more monotonic timer docs

covers

- initialization and configuration of the timer; this is now a responsibility of
  the application author
- correctness of `Monotonic::now()` in `#[init]`
- safety of `Monotonic::reset()`

closes #251

1 files changed, 9 insertions, 3 deletions

diff --git a/examples/schedule.rs b/examples/schedule.rs
index 27d3bd1f..97818e36 100644
--- a/examples/schedule.rs
+++ b/examples/schedule.rs
@@ -1,6 +1,5 @@
 //! examples/schedule.rs
 
-#![deny(unsafe_code)]
 #![deny(warnings)]
 #![no_main]
 #![no_std]
@@ -13,8 +12,15 @@ use rtfm::cyccnt::{Instant, U32Ext as _};
 #[rtfm::app(device = lm3s6965, monotonic = rtfm::cyccnt::CYCCNT)]
 const APP: () = {
     #[init(schedule = [foo, bar])]
-    fn init(cx: init::Context) {
-        let now = Instant::now();
+    fn init(mut cx: init::Context) {
+        // Initialize (enable) the monotonic timer (CYCCNT)
+        cx.core.DCB.enable_trace();
+        // required on devices that software lock the DWT (e.g. STM32F7)
+        unsafe { cx.core.DWT.lar.write(0xC5ACCE55) }
+        cx.core.DWT.enable_cycle_counter();
+
+        // semantically, the monotonic timer is frozen at time "zero" during `init`
+        let now = cx.start; // the start time of the system
 
         hprintln!("init @ {:?}", now).unwrap();