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Self-Calibration and Coordinate Conversion for a Touchpad or Touch Screen Computer Input Device

IP.com Disclosure Number: IPCOM000040055D
Original Publication Date: 1987-Sep-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 44K

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 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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Self-Calibration and Coordinate Conversion 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 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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To allow any downstream device driver and application program the maximum processing time, the coordinate conversion programs, which receive control when a hardware interrupt is first generated, must be extremely fast. These programs will see the "raw" X,Y data value, after it has passed through any software filtering, 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

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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. Application software that uses touchpad or touch screen input must be isolated from these devices and environment-dependent hardware considerations; this is the purpose of the coordinate conversion and self-calibration software. The goals of coordinate conversion and self-calibration are therefore as follows: 1. Extremely quick processing, so that subsequent

software data processing will have maximum time to

complete before the next data values are

presented.

2. Isolate application software from any calibration

procedure which accounts for unit-to-unit and

d...