From 0a38a9c25f24c480f1564fb9e3264ec0dd79faff Mon Sep 17 00:00:00 2001
From: Brandon Matthews <bmatthews@zipcar.com>
Date: Wed, 7 Nov 2018 09:26:53 -0800
Subject: [PATCH] Add example of opaque CountDown type usage

---
 .gitignore   |  1 +
 src/timer.rs | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/.gitignore b/.gitignore
index f8d7c8b49..2949c0d1d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,3 +2,4 @@
 .#*
 /target/
 Cargo.lock
+.*.sw*
diff --git a/src/timer.rs b/src/timer.rs
index 74cf088f3..b14ca5ba2 100644
--- a/src/timer.rs
+++ b/src/timer.rs
@@ -57,6 +57,23 @@ use void::Void;
 /// #     fn wait(&mut self) -> ::nb::Result<(), Void> { Ok(()) }
 /// # }
 /// ```
+///
+/// If you want to use a CountDown timer as an opaque type in a HAL-based driver, you have to add
+/// some type bounds to make sure arguments passed to `start` can be used by the underlying
+/// (concrete) `CountDown::Time` type:
+/// ```rust
+/// extern crate embedded_hal as hal;
+/// #[macro_use(block)]
+///
+/// pub fn uses_timer<T, U>(t: T)
+/// where
+///     T: hal::timer::CountDown<Time=U>,
+///     U: From<u32>,
+/// {
+///     // delay an arbitrary 1 time unit
+///     t.start(1);
+/// }
+/// ```
 pub trait CountDown {
     /// The unit of time used by this timer
     type Time;