Interrupt latency is the delay between the occurrence of an interrupt signal and the start of the corresponding interrupt service routine (ISR) execution. Minimizing interrupt latency is critical in embedded systems, especially in real-time applications where timely responses to events are essential.

Several strategies are used to reduce interrupt latency in firmware:

Prioritize Interrupts: Modern microcontrollers support interrupt priority levels. Assigning higher priority to critical interrupts ensures they are serviced immediately, preempting lower-priority tasks or interrupts.

Efficient ISR Design: Interrupt Service Routines should be as short and efficient as possible. Lengthy ISRs increase the time the processor is unavailable for other tasks or interrupts. Often, ISRs just set flags or buffer data for later processing by the main program, minimizing ISR workload.

Use of Nested Interrupts: Some microcontrollers allow nested interrupts, where a higher priority interrupt can interrupt a currently executing ISR. This reduces latency for urgent interrupts but requires careful design to avoid complexity and stack overflows.

Optimize Context Saving/Restoring: On entering an ISR, the processor must save its current context (registers, program counter, status registers) and restore it on exit. Using hardware features that automatically save minimal required context can reduce latency.

Disable/Enable Interrupts Selectively: Minimizing the time during which interrupts are globally disabled is important. Critical sections should be kept brief, allowing interrupts to be enabled as soon as possible.

Use of Direct Memory Access (DMA): DMA offloads data transfer from the CPU, reducing the frequency and duration of interrupts needed to move data.

Avoid Slow Operations in ISRs: Operations such as floating-point calculations, complex loops, or blocking I/O should be avoided within ISRs to reduce delay.

By carefully prioritizing interrupts, designing short ISRs, leveraging hardware features like nested interrupts, and optimizing context switching, firmware developers can significantly reduce interrupt latency. This ensures embedded systems respond quickly and reliably to external events, which is crucial for real-time performance.

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