diff --git a/CMakeLists.txt b/CMakeLists.txt
index 755e3d2..b77956a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,6 +5,8 @@
 # Description: Arduino CMake example                                          #
 #                                                                             #
 #=============================================================================#
+# The following line must be place before project(...).
+# If it was not there when first calling cmake, remove all cmake generated file.
 set(CMAKE_TOOLCHAIN_FILE cmake/ArduinoToolchain.cmake) # Arduino Toolchain