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Solving common I2C Communication Setup Issues

Updated: May 13, 2024

Introduction

I2C stands for Inter-Integrated Circuit and is a handy way to connect microcontrollers to all sorts of devices like sensors, screens, and even small memory chips. Although it was introduced by Philips Semiconductors over three decades ago, it's still widely used because it's simple and it works well. However, setting it up correctly can sometimes be a bit challenging. For more information about the I2C interface, you can check out the following references. Wikipedia.


Typical Issues When Setting up I2C

Let's walk through some usual troubles you might face when setting up an initial I2C interface communication.

Wiring Problems: The two main lines in I2C, SDA (data line) and SCL (clock line), need to be connected correctly. If they're not, your devices won't communicate.  

Pull-up Resistor Troubles: I2C needs so-called pull-up resistors to work. If these aren't in place or if they're the wrong kind, your I2C bus won't function properly. 

Short Circuits: Connecting the SDA and SCL lines by accident is problematic, as it can disrupt the signal and prevent communication between devices.

Capacitance Issues: If the SDA and SCL lines have too much electrical 'load,' it can slow down the communication, making it unreliable.

Mixing Up Pins: Getting the SDA and SCL lines mixed up (swapped) is an easy mistake to make, and it will stop your setup from working.

Interference: Sometimes other parts of your circuit can interfere with the I2C lines, which leads to errors in communication.

Target Address Errors: Every device on the I2C bus has a unique target address. Using the wrong target address will prevent the devices from recognizing each other's signals.


How to Tackle These Issues

Here are some steps to help you systematically fix these common issues based on a step-by-step approach.

Double-Check Connections: Always make sure the SDA and SCL wires are connected to their correct pins on both the microcontroller and the devices. Wiring issues are one of the most common causes why communication between devices does not work. Performing a visual inspection before starting debugging can easily solve this kind of problem. 

Test the Pull-Up Resistors: You'll want to check that the SDA and SCL lines can reach the voltage they're supposed to when they're pulled 'up' (towards the power supply voltage) and 'down' (towards zero volts). Use the following sequence to verify this:

  • Set SDA to high and measure voltage level at SDA. Must be VCC

  • Set SDA to low and measure voltage level at SDA. Must be 0V

  • Set SCL to high and measure voltage level at SCL. Must be VCC

  • Set SCL to low and measure voltage level at SCL. Must be 0V

Search for Shorts: Use a multimeter to see if the SDA is touching the SCL anywhere. They should be separate, not connected. Alternatively, you can do the following sequence:

  • Set SDA to high and SCL to low. Check that both levels (SCL and SDA) are correct

  • Set SDA to low and SCL to high. Check that both levels (SCL and SDA)are correct

Verify the achievable speed: Your I2C communication should work with a certain frequency. Make sure it's not too fast for your actual system. To verify this, you can do the following steps:

  • Apply a clock with the target frequency on SCL and measure the duty cycle. Check that the duty cycle is within the expected range

  • Apply a clock with the target frequency on SDA and measure the duty cycle. Check that the duty cycle is within the expected range

Look for Acknowledge from the target: When the controller sends out a command, the target acknowledges it to signal the transmitter that the data was successfully received. If it doesn't, the communication will not work properly. To ensure that it works as intended, follow the given steps:

  • Perform a read within a given range or based on a given list of target addresses

  • If no response (NACK) --> reduce speed (400kHz --> 100kHz)

  • If no response (NACK) --> swap SDA / SCL and retry

Identify Your Devices: In case more than one target is connected to the I2C bus, make sure you're actually communicating with the device you intend to. To verify this, you can do the following sequence: 

  • Perform a read test for known information (e.g. serial number, product ID…) about the target

  • Perform a write / read back test

Conclusion

Setting up an I2C communication can be time-consuming and can take some patience. By following the described step-by-step plan, this process could be sped up significantly. Following a systematic approach and not taking the second step before the first one is key to successfully getting your I2C communication to work.

We put together a checklist for setting up an I2C communication based on a step-by-step approach. The checklist can be downloaded from our GitHub repo for free. In an upcoming post, we'll explore how SmartWave can be used to speed up this process even more, making the I2C setup as simple as possible. Stay tuned, and you'll be well on your way to mastering I2C communication.




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