VHDL String?

Question

How do you do strings in VHDL?

Answer 1

Most often - VHDL strings are used in 2 scenarios :

1. During simulation :

Printing messages to the console and when reading and writing to files while simulating.

2. During synthesis :

Strings are also used to pass easy to read properties when compiling generic code.

Example :

entity some_entity is

generic 
(
  speed : string ; -- can be "fast" or "slow".
) ;

port
(
  clock : std_logic ;
  counter : unsigned ( 15 downto 0 ) ;
)

end some_entity ;

architecture rtl_some_entity of some_entity is

begin

  generate_slow_counter : if speed = "slow" 
  generate
    counting : process ( clock ) is
    begin 
      if rising_edge ( clock ) then
        counter <= counter + 1 ;
      end if ;
    end process counting ;    
  end generate ;

  generate_fast_counter : if speed = "fast" 
  generate
    counting : process ( clock ) is
    begin 
      if rising_edge ( clock ) then
        counter <= counter + 2 ;
      end if ;
    end process counting ;    
  end generate ;

end architecture rtl_some_entity ;

Answer 2

A string is an array of Character type elements. The type string is defined in the Standard package. The size of the string array must be specified at the time of declaration and it is important to remember that strings cannot have an index value of '0'. The index must always have a positive value. Strings are used for 'testbench' and cannot be used for synthesis. They are primarily useful for displaying messages during simulation. To explain the syntax, here is an example string declaration:

constant Exstring : String(1 to 25) := "This is an Example string";

The name of the string is Exstring and it is 25 characters long. Notice that the index starts from '1' and not '0'. Double quotation marks are used for strings and single quotation marks for characters. Different operations that can be performed on strings are : "=", "/=", "<", "<=", ">", ">=" and "&". A concatenation operator can be used to concatenate two strings. It is also possible to concatenate two characters or one character and a string.