SPI

Not every board-level peripheral link is worth squeezing into a shared-arbitration model like I2C just to save two wires. Devices like Flash, ADCs, and display controllers often care about something else: can the host send a stream of bits steadily, quickly, and directly, then get the result back?

SPI solves exactly that kind of problem. It optimizes first for clear timing, higher throughput, and simple controller implementation rather than for pin savings when many devices share the bus. If I2C tries to keep order on two shared wires, SPI goes the other way: it gives the master stronger control first, then accepts the cost of more wires and weaker scalability.