ARM Cortex M4 Return from Interrupt

Interrupts are a key part of embedded systems programming on ARM Cortex M4 microcontrollers. They allow the processor to respond quickly to events and requests from peripherals. When an interrupt occurs, the processor stops what it is currently doing, saves its state, and jumps to an Interrupt Service Routine (ISR) to handle the interrupt. Once the ISR finishes, the processor returns to where it left off. This is known as returning from the interrupt.

What Happens When an Interrupt Occurs

On the Cortex M4, interrupts are managed by the Nested Vectored Interrupt Controller (NVIC). When an interrupt occurs, the NVIC receives it and checks if interrupts are enabled globally and for that specific interrupt source. If enabled, the NVIC signals the processor core to pause execution and handle the interrupt.

The processor finishes executing the current instruction, then automatically does the following:

1. Saves the address of the next instruction that would have executed (the return address) on the stack.
2. Pushes the current Program Status Register (PSR) onto the stack. This saves important processor state like the current interrupt mask level.
3. Changes the Stack Pointer (SP) to point to the Interrupt Stack.
4. Sets the LR register to the interrupt exception return address.
5. Sets the PC to the address of the ISR vector table for that interrupt.

This sequence is known as an exception entry. The processor is now in Handler mode and executing the ISR code.

Interrupt Service Routine

The ISR code handles the event or request that caused the interrupt. This may involve reading status registers, writing data, or communicating with hardware. The ISR should execute quickly and have minimal overhead since interrupts pause the main program flow.

It is good practice to clear the interrupt at the start of the ISR, then clear the interrupt status flag at the end. This prevents the interrupt from retriggering itself repeatedly.

The ISR may need to preserve registers the main program depends on. This can be done by pushing them onto the stack at the start of the ISR and popping them off at the end. The stack allows independent register preservation for nested interrupts.

After completing the interrupt handling, the ISR execution ends. This triggers the exception return sequence to exit the ISR.

Exception Return Sequence

The processor performs the following steps automatically to return from the ISR:

1. Restore original Stack Pointer from the Interrupt Stack.
2. Load saved Program Status Register (PSR) from stack.
3. Load return address into PC.
4. Continue execution from return address.

This exception return sequence restores the stack, status registers, and program counter back to their pre-interrupt state. The processor resumes normal execution of the main program where it left off.

Configuring Interrupt Priority

The NVIC allows configuring the priority of interrupts on Cortex M4. Higher priority interrupts can interrupt lower priority ISRs. This prevents critical interrupts from being delayed by less important ones.

The NVIC has configurable priority levels from 0 (highest) to 255 (lowest). Level 0 disables that interrupt priority level. Interrupts at the same priority level follow round-robin scheduling.

Some key considerations when configuring interrupt priorities:

• Critical interrupts like timers should have higher priority
• Lower frequency interrupts can have lower priority
• Similar peripherals can share a priority level
• Too many high priority sources can stall lower priorities

Setting appropriate priority levels prevents critical system interrupts from being blocked for too long.

Latency and Nesting

Interrupt latency is the time from the interrupt occurring to its ISR starting. Fast interrupt response time is critical in embedded systems.

Cortex M4 has low latency as the processor automatically preserves context and enters the ISR on hardware signal without software intervention. Minimal latency allows meeting real-time requirements.

The stack architecture also permits nested interrupts. A higher priority interrupt can preempt a lower priority ISR by stacking a new context above it. This means critical interrupts get serviced even when the processor is already handling another interrupt.

However, too much nesting can overflow the stack. Nested interrupts should be avoided when possible by design and priority assignment.

Critical Sections in Interrupts

Sometimes the main program and the ISR need synchronized access to a shared resource like a global variable. This requires protecting critical sections of code from being interrupted.

On Cortex M4, critical sections can be implemented by:

• Temporarily raising the base interrupt mask level
• Using semaphores or mutexes to control access
• Disabling and re-enabling interrupts altogether

This prevents concurrency bugs when both the ISR and main program try to access the same resource. The interrupt mask levels offer a cleaner method than completely disabling interrupts.

Interrupts vs Threads

Interrupts provide an asynchronous way to react to events in embedded systems. An alternative model is using RTOS threads and synchronization primitives like semaphores.

Some key differences between interrupts and threads:

• Interrupts react faster with lower latency
• Threads allow more complex control flow and synchronization
• Interrupt handlers cannot sleep or block like threads
• Threads have separate stacks while interrupts nest on a single stack

In general, interrupts are best for simple, fast reactions to peripheral signals. Threads enable more structured concurrency and waiting for system events.

Guidelines for Efficient Interrupt Handling

Here are some guidelines for working with interrupts on Cortex M4:

• Keep ISR code short, fast, and atomic
• Avoid resource contention between main program and ISRs
• Prioritize interrupts appropriately
• Minimize nesting and stack usage
• Disable less important interrupts when handling critical ones
• Use peripherals with DMA capability to offload data transfers

Efficient interrupt handling ensures fast response times, avoids stack overflows, and prevents concurrency issues. This results in robust embedded systems that leverage interrupts for responsive real-time performance.

Conclusion

The Cortex M4 interrupt system allows assigning priority levels and preemption to handle asynchronous external events efficiently. Proper design practices like managing stack usage, preventing nesting, and using mutexes or semaphores when sharing data between ISRs and threads result in responsive real-time embedded systems performance.

Understanding the exception entry and return mechanism, NVIC priority configuration, critical sections, and differences from threads helps developers take full advantage of Cortex M4 interrupts for building robust embedded applications.