lowpass: i32, no multiplies

dsp/src/lockin.rs

@@ -4,12 +4,12 @@ use generic_array::typenum::U3;
 #[derive(Clone, Default)]
 pub struct Lockin {
     state: [Lowpass<U3>; 2],
-    k: u32,
+    k: u8,
 }
 
 impl Lockin {
     /// Create a new Lockin with given IIR coefficients.
-    pub fn new(k: u32) -> Self {
+    pub fn new(k: u8) -> Self {
         let lp = Lowpass::default();
         Self {
             state: [lp.clone(), lp.clone()],

dsp/src/lowpass.rs

@@ -15,27 +15,19 @@ impl<N: ArrayLength<i32>> Lowpass<N> {
     ///
     /// # Args
     /// * `x`: Input data
-    /// * `k`: Cutoff, `u32::MAX` being Nyquist
+    /// * `k`: Log2 time constant
     ///
-    /// # Returns
+    /// # Return
     /// Filtered output y
-    pub fn update(&mut self, x: i32, k: u32) -> i32 {
-        let mut x1 = self.xy[0];
-        self.xy[0] = x;
-        let mut x0 = x;
-
+    pub fn update(&mut self, x: i32, k: u8) -> i32 {
         // This is an unrolled and optimized first-order IIR loop
         // that works for all possible time constants.
+        // Note the zero(s) at Nyquist and the benign overflow (DF-I).
+        let mut x0 = x;
+        let mut x1 = self.xy[0];
+        self.xy[0] = x;
         for y1 in self.xy[1..].iter_mut() {
-            // Optimized first order lowpass expression
-            // Note the zero at Nyquist
-            let mut y0 =
-                ((x0 >> 1) as i64 + (x1 >> 1) as i64 - *y1 as i64) * k as i64;
-            y0 += (*y1 as i64) << 32;
-            y0 += 1i64 << 31; // Half-up rounding bias
-
-            // Store and advance
-            x0 = (y0 >> 32) as i32;
+            x0 = *y1 + (((x0 >> 1) + (x1 >> 1) - *y1 + (1 << k - 1)) >> k);
             x1 = *y1;
             *y1 = x0;
         }

src/bin/lockin-external.rs

@@ -34,7 +34,7 @@ const APP: () = {
                 + design_parameters::SAMPLE_BUFFER_SIZE_LOG2,
         );
 
-        let lockin = Lockin::new(1 << 22);
+        let lockin = Lockin::new(10);
 
         // Enable ADC/DAC events
         stabilizer.adcs.0.start();

src/bin/lockin-internal.rs

@@ -28,7 +28,7 @@ const APP: () = {
         // Configure the microcontroller
         let (mut stabilizer, _pounder) = hardware::setup(c.core, c.device);
 
-        let lockin = Lockin::new(1 << 22); // TODO: expose
+        let lockin = Lockin::new(10); // TODO: expose
 
         // Enable ADC/DAC events
         stabilizer.adcs.1.start();