Key Engineering Takeaways
- •UART is an asynchronous point-to-point protocol requiring only 2 wires (TX and RX) and a shared Baud rate (e.g. 115200).
- •I2C is a synchronous 2-wire multi-device bus (SDA and SCL) where up to 127 sensor breakout boards share the same two pins via unique 7-bit hexadecimal addresses.
- •SPI is a synchronous 4-wire high-speed master-slave protocol (MOSI, MISO, SCK, CS) capable of 10MHz - 80MHz speeds for displays and SD cards.
- •I2C requires 4.7kΩ pull-up resistors on both SDA and SCL lines because devices use open-drain outputs.
Prerequisites
- • Basic Microcontroller GPIO and Digital logic
Required Hardware / Tools
- • ESP32 / Arduino
- • I2C OLED Display (SSD1306) or MPU6050 IMU
- • SPI SD Card Module
Why Serial Communication? Parallel vs Serial Buses
In early computing, chips communicated over **Parallel buses**—sending 8, 16, or 32 bits simultaneously across 32 physical copper wires. However, parallel buses suffer from wire congestion and clock skew at high speeds.
Modern embedded systems use **Serial Communication**, streaming bits sequentially one after another over 1 to 4 wires at megahertz frequencies.
I2C (Inter-Integrated Circuit 2-Wire Bus)
Invented by Philips (NXP), **I2C (I-squared-C)** is the most popular bus for robotics sensors (IMUs, barometers, OLED displays):
- **Wires**: Only 2 lines: **SDA (Serial Data)** and **SCL (Serial Clock)**.
- **Topology**: Multi-device bus. You can connect up to 127 different sensor modules to the *exact same two pins* on your microcontroller!
- **Addressing**: Every I2C sensor has a factory hardcoded 7-bit address (e.g. `0x68` for MPU6050 gyroscope). The MCU broadcasts the target address before sending data.
- **Pull-Up Resistors**: Both SDA and SCL are open-drain lines and require $4.7\,\text{k}\Omega$ pull-up resistors to $3.3\,\text{V}$.
SPI (Serial Peripheral Interface High-Speed Bus)
When high throughput is required (such as color TFT LCD screens, SD card data loggers, or 1000Hz IMUs), **SPI** is the protocol of choice:
- **Wires (4 lines)**:
- **MOSI (Master Out Slave In)**: MCU transmits data to sensor.
- **MISO (Master In Slave Out)**: Sensor transmits data back to MCU.
- **SCK (Serial Clock)**: Synchronous clock generated by MCU.
- **CS / SS (Chip Select)**: Dedicated pin pulled LOW by MCU to select which slave device is currently active.
- **Speed**: $10\,\text{MHz}$ to $80\,\text{MHz}$ (over $100\times$ faster than standard I2C!).
Frequently Asked Questions
How do I know what I2C address my sensor is using?
You can flash a simple "I2C Scanner" sketch in Arduino IDE. The MCU scans all 127 possible addresses from 0x01 to 0x7F and prints the detected device address to the Serial Monitor.
Tags:#Basic Microcontrollers#UART#I2C#SPI#CAN Bus#Serial Protocols#Sensors