Reducing Interrupt Latency and Ensuring Memory Safety in Embedded C

The Problem: Race Conditions and Jitter in Hardware Events

When a microcontroller handles a hardware event via an Interrupt Service Routine (ISR), the CPU pauses the main program to execute a specific function. If the ISR takes too long to execute, it creates jitter—unpredictable delays in other system tasks. More critically, if the main loop and the ISR access the same variable, the compiler may optimize that variable into a CPU register, causing the main loop to miss updates from the hardware, or a multi-byte write may be interrupted halfway, leading to data corruption.

Prerequisites for Safe ISR Implementation

• A C compiler supporting volatile qualifiers and interrupt attributes (e.g., GCC for ARM or AVR).
• Hardware documentation specifying the interrupt vector table and priority levels.
• A debugger or logic analyzer for measuring execution timing.

Implementing a Low-Latency ISR Pattern

To maintain system stability, use a "Top-Half/Bottom-Half" architecture. The Top-Half (the ISR) performs only the absolute minimum work required to acknowledge the hardware, while the Bottom-Half (the main loop) handles the data processing.

Step 1: Declare Shared Variables as Volatile

Any variable modified inside an ISR and read in the main loop must be marked volatile. This tells the compiler that the value can change unexpectedly, forcing it to read from RAM every time rather than using a cached register value.

// Global shared flag
volatile bool event_pending = false;
volatile uint32_t sensor_reading = 0;

Step 2: Define the ISR with Compiler Attributes

Standard functions use a RET (return) instruction. ISRs require a specialized return instruction (like IRET or RETI) to restore the CPU state. Use the appropriate attribute for your toolchain.

// Example for GCC-based embedded compilers
void __attribute__((interrupt)) Timer_ISR_Handler(void) {
    // Top-Half: Minimal work
    sensor_reading = READ_HARDWARE_REG();
    event_pending = true;
    
    // Clear the hardware interrupt flag to prevent re-triggering
    CLEAR_INTERRUPT_FLAG();
}

Step 3: Implement Atomic Access in the Main Loop

If a shared variable is larger than the CPU's native word size (e.g., a 32-bit integer on an 8-bit MCU), the main loop could be interrupted while reading only half of the value. You must wrap these reads in an atomic block by temporarily disabling interrupts.

void main_loop(void) {
    while (1) {
        uint32_t local_copy;

        // Critical Section: Disable interrupts to prevent race conditions
        disable_global_interrupts(); 
        if (event_pending) {
            local_copy = sensor_reading;
            event_pending = false;
        }
        enable_global_interrupts();

        if (local_copy > 0) {
            process_data(local_copy); // Bottom-Half: Heavy processing here
        }
    }
}

Comparison: ISR Best Practices vs. Common Pitfalls

Action	Recommended (Safe)	Avoid (Risky)	Reason
Function Calls	Simple flag sets, register reads	printf(), malloc()	Non-reentrant functions cause deadlocks.
Math	Integer arithmetic	Floating-point (float/double)	FPU register saving increases latency.
Execution Time	Microseconds (Top-Half)	Milliseconds (Blocking loops)	Causes missed interrupts and system jitter.

Verification and Diagnostics

To verify the implementation, perform the following checks:

1. Latency Measurement: Toggle a GPIO pin high at the very start of the ISR and low at the end. Use an oscilloscope to measure the pulse width; this is your actual ISR execution time.
2. Stress Testing: Trigger the interrupt at the maximum rated hardware frequency. If the system crashes, check for stack overflow caused by nested interrupts (where a high-priority interrupt interrupts a lower-priority ISR).
3. Memory Check: Use a debugger to watch the volatile variable. Ensure it updates in real-time even when the main loop is performing a heavy calculation.

Rollback and Recovery

If the system becomes unstable after enabling interrupts:

• Disable the specific interrupt source in the hardware configuration register.
• Revert shared variables to non-volatile only if they are no longer accessed by the ISR.
• Increase the stack size in the linker script if nested interrupts are causing memory corruption.