March 27, 2023


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Programming embedded programs: How interrupts perform in ARM Cortex-M

3 min read

On most embedded CPUs, ISRs simply cannot be coded as normal C capabilities, but ARM Cortex-M makes it possible for you to do just that. In this lesson, you will see how Cortex-M pulls that off.

In the final lesson, you discovered that Interrupt Support Routines (ISRs) are asynchronous, whilst common C features are synchronous. For that reason, on most embedded CPUs, this sort of as MSP430 in the previous lesson, ISRs are not able to be coded as typical C features. But ARM Cortex-M will allow you to do just that. In the video for lesson 18, you are going to see how Cortex-M pulls that off.

Lesson 18 – How interrupts work on ARM Cortex-M

ISR Entry

The designers of the Cortex-M CPU certainly could not adjust the fundamental asynchronous mother nature of ISRs. But they could conceal the variations concerning ISRs and regular functions in hardware.

The initial portion of the alternative is preserving additional CPU registers. As demonstrated in the video clip, all through interrupt entry, the Cortex-M hardware saves all the registers that a regular purpose can potentially clobber according to the ARM Application Process Connect with Common (AAPCS) [1], which you figured out back again in lesson 9. Because the registers saved in components complement the CPU registers that a typical perform must maintain (according to the AAPCs), all CPU registers are preserved throughout interrupt processing.

ISR Return

The return from ISR is a far more tricky difficulty. As you recall from the final lesson, ISRs on the conventional MSP430 CPU must use a special RETI (return from interrupt) instruction instead of the standard RET instruction, which helps make them incompatible with frequent capabilities. But on ARM Cortex-M, ISRs need to use the identical (generally BX LR) return as regular capabilities.

Yet again, the alternative is in hardware. As shown in the video, the Cortex-M interrupt entry masses the LR website link register with a special benefit, these types of as 0xFFFF’FFF9, as a substitute the precise return handle. Later, when the ISR returns (e.g., by way of BX LR), the components acknowledges the unique LR value as an interrupt return and restores the CPU registers saved throughout the interrupt entry.

Conclusion Notes

Of training course, there are many far more nuances of the components interrupt entry and exit implementation on ARM Cortex-M, these types of as many CPU modes, stack alignment, the FPU (Floating Position Unit), and so on. Some of these are illustrated in the movie.

But the most significant takeaway is that the ARM Cortex-M CPU has been meticulously created to make ISRs programmable in common C, without any non-transportable extensions (like #pragma, __attribute__, and so on.) In actuality, it is possible to construct whole, very responsive, authentic-time purposes consisting predominantly of ISRs in pure C. What’s more, the Nested Vectored Interrupt Controller (NVIC, pointed out in the online video), a standard ingredient of all Cortex-M CPUs, can prioritize ISRs. This can be leveraged to develop a preemptive, precedence-dependent actual-time kernel applied solely in components [2].

[1] Joseph Yiu, “The Definitive Guidebook to ARM Cortex-M3 and Cortex-M4 Processors, 3rd Edition”, ISBN:‎ 978-0124080829

[2] GitHub: Super-Easy Tasker – hardware RTOS for ARM Cortex-M

Dr. Miro M. Samek is the creator of the open supply QP actual-time embedded frameworks and the freeware QM graphical product-dependent layout resource. He is also the founder and CEO of Quantum Leaps — the company of present day embedded software package based on energetic objects and hierarchical point out machines as effectively as tools for visible modeling, automatic code era, and unit tests of deeply embedded computer software. Miro teaches the well known YouTube “Modern Embedded Devices Programming” movie program on which this article series is dependent.

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