don't own DWT

This is fine as long as the user does not change or disable the cycle
counter. It allows the other DWT features to be used freely by the
application.

src/lib.rs

@@ -18,10 +18,11 @@ use rtic_monotonic::Monotonic;
 /// Note that the SysTick interrupt must not be disabled longer than half the
 /// cycle counter overflow period (typically a couple seconds).
 ///
+/// Note(Safety): Do not disable/enable or set/reset the DWT cycle counter.
+///
 /// When the `extend` feature is enabled, the cycle counter width is extended to
 /// `u64` by detecting and counting overflows.
 pub struct DwtSystick<const TIMER_HZ: u32> {
-    dwt: DWT,
     systick: SYST,
     #[cfg(feature = "extend")]
     last: u64,
@@ -34,7 +35,7 @@ impl<const TIMER_HZ: u32> DwtSystick<TIMER_HZ> {
     /// so the speed calculated at runtime and the declared speed (generic parameter
     /// `TIMER_HZ`) can be compared.
     #[inline(always)]
-    pub fn new(dcb: &mut DCB, mut dwt: DWT, mut systick: SYST, sysclk: u32) -> Self {
+    pub fn new(dcb: &mut DCB, dwt: &mut DWT, mut systick: SYST, sysclk: u32) -> Self {
         assert!(TIMER_HZ == sysclk);
 
         dcb.enable_trace();
@@ -50,7 +51,6 @@ impl<const TIMER_HZ: u32> DwtSystick<TIMER_HZ> {
         // We do not start the counters here but in `reset()`.
 
         DwtSystick {
-            dwt,
             systick,
             #[cfg(feature = "extend")]
             last: 0,
@@ -98,8 +98,8 @@ impl<const TIMER_HZ: u32> Monotonic for DwtSystick<TIMER_HZ> {
         self.systick.enable_counter();
 
         // Enable and reset the cycle counter to locate the epoch.
-        self.dwt.enable_cycle_counter();
-        self.dwt.set_cycle_count(0);
+        (*DWT::PTR).ctrl.modify(|r| r | 1);
+        (*DWT::PTR).cyccnt.write(0);
     }
 
     fn set_compare(&mut self, val: Self::Instant) {