This chapter briefly describes some of the devices which are present in most modern computer systems. It then describes in detail the clock management devices on the Raspberry Pi and the pcDuino. Next, it gives an explanation of asynchronous serial communications, and explains how there is some tolerance for mismatch between the clock rate of the transmitter and receiver. It then explains the Universal Asynchronous Receiver/Transmitter (UART) device. Next it covers in detail the UART devices present on the Raspberry Pi and the PcDuino. Once again, the reader is given the opportunity to do a comparison between two different devices which perform almost precisely the same functions.