Browse Prior Art Database

Dynamic Updating of Key-Sensing Thresholds

IP.com Disclosure Number: IPCOM000043532D
Original Publication Date: 1984-Sep-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 4 page(s) / 46K

Publishing Venue

IBM

Related People

Rowe, JT: AUTHOR

Abstract

Microprocessor-based keyboard scanners and control mechanisms are known to employ variable threshold level sensing amplifiers to determine when a given key in a key matrix has been depressed. For example, U.S. Patent 4,305,135 shows a typical example. In capacitive keyboard matrices, small dust or debris particles may lodge themselves beneath the capacitive coupling plate on top of the capacitive circuitboard and reduce the amount of capacitive coupling by increasing the space between the coupling plate and the capacitive circuit elements on the circuitboard. Extremely small electrical signals are encountered under the best of circumstances; hence, the occurrence of contamination can sometimes result in inability to sense a truly depressed key.

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Dynamic Updating of Key-Sensing Thresholds

Microprocessor-based keyboard scanners and control mechanisms are known to employ variable threshold level sensing amplifiers to determine when a given key in a key matrix has been depressed. For example, U.S. Patent 4,305,135 shows a typical example. In capacitive keyboard matrices, small dust or debris particles may lodge themselves beneath the capacitive coupling plate on top of the capacitive circuitboard and reduce the amount of capacitive coupling by increasing the space between the coupling plate and the capacitive circuit elements on the circuitboard. Extremely small electrical signals are encountered under the best of circumstances; hence, the occurrence of contamination can sometimes result in inability to sense a truly depressed key. The present article describes a dynamic method of setting an individual threshold into addressable memory accessed by the microprocessor for controlling the variable sense amplifier threshold individually for each key. The thresholds are updated after every full keyboard scan with a few of the keys being checked for threshold variations each time. The sense amplifier has its threshold adjusted and the appropriate level stored in memory so that the existing operative threshold for each key can be employed. In Fig. 1, the overall block diagram of a keyboard system is shown. The microprocessor 1 through drive lines 2 applies sensing signals to the columns of a key matrix 3, for example, one column at a time being provided with one or more pulses from the microprocessor 1. Sense lines 4 are connected to a variable threshold sense amplifier 5 which receives control signals over lines 6 to select a given row for sensing and to control the sense amplifier sensing threshold. The sense amplifier 5 provides an output over one of the group of lines 6 back to the microprocessor 1 to signal the detection of sensing an output from the key matrix 3 at or above whatever the sensing threshold for a given key has been. Fig. 2 illustrates a typical matrix output signal level for a depressed key, a released key and a depressed key having a five thousandths of an inch contamination debris between the coupling plate and the capacitive circuitboard. Also shown are a variety of typical threshold levels. For example, eight levels numbered 0 through 7 are depicted on the ordinate. The sense amplifier output of an undepressed key (released key) is approximately the 0 threshold level so that undepressed keys will not present a coupling sense amplifier output signal above the lowest sensible threshold level of the amplifier. The problem then becomes to select an appropriate sensing threshold for each key given that contamination will reduce the maximum output signal possible because it will prevent the coupling plate from coming into close coupling contact with capacitive pads on the circuitboard which would produce the high depressed key output level normally expected. Obviou...