18. Battery LED Bar

module battery_led_bar (
    input  [3:0] level,     // 0..8 inclusive
    output [7:0] led_bar
);
    // Build a "lowest-N-bits-one" mask:
    // For level=0:  ~(8'hFF << 0) = ~8'hFF = 8'h00
    // For level=8:  ~(8'hFF << 8) = ~8'h00 = 8'hFF
    // For level=N:  ~(8'hFF << N) = 00..011..1 (N ones at LSBs)
    assign led_bar = ~(8'hFF << level);
endmodule
💡RememberShifts + mask trick: ~(8'hFF << level) yields the lowest level bits = 1 (works for 0..8).
Sized literals help: 8'hFF, 4'd8 avoid width surprises.
Logical shift (<<) inserts zeros; shifting by the vector width (8) produces zero—exactly what we want before the ~.

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Testbench Code

`timescale 1ns/1ps

module tb_battery_led_bar;
    // Input
    reg  [3:0] level;

    // DUT output
    wire [7:0] led_bar;

    // Expected output
    reg  [7:0] expected_led_bar;

    // Mismatch signal
    wire mismatch = (led_bar !== expected_led_bar);

    integer TOTAL_TEST_CASES = 0;
    integer TOTAL_PASSED_TEST_CASES = 0;
    integer TOTAL_FAILED_TEST_CASES = 0;

    // Instantiate DUT
    battery_led_bar dut (
        .level(level),
        .led_bar(led_bar)
    );

    // VCD dump
    initial begin
        $dumpfile("tb_battery_led_bar.vcd");
        $dumpvars(0,
            tb_battery_led_bar.level,            // input
            tb_battery_led_bar.led_bar,          // output
            tb_battery_led_bar.expected_led_bar, // expected
            tb_battery_led_bar.mismatch          // mismatch flag
        );
    end

    // Golden model
    task compute_expected;
        input [3:0] t_level;
        begin
            expected_led_bar = ~(8'hFF << t_level);
        end
    endtask

    task run_test;
        input [3:0] t_level;
        begin
            level = t_level;
            compute_expected(t_level);
            #1;

            TOTAL_TEST_CASES++;
            if (!mismatch) begin
                TOTAL_PASSED_TEST_CASES++;
            end else begin
                TOTAL_FAILED_TEST_CASES++;
                $display("FAILED: level=%0d | led_bar=%b expected=%b mismatch=%b",
                         level, led_bar, expected_led_bar, mismatch);
            end
        end
    endtask

    integer i;
    initial begin
        // Exhaustive tests for all valid levels 0–8
        for (i = 0; i <= 8; i = i + 1) begin
            run_test(i[3:0]);
        end

        // Print truth table
        $display("======================================");
        $display("level | led_bar   | expected | mismatch");
        $display("--------------------------------------");
        for (i = 0; i <= 8; i = i + 1) begin
            level = i;
            compute_expected(i);
            #1;
            $display("%2d    | %b | %b | %b",
                     level, led_bar, expected_led_bar, mismatch);
        end
        $display("======================================");

        // Summary
        $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");
        $finish;
    end
endmodule