Publisher Summary This chapter explores the use of small-scale hardware design. The microcontroller needs to transfer digital data between its CPU and the outside world. Parallel input/output (I/O) is the workhorse for all the basic data interchange of a microcontroller, including interfacing with switches, LED, displays and so on. A group of parallel I/O interconnections, appearing on the pins of the microcontroller, is called a parallel port. The chapter illustrates how simple logic circuits can be developed to give a flexible interface between the microcontroller data bus and the outside world—these are the parallel ports. The chapter explains how external devices can be connected to the parallel port. In addition, the parallel input/output available on the PIC 16F84A is discussed. The 16F84A has diverse and flexible parallel ports. The essential hardware features of power supply and clock oscillator are also discussed in this chapter. A microcontroller needs a clock signal in order to operate. The characteristics of the clock oscillator determine speed of operation and timing stability, and strongly influence power consumption.