change Monotonic::ratio return type to Fraction

3 files changed, 29 insertions, 11 deletions

diff --git a/src/cyccnt.rs b/src/cyccnt.rs
index 468aa712..c8a1b7ee 100644
--- a/src/cyccnt.rs
+++ b/src/cyccnt.rs
@@ -10,9 +10,15 @@ use core::{
 
 use cortex_m::peripheral::DWT;
 
+use crate::Fraction;
+
 /// A measurement of the CYCCNT. Opaque and useful only with `Duration`
 ///
 /// This data type is only available on ARMv7-M
+///
+/// Note that this value is tied to the CYCCNT of one core and that sending it a different core
+/// makes it lose its meaning -- each Cortex-M core has its own CYCCNT counter and these are usually
+/// unsynchronized and they may even be running at different frequencies.
 #[derive(Clone, Copy, Eq, PartialEq)]
 pub struct Instant {
     inner: i32,
@@ -21,7 +27,6 @@ pub struct Instant {
 
 unsafe impl Sync for Instant {}
 
-#[cfg(not(feature = "heterogeneous"))]
 unsafe impl Send for Instant {}
 
 impl Instant {
@@ -182,15 +187,16 @@ impl U32Ext for u32 {
 }
 
 /// Implementation of the `Monotonic` trait based on CYCle CouNTer
-#[cfg(not(feature = "heterogeneous"))]
 pub struct CYCCNT;
 
-#[cfg(not(feature = "heterogeneous"))]
 impl crate::Monotonic for CYCCNT {
     type Instant = Instant;
 
-    fn ratio() -> u32 {
-        1
+    fn ratio() -> Fraction {
+        Fraction {
+            numerator: 1,
+            denominator: 1,
+        }
     }
 
     unsafe fn reset() {
diff --git a/src/lib.rs b/src/lib.rs
index decd2da1..22eff5ac 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -116,13 +116,25 @@ impl From<cortex_m::Peripherals> for Peripherals {
     }
 }
 
+/// A fraction
+pub struct Fraction {
+    /// The numerator
+    pub numerator: u32,
+
+    /// The denominator
+    pub denominator: u32,
+}
+
 /// A monotonic clock / counter
 pub trait Monotonic {
     /// A measurement of this clock
     type Instant: Copy + Ord + Sub;
 
     /// The ratio between the SysTick (system timer) frequency and this clock frequency
-    fn ratio() -> u32;
+    ///
+    /// The ratio must be expressed in *reduced* `Fraction` form to prevent overflows. That is
+    /// `2 / 3` instead of `4 / 6`
+    fn ratio() -> Fraction;
 
     /// Returns the current time
     fn now() -> Self::Instant;
diff --git a/src/tq.rs b/src/tq.rs
index 4f9b6e7e..4edb40a7 100644
--- a/src/tq.rs
+++ b/src/tq.rs
@@ -62,11 +62,11 @@ where
                     // set a new timeout
                     const MAX: u32 = 0x00ffffff;
 
-                    let dur = match (instant - now)
-                        .try_into()
-                        .ok()
-                        .and_then(|x| x.checked_mul(M::ratio()))
-                    {
+                    let ratio = M::ratio();
+                    let dur = match (instant - now).try_into().ok().and_then(|x| {
+                        x.checked_mul(ratio.numerator)
+                            .map(|x| x / ratio.denominator)
+                    }) {
                         None => MAX,
                         Some(x) => cmp::min(MAX, x),
                     };