diff --git a/include/polarssl/timing.h b/include/polarssl/timing.h
index 456508d..355c63c 100644
--- a/include/polarssl/timing.h
+++ b/include/polarssl/timing.h
@@ -39,7 +39,7 @@
 extern "C" {
 #endif
 
-extern int alarmed;
+extern volatile int alarmed;
 
 /**
  * \brief          Return the CPU cycle counter value