https://github.com/arduino/ArduinoCore-samd/blob/0b60a79c4b194ed2e76fead95caf1bbce8960049/cores/arduino/sync.h#L28 Reported here: https://github.com/terjeio/grblHAL/issues/224 The define should be prefixed to avoid this? Note that I am not going to change my code as it is used for a lot of other microcontrollers.