In this lesson, you will learn how to reduce system latency by allowing your microcontroller to skip unnecessary work when multiple interrupts trigger in rapid succession.
The Problem: Context Switching Overhead
When an interrupt occurs, the processor must save the current state (registers, program counter, status flags) to the stack before jumping to your Interrupt Service Routine (ISR). Once the ISR finishes, it must restore that exact same state.
If a new interrupt arrives just as your current ISR is finishing, the processor would normally "pop" all the saved registers back into the CPU, only to immediately "push" them back onto the stack to handle the next interrupt. This "pop-then-push" dance wastes precious clock cycles, adding jitter and increasing latency for the second interrupt.
The Solution: Tail-Chaining
Modern processors—specifically ARM Cortex-M cores—feature a hardware optimization called Tail-Chaining.
If the processor detects that another interrupt is pending while it is in the process of finishing an ISR, it realizes that restoring the context is a waste of time. Instead of returning to your main code, it "chains" directly into the next ISR. It keeps the CPU registers in their current state and simply jumps to the next handler.
Enabling the Optimization
Tail-chaining is an architectural feature of the Nested Vectored Interrupt Controller (NVIC). In most Cortex-M implementations, this is enabled by default because it is strictly a performance enhancement. You don't need to write custom assembly or complex logic to trigger it; the hardware handles it automatically as long as:
- The next pending interrupt has a high enough priority to preempt the completion of the current one.
- The interrupts are configured within the NVIC.
By keeping your ISRs short and efficient, you maximize the chance for the hardware to perform this optimization, keeping your system responsive even under high interrupt loads. If you see your interrupt latency drop during back-to-back events, you are witnessing the hardware skipping the "restore" and "save" cycles automatically.
Key Takeaway: Interrupt tail-chaining reduces latency by allowing the processor to jump directly from one pending ISR to the next, bypassing the redundant overhead of restoring and then re-saving the processor context.