bcd-tens-rollover

module TopModule (
    input  wire       clk,
    input  wire       rst,
    input  wire       inc_i,
    output reg  [3:0] tens_o,
    output reg        roll_o
);

    // Hidden units digit of the two-digit BCD counter.
    reg [3:0] units;

    // A rollover happens on an edge that increments the units digit from 9 to 0.
    wire rollover_now = inc_i && (units == 4'd9);

    // One-stage delay so the pulse appears the cycle AFTER tens_o changes,
    // and roll_o itself is one further register (registered output).
    reg rollover_d;

    always @(posedge clk) begin
        if (rst) begin
            units      <= 4'd0;
            tens_o     <= 4'd0;
            roll_o     <= 1'b0;
            rollover_d <= 1'b0;
        end else begin
            // Advance the BCD counter when enabled.
            if (inc_i) begin
                if (units == 4'd9) begin
                    units <= 4'd0;
                    if (tens_o == 4'd9)
                        tens_o <= 4'd0;      // 99 -> 00 wrap
                    else
                        tens_o <= tens_o + 4'd1;
                end else begin
                    units <= units + 4'd1;
                end
            end

            // Pipeline the rollover event: tens_o updates on this edge,
            // rollover_d captures it here, and roll_o pulses one edge later.
            rollover_d <= rollover_now;
            roll_o     <= rollover_d;
        end
    end

endmodule