Timofei’s traffic light

rtl/traffic_light.sv

// Synthesizable generic FSM. The state output uses one-hot encoding.
// Reset is asynchronous and active high; it selects S0.

module traffic_light
(
    input              clk,
    input              rst,

    output             red,
    output             yellow,
    output             green,
    output logic [9:0] state
);

    timeunit 1ns;
    timeprecision 1ps;

    wire [9:0] next_state = { state [8:0], state [9] };

    // State register

    always_ff @ (posedge clk or posedge rst)
        if (rst)
            state <= 10'b00_0000_0001;
        else
            state <= next_state;

    // Moore outputs depend only on the current state.

    assign red    = | state [3:0];
    assign yellow = | state [5:4];
    assign green  = | state [9:6];

endmodule

models/cd4017b.sv

// Synthesizable, zero-delay functional model of a powered CD4017B.
// Physical supply pins: 16 = VDD, 8 = VSS; they are not HDL ports.
// Binary encoding abstracts the chip's five-stage Johnson counter.
// Reset is required. Illegal Johnson-state recovery and electrical effects
// (current, setup/hold, pulse width and propagation delay) are not modeled.

module cd4017b
(
    input   p14_clock,
    input   p13_inhibit,
    input   p15_reset,

    output  p3_q0,
    output  p2_q1,
    output  p4_q2,
    output  p7_q3,
    output  p10_q4,
    output  p1_q5,
    output  p5_q6,
    output  p6_q7,
    output  p9_q8,
    output  p11_q9,
    output  p12_carry
);

    timeunit 1ns;
    timeprecision 1ps;

    // Falling INHIBIT while CLOCK is high is also a count event.
    // INHIBIT is tied low in this exercise, so synthesis removes the gate.
    // This models the chip; it is not a recommendation to gate FPGA clocks.

    wire effective_clock = p14_clock & ~ p13_inhibit;

    logic [3:0] count;

    always_ff @ (posedge effective_clock or posedge p15_reset)
        if (p15_reset)
            count <= '0;
        else if (count == 4'd9)
            count <= '0;
        else
            count <= count + 1'b1;

    assign p3_q0     = (count == 4'd0);
    assign p2_q1     = (count == 4'd1);
    assign p4_q2     = (count == 4'd2);
    assign p7_q3     = (count == 4'd3);
    assign p10_q4    = (count == 4'd4);
    assign p1_q5     = (count == 4'd5);
    assign p5_q6     = (count == 4'd6);
    assign p6_q7     = (count == 4'd7);
    assign p9_q8     = (count == 4'd8);
    assign p11_q9    = (count == 4'd9);
    assign p12_carry = (count <  4'd5);

endmodule

models/cd4072b.sv

// Synthesizable functional model: two four-input OR gates.
// Physical pins: 14 = VDD, 7 = VSS, 6/8 = NC. No electrical/timing model.

module cd4072b
(
    input   p2_a,
    input   p3_b,
    input   p4_c,
    input   p5_d,
    output  p1_j,

    input   p9_e,
    input   p10_f,
    input   p11_g,
    input   p12_h,
    output  p13_k
);

    timeunit 1ns;
    timeprecision 1ps;

    assign p1_j  = p2_a | p3_b  | p4_c  | p5_d;
    assign p13_k = p9_e | p10_f | p11_g | p12_h;

endmodule

models/cd4071b.sv

// Synthesizable functional model: four two-input OR gates.
// Physical supply pins: 14 = VDD, 7 = VSS. No electrical/timing model.

module cd4071b
(
    input   p1_a,
    input   p2_b,
    input   p5_c,
    input   p6_d,
    input   p8_e,
    input   p9_f,
    input   p12_g,
    input   p13_h,

    output  p3_j,
    output  p4_k,
    output  p10_l,
    output  p11_m
);

    timeunit 1ns;
    timeprecision 1ps;

    assign p3_j  = p1_a  | p2_b;
    assign p4_k  = p5_c  | p6_d;
    assign p10_l = p8_e  | p9_f;
    assign p11_m = p12_g | p13_h;

endmodule

rtl/traffic_light_chips.sv

// Generated by tools/build_diagrams.py from the same pins/nets as the drawings.
// Synthesizable with the ideal chip models; all supply pins are physical-only.

