Infrared-style touch screens are described having light emission means and light detection means along only two opposing sides of a touch input area, which are nevertheless able to detect and locate a touch object in two dimensions. Specifically, the present invention relates to expression of hepsin protease. The size of the persistence data is selectable by the user. The tap wall has an opening to receive the tap. Because the accumulated ticks are transferred to the core clock domain before the accumulator is reset, no ticks are ever lost using the circuitry of the illustrative embodiment.