When an Arduino needs to talk to more than one module or sensor in the same project, the connected parties need a structured language to exchange data; this is where the three most common serial communication protocols come in: UART, I2C, and SPI, each with characteristics that make it the best fit for a particular situation.
UART: the simplest and most widespread
UART relies on just two pins: RX for receiving data and TX for sending it (digital pins 0 and 1 on classic Arduino boards), and it is also the protocol responsible for connecting the board to a computer over USB during uploads and serial monitoring. Its simplicity and availability on every Arduino board make it the default choice for a connection between just two devices.
I2C: fewer wires for more devices
I2C relies on just two shared lines (data and clock) that up to 128 devices can connect to at once, under a master-slave architecture where each device is identified by a unique address. Trading wire count for speed makes it the preferred choice for projects that combine several sensors and port-expander modules on the same board.
SPI: the fastest option when high transfer speed matters
Unlike I2C, SPI dedicates a separate select line to each slave device, with a single master able to communicate with up to four slaves. This design makes it the fastest of the three protocols, so it's used in applications requiring large, fast data transfer such as SD memory cards, displays, and real-time temperature sensors.
How to pick the right protocol
The choice depends on the project's nature: how many devices need connecting, how many pins are available on the board, and the required data-transfer speed. A simple single-device project suits UART, a project with several sensors suits I2C to save pins, while fast applications like displays and storage cards need SPI.
You can find modules and sensors supporting all three communication protocols at Juthour Tech.
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