diff --git a/bacnet-stack/ports/bdk-atxx4-mstp/iar2gcc.h b/bacnet-stack/ports/bdk-atxx4-mstp/iar2gcc.h
index 4554012c..a325ea29 100644
--- a/bacnet-stack/ports/bdk-atxx4-mstp/iar2gcc.h
+++ b/bacnet-stack/ports/bdk-atxx4-mstp/iar2gcc.h
@@ -35,7 +35,7 @@
 #define IAR2GCC_H
 
 #if !defined(F_CPU)
-#define F_CPU (7372800)
+#error You must define F_CPU - clock frequency!
 #endif
 
 /* IAR */
@@ -53,6 +53,10 @@ static inline void _delay_us(
 }
 #endif
 
+#if defined(__GNUC__)
+#include <util/delay.h>
+#endif
+
 /* Input/Output Registers */
 #if defined(__GNUC__)
 #include <avr/io.h>