I2C Error Compendium

by OlliW

I2C errors are probably the most annoying side effect of the technology we are using in our brushless gimbals.

I2C errors occur then the signals on the I2C clock and data lines are disturbed beyond certain limits, and this happens due to two main reason:

• capacitive load on the I2C clock and/or data lines
• induced signal disturbances due to capacitive coupling to outside fields

Importantly, all error sources add up. One hence can't give decisive threshold values, such as, for instance, that the length of the I2C wires should not be longer than 40 cm. Without any further error sources one might be able to use even much longer wires, while if motor wires are nearby only shorter wires might be acceptable. It's like a glass of wine: When it's full, it's full. It doesn't matter if it had been filled with Bordeaux, Chianti, or any mixture. When the glass is filled only a tiny bit with Bordeaux, one can add a lot of Chianti before it overflows, but when the glass is nearly filled with Bordeaux, only a tiny drop of Chianti is acceptable.

So, you're not going to get an answer here to the question how long the cables can be, but "only" that the longer they are the more probable it is for i2c errors to occur.

Methods to Reduce I2C Error Rate

Before we look at the I2C errors, the methods shall be briefly summarized which can help to reduce the error-proneness of the I2C connection to the IMU module:

• reduce length of cable

• twist wires of the cable; this comes in many variants such as twisting all four, just twisting SCL and SDL, twisting SCL with GND and SDA with VCC, and so on

• shield the wires; using e.g. a coax cable or wrapping the wires with (self adhesive) conductive cloth EMI shielding

• add ferrite rings; this comes in many variants such as to put it to the I2C wires and/or the motor wires, or to the left and/or right ends of the wires, and so on

• lower the (motor) voltage

• increase spacing between I2C cables and motor wires

• reduce the I2C pullup resistors on e.g. the IMU module

Capacitive Load

Capacitive load decreases the slew rate of the signal, and eventually prevents the electronics to detect it correctly, and an I2C error occurs.

Capacitive load arises from the cables itself, and the longer the cables are the higher is the capacitive load. Thus, for any I2C system there is an "intrinsic" maximal length up to which the I2C communication works. In practice this maximal length is not achieved, since, as discussed in the introduction, other I2C error source may be present, reducing the acceptable length. The capacitive load by the cables depends on the cables. Thin wires have a smaller capacity than thicker wires. In practice, however, there is usually room to play with that factor. The acceptable capacitive load or maximal possible length depends also on other factors, such as the current capabilities of the I2C drivers and the pullup resistors.

Capacitive load however also arises from anything conducting in the surrounding. Drilling cables