Arduino, ESP32, & Raspberry Pi Pinout Reference for I2C, SPI, and UART

Microcontroller boards can communicate with a wide variety of sensors using several standard protocols. This guide serves as a practical resource for quickly referencing the pin connections for I2C, SPI, and UART communication on popular microcontroller boards, including Arduino, ESP32, and Raspberry Pi. If you're new to Arduino and need the basics on board setup and the IDE, start with our Arduino getting started guide for beginners.

I2C (Inter-Integrated Circuit)

I2C is a master-slave protocol where the Arduino acts as the master and sensors are slaves. Each device has a unique 7-bit or 10-bit address, allowing multiple devices to share the same SDA and SCL lines. The master initiates communication by sending the device address followed by read or write commands. Data is transmitted in bytes, synchronized by the clock line, and both master and slave can acknowledge receipt of each byte.

Pin Descriptions

Pin	Type	Description
SDA	Data	Carries data between master and slave devices
SCL	Clock	Synchronizes data transfer

Pin Mapping

Board	SDA	SCL	Notes
Arduino Uno / Nano / Mini	A4	A5	Fixed hardware pins
Arduino Mega 2560	20	21	Fixed hardware pins
Arduino Leonardo / Micro	2	3	Fixed hardware pins
Arduino Due / Zero	20	21	Fixed hardware pins
ESP32	21 (default)	22 (default) - configurable via code	Can be reassigned in software
ESP8266 (NodeMCU)	D2 (GPIO4)	D1 (GPIO5) - configurable via code	Can be reassigned in software
Raspberry Pi Pico / Pico 2	I2C0: GP0 I2C1: GP2	I2C0: GP1 I2C1: GP3	2 I2C buses (I2C0 default)

SPI (Serial Peripheral Interface)

SPI uses a master-slave setup with separate lines for data transmission (MOSI / PICO), data reception (MISO / POCI), clock (SCK), and a unique chip select (CS) for each device. The master controls the clock, and communication occurs full-duplex, meaning data can be sent and received simultaneously. Each slave listens only when its CS pin is active, allowing multiple devices to share the SPI bus.

Pin Descriptions

Pin	Type	Description
MOSI / PICO	Data Out	Sends data from master to slave
MISO / POCI	Data In	Receives data from slave to master
SCK	Clock	Synchronizes data transfer
CS / SS	Chip Select	Selects the active slave device

Pin Mapping

Board	MOSI / PICO	MISO / POCI	SCK	CS / SS
Arduino Uno / Nano / Mini	11	12	13	10 (default SS)
Arduino Mega 2560	51	50	52	53 (default SS)
Arduino Leonardo / Micro	D16	D14	D15	10 (default SS)
ESP32	23 (default)	19 (default)	18 (default)	5 (default) - configurable
ESP8266 (NodeMCU)	D7 (GPIO13)	D6 (GPIO12)	D5 (GPIO14)	D8 (GPIO15)
Raspberry Pi Pico / Pico 2	GP3 / GP7	GP4 / GP8	GP2 / GP6	Custom

UART / Serial

UART is a point-to-point, asynchronous protocol that sends data as a stream of bits using TX and RX lines. There are no addresses; each device connects directly to another. The data is framed with start and stop bits, and both devices must agree on a baud rate to ensure accurate timing.

Pin Descriptions

Pin	Type	Description
TX	Transmit	Sends data from this device to another UART device
RX	Receive	Receives data from another UART device

Pin Mapping

Board	TX	RX	Notes
Arduino Uno / Nano / Mini	1	0	Single hardware UART
Arduino Mega 2560	TX0: 1 TX1: 18 TX2: 16 TX3: 14	RX0: 0 RX1: 19 RX2: 17 RX3: 15	Four hardware UARTs
Arduino Leonardo / Micro	TX: 1 (USB Serial via MCU)	RX: 0 (USB Serial via MCU)	Hardware Serial available on pins 0/1, plus USB Serial
ESP32	TX0: 1 TX1: 17 TX2: 16	RX0: 3 RX1: 16 RX2: 17	Multiple UARTs, configurable pins
ESP8266 (NodeMCU)	TX: D10 (GPIO1)	RX: D9 (GPIO3)	Single hardware UART
Raspberry Pi Pico / Pico 2	TX0: GP0 TX1: GP4	RX0: GP1 RX1: GP5	Two UARTs, pins configurable

Comparison of I2C, SPI, and UART

Each communication protocol has its own advantages and typical use cases depending on the number of devices, speed requirements, and wiring complexity. Here's a detailed comparison:

Category	I2C	SPI	UART / Serial
Number of Wires	2 (SDA, SCL)	4 minimum (MOSI / PICO, MISO / POCI, SCK, CS per device)	2 (TX, RX)
Speed	Standard: 100 kbps Fast: 400 kbps Fast+: 1 Mbps	Fast: typically up to 10–50 Mbps (depending on MCU)	Standard baud rates: 9600–115200 bps, can be higher
Device Addressing	Each slave has a unique 7-bit or 10-bit address	No addressing, each slave has dedicated CS line	Point-to-point (no addressing)
Typical Use Cases	Multiple sensors, EEPROMs, RTCs, display modules	High-speed sensors, SD cards, displays, ADC/DAC modules	GPS modules, Bluetooth modules, serial debugging, simple devices
Pros	Few wires, supports multiple devices on same bus, widely used	Very fast, full-duplex communication, simple protocol	Simple to implement, widely supported, long cable distance possible
Cons	Slower than SPI, limited cable length, may need pull-up resistors	More wires required, limited number of CS lines, each device needs separate CS	Only supports 1-to-1 communication per hardware UART, slower than SPI for multiple devices

When to Use Which Protocol

• I2C: Use when you have multiple low-to-medium speed devices that need to share the same bus with minimal wiring, like temperature sensors, EEPROMs, RTCs, or small displays.
• SPI: Use when you need high-speed communication, full-duplex data transfer, or when working with SD cards, TFT displays, ADC/DAC chips, or any device where speed is critical.
• UART / Serial: Use for simple point-to-point communication, debugging via serial console, or modules like GPS, Bluetooth, or other serial peripherals where only two wires are needed.