Back to results
US 4,651,133 AGranted
Method and apparatus for capacitive keyboard scanning
AssigneeAT&T Technologies, Inc.
InventorsApparajan Ganesan, Wayne A. Weise
Life of this patent
Estimating the term…
Loading patent…
Loading patent…
AssigneeAT&T Technologies, Inc.
InventorsApparajan Ganesan, Wayne A. Weise
Estimating the term…
A capacitive keypad (24) is scanned in such a manner that the capacitance value (C) of an individual key in a given row is multiplied by the sum of the relative capacitance values of all the other keys in the same row by means of an amplifier (32) to generate a combined pulse. The combined pulse is then compared by a comparator (28) to a threshold voltage (Vref 3) to verify whether the individual key in question is in the closed or open position. The verification can be made independently of keypad-to-keypad variations in the capacitances of the keys, since the combined pulse value does not depend upon the absolute capacitance value of the keys, but only upon their relative values. Also disclosed is a method for scanning to determine the presence of multiple key closures. This involves comparing the capacitance of an interrogated key to not only the sum of the other capacitors of its row, but also to the sum of the capacitors of the other rows and columns and a comparison of the resulting value to that generated by previous scans to detect change.
A method of determining the switched state of one of a plurality of key capacitors in rows and columns of a capacitive keyboard, comprising the steps of:
A capacitive keyboard of the type having a plurality of key capacitors organized in an array of rows and columns and means for electronically scanning said capacitors to generate a logic output indicative of the switching condition of the capacitors, The improved scanning means therein comprising:
Dependencies are inferred from the claim text.