It seems the Serial Peripheral Protocol (SPI) was first introduced with the first microcontroller
deriving from the same architecture as the popular Motorola 68000 microprocessor, announced
in 1979. SPI defined the external microcontroller bus, used to connect the microcontroller
peripherals with 4 wires. Unlike I²C, it is hard to find a formal separate ‘specification’ of the SPI
bus – for a detailed ‘official’ description, one has to read the microcontrollers data sheets and
associated application notes.