module traffic_light_chips
(
    input              clk,
    input              rst,

    output             red,
    output             yellow,
    output             green,
    output       [9:0] state
);

    timeunit 1ns;
    timeprecision 1ps;

    cd4017b u1
    (
        .p3_q0        (state [0]),
        .p2_q1        (state [1]),
        .p4_q2        (state [2]),
        .p7_q3        (state [3]),
        .p10_q4       (state [4]),
        .p1_q5        (state [5]),
        .p5_q6        (state [6]),
        .p6_q7        (state [7]),
        .p9_q8        (state [8]),
        .p11_q9       (state [9]),
        .p12_carry    (),
        .p13_inhibit  (1'b0),
        .p14_clock    (clk),
        .p15_reset    (rst)
    );

    cd4072b u2
    (
        .p1_j         (red),
        .p2_a         (state [0]),
        .p3_b         (state [1]),
        .p4_c         (state [2]),
        .p5_d         (state [3]),
        .p9_e         (state [6]),
        .p10_f        (state [7]),
        .p11_g        (state [8]),
        .p12_h        (state [9]),
        .p13_k        (green)
    );

    cd4071b u3
    (
        .p1_a         (state [4]),
        .p2_b         (state [5]),
        .p3_j         (yellow),
        .p4_k         (),
        .p5_c         (1'b0),
        .p6_d         (1'b0),
        .p8_e         (1'b0),
        .p9_f         (1'b0),
        .p10_l        (),
        .p11_m        (),
        .p12_g        (1'b0),
        .p13_h        (1'b0)
    );

endmodule

tb/tb_traffic_light.sv

// Simulation only. Portable procedural checks; no classes, DPI or UVM.

module tb_traffic_light;

    timeunit 1ns;
    timeprecision 1ps;

    localparam period         = 20;
    localparam timeout_cycles = 500;

    logic clk = 1'b0;
    logic rst = 1'b0;

    always # (period / 2) clk = ~ clk;

    logic red_generic, yellow_generic, green_generic;
    logic red_chips, yellow_chips, green_chips;
    logic [9:0] state_generic, state_chips;

    int expected     = 0;
    int checks       = 0;
    int red_slots    = 0;
    int yellow_slots = 0;
    int green_slots  = 0;
    int visits [0:9];

    traffic_light generic_dut
    (
        .clk    (clk),
        .rst    (rst),
        .red    (red_generic),
        .yellow (yellow_generic),
        .green  (green_generic),
        .state  (state_generic)
    );

    traffic_light_chips chip_dut
    (
        .clk    (clk),
        .rst    (rst),
        .red    (red_chips),
        .yellow (yellow_chips),
        .green  (green_chips),
        .state  (state_chips)
    );

    task check_state;
        logic [9:0] wanted;
        logic [2:0] lamps;

        wanted = 10'b1 << expected;
        lamps  = expected < 4 ? 3'b100 : expected < 6 ? 3'b010 : 3'b001;

        if (   state_generic !== wanted
            || state_chips   !== wanted
            || { red_generic, yellow_generic, green_generic } !== lamps
            || { red_chips,   yellow_chips,   green_chips   } !== lamps)
        begin
            $fatal (1, "state=%0d generic=%b/%b chips=%b/%b",
                expected,
                state_generic, { red_generic, yellow_generic, green_generic },
                state_chips,   { red_chips,   yellow_chips,   green_chips   });
        end

        checks = checks + 1;
    endtask

    task tick;
        @ (posedge clk);
        expected = rst ? 0 : (expected + 1) % 10;
        # 1;  // Let register updates and lamp decoding settle before sampling.
        check_state ();
    endtask

