Publisher Summary The chapter explores how the parallel port permits the ready exchange of digital data between the outside world and the controller CPU. The chapter also deals with the electrical characteristics of the parallel port and how they interact with external elements. It is the considerable diversity in the logic design of ports—which is required to be followed in similar patterns. The chapter also reveals how the internal circuitry works, as it leads to effective use of ports. A microcontroller requires a clock signal in order to operate. The 16F84A has diverse and flexible parallel ports. The features of the clock oscillator determine speed of operation and timing stability, and strongly influence power consumption. Active elements of the oscillator are usually built in to a microcontroller, but the designer must select the oscillator type—and its frequency and configuration. Therefore, the requirements need to be understood and must be met by a supply of the appropriate type.