How to use SPI (Serial Peripheral Interface) in Arduino to Communication between two Arduino Boards

This project establishes a synchronous serial communication link between two Arduino Uno boards using the Serial Peripheral Interface (SPI) protocol. In an SPI configuration, one Arduino acts as the Master and the other acts as the Slave. The Master board initiates and controls the clock signal (SCK) and framing lines, while data is shifted synchronously between the two microcontrollers over the MOSI (Master Out Slave In) and MISO (Master In Slave Out) data lines. The Chip Select (SS) line determines which slave device is currently targeted by the master. To provide a functional interactive demonstration, each Arduino board is equipped with a push button, a 10k pull-down resistor for reliable digital input state reading, an LED, and a 2.2k current-limiting resistor to protect the LED. When a user presses the push button on one board, the local microcontroller detects the state change and transmits a command or data byte across the SPI bus to the other board. Upon receiving the data packet, the receiving microcontroller processes the command and updates the state of its local LED accordingly, effectively synchronizing or toggling outputs across the two separate hardware systems.