Document workaround for registering exceptions

Given the way this package abuses cortex-m-rt, it's not immediately
obvious how to register a Cortex-M exception handler with the
`#[exception]` macro. This commit documents and demonstrates the
workaround I use.

1 files changed, 23 insertions, 1 deletions

diff --git a/examples/blink-blocking.rs b/examples/blink-blocking.rs
index a3a71b9..1ae62b4 100644
--- a/examples/blink-blocking.rs
+++ b/examples/blink-blocking.rs
@@ -1,7 +1,10 @@
 //! Slowly blink an LED while blocking on a timer.
 //!
 //! Use this as the minimum-viable runtime support. You don't
-//! need interrupts for this example.
+//! need MCU-specific interrupts for this example.
+//!
+//! This example demonstrates how to register an exception
+//! handler. See the API documentation for more information.
 
 #![no_std]
 #![no_main]
@@ -16,3 +19,22 @@ fn main() -> ! {
         pit.blocking_delay();
     }
 }
+
+use imxrt_rt::exception;
+
+#[exception]
+unsafe fn DefaultHandler(_irqn: i16) {
+    uh_oh()
+}
+
+#[exception]
+unsafe fn HardFault(_: &imxrt_rt::ExceptionFrame) -> ! {
+    uh_oh()
+}
+
+#[inline(never)]
+fn uh_oh() -> ! {
+    loop {
+        core::sync::atomic::fence(core::sync::atomic::Ordering::SeqCst)
+    }
+}