VHDL Identifiers

VHDL IDENTIFIERS

·         An identifier in VHDL is composed of a sequence of one or more characters.

·         A legal character is an upper-case letter (A... Z), or a lower-case letter (a. .. z), or a digit (0 . . . 9) or the underscore ( _ ) character.

·         The first character in an identifier must be a letter and the last character may not be an underscore.

·         Lower-case and upper-case letters are considered to be identical when used in an identifier; Eg., Count, COUNT, and CouNT, all refer to the same identifier.

·         Two underscore characters cannot appear consecutively.

·         Comments in a description must be preceded by two consecutive hyphens (-); the comment extends to the end of the line.

·         Comments can appear anywhere within a description. Eg.,

--This is a comment; it ends at the end of this line.

--To continue a comment onto a second line, a separate

-- comment line must be started.

entity UART is end; --This comment starts after the entity declaration.

·         The language defines a set of reserved words, also called keywofds, have a specific meaning in the language, and therefore, cannot be used as identifiers.