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VHDL (VHSIC-HDL, Very High-Speed Integrated Circuit Hardware Description Language) is a hardware description language used in electronic des...

Sunday 16 December 2012

Use Clause

Formal Definition

Achieves direct visibility of declarations that are visible by selection.

Simplified Syntax

use library_name.package_name.item;

use library_name.package_name;

use library_name.package_name.all;


The use clause makes visible items specified as suffixes in selected names listed in the clause. In practice, the use clause makes visible declarations specified in packages and has the following form:

use library_name.package_name.item

If a designer wants to have all declarations in a package visible, then the 'item' clause should be substituted by the reserved word all.

The use clause is valid for the design unit immediately following it and for all secondary design units assigned to this design unit (if it is a primary design unit).


library IEEE;
use IEEE.Std_Logic_1164.all;
library IEEE;
use IEEE.Std_Logic_1164.Std_ulogic;
use IEEE.Std_Logic_1164.Rising_edge;

In the first example, all declarations specified in the package Std_Logic_1164 (which belongs to the library IEEE) have been made visible.

The second example makes visible the Rising_Edge function, which is declared in the same package. The function uses the type Std_ulogic, therefore declaration of this type is also made visible.

Important Notes

· Using multiple value logic and resolution functions requires using library clause and use clause like in the first example.

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