I2C Main

Note this guide was written for MicroPython.org firmware and not Adafruit CircuitPython firmware. This guide is not maintained and is probably no longer working! We're only maintaining CircuitPython libraries at this time, but you can run CircuitPython libraries on MicroPython using Adafruit_Blinka - check that out here https://github.com/adafruit/Adafruit_Blinka

The I2C protocol is a way for multiple devices to communicate with each other using just two wires, a clock and data line.  Unlike SPI or similar protocols there is no single 'main' which controls the clock and communication with each device.  Instead any device can be the main that takes control of the I2C clock and data lines to communicate with other devices.  Each I2C device is assigned a unique address that's used to identify it during read and write operations.  When a device sees its address sent on the I2C bus it responds to the request, and when it sees a different address it ignores it.  Using unique addresses many devices can share the same I2C bus without interferance.

With MicroPython you can interact with devices on an I2C bus using a few Python functions.  In most cases you'll act as an I2C 'main' that reads and writes data with other I2C devices.  However on some boards like the pyboard you can also act as an I2C 'secondary' or peripheral that's assigned an address and can listen for and respond to requests from other I2C devices.

Be aware with MicroPython there are some differences in how each board implements the I2C API.  As always consult each board's documentation:

Note I2C protocol support across MicroPython boards is under development and might change over time. Be sure to check your board's documentation to find the latest information on I2C usage!


To demonstrate acting as an I2C main this guide will look at how to read an I2C temperature sensor, the MCP9808, using MicroPython.  This sensor has a simple I2C interface that is similar to many other I2C devices and will help demonstrate basic I2C main usage.  

This guide will also focus on the ESP8266 MicroPython port, but you should be able to use similar code on other MicroPython boards.  Where other boards differ look for notes in bold that call out the differences.

To follow this guide you'll need the following hardware:

Connect the components as shown below:

  • Board 3.3V power to MCP9808 Vdd.
  • Board ground to MCP9808 ground.
  • Board SCL / GPIO #5 to MCP9808 SCL.
  • Board SDA / GPIO #4 to MCP9808 SDA.

With the ESP8266 it implements I2C using software 'bit-banging' so you can use other GPIO pins instead of the #5 and #4 pins above.  Other boards might not have this flexibility so be sure to check your board's documentation.

I2C Setup

To use I2C with MicroPython first you'll need to initialize access to the I2C bus.  Connect to the board's serial or other REPL and run the following commands:

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import machine
i2c = machine.I2C(scl=machine.Pin(5), sda=machine.Pin(4))

These commands will import the machine module which contains the API for hardware access.  Then an instance of the I2C class is created by passing to its initializer the following parameters:

  • SCL pin
  • SDA pin

Remember to pass each pin as an instance of the machine Pin class, not as a number!

pyboard note: The pyboard currently uses an older pyb module instead of the machine module for I2C access, be sure to read the pyboard documentation for more details.

pyboard & WiPy note: The pyboard and WiPy need an extra parameter to explicitly specify I2C main/secondary mode.  In addition these boards need to be told the hardware I2C bus ID number instead of explicit SCL & SDA pins (since these boards don't support software I2C on arbitrary GPIO pins).  Be sure to read each board's documentation for more details.

micro:bit note: The micro:bit I2C interface is a bit simplified and doesn't need to be initialized in the same manner as other boards.  Check out the micro:bit I2C documentation for more details.

Once the I2C bus is setup you can run a command that will scan for any devices connected to it and return their address.  This is useful to confirm the devices you expect are able to talk to the board.  To scan for devices call the scan function like:

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The scan function will return a list of addresses for devices that were found on the I2C bus.  With the MCP9808 connected as above you should see just one device with the address value 24.  

If you don't see anything returned double check your wiring, solder connections, and the power to the MCP9808 board.

Memory/Register Operations

For many I2C sensors and devices they expose data using memory or register addresses.  This means you interact with a device using both its I2C address, and the address of a register or memory location in the device.  Check your device's datasheet to understand exactly how it exposes data and the memory or register addresses to use.

For the MCP9808 first define a few variables that will hold the device and register address values used in this guide.  Creating variables for these values can help simplify and make your code more readable.  These values come from the MCP9808 datasheet:

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address = 24
temp_reg = 5
res_reg = 8

To read the temperature you need to get the value of the 16-bit temperature register (address 5).  With both the device address (24) and register address (5) you can read the register value by calling the readfrom_mem function:

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data = i2c.readfrom_mem(address, temp_reg, 2)

This function takes the following parameters:

  • Device address, this is the address of the I2C device.
  • Memory/register address, this is the address of the memory or register to read on the device.
  • Number of bytes to read, this is how many bytes from the register to read and return.

