56. Decoder

Testbench Code

`timescale 1ns/1ps

module tb_decoder3to8;
    // 1) Inputs
    reg  [2:0] s;

    // 2) DUT outputs
    wire [7:0] y;

    // 3) Expected (purely combinational from inputs)
    wire [7:0] expected_y = (8'b0000_0001 << s);

    // 4) Single mismatch flag (case-inequality catches X/Z differences)
    wire mismatch = (y !== expected_y);

    // Accounting
    integer TOTAL_TEST_CASES        = 0;
    integer TOTAL_PASSED_TEST_CASES = 0;
    integer TOTAL_FAILED_TEST_CASES = 0;

    // Limits
    integer VCD_MAX_CASES   = 64; // normal rows printed
    integer ERROR_MAX_CASES = 64; // error rows printed
    integer printed_rows    = 0;
    integer printed_errors  = 0;

    // DUT (pick one implementation)
    decoder3to8 dut(.s(s), .y(y));

    // VCD dump
    initial begin
        $dumpfile("tb_decoder3to8.vcd");
        $dumpvars(0,
            tb_decoder3to8.s,
            tb_decoder3to8.y,
            tb_decoder3to8.expected_y,
            tb_decoder3to8.mismatch
        );
        $dumpon;
    end

    // Header + init
    initial begin
        s = 3'b000;
        $display("  s  |   y     | expected_y | mismatch");
        $display("---------------------------------------------------");
    end

    // Apply + check (drive -> wait -> read combinational mismatch)
    task apply_and_check;
        input [2:0] ts;
        begin
            s = ts;
            #1; // settle

            TOTAL_TEST_CASES = TOTAL_TEST_CASES + 1;
            if (!mismatch) begin
                TOTAL_PASSED_TEST_CASES = TOTAL_PASSED_TEST_CASES + 1;
                if (printed_rows < VCD_MAX_CASES) begin
                    $display(" %0d  | 0x%02h  |   0x%02h    |    %0d",
                             s, y, expected_y, mismatch);
                    printed_rows = printed_rows + 1;
                end
            end else begin
                TOTAL_FAILED_TEST_CASES = TOTAL_FAILED_TEST_CASES + 1;
                if (printed_errors < ERROR_MAX_CASES) begin
                    $display("ERR: s=%0d => y=0x%02h (expected 0x%02h)",
                             s, y, expected_y);
                    printed_errors = printed_errors + 1;
                end
            end

            // Optional: stop VCD after cap of total vectors
            if (TOTAL_TEST_CASES == VCD_MAX_CASES) $dumpoff;
        end
    endtask

    integer i;
    initial begin
        // Exhaustive over valid 0..7
        for (i = 0; i < 8; i = i + 1)
            apply_and_check(i[2:0]);

        // Summary
        $display("---------------------------------------------------");
        $display("TOTAL_TEST_CASES=%0d", TOTAL_TEST_CASES);
        $display("TOTAL_PASSED_TEST_CASES=%0d", TOTAL_PASSED_TEST_CASES);
        $display("TOTAL_FAILED_TEST_CASES=%0d", TOTAL_FAILED_TEST_CASES);
        $display("ALL_TEST_CASES_PASSED=%s", (TOTAL_FAILED_TEST_CASES==0) ? "true" : "false");

        #2 $finish;
    end
endmodule