This chapter presents a comprehensive description of the on-chip hardware features of Atmel’s Flash-based microcontrollers. Included in this description are the following items. • The port drivers and how they function both as ports and, for Ports 0 and 2, in bus operations • The Timer/Counters • The Serial Interface • The Interrupt System • Reset • The Reduced Power Modes and Low Power Idle The devices under consideration are listed in Table 1. Figure 1 shows a functional block diagram of the AT89 Series microcontrollers.