Patent ID: 7196640

Claim:
A method of keypad decoding comprising the steps of: disposing a resistor ladder having a first portion of series connection of first number of elements of equal first resistance having first intermediate nodes between, a common node and a second portion of a series connection of equal second resistance having second intermediate nodes between, the second resistance equal to a product of the first resistance and the first number of elements; coupling a capacitor to said resistor ladder thereby forming a relaxation oscillator with a resistor/capacitor time constant; disposing pushbutton key switches relative to the resistor ladder to change a resistance value of the resistor ladder to unique resistance on a key press including disposing a first set of pushbutton key switches connected between respective first intermediate nodes and a common node, disposing a second set of pushbutton key switches connected between respective second intermediate nodes and the common node; detecting an oscillation frequency of the relaxation oscillator; and determining a key pressed from the oscillation frequency including determining which one key of said first set of pushbutton key switches is pressed in combination with which one key of said second set of pushbutton key switches is pressed.