A micro-computer keyboard circuit

Most microcontroller based applications use an on-board small keyboard and various switches. Using a large number of microcontroller I/O pins to handle these keys and switches results in a remaining small number of pins available for other uses. There are several methods to read keys and switches using fewer I/O pins than used in the classical matrix configuration, e.g.. the use of the internal A/D converter (if one exists), or the use of external circuits (diodes and resistors or capacitor and resistors). These methods are based on a polling procedure that checks which key (if any) was pressed. After a short analysis of the limits of these methods, a new interrupt procedure based method is presented. This method uses only one I/O pin, but one with interrupt capabilities. Within it there are two ways of determining which key was pressed, a "software" and a "hardware" way. This method was implemented to an x51 microcontroller, and it can be extended to most microcontrollers. The advantage of the proposed method is the combination of the use of a single I/O pin with the interrupt working mode.