An interface constant declared in
the block header of a block statement, a component declaration, or an
entity declaration. Generics provide a channel for static information
to be communicated to a block from its environment. Unlike constants,
however, the value of a generic can be supplied externally, either in
a component instantiation statement or in a configuration specification.
generic_interface_list ) ;
Generics support static
information to blocks in a similar way as constants, but unlike the
constants the values of generics can be supplied externally. Similar
to ports, they can be
declared in entities and component
declarations, and always before ports.
Values supported by generics declared in an entity can be read either
in entity or in architecture associated with the entity. In
particular, a generic can be used to specify the size of ports
(example 1), the number of subcomponents within a block, the timing
characteristics of a block (example 2), physical characteristics of a
design, width of vectors inside an architecture, number of loop
iterations, etc. In general, generic can be treated inside an
architecture in the same way as constant.
entity CPU is
(BusWidth : Integer := 16);
port(DataBus : inout
Std_Logic_Vector(BusWidth-1 downto 0));
. . .
The generic value BusWidth is used here to declare the width of the
port DataBus, and can be successively in all declarations of buses
inside associated architecture(s). This way the user supplies only
one value, which parameterizes complete design.
entity Gen_Gates is
generic (Delay : Time := 10 ns);
port (In1, In2 : in Std_Logic;
Output : out Std_Logic);
architecture Gates of
. . .
Output <= In1 or
In2 after Delay;
. . .
The Delay generic value Delay specifies here the delay through a
device or part of the device.