VHDL coding tips and tricks: April 2014

Wednesday, April 30, 2014

How to get the range of a record element?

Suppose you have a record type like this:

type record_type_1 is 
    slv_1 : std_logic_vector(15 downto 0); 
    slv_2 : std_logic_vector(7 downto 0); 
end record;

Now say we have one more record type where we need to declare elements with the same size as slv_1 and slv_2. How do we do that?

If we try the below method, it wont work: 

type record_type_2 is 
    slv_3     : std_logic_vector(record_type_1.slv_1'LEFT downto record_type_1.slv_1'RIGHT));
    slv_4     : std_logic_vector(record_type_1.slv_2'LEFT downto record_type_1.slv_2'RIGHT)); 
end record;

The reason is simple, record_type_1 is just the definition of a record type. What we need is a real object to access the range values.

So this is what we do:

--Declare a dummy signal with type record_type_1.
signal dummy : record_type_1;

--Now define the type.
type record_type_2 is 
    slv_3     : std_logic_vector(dummy.slv_1'LEFT downto dummy.slv_1'RIGHT));
    slv_4     : std_logic_vector(dummy.slv_2'LEFT downto dummy.slv_2'RIGHT)); 
end record;

This will compiler without any errors. Also you don't need to worry that you are wasting extra logic declaring a dummy signal. Almost all the synthesis tools will detect that the signal "dummy" is not used anywhere in the design and will trim it during the synthesis process.