bcd-tens-rollover

module TopModule(
    input        clk,
    input        rst,
    input        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;
    // Registered flag capturing a units 9->0 rollover so roll_o can be
    // delayed by exactly one clock relative to the tens_o change.
    reg       roll_pending;

    // A units rollover occurs this edge when we increment while units == 9.
    wire units_roll = inc_i && (units == 4'd9);

    always @(posedge clk) begin
        if (rst) begin
            units        <= 4'd0;
            tens_o       <= 4'd0;
            roll_o       <= 1'b0;
            roll_pending <= 1'b0;
        end else begin
            // roll_o pulses one full cycle after the tens_o change: the
            // rollover captured last edge (roll_pending) drives roll_o now.
            roll_o       <= roll_pending;
            roll_pending <= units_roll;

            if (inc_i) begin
                if (units == 4'd9) begin
                    units  <= 4'd0;
                    // Advance the tens digit, wrapping 9 -> 0 (99 -> 00).
                    tens_o <= (tens_o == 4'd9) ? 4'd0 : tens_o + 4'd1;
                end else begin
                    units  <= units + 4'd1;
                end
            end
        end
    end
endmodule