is there any possibility for "if" block to go out of given choices in vhdl?

Question

Below is the code of VHDL which uses IF block. In the last "elsif", the value of "my_choice" is assigned to "ch4". Then "else" block is executed, since no condition is satisfied. But, is there any chance that "my_choice" gets some other value other than (ch1,ch2,ch3,ch4), like high impedence (or anything else)? If so, how can i avoid that? Since this assignment can change the operation of the code.

    entity entity_name
    port
        .....
        .....
    end entity_name;

    architecture bhvr of entity_name

    type choices is (ch1, ch2, ch3, ch4);
    signal my_choice,next_choice : choices;

    begin
        process(clk, reset)
        begin
            if reset='1' tehn
                --------reset
            else
                  my_choice<=next_choice;
            end if;
        end process;

        process(my_choice)
        begin
            if my_choice = ch1 then
                next_choice <= ch2;
            elsif my_choice = ch2 then 
                next_choice <= ch3;
            elsif my_choice = ch3 then
                next_choice <= ch4;
            else  ------------------------------------coming to ch4
                ---- task for ch4
        end process;            
    end architecture;

score 2 · Accepted Answer · answered Jun 21 '16 at 06:31

Since my_choice is type choices, which is enumerated type with values 'ch1', 'ch2', 'ch3', 'ch4', then my_choice will always have one of these 4 values, at least in simulation.

In hardware, the available value space depends on the implementation, for example if the values are made as one-hot or binary. After start-up a reset is a good way to make sure that the value is well-defined to one of the 4 values, and if the circuit then adheres to the timing requirements, then the value will stay in the defined values space of the 4 values.

@Raviteja: If this answer solves your question, it is good practice to click accept ("V" mark to the left), so other users can prioritize their time. — Morten Zilmer, Jun 24 '16 at 02:30

is there any possibility for "if" block to go out of given choices in vhdl?

1 Answers1