Browse Prior Art Database

Data Filtering for a Touchpad or Touch Screen Computer Input Device

IP.com Disclosure Number: IPCOM000040057D
Original Publication Date: 1987-Sep-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 54K

Publishing Venue

IBM

Related People

Reed, MA: AUTHOR

Abstract

A touchpad or touch screen is attached to a computer via a hardware interface card occupying a slot on the computer's I/O bus. This interface card interrogates the analog membrane, interrupting the CPU at regular intervals with status information and positional data, as long as the membrane is being pressed. In addition, there is commonly a touch-gone interrupt whenever the membrane has not been pressed since the last interrogation. For effective use as a graphics input device, the sampling rate of the membrane should be approximately 100 samples per second; thus, the hardware must interrogate the device approximately every 10 milliseconds, and software making use of this information must clearly process the data in this time interval or less.

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Data Filtering for a Touchpad or Touch Screen Computer Input Device

A touchpad or touch screen is attached to a computer via a hardware interface card occupying a slot on the computer's I/O bus.

This interface card interrogates the analog membrane, interrupting the CPU at regular intervals with status information and positional data, as long as the membrane is being pressed. In addition, there is commonly a touch-gone interrupt whenever the membrane has not been pressed since the last interrogation. For effective use as a graphics input device, the sampling rate of the membrane should be approximately 100 samples per second; thus, the hardware must interrogate the device approximately every 10 milliseconds, and software making use of this information must clearly process the data in this time interval or less. To allow any downstream device driver and application program the maximum processing time, the filtering program, which receives control when a hardware interrupt is first generated, must be extremely fast. This filtering program will see the "raw" X,Y data value representing the current location being pressed on the touchpad or touch screen. These raw X,Y values are the result of an analog-to-digital conversion of the analog samples in each cartesian coordinate direction by the interrogation circuitry on the hardware adapter card. The raw X,Y values will have a range dependent on the resolution of the A/D process (e.g., a 10-bit A/D result will theoretically yield digital values in the range 0-1023, a 12-bit A/D in the range 0-4095). However, due to hardware self-calibration circuitry, reference voltage drift, clipping, housing design (possibly preventing access to the extreme boundaries of the input device), and other hardware considerations, the actual digital range presented to the software interrupt handler will be somewhat less. By physical design, a resistive touchpad or touch screen consists of two membranes separated by small spacers at regular intervals. Depression of the device's surface directly above one of these spacers can cause voltage spiking, and therefore yield erroneous data that cannot be detected (and therefore cannot be discarded) in hardware. As an instrument (a stylus or finger) is moved across the surface of the touchpad or touch screen, a stream of X,Y data values is generated. The faster one moves, the "smoother" these values will be, just as a penciled line is "smoother" if one draws it faster. Very...