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 (0..9) and the exposed tens digit (0..9)
    reg [3:0] units;
    // One-cycle staging register that remembers a units 9->0 rollover
    // event so roll_o can pulse one full cycle after tens_o updates.
    reg       roll_stage;

    wire units_rollover = inc_i && (units == 4'd9);

    always @(posedge clk) begin
        if (rst) begin
            units      <= 4'd0;
            tens_o     <= 4'd0;
            roll_stage <= 1'b0;
            roll_o     <= 1'b0;
        end else begin
            // Advance the two-digit BCD counter when requested.
            if (inc_i) begin
                if (units == 4'd9) begin
                    units <= 4'd0;
                    if (tens_o == 4'd9)
                        tens_o <= 4'd0;
                    else
                        tens_o <= tens_o + 4'd1;
                end else begin
                    units <= units + 4'd1;
                end
            end

            // Delay the rollover event by two edges: it is registered into
            // roll_stage on the edge that changes tens_o (roll_o stays low
            // there), then propagated to roll_o on the following edge.
            roll_stage <= units_rollover;
            roll_o     <= roll_stage;
        end
    end

endmodule