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| <h1 id="interrupts"><a class="header" href="#interrupts">Interrupts</a></h1> |
| <p>Interrupts differ from exceptions in a variety of ways but their operation and |
| use is largely similar and they are also handled by the same interrupt |
| controller. Whereas exceptions are defined by the Cortex-M architecture, |
| interrupts are always vendor (and often even chip) specific implementations, |
| both in naming and functionality.</p> |
| <p>Interrupts do allow for a lot of flexibility which needs to be accounted for |
| when attempting to use them in an advanced way. We will not cover those uses in |
| this book, however it is a good idea to keep the following in mind:</p> |
| <ul> |
| <li>Interrupts have programmable priorities which determine their handlers' execution order</li> |
| <li>Interrupts can nest and preempt, i.e. execution of an interrupt handler might be interrupted by another higher-priority interrupt</li> |
| <li>In general the reason causing the interrupt to trigger needs to be cleared to prevent re-entering the interrupt handler endlessly</li> |
| </ul> |
| <p>The general initialization steps at runtime are always the same:</p> |
| <ul> |
| <li>Setup the peripheral(s) to generate interrupts requests at the desired occasions</li> |
| <li>Set the desired priority of the interrupt handler in the interrupt controller</li> |
| <li>Enable the interrupt handler in the interrupt controller</li> |
| </ul> |
| <p>Similarly to exceptions, the cortex-m-rt crate exposes an <a href="https://docs.rs/cortex-m-rt-macros/0.1.5/cortex_m_rt_macros/attr.interrupt.html"><code>interrupt</code></a> attribute for declaring interrupt handlers. However, this |
| attribute is only available when the device feature is enabled. That said, this attribute is not intended to be used directly—doing |
| so will result in a compilation error.</p> |
| <p>Instead, you should use the re-exported version of the interrupt attribute provided by the device crate (usually generated using svd2rust). |
| This ensures that the compiler can verify that the interrupt actually exists on the target device. The list of available interrupts—and |
| their position in the interrupt vector table—is typically auto-generated from an SVD file by svd2rust.</p> |
| <pre><code class="language-rust ignore">use lm3s6965::interrupt; // Re-exported attribute from the device crate |
| |
| // Interrupt handler for the Timer2 interrupt |
| #[interrupt] |
| fn TIMER2A() { |
| // .. |
| // Clear reason for the generated interrupt request |
| }</code></pre> |
| <p>Interrupt handlers look like plain functions (except for the lack of arguments) |
| similar to exception handlers. However they can not be called directly by other |
| parts of the firmware due to the special calling conventions. It is however |
| possible to generate interrupt requests in software to trigger a diversion to |
| the interrupt handler.</p> |
| <p>Similar to exception handlers it is also possible to declare <code>static mut</code> |
| variables inside the interrupt handlers for <em>safe</em> state keeping.</p> |
| <pre><code class="language-rust ignore">#[interrupt] |
| fn TIMER2A() { |
| static mut COUNT: u32 = 0; |
| |
| // `COUNT` has type `&mut u32` and it's safe to use |
| *COUNT += 1; |
| }</code></pre> |
| <p>For a more detailed description about the mechanisms demonstrated here please |
| refer to the <a href="./exceptions.html">exceptions section</a>.</p> |
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