    // Called between rising edges, after tick or the focused low-phase check.
    // Checking before the next clock edge distinguishes asynchronous reset.
    task reset_now;
        rst      = 1'b1;
        expected = 0;
        # 1;  // Asynchronous register update and combinational output settling.
        check_state ();
        rst = 1'b0;
    endtask

    initial
    begin
        $dumpfile ("traffic_light.vcd");
        $dumpvars (0, tb_traffic_light);

        for (int i = 0; i < 10; i ++)
            visits [i] = 0;

        @ (posedge clk);
        # 1;  // Place the initial reset after the clock event, without a race.
        reset_now ();

        for (int round = 0; round < 12; round ++)
            for (int i = 0; i < 10; i ++)
            begin
                visits [expected] = visits [expected] + 1;

                if (red_generic)
                    red_slots = red_slots + 1;

                if (yellow_generic)
                    yellow_slots = yellow_slots + 1;

                if (green_generic)
                    green_slots = green_slots + 1;

                tick ();
            end

        if (red_slots != 48 || yellow_slots != 24 || green_slots != 48)
            $fatal (1, "4/2/4 duration mismatch");

        for (int i = 0; i < 10; i ++)
            if (visits [i] != 12)
                $fatal (1, "Missing state %0d", i);

        // Reset from every state while CLOCK is high, then while it is low.
        for (int low_phase = 0; low_phase < 2; low_phase ++)
            for (int i = 0; i < 10; i ++)
            begin
                tick ();
                reset_now ();

                repeat (i)
                    tick ();

                if (low_phase != 0)
                begin
                    // Necessary exception: exercise reset with CLOCK low and
                    // verify that a falling clock edge does not advance state.
                    @ (negedge clk);
                    # 1;  // Observe any erroneous falling-edge state update.
                    check_state ();
                end

                reset_now ();
            end

        rst      = 1'b1;
        expected = 0;

        repeat (3)
            tick ();  // Reset dominates the clock.

        rst = 1'b0;
        tick ();

        $display ("PASS traffic_light: %0d checks; 12 full cycles; red/yellow/green=48/24/48; reset and wrap",
            checks);
        $finish;
    end

    initial
    begin
        repeat (timeout_cycles) @ (posedge clk);
        $fatal (1, "Timeout after %0d clock cycles", timeout_cycles);
    end

endmodule

tb/tb_cmos.sv

// Simulation only. Chip pin truth tables and 4017 clock semantics.

module tb_cmos;

    timeunit 1ns;
    timeprecision 1ps;

    localparam period         = 20;
    localparam timeout_cycles = 500;

    logic clk     = 1'b0;
    logic inhibit = 1'b0;
    logic rst     = 1'b0;
    logic [7:0] a = '0;

    always # (period / 2) clk = ~ clk;

    logic [3:0] or2;
    logic [1:0] or4;
    logic [9:0] q;
    logic carry;

    int expected = 0;

    cd4071b g2
    (
        .p1_a  (a [0]),
        .p2_b  (a [1]),
        .p5_c  (a [2]),
        .p6_d  (a [3]),
        .p8_e  (a [4]),
        .p9_f  (a [5]),
        .p12_g (a [6]),
        .p13_h (a [7]),
        .p3_j  (or2 [0]),
        .p4_k  (or2 [1]),
        .p10_l (or2 [2]),
        .p11_m (or2 [3])
    );

    cd4072b g4
    (
        .p2_a  (a [0]),
        .p3_b  (a [1]),
        .p4_c  (a [2]),
        .p5_d  (a [3]),
        .p1_j  (or4 [0]),
        .p9_e  (a [4]),
        .p10_f (a [5]),
        .p11_g (a [6]),
        .p12_h (a [7]),
        .p13_k (or4 [1])
    );

    cd4017b counter
    (
        .p14_clock   (clk),
        .p13_inhibit (inhibit),
        .p15_reset   (rst),
        .p3_q0       (q [0]),
        .p2_q1       (q [1]),
        .p4_q2       (q [2]),
        .p7_q3       (q [3]),
        .p10_q4      (q [4]),
        .p1_q5       (q [5]),
        .p5_q6       (q [6]),
        .p6_q7       (q [7]),
        .p9_q8       (q [8]),
        .p11_q9      (q [9]),
        .p12_carry   (carry)
    );

