How do you interface the Raspberry Pi with external microcontrollers (e.g., via UART, I2C)?

Interfacing the Raspberry Pi with external microcontrollers (like Arduino, STM32, etc.) is a common and powerful technique to combine the strengths of both platforms.

Here's how you can do it using UART, I²C, and SPI—the most popular communication interfaces:

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1. UART (Serial Communication)

Wiring

Logic levels: Raspberry Pi is 3.3V — don’t connect directly to 5V devices without a level shifter!

Example (Python with PySerial)

Install:

bash

sudo apt install python3-serial

python

import serial

ser = serial.Serial('/dev/serial0', 9600)
ser.write(b'Hello MCU\n')
data = ser.readline()
print("Received:", data.decode())
ser.close()

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2. I²C (Inter-Integrated Circuit)

Wiring

Enable I²C:

bash

sudo raspi-config → Interface Options → I2C → Enable

Example (Python with smbus)

Install:

bash

sudo apt install python3-smbus i2c-tools

python

import smbus
bus = smbus.SMBus(1)
addr = 0x08  # Example slave address
bus.write_byte(addr, 0x42)
data = bus.read_byte(addr)
print("Received:", data)

Use i2cdetect -y 1 to scan devices.

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3. SPI (Serial Peripheral Interface)

Wiring

Enable SPI:

bash

sudo raspi-config → Interface Options → SPI → Enable

Example (Python with spidev)

Install:

bash

pip3 install spidev

python

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)  # Bus 0, CS0
spi.max_speed_hz = 1000000
spi.writebytes([0x01, 0x02])
response = spi.readbytes(2)
print("Received:", response)
spi.close()

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Which Interface Should You Use?

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Tips for Reliable Communication

• Always match logic levels (use level shifters if needed).

• Use pull-up resistors for I²C (typically 4.7kΩ).

• Keep ground lines connected between devices.

• Validate with tools: i2cdetect, minicom, logic analyzers.