ARM Cortex-M4 Interrupt Handling

The ARM Cortex-M4 processor has a flexible and configurable interrupt handling system that allows developers to respond quickly and efficiently to events. Interrupts are a key part of embedded systems programming and proper configuration is necessary for robust and responsive applications.

Cortex-M4 Interrupt Sources

There are several potential sources of interrupts in the Cortex-M4 system:

• Hardware interrupts from peripherals like timers, GPIO, serial interfaces, etc.
• Software interrupts triggered by code like SVC calls
• Internal exceptions like undefined instructions, bus faults, etc.

The processor core combines these into a single vector table with configurable priorities. Higher priority interrupts can interrupt lower priority code. The processor automatically stacks context to allow returning to the original execution flow.

NVIC and Interrupt Configuration

The Cortex-M4 Nested Vectored Interrupt Controller (NVIC) manages the enabling, masking, and priority configuration of interrupts. It provides a single point of configuration for the various interrupt sources.

Key aspects of NVIC configuration include:

• Setting priorities of interrupts
• Enabling or disabling interrupts
• Grouping interrupts for priority masking
• Registering handler functions to interrupts

Higher priority values correspond to higher logical priorities in the NVIC. For example, an interrupt with priority 4 will preempt an interrupt with priority 3. Priority 0 is the highest possible priority.

The NVIC provides up to 240 external interrupt sources with up to 16 priority levels and 32 total priority bits for precise configuration control. This flexible prioritization allows developers to create an interrupt structure optimized for their specific application.

Cortex-M4 Vector Table

The Cortex-M4 vector table defines the handler functions for each exception and interrupt source. It is normally located at the very beginning of code memory at address 0x0000 0000.

The vector table contains an entry for each exception like Reset, NMI, HardFault, and entries for each peripheral or configurable external interrupt. Each entry holds a 4-byte address pointer to the handler function.

When an interrupt occurs, the processor will jump to the corresponding vector table entry to find and execute the handler function. This provides a simple linkage between interrupt sources and their handlers.

Utilizing the vector table properly is key to configuring interrupt behavior. Developers must populate the table with pointers to valid handler functions for each interrupt they intend to use in their application.

Cortex-M4 Interrupt Handling Process

The general interrupt handling process on the Cortex-M4 is:

1. Interrupt occurs and the processor checks if its priority is higher than the currently executing code.
2. If higher priority, the processor suspends current execution, saves context to stack.
3. The processor loads the address for the corresponding handler from the vector table.
4. Handler function executes until completion.
5. The processor restores context from stack and resumes original execution flow.

The processor ensures atomic entry into handler code and transparent return to original execution. This creates the illusion that the handler was a simple function call.

The handler itself is written as a normal C function. It has access to all normal stack variables, static variables, and globals. Register values are stacked by hardware so r0-r3, r12, LR, PC, and xPSR can be freely used without saving.

Defining Interrupt Handlers

Here is an example handler definition: void TIM2_IRQHandler(void) { // Handle timer interrupt // … // Clear interrupt TIM2->SR = ~TIM_SR_UIF; }

Key aspects are:

• Function name matches the IRQ name, like TIM2_IRQHandler for Timer 2.
• Void input, void return to match interrupt function prototype.
• Clear interrupt source at handler end.

Handlers should be defined static inline or in separate C files. Inline allows the compiler to optimize without overhead. Separate C files can aid organization.

Connecting Handlers to NVIC

To activate an interrupt, its handler must be enabled in the NVIC. This connects the interrupt source to the handler code.

Example NVIC configuration: void enable_timer2_interrupt() { // Enable Timer 2 interrupt in NVIC NVIC_EnableIRQ(TIM2_IRQn); // Set Timer 2 priority 2 NVIC_SetPriority(TIM2_IRQn, 2); }

This enables the IRQ and sets a priority of 2. The handler will now be executed when the timer interrupt occurs.

Vector Table Initialization

The vector table must be properly populated at startup. This is often done in the reset handler executed on boot: void ResetHandler() { // Copy vector table from flash to RAM // Updates table in RAM with handler addr // Initialize data and BSS sections // Call main program }

Copying the vector table to RAM allows dynamic handler updates. A copy is still kept in flash.

Make sure no interrupts occur before the vector table is established. Initialize important handlers like hard fault first.

Interrupt Latency Considerations

Interrupt latency is the time from interrupt assertion to start of handler execution. Factors contributing to latency on Cortex-M4 include:

• Interrupt context saving – stacking register contents
• Priority filtering – determining higher vs lower priority
• Vector fetch – retrieving address of handler

Typical latencies are in the range of 10s of cycles. This small latency allows quick response from event to handler execution.

Latency can be minimized by:

• Efficient interrupt prioritization scheme
• Optimized context saving like using PendSV for stacking
• Caching vector table in fastest memory

ARM Cortex-M Interrupt Debugging

Debugging interrupt issues requires specialized techniques like:

• Monitoring interrupt vectors taken
• Tracking interrupt count occurrences
• Measuring temporal relationships like jitter or latency
• Injecting interrupts artificially

Debug/trace features like Embedded Trace Macrocell (ETM) or Instrumentation Trace Macrocell (ITM) can provide useful insight into interrupt behavior.

Logic analyzers and oscilloscopes triggered on interrupt signals can measure timing. This helps identify any latency issues.

Conclusion

The Cortex-M4 interrupt system provides a flexible foundation for responsive embedded applications. Key concepts include NVIC configuration, vector table setup, low-latency handling, and optimized prioritization.

Properly utilizing interrupts allows developers to build event-driven systems that are efficient, deterministic, and robust. With careful design and implementation, the Cortex-M4 interrupt architecture enables even complex high-performance embedded projects.