SPI
Serial Peripheral Interface.

SPI is the most common high-speed embedded bus. It is not standardized by a single specification body but is a widely adopted de-facto standard introduced by Motorola; clocking and bit-order conventions are described in NXP, ST, and other vendor reference manuals.

The bus uses four signals: MOSI (Controller → Target), MISO (Target → Controller), SCK (clock), and CS (chip-select, one per target). Communication is full-duplex — both directions clock data on every bit time. There is no bus-level addressing; target selection is handled entirely by the CS line.

Four clock modes are defined by the combinations of CPOL (clock polarity) and CPHA (clock phase). The maximum clock rate is determined by the target device, board layout, and trace length — modern flash, displays, and ADCs commonly accept clocks above 50 MHz.