diff --git a/defs.h b/defs.h
index 9b17b73..05d56f4 100644
--- a/defs.h
+++ b/defs.h
@@ -36,4 +36,10 @@
 #endif
 
 
+/// check if F_CPU is defined
+#ifndef F_CPU
+	#error "F_CPU is not defined!"
+#endif
+
+
 #endif // __DEFS_H__