Context switching on Cortex-M0 microcontrollers can be problematic due to the limited resources available on these low-cost ARM chips. While context switching is possible on Cortex-M0, it requires careful planning and programming to avoid issues. This article examines the challenges of context switching on Cortex-M0 and provides recommendations for successfully implementing preemptive multitasking.
The Problem with Cortex-M0 Context Switching
The Cortex-M0 is ARM’s lowest cost and most energy efficient microcontroller, intended for simple, low-power embedded applications. With only 12.5 CoreMark/MHz performance, no memory management unit, and limited RAM, the Cortex-M0 presents challenges for performing context switches between tasks.
On the Cortex-M0, context switching must be done manually in software. The processor has no hardware support for saving processor state on an interrupt or handling memory protection. This places a heavy burden on the programmer to implement preemptive multitasking properly.
The small RAM size, typically only 8-16KB on Cortex-M0 chips, makes it difficult to allocate sufficient stack space for multiple tasks. Fragmentation can also become a problem as tasks dynamically allocate and free memory.
Saving and restoring full processor context on each switch can be costly in terms of clock cycles. With no floating point unit, tasks that utilize floating point operations will run very slowly if context switched.
Given these constraints, blindly enabling interrupts and performing context switches whenever an interrupt occurs will likely lead to poor system performance, stack overflows, and other errors.
Guidelines for Cortex-M0 Context Switching
Here are some guidelines to follow for successful preemptive multitasking on Cortex-M0 microcontrollers:
- Minimize context switches whenever possible. Only switch tasks when necessary.
- Make tasks short and focused on a single job to limit execution time.
- Prioritize tasks appropriately to ensure important jobs complete on time.
- Allocate sufficient stack space for each task’s worst-case usage.
- Use stack watermarking to detect stack overflow conditions.
- Minimize dynamic memory allocation to avoid fragmentation.
- Carefully manage interaction between tasks accessing shared data structures.
- Utilize mutexes, semaphores, message queues and other RTOS objects to synchronize tasks.
- When switching context, save only the minimum CPU state necessary.
- Use the compiler’s smallest memory model that meets requirements.
- Consider using reduced context switching or cooperative multitasking models.
Reducing Context Switches
A key optimization approach is designing the application to reduce the number of context switches required. This involves:
- Disabling interrupts during critical sections of code
- Making ISR routines short and efficient
- Minimizing OS call frequency from tasks
- Configuring peripherals to operate via DMA rather than interrupts
- Polling inputs when appropriate instead of using interrupt-driven I/O
- Evaluating real-time requirements to determine if context switching is truly necessary
By only performing context switches when absolutely required, overall system reliability and response time can be improved.
Task State Data
When modifying tasks to reduce context switch overhead, one method is to use a state variable within each task to track what the code was doing. This allows restoring only a minimal CPU state on a switch.
For example, a task controlling a serial port receive buffer could have states like “idle”, “waiting for data”, “adding data”, “processing data”, etc. The state data can live in the task control block. Then when switching back to the task, the code simply checks the state variable to resume the appropriate work.
This technique works well for structured tasks that tend to execute linear sequences of operations. Each state change is explicitly coded, rather than relying on saving and restoring full CPU register contents.
Limited Preemptive Scheduling
Another technique is to only allow context switches at well-defined points in a task by creating a limited preemptive scheduler. This involves:
- Defining preemption points in tasks where switching is allowed
- Disabling interrupts on entry into key sections of code
- Checking for pending context switches at each preemption point
- Keeping preemption disabled code segments short
This forces context switches to only happen at controlled points, similar to cooperative multitasking but still maintaining preemptive capabilities.
RTOSes for Cortex-M0
Using a Real-Time Operating System (RTOS) can greatly assist with efficient preemptive multitasking on Cortex-M0. RTOSes like FreeRTOS provide standard solutions for:
- Task scheduling and prioritization
- Resource allocation
- Inter-task communication and synchronization
- Memory allocation and segmentation
RTOS hooks allow customizing context switching behavior and integrating device drivers and other hardware specific code.
However, an RTOS will consume some of the already limited Cortex-M0 RAM for data structures like task control blocks. So the memory footprint versus benefit tradeoff should be evaluated per application.
Example Context Switcher for Cortex-M0
Here is example code for a simple cooperative context switcher for Cortex-M0 written in C: // Task control block structure struct tcb { void (*taskfunc)(void); // Function to call volatile char state; // Current state }; // Current TCB struct tcb *current; // Context switcher void switchcontext(struct tcb *next) { // If task is same, just return if (current == next) { return; } // Save state of current task current->state = state_running; // Switch tasks current = next; // Restore state of next task switch(current->state) { case state_ready: break; case state_running: return; // etc… } // Call function for next task current->taskfunc(); } int main(void) { // Init TCBs while (1) { switchcontext(&tcb1); switchcontext(&tcb2); } }
This simple scheduler alternates between 2 tasks using explicit context switching. To make it preemptive, switchcontext() could be called from the ISR vector table instead.
In summary, with careful design and programming, even highly resource constrained Cortex-M0 microcontrollers can support preemptive multitasking. By optimizing context switches, task structure, RTOS integration, and memory usage, real-time performance can be achieved.
