I2C Essentials

# I2C Essentials

**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.

The Embedded Rustacean · Rust Week 2026