    task check_counter;
        if (q !== (10'b1 << expected) || carry !== (expected < 5))
            $fatal (1, "4017 state/carry mismatch: expected=%0d q=%b carry=%b",
                expected, q, carry);
    endtask

    task tick;
        @ (posedge clk);
        expected = rst ? 0 : inhibit ? expected : (expected + 1) % 10;
        # 1;  // Counter register and decoded output settling.
        check_counter ();
    endtask

    initial
    begin
        @ (posedge clk);
        # 1;  // Assert reset after the clock event, without a stimulus/DUT race.
        rst      = 1'b1;
        expected = 0;
        # 1;  // Check asynchronous reset before the next clock edge.
        check_counter ();

        // Inputs are changed after sampling, safely before the next clock edge.
        for (int i = 0; i < 256; i ++)
        begin
            a = 8' (i);
            tick ();

            if (   or2 !== { (| a [7:6]), (| a [5:4]), (| a [3:2]), (| a [1:0]) }
                || or4 !== { (| a [7:4]), (| a [3:0]) })
                $fatal (1, "OR pin truth-table mismatch input=%h", a);
        end

        rst = 1'b0;

        repeat (30)
            tick ();

        inhibit = 1'b1;
        # 1;  // Raising inhibit while CLOCK is high must not count.
        check_counter ();

        repeat (4)
            tick ();

        // Necessary exception: falling INHIBIT with CLOCK low must not count.
        // Posedge-only stimulus cannot test this half of the chip's truth table.
        @ (negedge clk);
        inhibit = 1'b0;
        # 1;  // Settle the effective clock and output decode before checking.
        check_counter ();

        tick ();

        inhibit = 1'b1;
        # 1;  // Raising inhibit with CLOCK high must not count.
        check_counter ();

        inhibit  = 1'b0;
        expected = (expected + 1) % 10;
        # 1;  // Falling inhibit with CLOCK high IS a count event.
        check_counter ();

        rst      = 1'b1;
        expected = 0;
        # 1;  // Reset must act without waiting for the next clock edge.
        check_counter ();

        inhibit = 1'b1;
        tick ();
        inhibit = 1'b0;
        # 1;  // Reset also dominates the falling-INHIBIT count event.
        check_counter ();

        repeat (2)
            tick ();

        $display ("PASS CMOS: 256 OR input combinations; 30 count edges; carry, inhibit and asynchronous reset");
        $finish;
    end

    initial
    begin
        repeat (timeout_cycles) @ (posedge clk);
        $fatal (1, "Timeout after %0d clock cycles", timeout_cycles);
    end

endmodule

tb/tb_integrations.sv

// Simulation only. Both core versions in all three integration wrappers.

module tb_integrations;

    timeunit 1ns;
    timeprecision 1ps;

    localparam period         = 20;
    localparam timeout_cycles = 200;

    logic clk = 1'b0;
    logic rst = 1'b0;
    logic ena = 1'b1;

    always # (period / 2) clk = ~ clk;

    logic [7:0] ui  = '0;
    logic [7:0] key = '0;

    logic [7:0] lab_led  [0:1];
    logic [7:0] hack_led [0:1];
    logic [7:0] tt_out   [0:1];
    logic [7:0] tt_io    [0:1];
    logic [7:0] tt_oe    [0:1];

    int n_bgm = 0;
    int n_tt  = 0;
    logic [7:0] seven [0:9];

    generate
        genvar k;

        for (k = 0; k < 2; k ++)
        begin : versions
            lab_top # (.CHIP_MODEL (k)) lab
            (
                .clk      (clk),
                .slow_clk (clk),
                .rst      (rst),
                .key      (key [3:0]),
                .sw       (8'b0),
                .led      (lab_led [k]),
                .abcdefgh (),
                .digit    (),
                .x        (10'b0),
                .y        (9'b0),
                .red      (),
                .green    (),
                .blue     (),
                .mic      (24'b0),
                .sound    (),
                .uart_rx  (1'b0),
                .uart_tx  (),
                .gpio     ()
            );

