SPI protocol: GND VCC MISO MOSI SCK

Choosing I2C to make two boards communicate to eachother. They need connect the GND in common at least and make this two boards connect to power suply. anther way is using master board to provide both VCC and GND to slave board.（The A4 pin = SDA ，A5 pin = SDL）

Here is the code file

then we used the Serial busthrough TX and RX to comunacate eachother.

Here is the code file-SoftwareSerial