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

@@ -50,16 +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?!
-//TODO: Optimize this code, currently it does 2 add, 1 sub, 1 mul and 1 div all on 32bit numbers!
-#[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>;
@@ -95,7 +85,7 @@ unsafe fn TIMER0_COMPA() {
         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(scale_brightness(brightness));
+                let elapsed = st.segment_on_time(brightness.scale_brightness());
                 io_loop.frame_data = Some((elapsed, brightness.unwrap()));
             });
         });
@@ -638,21 +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);
-                let (frame, index) = io_loop.frame();
+                (io_loop.frame(), io_loop.frame_data())
+            })
+        });
         if frame % 64 == 0 && index == frame_data_index {
-                    ufmt::uwriteln!(
-                        &mut serial,
-                        "[{}, {}] {:?}",
-                        frame,
-                        index,
-                        io_loop.frame_data()
-                    )
-                    .ok();
+            ufmt::uwriteln!(&mut serial, "[{}, {}] {:?}", frame, index, frame_data).ok();
         }
-            });
-        });
     }
 }

src/timer.rs

@@ -44,15 +44,12 @@ impl SegmentTimer {
     // 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) -> u8 {
-        let ocrb = us_to_ticks(segment_on_us);
+        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.timer.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;
     }
 }