diff --git a/include/i2c.h b/include/i2c.h
index 54c6f9fbc6c476cd6244dc57b19854602d81b722..d31c72d2475c48e42f977a63b48e68875620a924 100644
--- a/include/i2c.h
+++ b/include/i2c.h
@@ -56,6 +56,14 @@
 #define I2C_SET_BUS(a)
 #endif
 
+/* define the I2C bus number for RTC and DTT if not already done */
+#if !defined(CFG_RTC_BUS_NUM)
+#define CFG_RTC_BUS_NUM		0
+#endif
+#if !defined(CFG_DTT_BUS_NUM)
+#define CFG_DTT_BUS_NUM		0
+#endif
+
 /*
  * Initialization, must be called once on start up, may be called
  * repeatedly to change the speed and slave addresses.