diff --git a/inc/core/NotifyEvents.h b/inc/core/NotifyEvents.h
new file mode 100644
index 0000000..b54ef1b
--- /dev/null
+++ b/inc/core/NotifyEvents.h
@@ -0,0 +1,7 @@
+#ifndef NOTIFY_EVENTS_H
+#define NOTIFY_EVENTS_H
+
+#define MICROBIT_DISPLAY_EVT_FREE           1
+#define MICROBIT_SERIAL_EVT_TX_EMPTY        2
+
+#endif
diff --git a/inc/drivers/MicroBitDisplay.h b/inc/drivers/MicroBitDisplay.h
index 96cc1e6..53f3187 100644
--- a/inc/drivers/MicroBitDisplay.h
+++ b/inc/drivers/MicroBitDisplay.h
@@ -14,8 +14,7 @@
   * Event codes raised by MicroBitDisplay
   */
 #define MICROBIT_DISPLAY_EVT_ANIMATION_COMPLETE         1
-#define MICROBIT_DISPLAY_EVT_FREE                       2
-#define MICROBIT_DISPLAY_EVT_LIGHT_SENSE                4
+#define MICROBIT_DISPLAY_EVT_LIGHT_SENSE                2
 
 //
 // Internal constants
diff --git a/inc/drivers/MicroBitSerial.h b/inc/drivers/MicroBitSerial.h
index 3ac0f02..a4097ad 100644
--- a/inc/drivers/MicroBitSerial.h
+++ b/inc/drivers/MicroBitSerial.h
@@ -7,10 +7,9 @@
 #define MICROBIT_SERIAL_DEFAULT_BAUD_RATE   115200
 #define MICROBIT_SERIAL_DEFAULT_BUFFER_SIZE 20
 
-#define MICROBIT_SERIAL_EVT_TX_EMPTY        1
-#define MICROBIT_SERIAL_EVT_DELIM_MATCH     2
-#define MICROBIT_SERIAL_EVT_HEAD_MATCH      3
-#define MICROBIT_SERIAL_EVT_RX_FULL         4
+#define MICROBIT_SERIAL_EVT_DELIM_MATCH     1
+#define MICROBIT_SERIAL_EVT_HEAD_MATCH      2
+#define MICROBIT_SERIAL_EVT_RX_FULL         3
 
 #define MICROBIT_SERIAL_RX_IN_USE           1
 #define MICROBIT_SERIAL_TX_IN_USE           2
diff --git a/source/drivers/MicroBitDisplay.cpp b/source/drivers/MicroBitDisplay.cpp
index 1aa6ae1..8c616c7 100644
--- a/source/drivers/MicroBitDisplay.cpp
+++ b/source/drivers/MicroBitDisplay.cpp
@@ -8,6 +8,7 @@
 #include "MicroBitSystemTimer.h"
 #include "MicroBitFiber.h"
 #include "ErrorNo.h"
+#include "NotifyEvents.h"
 
 const int greyScaleTimings[MICROBIT_DISPLAY_GREYSCALE_BIT_DEPTH] = {1, 23, 70, 163, 351, 726, 1476, 2976};
 
@@ -105,7 +106,7 @@ void MicroBitDisplay::renderFinish()
 
 void MicroBitDisplay::render()
 {
-    // Simple optimisation. 
+    // Simple optimisation.
     // If display is at zero brightness, there's nothing to do.
     if(brightness == 0)
         return;
diff --git a/source/drivers/MicroBitSerial.cpp b/source/drivers/MicroBitSerial.cpp
index 111747b..011db0f 100644
--- a/source/drivers/MicroBitSerial.cpp
+++ b/source/drivers/MicroBitSerial.cpp
@@ -3,6 +3,7 @@
 #include "ErrorNo.h"
 #include "MicroBitComponent.h"
 #include "MicroBitFiber.h"
+#include "NotifyEvents.h"
 
 uint8_t MicroBitSerial::status = 0;