diff --git a/libboard_zynq/src/timer/global.rs b/libboard_zynq/src/timer/global.rs
index 616ae03..e671e06 100644
--- a/libboard_zynq/src/timer/global.rs
+++ b/libboard_zynq/src/timer/global.rs
@@ -1,3 +1,4 @@
+use void::Void;
 use libregister::{RegisterR, RegisterW};
 use crate::{
     clocks::Clocks,
@@ -5,6 +6,7 @@ use crate::{
     time::Milliseconds,
 };
 
+/// "uptime"
 pub struct GlobalTimer {
     regs: &'static mut mpcore::RegisterBlock,
 }
@@ -14,7 +16,7 @@ impl GlobalTimer {
         let regs = mpcore::RegisterBlock::new();
         GlobalTimer { regs }
     }
-    
+
     pub fn reset(&mut self) {
         // Disable
         self.regs.global_timer_control.write(
@@ -32,28 +34,64 @@ impl GlobalTimer {
         // Start
         self.regs.global_timer_control.write(
             mpcore::GlobalTimerControl::zeroed()
+                // maximum prescaler is still enough for millisecond
+                // precision while overflowing after centuries.
                 .prescaler(255)
                 .auto_increment_mode(true)
                 .timer_enable(true)
         );
     }
 
+    /// read the raw counter value
     pub fn get_counter(&self) -> u64 {
         loop {
-            let c1 = self.regs.global_timer_counter1.read().value();
+            let c1_pre = self.regs.global_timer_counter1.read().value();
             let c0 = self.regs.global_timer_counter0.read().value();
             let c1_post = self.regs.global_timer_counter1.read().value();
 
-            if c1 == c1_post {
-                return ((c1 as u64) << 32) | (c0 as u64);
+            if c1_pre == c1_post {
+                return ((c1_pre as u64) << 32) | (c0 as u64);
             }
+            // retry if c0 has wrapped while reading.
         }
     }
 
+    /// read and convert to time
     pub fn get_time(&self) -> Milliseconds {
         let prescaler = self.regs.global_timer_control.read().prescaler() as u64;
         let clocks = Clocks::get();
 
         Milliseconds(self.get_counter() * (prescaler + 1) / (clocks.cpu_3x2x() as u64 / 1000))
     }
+
+    /// return a handle that has implements
+    /// `embedded_hal::timer::CountDown`
+    pub fn countdown(&self) -> CountDown {
+        CountDown {
+            timer: &self,
+            timeout: Milliseconds(0),
+        }
+    }
+}
+
+#[derive(Clone)]
+pub struct CountDown<'a> {
+    timer: &'a GlobalTimer,
+    timeout: Milliseconds,
+}
+
+impl embedded_hal::timer::CountDown for CountDown<'_> {
+    type Time = Milliseconds;
+
+    fn start<T: Into<Self::Time>>(&mut self, count: T) {
+        self.timeout = count.into();
+    }
+
+    fn wait(&mut self) -> nb::Result<(), Void> {
+        if self.timer.get_time() < self.timeout {
+            Err(nb::Error::WouldBlock)
+        } else {
+            Ok(())
+        }
+    }
 }
diff --git a/libboard_zynq/src/timer/mod.rs b/libboard_zynq/src/timer/mod.rs
index 2bb4e94..88026ff 100644
--- a/libboard_zynq/src/timer/mod.rs
+++ b/libboard_zynq/src/timer/mod.rs
@@ -1,2 +1,2 @@
-mod global;
+pub mod global;
 pub use global::GlobalTimer;