VHDL Identifiers and Types

VHDL Identifiers and Types

There are two types of identifiers in VHDL :

• Basic identifiers

• Extended identifiers

Basic identifier : The base identifier in VHDL is composed of a sequence of one or more characters. Legal characters are ;

• Lower case letter (a ... z)

• Upper case letter (A ... Z)

• Digit (0 ... 9)

• Underscore (_) 

Note 1. The first character in a basic identifier must be a letter, and the last character may not be an underscore.

2. The characters are not case sensitive. Therefore, as an example, NUM, num and Num are considered to be same identifier.

Examples : Signal l, H Adder

Extended identifier : The extended identifier in VHDL is a sequence of characters written between two backslashes. Here, any allowable characters can be used including characters like . , !, @, ', and $. Examples : /COUNT/ , /- 125/ , /TTL/ /Process/

Note 1. Extended identifiers are case sensitive. Thus Num and NUM are distinct identifiers.

2. Even though two blackslashes includes keyword it is a valid identifier. For example, /process/ is distinct from keyword process.

Other important points while representing any module in VHDL

1. Each statement in VHDL is terminated with a semicolon (;).

2. The name of the ports must be followed by a colon (:).

3. The architecture body starts with the predefined word begin, followed by statements that detail the relationship between the outputs and inputs.

4. The comment should begin with two hyphens (“)•

5. Leaving the blank spaces between two words or at the beginning of the line are allowed.

6. Leaving the blank line(s) is allowed in the module.

Review Question

1. Explain identifiers and its types with example.