FreeRTOS Scheduling: Start, Preempt, Yield (and What’s Happening in RAM)

Your tasks are in Ready; the scheduler decides who runs. Here’s the practical path to start it, pick a policy (pre-emptive vs co-operative), and understand the heap/TCB story behind xTaskCreate().

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✅ Start the Scheduler

vTaskStartScheduler() launches the kernel; it never returns unless heap is insufficient to create the Idle (and timer) task.

configASSERT(xTaskCreate(Task1,"T1",256,NULL,2,NULL)==pdPASS);
configASSERT(xTaskCreate(Task2,"T2",256,NULL,2,NULL)==pdPASS);
vTaskStartScheduler(); // returns only on failure

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⚙️ Scheduling Policies

• Pre-emptive (configUSE_PREEMPTION 1): the running task can be replaced on ticks or when a higher-priority task becomes ready.

• Co-operative (configUSE_PREEMPTION 0): tasks voluntarily give up the CPU with taskYIELD() (no forced pre-emption).

// FreeRTOSConfig.h
#define configUSE_PREEMPTION 1
#define configTICK_RATE_HZ   1000  // 1 ms tick

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🔁 Round-Robin & ⏫ Priority-Based

• Round-robin: equal-priority ready tasks share the CPU in time slices tied to the RTOS tick.

• Priority-based: the highest priority ready task runs; when it blocks, the next highest takes over; if a higher priority task unblocks, it pre-empts immediately.

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🤝 Co-operative Example (clean prints)

static void Task1(void* pv){ for(;;){ puts("T1"); taskYIELD(); } }
static void Task2(void* pv){ for(;;){ puts("T2"); taskYIELD(); } }

If your output looks garbled under pre-emption, protect the shared sink (UART/ITM) with a mutex.

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🧠 Behind the Scenes: Heap, TCB, Stacks

• xTaskCreate() dynamically allocates the TCB and the task’s private stack from the RTOS heap and then links the TCB into the Ready list.

• Heap size is set by configTOTAL_HEAP_SIZE; many projects use heap4.c (malloc/free over a single heap array).

• On Cortex-M, task stacks are tracked with PSP (process stack pointer); the TCB stores TopOfStack.

// FreeRTOSConfig.h
#define configTOTAL_HEAP_SIZE (75*1024)  // tune to your MCU RAM

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⚡ Pro Tips

• Keep for(;;){ … vTaskDelay(...) } or block on queues; avoid busy loops.

• Few priority levels → fewer context switches (less RAM/time overhead).

• Always check xTaskCreate() and enable overflow/malloc-fail hooks.

• Put app code inside USER CODE BEGIN/END in Cube projects to survive regen.

🎯 Conclusion

Choose pre-emptive for responsiveness or co-operative for deterministic prints, start the scheduler, and size the heap/stack sanely, your tasks will play nicely and your logs will, too.

Written By: Musaab Taha

This article was improved with the assistance of AI.