rpll: auto-align counter

dsp/src/rpll.rs

@@ -18,14 +18,12 @@ impl RPLL {
     ///
     /// Args:
     /// * dt2: inverse update() rate. 1 << dt2 is the counter rate to update() rate ratio.
-    /// * t: Counter time. Counter value at the first update() call. Typically 0.
     ///
     /// Returns:
     /// Initialized RPLL instance.
-    pub fn new(dt2: u8, t: i32) -> RPLL {
+    pub fn new(dt2: u8) -> RPLL {
         RPLL {
             dt2,
-            t,
             ..Default::default()
         }
     }
@@ -69,7 +67,19 @@ impl RPLL {
             // Update frequency lock
             self.ff = self.ff.wrapping_add(p_ref.wrapping_sub(p_sig));
             // Time in counter cycles between timestamp and "now"
-            let dt = self.t.wrapping_sub(x);
+            let mut dt = self.t.wrapping_sub(x);
+
+            if dt < 0 {
+                // Timestamp is in the future
+                // Advance phase and time until we are just past the most recent
+                // timestamp.
+                let mut dt_ceil = dt >> self.dt2;
+                self.y = self.y.wrapping_sub(self.f.wrapping_mul(dt_ceil));
+                dt_ceil <<= self.dt2;
+                self.t = self.t.wrapping_sub(dt_ceil);
+                dt = dt.wrapping_sub(dt_ceil);
+            }
+
             // Reference phase estimate "now"
             let y_ref = (self.f >> self.dt2).wrapping_mul(dt);
             // Phase error

src/bin/lockin.rs

@@ -53,7 +53,7 @@ const APP: () = {
         // Configure the microcontroller
         let (mut stabilizer, _pounder) = hardware::setup(c.core, c.device);
 
-        let pll = RPLL::new(ADC_SAMPLE_TICKS_LOG2 + SAMPLE_BUFFER_SIZE_LOG2, 0);
+        let pll = RPLL::new(ADC_SAMPLE_TICKS_LOG2 + SAMPLE_BUFFER_SIZE_LOG2);
 
         let lockin = Lockin::new(
             &iir_int::IIRState::lowpass(1e-3, 0.707, 2.), // TODO: expose