            hackathon_top # (.CHIP_MODEL (k)) hack
            (
                .clock      (clk),
                .slow_clock (clk),
                .reset      (rst),
                .key        (key),
                .led        (hack_led [k]),
                .abcdefgh   (),
                .digit      (),
                .x          (9'b0),
                .y          (9'b0),
                .red        (),
                .green      (),
                .blue       (),
                .gpio       ()
            );

            tt_um_timofei_traffic_light # (.USE_CHIP_MODEL (k)) tt
            (
                .ui_in   (ui),
                .uo_out  (tt_out [k]),
                .uio_in  (8'b0),
                .uio_out (tt_io [k]),
                .uio_oe  (tt_oe [k]),
                .ena     (ena),
                .clk     (clk),
                .rst_n   (~ rst)
            );
        end
    endgenerate

    task check_all;
        logic [2:0] bgm_lamps;
        logic [2:0] tt_lamps;
        logic [7:0] segments;

        bgm_lamps = n_bgm < 4 ? 3'b001 : n_bgm < 6 ? 3'b010 : 3'b100;
        tt_lamps  = n_tt  < 4 ? 3'b001 : n_tt  < 6 ? 3'b010 : 3'b100;
        segments  = ui [0] ? seven [n_tt] : n_tt < 4 ? 8'h01 : n_tt < 6 ? 8'h40 : 8'h08;

        for (int j = 0; j < 2; j ++)
        begin
            if (   lab_led  [j] !== { 5'b0, bgm_lamps }
                || hack_led [j] !== { 5'b0, bgm_lamps })
                $fatal (1, "BGM output mismatch state=%0d model=%0d", n_bgm, j);

            if (   tt_out [j] !== (ena ? segments : 8'b0)
                || tt_io  [j] !== (ena ? { 5'b0, tt_lamps } : 8'b0)
                || tt_oe  [j] !== (ena ? 8'h07 : 8'b0))
                $fatal (1, "Tiny Tapeout output mismatch state=%0d model=%0d", n_tt, j);
        end
    endtask

    task tick;
        @ (posedge clk);
        n_bgm = rst || key [0] ? 0 : (n_bgm + 1) % 10;
        n_tt  = rst           ? 0 : (n_tt  + 1) % 10;
        # 1;  // Register updates and wrapper output decoding must settle.
        check_all ();
    endtask

    initial
    begin
        seven [0] = 8'h3f;
        seven [1] = 8'h06;
        seven [2] = 8'h5b;
        seven [3] = 8'h4f;
        seven [4] = 8'h66;
        seven [5] = 8'h6d;
        seven [6] = 8'h7d;
        seven [7] = 8'h07;
        seven [8] = 8'h7f;
        seven [9] = 8'h6f;

        @ (posedge clk);
        # 1;  // Drive reset after the clock event, without a stimulus/DUT race.
        rst   = 1'b1;
        n_bgm = 0;
        n_tt  = 0;
        # 1;  // Asynchronous reset must be observable before the next clock.
        check_all ();
        rst = 1'b0;

        // Each display mode sees all ten states; stimulus changes after sampling.
        for (int i = 0; i < 30; i ++)
        begin
            ui = 8' ((i / 10) % 2);
            tick ();
        end

        // ena blanks TT outputs but must not stop the internal state sequence.
        ena = 1'b0;

        repeat (3)
            tick ();

        ena = 1'b1;
        # 1;  // Combinational output-enable and display settling.
        check_all ();

        // Check the BGM reset button independently of the dedicated TT reset.
        key   = 8'd1;
        n_bgm = 0;
        # 1;  // Asynchronous button reset; TT must retain its own state.
        check_all ();

        repeat (2)
            tick ();

        key = '0;
        tick ();

        rst   = 1'b1;
        n_bgm = 0;
        n_tt  = 0;
        # 1;  // Asynchronous dedicated reset in all wrappers.
        check_all ();

        repeat (2)
            tick ();

        rst = 1'b0;
        tick ();

