diff --git a/core/sys/timer.h b/core/sys/timer.h
index 88a45850f..3bfdfa81f 100644
--- a/core/sys/timer.h
+++ b/core/sys/timer.h
@@ -88,10 +88,10 @@ struct timer {
   clock_time_t interval;
 };
 
-void timer_set(struct timer *t, clock_time_t interval);
+CCIF void timer_set(struct timer *t, clock_time_t interval);
 void timer_reset(struct timer *t);
 void timer_restart(struct timer *t);
-int timer_expired(struct timer *t);
+CCIF int timer_expired(struct timer *t);
 clock_time_t timer_remaining(struct timer *t);