Using Verilog Parameter keyword

Question

I am using the parameter keyword to define a state ie RESET = 5'b00000. If I want to use $display to printout the state name instead of the binary representation, or display the state name in my simulation wave viewer, how can I do this? It doesn't work to try to print it out as a string (as you would expect), so I'm wondering if this can be done.

Answer 1

There can be several parameters with the same value, so you cannot in general go from a numeric value to the name of a parameter with that value. Parameters are like C #defines; although you can use them to define a set of states, there is no formal association between the variable and the list of parameters you are using to represent the possible states. Because there was no better option it used to be common to (mis)use parameters in this way, but then SystemVerilog introduced an enum type which does not have the issues of the parameter method. You did not mention what tools you are using, but assuming that your tools support SystemVerilog enum types then that would be a better choice for a state value.

Answer 2

I do not know of a way to automatically $display the name of a parameter. However, if you don't mind duplicating your code, you could create a task (or function) to accomplish your goal:

    task show_name_state;
        case (state)
            5'b00000: $display("RESET");
            5'b00001: $display("WAIT");
        endcase
    endtask

    $display(state); show_name_state();

I know of at least one (expensive) Verilog debugger which has the capability to recognize parameters and automatically display their names in its waveform viewer: the Verdi (formerly Debussy) nWave tool can do this.

Answer 3

If your goal is just the display the name of the state during simulation, I will usually do something like

`ifdef SIMULATION
reg [127:0] __state__;
case (state)
  STATE_1 : __state__ = "STATE_1";
  STATE_2 : __state__ = "STATE_2";
  default : __state__ = "error";
endcase
`endif

Where state is the state register that has the parameter in question.