iverilog testbench error: input is declared as wire, but it isn't

Question

I am very new to iverilog and am creating a counter to reduce a 100Mhz clock frequency to something easier to work with, as part of a larger project. I found some code that does that and so I tried to write a testbench for it. Here is the code I found:

    module slowClock(clk, reset, clk_1Hz);
input clk, reset;
output clk_1Hz;

reg clk_1Hz;
reg [27:0] counter;

always@(posedge reset or posedge clk)
begin
     if (reset == 1'b1)
         begin
             clk_1Hz <= 0;
             counter <= 0;
         end
     else
         begin
             counter <= counter + 1;
             if ( counter == 25_000_000)
                 begin
                     counter <= 0;
                     clk_1Hz <= ~clk_1Hz;
                 end
         end
end
endmodule   

and here is the testbench I wrote:

module slowClock_tb(clk, reset, clk_1Hz);
    input  clk;
    input  reset;
    output  clk_1Hz;

initial 
begin
    clk = 1'b0; 
    reset = 1'b0;
#2 reset = ~reset;

end

    always #3 clk = ~clk;

slowClock clock_generator(clk, reset, clk_1Hz);


endmodule

Here are the error messages:

$ iverilog  slowClock.v slowClock_tb.v 
slowClock_tb.v:8: error: clk is not a valid l-value in slowClock_tb.
slowClock_tb.v:2:      : clk is declared here as wire.
slowClock_tb.v:9: error: reset is not a valid l-value in slowClock_tb.
slowClock_tb.v:3:      : reset is declared here as wire.
slowClock_tb.v:10: error: reset is not a valid l-value in slowClock_tb.
slowClock_tb.v:3:      : reset is declared here as wire.
slowClock_tb.v:14: error: clk is not a valid l-value in slowClock_tb.
slowClock_tb.v:2:      : clk is declared here as wire.
4 error(s) during elaboration.

The first error message: clk is declared here as wire. But it hasn't been declared as a wire in either the original code or the testbench. Same goes for reset. I have tried getting help from the on-campus tutors, but they didn't know why this is happening or were able to advise on how to fix it.

Can anyone suggest how to fix this?

Answer 1

When you don't include a type all variables/signals are inferred as wires. You haven't given them a type, so they're assumed to be wires.

You've also defined clk and reset as inputs in your testbench module, but then you're assigning to them inside the testbench so that's why they're not valid l-values.

Try this:

module slowClock(
    input  wire clk,
    input  wire reset,
    output reg clk_1Hz
    );

    reg [27:0] counter;

    always@(posedge reset or posedge clk) begin
        if (reset == 1'b1) begin
            clk_1Hz <= 0;
            counter <= 0;
        end else begin
            counter <= counter + 1;
            if ( counter == 25_000_000) begin
                counter <= 0;
                clk_1Hz <= ~clk_1Hz;
            end
        end
    end
endmodule

module slowClock_tb;
    reg clk = 1'b0;
    reg reset = 1'b0;
    integer counter = 0;
    wire clk_1Hz;

    initial begin
        #2 reset <= ~reset;
    end

    always #3 clk <= ~clk;

    slowClock clock_generator(clk, reset, clk_1Hz);

    always @(posedge clk) begin
        counter <= counter + 1;
        $display("%0d", counter);

        if (counter > 100) $finish;
    end

endmodule