Idle: Switch to NOP instead of WFI

Add example how to get old WFI behaviour

examples/idle-wfi.rs

@@ -0,0 +1,47 @@
+//! examples/idle-wfi.rs
+
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+use panic_semihosting as _;
+
+#[rtic::app(device = lm3s6965)]
+mod app {
+    use cortex_m_semihosting::{debug, hprintln};
+
+    #[shared]
+    struct Shared {}
+
+    #[local]
+    struct Local {}
+
+    #[init]
+    fn init(mut cx: init::Context) -> (Shared, Local, init::Monotonics) {
+        hprintln!("init").unwrap();
+
+        // Set the ARM SLEEPONEXIT bit to go to sleep after handling interrupts
+        // See https://developer.arm.com/docs/100737/0100/power-management/sleep-mode/sleep-on-exit-bit
+        cx.core.SCB.set_sleepdeep();
+
+        (Shared {}, Local {}, init::Monotonics())
+    }
+
+    #[idle(local = [x: u32 = 0])]
+    fn idle(cx: idle::Context) -> ! {
+        // Locals in idle have lifetime 'static
+        let _x: &'static mut u32 = cx.local.x;
+
+        hprintln!("idle").unwrap();
+
+        debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
+
+        loop {
+            // Now Wait For Interrupt is used instead of a busy-wait loop
+            // to allow MCU to sleep between interrupts
+            // https://developer.arm.com/documentation/ddi0406/c/Application-Level-Architecture/Instruction-Details/Alphabetical-list-of-instructions/WFI
+            rtic::export::wfi()
+        }
+    }
+}