This application note shows how to use the PCF8584 I2C-bus controller with 80C51 family microcontrollers. One typical way of connecting the PCF8584 to an 80C31 is shown. Some basic software routines are described showing how to transmit and receive bytes in a single master system. An example is given of how to use these routines in an application that makes use of the I2C circuits on an I2C demonstration board. The PCF8584 is used to interface between