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
);

    reg [3:0] units_q;   // hidden units digit
    reg       roll_d;    // one-cycle delay stage for the rollover pulse

    // A units 9 -> 0 rollover (which advances the tens digit) happens on any
    // incrementing edge where the units digit is currently 9.
    wire rollover = inc_i && (units_q == 4'd9);

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

            // Two-stage pipeline: the pulse appears one full cycle after the
            // tens digit changes, and is never asserted on the same edge that
            // changes tens_o.
            roll_d <= rollover;
            roll_o <= roll_d;
        end
    end

endmodule