I2C Essentials

~10 min slides

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

How I2C Works

Two wires:

SDA — Serial Data (bidirectional)
SCL — Serial Clock (driven by the controller)

Multiple devices can share the same two wires. Each device has a unique address (7 bits = up to 128 devices on one bus).

  Controller (ESP32-C3)
       │  │
   SDA ┤  ├ SCL
       │  │
  ┌────┴──┴────┐
  │  I2C Bus   │
  ├────────────┤
  │            │
Sensor A    Sensor B
(addr 0x68) (addr 0x44)

Key Concepts

Concept	What It Means
Controller	The device that initiates communication (our ESP32-C3)
Target	The device being addressed (sensors on the uFerris board)
Address	7-bit identifier for each device (e.g., `0x68`)
Write	Controller sends data to a target
Read	Controller reads data from a target
Clock Speed	How fast data transfers: 100 kHz (standard), 400 kHz (fast)

The Driver Crate Pattern

This is where the embedded-hal abstraction becomes powerful:

esp-hal ──implements──→ embedded_hal::i2c::I2c trait
                              ↑
                              │ uses
                              │
Driver crate ──────→ takes generic `impl I2c`
(e.g., icm42670)

The driver crate doesn't import esp-hal. It only depends on the embedded-hal trait. This means:

The driver works on ESP32, nRF52, STM32, RP2040 — any chip with an I2c implementation
You can switch chips without changing your driver code
This is the whole point of the abstraction layers from Part 1

Configurations

Setting	Options	docs.rs Location
Clock frequency	100 kHz, 400 kHz, custom	`Config::with_frequency()`
SDA/SCL pins	Any GPIO with I2C capability	`.with_sda()`, `.with_scl()`
Timeout	Bus timeout duration	`Config::with_timeout()`

Controls

Method	What It Does
`write(addr, &[u8])`	Send bytes to a device
`read(addr, &mut [u8])`	Read bytes from a device
`write_read(addr, &[u8], &mut [u8])`	Write then read in one transaction

What's on the uFerris Board?

Your uFerris board has I2C sensor(s) connected. You'll discover exactly which ones in the first exercise — by scanning the bus.

Check your pinout card for the SDA and SCL pin assignments.