        $display ("PASS integrations: both implementations in BGM lab/hackathon and Tiny Tapeout; 30 steps, both display modes, 3 disabled cycles, independent button and dedicated resets");
        $finish;
    end

    initial
    begin
        repeat (timeout_cycles) @ (posedge clk);
        $fatal (1, "Timeout after %0d clock cycles", timeout_cycles);
    end

endmodule

integration/bgm_lab/lab_top.sv

// Full BGM lab interface. Requires w_key >= 1 and w_led >= 3.

module lab_top
# (
    parameter CHIP_MODEL    = 0,
    parameter clk_mhz       = 50,
    parameter w_key         = 4,
    parameter w_sw          = 8,
    parameter w_led         = 8,
    parameter w_digit       = 8,
    parameter w_gpio        = 100,

    parameter screen_width  = 640,
    parameter screen_height = 480,

    parameter w_red         = 4,
    parameter w_green       = 4,
    parameter w_blue        = 4,

    parameter w_x           = $clog2 (screen_width),
    parameter w_y           = $clog2 (screen_height)
)
(
    input                  clk,
    input                  slow_clk,
    input                  rst,

    // Keys, switches, LEDs

    input  [w_key   - 1:0] key,
    input  [w_sw    - 1:0] sw,
    output [w_led   - 1:0] led,

    // A dynamic seven-segment display

    output [          7:0] abcdefgh,
    output [w_digit - 1:0] digit,

    // Graphics

    input  [w_x     - 1:0] x,
    input  [w_y     - 1:0] y,

    output [w_red   - 1:0] red,
    output [w_green - 1:0] green,
    output [w_blue  - 1:0] blue,

    // Microphone, sound output and UART

    input  [         23:0] mic,
    output [         15:0] sound,

    input                  uart_rx,
    output                 uart_tx,

    // General-purpose Input/Output

    inout  [w_gpio  - 1:0] gpio
);

    timeunit 1ns;
    timeprecision 1ps;

    logic lamp_red, lamp_yellow, lamp_green;
    logic [9:0] state;

    wire exercise_reset = rst | key [0];

    generate
        if (CHIP_MODEL == 0)
        begin : generic_version
            traffic_light dut
            (
                .clk    (slow_clk),
                .rst    (exercise_reset),
                .red    (lamp_red),
                .yellow (lamp_yellow),
                .green  (lamp_green),
                .state  (state)
            );
        end
        else
        begin : chip_version
            traffic_light_chips dut
            (
                .clk    (slow_clk),
                .rst    (exercise_reset),
                .red    (lamp_red),
                .yellow (lamp_yellow),
                .green  (lamp_green),
                .state  (state)
            );
        end
    endgenerate

    // Infrastructure LEDs and graphics use active-high logical signals.

    assign led      = w_led' ({ 5'b0, lamp_green, lamp_yellow, lamp_red });
    assign red      = { w_red   { lamp_red   | lamp_yellow } };
    assign green    = { w_green { lamp_green | lamp_yellow } };
    assign blue     = '0;
    assign abcdefgh = '0;
    assign digit    = '0;
    assign sound    = '0;
    assign uart_tx  = 1'b1;
    assign gpio     = 'z;

    wire unused_inputs = & { 1'b0, clk, sw, key, x, y, mic, uart_rx, state };

endmodule

integration/bgm_hackathon/hackathon_top.sv

// The BGM module is spelled hackathon_top, with "h".

