How Long Can My Cable Be for Usage With I2C?

Updated Oct 14, 2023

Reported In

Hardware

  • USB-8451
  • USB-8452

Issue Details

I want to use a cable longer then 1 Meter. What is the maximum cable length that I can use with I2C?

Solution

The maximum length of your cable depends largely on its capacitance. It is a strict requirement to use a cable that does not exceed a capacitance value of 400 pF. Using a cable with higher capacitances can cause the maximum frequency possible over I2C to significantly drop.
This is due to the fact that a rising capacitance will also affect the rise and fall times of the signal.
As all cables are not manufactured equally, it is your responsibility to check in advance the meter-to-capacitance ratio of your selected cable.

For example, CAT5 cabling comes with strict requirements. Such a cable should nominally have a capacitance rating of 52 pF. As such, you can comfortably use a 3-meter-long CAT5 cable. At 4 m in length, you will start to exceed the 400 pF maximum rating, so afterwards, your milage may vary and the intended performance cannot be guaranteed anymore.

In order to ensure the best performance, please choose a cable type that comes with shielding (to eliminate stray capacitances) and use pull-up resistors on each end of the cable.

Should you wish to use distances larger than 4 meters, you should consider I2C bus extenders. Such extenders can boost the signal strength and allow you to use I2C over larger distances.