Assignment 2: I2C

In this module, you’ll move beyond GPIO to a communication protocol — I2C. You’ll set up an I2C bus, scan for devices, and control GPIO pins through an I/O expander.

I2C Foundations

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What is I2C?

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I2C (Inter-Integrated Circuit) is a two-wire bus protocol for communicating with sensors, displays, and other devices.

The two wires are:

• SDA — Serial Data (bidirectional). Used to propagate data bits.
• SCL — Serial Clock (driven by the controller). Used to synchronize data exchanges on the bus.

Since it’s a bus, multiple devices can share the same two wires. Each device has a unique address (7-bit) that the controller uses to address any device on the bus.

Theory of Operation

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• Each bus has at least one master and one or more slaves
• The master orchestrates operations on the bus and addresses slaves using the 7-bit address
• Data exchange speed is governed by the clock speed (propagated on SCL)
• A master can perform two operations: read or write
• I2C exchanges data in one-byte chunks

Write Operations

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A master writes data to a slave. You need:

• The address of the slave
• An array of bytes to write

Read Operations

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A master reads data from a slave. You need:

• The address of the slave
• A byte array buffer for the received data

Configurations

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Exercise A: Bus Scan

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Goal: Set up an I2C bus and scan for connected devices.

1. Create a New Project

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esp-generate --chip esp32c3 -o unstable-hal -o vscode -o esp-backtrace -o log --headless i2c_scan
cd i2c_scan

2. Find an I2C Example

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Navigate to the esp-hal I2C documentation.

Look for an I2C example in:

• The module-level documentation (esp_hal::i2c)
• The I2c struct page
• The esp-hal examples directory on GitHub

3. Apply the Mental Model

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Read the example and identify:

• Instantiate — How is the I2c created? What peripheral and pins does it need?
• Configure — What configuration is applied? (Clock speed, pins)
• Control — What methods are available for reading/writing?

4. Adapt to Your Hardware

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• Check your board’s pinout for the I2C SDA and SCL pins
• Update the pins in the example to match

5. Scan the Bus

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Write a loop that attempts to communicate with every address from 0x01 to 0x7F:

• For each address, try a zero-length write
• If the write succeeds, a device is present at that address
• Print the address of each device found

6. Build and Flash

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cargo build --release
espflash flash target/riscv32imc-unknown-none-elf/release/i2c_scan --monitor

Exercise B: GPIO over I2C

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Goal: Use the TCA6424A I/O expander to control GPIO pins over I2C, replicating what you did with direct GPIO.

Background

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The TCA6424A is a 24-bit I/O expander connected via I2C. It provides three banks (ports) of 8 GPIO pins each (P0, P1, P2), giving you 24 additional GPIO pins over just two I2C wires.

How I2C Communication Works with the TCA6424A

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Communication happens in two cycles:

1. First cycle (write): Send the register address — tells the device which internal register to access
2. Second cycle (read or write): Write a value to that register or read back the current value

For a write operation, send the register address followed by the data byte in the same I2C write transaction.

For a read operation, first write the register address, then perform a separate I2C read.

TCA6424A Register Map

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Steps

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1. Create a New Project

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esp-generate --chip esp32c3 -o unstable-hal -o vscode -o esp-backtrace -o log --headless i2c_expander
cd i2c_expander

2. Configure Pin Direction

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Write to the Configuration register for the appropriate port. Set the corresponding bit to 0 for output.

3. Set a Pin High

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Write to the Output register for the appropriate port. Set the corresponding bit to 1 for high.

4. Blink an LED

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Combine the above in a loop:

1. Configure the port direction as output (once at startup)
2. In a loop: set the pin high, delay, set the pin low, delay

5. Build and Flash

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cargo build --release
espflash flash target/riscv32imc-unknown-none-elf/release/i2c_expander --monitor

Exercise C: Adaptation Challenge

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Goal: Read a button input through the I/O expander, controlling an LED entirely over I2C.

Steps

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1. Create a New Project

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esp-generate --chip esp32c3 -o unstable-hal -o vscode -o esp-backtrace -o log --headless i2c_challenge
cd i2c_challenge

2. Configure an Input Pin

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Using the TCA6424 registers:

• Set a pin’s configuration bit to 1 (input)
• This pin should be connected to a button on your board

3. Read the Button State

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• Read the input register for the relevant bank
• Check the bit corresponding to your input pin
• Detect when the button is pressed

4. Control the LED

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Combine input reading with output control:

• Read the button state from the I/O expander
• When pressed, turn on the LED (on the I/O expander)
• When released, turn off the LED

5. Build and Flash

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cargo build --release
espflash flash target/riscv32imc-unknown-none-elf/release/i2c_challenge --monitor

Cross-HAL Comparison

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How do other HALs handle I2C? Navigate the documentation for these two libraries:

• rp2040-hal
• stm32f4xx-hal

Compare how each HAL handles Instantiate, Configure, and Control for I2C.