always @(posedge clk) begin
    if(rst) begin
        xpos <= 0;
        ypos <= 0;
        state <= DOWN;
    end
    else begin
        xpos <= xpos_nxt;
        ypos <= ypos_nxt;
        state <= state_nxt;
    end
end

always @* begin

    xpos_nxt = XPOS_INIT;

    case (state)

    DOWN: begin

        ypos_nxt = ypos + 1;

        if ( ypos >= 300) begin
            GO_UP = 1'b1;
            state_nxt = CHANGE_DIRECTION;
        end
        else begin
            GO_UP = 1'b0;
            state_nxt = DOWN;
        end

    end

    UP: begin

        ypos_nxt = ypos - 1;

        if ( ypos <= 1) begin
            state_nxt = CHANGE_DIRECTION;
            GO_UP = 1'b0;
        end
        else begin
            GO_UP = 1'b1;
            state_nxt = UP;
        end
    end

    CHANGE_DIRECTION: begin

        ypos_nxt = ypos;
        state_nxt = GO_UP ? UP : DOWN;
        GO_UP = 1'b0;

    end

    default: begin
        GO_UP = 1'b0;
        state_nxt = DOWN;
    end

endcase
end

endmodule