Functions in Generics - 2024.1 English

Vivado Design Suite User Guide: Synthesis (UG901)

Document ID
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2024.1 English

In VHDL-2008, you can declare undefined functions inside of entities. For example

entity bottom is
generic (
function my_func (a,b : unsigned) return unsigned);
port ...
end entity bottom;

Later in the architecture of the entity:

process(clk) is
if rising_edge(clk) then
y <= my_func(a,b);
end if;
end process;

This uses the my_func function, inside of the entity, but it still has not defined what this function actually accomplishes. That is defined as when the bottom is instantiated in an upper-level RTL.

inst_bot1 : bottom
generic map (
my_func => my_func1 )
port map ...

This ties the function my_func1 that was declared in a VHDL file or a package file to the generic function my_func. As long as my_func1 has two inputs called a and b that are both unsigned, it is able to work.