brightness scaling optimization

src/display.rs

@@ -5,7 +5,7 @@ use arduino_hal::{
     port::{mode::Output, Pin},
 };
 
-use crate::DISPLAY_SEGMENTS;
+use crate::{DISPLAY_SEGMENTS, IO_SEGMENT_ON_MAX_US, IO_SEGMENT_ON_MIN_US};
 
 pub struct SegmentPins {
     kd_seg_a: Pin<Output, PD5>,
@@ -96,6 +96,12 @@ impl Brightness {
     pub const fn unwrap(self) -> u8 {
         self.0
     }
+
+    // Scales brightness (0-255) to range between IO_SEGMENT_ON_MIN_US and IO_SEGMENT_ON_MAX_US
+    pub fn scale_brightness(self) -> u32 {
+        // Using >> to avoid 32bit division which take ~576 cycles
+        IO_SEGMENT_ON_MIN_US + ((IO_SEGMENT_ON_MAX_US - IO_SEGMENT_ON_MIN_US) * self.0 as u32 >> 8)
+    }
 }
 
 #[derive(Clone, Copy, PartialEq, Eq)]

src/main.rs

@@ -50,15 +50,6 @@ pub const IO_SEGMENT_RATE_US: u32 = 1000; // Time in μs between segment updates
 pub const IO_SEGMENT_ON_MIN_US: u32 = 80; // How long in μs to hold segment LEDs on (dark)
 pub const IO_SEGMENT_ON_MAX_US: u32 = 700; // How long in μs to hold segment LEDs on (bright)
 
-//TODO: When inline it is much slower due to use of Brightness instead of const 0xFF?!
-#[inline(never)]
-fn scale_brightness(b: Brightness) -> u32 {
-    assert_eq!((Brightness::full().unwrap() as u32), 0xFF);
-    IO_SEGMENT_ON_MIN_US
-        + (IO_SEGMENT_ON_MAX_US - IO_SEGMENT_ON_MIN_US) * b.unwrap() as u32
-            / (Brightness::full().unwrap() as u32)
-}
-
 // Calculator setup
 pub const STACK_DEPTH: usize = 7;
 type Calc = StackCalc<f32, STACK_DEPTH, 5, u8>;
@@ -91,9 +82,12 @@ fn access_global<'cs, 'v: 'cs, T, O>(
 #[avr_device::interrupt(atmega328p)]
 unsafe fn TIMER0_COMPA() {
     avr_device::interrupt::free(|cs| {
-        let brightness = access_global(&IO_LOOP, cs, |io_loop| io_loop.display_on());
-        access_global(&SEGMENT_TIMER, cs, |st| {
-            st.segment_on_time(scale_brightness(brightness))
+        access_global(&IO_LOOP, cs, |io_loop| {
+            access_global(&SEGMENT_TIMER, cs, |st| {
+                let brightness = io_loop.display_on();
+                let elapsed = st.segment_on_time(brightness.scale_brightness());
+                io_loop.frame_data = Some((elapsed, brightness.unwrap()));
+            });
         });
     });
 }
@@ -115,24 +109,28 @@ unsafe fn TIMER0_COMPB() {
 
 pub struct IOLoop {
     index: usize,
+    frame: u16,
     io_pins: IOPins,
     segment_pins: SegmentPins,
     dispaly: DispalyState,
     keyboard: Keyboard,
     readount: Option<KeyReadout>,
     debounce: Debounce,
+    frame_data: Option<(u8, u8)>,
 }
 
 impl IOLoop {
     pub fn new(io_pins: IOPins, segment_pins: SegmentPins, keyboard: Keyboard) -> IOLoop {
         IOLoop {
             index: 0,
+            frame: 0,
             io_pins,
             segment_pins,
             dispaly: Default::default(),
             keyboard,
             readount: None,
             debounce: Default::default(),
+            frame_data: None,
         }
     }
 
@@ -151,6 +149,8 @@ impl IOLoop {
     pub fn advance(&mut self) -> Option<KeyPress> {
         self.index += 1;
         if self.index == self.io_pins.len() || self.index == self.dispaly.len() {
+            // Frame done
+            self.frame = self.frame.wrapping_add(1);
             // Start from first segment
             self.index = 0;
 
@@ -181,6 +181,14 @@ impl IOLoop {
         }
         self.select_off();
     }
+
+    pub fn frame(&self) -> (u16, usize) {
+        (self.frame, self.index)
+    }
+
+    pub fn frame_data(&self) -> Option<(u8, u8)> {
+        self.frame_data
+    }
 }
 
 #[derive(uDebug)]
@@ -526,6 +534,7 @@ fn main() -> ! {
     let mut display = DispalyState::default();
     number_input.show(&mut display);
 
+    let mut frame_data_index: usize = 0;
     let mut calc = Calc::default();
     let mut state = State {
         transient: TransientState::Done,
@@ -557,6 +566,12 @@ fn main() -> ! {
                     state.calculator
                 )
                 .ok();
+                match key {
+                    KeyPress::Num(n) => {
+                        frame_data_index = n as usize;
+                    }
+                    _ => (),
+                }
                 match state.calculator.on_key(key, &mut number_input, &mut calc) {
                     Ok(new_state) => state.calculator = new_state,
                     Err(err) => {
@@ -613,10 +628,14 @@ fn main() -> ! {
             TransientState::Err { .. } => display.error(),
         }
 
-        avr_device::interrupt::free(|cs| {
+        let ((frame, index), frame_data) = avr_device::interrupt::free(|cs| {
             access_global(&IO_LOOP, cs, |io_loop| {
                 io_loop.update_display(&display);
-            });
+                (io_loop.frame(), io_loop.frame_data())
+            })
         });
+        if frame % 64 == 0 && index == frame_data_index {
+            ufmt::uwriteln!(&mut serial, "[{}, {}] {:?}", frame, index, frame_data).ok();
+        }
     }
 }

src/timer.rs

@@ -10,7 +10,9 @@ const fn us_to_ticks(us: u32) -> u32 {
 }
 
 // Timer 0 (8bit)
-pub struct SegmentTimer(TC0);
+pub struct SegmentTimer {
+    timer: TC0,
+}
 
 impl SegmentTimer {
     // Sets up timer to rise interrupts:
@@ -36,19 +38,18 @@ impl SegmentTimer {
         tc0.timsk0
             .write(|w| w.ocie0a().set_bit().ocie0b().set_bit());
 
-        SegmentTimer(tc0)
+        SegmentTimer { timer: tc0 }
     }
 
     // Set for how long the segment LEDs should be on in μs
     // Controls TIMER0_COMPB interrupt time after TIMER0_COMPA
-    pub fn segment_on_time(&mut self, segment_on_us: u32) {
-        let ocrb = us_to_ticks(segment_on_us);
+    pub fn segment_on_time(&mut self, segment_on_us: u32) -> u8 {
+        let delay: u8 = us_to_ticks(segment_on_us)
+            .try_into()
+            .expect("timer init segment_on_us out of rage");
+        let elapsed = self.timer.tcnt0.read().bits();
         // Set the compare value for B match
-        self.0.ocr0b.write(|w| {
-            w.bits(
-                ocrb.try_into()
-                    .expect("timer init segment_on_us out of rage"),
-            )
-        });
+        self.timer.ocr0b.write(|w| w.bits(elapsed + delay));
+        return elapsed;
     }
 }