Patent ID: 7133029

Claim:
A keypad including; a plurality of keys for actuation by a user to input data into the keypad; a matrix electrical interconnection interconnecting the keys so that actuation of a key can be detected by scan signals applied to the matrix and by reading signals from the matrix electrical interconnection; control means for causing the matrix electrical interconnection to change state independently of the depression of a key to thereby disguise which of the keys has been pressed, and to interrogate the matrix electrical interconnection having regard to the caused change of state of the keys to determine which of the keys has actually been pressed; and state actuating means for causing the matrix interconnection to take up a predetermined state when a key is pressed to prevent the identity of the pressed key from being determined by an outside observer attempting to determine the duration of depression of the pressed key, wherein the matrix electrical interconnection includes a plurality of read lines and a plurality of scan lines, the scan lines and read lines being interconnected by the keys so that when a key is depressed a scan line is connected to a read line, and wherein the scan lines are connected to open drain ports of the control means so that when the scan lines are connected to a read line by the depression of a switch the read line will maintain a predetermined state notwithstanding the control means attempting to change the state of that read line, and wherein the control means applies signals to the read lines to make the read lines change state and identifies a read line associated with a depressed key by the read line not changing state when an attempt is made to make the read line change state because of the connection of that read line to the open drain port via the depressed key.