The readfrom_mem function will return a byte string with the data retrieved from the device.  The length of the byte string should match the number of requested bytes.

Note the I2C protocol acknowledges each byte is received so an operation might fail with an exception (I2C bus error) if the requested device doesn't exist or never responds!

pyboard note: The pyboard currently uses a slightly different function for reading from memory, mem_read. Check the pyboard documentation for more details.

Another way to read data is into a buffer using the readfrom_mem_into function, for example:

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data = bytearray(2)
i2c.readfrom_mem_into(address, temp_reg, data)

This function is similar to readfrom_mem and takes the address and register value as the first two parameters.  However the third parameter is a buffer or bytearray object which will be filled with the data received from the device.  You don't need to pass in the number of bytes to read since the buffer you create knows its size and will fill itself with data.

Using the readfrom_mem_into function can help you save memory and increase performance by creating a buffer to receive data ahead of time.  This is useful if you're reading I2C data in a loop that's constantly calling read functions.  If you're just using the readfrom_mem function in a loop you'll actually be allocating memory to hold the results and that can impact the performance of your program.  Instead try creating a buffer once before the loop and use the readfrom_mem_into function to fill the buffer with data as needed.  This can help prevent the need to 'garbage collect' unused memory (which can be very slow) inside the loop.

Once you have the temperature register value you can pull out the signed 12-bit value in degrees Celsius with a simple Python function:

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def temp_c(data):
    value = data[0] << 8 | data[1]
    temp = (value & 0xFFF) / 16.0
    if value & 0x1000:
        temp -= 256.0
    return temp


Notice the temperature in degrees Celsius is printed!

Try holding your finger on the MCP9808 to heat it up and then read and print the temperature again:

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temp_c(i2c.readfrom_mem(address, temp_reg, 2))

You should see the temperature increase a few degrees!

In addition to reading data from memory or registers you can also write data to using the writeto_mem function.  For example the MCP9808 has a resolution register that lets you configure the accuracy and speed of temperature measurements.  By default the device uses the most accurate but also slowest measurement resolution.  You can tell the device to use faster but less accurate measurements by writing a 0 value to the resolution register (address 8):

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i2c.writeto_mem(address, res_reg, b'\x00')

The writeto_mem function is similar to readfrom_mem and takes the device address and memory/register address as the first two parameters.  The third parameter is a byte string with the bytes to send to the device, in this case the hex value 0x00 (the \x syntax in a byte string lets you specify a raw hex value instead of a printable character).

pyboard note: The pyboard uses a slightly different function for writing to memory, mem_write. See the pyboard documentation for more details.

Now try reading the temperature again and notice there are less digits after the decimal point, i.e. the reading is less accurate:

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temp_c(i2c.readfrom_mem(address, temp_reg, 2))

You can put the device back into its most accurate resolution by setting the register to the value 3:

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i2c.writeto_mem(address, resolution, b'\x03')
temp_c(i2c.readfrom_mem(address, temperature, 2))

That's all there is to reading and writing data with an I2C device that exposes memory & registers!  You'll find almost all I2C sensor use this memory/register mode of operation.  Check the device datasheet to see which register addresses and values to use when interacting with the device!

Device Operations

Some I2C devices don't expose data with memory or registers.  For these devices you simply send and receive bytes of data with no associated memory/register address.  To work with these devices you'll use the following functions that are similar to the register/memory operations above:

  • readfrom function: Read a number of bytes from the device and return them as a new byte string, just like readfrom_mem but without the memory/register address parameter.
  • readfrom_into function: Read a number of bytes from the device and save them in a buffer, just like readfrom_mem_into but without the memory/register address parameter.
  • writeto function: Write a byte string to the device, just like writeto_mem but without the memory/register address parameter.

pyboard note: The pyboard device operations are slightly different and use the name recv and send. See the pyboard documentation for more details.

Also for the ESP8266 MicroPython port you can even perform 'raw' or primitive I2C operations that act at an even lower level then the device and register operations.  Check out the ESP8266 I2C primitive operation documentation for more details.  In most cases you'll want to stick to the higher-level device & register operations above!

This guide was first published on Sep 14, 2016. It was last updated on Sep 14, 2016.
This page (I2C Main) was last updated on Jul 14, 2020.