bcd-tens-rollover

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

    reg [3:0] units;
    reg [3:0] tens;
    reg       roll_pending;
    reg       roll_q;

    // A rollover occurs when the hidden units digit steps from 9 back to 0.
    wire rollover = inc_i && (units == 4'd9);

    always @(posedge clk) begin
        if (rst) begin
            units        <= 4'd0;
            tens         <= 4'd0;
            roll_pending <= 1'b0;
            roll_q       <= 1'b0;
        end else begin
            if (inc_i) begin
                if (units == 4'd9) begin
                    units <= 4'd0;
                    tens  <= (tens == 4'd9) ? 4'd0 : (tens + 4'd1);
                end else begin
                    units <= units + 4'd1;
                end
            end

            // Stage 1: capture the rollover that just updated tens_o.
            roll_pending <= rollover;
            // Stage 2: emit the pulse one full cycle after tens_o changed.
            roll_q       <= roll_pending;
        end
    end

    assign tens_o = tens;
    assign roll_o = roll_q;

endmodule