A round-robin scheduler is a scheduling algorithm that sequentially cycles through a list of tasks, giving each task a slice of time to execute before moving on to the next task. Implementing a round-robin scheduler on Cortex-M microcontrollers can provide predictable multitasking capabilities for real-time embedded applications.
Overview of Round-Robin Scheduling
The key features of round-robin scheduling are:
- Tasks are organized in a queue data structure
- Scheduler cycles through the queue, executing each task for a pre-defined time slice
- After a task’s time slice expires, it is preempted and moved to the back of the queue
- Scheduler continues cycling through tasks indefinitely
This ensures that all tasks get equal access to the CPU at regular intervals. The time slice duration is an important parameter that impacts responsiveness and context switching overhead.
Challenges on Cortex-M
Implementing round-robin scheduling on Cortex-M poses some challenges:
- No built-in scheduler – Cortex-M CPUs have no native scheduling hardware
- Manual context switching – scheduler must save/restore context when preempting tasks
- Timers for preemption – need periodic interrupts to trigger preemptions
- Fixed priority levels – Cortex-M NVIC has fixed interrupt priorities
These factors mean the scheduler must be implemented fully in software. Care must be taken to minimize context switching overhead and maintain deterministic behavior.
Scheduler Implementation
Here is one approach to implementing a round-robin scheduler on Cortex-M:
Task Structure
Each task is defined with a structure containing:
- Task function pointer
- Task arguments
- Task stack pointer
- Task stack size
- Task priority
Task Queue
A queue data structure stores the list of tasks to be scheduled. New tasks are added to the back of the queue.
Context Switching
Manually save context (registers, stack pointer) of old task and restore context of new task on preemptions.
Preemption Timer
A periodic SysTick timer triggers an interrupt handler at the end of each time slice to force a context switch.
Scheduler Loop
Main scheduler loop runs the highest priority ready task. It yields to the preemption handler at each time slice.
Optimizations
Some optimizations can improve performance and determinism:
- Use dedicated stack per task to simplify context switching
- Adjust time slice duration to balance responsiveness and overhead
- Consider task priorities when ordering queue
- Use compiler intrinsics for context switching for speed
- Configure preemption timer in Cortex-M systick
Example Code
Here is some example pseudo-code to illustrate the implementation: // Task structure struct task { void (*function)(void*); void* args; char* stack; int priority; }; // Task queue task_t tasks[MAX_TASKS]; int head, tail; // Current running task task_t* current; // Scheduler initialization void scheduler_init() { // Setup systick for preemption timer // … // Initialize task queue head = tail = 0; } // Add task to scheduler void scheduler_add(task_t* task) { tasks[tail] = task; tail++; } // Context switching void switch_context(task_t* next) { // Save context of old task // Restore context of next task } // Preemption handler void preempt_handler() { task_t* next = tasks[head]; switch_context(next); head++; if (head >= MAX_TASKS) { head = 0; } } // Scheduler loop void scheduler_run() { while (1) { task_t* next = tasks[head]; switch_context(next); next->function(next->args); // Yield to preemption handler preempt_handler(); } }
This demonstrates the key steps of queueing tasks, timer-based preemption, context switching, and running the scheduler loop. Real implementations would also need to handle task completion, task priorities, and other features.
Conclusion
Implementing a round-robin scheduler on Cortex-M requires careful software design due to the lack of hardware scheduling support. With an efficient preemption timer, optimized context switching, and a well-structured task queue, real-time multitasking can be achieved. The result is a fair and deterministic scheduling mechanism suitable for many embedded applications.
