Fix volume dial backlash

Fixes tonarino/portal#2666

Cargo.toml

@@ -6,6 +6,7 @@ license = "MIT"
 edition = "2018"
 
 [dependencies]
+arrayvec = { version = "0.5.2", default-features = false }
 stm32f4xx-hal = { version = "0.9", features = ["rt", "stm32f411", "usb_fs"] }
 embedded-hal = "0.2"
 cortex-m = "0.7"

src/counter.rs

@@ -1,26 +1,51 @@
-use hal::{prelude::*, qei::Qei, stm32::TIM1};
+use hal::{
+    prelude::*,
+    qei::Qei,
+    stm32::TIM1,
+    timer::{Instant, MonoTimer},
+};
 use stm32f4xx_hal as hal;
 
+const DETENT_RESET_TIMOUT_MS: u32 = 200;
+
 pub struct Counter<PINS> {
     qei: Qei<TIM1, PINS>,
     last_count: u16,
+    timer: MonoTimer,
+    last_update: Instant,
 }
 
 impl<PINS> Counter<PINS> {
-    pub fn new(qei: Qei<TIM1, PINS>) -> Self {
+    pub fn new(qei: Qei<TIM1, PINS>, timer: MonoTimer) -> Self {
         let last_count = qei.count();
-        Counter { qei, last_count }
+        Counter { qei, last_count, last_update: timer.now(), timer }
     }
 
     pub fn poll(&mut self) -> Option<i8> {
         let count = self.qei.count();
         let diff = count.wrapping_sub(self.last_count) as i16;
 
+        // Sometimes it happens that last_count gets out of sync with the physical detents.
+        // We require count to increment by 4 to fire a single tick but when last_count lands in between
+        // detents 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 last_count to current position.
+        if self.last_update.elapsed() > self.timer.frequency().0 * (DETENT_RESET_TIMOUT_MS / 1000) {
+            self.last_count = count;
+        }
+
+        if diff != 0 {
+            self.last_update = self.timer.now();
+        }
         if diff.abs() >= 4 {
             self.last_count = count;
             Some((diff / 4) as i8)
         } else {
             None
         }
     }
+
+    pub fn current_count(&self) -> u16 {
+        self.qei.count()
+    }
 }

src/main.rs

@@ -14,6 +14,8 @@ use crate::{
     rgb_led::{LedStrip, Pulser},
     serial::{Command, Report, SerialProtocol},
 };
+use arrayvec::ArrayString;
+use core::fmt::Write;
 use cortex_m_rt::entry;
 use embedded_hal::digital::v2::OutputPin;
 use hal::{
@@ -184,6 +186,10 @@ fn main() -> ! {
             _ => {},
         }
 
+        let mut buf = ArrayString::<[u8; 256]>::new();
+        write!(&mut buf, "count: {}", counter.current_count()).expect("can write count");
+        protocol.debug(buf.as_str());
+
         if let Some(diff) = counter.poll() {
             if !encoder_button.is_pressed() {
                 protocol.report(Report::DialValue { diff }).unwrap();