How to optimize MCU interrupt response time?

 Optimizing MCU interrupt response time is crucial for real-time performance and accurate signal processing. Below are practical, layered strategies to reduce latency and improve the efficiency of your interrupt handling:

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 1. Minimize Interrupt Latency (Time Until ISR Starts)

 What causes latency?

• Instruction execution delay

• Stack saving (context switch)

• Nested interrupts disabled

• Peripheral delay (e.g., slow flag clearing)

 Optimization Techniques:

Tip	Description
Enable fast interrupt mode (if supported)	Some MCUs (e.g. ARM Cortex-M3+) support tail-chaining or fast ISR entry
Use high-priority interrupts	Assign highest priority to critical ISRs
Avoid global interrupt disable (__disable_irq())	Keep interrupts globally enabled as much as possible
Use vectorized interrupt handling	Avoid shared interrupt vectors; direct vector → faster
Place ISR in RAM (if allowed)	Running code from RAM can be faster than Flash on some MCUs

 2. Write Efficient ISRs (Interrupt Service Routines)

 Keep It Short and Fast:

• Avoid blocking operations (e.g., delays, loops)

• Avoid memory allocation or printf

• Just set flags or buffers, do the real work in the main loop or RTOS task

Example:

c

volatile bool adc_ready = false;

void ADC_IRQHandler(void) {
    adc_ready = true; // fast!
    ADC->SR &= ~ADC_SR_EOC; // Clear interrupt flag
}

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 3. Use Priority and Preemption Properly

• Use nested interrupt capability if the MCU supports it (e.g., Cortex-M with NVIC)

• Assign lower priority to non-critical ISRs

• Configure preemptive priority levels carefully to avoid blocking time-sensitive tasks

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 4. Peripheral-Specific Optimizations

Peripheral	Optimization
UART	Use DMA for RX/TX instead of byte-wise interrupts
ADC	Enable DMA+interrupt on completion for fast sampling
Timer	Use one-shot timers for precise timeouts, and minimize overhead
GPIO interrupt	Use edge-triggered rather than level-triggered to avoid extra CPU cycles

 5. Use DMA to Offload Work

• DMA lets you transfer data between peripherals and memory without CPU

• Ideal for SPI, UART, ADC, etc.

• Interrupts only when the DMA completes, not per byte → fewer interrupts → faster response elsewhere

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 6. Measure & Profile Interrupt Latency

Use oscilloscope or logic analyzer to:

• Toggle a GPIO at ISR entry and exit

• Measure latency and execution time

• Refine code where needed

c

void EXTI0_IRQHandler(void) {
    GPIO_SetBit(GPIOB, GPIO_PIN_1); // Measure start
    ...
    GPIO_ResetBit(GPIOB, GPIO_PIN_1); // Measure end
}

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 Summary Table: Optimization Checklist

Goal	Techniques
Reduce ISR entry latency	Fast mode, priority, avoid global disable
Make ISR code faster	No delays, just flags, keep minimal
Allow faster critical ISRs	Use nested interrupts with correct priorities
Offload routine data movement	Use DMA
Reduce number of interrupts	Batch data or use DMA
Measure and test	Use GPIO toggle profiling