- Section 10.4
The use clause must appear before the start of the design unit itself. If the unit to be used is not in the library work, it must be preceded by a library clause:
library IEEE; use IEEE.STD_LOGIC_1164.all; entity WIDGET is -- etc. end WIDGET;
The use clause is valid only for the design unit immediately following it in the design file. However it also automatically applies to any secondary units associated with primary units for which it is valid.
Where multiple primary design units in the same design file need to use the same items, the use clause(s) must be repeated before each unit.
The last field in a use clause may be replaced by all to access all items in a design unit, or all design units in a library:
library MY_LIB; use MY_LIB.MY_PACK.all;
The shorter form of the use clause may appear in a configuration, to select which library entities are selected from:
library ASIC_LIB; configuration ASIC_TECH of ENT is for GATE_ARCH use ASIC_LIB.all; end for; end ASIC_TECH;
The shorter form of the use clause may also appear in the declarative part of an entity, architecture or block.
The use clause is usually supported by synthesis tools, but design libraries are often not.
Design files corresponding to any design units referenced by a use clause must be analyzed before the design units referencing them can be synthesized.