Demystifying UART, SPI, and I2C: The High-Speed Communication Protocols of Hardware

Demystifying UART, SPI, and I2C: The High-Speed Communication Protocols of Hardware

When you connect a sensor, an OLED display, or a GPS module to an ESP32 or STM32 microcontroller, how do they talk to each other? They don't have ethernet ports or HTTP REST APIs.

Instead, microchips communicate using low-level serial protocols. The three most common hardware buses in embedded systems are UART, I2C, and SPI.

At Kone Lab, we teach students to diagnose these physical waveforms using oscilloscopes and logic analyzers. Here is how they compare.

1. UART: The Asynchronous Point-to-Point Wire

UART (Universal Asynchronous Receiver-Transmitter) is an asynchronous, point-to-point communication channel.

• Wires Needed: 2 wires (TX transmit, RX receive) + Ground.
• Clock Line: None! Both sender and receiver must pre-agree on the speed (Baud Rate, e.g., 9600, 115200 bps).
• Best Used For: Debug console logs, GPS modules, Bluetooth modules (HC-05).

flowchart LR
    subgraph MCU["Host Microcontroller (Transceiver 1)"]
        TX1["UART TX (Transmit)"]
        RX1["UART RX (Receive)"]
        GND1["GND (Signal Ground)"]
    end

    subgraph Sensor["GPS / Peripheral (Transceiver 2)"]
        RX2["UART RX (Receive)"]
        TX2["UART TX (Transmit)"]
        GND2["GND (Signal Ground)"]
    end

    TX1 -->|Asynchronous Serial Data| RX2
    TX2 -->|Asynchronous Serial Data| RX1
    GND1 <-->|Common Voltage Reference| GND2

    style MCU fill:#080d1a,stroke:#334155,stroke-width:1px,color:#f8fafc
    style Sensor fill:#080d1a,stroke:#334155,stroke-width:1px,color:#f8fafc
    style TX1 fill:#0f172a,stroke:#00d4ff,stroke-width:1px,color:#f8fafc
    style RX1 fill:#0f172a,stroke:#00d4ff,stroke-width:1px,color:#f8fafc
    style GND1 fill:#0f172a,stroke:#64748b,stroke-width:1px,color:#f8fafc
    style TX2 fill:#0f172a,stroke:#10b981,stroke-width:1px,color:#f8fafc
    style RX2 fill:#0f172a,stroke:#10b981,stroke-width:1px,color:#f8fafc
    style GND2 fill:#0f172a,stroke:#64748b,stroke-width:1px,color:#f8fafc

2. I2C: The Multi-Device Addressable Bus

I2C (Inter-Integrated Circuit) is a synchronous, two-wire multi-device bus developed by Philips (NXP).

• Wires Needed: 2 wires:

* SDA (Serial Data): Carries the binary payload bits.

* SCL (Serial Clock): Pulsed by the master to synchronize data bit sampling.

• Addressing: Each peripheral chip on the bus has a unique 7-bit hardware address (e.g. 0x3C for OLED screens). Up to 127 devices can share the same two pins!
• Pull-up Resistors: I2C is an open-drain line. Both SDA and SCL require 4.7kΩ pull-up resistors to 3.3V to function properly.

// Reading an I2C Temperature Sensor (Arduino C++)
#include <Wire.h>

#define SENSOR_I2C_ADDR 0x48

void setup() {
  Wire.begin();
  Serial.begin(115200);
}

void loop() {
  Wire.beginTransmission(SENSOR_I2C_ADDR);
  Wire.write(0x00); // Request temperature register
  Wire.endTransmission();

  Wire.requestFrom(SENSOR_I2C_ADDR, 2);
  if (Wire.available() == 2) {
    int msb = Wire.read();
    int lsb = Wire.read();
    float tempC = ((msb << 8) | lsb) * 0.0625;
    Serial.printf("Current Temperature: %.2f C\n", tempC);
  }
  delay(1000);
}

3. SPI: The High-Speed Full-Duplex Bus

SPI (Serial Peripheral Interface) is designed for high bandwidth (typically 10 MHz to 80 MHz+).

• Wires Needed: 4 wires:

* SCK (Serial Clock)

* MOSI (Master Out, Slave In)

* MISO (Master In, Slave Out)

* CS / SS (Chip Select / Slave Select)

• Full Duplex: Data is transmitted and received simultaneously on every clock pulse.
• Best Used For: SD card storage, high-framerate TFT LCD displays, RFID readers.

Summary Comparison

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