initial setup to use interrupts for I/O

src/main.rs

@@ -1,3 +1,4 @@
+#![feature(abi_avr_interrupt)]
 #![no_std]
 #![no_main]
 
@@ -40,6 +41,20 @@ pub const STACK_DEPTH: usize = 7;
 
 type Calc = StackCalc<f32, STACK_DEPTH, 5, u8>;
 
+// Interrupt driven I/O
+//
+// * Run timer at 100Hz (10ms) to step through segments.
+// * On each segment interrupt configure segments and enable output (LEDs on).
+// * Set another timer to run for 1ms.
+// * On another timer interrupt expire disable display (LEDs off) and handle keyboard input.
+
+#[avr_device::interrupt(atmega328p)]
+fn TIMER1_OVF() {
+    //TODO: Handle timer interrupt to display next segment
+    // file:///home/hxd/projects/avr-calc/target/avr-none/doc/avr_device/interrupt/index.html
+    // ...
+}
+
 struct IOSelect<'p> {
     display_no: usize,
     pin: &'p mut Pin<Output>,
@@ -442,8 +457,15 @@ fn main() -> ! {
         calculator: CalcluclatorState::EnterSignificant,
     };
 
+    // TODO: configure timer, enable it to interrupt: dp.TC1.tifr1
+
+    unsafe {
+        avr_device::interrupt::enable();
+    }
+
     loop {
         let mut last_key_readout: Option<KeyReadout> = None;
+        // Timing: 1ms display, 0.230ms read key; every 12ms/83Hz
         for (mut io_select, ss) in io.iter_mut().zip(display.iter_segments()) {
             ss.apply(&mut seg);
             io_select.set_on();