VHDL Array Type Case Handling

Question

Lets consider the following (and ignore synthesis for the moment):

SIGNAL sig1 : std_logic_vector( 3 DOWNTO 0 ) := "0000";

CASE sig1 IS

    when "0000" => do something
    when "0001" => do something
    when others => do something

END CASE;

I know that is very good practice to have a conditional for each potential "valid" value of sig1 ( "0000", "0001", "0010", ..., "1111" ) because the others check would include 'U', 'X', and 'Z'. So, this good practice tells us that the example above is not good VHDL code.

Now lets consider the following (keeping synthesis in mind):

TYPE state_type IS ( state0, state1, state2, state3, state4 );

SIGNAL sig2 : state_type;

CASE sig2 IS
    when state0 => do something
    when state1 => do something
    when state3 | state4 => do something
    when others => do something
END CASE;

The same action would be taken state3 and state4 have the same action. The intent for others is to have the same action for states 2 and 5.

Is it still "required" to do something similar to the follow

when state2 | state5 => do something
when others => null;

My goal is to avoid having to have a huge list of states OR'd together for the same action.

Thanks!

Note that the case statement is not a concurrent statement but a sequential statement (to be used in processes). The concurrent form of this is the with ... select statement. — wap26

Martin Thompson Martin Thompson · Accepted Answer · 2012-07-19T08:33:53

Whether you use others or describe the operation with explicit reference to the states, you are describing the same behaviour. The synthesiser will interpret the code as producing same output in terms of function and therefore the logic will be identical.

If you need a huge list of states ORed together for the function then you have to have them.

If you are talking in terms of "code readability" then use whichever form suits the situation best.

VHDL Array Type Case Handling

1 Answers