SPI (Serial Peripheral Interface)

SPI uses separate lines for clock, data out and data in, plus a chip-select line for each device. With no addressing overhead and actively driven signals, it runs much faster than I²C, which is why it is used for flash memory, displays, SD cards, radio chips and fast ADCs. Quad-SPI variants move four bits at a time, mainly for external memory.

SPI has no single formal standard, so devices differ in clock polarity and phase (the "SPI modes"), word length and timing; reading each datasheet carefully is part of the job. Each additional device usually needs its own chip-select pin.

Where it bites: fast SPI over cables or long traces suffers signal-integrity problems, and mismatched SPI modes produce intermittent garbage that is hard to diagnose.