1 Each term should be used in one and only one way throughout the dissertation.  The easiest way to avoid a long series of definitions is to include a statement: ``the terminology used throughout this document follows that given in [CITATION].'' Then, only define exceptions.
Serial Peripheral Interface Bus. The Serial Peripheral Interface bus (SPI) is a synchronous serial communication interface specification used for short distance communication, primarily in embedded systems. The interface was developed by Motorola in the late 1980s and has become a de facto standard. Typical applications include Secure Digital cards and liquid crystal displays.
PCI Express (PCIe) is a serial bus, with each lane sending digital signals as a 2-pin differential signal. To transmit the same amount of data, the serial bus needs to run at a much higher clock rate, but synchronizing that single signal is easier than making sure all 32 parallel signals arrive at exactly the same time, and the differential signal is more resilient to electrical noise.