fw-rust: Create a PWM signal for the backlight

To have a dimmable backlight, generate a variable duty-cycle PWM
signal which is consumed by the backlight driver circuit.

firmware/rust/src/main.rs

@@ -1,20 +1,39 @@
 #![no_std]
 #![no_main]
 
-use atmega_hal::{clock::MHz8, delay::Delay, pins, Peripherals};
-use embedded_hal::blocking::delay::DelayMs;
+use atmega_hal::{pins, Peripherals};
+use avr_device::interrupt;
 use panic_halt as _;
 
 #[atmega_hal::entry]
 fn main() -> ! {
+    // Disable interrupts when initializing
+    interrupt::disable();
+
+    // Get peripherals and pins
     let dp = Peripherals::take().unwrap();
     let pins = pins!(dp);
 
-    let mut led = pins.pd5.into_output();
-    let mut delay = Delay::<MHz8>::new();
+    // Init display backlight
+    let tc0 = dp.TC0;
+    tc0.tccr0a.write(|w| {
+        w.wgm0().pwm_fast();
+        w.com0b().match_clear();
+        w
+    });
+    tc0.tccr0b.write(|w| {
+        w.wgm02().set_bit();
+        w.cs0().prescale_64();
+        w
+    });
+    tc0.ocr0a.write(|w| unsafe { w.bits(255) });
+    // TODO: Use EEPROM
+    tc0.ocr0b.write(|w| unsafe { w.bits(0) });
+    pins.pd5.into_output();
 
-    loop {
-        led.toggle();
-        delay.delay_ms(255_u8);
-    }
+    // Enable interrupts
+    unsafe { interrupt::enable() };
+
+    #[allow(clippy::empty_loop)]
+    loop {}
 }