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 (00..99).
    reg [3:0] units;
    // One-stage pipeline so roll_o appears exactly one cycle after the
    // tens digit changes (never on the same edge as the tens change).
    reg       roll_stage;

    // A units 9->0 rollover happens this edge when we increment at units==9.
    wire rollover = inc_i && (units == 4'd9);

    always @(posedge clk) begin
        if (rst) begin
            units      <= 4'd0;
            tens_o     <= 4'd0;
            roll_o     <= 1'b0;
            roll_stage <= 1'b0;
        end else begin
            // Delay-by-one-cycle pipeline for the rollover pulse.
            roll_o     <= roll_stage;   // pulses one full cycle after tens_o changed
            roll_stage <= rollover;     // captured on the edge that changes tens_o

            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
        end
    end
endmodule