What is SPI communication protocol?

SPI (Serial Peripheral Interface) is a synchronous serial protocol with four signals: SCLK (clock from master), MOSI (Master Out Slave In), MISO (Master In Slave Out), and SS/CS (Slave Select, active low). The master generates clock and selects slave. Data is shifted simultaneously in both directions (full-duplex). SPI is faster than I2C (MHz range), supports multiple slaves via individual SS lines, and is simpler than I2C (no addressing). Common uses: Flash memory, displays, sensors, and SD cards.