Hi Vishnu, To use token pasting preprocessor operator, you must be pasting a token and a token to form a single new token. You cannot be pasting two token together. #define DO_SET_1(X, Y) PORT ## X ##.OUTSET = PIN ## Y ## _bm DO_SET_1(D, 0); I am going to list out the expansion with one token per line: PORTD . OUTSET = PIN0_bm Notice in your macro you have this token pasting: X ## . You cannot token paste X substitution argument conjoined to . (dot). That does not form a token, it forms two tokens. If you change your #define macro to: #define DO_SET_1(X, Y) PORT ## X .OUTSET = PIN ## Y ## _bm Then you will no longer be trying to form an invalid token paste between X and . (dot). Sincerely, --Eljay
