SysTick Interrupt Handler Design Tips for Cortex-M

The SysTick timer is an essential peripheral found in all Cortex-M processors. It allows precise timing of events and periodic software execution. The SysTick interrupt handler, which executes when the timer expires, is a critical part of any Cortex-M firmware. Well-designed interrupt handlers improve system robustness and determinism. This article provides practical design tips for writing efficient and reliable SysTick interrupt handlers on Cortex-M cores.

Understanding the SysTick Timer

The SysTick timer built into Cortex-M processors is a 24-bit down counter that decrements at the system clock rate. It can generate interrupts when counting down to zero. The SysTick has several key registers:

• SYST_CSR: Control/status register to enable SysTick, generate interrupts, etc.
• SYST_RVR: Reload value register sets the start counter value.
• SYST_CVR: Current value register indicates current counter value.

To use the SysTick timer:

1. Program SYST_RVR with the desired countdown value.
2. Configure SYST_CSR to enable counting and interrupts.
3. The timer will decrement on each clock cycle and fire interrupt when reaching 0.
4. In the ISR, reload SYST_RVR to reset timer for periodic operation.

This allows periodic interrupts at fixed intervals, e.g. for task scheduling in a real-time OS. The SysTick timer forms the basis for precise timing in Cortex-M firmware.

SysTick Handler Design Considerations

Here are some key considerations when designing the SysTick interrupt handler:

Keep It Short

The SysTick ISR should be kept as short as possible, ideally just a few instructions. This reduces interrupt latency for other system events. Move any non-trivial processing out of the ISR into tasks.

Avoid Function Calls

Calling functions from an ISR incurs significant overhead. It also creates reentrancy issues if the function itself causes an interrupt. Ideally, implement the ISR in a straight-line code sequence.

Use Stack with Caution

Many Cortex-M devices have separate stacks for thread and handler modes. Restrict stack usage in ISRs to avoid collisions with thread stack usage.

Prioritize Consistent Timing

Aim for consistent interrupt latencies on each SysTick event. Avoid any branching logic in the ISR that could cause variable execution times.

Re-enable Interrupts ASAP

Interrupts are automatically disabled on ISR entry. Re-enable interrupts as soon as possible to allow nesting of same priority interrupts.

Use Atomic Access for Shared Resources

Use atomic instructions like LDREX/STREX to safely access variables shared between ISR and threads. Avoid corruption.

Carefully Select Trigger Events

Choose the SysTick reload value appropriately to trigger interrupts at the desired frequency. Balance power and precision.

Handle Nested Interrupts

Code defensively for nested interrupts sharing resources. Use stack for safe variable backup/restore.

Example SysTick Handler Pseudocode

Here is some example pseudocode for a simple yet effective SysTick interrupt handler: void SysTick_Handler() { // Re-enable interrupts ASAP __enable_irq(); // Trigger thread context switch os_ctx_switch_needed = true; // Atomic increment of tick counter tick_count++; // Reload SysTick counter SYST_RVR = RELOAD_VAL; }

This demonstrates some of the design principles discussed:

• Short handler with just 4 lines of executable code.
• Does not call any functions, just straight-line code.
• Shared variable access uses atomic instruction.
• Interrupts re-enabled immediately.
• Consistent timing by avoiding branches.

Typical SysTick Handler Operations

Here are some typical operations that might be performed within the SysTick interrupt handler:

Context Switch Trigger

Set a flag to indicate it’s time for the OS scheduler to perform a thread context switch. This allows running threads at precisely timed slices.

Update System Tick Counter

Increment a global tick counter variable used throughout the system to track passage of time. This may require atomic access.

Reload SysTick Timer

Write the SYST_RVR register to reset the timer to its initial value for the next period. This reloads and restarts the timer.

Set Status Indicators

Toggle LEDs or update diagnostic variables to provide visual feedback of timer operation. For example, an LED can blink each time the timer expires.

Trigger Time-Dependent Tasks

If certain application tasks need to run periodically, trigger or schedule them from the SysTick handler based on elapsed ticks.

Monitor Execution Deadlines

Check if threads have exceeded their execution deadlines and take appropriate error recovery actions as needed.

Optimizing SysTick Handler Performance

To optimize SysTick handler execution time and performance, consider these techniques:

• Inline Assembly: Code the ISR directly in efficient assembly instead of C.
• Target a Specific Core: If available, target a specific low-power core for SysTick handling.
• Tuned Linker Script: Align critical ISR code sections for improved caching.
• Fast GPIO Libraries: Optimize any GPIO operations for fast bit toggling.
• Minimize ISR Operations: Cut unnecessary operations and variables reads/writes.
• Pre-compute Values: Do calculations outside ISR to minimize internal processing.
• Assign ISR Core Affinity: Prevent OS from migrating ISR across cores if applicable.

Testing SysTick Handler Logic

Thoroughly test any custom SysTick interrupt handler code before deployment. Here are some useful techniques:

• Static Analysis: Use lint tools and formal verification to reduce bugs.
• Dynamic Analysis: Insert debug/log statements and runtime instrumentation.
• Simulation Platform: Model system timing behavior with virtual test bench.
• Target Testing: Validate handler operation on real target hardware.
• Stress Testing: Perform tests under heavy simulated load and corner cases.
• Code Coverage: Ensure all branches and logic paths are fully exercised during testing.

Rigorous testing provides confidence in the interrupt handler’s functionality and robustness prior to field deployment.

Debugging Errant SysTick Handlers

Some techniques for debugging problematic SysTick interrupt handling:

• Interrupt Spy: Logic analyzer tracks handler entry/exit timing.
• Stack Overflow Detection: Run-time stack monitoring for collisions.
• Variable Monitoring: Probe shared variables to detect access conflicts.
• Trace Capabilities: Embedded trace modules pinpoint timing issues.
• Core Hang Detection: Watchdogs identify handler lock-ups.
• Baseline Profiling: Compare with known good handler flow.
• Throttled Operation: Slow down system clock to simplify debugging.

With careful design, testing, and debugging methodologies, developers can create optimized SysTick interrupt handlers that maximize system reliability and performance.

Conclusion

The SysTick interrupt handler is a critical component in Cortex-M based systems. Following the design, implementation, and testing best practices described in this article will help developers create robust, efficient SysTick handlers. Key takeaways include minimizing handler code, safeguarding shared resources, managing timing determinism, and thoroughly testing logic. By leveraging the Cortex-M’s SysTick capabilities and crafting high-quality ISR code, developers can build featurerich embedded systems that fully utilize the hardware’s capabilities.