How to Fix Improper Interrupt Prioritization: Step-by-Step Guide

Analyze Current Interrupt Configuration

• Review the existing interrupt handling mechanism in your firmware. Examine the interrupt vector table and ensure that all interrupts and exceptions are correctly mapped.
• Check the configuration of the microcontroller/processor, focusing on the interrupt levels and priority registers.
• Utilize debugger tools to pause execution at various interrupt priority levels, observing order and precedence. Confirm that priority levels correlate with the system design.

Identify Prioritization Issues

• Examine symptoms such as unresponsive hardware or tasks, missed data, and overrunning interrupts, which could indicate improper prioritization.
• Investigate specific cases where low-priority interrupts preempt more critical tasks. This can be observed using tracing tools and logging mechanisms in your development environment.
• Check for priority inversion situations, where higher priority interrupts wait indefinitely for lower priority tasks.

Refactor Interrupt Service Routines (ISRs)

• Ensure your ISRs are designed to execute quickly and perform minimal processing. Offload complex logic to lower priority tasks or deferred work using task schedulers or event flags.
• Refactor lengthy code within ISRs, utilizing flags or semaphore mechanisms that allow main tasks or background tasks to handle in-depth processing.
• Consider re-architecting interrupt flows by using techniques like nested interrupts, where supported, to handle high-priority tasks immediately.

Adjust Interrupt Priorities

• Map out required priority levels for each interrupt according to the system's real-time requirements. This involves distinguishing between critical and non-critical tasks.
• Update hardware-specific priority registers, which may involve modifying specific bits in register addresses based on your MCU documentation.
• Use interrupt controller libraries provided by your vendor (e.g., the ARM CMSIS library) to configure interrupt priorities in a higher level and more readable fashion. Example in C:

  ```c
  NVIC_SetPriority(USART1_IRQn, 1); // Set USART1 interrupt priority to 1
  NVIC_SetPriority(TIM2_IRQn, 2); // Set Timer 2 interrupt priority to 2
  ```

Test and Validate Changes

• Conduct thorough testing using simulations of interrupt storms, edge cases, or peak load conditions to verify proper interrupt prioritization.
• Utilize hardware debuggers or logic analyzers to measure interrupt latency and ensure that high-priority interrupts are serviced as expected.
• Review performance metrics and confirm that all critical tasks meet their timing requirements without interruption overruns or missed deadlines.

Implement a Priority Handling Strategy

• Incorporate a dynamic priority adjustment mechanism, if applicable, to handle varying system loads where interrupt priorities might need real-time adjustments.
• Develop a priority ceiling protocol or similar strategy to prevent priority inversion in more sophisticated systems.
• Consider implementing a watchdog for ISR latency, which can trigger system alerts if certain timing thresholds are exceeded.