diff --git a/src/serial.h b/src/serial.h
index 5265026..a49dfdc 100644
--- a/src/serial.h
+++ b/src/serial.h
@@ -83,6 +83,8 @@
 
 .- */
 
+#include "cpu.h"
+
 /*---------------------------------------------------------------------------
         Macro/Data type definitions
   ---------------------------------------------------------------------------*/