to choose an RTOS
  • Concurrency
  • Preemption
    • a task to be interrupt by higher priority to run on the CPU
    • In bare-metal, using preemption will make code too complex.
  • Available RAM
  • Available flash
  • Sync tools
  • 3rd party software
  • Ease of use

FreeRTOS

Task Fundamentals

Create Task

xTaskCreate(
    Led_BlueBlink,                       /* Task Pointer */
    (const char *const) "led_blue",      /* Task Name */
    configMINIMAL_STACK_SIZE,            /* Stack Depth */
    0,                                   /* Parameters to Pass to task */
    1,                                   /* Task Priority */
    0                                    /* Pass handle to create task */
);

Task States

FreeRTOS Resource