diff --git a/README.md b/README.md
index bc469ec..90813d8 100644
--- a/README.md
+++ b/README.md
@@ -38,7 +38,12 @@ This library provides for this use case some Arduino interfaces, but not all yet
     lib_deps =
         BlueAndi/ArduinoNative @ ~0.1.0
     ```
-2. Call the ```Arduino::setup()``` once and the ```Arduino::loop()``` in a infinite loop in your main entry point function.
+2. Add this build flags to your _platformio.ini_ in your environment:
+    ```
+    build_flags =
+        -D _USE_MATH_DEFINES
+    ```
+3. Call the ```Arduino::setup()``` once and the ```Arduino::loop()``` in a infinite loop in your main entry point function.
 
 ## Example
 See [minimal example](./examples/example/).
diff --git a/examples/example/platformio.ini b/examples/example/platformio.ini
index 0762229..3207351 100644
--- a/examples/example/platformio.ini
+++ b/examples/example/platformio.ini
@@ -16,6 +16,7 @@
 platform = native @ ~1.2.1
 build_flags =
     -std=c++11
+    -D _USE_MATH_DEFINES
 lib_deps =
     MainNative
     BlueAndi/ArduinoNative @ ~0.1.1
diff --git a/library.json b/library.json
index 4b6fc46..e842ca8 100644
--- a/library.json
+++ b/library.json
@@ -1,6 +1,6 @@
 {
     "name": "ArduinoNative",
-    "version": "0.2.0",
+    "version": "0.2.1",
     "description": "Arduino for the native environment. It doesn't support every interface, just some of them!",
     "authors": [{
         "name": "Andreas Merkle",
diff --git a/src/Arduino.h b/src/Arduino.h
index 932a124..84f2287 100644
--- a/src/Arduino.h
+++ b/src/Arduino.h
@@ -40,7 +40,9 @@
  * Compile Switches
  *****************************************************************************/
 
-#define _USE_MATH_DEFINES
+#ifndef _USE_MATH_DEFINES
+#error Please define _USE_MATH_DEFINES in your build flags.
+#endif /* _USE_MATH_DEFINES */
 
 /******************************************************************************
  * Includes
@@ -53,6 +55,9 @@
  * Macros
  *****************************************************************************/
 
+/**
+ * Constrain the given value to min. and max.
+ */
 #define constrain(amt,low,high) ((amt)<(low)?(low):((amt)>(high)?(high):(amt)))
 
 #ifndef PSTR
@@ -63,6 +68,9 @@
 #define PROGMEM
 #endif
 
+/**
+ * The constant PI.
+ */
 #define PI  M_PI
 
 /******************************************************************************