🔗 I²C Communication

Why it matters

I²C lets you connect many peripherals (IMUs, OLEDs, environmental sensors) using only two wires.

The idea

The core idea

I²C (Inter-Integrated Circuit) is a two-wire bus protocol that lets you connect many devices to a microcontroller:

• SDA (Serial Data): Carries the actual data bits
• SCL (Serial Clock): Synchronizes data transfer

Both lines are open-drain and require pull-up resistors (typically 2.2kΩ–10kΩ) to 3.3V.

        <h3>Why Open-Drain?</h3>
        Open-drain means devices can only pull the line LOW, never HIGH. The pull-up resistor pulls it HIGH when no device is pulling LOW.
        This allows multiple devices to share the same bus safely:
        <ul>
          <li>Any device can pull the line LOW (wired-AND logic)</li>
          <li>If one device pulls LOW, the whole bus goes LOW</li>
          <li>All devices must release for the bus to go HIGH</li>
          <li>No conflicts — devices can't fight over the bus</li>
        </ul>

        <h3>Addressing: How Devices Are Selected</h3>
        Each I²C device has a unique 7-bit address (0x08 to 0x77). The master sends the address first:
        <ul>
          <li>7 address bits identify the device</li>
          <li>1 R/W bit: 0 = write (master→slave), 1 = read (slave→master)</li>
          <li>Combined into one 8-bit byte on the wire</li>
        </ul>
        Example: Address 0x3C (0b0111100) + Write (0) = byte 0x78 (0b01111000)

        <h3>Protocol Flow</h3>
        <div class=

Demo

Top line is SCL, bottom is SDA. Watch for:

• START: SDA falls while SCL is high
• STOP: SDA rises while SCL is high
• ACK/NACK: the 9th clock after each byte

Key takeaways

• I²C uses two wires (SDA, SCL) with open-drain outputs requiring pull-up resistors
• 7-bit addresses (0x08–0x77) + 1 R/W bit = 8 bits on wire
• START: SDA falls while SCL high; STOP: SDA rises while SCL high
• ACK (SDA LOW) = success; NACK (SDA HIGH) = error or end of read
• Clock stretching lets slow slaves pause the master by holding SCL LOW
• Multiple devices share the bus; addressing selects which responds
• Standard speed: 100 kHz; Fast: 400 kHz; bus capacitance limits max speed

Going deeper

Troubleshooting I²C Issues

• No response (NACK): Check address, power, wiring, pull-ups
• Garbled data: Reduce bus speed, check for loose connections
• Bus stuck LOW: A device may be holding SDA/SCL LOW; power cycle
• Address conflicts: Some devices have address-select pins (A0, A1)

        <h4>Address Scanning</h4>
        Scan addresses 0x08–0x77: send START + address + R/W, check for ACK.
        This identifies all devices on the bus — essential for debugging.

        <h4>Pull-up Resistor Selection</h4>
        <ul>
          <li>Too weak (high resistance): Bus rises slowly, limits speed</li>
          <li>Too strong (low resistance): High current when pulled LOW, wastes power</li>
          <li>Sweet spot: 2.2kΩ–10kΩ for 3.3V (depends on bus capacitance)</li>
          <li>Long wires need stronger pull-ups (lower resistance)</li>
        </ul>

        <h4>Reading vs Writing</h4>
        <ul>
          <li><strong>Write</strong>: Master sends data bytes, slave ACKs each</li>
          <li><strong>Read</strong>: Master sends address+R, slave sends data, master ACKs (or NACKs last byte)</li>
          <li>Some devices need register writes before reads (e.g.,

Math details

I²C frame (write, 7-bit address):
  START
  [A6 A5 A4 A3 A2 A1 A0 W]
  ACK
  [D7 D6 D5 D4 D3 D2 D1 D0]
  ACK
  STOP

Implementation

LLM Prompt: I²C Sensor Reading