The I2C Bus Standard

Most microcontrollers use the I2C bus as a standard way to communicate with peripheral devices such as sensors, real-time clocks and displays. In this section we cover how to connect your I2C devices to the main microcontroller and test that the connections are working.

Note that I2C is a two-way bus. Although in many of our display examples, there is only data moving from the microcontroller to the device. When we study real-time clocks we will see data going back and forth from the microcontroller to peripheral device.

Connections

The I2C bus has four connections:

1. GND - Ground
2. VCC - Power - (usually 3.3 or 5 volts)
3. SDA - Data
4. SCL - Clock

Ground (GND)

The common reference point for electrical signals that completes the circuit and ensures stable voltage measurements.

Connecting the display's GND pin to the microcontroller's GND pin provides a shared zero-voltage reference.

Power (VCC)

The voltage supply line that provides electrical power to operate the display, typically accepting either 3.3V or 5V depending on the model.

Connecting a 3.3V supply from the microcontroller to power a small OLED display.

Serial Data (SDA)

A bidirectional line that carries data bits between devices using a specific protocol for addressing and acknowledgment.

In the example code above, we use GPIO pin 0 to transmit display content and receive status information.

Serial Clock (SCL)

A timing signal line generated by the master device that synchronizes data transfer and defines when the data line should be read or written. In our work, the master device is the microcontroller which sends the clock signal to the display.

In the sample code, we use GPIO pin 1 to coordinate data transfers at rates typically between 100kHz and 400kHz.

Some important points about these connections:

1. Both SDA and SCL require pull-up resistors (typically 4.7kΩ)
2. All devices share the same GND connection
3. Multiple I2C devices can share the same SDA and SCL lines
4. Power must match the display's voltage requirements (check datasheet)

Sample I2C Display Initialization Code

from machine import Pin, I2C
from ssd1306 import SSD1306_I2C

i2c = I2C(0, sda=Pin(0), scl=Pin(1))
display = SSD1306_I2C(128, 64, i2c)

# drawing code here
display.fill(0)
display.text("MicroPython", 0, 0)
display.text("Rocks!", 20, 20)
display.show()

Let's break down this I2C initialization code:

This code sets up a communication interface to control an OLED display using I2C (Inter-Integrated Circuit) protocol instead of SPI. Here's what each part does:

1. The imports:

from machine import Pin, I2C            # Gets I2C and Pin control classes
from ssd1306 import SSD1306_I2C         # Gets the OLED display driver

1. The I2C bus initialization:

i2c = I2C(0,                           # Use I2C bus 0 
          sda=Pin(0),                  # Data line on GPIO pin 0
          scl=Pin(1))                  # Clock line on GPIO pin 1

1. The display initialization:

display = SSD1306_I2C(128,             # Display width in pixels
                      64,              # Display height in pixels 
                      i2c)             # I2C bus we created

Key differences from the SPI version: - Uses only 2 pins instead of 5 (no CS, DC, or RES needed) - Simpler initialization code - Slightly slower than SPI but adequate for most uses - Can share bus with other I2C devices using different addresses

Worth noting that I2C is often preferred for simple display projects because it requires fewer pins and simpler wiring, even though it's not as fast as SPI.

A common issue to watch for is making sure your I2C connections have appropriate pull-up resistors, though many development boards include these built-in.

Testing Your Connections

The I2C Scanner

from machine import I2C, Pin
import time

# I2C setup
i2c = I2C(0, sda=Pin(0), scl=Pin(1))

# Run the scanner
devices_found = i2c.scan()
for device in devices_found:
    print(device, hex(device))

This code is useful for testing connections on your clock projects.

Imagine you've just connected a new display or real-time clock chip to your Raspberry Pi Pico W, and you want to make sure it's properly connected before you start writing more complex code. This is where this I2C scanner comes in handy - it's like a metal detector that beeps when it finds something, but for electronic components!

Let's break it down step by step:

Imports

First, we import the tools we need:

from machine import I2C, Pin
import time

- I2C is for communicating with components using just two wires - Pin lets us control the physical pins on the Pico W - time gives us timing functions (though we don't use it in this example)

Connection Setup

Next, we set up I2C communication:

i2c = I2C(0, sda=Pin(0), scl=Pin(1))

- I2C(0) means we're using the first I2C channel - sda=Pin(0) connects the data line to pin GP0 - scl=Pin(1) connects the clock line to pin GP1

Run the Scan Function

Finally, we scan for devices and print what we find:

devices_found = i2c.scan()
for device in devices_found:
    print(device, hex(device))

• i2c.scan() looks for any I2C devices connected to those pins
• For each device found, it prints both the decimal number and hexadecimal address
• For example, if you see 104 0x68, that's the same number in two formats
• 0x68 (hex 68) is the address for a DS3231 real-time clock
• 0x3C (hex 3C) is common for OLED displays

This code is super helpful because: 1. It confirms your wiring is correct - if you see a device, your connections work! 2. It tells you the address of your device, which you need for the rest of your code 3. It helps troubleshoot - if you don't see your device, you know to check your wiring

Think of it like checking if your game controller is properly connected to your computer - this code does the same thing for I2C devices connected to your Pico W.

Common I2C Addresses for Clock and Watches

I'll create a comprehensive list of common I2C addresses you might encounter when building clocks and watches with the Raspberry Pi Pico W.

Real-Time Clock (RTC) Modules

• 0x68: DS3231 Temperature-compensated RTC
• 0x68: DS1307 Basic RTC
• 0x6F: PCF8563 RTC

OLED & LCD Displays

• 0x3C: SSD1306 OLED display (most common setting)
• 0x3D: SSD1306 OLED display (alternate address)
• 0x27: LCD displays with PCF8574 I2C adapter
• 0x3F: LCD displays with PCF8574A I2C adapter

Environmental Sensors (for clock/weather stations)

• 0x76: BME280 Temperature/Humidity/Pressure (primary address)
• 0x77: BME280 Temperature/Humidity/Pressure (alternate address)
• 0x40: HDC1080 Temperature/Humidity
• 0x44: SHT31 Temperature/Humidity (primary address)
• 0x45: SHT31 Temperature/Humidity (alternate address)

User Input Expanders

• 0x20 - 0x27: MCP23017 16-bit I/O expander (configurable)
• 0x38 - 0x3F: PCF8574 8-bit I/O expander (configurable)

Pro Tips:

1. If you see address conflicts (two devices wanting to use 0x68 for example), many devices have an address select pin or jumper to change their address.
2. Some devices like the SSD1306 OLED have a built-in address selection based on whether you connect their SA0 pin to ground or VCC.
3. Always run an I2C scanner before starting a new project to confirm your device addresses match what you expect.

Remember: These addresses are shown in hexadecimal format (hence the 0x prefix). When you see these in scan results, they'll often show both decimal and hex formats (like 104 0x68 for a DS3231).