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RM44L920: I2C communication issues due to EMI

Matthias Schmitt
Expert 5670 points

Part Number: RM44L920
Other Parts Discussed in Thread: TLC59116, P82B715

Dear Hercules team,

My customer is seeing some I2C issues on a RM44 based H/W due to EMI.
They do have a small driver boards (with a P82B715 on it) attached to the RM44 and long wires with two long  small LED-driver boards (with some TLC59116).
The following oscilloscope plots are made with an USB-Oscilloscope directly on the traces.


They are working on damping the HF-harmonics but also try to understand what happen to the communication itself.
So they try to figure out what creates the behavior of the RM44 related to the observed “short clocks”.
They assume this is related to the “clock stretching” feature of an I²C-bus, but in their case the clocks are shortened…

Can you help them to understand why the RM44x-I2C is breaking the communication in this case?

Best Regards,
Matthias

  • 0
    TI__Guru**** 199216 points
    Hello Mathias,

    Are those waveform captured at slave side or on RM44 MCU I2C pins?

    The Outputs (SDA & SCL) from a I2C device are ‘Open-Collector/Drain’ which means they need to be ‘Pulled-Up’ externally from the device by a resistor. A slow rising of the signals and/or a floating high level (not saturated to bus-Vcc) indicates a too capacitive bus and pull-up resistors needs to be changed/added. The total pull-up resistance is normally in the range of 10 kohm till 1 kohm. Undershoots mostly indicates a long, inductive, bus wire. Add series resistors in the bus-signal lines to eliminate this.

    Noise in the signals can be caused by a bad ground connection and/or cross-talk from other signals. The boards (MCU, I2C slave) should have the same GND.

    Clock Stretching is NOT generated by the Master but normally done by the Slave after the ACK-bit or by a 2nd Master during Arbitration with each address CLK, this to Hold (Pause) the Master.