Circuit for detecting keypad conditions in a microprocessor controlled telephone instrument

Abstract From the Canadian patent

A technique for evaluating keypad conditions while utilizing only a minimum number of ports in a microprocessor involved in control of a telephone instrument. Because of the limited number of ports available and necessity to also evaluate conditions of mode selecting strap switches, the utilization of a reduced number of ports is highly desirable in such an enviroment. Such a reduction becomes an absolute necessity particularly in dealing in those enviroments that include random access memories read, only memories, pulse dialers, integrated circuits and liquid crystal displays. A combination of diode gating, resistive decoupling and time multiplexing techniques are used to match the …