What is SPI and when would you use it?

SPI (Serial Peripheral Interface) is a synchronous serial communication protocol using master-slave architecture. Four signals: SCLK (clock from master), MOSI (Master Out Slave In), MISO (Master In Slave Out), SS/CS (Slave Select, active low). Characteristics: Full-duplex synchronous communication. High speed (MHz range, often 10-50 MHz). No addressing; devices selected by chip select lines. No acknowledgment mechanism. Simple hardware implementation. Use SPI for: High-speed data transfer (SD cards, displays). Daisy-chainable devices. When multiple bytes need fast transfer. Flash memory, ADCs, DACs, displays, and sensors commonly use SPI.