diff --git a/libraries/AP_HAL/HAL.cpp b/libraries/AP_HAL/HAL.cpp
index deef2581c3..6f796c3ab9 100644
--- a/libraries/AP_HAL/HAL.cpp
+++ b/libraries/AP_HAL/HAL.cpp
@@ -11,12 +11,3 @@ HAL::FunCallbacks::FunCallbacks(void (*setup_fun)(void), void (*loop_fun)(void))
 }
 
 }
-
-// access serial ports using SERIALn numbering
-AP_HAL::UARTDriver* AP_HAL::HAL::serial(uint8_t sernum) const
-{
-    if (sernum >= ARRAY_SIZE(serial_array)) {
-        return nullptr;
-    }
-    return serial_array[sernum];
-}
diff --git a/libraries/AP_HAL/HAL.h b/libraries/AP_HAL/HAL.h
index d56aeaa724..063871d064 100644
--- a/libraries/AP_HAL/HAL.h
+++ b/libraries/AP_HAL/HAL.h
@@ -155,3 +155,12 @@ public:
 #endif
 
 };
+
+// access serial ports using SERIALn numbering
+inline AP_HAL::UARTDriver* AP_HAL::HAL::serial(uint8_t sernum) const
+{
+    if (sernum >= ARRAY_SIZE(serial_array)) {
+        return nullptr;
+    }
+    return serial_array[sernum];
+}