VHDL Programming

114 Chapter Five

‘ 0 ’—Logical 0 value

‘ 1 ’—Logical 1 value

‘Z’—High-impedance or open-collector value

From the description of the two value systems, the conversion function

is trivial. Following is an example of one:

FUNCTION convert4val(S : fourval) RETURN fourvalue IS

BEGIN

CASE S IS

WHEN X =>

RETURN ‘X’;

WHEN L =>

RETURN ‘ 0 ’;

WHEN H =>

RETURN ‘ 1 ’;

WHEN Z =>

RETURN ‘Z’;

END CASE;

END convert4val;

This function accepts a value of type fourvaland returns a value of

type fourvalue. The next example shows where such a function might

be used:

PACKAGE my_std IS

TYPE fourval IS (X, L, H, Z);

TYPE fourvalue IS (‘X’, ‘ 0 ’, ‘ 1 ’, ‘Z’);

TYPE fvector4 IS ARRAY(0 TO 3) OF fourval;

END my_std;

USE WORK.my_std.ALL;

ENTITY reg IS

PORT(a : IN fvector4;

clr : IN fourval;

clk : IN fourval;

q : OUT fvector4);

FUNCTION convert4val(S : fourval)

RETURN fourvalue IS

BEGIN

CASE S IS

WHEN X =>

RETURN ‘X’;

WHEN L =>

RETURN ‘ 0 ’;

WHEN H =>

RETURN ‘ 1 ’;

WHEN Z =>