Identifiers in VHDL An identifier represent a token in VHDL. VHDL has token in the form of reserve word and user defined word. Further, identifier may be : Basic identifier Extended identifiers. Basic Identifiers: These are used to define user defined names and following rules applies to the valid basic identifiers: The valid character set in The first letter of an identifier is always an alphabet (‘a’…’z’ or ‘A’…..’Z’) and digits 0 – 9. the first letter cannot be a special character or digit or underscore The last letter may be a letter or a digit but cannot be an underscore Double underscore anywhere is not permitted Graphical character/special symbols are not permitted Keywords cannot be used as basic