1 Common Customer Issue: I2C Cable Glitches By Art KayApr 10, 2012
2 Simple Cable Design I2C Master (microcontroller) I2C Slave (e.g. Temp Sensor)One issue with this design is that capacitance between the communication lines increases as the transmission distance increases.
3 Issue with Simple Cable Design The distributed parasitic capacitance in the cable between the SCL and SDA lines can cause cross-talk between “SCL” and “SDA”. The edges of the SCL will feed through to the SDA, and edges of the SDA will feed through to the SCL.
4 Better Cable DesignUsing shielded (coax) cables for SCL and SDA eliminate the issue with the parasitic capacitance (i.e. no more crosstalk). Using twisted pair for Vdut and GND help to eliminate noise pick up in the supply. The better design is especially important for long cables (length > 20cm).
6 Photo of Cable Design Coax for SDA and SCL Shield connected to GND Male solder cup D-SUB 35 pin connector.Connects to J3 on USB DIG Platform.Power supply connectionsShielded twisted pair.
7 Cable EndEnds of the cable connect to your system.
8 Cable LengthI2C is not designed for long distance communication. However, using Coax will allow for lengths up to 1meter.
9 Real World ResultsFor this picture I used two separate shielded cables for SCK and SDA. Power and Ground were on a shielded twisted pair cable. This is a 1m Cable.For this picture I used a twisted pair cable for SCK, SDA, Power, and GND. The glitches occur on the falling edge of clock and data. The rapid change in the signal feeds through the parasitic capacitance between the twisted pair cables. This is a 1m Cable.