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
);

    // Hidden units digit of the two-digit BCD counter.
    reg [3:0] units;

    // Registered "rollover happened" flag, delayed one cycle so that
    // roll_o pulses one full clock after the new tens_o first appears.
    reg roll_pending;

    // Combinational detection of a units 9 -> 0 rollover on this edge.
    wire rollover_now = 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
            // Second pipeline stage: emit the delayed rollover pulse.
            roll_o <= roll_pending;

            // First pipeline stage plus the actual counting.
            if (inc_i) begin
                roll_pending <= rollover_now;
                if (units == 4'd9) begin
                    units  <= 4'd0;
                    tens_o <= (tens_o == 4'd9) ? 4'd0 : (tens_o + 4'd1);
                end else begin
                    units <= units + 4'd1;
                end
            end else begin
                roll_pending <= 1'b0;
            end
        end
    end

endmodule