Merge #238

238: Port MonoTimer from stm32f1xx-hal r=therealprof a=bschwind

This is a straight-up copy of the `MonoTimer` from the `stm32f1xx-hal` crate:

https://github.com/stm32-rs/stm32f1xx-hal/blob/d976ba2312fae2f1b15707573bc5d82ea00ba73f/src/time.rs#L229-L278

I wanted the same functionality in a stm32f4-based project. I tested it on an `stm32f411CE` [WeAct USB C board](https://stm32-base.org/boards/STM32F401CEU6-WeAct-Black-Pill-V3.0), and it seems to behave properly.

Co-authored-by: Brian Schwind <[email protected]>
Co-authored-by: Daniel Egger <[email protected]>

Author: 3 people

CHANGELOG.md

@@ -37,6 +37,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Add initial DMA support
 - Allow specification of ADC reference voltage in ADC configuraton structure
 - Added support for hardware-based CRC32 functionality
+- Add `MonoTimer` and `Instant` structs for basic time measurement.
 
 ### Fixed
 

src/timer.rs

@@ -2,7 +2,7 @@
 
 use cast::{u16, u32};
 use cortex_m::peripheral::syst::SystClkSource;
-use cortex_m::peripheral::SYST;
+use cortex_m::peripheral::{DCB, DWT, SYST};
 use embedded_hal::timer::{Cancel, CountDown, Periodic};
 use void::Void;
 
@@ -170,6 +170,57 @@ impl Cancel for Timer<SYST> {
 
 impl Periodic for Timer<SYST> {}
 
+/// A monotonic non-decreasing timer
+///
+/// This uses the timer in the debug watch trace peripheral. This means, that if the
+/// core is stopped, the timer does not count up. This may be relevant if you are using
+/// cortex_m_semihosting::hprintln for debugging in which case the timer will be stopped
+/// while printing
+#[derive(Clone, Copy)]
+pub struct MonoTimer {
+    frequency: Hertz,
+}
+
+impl MonoTimer {
+    /// Creates a new `Monotonic` timer
+    pub fn new(mut dwt: DWT, mut dcb: DCB, clocks: Clocks) -> Self {
+        dcb.enable_trace();
+        dwt.enable_cycle_counter();
+
+        // now the CYCCNT counter can't be stopped or reset
+        drop(dwt);
+
+        MonoTimer {
+            frequency: clocks.hclk(),
+        }
+    }
+
+    /// Returns the frequency at which the monotonic timer is operating at
+    pub fn frequency(self) -> Hertz {
+        self.frequency
+    }
+
+    /// Returns an `Instant` corresponding to "now"
+    pub fn now(self) -> Instant {
+        Instant {
+            now: DWT::get_cycle_count(),
+        }
+    }
+}
+
+/// A measurement of a monotonically non-decreasing clock
+#[derive(Clone, Copy)]
+pub struct Instant {
+    now: u32,
+}
+
+impl Instant {
+    /// Ticks elapsed since the `Instant` was created
+    pub fn elapsed(self) -> u32 {
+        DWT::get_cycle_count().wrapping_sub(self.now)
+    }
+}
+
 macro_rules! hal {
     ($($TIM:ident: ($tim:ident, $en_bit:expr, $reset_bit:expr, $apbenr:ident, $apbrstr:ident, $pclk:ident, $ppre:ident),)+) => {
         $(