module hackathon_top
# (
    parameter CHIP_MODEL = 0
)
(
    input        clock,
    input        slow_clock,
    input        reset,

    input  [7:0] key,
    output [7:0] led,

    // A dynamic seven-segment display

    output [7:0] abcdefgh,
    output [7:0] digit,

    // LCD screen interface

    input  [8:0] x,
    input  [8:0] y,

    output [4:0] red,
    output [5:0] green,
    output [4:0] blue,

    inout  [3:0] gpio
);

    timeunit 1ns;
    timeprecision 1ps;

    logic lamp_red, lamp_yellow, lamp_green;
    logic [9:0] state;

    wire exercise_reset = reset | key [0];

    generate
        if (CHIP_MODEL == 0)
        begin : generic_version
            traffic_light dut
            (
                .clk    (slow_clock),
                .rst    (exercise_reset),
                .red    (lamp_red),
                .yellow (lamp_yellow),
                .green  (lamp_green),
                .state  (state)
            );
        end
        else
        begin : chip_version
            traffic_light_chips dut
            (
                .clk    (slow_clock),
                .rst    (exercise_reset),
                .red    (lamp_red),
                .yellow (lamp_yellow),
                .green  (lamp_green),
                .state  (state)
            );
        end
    endgenerate

    assign led      = { 5'b0, lamp_green, lamp_yellow, lamp_red };
    assign red      = { 5 { lamp_red   | lamp_yellow } };
    assign green    = { 6 { lamp_green | lamp_yellow } };
    assign blue     = '0;
    assign abcdefgh = '0;
    assign digit    = '0;
    assign gpio     = 'z;

    wire unused_inputs = & { 1'b0, clock, key, x, y, state };

endmodule

integration/tinytapeout/project.v

// Synthesizable Tiny Tapeout top; compile as SystemVerilog.
// Set the external project clock to 1 Hz.
// Set USE_CHIP_MODEL to 1 to synthesize the pin-connected version.

module tt_um_timofei_traffic_light
# (
    parameter USE_CHIP_MODEL = 0
)
(
    input  [7:0] ui_in,
    output [7:0] uo_out,
    input  [7:0] uio_in,
    output [7:0] uio_out,
    output [7:0] uio_oe,
    input        ena,
    input        clk,
    input        rst_n
);

    timeunit 1ns;
    timeprecision 1ps;

    logic red, yellow, green;
    logic [9:0] state;

    generate
        if (USE_CHIP_MODEL == 0)
        begin : generic_version
            traffic_light dut
            (
                .clk    (clk),
                .rst    (~ rst_n),
                .red    (red),
                .yellow (yellow),
                .green  (green),
                .state  (state)
            );
        end
        else
        begin : chip_version
            traffic_light_chips dut
            (
                .clk    (clk),
                .rst    (~ rst_n),
                .red    (red),
                .yellow (yellow),
                .green  (green),
                .state  (state)
            );
        end
    endgenerate

    // TT output order [7:0] = { dp, g, f, e, d, c, b, a }; active high.
    // DIP 0 = 0: top/middle/bottom bar represents red/yellow/green.
    // DIP 0 = 1: show state number 0..9, not seconds remaining.

    logic [7:0] decimal_segments;

    always_comb
    begin
        decimal_segments = '0;

        case (state)
        10'b00_0000_0001: decimal_segments = 8'h3f;
        10'b00_0000_0010: decimal_segments = 8'h06;
        10'b00_0000_0100: decimal_segments = 8'h5b;
        10'b00_0000_1000: decimal_segments = 8'h4f;
        10'b00_0001_0000: decimal_segments = 8'h66;
        10'b00_0010_0000: decimal_segments = 8'h6d;
        10'b00_0100_0000: decimal_segments = 8'h7d;
        10'b00_1000_0000: decimal_segments = 8'h07;
        10'b01_0000_0000: decimal_segments = 8'h7f;
        10'b10_0000_0000: decimal_segments = 8'h6f;
        default:          decimal_segments = '0;
        endcase
    end

    wire [7:0] bars = { 1'b0, yellow, 2'b00, green, 2'b00, red };

    assign uo_out  = ena ? (ui_in [0] ? decimal_segments : bars) : '0;
    assign uio_out = ena ? { 5'b0, green, yellow, red } : '0;
    assign uio_oe  = ena ? 8'b0000_0111 : '0;

    wire unused_inputs = & { 1'b0, ui_in [7:1], uio_in };

endmodule