Chapter 13. Communicating Using I2C and SPI
The I2C (Inter-Integrated Circuit) and SPI (Serial Peripheral Interface) standards were created to provide simple ways for digital information to be transferred between sensors and microcontrollers such as Arduino. Arduino libraries for both I2C and SPI make it easy for you to use both of these protocols.
The choice between I2C and SPI is usually determined by the devices you want to connect. Some devices provide both standards, but usually a device or chip supports one or the other.
I2C has the advantage that it only needs two signal connections to Arduino—using multiple devices on the two connections is fairly easy, and you get acknowledgment that signals have been correctly received. The disadvantages are that the data rate is slower than SPI and data can only be traveling in one direction at a time, lowering the data rate even more if two-way communication is needed. It is also necessary to connect pull-up resistors to the connections to ensure reliable transmission of signals (see the introduction to Chapter 5 for more on pull-ups).
The advantages of SPI are that it runs at a higher data rate, and it has separate input and output connections, so it can send and receive at the same time. It uses one additional line per device to select the active device, so more connections are required if you have many devices to connect.
Most Arduino projects use SPI devices for high data rate applications such as Ethernet and memory ...