Updating timestamp timer to be more precise

src/main.rs

@@ -289,14 +289,21 @@ const APP: () = {
         let sampling_timer_channels = sampling_timer.channels();
 
         let mut timestamp_timer = {
-            // TODO: This needs to be precisely controlled via the prescaler of the timer.
-            let timer5 = dp.TIM5.timer(
-                (SAMPLE_FREQUENCY_KHZ / SAMPLE_BUFFER_SIZE as u32).khz(),
-                ccdr.peripheral.TIM5,
-                &ccdr.clocks,
+            // The timer frequency is manually adjusted below, so the 1KHz setting here is a
+            // dont-care.
+            let timer5 =
+                dp.TIM5.timer(1.khz(), ccdr.peripheral.TIM5, &ccdr.clocks);
+
+            // The time stamp timer must run at exactly a multiple of the sample timer based on the
+            // batch size. To accomodate this, we manually set the period identical to the sample
+            // timer, but use a prescaler that is `BATCH_SIZE` longer.
+            let mut timer = timers::TimestampTimer::new(timer5);
+            timer.set_period(sampling_timer.get_period());
+            timer.set_prescaler(
+                sampling_timer.get_prescaler() * SAMPLE_BUFFER_SIZE as u16,
             );
 
-            timers::TimestampTimer::new(timer5)
+            timer
         };
 
         let timestamp_timer_channels = timestamp_timer.channels();

src/timers.rs

@@ -32,8 +32,36 @@ macro_rules! timer_channels {
                     self.channels.take().unwrap()
                 }
 
-                /// Start the sampling timer.
-                pub fn start(&mut self) {
+                #[allow(dead_code)]
+                pub fn get_prescaler(&self) -> u16 {
+                    let regs = unsafe { &*hal::stm32::$TY::ptr() };
+                    regs.psc.read().psc().bits() + 1
+                }
+
+                #[allow(dead_code)]
+                pub fn set_prescaler(&mut self, prescaler: u16) {
+                    let regs = unsafe { &*hal::stm32::$TY::ptr() };
+                    assert!(prescaler >= 1);
+                    regs.psc.write(|w| w.psc().bits(prescaler - 1));
+                }
+
+                #[allow(dead_code)]
+                pub fn get_period(&self) -> u32 {
+                    let regs = unsafe { &*hal::stm32::$TY::ptr() };
+                    regs.arr.read().arr().bits()
+                }
+
+                #[allow(dead_code)]
+                pub fn set_period(&mut self, period: u32) {
+                    let regs = unsafe { &*hal::stm32::$TY::ptr() };
+                    regs.arr.write(|w| w.arr().bits(period));
+                }
+
+                /// Start the timer.
+                pub fn start(mut self) {
+                    // Force a refresh of the frequency settings.
+                    self.timer.apply_freq();
+
                     self.timer.reset_counter();
                     self.timer.resume();
                 }