Fix dial backlash

Fixes tonarino/portal#2666

src/counter.rs

@@ -1,26 +1,64 @@
-use hal::{prelude::*, qei::Qei, stm32::TIM1};
+use hal::{
+    prelude::*,
+    qei::Qei,
+    stm32::TIM1,
+    timer::{Instant, MonoTimer},
+};
 use stm32f4xx_hal as hal;
 
-pub struct Counter<PINS> {
+const DETENT_RESET_TIMOUT_MS: u32 = 200;
+
+pub struct Counter<'a, PINS> {
     qei: Qei<TIM1, PINS>,
+    // Cache for the count returned by qei for detecting changes
     last_count: u16,
+    timer: &'a MonoTimer,
+    last_update: Instant,
+    diff_accumulator: i16,
 }
 
-impl<PINS> Counter<PINS> {
-    pub fn new(qei: Qei<TIM1, PINS>) -> Self {
+impl<'a, PINS> Counter<'a, PINS> {
+    pub fn new(qei: Qei<TIM1, PINS>, timer: &'a MonoTimer) -> Self {
         let last_count = qei.count();
-        Counter { qei, last_count }
+        Counter { qei, last_count, last_update: timer.now(), timer, diff_accumulator: 0 }
     }
 
     pub fn poll(&mut self) -> Option<i8> {
         let count = self.qei.count();
-        let diff = count.wrapping_sub(self.last_count) as i16;
+
+        let diff = self.update_counts(count);
 
         if diff.abs() >= 4 {
-            self.last_count = count;
+            self.diff_accumulator = 0;
             Some((diff / 4) as i8)
         } else {
             None
         }
     }
+
+    // Sometimes the accumulator gets out of sync with the physical detents. We require count to
+    // increment by 4 to fire a single tick but when the zero of accumulator lands in between
+    // detents (because of noise) we get backlash or missed ticks.
+    // Assume that when the dial rests for more than DETENT_RESET_TIMOUT_MS it is aligned to a detent
+    // and reset the accumulator.
+    fn update_counts(&mut self, count: u16) -> i16 {
+        if count != self.last_count {
+            self.last_update = self.timer.now();
+        }
+
+        if self.accumulator_timed_out() {
+            self.diff_accumulator = 0;
+        }
+
+        self.diff_accumulator += self.last_count.wrapping_sub(count) as i16;
+        self.last_count = count;
+
+        self.diff_accumulator
+    }
+
+    fn accumulator_timed_out(&self) -> bool {
+        let timeout_ticks =
+            self.timer.frequency().0 as f32 * (DETENT_RESET_TIMOUT_MS as f32 / 1000.0);
+        self.last_update.elapsed() > timeout_ticks as u32
+    }
 }

src/main.rs

@@ -118,7 +118,7 @@ fn main() -> ! {
     let rotary_encoder_pins = (gpioa.pa8.into_alternate_af1(), gpioa.pa9.into_alternate_af1());
     let rotary_encoder = Qei::new(rotary_encoder_timer, rotary_encoder_pins);
 
-    let mut counter = Counter::new(rotary_encoder);
+    let mut counter = Counter::new(rotary_encoder, &timer);
 
     let button_pin = gpioa.pa10.into_pull_up_input();
     let debounced_encoder_pin = Debouncer::new(button_pin, Active::Low, 30, 3000);