timestamp: pass overflows to the top and ignore them there

src/bin/lockin-external.rs

@@ -117,8 +117,14 @@ const APP: () = {
         let iir_state = c.resources.iir_state;
         let lockin = c.resources.lockin;
 
+        let t = c
+            .resources
+            .timestamper
+            .latest_timestamp()
+            .unwrap_or_else(|t| t) // Ignore timer capture overflows.
+            .map(|t| t as i32);
         let (pll_phase, pll_frequency) = c.resources.pll.update(
-            c.resources.timestamper.latest_timestamp().map(|t| t as i32),
+            t,
             22, // frequency settling time (log2 counter cycles), TODO: expose
             22, // phase settling time, TODO: expose
         );

src/hardware/digital_input_stamper.rs

@@ -62,15 +62,12 @@ impl InputStamper {
     /// Get the latest timestamp that has occurred.
     ///
     /// # Note
-    /// This function must be called sufficiently often. If an over-capture event occurs, this
-    /// function will panic, as this indicates a timestamp was inadvertently dropped.
-    ///
-    /// To prevent timestamp loss, the batch size and sampling rate must be adjusted such that at
-    /// most one timestamp will occur in each data processing cycle.
+    /// This function must be called at least as often as timestamps arrive.
+    /// If an over-capture event occurs, this function will clear the overflow,
+    /// and return a new timestamp of unknown recency an `Err()`.
+    /// Note that this indicates at least one timestamp was inadvertently dropped.
     #[allow(dead_code)]
-    pub fn latest_timestamp(&mut self) -> Option<u32> {
-        self.capture_channel
-            .latest_capture()
-            .expect("DI0 timestamp overrun")
+    pub fn latest_timestamp(&mut self) -> Result<Option<u32>, Option<u32>> {
+        self.capture_channel.latest_capture()
     }
 }

src/hardware/timers.rs

@@ -281,28 +281,26 @@ macro_rules! timer_channels {
             impl [< Channel $index InputCapture >] {
                 /// Get the latest capture from the channel.
                 #[allow(dead_code)]
-                pub fn latest_capture(&mut self) -> Result<Option<$size>, ()> {
+                pub fn latest_capture(&mut self) -> Result<Option<$size>, Option<$size>> {
                     // Note(unsafe): This channel owns all access to the specific timer channel.
                     // Only atomic operations on completed on the timer registers.
                     let regs = unsafe { &*<$TY>::ptr() };
-                    let sr = regs.sr.read();
 
-                    let result = if sr.[< cc $index if >]().bit_is_set() {
+                    let result = if regs.sr.read().[< cc $index if >]().bit_is_set() {
                         // Read the capture value. Reading the captured value clears the flag in the
                         // status register automatically.
-                        let ccx = regs.[< ccr $index >].read();
-                        Some(ccx.ccr().bits())
+                        Some(regs.[< ccr $index >].read().ccr().bits())
                     } else {
                         None
                     };
 
                     // Read SR again to check for a potential over-capture. If there is an
                     // overcapture, return an error.
-                    if regs.sr.read().[< cc $index of >]().bit_is_clear() {
-                        Ok(result)
-                    } else {
+                    if regs.sr.read().[< cc $index of >]().bit_is_set() {
                         regs.sr.modify(|_, w| w.[< cc $index of >]().clear_bit());
-                        Err(())
+                        Err(result)
+                    } else {
+                        Ok(result)
                     }
                 }