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Dual Drive Capacitor Keyboard

IP.com Disclosure Number: IPCOM000081502D
Original Publication Date: 1974-Jun-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 80K

Publishing Venue

IBM

Related People

Fox, JE: AUTHOR

Abstract

As logic densities increase, the number of pins required on a given electronic module becomes a more critical consideration. In various keyboard systems, the number of pins required is reduced from one per key to one for several keys by matrixing the drive and sense lines. Typically, a configuration would have 2O drive and 4 sense lines for an 80-key keyboard unit. Assuming that all the logic for the keyboard were in a single module, and one pin was required for each drive and each sense line, then 24 pins would be utilized in the 80 key example given. This is a considerable improvement over 81 pins normally required for an unmatrixed 80-key keyboard, but it would be desirable to reduce the number of pins even further.

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Dual Drive Capacitor Keyboard

As logic densities increase, the number of pins required on a given electronic module becomes a more critical consideration. In various keyboard systems, the number of pins required is reduced from one per key to one for several keys by matrixing the drive and sense lines. Typically, a configuration would have 2O drive and 4 sense lines for an 80-key keyboard unit. Assuming that all the logic for the keyboard were in a single module, and one pin was required for each drive and each sense line, then 24 pins would be utilized in the 80 key example given. This is a considerable improvement over 81 pins normally required for an unmatrixed 80-key keyboard, but it would be desirable to reduce the number of pins even further.

An additional reduction can be obtained by utilizing a dual-drive approach. Referring to Fig. 1a and 1b, a typical capacitive coupling pad arrangement is illustrated in which plural coplanar drive pads and a sense pad are shown. The configuration of Fig. 1a is similar to the usual single drive and sense line configuration for the capacitive pads, but two drive pads labeled drive 1 and drive 2 are shown. It should be understood that a cooperating capacitive coupling plate would be brought down into coupling operation with the drive pads and the sense pad when a key is depressed, and this is not shown for the sake of clarity.

Fig. 1b illustrates an alternative configuration for the drive and sense pads utilizing approximately the same area as Fig. 1a. It will be appreciated that the method of using more than one drive line can be expanded and that numerous drive lines could be used, as will be detailed later. Although the illustration is for capacitively coupled drive and sense plates, it should be equally clear that purely conductive drive and sense pads work as well, where a cooperating bridging or shorting contact would be brought down into contact with all three pads (if the drivers can be logically dotted together, or are diode isolated to prevent component damage due to overloads).

For example, utilizing two lines out of eight as a two out of eight code, 28 unique combinations can be created. Therefore, if eight drive lines were used in conjunction with four amplifiers, a maximum of 4 times 28 keys, which equals 112, could be used while requiring only 8 plus 4, equals 12, module pins. There would be some increase in the logic complexity in order to work with a 2 out of 8 code, but the savings in pins would become an important consideration when large-scale integrated (LSI) modules are used. The unique definition of each particular key would be given by a sense amplifier address in two drive line addresses as will appear below.

Fig. 2 illustrates the logic in a block form for a dual-drive approach. A 7-bit binary counter 1 is utilized as the basic scanner. The low order 5 bits, for example, from binary counter 1 are converted to a six-bit code in the converter 2. Three of these 6 bit...