Scheduler, Priorities & Context Switch

Overview

This is the foundation of the entire FreeRTOS series. Before you can use queues, mutexes, or timers you need to understand how the scheduler works, how tasks are created, and exactly what happens during a context switch on ARM Cortex-M. Everything else builds on this.

• ✓Create and delete FreeRTOS tasks using xTaskCreate()
• ✓Understand priorities, the ready list, and how the preemptive scheduler selects tasks
• ✓Measure context switch overhead on real hardware with an oscilloscope
• ✓Know when to use vTaskDelay() vs vTaskDelayUntil() for jitter-free periodic tasks

How the Scheduler Works

FreeRTOS uses a fixed-priority preemptive scheduler. At every tick interrupt (typically every 1ms), it checks whether a higher-priority task has become ready. If one has, it immediately context-switches to it. Equal-priority tasks share the CPU in round-robin.

Code

void vHighTask(void *pv), vLowTask(void *pv);

int main(void) {
    HAL_Init(); SystemClock_Config();
    // xTaskCreate(func, name, stack_words, param, priority, handle)
    xTaskCreate(vHighTask, "High", 256, NULL, 3, NULL);
    xTaskCreate(vLowTask,  "Low",  256, NULL, 1, NULL);
    vTaskStartScheduler();
    while(1);
}

void vHighTask(void *pv) {
    TickType_t xLast = xTaskGetTickCount();
    while(1) {
        HAL_GPIO_TogglePin(GPIOD, GPIO_PIN_12);
        // Exact 100ms period — compensates for execution time
        vTaskDelayUntil(&xLast, pdMS_TO_TICKS(100));
    }
}

void vLowTask(void *pv) {
    while(1) {
        DoBackgroundWork();
        // 500ms AFTER execution — period drifts
        vTaskDelay(pdMS_TO_TICKS(500));
    }
}